ANALYSIS OF HYBRID SOFC-STIRLING ENGINE PLANTSetd.dtu.dk/thesis/316846/ANALYS_1.pdf · SOFC...

ANALYSIS OF HYBRID SOFC-STIRLING ENGINE PLANTS

MASTER THESIS

10-FEBRUARY-2011

Author: Carlos Boigues Muñoz Supervisor: Masoud Rokni

Co-supervisor: Henrik Carlsen

Analysis of hybrid SOFC-Stirling engine plants

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PREFACE

This Master Thesis was carried out from October 2010 to February 2011 at the Technical University of Denmark, and more precisely in the Mechanical Engineering Department, under the supervision of Professor Masoud Rokni and Professor Henrik Carlsen.

First of all I would really like to thank my family for their unconditional support, not only in studies, but in life itself. Words are not enough to express my gratitude for all the things they have done for me in these 23 years.

Special thanks to Professor Masoud Rokni and Professor Henrik Carlsen for their help and advises.

I would like to thank all the great friends I made during the Erasmus program, and with who I spent one of the best times of my life, if not the best. I hope this is just the beginning of a lifetime friendship.

Last but not least I would like to thank my friends in Spain for their continuous support and unwavering friendship.

Lyngby (Copenhagen), 10th of February 2011

Carlos Boigues Muñoz


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ABSTRACT

The wish of producing electrical energy with high efficiencies and low contamination levels has been motivation enough to undertake the study of the hybrid SOFC-Stirling engine power plant.

The purpose of the project is to analyse the optimum power plant configuration and type of fuel to obtain 10 electrical kilowatts.

Three fuels have been studied for fuelling the SOFC stacks: Natural Gas, ammonia and methanol. The configuration of the power plant is different for each fuel type, as they require different processes in the plant cycle. The optimization for each configuration has been carried out looking for a compromise between maximum efficiency and low cost.

The calculations have been carried out in DNA (Dynamic Network Analysis), a component-based simulation tool for energy systems, created and developed in the Thermal Energy Systems Department of DTU.

The first attempt of integrating an SOFC plant with a Stirling cycle has been developed, obtaining an improved Natural Gas power plant with an efficiency value of 59,01%, which represents an efficiency increase of 16,46% with respect to the original Natural Gas power plant (50,67%). In both plants the fuel has to be desulphurized and pre-reformed before entering the SOFC stack. For the ammonia power plant, the optimum configuration operating at nominal load has a thermal efficiency of 57,89%, and the fuel does not have to undergo any physicochemical treatment before entering the SOFC, it only needs to increase its temperature to 650ºC. The methanol power plant has an efficiency of 55,12%. The methanol needs to be steam reformed before entering the anode inlet of the SOFC, so the steam present in the used fuel mass flow is used for the purpose. This is accomplished by recirculating part of the used fuel mass flow to the methanol stream.

The thermoeconomic analysis carried out demonstrates that currently none of the power plants analysed can compete with the traditional power generation systems, as the cost of electricity is higher than the market price. In the actual time, the lowest cost of generated electricity corresponds to the ammonia power plant, with 0,2742€/kWh, whilst the market price was 0,2462€/kWh in 2010. For a conservative hypothesized future scenario, were the price of the SOFC stacks and Stirling engine is lower than the actual price, the electricity generated by the ammonia power plant has a cost of 0,1828€/kWh.


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CONTENTS

PREFACE 3

ABSTRACT 5

CONTENTS 7

1. INTRODUCTION TO THE SOFC AND STIRLING ENGINE TECHNOLOGY 10 1.1. Introduction 10 1.2. General overview of fuel cells 11

1.2.1. Operating basis of fuel cells 11 1.2.2. Development and classification of fuel cells 12 1.2.3. Thermodynamic aspects 12 1.2.4. SOFC principles and characteristics 13

1.3. General overview of the Stirling engine 16 1.3.1. Introduction to the operation of the Stirling engine 16 1.3.2. Ideal Stirling cycle 17 1.3.3. Advantages and disadvantages of the Stirling engine 18 1.3.4. Applications of the Stirling engine 18

1.4. General overview of the hybrid power plant 19

2. CONFIGURATION, STUDY AND ANALYSIS OF THE HYBRID SOFC-STIRLING ENGINE POWER PLANTS 21 2.1. SOFC-Stirling engine power plant fuelled with Natural Gas 21

2.1.1. Natural gas as a fuel 21 2.1.2. Configuration of the basic Natural Gas power plant 23 2.1.3. Optimization of the basic Natural Gas power plant 25 2.1.4. Study and analysis of the power output and plant efficiency 32 2.1.5. Results of the optimized basic Natural Gas power plant 34 2.1.6. Improved SOFC-Stirling engine plant fuelled with Natural Gas 36 2.1.7. Configuration of the improved Natural Gas power plant 40 2.1.8. Study and analysis of the power output and plant efficiency 43 2.1.9. Results of the optimized improved Natural Gas power plant 44 2.1.10. Comparison between the basic and improved Natural Gas power plants 45

2.2. SOFC-Stirling engine power plant fuelled with Ammonia 46 2.2.1. Ammonia as a fuel 46 2.2.2. Configuration of the ammonia power plant 47 2.2.3. Optimization of the ammonia power plant 49 2.2.4. Study and analysis of the power output and plant efficiency 53 2.2.5. Results of the optimized ammonia power plant 54

2.3. SOFC-Stirling engine power plant fuelled with Methanol 56 2.3.1. Methanol as a fuel 56 2.3.2. Methanol steam reforming 57 2.3.3. Configuration of the methanol power plant 58 2.3.4. Optimization of the methanol power plant 60 2.3.5. Study and analysis of the power output and plant efficiency 66 2.3.6. Results of the optimized methanol power plant 67


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2.4. Comparison between the improved Natural Gas, Ammonia and Methanol power plants operating at nominal load (10kW) 69

2.4.1. Power and efficiency 69 2.4.2. Heating power 69 2.4.3. Hot water generation 70

2.5. Comparison between the improved Natural Gas, Ammonia and Methanol power plants operating at partial load 71

2.5.1. Concept of partial load 71 2.5.2. Efficiency 71 2.5.3. SOFC and Stirling engine power output 73 2.5.4. Optimum recirculation in the power plant fuelled with methanol 74

3. THERMOECONOMIC ANALYSIS 76 3.1. Introduction to thermoeconomics 76 3.2. Fundamentals of the thermoeconomic model 76

3.2.1. Equivalent hours of utilization 76 3.2.2. Total capital investment 77 3.2.3. Amortization 78 3.2.4. Calculations of the capital investment cost rate and the O&M cost rate 80 3.2.5. Estimation of purchase cost 81

3.3. Results of the thermoeconomic analysis 84 3.3.1. Improved Natural Gas power plant 84 3.3.2. Ammonia power plant 86 3.3.3. Methanol power plant 87 3.3.4. Specific capital cost of power plants 88 3.3.5. Specific costs of electricity and water 88

3.4. Future scenario 89 3.4.1. Description of the future scenario 89 3.4.2. Cost rates for capital investment and operation and maintenance in the

future scenario 89 3.4.3. Specific costs of electricity and water in the future scenario 90

4. CONCLUSION 91

NOMENCLATURE 92

REFERENCES 94

APPENDIX I 96

APPENDIX II 100

APPENDIX III 111

APPENDIX IV 115


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1. INTRODUCTION TO THE SOFC AND STIRLING ENGINE TECHNOLOGY

1.1 INTRODUCTION

The aim of this project is to study and analyze the performance of a 10 electric kilowatt hybrid SOFC-Stirling engine power plant where electricity is generated as the main product, and heat is produced as a by-product.

Several kinds of fuels have been used, varying for each kind the plant configuration to adapt it to the specifications each one of them requires, and searching to obtain a high efficiency with a low capital investment and operation costs.

Simulations were carried out in DNA (Dynamic Network Analysis), computer software designed and developed by the Thermal Energy Systems department of DTU. DNA is a component-based simulation tool for energy system analysis.

As a 10kW power system is suitable to be installed in a house, shop or small building; a thermoeconomic study has been carried out to determine if it is economically advantageous to use this isolated power system rather than being connect to the electrical grid and district heating net.


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1.2 GENERAL OVERVIEW OF FUEL CELLS

1.2.1 Operating basis of fuel cells

A fuel cell is a power device which uses a source of fuel and an oxidant to provide electrical energy by means of an electrochemical reaction. Heat is generated as a by-product. The fuels used are pure hydrogen or hydrocarbons, depending on the type of the cell, whilst air or pure oxygen is used as oxidant.

Fuel cells basically consist of two electronically conducting electrodes, cathode and anode porous gas diffusion catalyst layers and an ionically conducting electrolyte. For the electrical current to flow, interconnections are needed, and some fuel cells are built with a gasket to forbid gas leakage between anode and cathode.

Figure 1.1- Fuel cell

The total electrochemical reaction can be divided into two semi-reactions: oxidation that takes place in the anode, and reduction which takes place in the cathode. In the oxidation reaction the liberated electrons of the fuel pass through the external circuit and flow to the cathode, where they reduce the oxygen present in the air. The electrochemical circuit is closed by the migration and diffusion of the electrolyte ions inside the fuel cell. Catalysts are used to make more favourable the reaction, however, they are not consumed. [3]

As the voltage of a single SOFC is below 1V, such fuel cells can be connected in series and/or parallel to obtain the desired current and voltage. The build up of the individual cells and the interconnections is called a stack.


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1.2.2 Development and classification of fuel cells

Sir William Grove demonstrated the basic principle of fuel cell operation in 1839. His prototype operated with two separate platinum rods surrounded by closed tubes containing oxygen and hydrogen gas respectively, submerged in a dilute sulphuric acid electrolyte solution. However it was not until the 1960’s when the development and application of fuel cells occurred, thanks to the economical injection of the United States in the space race program.

This energy conversion technology is usually classified by the nature of the electrolyte. Table 1.1 illustrates the main operating parameters of the different fuel cell technologies currently available.

FUEL CELL TEMPERATURE [ºC]

FUEL OXIDANT ION TRANSFERRED

Alkaline fuel cell (AFC)

60-250 H2 O2 2OH-

Polymer electrolyte fuel

cell (PEFC)

30-100 H2 Air 2H+

Phosphoric acid fuel cells (PAFC)

160-220 H2 Air 2H+

Direct metanol fuel cells (DMFC)

60-200 Methanol Air 2H+

Molten carbonate fuel

cell (MCFC)

600-800 Hydrocarbons Air+ O2 CO32-

Solid oxide fuel cell (SOFC)

600-1000 Hydrocarbons Air O2-

Table 1.1

The two main structural designs of fuel cells are planar and tubular. In the first one the components are assembled in laminar layers, with the air and fuel flowing through the anode and cathode cannels. In this configuration, air and fuel can flow co-directionally, anti-directionally or by cross-flow.

In the tubular design the components are assembled as a cylinder, with the cell constructed in layers around a tubular cathode. The air is the fluid that flows through the inside of the tube, whilst the fuel flows around the external part of the tube. The tubular fuel cells have a low power density and the fabrication costs are very high in comparison with the other designs.

1.2.3 Thermodynamic aspects

The production of electrical energy in a fuel cell is a direct consequence of the Gibbs free energy variation during the electrochemical reaction that takes place in the interior of such fuel cell.


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Where Wel,max is the maximum achievable electrical energy

The variations of the reversible potential (Er) with temperature and pressure are expressed by the equations (1.2) and (1.3) respectively:

Er0 is the standard electrode potential compared with the standard hydrogen electrode (SHE)

which is used as a reference. The standard conditions are those with a temperature of 25ºC and 1 atmosphere of pressure.

The maximum efficiency of the process is given by the following expression:

However in practice the fuel cell efficiency is lower due to the internal resistances and the irreversible overpotential losses, which generally include three main sources: activation overpotential, ohmic overpotential and concentration overpotential. [Y.Zhao, N. Shah, N. Brandon].

The utilization factor (Uf) defines the relationship between the reacted fuel in the electrochemical reaction and the input fuel in the system.

1.2.4 Solid Oxide Fuel Cells principles and characteristics

The solid oxide fuel cells dissociate the oxygen present in the air (cathode), yielding O2- anions. These migrate through the electrolyte and oxidize the hydrogen atoms carried to the anode by the fuel. The electrochemical reaction inside the cell produces water in the anode side, and liberates electrons that flow from anode to cathode, which enable the reaction to keep going, as these are responsible of reducing the oxygen in the cathode.


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Figure 1.2- SOFC electrochemical reaction

At the cathode oxygen is reduced according to the following reaction scheme:

The electrolyte enables the transport of pure oxygen ions to the anode, where they oxidize the fuel, resulting in the generation of water and two electrons, as shown in reaction (1.8).

The high working temperatures combined with the harsh environments inside the cell require the components used in the fuel cell stacks to possess specific qualities which assure the perfect functioning of the system. The thermal expansion coefficient of each component must be as similar to one another as possible, in order to avoid thermal stresses. Inertness between components must also be a priority.

• Cathode: has to be porous to allow oxygen molecules to reach the electrolyte. In some designs it provides structural support for the cell as it is the heaviest component. As the cathode is exposed to an oxidizing atmosphere, lanthanum manganite (LaMnO3) is usually used for the cathode. It is doped with rare earth elements to improve the electronic conductivity.

• Anode: a nickel-zirconia cermet (Ni-YSZ) is used, amongst other things because of its high electronic conductivity and chemical stability under reducing atmospheres. The nickel also plays an important role in the internal reforming of the fuel, as it acts as a catalyst.


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• Electrolyte: must have a high ionic conductivity to allow the oxygen ions to migrate through it, but no electrical conductivity. It must be as thins as possible to minimize the ohmic losses in the cell. Zirconia doped with 8 to 10% mole of yttria Yttria-stabilized zirconia (YSZ) is the most effective material for electrolyte construction, as it is stable for both oxidizing and reducing atmospheres. However, it only conducts ions above approximately 650ºC, therefore the SOFC must work with higher temperatures than this one.

• Interconnect: must have 100% electric conductivity in order to reduce electrical losses, and no porosity to prevent gases reacting inside it. LaCrO3 doped with rare earth elements is the material used to build the interconnect as it has all the above desired qualities, plus it works perfectly above 700ºC.

Solid oxide fuel cells have an important advantage with respect to the rest of fuel cells, and it is that due to the high temperatures reached during the functioning of the system, the low molecular weight hydrocarbons can be internally reformed, without the needs of an external reformer.

Reaction (1.9) is endothermic, being the specific energy needed to make the reaction occur ΔH=206 kJ/mol, whilst the second one (reaction (1.10)) is exothermic with a specific energy of ΔH=-41 kJ/mol. As it can be seen, the heat of the SOFC is used to reform the methane into carbon monoxide and hydrogen, whilst the carbon monoxide reacts with water to obtain carbon dioxide and hydrogen, providing energy, and eliminating the harmful CO from the outlet gases.


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1.3 GENERAL OVERVIEW OF THE STIRLING ENGINE

1.3.1 Introduction to the operation of the Stirling engine

A Stirling engine is an energy converter of the heat engine type developing shaft work output from a thermal input [4]. It was invented in 1816 by Dr. Robert Stirling, although the initial name for the invention was hot-air engine.

The Stirling engine compresses a light molecular weight gas such as helium (He) or hydrogen (H2) at low temperatures and expands it after a heating process, obtaining output mechanical work. This gas is totally contained, thus it receives the energy from an external heat source and the energy rejection process with the surroundings is done using a heat exchanger. Regenerative heat exchangers are an essential element in the engine as they store and return heat that would otherwise be exchanged with the environment to the working fluid, reducing the irreversibility of the system, thus increasing the thermal efficiency. The regenerative effect keeps the heat input to the system to a minimum, maintaining the desired power output.

The displacer type Stirling engines have two pistons in tandem: the power piston and the displacer piston. The first one is tightly sealed and moves up and down with the gas expansion, whilst the displacer is loosely fitted and is responsible of moving the fluid between the hot space and the cold space.

Figure 1.3- Displacer type Stirling engine


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1.3.2 Ideal Stirling cycle

1-2 Isothermal expansion: heat is needed to compensate the cooling effect of expansion and maintain constant the temperature.

2-3 Isochoric heat removal: the gas flows through the regenerator transferring heat to it and cooling down.

3-4 Isothermal compression: Heat needs to be removed to compensate the heating effect of the compression to maintain constant the temperature.

4-1 Isochoric heat addition: the gas flows through the regenerator in the opposite direction as in 2-3 and heat is returned to it, increasing like this its temperature.

Figure 1.4- Stirling engine ideal cycle P-V diagram

The equation for the theoretical thermal efficiency is expressed by means of Carnot’s efficiency equation.

In the ideal Stirling engine a regenerator is used to recover the heat rejected in process 2-3 and return it to process 4-1. If the regenerator is not 100% efficient, the returned heat will generate the process 4-4’, where 4’ is between 4 and 1, and process 4’-1 will need an external heat energy transfer to occur.

The efficiency of the regenerator may be expressed by means of:


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Defining the temperature ratio (ξ) and the compression ratio (rv) by expression (1.13) and (1.14) respectively, the thermal efficiency can be expressed by equation (1.15). If the regenerator efficiency is 100%, this last equation transforms directly into Carnot’s efficiency expression (equation (1.11)).

1.3.3 Advantages and disadvantages of the Stirling engine

The principal advantage of the Stirling engines is that they can run on any heat source, so any non-renewable fuel can be used for external combustion, as well as renewable sources like solar, geothermal and nuclear. As there is no internal combustion, the engine is very quiet and because there are no continuous explosions inside the motor, the vibrations are minimal.

One disadvantage of the Stirling engine is that it needs a long time to warm up, therefore it is impossible to obtain an immediate response in the start up, or when changing the power load. Because of this it is better to run the motor continuously. This motor has also a low power/weight ratio so it is not suitable for non-stationary purposes.

1.3.4 Applications of the Stirling engine

In the last lustrum, the use of the Stirling engine for large energy production has been proven not to be a utopia, but a reality. These motors, integrated with a parabolic mirror can convert solar energy into electricity with a high efficiency. A megaproject is currently being developed in the Mojave Desert (U.S.A), to produce 500MW from Stirling engines integrated with mirrors.

Swedish shipbuilder Kockums has built around 10 submarines operating with Stirling engines. Swedish, Danish and Japanese armed forces are its main clients.

Stirling engines can be used as cryocoolers if mechanical power is introduced into the system. The Rankine cooling cycle is not effective for temperatures below -40ºC as there are no suitable refrigerants with boiling points that low, so Stirling cryocoolers can be used instead to reach temperatures of -200ºC [5].


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Figure 1.5- Stirling engine integrated with a parabolic mirror

1.4 GENERAL OVERVIEW OF THE HYBRID POWER PLANT

Solid oxide fuel cells operate under high temperatures, ranging from 750ºC to 1100ºC, releasing exhaust gases at very high temperature. The cathode off gas is basically air with a low content of oxygen therefore it can be used for heating purposes. As the utilization factor of the fuel cells is never 100%, the anode off gas usually contains traces of fuel that need to be eliminated for ecological reasons. A combustion process can take place for the anode off gases to eliminate the unused fuel in the SOFC, producing combustion products at very high temperatures which can be used to operate gas turbines, produce steam for steam turbines, heat source for a Stirling engine or district heating.

Gas and steam turbines require large dimensions to achieve high efficiencies, thus they are suitable for power plants that generate hundreds of kilowatts or tens of Megawatts. For much smaller power plants, efficiency is drastically reduced and the economic costs are too elevated when compared with the benefits from the power output. The use of a Stirling engine can be advantageous in the kilowatt magnitude, and specially in the nearby future with a reduction of its cost.


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Figure 1.6- Power outputs of the hybrid power plant

The main purpose of the SOFC-Stirling engine hybrid power plant is to produce electricity, however heat is released as a by-product, and can be used for heating purposes such as increasing the temperature of water for domestic consumption. This converts the power plant in a cogeneration system.

The electrical energy demanded by the components in the power plant, is supplied by the power plant itself. The electrical efficiency of the overall power plant is thus expressed by equation (1.16).

• is the electrical power generated in the solid oxide fuel cell

• is the electrical power generated by generator attached to the Stirling engine.

• is the electrical power consumed by a generic component i.

• is the fuel mass flow.

• is the lower heating value of the fuel.


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2. CONFIGURATION, STUDY AND ANALYSIS OF THE HYBRID SOFC-STIRLING ENGINE POWER PLANTS

2.1 SOFC-STIRLING ENGINE POWER PLANT FUELLED WITH NATURAL GAS

2.1.1 Natural Gas as a fuel

The world, headed by North America, is shifting towards natural gas for power generation as a substitute for coal and oil [6].

Natural Gas (from now on NG) is an underground natural energy source under pressure, which can be found dissolved in heavier hydrocarbons or by itself in rock cavities in the Earth’s crust. It is a mix of light gases, being its principal constituent methane. Heavier hydrocarbons such as propane and butane are present in the natural gas in much smaller quantities, whilst depending on the oilfield it may also contain nitrogen, carbon dioxide and/or hydrogen sulphide. The following table shows the typical composition of natural gas.

Table 2.1

The composition of the natural gas used in this thesis is illustrated in table 2.2.

FORMULA MOLAR FRACTION (%) CH4 87 C2H6 8,1 C3H8 1

C4H10-N 0,6 CO2 2,925 H2S 0,375

Table 2.2


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The pie chart below illustrates the different uses of NG, and the specific weight of each one of them in the total consumption. As can be observed, the production of electricity represents nearly one quarter of the NG consumption.

Figure 2.1- Pie chart of different uses of NG [1]


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2.1.2 Configuration of the basic Natural Gas power plant

Figure 2.3 illustrates the configuration of the basic hybrid SOFC-Stirling engine power plant fuelled by NG. The reader must not be confused by the word <<basic>>, as it does not refer to simplicity, but to a first attempt to integrate the SOFC cycle with the Stirling engine cycle. The SOFC cycle represents the Topping cycle, whilst the Stirling engine, burner and the water heating are considered to be the Bottoming cycle.

Figure 2.3- Basic Natural Gas power plant


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LETTER COMPONENT LETTER COMPONENT

A Cathode air compressor

H CPO air compressor

B Cathode air preheater

I CPO reformer

C SOFC J Fluegas splitter D Fuel Valve K Burner E Fuel Preheater L Stirling engine F Desulphurizer M Water pump

G CPO reformer

preheater N Water heater

Table 2.3- Component legend for the basic NG power plant

TOPPING CYCLE

1-2 air in standard conditions is compressed in order to reach the SOFC cathode’s working pressure.

2-3 standard air is heated by means of a heat exchanger, as it needs to be introduced into the cathode side of the SOFC with a similar temperature to the SOFC working temperature.

3-4 many of the oxygen present in the standard air is ionized inside the SOFC and migrates to the anode mass flow. The air leaves then the fuel cell stack with a low content of oxygen (fluegas).

4-5 the fluegas is used to heat up the air flow coming from the cathode blower.

5-6-7 fluegas is splitted into two streams, one of them enters the burner and the other is liberated to the environment.

8-9 the pressure of the NG is decreased to the working pressure of the system.

9-10 NG increases its temperature before entering the desulphurizer.

10-11-12 fuel is desulphurized obtaining the clean gas, which has the same composition as the fuel but without hydrogen sulphide (H2S).

12-13 the clean gas is heated up in the CPO reformer preheater by the high temperature usedfuel.

14-15 standard air is compressed to the required working pressure of the steam reformer.

13-15-16 clean gas is pre-reformed, reforming all the high-weight hydrocarbons, obtaining the reformed gas.

16-17 the SOFC internally reforms and electrochemically oxidizes the pre-reformed fuel (reformed gas) obtaining electricity. Heat is obtained as a by-product. The unreacted reformed gas along with water exits the anode side of the fuel cell. This outlet fluid is known as used fuel.


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17-18 the high temperature used fuel transfers part of its thermal energy to the desulphurized fuel mass flow.

18-19 the low temperature used fuel transfers part of its remaining thermal energy to the fuel mass flow.

BOTTOMING CYCLE

7-19-20 the unreacted fuel in the used fuel reacts with the unreacted oxygen in the fluegas in a combustion process, obtaining combustion products.

20-21 the high temperature combustion products are used to make the Stirling engine work.

22-23 the cooling water of the Stirling engine has as objective to decrease the temperature inside the machine, and is also responsible for providing heat.

27-22 the cooling water is pumped back again into the Stirling engine after returning from the domestic heating system.

21-24 the combustion off gases still have a high temperature, which is used to heat domestic water.

25-26 hot water is heated and stored for its use in domestic purposes.

2.1.3 Optimization of the basic Natural Gas power plant

2.1.3.1 SOFC

Thermal restrictions of ceramic components in the SOFC limit the maximum temperature difference between the anode and cathode inlet mass flows, and the working temperature of the fuel cell to a value around 180ºC [7].

The fuel cell working temperature is fixed to 780ºC, as the insulating materials required for this temperature levels are cheaper than the ones used for higher operating temperatures. The following graphs show the values of the plant efficiency for different temperatures in the anode and cathode inlet. For the anode study, the cathode’s temperature was set to 600ºC whilst for the cathode study the anode’s temperature was fixed to 650ºC.


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Figure 2.4- Response of the plant’s efficiency with the variation of the anode inlet temperature

Figure 2.5- Response of the plant’s efficiency with the variation of the cathode inlet temperature

Increasing the temperature of the cathode and anode inlets of the fuel cell affects negatively the efficiency of the power plant. The immediate effect of increasing the temperature in both cases is an increase in the air mass flow entering the cathode, thus more power is generated in the SOFC, however the air compressor has a bigger power demand. The new overall balance results to be negative, with more extra power being demanded than extra power being generated.

The variation in the anode’s temperature appears to be more critical, as the reduction of the efficiency is of around 4% in only 30ºC difference. For the plant to work in its optimum conditions, the inlet temperatures should be as low as possible, always taking into account the

0.46

0.47

0.48

0.49

0.5

0.51

0.52

645 650 655 660 665 670 675 680 685

Plan

t eff

icie

ncy

Anode inlet temperature (ºC)

0.504

0.506

0.508

0.51

0.512

0.514

0.516

0.518

580 600 620 640 660 680 700 720

Plan

t eff

icie

ncy

Cathode inlet temperature (ºC)


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material restrictions. Thus, the optimum temperatures for the SOFC inlets are 600ºC for the cathode and 650ºC for the anode.

2.1.3.2 Desulphurizer

Most fossil fuels contain organic sulphur compounds that usually have to be removed before any further fuel processing can be carried out. Sulphur reacts with oxygen forming sulphur dioxide, which is an intermediate in the production of sulphuric acid and responsible for the acid rain if generated in the atmosphere. Sulphur also reacts with the SOFC’s reforming catalysts inactivating them in matter of minutes.

For the desulphurizer to decrease the sulphur’s part per million (ppm) in the fuel stream, the temperature of the inlet fuel gas must be at least 200ºC. This explains the presence of the heat exchanger previous to the desulphurizer, as Natural Gas is supplied at 20ºC. At the mentioned 200ºC, the lighter sulphur compounds such as H2S can be absorbed onto a bed of zinc oxide, forming zinc sulphide and water. This process is shown in equation (2.1).

The following table illustrates the molar composition of the inlet and outlet flow of the desulphurizer. The model completely removes the sulphur compounds from the stream, being a realistic assumption, as in practice the hydrogen sulphide ppm is reduced to negligible amounts.

COMPONENT INLET OUTLET CH4 0,87 0,8733 H2S 0,00375 0 C2H6 0,081 0,0813 CO2 0,02925 0,02936 C3H8 0,01 0,01004

C4H10-N 0,006 0,0060 Table 2.4

2.1.3.3 CPO pre-reformer

The catalytic partial oxidation (from now on CPO) pre-reformer uses air to break down the heavy hydrocarbons of the fuel and obtain hydrogen and methane instead, which can be introduced in the anode side of the solid oxide fuel cell. The advantages of this reformer are that it does not need super-heated steam in the start-up, therefore its working temperature is relatively low and the design of the system is simple. The partial oxidation produces less hydrogen per mole of fuel than the steam reforming, however this is not a problem in the analyzed system, as there is an internal reforming in the SOFC which transforms all the low weight hydrocarbons into hydrogen. An important disadvantage with the CPO pre-reformer is


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that the injected air dilutes the hydrogen with nitrogen, therefore the fuel cell stacks need to be slightly larger in order to obtain the same power as if a steam reformer was used.

Equation (2.2) expresses the partial oxidation for any hydrocarbon. It is an exothermic reaction (ΔH<0), therefore heat is produced.

Attending to graph 2.6, increasing the temperature of the inlet clean gas shifts the reaction towards a higher molar composition of methane in the outlet stream. This can be explained because the amount of air entering the reformer decreases and therefore it introduces less oxygen.

The efficiency of the plant increases with the CPO reformer inlet fuel temperature, as the amount of elements suitable for electrochemical reaction in the SOFC are increased, therefore producing more electricity per unit mass flow of natural gas. Also the CPO air compressor decreases its electrical power demand as less mass flow has to be compressed. Traditional Nickel catalysts need a minimum temperature of approximately 350ºC to start working and assure a correct functioning of the system until temperatures of 550ºC to 600ºC [8]. The CPO pre-reformer inlet clean gas temperature is thus set to 525ºC.

Figure 2.6

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

400 420 440 460 480 500 520 540

Refo

rmed

gas

com

posi

tion

CPO reformer inlet fuel temperature (ºC)

H2

N2

CO

CO2

H2O-G

CH4


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Figure 2.7

2.1.3.4 Splitter and burner

The objectives of the combustion process are the following:

• Eliminate the traces of fuel in the usedfuel mass flow.

• Avoid the liberation of carbon monoxide (CO) into the atmosphere by oxidizing it completely.

• Use the high temperature combustion products as the hot source for the Stirling engine.

The fluegas splitter is used to allow a desired quantity of mass flow into the burner, and thus control the ratio fuel/oxygen of the combustion reaction. The rest of the fluegas can be liberated into the atmosphere as it is air with a low oxygen composition. The fluegas entering the burner is established so that the combustion products temperature reaches 1200ºC, which is considered to be a suitable temperature for the hot side of the Stirling engine.

49.50%

50.00%

50.50%

51.00%

51.50%

52.00%

400 420 440 460 480 500 520 540

Plan

t eff

icie

ncy

CPO reformer inlet fuel temperature (ºC)


30

Figure 2.8

2.1.3.5 Stirling engine

In Appendix I, section A.I-11 the election of the different values for the engine’s parameters is explained.

The cooling water of the Stirling engine can be used for heating purposes. By circulating in a closed system, the water removes heat from the Stirling engine, and transmits it to the room’s air by means of heat exchangers, and is then pumped back to the engine to restart the cycle. The water must reach in some point of the cycle at least 60ºC to avoid problems with Legionnaires disease, caused by the Legionella bacteria which can grow at temperatures from 25ºC to 50ºC, but is killed at temperatures above 60ºC.

Current Stirling engines using water as cooling fluid operate with a water stream that produces a temperature difference between inlet and outlet not bigger than 20ºC. Table2.5 shows the net power and efficiency of the power plant for different outlet cooling water temperatures and different admissible temperature variations between inlet and outlet.

ΔT cooling water (ºC)

5 10 15 20

Cooling water outlet temperature

(ºC)

60 Net power

(kW) 9,4556 9,4658 9,4748 9,4836

Efficiency 58,97% 59,03% 59,09% 59,14%

65 Net power

(kW) 9,43 9,4402 9,4493 9,4581

Efficiency 58,81% 58,87% 58,93% 58,98%

70 Net power

(kW) 9,4044 9,4147 9,4238 9,4326

Efficiency 58,65% 58,71% 58,77% 58,83% Table 2.5

7.40%

7.42%

7.44%

7.46%

7.48%

7.50%

7.52%

7.54%

0.405 0.41 0.415 0.42 0.425 0.43 0.435 0.44 0.445 0.45 0.455

Perc

enta

ge o

f flu

egas

goi

ng in

to th

e bu

rner

Natural gas mass flow (g/s)


31

The optimum pair of values corresponds to a cooling water outlet temperature of 60ºC and a difference of 20ºC, as it achieves the highest net power output and efficiency.

The maximum temperature of the cycle must be reached in the outlet of the Stirling engine, whilst the minimum is the inlet temperature. Therefore the minimum inlet cooling temperature admissible, and thus the optimum one is 40ºC.

2.1.3.6 Water Heating

As the combustion products still have a high temperature at the outlet of the Stirling engine, they can be used to heat a fluid stream. In this project a heat exchanger is responsible of heating the consumption water. This water can be then stored in a water tank and be used when there is a demand of hot water. It is not the objective of this project to design the water piping system for domestic water consumption. The temperature at which the exhaust gases are liberated into the atmosphere has to be above 90ºC to prevent water condensation and thus corrosion problems in the piping.

Hot water for consumption must be stored at least at 60ºC to avoid the Legionnaires disease, therefore this is the minimum temperature at which the water must exit the heat exchanger.

The mass flow of water heated is expressed by means of equation (2.3).

The optimization of the heat exchanger system is accomplished by maximizing the water stream. As the inlet water temperature and the inlet gas temperature are fixed, just like the gas mass flow and the pressures, the only parameters that can be varied are the outlet gas temperature and the outlet water temperature. The gas outlet temperature should be as low as possible in order to achieve a higher water mass flow, however the minimum temperature is imposed to be 95ºC to avoid corrosion in the off gas piping. The outlet water temperature should be as low as possible in order to achieve a big water mass flow, always taking into account that the temperature should be high enough for the hot water to be used for domestic purposes such as showering, dishwashing and clothes washing, which usually demand water in the range of 35ºC to 45ºC. As previously commented, regulations dictate that the water must be at least at 60ºC for healthy issues, thus the minimum temperature of the water has to be 60ºC.

The resume of the temperatures inherent to the water heat exchanger is exposed below:

•


32

•

•

2.1.4 Study and analysis of the power output and plant efficiency

The desired net power output of 10kW can be obtained by using different combinations of number of stacks and fuel mass flow. As the considered fuel cell stack is of 1kW, a minimum number of 10 stacks are considered.

A study of the fuel consumption was made for the net power region of 10 kW ± 0,5 kW for 10, 11 and 12 stacks of fuel cell respectively. Graphical results are contained in Appendix III. The net power output has a linear relationship with the natural gas mass flow in the studied region therefore its mathematical expression is represented as follows: , where

a and b are constants.

The slope for the system with a high number of fuel cell stacks is slightly bigger than for a low number of fuel cell stacks, meaning that more extra power can be obtained for the same fuel increase. Analyzing the plant for a constant fuel mass flow, increasing the number of stacks increases the power output, however the power gain with the addition of an extra fuel cell is smaller as the number of cells increases. The following equation depicts the situation, where x is the number of SOFC stacks.

As mentioned previously, increasing the amount of fuel cells makes the quantity of fuel necessary to obtain the desired power output to decrease. In one hand the cost raises due to the purchase of the extra fuel cell stack, but on the other hand the fuel consumption is lower, thus the operation cost diminishes. As the fall in fuel consumption is very small, the price of the rest of the components in the plant is considered to remain constant, therefore the critical component to be studied is the SOFC.

The cost associated to the solid oxide fuel cell can be seen as the cost of the purchase, the cost of the fuel consumed and the direct and indirect costs linked to the installation and maintenance of the cell stacks.

The cost of the fuel for a period of n years and a cost escalation rate of a is given by the geometric progression equation (2.7). HWY stands for Hours Worked per Year and is the

specific cost of the fuel.


33

The total cost associated with the fuel cell stacks is therefore the sum of the equation (2.7) and equation (x) in the thermoeconomic chapter.

The numerical values of the parameters used in the equation above are shown in the table 2.6. The explanation to each one of them is done in Chapter 3 –Thermoeconomic analysis.

PARAMETER VALUE Cost of fuel cell stack (€) 3000 Cost of natural gas (€/kg) 1,36 Cost escalation rate (%) 2,22

Hours Worked per Year (h) 4000 SOFC life expectancy (years) 15

Table 2.6

Table 2.7 displays the amount of fuel needed per hour to achieve a net power of 10kW and the total cost associated to the fuel cells.

NUMBER OF STACKS NATURAL GAS MASS FLOW (kg/h)

TOTAL COST ASSOCIATED TO THE FUEL CELLS (€)

10 1,5516 192059,5 11 1,5408 195412,5 12 1,5336 199100,1

Table 2.7

Installing 10 fuel cell stacks in the power generating system is the best option from the economy point of view for the current life expectancy of fuel cells. The other options are more expensive even though the efficiency is higher.


34

2.1.5 Results of the optimized basic Natural Gas power plant

Table 2.8 expresses the value of the parameters of each component of the plant. Table 2.9 shows the obtained results for power outputs and efficiency of the optimized basic NG power plant.

ELEMENT PARAMETER VALUE

SOFC

Utilization factor 0,8

Nº of cells per stack 74

Number of stacks 10

Working temperature (ºC) 780

Anode inlet temperature (ºC) 650

Cathode inlet temperature (ºC) 600

Compressor

Isoentropic efficiency 0,7

Mechanical efficiency 0,95

Inlet temperature (ºC) 25

Inlet pressure (bar) 1

CPO reformer Inlet fuel temperature (ºC) 525

Desulphurizer Working temperature (ºC) 200

Fuel Valve Fuel inlet temperature (ºC) 25

Fuel inlet pressure (bar) 8

Burner Products temperature (ºC) 1200

Pressure ratio 0,97

Stirling Engine

Temperature of inlet cooling water (ºC)

40

Temperature of outlet cooling water (ºC)

60

Pressure of cooling water (bar) 1,3

Water Heating

Inlet water temperature (ºC) 20

Inlet water pressure (bar) 1,2

Outlet water temperature (ºC) 60 Exhaust gases outlet temperature

(ºC) 95

Exhaust gases outlet pressure (bar) 1,02 Table 2.8

PARAMETER VALUE UNITS Electric power production 10,2089 kW Total power consumption 0,2086 kW

Net Power 10,0003 kW Fuel consumption (LHV) 19,7367 kJ/s Thermal efficiency (LHV) 0,5067 -

Table 2.9


35

The following pie chart illustrates the amount of power supplied by the solid oxide fuel cells, and by the Stirling engine.

Figure 2.9- SOFC and Stirling engine power output

91%

9%

TOTAL POWER OUTPUT

SOFC POWER OUTPUT

STIRLING ENGINE POWER OUTPUT


36

2.1.6 Improved SOFC-Stirling engine plant fuelled with Natural Gas

The previously studied power plant offers an acceptable efficiency value, however this can be raised by improving the following:

• The way in which the air is preheated in the cathode side.

• Increasing the temperature of the air which is introduced into the CPO reformer, as in the current plant air is introduced to the reformer at very low temperature therefore part of the energy liberated in the exothermic reaction is lost in heating the inlet air instead of the products.

2.1.6.1 Improved cathode preheating system

Installing an additional heat exchanger in the cathode side that pre-heats the air that exits the compressor with the non-used fluegas of the splitter has been proved to be more efficient than just using one heat exchanger which uses the total fluegas out of the SOFC. To decide how much heat must be transferred in the first air preheater to obtain the maximum efficiency of the plant, a study of the model took place.

Figure 2.10- Cathode side of the power plant with 2 air preheaters


37

Figure 2.11- Diagram of the fluegas splitter

The higher the temperature at the exit of the heated mass flow of the cathode preheater 1, the higher the plant efficiency. It is a lineal relation as shown in figure 2.12. This trend continues until the outlet temperature of the first heat exchanger equals the desired temperature in the inlet cathode side of the SOFC, which in this case is 600ºC. In this point the second heat exchanger does not exchange any heat, and therefore does not work and can be removed from the system. Removing it from the design also increases the efficiency even more, as the compressor does not have to deal anymore with the pressure drops the air preheater 2 caused, and therefore consumes less power. The most efficient design is therefore the one with a single air preheater in the cathode side, in which the heating fluid is the fluegas that comes out from the splitter and is not used in the burner.

Figure 2.12

0.52

0.525

0.53

0.535

0.54

0.545

0.55

0.555

200 250 300 350 400 450 500 550 600

Effic

ienc

y of

the

pla

nt

Outlet Temperature of Heat exchanger 1


38

The efficiency of the power plant increases as the fluegas that enters the burner (Fluegas_1) increases, however, there is an imposed limit, which is given by the minimum mass flow of Fluegas_2 needed to heat the air entering the SOFC to the desired temperature.

The heat exchanger is considered to be adiabatic therefore there are no heat losses to the surroundings. The energy balance between fluids is given by the following equation:

As the DNA program gives the solution to the specific enthalpy, from now on equation (2.10) will be used.

By rearranging the previous equation the needed mass flow of fluegas can be obtained.

The theoretical minimum mass flow of Fluegas_2 that can be used to achieve the desirable temperature in the entrance of the SOFC anode, corresponds to the situation when the outlet temperature of the heating fluid is equal to the inlet temperature of the cooling fluid of heat exchanger (T2=T6). As the efficiency of the heat exchangers is not 100%, the outlet temperature of the heating fluid will always be slightly higher than the theoretical one. Assuming a ΔT=7ºC, the optimum mass flows of fluegas are expressed by:

2.1.6.2 Improved air inlet into CPO reformer

Figure 2.13 displays the response of the power plant’s efficiency as the temperature of the inlet air into the CPO reformer is increased. The results show that it is convenient to introduce a heat exchanger in the CPO air system as the efficiency is increased considerably with respect to the original plant.

The natural gas is reformed at temperatures above 350ºC, however there is a topping limiting temperature imposed by the deactivation of catalysts, which is around 550ºC. Observing the plotted results, the higher the natural gas temperature inlet into the reformer, the higher the plant efficiency is. The optimum temperature therefore should lie somewhere near the topping limit. In this plant a temperature of 525ºC has been chosen. Regarding to the CPO air


39

temperature, it has been demonstrated that high temperatures involve high efficiencies (coloured lines in figure 2.13).

The inlet fuel temperature was set to 525ºC and the inlet air temperature was set to 550ºC.

Figure 2.13

55.0%

56.0%

57.0%

58.0%

59.0%

60.0%

61.0%

400 420 440 460 480 500 520 540

Plan

t eff

icie

ncy

Natural gas inlet temperature (ºC)

300

350

400

450

500

550


40

2.1.7 Configuration of the improved Natural Gas power plant

Figure 2.14 illustrates the configuration of the improved hybrid SOFC-Stirling engine power plant fuelled with NG.

Figure 2.14- Configuration of the improved Natural Gas power plant



H CPO air preheater


I CPO reformer

C SOFC J Fluegas splitter D Fuel preheater K Burner E Desulphurizer L Stirling engine

F CPO reformer

preheater M Water pump

G CPO air compressor N Water heater Table 2.10- Component legend for the improved NG power plant


41

TOPPING CYCLE


2-3 standard air is heated by means of a heat exchanger (cathode pre-heater), as it needs to be introduced into the cathode side of the SOFC with a similar temperature to the SOFC working temperature.


4-5-7 the fluegas is split into two mass flows.

5-6 the fluegas cools down as it transfers part of its thermal energy to the standard air entering the SOFC.

8-9 Natural Gas increases its temperature in the fuel pre-heater.

9-10-11 Natural Gas is desulphurized obtaining the clean gas, which has the same composition as the fuel but without hydrogen sulphide (H2S).

11-12 the clean gas is heated up in the reformer pre-heater by the usedfuel.

13-14 simple air is compressed until the SOFC working pressure in the anode side.

14-15 the compressed simple air is pre-heated before entering the CPO reformer.

12-15-16 clean gas is pre-reformed, reforming all the high-weight hydrocarbons, obtaining the reformed gas.

16-17 the SOFC internally reforms and electrochemically oxidizes the pre-reformed fuel (reformed gas) obtaining electricity. Heat is obtained as a by-product. The unreacted reformed gas along with water exits the anode side of the fuel cell. This outlet fluid is known as used fuel.

17-18 the high temperature used fuel transfers part of its thermal energy to the desulphurized fuel mass flow in the reformer pre-heater.

18-19 the used fuel transfers part of its thermal energy to the Natural Gas mass flow in the fuel pre-heater.

19-20 used fuel transfers part of its thermal energy to the simple air massflow.


42

BOTTOMING CYCLE



23-24 the cooling water of the Stirling engine is used to lower the temperature inside the machine.

28-23 the cooling water is pumped back again into the Stirling engine after returning from the domestic heating system.

22-25 the combustion products are used to heat water for domestic purposes, as they still have a high thermal energy.

26-27 water is heated up and stored for domestic uses.


43

2.1.8 Study and analysis of the improved plant power output and efficiency

The net power output for the improved power plant follows the same trend as the original power plant’s net power output: kilowatts increase with the fuel mass flow inlet.

The expressions for the net power output for 10, 11 and 12 stacks in the region of the 10 kW are written below:

Analogously to the basic natural gas power plant, the cost associated to the SOFC has been calculated for a life expectancy of 15 years by means of equation (2.8). The table below also shows the natural gas mass flow needed to obtain a net power output of 10 kW.

NUMBER OF STACKS NATURAL GAS MASS FLOW (kg/h)


10 1,3316 171040,2 11 1,3273 175004,9 12 1,3219 178835,9

Table 2.11

The installation of 10 stacks has the best economical results, therefore can be considered as the optimum number of fuel cell stacks for this power plant. The natural gas mass flow for a nominal power of 10kW is 1,3316kg/h.


44

2.1.9 Results of the optimized improved Natural Gas power plant

The following table shows the imposed parameters in the different components of the power plant. Table (2.13) illustrates the power outputs and efficiency of the power generation system.

COMPONENT PARAMETER VALUE

SOFC

Utilization factor 0,8 Operation temperature (ºC) 780

Anode inlet temperature (ºC) 650 Cathode inlet temperature (ºC) 600

CATHODE HEAT EXCHANGER

Heating fluid outlet temperature (ºC) 40

AIR COMPRESSOR


Mechanical efficiency 0,95 Inlet pressure (bar) 1


CPO REFORMER Clean fuel inlet temperature (ºC) 525

Air inlet temperature (ºC) 550 DESULPHURIZER Operation temperature (ºC) 200

FUEL PREHEATER


BURNER Pressure ratio 0,97

STIRLING ENGINE

Cooling water outlet temperature (ºC) 60

WATER PUMP

Temperature of inlet water (ºC) 40


Outlet pressure (bar) 1,3 Electrical efficiency 0,95

HEAT EXCHANGER

FOR HEATING WATER

Heating fluid outlet temperature (ºC) 95 Water inlet temperature (ºC) 20

Water outlet temperature (ºC) 60 Water inlet pressure (bar) 1,5

Table 2.12

Natural Gas is supposed to be supplied by the local NG supplying company. The inlet pressure is regulated by means of an isothermal valve at the entrance of the system, but it is not taken into account in the calculations of this plant.

The pressure at the outlet of the heating fluid of the water heat exchanger is set to 1,02bar so the combustion products are released into the atmosphere naturally, but do not generate a pushing force big enough to cause mechanical stress in the pipes and components.


45

The relation between the Stirling power output and the SOFC power output is approximately the same as for the basic NG power plant.


2.1.10 Comparison between the basic and improved Natural Gas power plant

The results of the comparison between the two natural gas power plants are shown in table (2.14). Equation (2.17) has been used for the calculation of the variation of the generic parameter , where can be any of the tabulated parameters in table (2.14).

PARAMETER VARIATION WITH

RESPECT TO ORIGINAL PLANT

Total power consumed -34,28% Fuel consumed -14,14%

Efficiency 16,46% Table 2.14

89%

11%

TOTAL POWER OUTPUT

SOFC POWER OUTPUT




Table 2.13


46

2.2 SOFC- STIRLING ENGINE POWER PLANT FUELLED WITH AMMONIA

2.2.1 Ammonia as a fuel

Ammonia (NH3) is a gas in normal conditions; this is at ambient temperature and atmospheric pressure. However it can be liquefied at slightly higher pressures, around 8atm, so its density increases by approximately 850 times, making it a cost-effective fuel for transportation and storage. In this condition, the volumetric energy density of the ammonia is comparable with that of gasoline or methanol [9]. As ammonia has no carbons, it releases no carbon dioxide to the atmosphere in its combustion. It does however generate NOx but they are easily neutralized. Ammonia is non flammable and does not damage the ozone layer.

The Haber-Bosch process synthesizes ammonia from nitrogen and hydrogen. Currently the hydrogen is mainly obtained from non-renewable sources such as Natural Gas and coal, however new technologies make possible to obtain ammonia from renewable sources such as water or biogas.

Figure 2.16- The X-15 rocket plane set speed and altitude records in the 1960’s powered by anhydrous ammonia fuel [22]

Reforming of ammonia can be carried out externally, so hydrogen is the element that enters the fuel cell. The reaction takes place at over 600ºC, therefore an external heat source may be required. The reformer can be integrated with the SOFC stacks and obtain the heat required from the fuel cell heat output [10].

http://www.ammoniafuelnetwork.org/�


47

2.2.2 Configuration of the ammonia power plant

Figure (2.17) shows the configuration of the ammonia power plant.

Figure 2.17- Configuration of the ammonia power plant

LETTER COMPONENT A Cathode air compressor B Cathode air preheater C SOFC D Fuel preheater E Fluegas splitter F Burner G Stirling engine H Water pump I Water heater

Table 2.15- Component legend for the ammonia power plant


48

TOPPING CYCLE

1-2 air in standard conditions is compressed to reach the pressure demanded in the cathode side of the power plant.

2-3 the pressurized air is heated by means of a heat exchanger, as it needs to be introduced into the cathode side of the SOFC with a similar temperature to the SOFC working temperature.


4-5-7 the fluegas is split into two mass flows, one into the burner and the other to the cathode air preheater.

8-9 Ammonia increases its temperature in the fuel pre-heater, reaching the desired temperature in the SOFC anode inlet.

9-10 the SOFC internally reforms and electrochemically oxidizes the ammonia obtaining electricity. Heat is obtained as a by-product. The unreacted fuel along with water, nitrogen and traces of hydrogen exits the anode side of the fuel cell. This outlet fluid is known as usedfuel.

10-11 the high temperature usedfuel transfers part of its thermal energy to the ammonia mass flow in the fuel pre-heater.

BOTTOMING CYCLE

7-11-12 the unreacted fuel in the usedfuel reacts with the oxygen in the combined mass flow of fluegas and air in a combustion process, obtaining combustion products.

12-13 the high temperature combustion products are used as the heating source to make the Stirling engine work.

14-15 the cooling fluid of the Stirling engine is used to lower the temperature inside the machine.

19-14 water is pumped back into the Stirling engine after being used for domestic heating.


17-18 water is heated up by means of a heat exchanger.


49

2.2.3 Optimization of the ammonia power plant

2.2.3.1 SOFC

Increasing the temperature of the air entering the cathode side of the fuel cells affects negatively the efficiency of the power plant as can be observed from table 2.16. The reasons are the same as for the natural gas power plant; more fluegas is needed in the cathode heat exchanger in order to obtain the desired temperature in the outlet of the heated fluid, thus more air has to be injected. This increases the SOFC power output, however the air compressor demands more power, and the Stirling engine produces less power as the mass flow of the combustion products produced in the burner is lower. The overall consequence is that the increase in power consumption is bigger than the increase in power production.

Anode temperature set constant to 650ºC

SOFC power output (kW)

STIRLING power output (kW)

COMPRESSOR power consumption (kW)

EFFICIENCY

INLET TEMPERATURE

INTO SOFC CATHODE SIDE

600 9,074 1,061 0,1336 57,9% 620 9,083 1,059 0,1497 57,8% 640 9,092 1,058 0,1703 57,8% 660 9,101 1,056 0,1978 57,6% 680 9,109 1,053 0,2364 57,5% 700 9,117 1,049 0,2942 57,1%

Table 2.16

Diminishing the inlet temperature into the anode side of the SOFC has a positive effect on the overall efficiency of the plant. The reason is that the stack needs to heat up the anode stream and it achieves so by reducing the amount of air into the cathode, which acts as a cooler in the system. By doing so, the fuel cell power output is decreased as there is less oxygen inside it for the electrochemical reaction; however the power input into the compressor decreases as there is less air to be compressed, and the output power of the Stirling engine increases due to a higher composition of products in the usedfuel stream which combust in the burner obtaining higher temperatures.

Cathode temperature set constant to 600ºC

SOFC power output (kW)

STIRLING power output (kW)

COMPRESSOR power consumption (kW)

EFFICIENCY

INLET TEMPERATURE

INTO SOFC ANODE SIDE

650 9,074 1,061 0,1336 57,89% 660 9,075 1,055 0,1348 57,86% 670 9,076 1,05 0,136 57,82% 680 9,076 1,044 0,1371 57,78% 690 9,077 1,038 0,1383 57,75% 700 9,078 1,032 0,1395 57,71%

Table 2.17


50

2.2.3.2 Cathode air preheater

The cathode air preheater (from now on CAP) uses a certain quantity of fluegas to heat up the air stream entering the cathode of the SOFC up to 600ºC. The air coming out from the compressor into the CAP has a temperature of 32,72ºC, so the theoretical minimum temperature the outlet fluegas could have corresponds to this same value. However, in reality is impossible to obtain the same temperature in the outlet heating fluid and inlet cooling fluid, as the heat exchanger efficiency is never 100%. For this thesis, a minimum temperature difference of 7ºC has been accounted.

Figure 2.18- Diagram of the cathode air preheater

Table 2.18 shows how the different parameters of the power plant such as power outputs and efficiency are affected when the outlet temperature of the heating fluid in the CAP is increased, for a fix fuel mass flow of 0,00094kg/s.

Outlet temperature of air preheater

heating fluid [ºC]

Net Power [kW]

Plant efficency

Stirling engine power [kW]

Temperature of products out of the

burner [ºC]

Fluegas mass flow entering

the burner [kg/s]

40 10,1217 57,86% 1,075 1151,93 0,003108 50 10,1136 57,82% 1,066 1160,69 0,002949 60 10,1053 57,77% 1,058 1170,12 0,002785 70 10,0968 57,72% 1,05 1180,29 0,002618 80 10,0881 57,67% 1,041 1191,31 0,002445 90 10,0791 57,62% 1,032 1203,21 0,002268

100 10,0698 57,57% 1,023 1216,35 0,002085 110 10,0603 57,51% 1,013 1230,65 0,001898 120 10,0506 57,46% 1,003 1246,38 0,001705

Table 2.18


51

The following graph (figure 2.19) shows the trend of the fluegas into the burner and the temperature of the products out of the burner as the outlet temperature of the heating fluid increases.

Figure 2.19

The higher the temperature at the outlet of the CAP in the heating fluid stream, the higher the temperature of the burner products is. This responds to the fact that more heating fluid mass flow is needed in order to achieve a higher temperature, thus according to equation (2.13) the fluegas mass flow into the burner decreases (as shown in the previous graph) and the combustion reaction inside the burner occurs with less excess of air.

Figure 2.20 represents the variation of the Stirling engine power output and the plant efficiency with the CAP outlet heating fluid temperature.

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

1140

1150

1160

1170

1180

1190

1200

1210

1220

1230

1240

40 60 80 100

Tem

pera

ture

of p

rodu

cts

out o

f bur

ner [

ºC]

Outlet temperature of air prehetear heating fluid [ºC]

Temperature of products out of the burner [ºC]

Fluegas mass flow entering the burner [kg/s]


52

Figure 2.20


The optimization of the Stirling engine attends to the same reasons as for the Natural Gas power plant.

2.2.3.4 Water heating

The optimization of the water heating attends to the same reasons as for the Natural Gas power plant.

57.45%

57.50%

57.55%

57.60%

57.65%

57.70%

57.75%

57.80%

57.85%

57.90%

0.99

1

1.01

1.02

1.03

1.04

1.05

1.06

1.07

1.08

0 50 100 150

Pow

er [k

W]

Outlet temperature of air preheater heating fluid [ºC]

Stirling engine power [kW]

Plant efficency


53


Analogously to the study of the NG power plant, a study of the ammonia power plant took place varying the number of fuel cell stacks and inlet fuel mass flow to obtain the optimum configuration for a net power output of 10kW. Results are plotted in Appendix III.

For the analyzed region, the trend can be considered as lineal. The following equations adjust to the net power output in the region of the 10 kW 0,5 kW.

PARAMETER VALUE Cost of fuel cell stack (€) 3000 Cost of ammonia (€/kg) 0,45 Cost escalation rate (%) 2,22


Table 2.19

Executing the same steps as in the analysis of the optimized natural gas power plant, the cost associated to the purchase, operation and maintenance of the solid oxide fuel cell has been calculated for a life expectancy of 15 years. The rest of parameters used are tabulated in table 2.19.

NUMBER OF STACKS AMMONIA MASS FLOW (kg/h)


10 3,3417 149448,2 11 3,3300 153454,0 12 3,3203 157523,0

Table 2.20

The best solution for the power generation system is to install 10 fuel cell stacks. The efficiency is lower than for a higher number of stacks, but the monetary savings in fuel purchase are not big enough to justify the purchase of an extra stack.


54

2.2.5 Results of the optimized ammonia power plant

The following table shows the imposed parameters in the different components of the power plant. Table 2.22 depicts the power outputs and efficiencies of the power generation system.

COMPONENT PARAMETER VALUE

SOFC

Utilization factor 0,8 Operation temperature (ºC) 780

Anode inlet temperature (ºC) 650 Cathode inlet temperature (ºC) 600

CATHODE HEAT EXCHANGER

Heating fluid outlet temperature (ºC) 40

AIR COMPRESSOR


Mechanical efficiency 0,95 Inlet pressure (bar) 1

Inlet temperature (ºC) 25 BURNER Pressure ratio 0,97

STIRLING ENGINE

Cooling water outlet temperature (ºC) 60

WATER PUMP

Temperature of inlet water (ºC) 40


Outlet pressure (bar) 1,3 Electrical efficiency 0,95

HEAT EXCHANGER

FOR HEATING WATER

Heating fluid outlet temperature (ºC) 95 Water inlet temperature (ºC) 20

Water outlet temperature (ºC) 60 Water inlet pressure (bar) 1,5

Table 2.21

The ammonia is supposed to be stored under pressure in a tank with a pressure regulator at the outlet tube so it can be introduced to the power plant at the demanded pressure.




Table 2.22


55


90%

10%

Total power productionSOFC Stirling engine


56

2.3 SOFC-STIRLING ENGINE POWER PLANT FUELLED WITH METHANOL

2.3.1 Methanol as a fuel

Methanol (CH3OH) is an alcohol which is liquid at atmospheric pressure and standard temperature, with a boiling point of 65ºC, therefore its storage and handling is easier than for other fuels, such as hydrogen. It can be produced from Natural Gas, coal or biomass, being the most common source the first mentioned. A limited toxicity, high energy density and a competitive cost make methanol a good candidate to replace the current pollutant fossil fuels. However, the idea of using this alcohol as a fuel is not new, during the oil crisis of the 1970’s important car manufacturers such as General Motors focused their attention in the use of methanol as a motor fuel due to its low cost and commercial availability.

Figure 2.22- Daimler’s NECAR 5 prototype getting a methanol fill up

There are two ways in which methanol can be used in fuel cells, directly and indirectly. In the direct methanol fuel cells, known as DMFC, the alcohol is electrochemically oxidized to carbon dioxide and water without any initial reforming. Compared with ethanol, methanol has the significant advantage of high selectivity to CO2 formation in the electrochemical oxidation process [11]. In indirect methanol fuel cells (RMFC), methanol is reformed to hydrogen first, and is this element the one that enters the anode side of the cell. Hydrogen production from methanol can be obtained at moderate temperatures (250-350ºC) because it contains no carbon-carbon bonds that need to be broken therefore it seems a suitable fuel for systems which cannot work under high temperatures.

In the present study the indirect methanol plant configuration has been analyzed.


57

2.3.2 Methanol steam reforming

Methanol steam reforming occurs in two steps. The first one is methanol decomposition, equation (2.22), in which CH3OH is decomposed to carbon monoxide and hydrogen, whilst the second step is known as water-gas shift reaction, equation (2.23), where the carbon monoxide reacts with the steam to form hydrogen and carbon dioxide. The resulting reaction is endothermic with an enthalpy variation of , therefore a small amount of heat needs to be supplied to maintain the reaction going on. Temperatures above 250ºC and the presence of catalysts such as copper supported on zinc oxide in the reformer are enough for the reaction to happen. The pressure range for methanol steam reforming is between 1 and 5 bar.

The total reaction is given by the following equation:

Methane is formed by the methanation reaction written below. In this thesis the formation of higher molecular weight compounds is not taken into account. This corresponds to the fact that in practice a negligible amount of these compounds are formed, as L. Laosiripojana and S. Assabumrungrat demonstrated in their work <<Catalytic steam reforming of methane, methanol and ethanol over Ni/YSZ: the possible use of these fuels in internal reforming SOFC>> .


58

2.3.3 Configuration of the methanol power plant

Figure 2.23 illustrates the configuration of the reformed methanol power generation plant.

Figure 2.23- Configuration of the methanol power plant



H Usedfuel splitter


I Fluegas splitter

C SOFC J Burner D Fuel preheater K Stirling engine

E Fuel and usedfuel

mixer L Water pump

F Steam reformer M Water heater

G Anode reformed fuel

preheater

Table 2.23- Component legend for the methanol power plant


59

TOPPING CYCLE


2-3 standard air is heated by means of a heat exchanger (cathode pre-heater), as it needs to be introduced into the cathode side of the SOFC with a similar temperature to the SOFC working temperature.


4-5-7 the fluegas is split into two mass flows.

5-6 the fluegas cools down as it transfers part of its thermal energy to the standard air entering the SOFC.

8-9 Methanol increases its temperature in the fuel pre-heater.

9-10-11 Methanol and the recirculated usedfuel converge into a single mass flow

11-12 The mix fluid of methanol and usedfuel enters the reformer where it is converted into suitable fuel for the fuel cell.

12-13 The reformed gas is heated to enter the SOFC anode at an optimum temperature.

13-14 the SOFC internally reforms and electrochemically oxidizes the reformed gas obtaining electricity. Heat is obtained as a by-product. The unreacted reformed gas along with water exits the anode side of the fuel cell. This outlet fluid is known as used fuel.

14-15 the high temperature used fuel transfers part of its thermal energy to the reformed gas fuel mass flow.

15-16-17 The usedfuel is separated into two different massflows, one directed towards the burner and the other to the mixer, where it mixes with methanol.

16-101

17-18 the low temperature used fuel transfers part of its remaining thermal energy to the fuel mass flow.

The recirculated mass flow that exits the used fuel splitter enters the mixer.

BOTTOMING CYCLE


1 Nodes 16 and 10 are in practice the same one, however two different nodes had to be considered in the DNA model for it to work correctly.


60


21-22 the cooling fluid of the Stirling engine is used to lower the temperature inside the machine.

26-21 recirculated water is pumped back into the Stirling engine.


24-25 water is heated up for its use in domestic heating, and other domestic uses.

2.3.4 Optimization of the methanol power plant

2.3.4.1 SOFC

The optimization of the SOFC attends to the same criteria explained in paragraph 2.1.3.1 as the inlet composition to the fuel cell stacks is the same as for the NG power plant. The inlet temperature of the cathode side is set to 600ºC, whilst the fuel inlet temperature to the anode side is set to 650ºC.

2.3.4.2 Cathode air preheater

The cathode side of the power system is identical to the cathode side of the previous analyzed power plants, therefore the optimum value for the heating fluid exiting the cathode preheater is 40ºC.

2.3.4.3 Splitter, mixer and reformer

The usedfuel splitter controls the amount of steam entering the methanator, and the amount of non reacted fuel entering the burner. The first stream is responsible for the SOFC fuel production (mainly methane and hydrogen); whilst the second is the source of energy for the Stirling engine. As the addition of both streams must be a constant value, the increase of one mass flow causes the decrease of the other. In order to determine the optimum working conditions of the power plant, the net power and efficiency were calculated for different usedfuel recirculation mass flows, and for different inlet temperatures.

Methane and hydrogen are obtained by processes (2.24) and (2.25) in the reformer, along with by-products such as water and carbon monoxide. As the usedfuel stream not only contains steam, but also H2, CO, CO2 and H2O the formation of each of the products is dependent on the molar base composition of the usedfuel.

For a complete methanol steam reforming, the molar relationship between methanol and water must stand in the range of 1 to 1/5, depending on the catalyst nature. This means that water mass flow should be from 0,5625 to 2,8125 times the methanol mass flow attending to equation (2.27), thus according to equation (2.29) the used fuel mass flow should approximately lie in the range of 1,4 to 6,9 times the methanol mass flow.


61

The molar ratio between methanol and steam is given by the following equation:

The mass flow of steam needed to achieve a 100% methanol conversion is expressed by equation (2.27).

As in this power plant the steam comes from the recirculation of usedfuel, the molar ratio can be expressed in terms of the usedfuel massflow in the following way:

Finally, rearranging the previous equation and knowing that the molecular mass of methanol is 32 kg/mol, the necessary mass flow of usedfuel for a complete methanol conversion is expressed by the following formula.

Figure 2.24 illustrates the net power and the efficiency of the power plant when a methanol mass flow of 0,0009kg/s is introduced into the system, and the recirculated stream is varied from 0,00126kg/s (1,4*mmethanol) to 0,00522kg/s (5,8*mmethanol). The decrease of the efficiency when the recirculated mass flow increases supports the fact that the decrease in power output from the Stirling engine is bigger than the power gained in the SOFC.


62

Figure 2.24

The optimum usedfuel recirculated mass flow is the one that achieves the maximum plant’s efficiency. Assuming a 100% methanol conversion, the best efficiency is achieved for a molar ratio (r) of 1 between methanol and water. However, for the current reformers in the market, this is not possible, so there must be an excess of water. Assuming 25% more moles of water than the stoichiometric necessary, the methanol/water ratio is 0,8. Therefore, according to equation (2.29), the optimum recirculated mass flow is expressed by the following formula.

The molar composition of the products in the reformed gas which are subject to react electrochemically in the SOFC (CH4, CO and H2) decrease when the recirculated usedfuel increases. This trend is reported in graph 2.25.

0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

0

2

4

6

8

10

12

0 0.001 0.002 0.003 0.004 0.005 0.006

Pow

er [k

W]

Recirculated usedfuel [kg/s]

Net Power

Efficiency


63

Figure 2.25

For constant fuel mass flow of 0,0009kg/s and a recirculated mass flow of 0,002kg/s the power plant efficiency and power output was analyzed for different fuel inlet temperatures into the mixer. Figure 2.26 illustrates the obtained results.

Figure 2.26

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 0.001 0.002 0.003 0.004 0.005 0.006

Mol

ar c

ompo

siti

on

Recirculated usedfuel [kg/s]

H2

CO

CO2

H2O-G

CH4

0.5330

0.5335

0.5340

0.5345

0.5350

0.5355

0.5360

0.5365

0.5370

9.55

9.56

9.57

9.58

9.59

9.6

9.61

9.62

150 200 250 300 350

Pow

er [k

W]

Fuel inlet temperature to mixer [ºC]

Net Power

Efficiency


64

The range of temperatures for which the methanol is completely converted lay in the range of 250ºC to 350ºC (Figure 2.27). The different nature of the catalyst determines how fast the methanol reaches a 100% conversion, but all the catalysts used currently for steam reforming achieve a complete conversion for the range of temperatures mentioned above. Copper based catalysts are active and selective for the methanol steam reforming until 280ºC, above this temperature other catalysts such as Pd/ZnO based must be used.

For the purpose of this project, a temperature of 280ºC is chosen as the optimum one for the fuel inlet to the mixer.

Figure 2.27

For an increase in the inlet methanol temperature into the mixer, the molar composition of the reformedgas remains practically constant in for the first range of temperatures analyzed, whilst slightly changes for the range of 250 to 350ºC as can be seen in figure 2.28.


65

Figure 2.28


The optimization of the Stirling engine attends to the same reasons as for the Natural Gas power plant.

2.3.4.5 Water heating

The optimization of the water heating attends to the same reasons as for the Natural Gas power plant.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

150 200 250 300 350

Mol

ar c

ompo

siti

on

Fuel inlet temperature to mixer [ºC]

H2

CO

CO2

H2O-G

CH4


66


The net power output of the power plant was studied for a variation in the methanol mass flow for 10, 11 and 12 fuel cell stacks respectively. Results are contained in Appendix III.

The net power of the energy system can be adjusted to a linear equation for the studied power range: 10kW 0,5 kW. The following equations show the relation between this net power and the inlet fuel mass flow.

Table 2.24 contains the value of the parameters used in equation (2.8) to calculate the cost associated to the fuel cell purchase and operation.

PARAMETER VALUE Cost of fuel cell stack (€) 3000 Cost of methanol (€/kg) 0,465 Cost escalation rate (%) 2,22


Table 2.24

Table 2.25 shows the results obtained when applying equation (2.8) for 10, 11 and 12 fuel cell stacks with the above parameters and the methanol mass flow that generates a net power output of 10kW.

NUMBER OF STACKS METHANOL MASS FLOW (kg/h)


10 3,279 150922 11 3,238 153958 12 3,207 157321

Table 2.25

Installing 10 fuel cell stacks is the most convenient thing to do, as less would not be enough to supply the demand of electricity, and more fuel cells would make the plant more expensive, even if the systems works with a higher efficiency.


67

2.3.6 Results of the methanol power plant

The following table shows the imposed parameters in the different components of the power plant. Table 2.27 contains the results of the power outputs and efficiency of the power generation system fuelled with methanol.

ELEMENT PARAMETER VALUE

SOFC

Utilization factor 0,8

Nº of cells per stack 74

Number of stacks 10

Working temperature (ºC) 780

Anode inlet temperature (ºC) 650

Cathode inlet temperature (ºC) 600

Cathode air compressor


Mechanical efficiency 0,95


Inlet pressure (bar) 1 Outlet temperature of heating

fluid (ºC) 40

Mixer Fuel inlet temperature (ºC) 280

Usedfuel recirculated (kg/s) 0,001478

Burner Usedfuel into burner (kg/s) 0,0021

Pressure ratio 0,97

Stirling Engine

Temperature inlet cooling water (ºC)

40

Temperature outlet cooling water (ºC)

60

Pressure of cooling water (bar) 1,3

Water Heating

Inlet water temperature (ºC) 20

Inlet water pressure (bar) 1,2

Outlet water temperature (ºC) 60 Exhaust gases outlet temperature

(ºC) 95

Exhaust gases outlet pressure (bar) 1,02

Fuel Preheater

Inlet fuel temperature (ºC) 25

Fuel mass flow (kg/s) 0,000901 Table 2.26

The methanol is supposed to be stored under pressure in a tank with a pressure regulator at the outlet tube so it can be introduced to the power plant at the demanded pressure.


68




Table 2.27

The pie chart represented in figure 2.29 illustrates how much is the power output by the SOFC and how much by the Stirling engine.


90%

10%

TOTAL POWER OUTPUT

SOFC POWER OUTPUT



69

2.4 COMPARISON BETWEEN THE IMPROVED NATURAL GAS, AMMONIA AND METHANOL POWER PLANTS OPERATING AT NOMINAL LOAD (10KW)

2.4.1 Power and efficiency

Table 2.28 resumes the values obtained for power generation and consumption, as well as the thermal efficiency for the different studied power plants.

PARAMETER NATURAL GAS AMMONIA METHANOL ELECTRICITY

PRODUCTION (kW) 10,1371 10,1349 10,2445

SOFC POWER OUTPUT (kW)

9,032 9,074 9,23


(kW) 1,106 1,061 1,015

TOTAL POWER CONSUMPTION (kW)

0,1371 0,1345 0,2450

LHV (kJ/s) 16,9466 17,2748 18,1418 THERMAL

EFFICIENCY (LHV) 0,5901 0,5789 0,5512

Table 2.28

The configuration of the NG plant enables the power generation system to obtain more power from the Stirling engine only elevating the power consumption a small amount; this is the main reason for the improved NG power plant to present the highest efficiency.

The elevated power consumption of the methanol power plant is due to the cathode air compressor, more air needs to be pumped into the cathode side because the quantity of mass flow entering the anode of the SOFC is bigger due to the recirculation. This explains also the high power output of the fuel cell when compared with the other two power plants, as part of the fuel that did not react electrochemically inside the cell enters again with the recirculation.

2.4.2 Heating power

The cooling water of the Stirling engine can be used for heating purposes, as it exits at a high temperature from the motor. Attending to the reasons explained in paragraph 2.1.3.5, the maximum theoretical heating power of the Stirling engine cooling water mass flow is expressed by the following equation.


70

Table 2.29 contains the heating power values for the different power plants.

NATURAL GAS AMMONIA METHANOL STIRLING COOLING

WATER MASS FLOW (kg/s)

0,0273 0,02617 0,02502

HEATING POWER (kW)

2,27 2,18 2,08

Table 2.29

2.4.3 Hot water generation

The volume of hot water produced in a certain period of time t can be calculated by means of equation (2.35).

Figure 2.30 shows the hot water consumption per occupant in the most common types of buildings.

Figure 2.30- Hot water consumption per occupant [http://www.engineeringtoolbox.com/hot-water-consumption-person-d_91.html]

If the consumption per occupant and day is considered to be the maximum tabulated for houses and flats, and an assumption is made that there are 5 inhabitants, the total daily consumption of hot water is 800 litres.

http://www.engineeringtoolbox.com/hot-water-consumption-person-d_91.html�

http://www.engineeringtoolbox.com/hot-water-consumption-person-d_91.html�


71

Rearranging equation (2.35), the necessary hours per day to fulfil the hot water demand can be calculated. The density of water at 60ºC and 1,5 bar is approximately 0,9832kg/litre

NATURAL GAS AMMONIA METHANOL HEATED WATER

MASS FLOW (kg/s) 0,0191 0,02175 0,03172

HOURS TO SUPPLY THE DEMAND OF

WATER (h) 11,44 10,05 6,89

Table 2.30

If the power plants worked at their nominal load at least for the periods of time expressed in the table above, the water demand would be supplied in its totality by the hybrid-power plant. The configuration of the water piping system should allow a recirculation of the stored water to heat it again when it cools down, and also avoid the boiling of water inside the water heater when there is no demand.

2.5 COMPARISON BETWEEN THE IMPROVED NATURAL GAS, AMMONIA AND METHANOL POWER PLANTS OPERATING AT PARTIAL LOAD

2.5.1 Concept of partial load

Electricity generation power plants are designed to work in an optimum point which corresponds with the usual power demand. However the electrical demand is variable with the time in the great majority of the households, therefore the power plant has to work outside its optimum conditions when this happens. In this project, the hybrid power plants have been designed for a nominal electrical power of 10kW. In the analysis of the partial loads, the assumption of the pressure drop in each of the elements remaining constant has been made.

• • •

2.5.2 Efficiency

Due to the intrinsic characteristics of the fuel cell stacks, and the configuration of the plants; the thermal efficiency increases as the partial load decreases. For the simplification of the analysis and calculations of the plant’s efficiency at partial loads, the parameters set by the user in the DNA file for the different components have not been changed with respect to the


72

nominal power configuration. This is not completely realistic, as some parameters, such as the compressor isoentropic efficiency decrease with the temperatures and mass flows.

The following quadratic equations adjust to the curves obtained when calculating the efficiency of the plants for different partial loads. Figure 2.31 is the representation of such equations.

Figure 2.31

The improved NG power plant has the best efficiency for all the range of partial loads as can be observed in the previous graph. Methanol’s power plant presents a better efficiency than ammonia for low partial loads, whilst ammonia works better for the range of loads above 40% approximately, as can be observed in figure 2.31.

56.5

57

57.5

58

58.5

59

59.5

60

60.5

61

61.5

0 2 4 6 8 10

Effic

ienc

y of

pla

nt (%

)

Electrical demand (kW)

Natural Gas

Ammonia

Methanol


73

2.5.3 SOFC and Stirling engine power output

The following equations adjust to the curves obtained in the analysis of the SOFC power output and Stirling engine power output for different power loads.

Natural Gas

Ammonia

Methanol

Figure 2.32 represents the relationship between the fuel cell power output and the Stirling engine power output for different electrical demands. All the curves present a maximum in the region of low electrical demand.


74

Figure 2.32

2.5.4 Optimum recirculation in the power plant fuelled with methanol

As in the methanol power plant there is a recirculation, there are two ways to achieve the supply of the desired electrical power; the first one is to vary the recirculation maintaining the fuel mass flow constant, whilst the second way is to reduce the fuel inlet. Obviously the second option is the best one from the cost and efficiency point of view, as less fuel is consumed, therefore it is the only option to be studied in this thesis. According to equation (2.30), there is an optimum recirculation mass flow which makes the efficiency maximum for each demand.

88.5%

89.0%

89.5%

90.0%

90.5%

91.0%

91.5%

0 2 4 6 8 10

SOFC

pow

er o

utpu

t/To

tal p

ower

out

put

Electrical demand (kW)

NG

AMMONIA

METHANOL

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.00% 20.00% 40.00% 60.00% 80.00% 100.00%

Mas

s flo

w (k

g/s)

Partial load

Opt. Mass flow


75

Figure 2.33

The following quadratic equation adjusts to the optimum recirculated mass flow curve, plotted in the graph above (figure 2.33).


76

3. THERMOECONOMIC ANALYSIS

3.1 INTRODUCTION TO THERMOECONOMICS

The Second Law of thermodynamics has an important role in the design of thermal systems, as it provides relevant information to the designer about the real available energy (exergy), which would be impossible to obtain carrying out a conventional energy analysis. Thermoeconomics is the branch of engineering that combines exergy analysis and economic principles [12]. The objective of this thermoeconomic analysis is to calculate the cost of the different mass flows of the system.

3.2 FUNDAMENTALS OF THE THERMOECONOMIC MODEL

For a power generating plant operating at a steady state, the total cost balance is given by the equation (3.1).

In the equation above, where is the cost rate associated with the product i

of the system and is the total rate of expenditures made to generate the product, which

in this particular case is the cost rate of the fuel. is the cost rate associated with the

capital investment and is the cost rate of operating and maintenance. These two last terms can be integrated in just one, equation (3.2), which represents the costs of the plant not associated with exergy stream, stream of matter, power or heat transfer.

The cost rate associated with the exergy, power, stream of matter and heat transfer is

expressed as the product of the total exergy flow and the specific exergetic cost . The exergy units are [kJ/s] and the specific exergetic cost is expressed in [€/kJ].

3.2.1 Equivalent hours of utilization

The equivalent hours of utilization are the number of hours the electric power system should work at its maximum capacity to produce the amount of electrical energy demanded during a certain period. It is normally calculated for a period of one year.

ehu is the equivalent hours of utilization


77

Ei is the power demand during the sub-period i. It may be that during some of these sub-periods there is no electrical demand.

ti is the period of time of the sub-period i

Emax is the maximum power of the system

If the period of time considered is of one year:

It must be taken into account that during the night the electricity demand is very low, and during the day not all of the apparatus are connected at the same time, nor working at their maximum capacity. For the purpose of this project, 4000 equivalent hours of utilization have been chosen.

3.2.2 Total capital investment

According to the dictionary definition, total capital investment is the expenditure made for income producing assets. In this particular case, the analysed power plants do not produce an income, but in compensation try to decrease the outcome.

The direct costs associated to the construction of the plants have been expressed as a percentage of the purchase equipment cost. However, as the cost of the fuel cell is so high, the assumption of it already containing electrical instrumentation and control has been made. Also the purchase and installation cost of the fuel and air pipes into the fuel cell inlets and outlets has been considered to be negligible, as fuel cells already come prepared with their own tubes.

DIRECT COSTS

Purchase equipment cost PEC

Installation of equipment 35% of PEC

Purchase and installation of pipes

65% of topping cycle PEC not including the cost of the SOFC

30% of bottoming cycle PEC

Instrumentation, control and electrical equipment

20% of PEC not including the cost of the SOFC

INDIRECT COSTS

Engineering 7% of the Direct Costs

Contingency 15% of the rest of Indirect Costs Table 3.1

• The total capital investment for the component k in the topping cycle (except the SOFC) is:


78

• For the SOFC, the total capital investment is expressed by:

• The total capital investment for the component k in the bottoming cycle is:

3.2.3 Amortization

EPPk is an end-of-period payment for the component k in a uniform series of payments over n periods with an interest factor a. The interest factor is calculated by means of the interest rate i and the cost escalation rate (also known as inflation rate) r. The construction period is considered to be zero, as it is a small power plant that can be built in a negligible number of days.

Equation (3.8) expresses the end-of-period payment for component k.

The interest factor is given by equation (3.9). In this case the inflation rate is taken into account.

Table 3.2 and its annexed graph (figure 3.1) show the monthly bank interest rates for Denmark in the year 2010.

Interest rates for the year 2010 in Denmark

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec AVERAGE i[%] 3,57 3,5 3,4 3,34 2,93 2,7 2,72 2,45 2,4 2,46 2,65 3,01 2,93

Table 3.2- Interest rates in Denmark (2010) [13]


79

Figure 3.1

Table 3.3 and figure 3.2 show the values of the inflation rate in Denmark for the period 2000-2009.

Inflation rate in Denmark for the period 2000-2009

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 AVERAGE r [%] 2,5 2,9 2,3 2,3 2,1 1,4 1,8 1,8 1,7 3,4 2,22

Table 3.3- Inlfation rate in Denmark (2000-2009) [13]

Figure 3.2

0

0.5

1

1.5

2

2.5

3

3.5

4

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Inte

rest

rate

[%]

Year 2010

0

0.5

1

1.5

2

2.5

3

3.5

4

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

Infla

tion

rat

e [%

]


80

Table 3.4 illustrates the average values of the interest rate and inflation rate in Denmark for the year 2010, and the period 2000-2009 respectively. This table also contains the amortization period; this is the period of time needed to repay the mortgage. For the purpose of this project 15 years have been chosen, as it is the estimated life expectancy for the SOFC and Stirling engine. The rest of the components in the power plant estimated life expectancy can be considered to be longer, as a correct operation has been proved in thousands of other power plants for longer periods than the one accounted.

PARAMETER VALUE UNITS Interest rate (i) 2,93 % Inflation rate (r) 2,22 %

Amortization period (n) 15 years Table 3.4

For the tabulated values, the interest factor a and the end-of-period payment (EPP) have the following expression:

3.2.4 Calculations of the capital investment cost rate and the O&M cost rate

The cost rate of the capital investment of the component k in the power generation system is calculated by dividing the end-of-period payment by the number of hours worked per period (WHP). The period considered for each payment is 1 year, and the equivalent number of hours that the power plant is working at its nominal load is 4000 h/year.

The operation and maintenance cost rate can be calculated by multiplying by a factor (fOM). An operation and maintenance factor of 0,05 has been chosen for this plant, as it is of a small size.


81

Substituting equations (3.12) and (3.13) into equation (3.2) the cost rate for capital investment and operation and maintenance is obtained:

3.2.5 Estimation of purchase costs

3.2.5.1 Solid Oxide Fuel Cell

To manufacture a SOFC today, it is necessary to spend about 3000€/kW [14], thus the purchase cost of the 10 SOFC stacks can be considered to be 30.000€.


The purchase cost of a 1,2 kW Stirling engine is set 2.500€.

3.2.5.3 Heat exchangers

The price of the heat exchangers is directly dependent to the heat exchange surface. According to the book Heat exchanger design (Arthur P. Fraas), the cost equation for a plane and frame heat exchanger type is calculated by means of equation (3.15).

Where A is the heat exchanging surface expressed in m2.

This surface can be calculated by the following equation:

Where is the heat transferred per unit of time from the heating fluid to the cold stream. K stands for the heat exchange coefficient and is the mean arithmetic temperature difference, expressed by equation (3.17).

The following table shows the values of the heat exchange coefficient.

HEAT EXCHANGE COEFFICIENT (K) VALUE [W/m2K] Heat exchange between GAS-GAS 35

Heat exchange between GAS-LIQUID 135 Table 3.5


82

If the cost obtained by means of equation (3.15) is below 500€, the purchase cost is set to this value as in real life heat exchangers that work with the temperatures in this project are considered not to have a lower cost. Table 3.6 contains the prices of the heat exchanger in each one of the studied power plants.

NATURAL GAS POWER PLANT Cathode air preheater 1.482 €

CPO air preheater 500 € CPO cleangas preheater 500 €

Fuel preheater 500 € Water heating 500 € AMMONIA POWER PLANT

Cathode air preheater 1.487 € Fuel preheater 500 € Water heating 500 €

METHANOL POWER PLANT Anode reformed fuel preheater 500 €

Cathode air preheater 2.426 € Fuel preheater 500 € Water heating 500 €

Table 3.6

3.2.5.4 Desulphurizer

The purchase cost of Zinc oxide based desulphurizers can be considered to be 70% of that of the fuel reformer, therefore the desulphurizer cost is set to 800€.

3.2.5.5 Fuel reformer

Compact reformers cost about $150/kW of H2 produced. Using a conversion dollar-euro of one US Dollar equals 0,7288 Euros, and making an assumption that the compact reformers cost $150/kW produced in the SOFC, the price of the reformers used in this project is 1100€.

3.2.5.6 Air compressor

The purchase cost of the air compressor is set to 100€ [15].

3.2.5.7 Burner

The purchase cost of the burner is set to 200€.


83

Figure 3.3- Stirling engine burner [16]

3.2.5.8 Water pump

The purchase cost of the water pump is negligible, as the power it needs is very small and the price in the market would be around 15€.

3.2.5.9 Splitter

The cost of the splitter is negligible

3.2.5.10 Mixer

The cost of the mixer is negligible

3.2.5.11 Valve

The cost of the valve is negligible


84

3.3 RESULTS OF THE THERMOECONOMIC ANALYSIS

The detailed results of the calculations are illustrated in APPENDIX II.

3.3.1 Improved NG power plant

3.3.1.1 Natural Gas cost rate

The gas price for the Danish households was of 29,7€/GJ in the year 2010, the highest value in the whole Europe.

Figure 3.4- Electricity and Gas prices for European countries (2010) [17]

PARAMETER VALUE UNITS Cost 0,1069 €/kWh

Specific energy 12,726 kWh/kg Mass flow 0,0003666 kg/s

FUEL COST RATE (CF) 1,7928 €/h Table 3.7- Fuel cost rate of Natural Gas


85

3.3.1.2 Cost rates for capital investment and operation and maintenance

COMPONENT (€/h)

SOFC 1,1228 Stirling engine 0,1282

Cathode air preheater 0,0897 CPO air preheater 0,0305

CPO clean fuel preheater 0,0305 Fuel preheater 0,0305 Water heating 0,0256 Desulphurizer 0,0488 CPO reformer 0,0671

Air compressor 0,0122 Burner 0,0103

Table 3.8

3.3.1.3 Total purchase cost and total capital investment

COST VALUE (€) TOTAL PURCHASE COST 38270

Topping cycle 91,06% Bottoming cycle 8,94%

TOTAL CAPITAL INVESTMENT 64821,19 Topping cycle 89,02%

Bottoming cycle 10,98% Table 3.9


86

3.3.2 Ammonia power plant

3.3.2.1 Ammonia cost rate

Ammonia prices in Europe fluctuated from the $400 in mid September 2010 to the $430 in the beginning of October and back to $390 in mid November [18]. Using a conversion dollar-euro as $1 = 0,7298€ and assuming an increase in the ammonia cost of 50% for the time the ammonia reaches the consumer due to transport and storage, the cost of ammonia is set to 450€/ton.

PARAMETER VALUE UNITS Cost 450 €/ton

Mass flow 0,00092 kg/s FUEL COST RATE (CF) 1,4904 €/h

Table 3.10- Fuel cost rate of ammonia


COMPONENT (€/h)


Cathode air preheater 0,0907 Fuel preheater 0,0305 Water heating 0,0256 Air compressor 0,0061

Burner 0,0103 Table 3.11







87

3.3.3 Methanol power plant

3.3.3.1 Methanol cost rate

Methanol prices in Europe were of 230€/ton at the beginning of the last quarter of the year 2010, rising to 280€/ton by mid-November. The growing Chinese demand pushed European prices up even more, towards the Asiatic prices that were of $445/ton by mid-November [18]. Assuming an increase in the price of methanol of 50% due to transport and storage, the methanol cost is set to 465€/ton.

PARAMETER VALUE UNITS Cost 465 €/ton

Mass flow 0,000901 kg/s FUEL COST RATE (CF) 1,5083 €/h

Table 3.13- Fuel cost rate for methanol


COMPONENT (€/h)


Anode reformed fuel preheater 0,0305 Cathode air preheater 0,1492

Fuel preheater 0,0305 Water heating 0,0256 Air compressor 0,0061

Burner 0,0103 Methanol reformer 0,0671

Table 3.14







88

3.3.4 Specific capital cost of the power plants

The specific capital cost is expressed by means of equation (3.18)

Natural Gas combined power plants have a specific cost of approximately $1200/kW, coal power plants have estimated costs of $2100/kW or greater, whilst nuclear, geothermal and IGCC plants have estimated costs in excess of $3000/kW [19].

Table 3.16 contains the specific capital costs for the NG, ammonia and methanol power plants. The values have been obtained using equation (3.18) and knowing that the net power output is 10kW.

POWER PLANT SPECIFIC COST (€/kW) NATURAL GAS 6482,1

AMMONIA 5511,5 METHANOL 6012,8

Table 3.16

The specific capital costs for the hybrid SOFC-Stirling engine power plants are currently approximately two to three times higher than the nuclear power plant specific capital cost. This is because the purchase cost of the Stirling engine and specially the fuel cells is far too high.

3.3.5 Specific costs of electricity and water

Equations (3.1), (3.2) and (3.3) applied to each one of the components in the power plant, constitute a linear equation system, where the unknown factors are the specific costs of the fluids, electricity and water.

POWER PLANT SOFC ELECTRICITY COST (€/kWh)

STIRLING ENGINE ELECTRICITY

COST(€/kWh)

AVERAGE ELECTRICITY COST

(€/kWh) NATURAL GAS 0,2981 0,3744 0,3064

AMMONIA 0,2708 0,3032 0,2742 METHANOL 0,2827 0,2506 0,2795

Table 3.17

Electricity had an average price of 0,2462€/kWh for Danish households in 2010 and of 0,1720€/kWh for Spanish households in the same year [20].


89

POWER PLANT HOT WATER (€/kWh) STIRLING ENGINE COOLING WATER (€/kWh)

NATURAL GAS 1,3600 0,324 AMMONIA 1,0270 0,2923 METHANOL 0,7668 0,2966

Table 3.18

The district heating price for Denmark in 2009 was of approximately 0,049€/kWh.

3.4 FUTURE SCENARIO

3.4.1 Description of the future scenario

In a mid-time period, from 5 to 10 years time, it is expected that both the fuel cells and Stirling engine prices decrease in an exponential way. With a mass production the economies of scale make the unit price cheaper.

The objective for 2020 is that fuel cells cost no more than $450/kW [21], however at the moment this seems to be more near to a utopia than to reality. A conservative price of 1000€/kW has been considered for the SOFC stacks for the study in the future scenario.

Future price of the Stirling engine is set to 850€/kW, not including the burner.

The purchase cost of the rest of the elements is maintained constant, as it is considered that the technologies associated to their production have already achieved the lowest possible construction price.

A thermoeconomic study of the Ammonia power plant has been carried out for the future scenario, as it is the power plant that presents the best specific costs for the current period of time.

3.4.2 Cost rates for capital investment and operation and maintenance in the future scenario

COMPONENT (€/h)


Cathode air preheater 0,0907 Fuel preheater 0,0305 Water heating 0,0256 Air compressor 0,0061

Burner 0,0103 Table 3.19


90

3.4.3 Specific costs of electricity and water in the future scenario

Fourth column in Table 3.20 shows the average electricity cost in the future scenario for the ammonia power plant. It is a reduction of 33,33% with respect to the current cost associated to the same power plant.

It can be seen that future prices will be competitive with the market electrical prices.

POWER PLANT SOFC (€/kWh) STIRLING ENGINE (€/kWh)

ELECTRICITY COST (€/kWh)

AMMONIA 0,1753 0,2473 0,1828 Table 3.20

The cost for the hot water generated in the water heater remains constant, whilst the cooling water of the Stirling engine reduces its cost in 33,25%. However, it is still not a reduction big enough to become competitive with the current district heating systems.

POWER PLANT HOT WATER (€/kWh) STIRLING ENGINE COOLING WATER (€/kWh)

AMMONIA 1,027 0,1951 Table 3.21


91

4. CONCLUSION

Integrating a Stirling engine cycle with an SOFC power plant has been proved to be a suitable option to produce electrical power in a nearby future. For a small power system such as the one studied in this thesis, efficiencies of nearly 60% have been achieved, expecting higher efficiencies for much larger power generation plants. This hybrid plant offers not only satisfactory efficiencies at nominal load, as at partial loads the performance of the plant remains quasi-constant for a large range of loads.

The utilization of fuels that do not need previous physicochemical treatments before entering the solid oxide fuel cell stack have been proved to be an acceptable alternative to the fossil fuels used today in the fuel cell power plants. The ammonia power plant for example, presents efficiencies higher than the methanol one; and very similar to the studied Natural Gas power plant. The capital investment needed for the ammonia plant is lower than for the other two, plus no CO2 is released with ammonia as a fuel, only nitrogen and steam, therefore it is a much more environmentally friendly choice.

The thermoeconomic analysis carried out shows that today the integrated SOFC-Stirling engine power plants cannot compete with the traditional power generation systems, as the cost of the electricity produced is higher than the market price. However, with the conservative hypothetical future scenario studied, the costs of the electricity generated will be competitive with the market prices (actual ones).


92

NOMENCLATURE

Latin alphabet

A Surface area

Er Reversible Potential

E Electrical power

F Faraday’s constant (96.485 C)

Change in Gibbs free energy

LHV Lower heating value

n Number of electrons in the reaction

NG Natural Gas

p Pressure

Q Heat energy

R Molar gas constant (8,314 J/kmol)

ΔS Change in entropy

SOFC Solid Oxide Fuel Cell

T Temperature

t time

∀ Volume

Greek Alphabet

ρ Density

η Efficiency

φ Partial Load

Superscript

TOT Total

º Standard conditions


93

Underscript

dem Demand

k Element or component k in the powerplant

MAX Maximum

MIN Minimum

rec Recirculated


94

REFERENCES

[1] www.Naturalgas.org

[2] Matthew M. Mench – FUEL CELL ENGINES. John Wiley & Sons; 2008, ISBN 978-0-471-68958-4

[3] Leo J.M.J Blomen and Michael N. Mugerwa – FUEL CELL SYSTEMS. Plenum Press; 1993, ISBN 0-306-44158-6

[4] G.T Reader & C.Hooper – STIRLING ENGINES. E&F.N Spon; 1993, ISBN 0-419-12400-4

[5] www.moteurstirling.fr

[6] Saeid Mokhatab, William A.Poe and James G. Speight – HANDBOOK OF NATURAL GAS TRANSMISSION AND PROCESSING. Elsevier inc; 2006, ISBN 978-0-7506-7776-9

[7] www.mp.haw-hamburg.de

[8] Ke Lu, Chunshan Song, Velo Subramani – HYDROGEN AND SYNGAS PRODUCTION AND PURIFICATION TECHNOLOGIES. John Wiley & Sons; 2010, ISBN 978-0-471-71975-5

[9] C. Zamfirescu, I. Dincer- USING AMMONIA AS SUSTAINABLE FUEL. Journal of Power sources 185; 2008, pp 459-465

[10] Colleen Spiegel – DESIGNING AND BUILDING FUEL CELLS. Mc.Graw Hill Professional; 2007, ISBN 0-07-146977-0

[11] Hansan Liu and Jiujun Zhang – ELECTROCATALYSTS OF DIRECT METHANOL FUEL CELLS. FROM FUNDAMENTALS TO APPLICATIONS. Wiley-VCH; 2009, ISBN 978-3-527-32377-7

[12] Adrian Bejan, Georgen Tsatsaranis, Michael Moran – THERMAL DESIGN AND OPTIMIZATION. John Wiley & sons; 1996, ISBN 0-471-58467-3

[13] http://stats.oecd.org/index

[14] http://ec.europa.eu/research/energy/pdf/efchp_fuelcell5.pdf

[15] http://www.nextag.com/air-blower/shop-html

[16] http://www.precision-combustion.com/bstirlingb.html

[17] http://epp.eurostat.ec.europa.eu/statistics_explained/

[18] http://www.icis.com/v2/chemicals/

[19] http://www.fas.org/sgp/crs/misc/RL34746.pdf


95

[20] http://www.energy.eu/#Domestic

[21] http://www.hydrogen.energy.gov/pdfs/48265.pdf

[22] http://www.ammoniafuelnetwork.org/

[23] http://www.rpi.edu/dept/chem-eng/Biotech-Environ/FERMENT/index.html


96

APPENDIX I- DNA MODELS

A.I-1 Solid Oxide Fuel Cell (SOFC)

The models of solid oxide fuel cell used in DNA are a modified version of SOFCEQ_1D, as the original version does not work when there is nitrogen present in the anode mass flow. The changes made with respect to the original one have been very small, and the basis of the model remains the same. The outlet gas composition calculation is based on equilibrium in the used SOFC model, whilst the generated power is evaluated using a detailed electrochemical model which is calibrated using empirical data. This precise calibration makes the model valid only in the range of temperatures between 650-800ºC. The SOFC models allow the user to introduce the fuel utilization, the working temperature of the cell, the pressure losses at anode and cathode sides, the number of fuel cells per stack and the total number of stacks required. The user can also specify the heat losses. In this project the SOFC is supposed to be adiabatic, therefore there are no losses and the generated heat is fully transferred to the air and fuel mass flows. For the purpose of this project, a number of 74 cells per stack was considered.

• The modified version used in the Natural Gas power plant and the methanol power plant is SOFCEQ_1DN2. The electrochemical reaction occurs for the methane (CH4), hydrogen (H2) and carbon monoxide (CO).

• The modified version used in the ammonia power plant is SOFCEQ_1DNH3. The electrochemical reaction occurs for ammonia (NH3) and hydrogen (H2).

A.I-2 Desulphurizer

The model of desulphurizator used in DNA is SEPARA_1. It is a simple model in which the fuel is introduced by one node, hydrogen sulphide (H2S) is removed from the fuel and the outlet is the clean gas, which is the inlet fuel without the removed gas.

A.I-3 CPO reformer

The model used in DNA for the CPO pre-reformer stands for STEAM_REFORMER. However, the reader must not be confused by the name of the model as it really does not use steam; it is a catalytic partial oxidation reformer. Ethane, propane and N-butane are converted into hydrogen and carbon monoxide, whilst nitrogen is incorporated to the mass flow as air is pumped into the pre-reformer to provide the oxygen needed for the partial oxidation reaction to take place.


97

A.I-4 Methanator

The DNA model for the component in which the methanol is reformed into methane, hydrogen, carbon dioxide and carbon monoxide is METHANATOR. It has one fluid inlet and one fluid outlet. The heat losses can also be specified in the model, however the systems is supposed to be adiabatic in this project. The fluid inlet must have steam in its composition in order for the model to work.

A.I-5 Burner

The model used in DNA for the burner is GASBUR_3. It is a model with two inlets (fuel and oxidant) and one outlet (products); where the composition of the products is calculated based on flows and compositions of oxidant and fuel. Heat evacuated to the surroundings is a parameter to be specified, however, in this thesis the assumption of the burner being adiabatic has been made, therefore The ratio between inlet and outlet pressures is a parameter to be introduced in the model.

A.I-6 Splitter

The objective of the splitter is to separate a mass flow into two different flows, so each one of these can be used for different purposes. The DNA model SPLITTER accepts an inlet flow, and two outlet flows. Pressure and temperature are the same in the inlet and in the outlet.

A.I-7 Mixer

The mixer makes two fluids to converge into a single one. The DNA model for the mixer stands for MIXER_01 and allows the two inlet fluids to have different composition, however the inlet pressure must be the same. There is no pressure loss in the system.

A.I-8 Heat exchangers

HEATEX_1 is the DNA model used in the power plant. It recognises pressure losses both in the cold and hot fluids. The temperature in the hot fluid inlet must be bigger than the temperature in the hot fluid outlet (Thi>Tho); the hot fluid inlet temperature must be higher than the cold


98

fluid outlet temperature (Thi>Tco), and the hot fluid outlet temperature must be higher than the cold fluid inlet temperature (Tho>Tci).

Figure A.I.1 [23]

A.I-9 Water pump

The simple model LIQPUM_1 increases the pressure of a real fluid until the desired value. It has one mass flow inlet and one mass flow outlet. The electrical efficiency of the pump’s electrical motor has to be defined too.

A.I-10 Valve

The DNA model for the valve is VALVE_01. It reduces the pressure to the desired value, maintaining constant the rest of parameters.

A.I-11 Stirling engine

The Stirling engine model used in this master thesis was created by former DTU student Thomas Frank Petersen for his master thesis. In case a complete understanding of the model was needed, please turn to his thesis.

STIRLING_1 refers to the DNA component which models the operation of a Stirling engine. It works with a heating fluid which transfers the heat to the helium enclosed in the engine; and


99

with a cooling fluid which removes the heat. The parameters to be introduced are the pressure losses in the heating and cooling fluid, the heater wall temperature, the temperature difference between the heater and the helium, the heater efficiency, temperature difference between cooler temperature and the helium, compression ratio, regenerator effectiveness and a loss factor for the heat.

The following considerations have been made for the Stirling engine model throughout the project:

• The pressure loss has considered negligible, both in the cooling fluid pipe and in the heating fluid tube.

• The heater wall temperature is set to 600ºC, so the engine can be built with non-expensive materials, thus the cost is lower than if it operated at higher temperatures. The big enthalpy change between the entrance of the combustion products and the exit, is beneficial for the power output, but is detrimental for the efficiency of the Stirling engine.

• The compression ratio is set to 1,44 which is a value slightly superior to the current compression ratios used in the scarce commercially available Stirling engines.

• The regenerator efficiency is fixed to 0,98.


100

APPENDIX II – EQUATIONS AND RESULTS FOR THE THERMOECONOMIC ANALYSIS

A.II-1 Thermoeconomic balance equations of the components

A.II-1.1 SOFC

The air introduced into the Solid Oxide Fuel Cell exits the cathode side with a low content of oxygen, therefore the fluegas can be considered as a waste product and its specific cost is fixed to 0 €/kJ. The usedfuel however still contains unreacted fuel that is capable of reacting in a combustion process, and therefore the specific cost of this stream cannot be considered as zero.

Figure A.II.1


101

A.II-1.2 Stirling engine

The equation of the Stirling engine is shown below.

Figure A.II.2

A.II-1.3 Heat exchanger

The purpose of the totality of the heat exchangers used in the analyzed power generation plants is to heat a cold fluid by means of heat transfer from a hot fluid. The exergetic specific cost of the heated fluid (fluid 2 in the diagram) will thus vary, however the specific cost of the heating fluid (fluid 1 in the diagram) is considered to remain constant [12].

Figure A.II.3


102

A.II-1.4 Desulphurizer

The aim of the desulphurizer is to remove the hydrogen sulphide from the fuel stream. The assumption that pouring the hydrogen sulphide to the environment is costless has been made, despite the fact that it is a very poisonous and flammable gas, therefore it should be removed safely and thus with an economic cost.

Figure A.II.4


103

A.II-1.5 Fuel reformer

The CPO reformer thermo-economic balance equation is as follows:

Figure A.II.5a

Equation (A.II.10) expresses the thermo-economic balance of the methanator used in the methanol power plant.

Figure A.II.5b

A.II-1.6 Air compressor

The equation of the air compressor is expressed as follows. The air that enters the compressor is in standard conditions, therefore is available in the environment thus its price is set to zero.


104

Figure A.II.6

A.II-1.7 Burner

The burner’s cost rate equation is shown below.

Figure A.II.7


105

A.II-1.8 Water pump

Figure A.II.8

A.II-1.9 Splitter

Figure A.II.9


106

A.II-1.10 Mixer

Figure A.II.10

A.II-1.11 Valve

Figure A.II.11


107

A.II-2 Auxiliary equations

• The specific cost of the mechanical power required by the air compressor and the water pump is assumed to be expressed by the following equation:

• The specific cost of the cooling water entering the pump and the cooling water exiting the Stirling engine is the same, as it is the heating fluid the heat exchanger responsible for the domestic heating (not represented in the illustrations).


108

A.II-3 Numerical results from the thermoeconomic analysis

A.II-3.1 Natural Gas

COMPONENT UNIT

PRICE (€) NUMBER OF UNITS

PURCHASE COST (€)

(€)

EPP (€/year)

(€/h)

SOFC 3000 10 30000 43758 4277,21 1,1228 Stirling engine 2500 1 2500 4997,25 488,47 0,1282

Cathode air preheater

1470 1 1470 3494,34 341,56 0,0897

CPO air preheater

500 1 500 1188,55 116,18 0,0305

CPO clean fuel preheater

500 1 500 1188,55 116,18 0,0305

Fuel preheater

500 1 500 1188,55 116,18 0,0305

Water heating 500 1 500 999,45 97,69 0,0256 Desulphurizer 800 1 800 1901,68 185,88 0,0488 CPO reformer 1100 1 1100 2614,81 255,59 0,0671

Air compressor

100 2 200 475,42 46,47 0,0122

Burner 200 1 200 399,78 39,08 0,0103 Table A.II.1

FLUID c (€/GJ) FLUID c (€/GJ) 1 0 15 134,4 2 120,4 16 33,09 3 2,672 17 33,09 4 0 18 33,09 5 0 19 33,09 6 0 20 33,09 7 0 21 30,4 8 28,28 22 30,4 9 28,9 23 89,98

10 0 24 89,98 11 29,77 25 30,4 12 30,41 26 0 13 0 27 377,8 14 3,55

Table A.II.2- Specific costs for the different fluids in the NG power plant


109

A.II-3.2 Ammonia

COMPONENT UNIT


PURCHASE COST (€)

(€)

EPP (€/year)

(€/h)



1487 1 1487 3534,75 345,51 0,0907

Fuel preheater

500 1 500 1188,55 116,18 0,0305

Water heating 500 1 500 999,45 97,69 0,0256 Air

compressor 100 1 100 237,71 23,24 0,0061

Burner 200 1 200 399,78 39,08 0,0103 Table A.II.3

A.II-3.3 Methanol

COMPONENT UNIT


PURCHASE COST (€)

(€)

EPP (€/year)

(€/h)


Anode reformed fuel

preheater 500 1 500 1188,55 116,18 0,0305


2246 1 2246 5814,39 568,34 0,1492

Fuel preheater

500 1 500 1188,55 116,18 0,0305


compressor 100 1 100 237,71 23,24 0,0061

Burner 200 1 200 399,78 39,08 0,0103 Methanol reformer

1100 1 1100 2614,81 255,59 0,0671


110

A.II-3.4 Future scenario for Ammonia power plant

COMPONENT UNIT


PURCHASE COST (€)

(€)

EPP (€/year)

(€/h)

SOFC 1000 10 10000 14586 1425,9 0,3743 Stirling engine 850 1 850 1699 199,24 0,0523


1487 1 1487 3534,75 345,51 0,0907

Fuel preheater

500 1 500 1188,55 116,18 0,0305


compressor 100 1 100 237,71 23,24 0,0061

Burner 200 1 200 399,78 39,08 0,0103


111

APPENDIX III – GRAPHS OBTAINED IN THE ANALYSIS OF POWER OUTPUT FROM THE POWER PLANTS

A.III-1 Basic NG power plant

Figure A.III.1

Figure A.III.2

9.5

9.6

9.7

9.8

9.9

10

10.1

10.2

10.3

0.405 0.41 0.415 0.42 0.425 0.43 0.435 0.44

Net

Pow

er (k

W)

Natural gas massflow (g/s)

10

11

12

0.5050

0.5060

0.5070

0.5080

0.5090

0.5100

0.5110

0.5120

0.5130

0.5140

0.5150

0.405 0.41 0.415 0.42 0.425 0.43 0.435 0.44

Effic

ienc

y

Natural gas massflow (g/s)

10

11

12


112

A.III-2 Improved NG power plant

Figure A.III.3

Figure A.III.4

9.7

9.8

9.9

10.0

10.1

10.2

10.3

10.4

10.5

10.6

0.355 0.36 0.365 0.37 0.375 0.38 0.385 0.39

Net

Pow

er (k

W)

Natural Gas massflow (g/s)

10

11

12

0.5880

0.5890

0.5900

0.5910

0.5920

0.5930

0.5940

0.5950

0.5960

0.355 0.36 0.365 0.37 0.375 0.38 0.385 0.39

Effic

ienc

y

Natural Gas massflow (g/s)

10

11

12


113

A.III-3 Ammonia power plant

Figure A.III.5

Figure A.III.6

9.8500

9.9000

9.9500

10.0000

10.0500

10.1000

10.1500

10.2000

10.2500

10.3000

0.92 0.92 0.93 0.93 0.94 0.94 0.95 0.95

Net

Pow

er (k

W)

Ammonia mass flow (g/s)

10

11

12

0.5780

0.5785

0.5790

0.5795

0.5800

0.5805

0.5810

0.5815

0.5820

0.5825

0.5830

0.915 0.92 0.925 0.93 0.935 0.94 0.945 0.95

Effic

ienc

y

Ammonia mass flow (g/s)

10

11

12


114

A.III-4 Methanol power plant

Figure A.III.7

Figure A.III.8

9.8000

9.9000

10.0000

10.1000

10.2000

10.3000

10.4000

10.5000

10.6000

0.895 0.9 0.905 0.91 0.915 0.92 0.925 0.93 0.935 0.94 0.945

Net

pow

er (k

W)

Methanol mass flow (g/s)

10

11

12

0.5480

0.5500

0.5520

0.5540

0.5560

0.5580

0.5600

0.5620

0.5640

0.895 0.9 0.905 0.91 0.915 0.92 0.925 0.93 0.935 0.94 0.945

Effic

ienc

y

Methanol mass flow (g/s)

10

11

12


115

APPENDIX IV- DNA CODES USED FOR THE MODELLING OF THE POWER PLANTS

A.IV-1 Basic Natural Gas Powerplant2

Figure A.IV.1



H CPO air preheater


I CPO reformer


F CPO reformer


G CPO air compressor N Water heater Table A.IV.1

2 The numeration of the different fluids is different from the illustration in paragraph 2.1.2. Use figureA.III-1 to follow the Basic Natural Gas power plant code.


116


117


118

A.IV-2 Improved Natural Gas power plant

Figure A.IV.2



H CPO air preheater


I CPO reformer


F CPO reformer


G CPO air compressor N Water heater Figure A.IV.2


119


120


121


122

A.IV-3 Ammonia power plant3

Figure A.IV.3

LETTER COMPONENT A Cathode air compressor B Cathode air preheater C SOFC D Fuel preheater E Fluegas splitter F Burner G Stirling engine H Water pump I Water heater

Table A.IV.3

3 The numeration of the different fluids is different from the illustration in paragraph 2.2.2. Use figureA.III-3 to follow the Ammonia power plant code.


123


124


125

A.IV-4 Methanol power plant4

Figure A.IV.4



H Usedfuel splitter


I Fluegas splitter

C SOFC J Burner D Fuel preheater K Stirling engine

E Fuel and usedfuel

mixer L Water pump

F Steam reformer M Water heater

G Anode reformed fuel

preheater

Table A.IV.4

4 A virtual component had to be added to the DNA model for it to work correctly, between nodes 16 and 10. In Figure A.IV.4 only node 10 is represented, as 16 is in reality the same one.


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