A Carbon Neutral SOFC (Solid Oxide Fuel Cell)-Based...

MULTI-GENERATION SYSTEM FOR BUILDING APPLICATION 1

A Carbon Neutral SOFC (Solid Oxide Fuel Cell)-Based

Multi-generation System for Building Application

A Briefing Summary

The Hong Kong Polytechnic University

Submitted by:

HONG Jingke Chris

CHEN Mengpei Julia

CHEN Tingting Cindy

LEE Pan

JIANG Chen David


Abstract

Global energy crisis urges the development of alternative energy and enhancement of energy

utilization efficiency. This study proposes a Carbon Neutral SOFC Multi-generation System to utilize

the wastes for the provision of electricity, cooling and hot water. This system consists of four major

components: 1) biogas digestion plant where the organic waste (e.g. food waste and sewage) is

utilized; 2) SOFC energy server for uninterrupted electricity generation; 3) absorption cooling system

recovering the exhaust heat from high temperature SOFC; 4) hot water production from the residual

thermal energy by a bottoming cycle. A real case simulation of Hong Kong International Airport is

used to demonstrate the applicability of this proposed system. This system will be analysed through

three different perspectives, namely economic, environmental and social aspects. Firstly, cost and

saving analysis was conducted to investigate the financial feasibility of the proposed system.

Sensitivity analysis was then carried out to identify the significant factors affecting the payback period

of the system. Secondly, life cycle energy performance assessment and greenhouse gas emission

assessment were also performed to compare the environmental performance between the multi-

generation system and the traditional power plant in terms of energy consumption, CO2, CH4 and N2O

emission. Thirdly, social factors were considered to solicit the views from difference stakeholders on

the use of the proposed system. Finally, the recommendations in policy-making level were made to

enhance the further use of this Carbon Neutral SOFC Multi-generation System.

Keywords: Multi-generation system, SOFC, Absorption Chiller, Biogas, Sustainable


1. INTRODUCTION

Energy crisis has become an undeniable global issue. Not only developing countries as far apart as

South Africa are plagued by electricity disruptions, developed countries are also facing periodic

blackouts and concerning security of energy supply (ElBaradei, 2010). In China, the Electricity

Council expected a power deficit of 40 million kilowatts in 2012 (Caixin Online, 2012), and the coal

shortage has become the most significant factor restricting the power supply (International Society of

Automation, 2008). Besides, the large consumption of fossil fuels causes serious pollution and climate

changes around the world (Jiang et al, 2011).

To address the energy crisis, there are two options: 1) searching for alternative energy sources, 2)

increasing system efficiency during energy generation thus reducing the energy loss. Biogas

production from municipal sewage sludge and food waste is considered as a means to supplement the

supply of energy and reduce the demand of waste treatment in many developed countries (Curry and

Pillay, 2012). Furthermore, developing technologies that increase energy efficiency and recovery of

waste heat have been gaining attention. Multi-generation, most commonly in form of cogeneration to

provide power and heating at the same time, is one of such technologies. This system can raise the

system efficiency to over 80% while the conventional electricity generation systems have the average

efficiency of 30-35%. Apart from the fossil-fuel combustion generation, there have been promising

conversion technologies replacing the direct chemical reactions to reduce energy loss, such as fuel

cells, which produce electricity with high efficiency through electrochemical reactions. Another way

to increases system efficiency is by heat-recovery such as absorption cooling, which is commonly

adopted to recover high quality exhaust heat for space cooling.

Up to present, there is no previous study on the SOFC-based multi-generation for condition in

Mainland China or Hong Kong, let alone a carbon-neutral SOFC multi-generation system using biogas

as fuel. Therefore, an investigation for large-scale application of such system is worth conducting.

2. SELECTION OF PREMISES - HONG KONG INTERNATIONAL AIRPORT

In order to investigate the suitability of the Carbon Neutral Multi-generation System, a set of criteria

is established, with considerations on the daily and monthly energy demand patterns, reusable waste

generation, composition of energy resources as well as the space available for the installation. These

criteria are detailed as follows:


Electricity demand should be relatively consistent in daily basis, as the SOFC energy server

will be operating uninterruptedly.

Preferably there should be consistent demand in space cooling and hot water around the clock,

as the power generation and the provision of cooling and hot water are simultaneously.

For the biogas digestion, large amount of organic wastes from mainly three sources (i.e. used

cooking oil, food waste and sewage) are required to provide sufficient amount of gas fuel for

the energy server.

As the installation of the system will take up considerable space within the buildings/

development, especially for the digestion plant, it is suggested that non-commercial buildings

with lower building density are of higher applicability.

Taking the above four aspects into consideration, transport buildings with comprehensive customer

services operating around the clock will be of highest applicability for the proposed multi-generation

system. In this study, Hong Kong International Airport (HKIA) is chosen with its waste profile targeted

for design of the carbon-neutral multi-generation system and the following feasibility study.

The target wastes to be utilized for the multi-generation system are used cooling oil, food waste

and the sewage from the toilet facilities. The estimation of target wastes is listed in Table 1.

To sum up, the organic waste from used cooking oil, food waste and the sewage amounts to

34,667kg/day from HKIA.

3. SYSTEM CONFIGURATION - CARBON NEUTRAL MULTI-GENERATION SYSTEM

The Carbon Neutral Multi-Generation System consists of four major components, namely Biogas

Production Plant, SOFC Power Generator, Absorption Chiller and Hot Water System.

3.1 Biogas Production Plant

The methane-rich biogas is produced from the biomass resources through the process of anaerobic

digestion. Anaerobic digestion is a process in which micro-organism breaks down biodegradable

material in the absence of oxygen (Curry and Pillay, 2011). Through this complicated biochemical

Table 1: The estimation of target wastes

Target wastes Original volume Density Quantity Data source

Cooling oil 120 liter 0.92 kg/liter 110.4 kg/day Airport Authority

Food waste 200 kg/day ———— 200 kg/day Airport Authority

Sewage 156164 passengers/day *1.1 kg /per

persom=171781 kg/day

0.2 (utilization rate) 34,356kg/day Xiao & Huang, 2010

Airport Authority


process, different strains of bacteria digest complex chains of carbohydrates, fats and proteins into

roughly 60% of methane (CH4) and 40% of carbon dioxide (CO2). Theoretically, the estimated

amount of biogas can be calculated using Buswell’s Equation (George et al, 1993):

a 2 4 2 3

4 2 3 4 2 3 4 2 3

4 8 8b c d

a b c d a b c d a b c dC H O N H O CH CO dNH

However, the above calculation is assumed that 100% of the food waste is utilized. In practice,

when no additional heat input to the system, around only 40-65% of the food waste is broken down

and converted to the biogas.

As discussed in the previous section, around 34,667 kg/day of the usable organic waste,

including sewage, cooking oil and food waste, is collected per day in HKIA. Based on the value of

biogas yield for mesophilic digestion (367 m3/tonne), the daily biogas produced from the organic

waste is 12,723 m3, with regards to the incomplete breakdown of organic waste.

3.2 SOFC Power Generation

A SOFC consists of two porous electrodes, namely the anode and the cathode, which are separated by

electrolyte to conduct oxide-ion (Singhal, 2008). During the operation, oxygen is supplied at the

cathode to react with electrons from circuit. The resulting oxide ions will pass through the electrolyte

to the anode. At the anode side, gas fuels such as hydrogen, natural gas and biogas are supplied and

internally reformed under the high operating temperature.

Bloom Energy’s ES-5700 Energy Server will be adopted for this study as it is the largest SOFC

energy server commercially available at present. Given the technical specification of ES-5700,

1.32MMBtu/h (equals to 1393.7484MJ/h) fuel input generates a base output of 200kWh; on average

an efficiency of 51.66% can be expected. According to the organic waste collection data in HKIA, the

biogas digestion plant should be able to produce 12,722.63m3 biogas on daily basis, which is capable

of providing power at 1,597kW given the electrical efficiency of ES-5700. To cope with potential

fluctuation in gas supply, the multi-generation is designed with 7 units of ES-5700 server, providing

electricity at 1,400kW uninterruptedly.

3.3 Configuration of Absorption Cooling System

The multi-generation system in this study adopts a double-effect absorption chiller to recovery the

high quality exhaust heat from the SOFC power generator.


As mentioned, seven ES-5700 energy servers will be employed in the multi-generation system.

Considering each of the servers has the fuel requirement of 1.32MMBtu/hour and a base load output

of 200kW, and it is assumed that the energy server box is thermally enclosed, the residual energy will

be exhausted in form of steam and carbon dioxide mixture. The high temperature exhaust steam will

then be directed to a flow controller to achieve a desired temperature of 500°C. The efficiency of the

flow controller is taken as 100%. The exhaust gas at 500°C serves as the energy input for the

absorption chiller. By the calculation (the details, please refer to the full report), the hourly exhaust

flow rate = 4,716,238.8 kJ/hour ÷ [1.70kJ/(kg∙K) * (500-20)K] = 5,780 kg/hour, and the Broad X

Non-electric Packaged Hot W/Exhaust Chiller BE Mode 75 is used for the cogeneration system,

providing cooling at 872kW .

3.4 Hot Water System

One characteristic of SOFC is the high operating temperature. Although the temperature of the

exhaust gas passing through the absorption chiller is reduced for space cooling, it is still high enough

to provide hot water for water heating system in HKIA. This process can be simply realized by a heat

exchanger in which the heat from the exhaust gas is transferred to the water for hot water provision.

4. RESULTS AND DISCUSSIONS

4.1 Economic Analysis

4.1.1 Cost and Savings Analysis

Cost and savings analysis was conducted to calculate the total costs and savings of the system (See

Table 2). The results show that it is possible for the system to offset the initial investment through

energy saving in around 3 years if the government subsidy is provided at 50% of system cost.

4.1.2 Sensitivity Analysis

A set of cost and savings are summarized above as the baseline profile for the sensitivity analysis. In

regard to the uncertainties behind the assumptions made in compiling this profile, 10% variance on

the cost of the biogas production plant, SOFC energy server, absorption chiller, auxiliary components,

the level of government subsidy, unit price of fuel and electricity as well as the maintenance cost will

be considered for the sensitivity study. The results are presented in the Table 3 and Figure 1 (For

details, please refer to the full report, Appendix 3).


Table 2 Summary of costs and savings for multi-generation system

System Costs Data Sources for Cost and Saving Calculations

Amount (HK$)

Biogas Production Plant 4,923,000 The unit cost of unheated biogas plant ranged from 50 to 70 US dollar per m3 capacity

(Advisory Service on Appropriate Technology)

SOFC Energy Server 54,180,000 Apple’s 4.8 megawatts fuel cell farm in North Carolina (24 ES-5700 units) was set at

the cost of US$ 6.7 million (Ginter, 2012)

The small units (1kW) for residential use is expected at the cost less than US$ 3,000

(Dr. K. R. Sridhar, the CEO of Bloom Energy)

Absorption Chiller 1,940,000 The chosen Two-stage Non-Electric Chiller costs 1.42 million RMB in 2011 price

(Broad Air Conditioning, 2011)

5% inflation rate and currency exchange rate of 1.24 are considered in the cost

calculation

Hot Water Production System 610,434 1% of the cost of the biogas production plant, SOFC energy server and the chiller

Auxiliary Components 6,104,341 10% of the cost of the biogas production plant, SOFC energy server and the chiller

Total Initial Cost 62,224,341

Initial Cost at Government Subsidy of 50% 31,112,170 The purchase cost of the system and the related project can be reduced by 80% through

government (Seattle City Light, 2010)

Operational Costs

Amount (HK$/month)

Biogas Production Plant 3,154 0.2-0.3 RMB/m3 (Biomass Utilization Project)

Electrical Cost for Absorption Chiller 3,766 The tariff rates offered by the China Light and Power Company

Maintenance Cost 311,122 An annual maintenance cost at 6% of the total system cost is set (0.5%/month)

Total Operational Cost 318,042

Operational Savings

Amount (HK$/month)

Electricity Savings by SOFC Power Generation 831,563 Based on the selected model ES-5700, the whole SOFC system generates

1,008,000kWh per month, and the bulk tariff is on the basis that the On-Peak period is

312 hours per month whilst the Off-Peak Period is 408 hours (For details, please refer

to the full report, Table 6.1)

Electricity Savings on Cooling by Absorption Cooling 111,025 Electricity savings are calculated if the water-cooled chillers with the COP of 4.6 is

replaced by the absorption chillers under the same amount of cooling load (For details,

please refer to the full report, Table 6.2)

Fuel Savings by Hot Water Production 173,708 Fuel savings are calculated if the Commercial Heater (Blue Flame NJW321FEL)

manufactured by Towngas is replaced by hot water system under the same amount of

hot water being produced per day. (For details, please to refer the full report, Table 6.3)

Total Operational Savings 1,116,296

Payback Period in Months 39.358

Payback Period in Years 3.28


Table 3 Summary of single factor sensitivity analyses for cogeneration system

+10% Factor Change -10% Factor Change

Payback Period (years) Changes Payback Period (years) Changes

Level of Government Subsidy 2.83 -20.00% 4.24 20.00%

Rate of Electricity 3.16 -10.52% 4.01 13.33%

Cost of Auxiliary Components 3.89 9.91% 3.36 -4.95%

Cost of SOFC Energy Server 3.85 8.88% 3.22 -8.88%

Cost of Maintenance 3.68 4.06% 3.40 -3.75%

Cost of fuel 3.46 -2.13% 3.62 2.23%

Cost of Biogas Production Plant 3.57 0.81% 3.51 -0.81%

Cost of Absorption Chiller 3.55 0.32% 3.53 -0.32%

Operational Cost for Biogas Plant 3.54 0.04% 3.54 -0.04%

-20.00%

-15.00%

-10.00%

-5.00%

0.00%

5.00%

10.00%

15.00%

20.00%

Level of

Government

Subsidy

Rate of

Electricity

Cost of

Auxiliary

Components

Cost of SOFC

Energy Server

Cost of

Maintenance

Cost of fuel Cost of Biogas

Production Plant

Cost of

Absorption

Chiller

Operational

Cost for Biogas

Plant

+10% Factor Change -10% Factor Change

Figure 1 Results of sensitivity analysis

The combined effects of these factors are remarkable. While single factor can extend or shorten the

payback period by 20% at most, the time required for the operational savings to compensate the initial

cost can be more than doubled if adverse changes occur altogether, prolonging the payback period to

over 5 years. Similarly, in a most favourable circumstance, it is possible for the payback period to be

almost halved, leaving less than 3 years for the initial investment to be financially justified.

4.2 Environmental Analysis

The environmental analysis focuses on the comparison of energy performance and greenhouse gas

emissions between the multi-generation system and the traditional power plant, aiming to evaluate the

energy consumption and air emissions during the whole life cycle. Life-cycle assessment (LCA) is a

method for evaluating the environmental load and energy consumption of processes or products, goods,

and services during their life cycle from cradle to grave (ISO 2006). This study adopts hybrid LCA

analysis method to evaluate the life cycle energy and air emission performance of two alternatives

namely multi-generation system and tradition power plant.

4.2.1 Embodied Energy Analysis

In general, embodied phase of a certain product includes the extraction of raw materials,

transportation, manufacturing process of component, and production. All the infinite interrelationships


evolved in this process have been reflected in Input-Output Table. By using the cost of each

component of the system (See Table 2) and energy intensity of the industry to which the

corresponding component of multi-generation system belongs, the embodied energy of multi-

generation energy supply system is 7,332.5 tce (tonnes coal equivalent). Figure 2 shows that SOFC

system accounts for around 80% of all energy use, and followed by transformer, flower controller and

biogas digester with the proportion of 9.16% and 5.52% respectively. The embodied energy of the

equivalent cost of the construction of electric power plant by generating 122.64 million kWh is 191.69

tce, which is only 2.6% of embodied energy consumption of the multi-generation system (Figure 3).

4.2.2 Operational Energy Analysis

During the operation, the energy supply system generates electricity by using biogas from biogas

digester. Biogas digestion recovers the energy from the used cooking oil, food waste and sewage

sludge; no additional energy source is required to fuel the SOFC energy server. Therefore, the

operational energy consumption of multi-generation system is -5515.38 tce, which is equal to the sum

of the energy savings from air-conditioning and hot water heating. For the conventional power plant,

according to the energy intensity of traditional electricity generation, the energy required for

generating 122.64 million kWh electricity is 4,586.74 tce.

4.2.3 Life Cycle Energy Performance of Two Alternatives

When considering the whole life cycle, the energy performances of the proposed system and the

conventional system are summarized in Table 4. One unit of kilowatt-hour electricity generated by the

multi-generation system consumes 0.151 kgce while the traditional power plant consumes 0.398 kgce.

The halved energy requirement indicates that this innovative energy supply system is much more

energy-efficient and sustainable in long term use.

SOFC

81.40%

Transformer+Flo

w controller

9.16%

Absorption Chiller

2.98%

Heating exchanger

0.94%

Biogas digester

5.52%

0.00%

1.00%

2.00%

3.00%

4.00%

5.00%

6.00%

Figure 2 Embodied energy contributions from 7 components Figure3 Ratio of embodied energy between traditional

power plant and multi-generation system


4.2.4 Greenhouse Effect Analysis

In this study, three types of gas emissions (CO2, CH4, N2O), which are the primary causes of the

greenhouse effect are studied. According to the conversion coefficients from the Intergovernmental

Panel on Climate Change (IPCC) report, the volume of life cycle the emissions is listed in Table 5.

From the perspective of the whole life cycle, traditional electricity generation method will

produce more CO2 and N2O. In contract, multi-generation system emits more CH4 due to the large

volume in the embodied phase. However, it should be noticed that as the SOFC technology is new to

the market, the unit price of the energy server may not realistically reflect the embodied energy.

4.3 Social Factors Analysis

In order to establish the comprehensiveness of this study, other influential factors for the prospect of

this Carbon Neutral Multi-Generation System are investigated through extensive literature reviews

concerning the technology of biogas digestion, SOFC and absorption chiller to solicit the views from

different stakeholders towards the proposed system. Economic, environmental, technological and

social aspects are considered for circumstances in China. Meanwhile, interviews (For details, please

refer to the full report Appendix 3) with the government official, scholars in relevant fields,

engineering consultant and the end-user were conducted to discuss the feasibility, benefits, and

attitudes towards the prospect of carbon neutral multi-generation system in China, energy

management performance as well as distributed power generation in a broader sense. Key points

raised during the discussion are summarised in the Table 6.

For the prospect of the carbon neutral multi-generation system, the environmental and public

safety impacts are at the heart of the Government’s concern. From the users’ perspective, noticeable

financial benefit is always the primary concern in large-scale commercialization. The amount of initial

Table 4 Life cycle energy performance of two electricity supply system

Embodied phase

(tce)

Operational phase

(tce)

Whole life cycle

(tce)

Quo-generation system 7332.5 -5515.38 1817.12

Power plant 191.69 4586.74 4778.43

Table 5 Life cycle greenhouse gas emissions

Embodied phase Operation phase Life cycle Intensity (g/KWh)

CO2 CH4 N2O CO2 CH4 N2O CO2 CH4 N2O CO2 CH4 N2O

Multi-generation system 28309.8 410.0 393.3 -16546.1 -165.5 -248.2 11763.7 244.5 145.1 98.0 2.0 1.2

Power Plant 665.1 10.0 9.1 13760.2 137.6 206.4 14425.3 147.6 215.5 120.2 1.2 1.8


Table 6 Summary of indicators of utilization of carbon neutral multi-generation system from different stakeholders

Economic Environmental Social Technology

Government Energy loans

Cost

competitiveness

in the market

Project cost

Stimulus of the

renewable

industry

Environmental impact

assessment

The environmental

protection facilities

Pollution Prevention

The reduction of

greenhouse gas emissions

and global warming

Energy supply

Energy demand

The reduction of waste

Energy security

The emerging social

expectation towards

green technology

Public interest

The awareness of

environmental

protection

Education

The increase of the

renewable energy

generation

The reduction of

carbon emission

The use of biogas

technology

End-users

(Clients) Government

support

(subsidies)

Operating and

maintenance

costs

The economic

costs and

benefits

Energy consumption

Compliance of local

regulations

Enhancement of

corporation image

Corporate reputation

Social responsibility

Enterprise culture

Engineering

equipment and

material

Constant power

production

Choice of fuel

Technological

breakthrough

System reliability

Citizen The

justifications

of public

expenditures

for

government

subsidies

The reduction

of electricity

costs

The reduction of

greenhouse gas

emissions and global

warming

The reduction of carbon

emissions

The alleviation of

landfill burden

Improvement of air and

water quality

Enhancement of living

standard and public

health

Levels of trust in

public and private

sector organizations,

The support for future

local energy

development

Social consequences

of renewable energy

The impact of modern

and clean city image

Long-term

performance

capital investment and the length of payback period may carry different weight for users from various

backgrounds. On the other hand, the ordinary citizens are more likely to consider the impacts of the

system on the enhancement of the living standard and the public health and safety.

5. Conclusion

Global energy crisis urges the development of alternative energy and enhancement of energy

utilization efficiency. In this study, an uninterrupted SOFC based multi-generation system, integrated

with biogas digestion, is proposed for building application. Apart from cleaner power generation and

minimal wastage, the system is able to achieve carbon-neutral as fuelled by biogas, and serves as an

alternative for urban waste treatment.


Economic analysis based on a case study of Hong Kong International Airport shows that the

system is of noticeable financial feasibility; it is possible for the system to offset the initial investment

through energy savings in around 3 years. Sensitivity analyses incorporating 9 influential factors

indicate that a payback period of around 2 years in the most optimistic cases and 5 years in the

pessimistic scenario.

From the environmental perspective, the multi-generation system has demonstrated advantages

in life cycle energy performance and greenhouse gas emissions. For per unit of kilowatt-hour power,

the energy use in generating the equivalent electricity by proposed system are less than half of the

energy use in conventional power generation.

According to the interviews conducted with different stakeholders towards the proposed system,

popularity of this multi-generation system to a large extent relies on the government support at starting

stage. In the long-term, the payback period is still vital for the prospect system application. Citizens put

more emphasis on the health and safety issues, as well as the enhancement of the living standard brought

by the system. To sum up, the proposed system brings the advantages ofis promising for the reduction of

greenhouse gas emission; it eases the energy crisis by renewable energy application and a efficient

energy utilization; it is also a possible solution for urban waste treatment.

References

Airport Authority Hong Kong. (2013). HKIA Master

Plan 2030 Technical Report. HKSAR.

Broad Air Conditioning. (2011, 3). Model Selection &

Design Manual. Changsha, China.

Caixin Online. (2012). Power Shortage to Continue in

2012. Beijing, China.

Curry, N., & Pillay, P. (2011). Biogas prediction and design

of a food waste to energy system for the urban

environment. Renewable Energy , 41, pp. 200-209.

ElBaradei, M. (2010). Tackling the Global Energy Crisis.

International Atomic Energy Agency Bulletin, 50 (1).

Ginter, S. (2012, 4 24). Apple Joins Clean Energy

Production Movement. Retrieved 2 22, 2013, from

Energy & Capital:

International Organization for Standardization (ISO).

(2006). Environmental management –life cycle

assessment-principles and framework (ISO14040:

2006), Geneve. International Society of Automation. (2008). Coal

shortage has China living on the edge. Retrieved

November 4, 2012, from ISA:

http://www.isa.org/InTechTemplate.cfm?Section=In

Tech&template=/ContentMa

nagement/ContentDisplay.cfm&ContentID=68134

Seattle City Light. (2010). Integrated Resources Plan.

USA.

Xiao, D., & Huang, D. (2010). Strategy for Water

Resrouces Demand and Supply Management in

Taiwan. Taiwan.

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