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CHAPTER 1

INTRODUCTION

Energy is essential and vital for economic activity. Building a

storage base of energy resources is a pre-requisite for the sustainable

economic and social development of a country. Indiscriminate extraction and

increased consumption of fossil fuels have led to a reduction in the

underground carbon resources. Energy crises due to the rapid depletion of

fossil fuel, and environmental air pollution due to fossil fuel combustion, are

of alarming concern worldwide. The rapid industrial and economical growth

in recent years in some of the thickly populated nations has stimulated the

utilization of sustainable energy sources and energy conservation

methodologies considering environmental protection. Hence, scientists,

researchers and technocrats are forced to concentrate on finding renewable,

environment-friendly alternative sources of energy the ways and means to

conserve the depleting energy sources, and to recover some of the energy that

would otherwise be wasted.

1.1 THE ENERGY SCENARIO

The energy demand of the world is increasing at an alarming rate

due to industrial growth, increasing mobility, modern means of transport,

changing life style and mechanization of labour. Hydrocarbon fuels (fossil),

have been the main sources of energy for the transport and other sectors for

more than a century. The rapidly increasing consumption and consequent

depletion of reserves show that the end of ‘fossil fuel age’ is not very far off.

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These fuels are the chief contributors to urban air pollution and a major

source of green house gases – the prime cause behind the global climate

change.

Presently, the majority of the world energy needs are met through

non- renewable (fossil) resources, such as petrochemicals, natural gas and

coal. The trend showing the utilization of the various energy resources as also

the projected future demand is shown in Figure 1.1. Since the demand and

cost of petroleum based fuel is growing rapidly, and if the pattern of

consumption continues, these resources will be depleted in few years. Fossil

fuels account for about 90% of the world energy consumption. Fossil fuels are

currently the most economically available source of power for both personal

and commercial uses. However, there are environmental challenges associated

with extracting, transporting and using fossil fuel. In particular, in the process

of burning of fossil fuels, compounds are emitted into the air, which can cause

harm to humans, plants, animals, and the entire ecosystem. In addition, with

the rising prices of crude oil and petroleum products in the world market, and

the increasing dependence on imports, countries like India are becoming more

vulnerable in matters of energy security.

Source : National Bureau of Standards

Figure 1.1 World Primary Energy Demand

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The transportation sector in India is the fastest growing energy

consumer, consuming 90% of the total available oil. It consumes nearly 112

million tonnes of oil annually, and is critically important for Indian economy

and security. On seeing the power generation scenario, the installed capacity

in India the installed capacity of production is 1,34,568 MW with the mixed

proportion of 72 % thermal, 25 % hydel and 3 % nuclear. Only 7% of the

energy is obtained from renewable energy sources as against the target of 25

%. It would be evident that for true energy independence, a major shift in the

structure of energy resources from fossil to renewable energy source is

inevitable. Being one of the fastest growing economies in the world, India is

presently witnessing an unprecedented demand for energy. The projected

increase in demand for all energy resources is shown in comparison with

China and the rest of the world in Figure 1.2.

China & India will contribute more than 40% of the increasein global energy demand to 2030 on current trends

WHERE INDIA STANDS ON ENERGY DEMAND

Source : National Bureau of Standards

Figure 1.2 Where India stands on Energy Demand

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In order to achieve despite the upheaval in economic activities, India

is consuming over a whopping 130 million tonnes of crude oil a year and is

forced to import about 70 % of its needs (PCRA). A sustained growth rate of

8% through 2031, India needs to grow its primary energy supply by 3 to 4

times, and the electricity supply by 5 to 7 times of today’s consumption. The

total Indian energy consumption is expected to grow up to 1500-2000 MT by

2025 and it is imperative to aggressively promote energy efficiency (UPC).

1.2 ENVIRONMENT SCENARIO

Environmental degradation due to fossil fuel combustion includes

global warming, ozone depletion, acid precipitation and others, resulting in a

gradual increase in global temperature, acidification of lakes, streams and

ground water, damage to fish and aquatic life, to forests and agriculture crops,

and deterioration of materials. Most of the global air pollution is caused by

the use of fossil fuels for transportation. Diesel engines are a major source of

air pollution. The exhaust gases from diesel engines contain oxides of

nitrogen (NOx), carbon monoxide (CO), carbon dioxide (CO2), unburned or

partially burned hydrocarbons (HC), and particulate matter (PM).

India is one among the most CO2 emitting countries of the world,

which is not a fact to feel proud of. The present atmospheric CO2 status is

shown in Figure 1.3. As seen from the figure, the present level of the

atmospheric CO2 is 365 ppm, which is not very far from reaching the

dangerous upper limit of 400 ppm (MNRE).

5

0

50

100

150

200

250

300

350

400

450

Low er Limit At the time of

evolution

Present Upper Limit

Limits

CO

2 L

evel

(pp

m)

Figure 1.3 Atmospheric CO2 status

This emphasizes the need of the entire world to curtail the emission

of CO2, and hence save fossil fuel. The proportion of CO2 in the atmosphere

has risen by about one third since industrialization began. The number of

disasters has tripled since the 1960s and the resulting economic damage has

increased by a factor of nine. The eight warmest years over the last 130 years

were recorded during the past 11 years (Munich 2006). The economic damage

of this climate change will touch an annual figure of $ 300 billion by 2050.

Sea levels have risen by 10-20 cm in the last 100 years, 9-12 cm of this in the

last 50 years. A mid range level of global warming could result in the

extinction of 1,000,000 terrestrial species by the middle of this century.

1.3 ENERGY CONSERVATION

Energy conservation and energy efficiency are separate, but related

concepts. Energy conservation is achieved when the growth of energy

consumption is reduced, measured in physical terms. Energy conservation

can, therefore, be the result of several processes or developments, such as

productivity increase or technological progress. On the other hand, energy

efficiency is achieved when energy intensity in a specific product, process or

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area of production or consumption is reduced without affecting the output,

consumption or comfort levels. The promotion of energy efficiency will

contribute to energy conservation, and is therefore, an integral part of energy

conservation promotional policies. Waste Heat Recovery (WHR) is one of the

energy conservation options which is as important as developing a new source

of energy.

1.3.1 Waste Heat Recovery

Waste heat is the heat generated in a process by way of fuel

combustion or chemical reactions, which is then 'dumped' into the

environment, and not reused for useful and economic purposes. The essential

fact is not the amount of heat, but rather its ‘value’. The mechanism to

recover the unused heat depends on the temperature of the waste heat gases

and the economics involved. Large quantities of hot flue gases are generated

from boilers, kilns, ovens and furnaces. If some of the waste heat could be

recovered, a considerable amount of primary fuel could be saved. When

recovering waste heat, its quality must be considered first. Depending upon

the type of process, waste heat can be discarded at virtually any temperature

from that of chilled cooling water to high temperature waste gases in an

industrial furnace or kiln. Usually, higher temperatures equate to higher

quality of heat recovery and greater cost effectiveness. In any study of WHR,

it is absolutely necessary that there is some use for the recovered heat. Typical

examples of use would be pre-heating of combustion air, space heating, or

pre-heating boiler feed water or process water.

A waste heat recovery unit is an energy recovery heat exchanger

that recovers heat from hot streams with potential high energy content, such

as hot flue gases from a diesel generator or steam from cooling towers or even

waste water from different cooling processes. Depending on the temperature

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level of the waste heat and the proposed applications, different heat exchanger

devices can be employed to facilitate the use of the recovered heat.

The major applications of WHR are:

Waste heat of low temperature range (0-120oC) could be used

for the production of bio-fuel by growing algae farms, or it

could be used in green houses, or even in Eco-industrial parks.

Waste heat of medium (120-650oC) and high (>650oC)

temperature could be used for the generation of electricity via

different capturing processes.

A WHR system has many direct and indirect benefits:

The recovery of waste heat has a direct effect on the efficiency

of the process. This results in a reduction of the size of all the

flue gas handling equipments, such as fans, stacks, ducts,

burners etc., that reduce the auxiliary energy consumption: The

reduction in the equipment size gives additional benefits in the

form of reduction in auxiliary energy consumption, like

electricity for fans, pumps etc.

The improvement in the efficiency of the system by heat

recovery reduces the energy generation requirement, and also

depending on the temperature level of the exhaust and the

proposed application, different heat exchange devices, heat

pipes and combustion equipments can be employed to facilitate

the use of the recovered heat. The shell-and-tube heat exchanger

is the most widely used type of industrial heat transfer

equipment. Initially, only plain tubes were used in shell-and-

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tube heat exchangers. However, due to the increasing incentives

for more efficient heat exchangers, considerable emphasis has

been placed on the development of various augmented or

enhanced heat transfer surfaces. The use of enhanced surfaces

allows the designer to increase the heat duty for a given

exchanger to reduce the size of the exchanger, the pumping

power and also the approach temperature difference. Shah and

Sekulic (2003) have reported that the heat transfer coefficient

(h), for gases, is generally several orders of magnitude lower

than that for water, oil and other liquids. In order to minimize

the size and weight of a gas-to-liquid heat exchanger, the

thermal conductance (hA) on both sides of the exchanger should

be approximately the same. Hence, the heat transfer surface on

the gas side needs to have a much larger area and be more

compact than the circular tubes commonly used in shell-and-

tube exchangers.

1.3.2 Heat Recovery from Diesel Engines

Large capacity diesel engines are one of the most widely used stand

alone power generation units. Nearly two-thirds of the input energy is wasted

through the exhaust gas and cooling water of these engines. It is imperative

that a serious and concrete effort should be launched for conserving this

energy through WHR techniques. Such a system would ultimately reduce the

overall energy requirement. There are three sources from which heat can be

recovered, namely, jacket water, exhaust gases and lubricating oil in diesel

engines. Among these sources, high quality heat can be extracted from the

exhaust gas which will be useful for many process applications, as the exhaust

gas temperature is at a higher level. In a four stroke diesel engine, the

temperature of the exhaust gas is approximately 400 to 500oC at full load

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conditions. Hence, it is possible to recover a large quantity of useful heat from

these exhaust gases.

1.4 THERMAL STORAGE FOR WASTE HEAT RECOVERY

The major technical constraint that prevents the successful

implementation of a heat recovery system is the intermittent and time

mismatched demand and availability of energy. In order to overcome the

above constraint, WHR systems should be integrated with energy storage

units.

1.4.1 Types of Energy Storage

There are many types of energy storage systems and they are

broadly classified as below.

1. Mechanical Energy Storage

Hydro storage (pumped storage)

Compressed air storage

Flywheels

2. Chemical Energy Storage

Electrochemical batteries

Lead acid batteries

Lithium iron sulfide batteries

Sodium sulfur batteries

Organic molecular storage

Chemical heat pump storage

3. Biological Storage

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4. Magnetic Storage

5. Thermal Energy Storage

Among these storage methods, Thermal Energy Storage (TES) is

one of the key technologies for energy conservation, and therefore, is of great

practical importance. TES systems can contribute significantly to meet the

society’s needs for more efficient, environmentally benign energy use in

building heating and cooling, aerospace power, and utility applications. TES

is perhaps as old as civilization itself. Since recorded time, people have

harvested ice and stored it for later use. Large TES systems have been

employed in recent years for numerous applications, ranging from solar hot

water storage to building air conditioning systems. The TES technology has

only recently been developed to a point where it can have a significant impact

on modern technology.

TES systems have an enormous potential to increase the

effectiveness of energy conversion equipment use, and for facilitating large

scale fuel substitutions in the world’s economy. The use of the TES system

has the following significant benefits:

Reduced energy costs

Reduced energy consumption

Improved indoor air quality

Increased flexibility of operation

Reduced initial and maintenance costs

In addition, Dincer and Rosen (2002) pointed out some further

advantages of the TES system:

Reduced equipment size

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Charging

Time

T3

T4

QrQr T Constant

T1>T2 T4>T3

T2

T1

DischargingStoringCharging

More efficient and effective utilization of equipment

Conservation of fossil fuels (by facilitating more efficient

energy use and/ or fuel substitution)

Reduced pollutant emissions (e.g. CO2 and CFCs)

TES can be achieved in the form of the sensible heat of a solid or

liquid medium, the latent heat of a phase change substance, or by a chemical

reaction. Energy is supplied to a storage system for removal and use at a later

time.

Figure 1.4 shows the schematic of a complete storage process that

involves charging, storing and discharging processes. The choice of the

storage medium depends on the amount of energy to be stored in unit volume

or weight of the medium, and the temperature range at which it is required for

a given application.

Figure 1.4 Charging, storing and discharging processes

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Sensible Heat Storage System

Sensible Heat Storage (SHS) is a simple and well developed

technology. In this storage system, energy is stored by heating or cooling a

liquid or a solid, which does not change its phase during the process. A

variety of materials have been used in such systems. The commonly used

materials in the sensible heat storage system are water, pebble beds, packed

solid beds, refractory materials, hydrocarbon oils, organics and metal salts.

The amount of heat stored depends on the specific heat of the medium, the

temperature change and the quantity of the storage material. An SHS system

consists of a storage medium, a container and input /output ports. The

containers must retain the storage material and prevent the loss of thermal

energy.

The main advantage of the SHS system is that, the energy can be

recovered very easily, as the surface convective heat transfer coefficient is

very high. However, the SHS materials have very low heat storage capacity

per unit volume.

Latent Heat Storage System

Latent Heat Storage (LHS) is based on heat absorption or release,

when a storage material undergoes a phase change process. The LHS unit is

particularly attractive due to its high-energy storage capacity and its

isothermal behaviour during the charging and discharging processes. A wide

range of Phase Change Materials (PCMs) have been investigated, including

salt hydrates, paraffin waxes and non-paraffin organic compounds, for heating

and cooling applications. Any LHS system must possess at least the

following three components.

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A heat storage substance (PCM) that undergoes a phase

transition within the desired operating temperature range,

wherein the bulk of the heat added is stored as latent heat

Containment for the storage substance

A Heat Transfer Fluid (HTF) for transferring heat from the

heat source to the storage substance, and from the latter to

the application.

Thermo Chemical Storage System

In a thermo chemical storage system, thermal energy is used to

produce a certain endothermic chemical reaction and the products of the

reaction are stored. When the energy is required to be released, the reverse

exothermic reaction is made to take place. However, such systems are

expensive and are not suitable for most commercial and domestic uses.

Combined Storage System

SHS systems are simpler in design compared to the latent heat (or)

thermo chemical storage systems. However, they suffer from the

disadvantages of the low heat storage capacity per unit volume of the storage

medium, and their non-isothermal behaviour during the charging (heat

storage) and discharging (heat recovery) processes. On the other hand, LHS

systems have received considerable attention due to their advantages, such as

storing a large amount of energy in a small volume, i.e., high storage density

and charging /discharging at a nearly constant temperature. Though the LHS

systems have desirable characteristics, they are not in commercial use as

much as the SHS systems, because of the poor heat transfer rate during the

heat storage and recovery processes. The main reason is that in an LHS unit

during phase change, the solid-liquid interface moves away from the

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convective heat transfer surface (during charging in the cool storage process

and during discharging in the hot storage process), due to which the thermal

resistance of the growing layer of the solidified PCM increases, resulting in a

poor heat transfer rate.

The combined storage system possesses the advantages of both the

sensible and latent heat storage systems. In this system, the PCM containers /

capsules are always surrounded by the HTF that also performs as an SHS

material, and it is a better alternative, which offers the following benefits:

Higher heat capacity

Isothermal charging and discharging

Eliminating variations in the surface heat transfer rate, due to

the poor thermal conductivity of the PCM

Compact size

Economical operation

The combined sensible and latent heat storage system has found

manifold applications in the domestic, commercial and industrial sectors. In

air-conditioning applications, the combined sensible and latent heat storage

system has been introduced successfully.

1.4.2 Thermal Energy Storage Materials

In recent years, there is a keen interest among researchers to develop

storage materials for various applications. The classification of the storage

materials and the criteria for the selection of the storage material for the

required applications are given in this section.

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Sensible Heat Storage Materials:

In SHS systems, energy is stored by increasing the temperature of a

storage medium, such as water, air, oil, rock beds, bricks and sand or soil. In

the case of a liquid medium, water appears to be the most convenient, because

it is inexpensive and has a high specific heat. However, the storage tank cost

increases considerably beyond 100oC. Organic oils, molten salts and liquid

metals do not exhibit the same pressure problems but their use is limited

because of their handling, containment, storage capacities and cost. However,

for storage at higher temperatures, liquids having a low vapour pressure are

used.

The difficulties and limitations relative to liquids can be avoided by

using solid materials for storing thermal energy as sensible heat. But larger

amounts of solids are needed than while using water, due to the fact that

solids, in general, exhibit a lower storing capacity than water. The cost of the

storage media per unit energy stored is, however, still acceptable for rocks.

The most commonly used solid and liquid sensible heat materials are given in

Appendix 1 (Tables A 1.1 and A 1.2).

Phase Change Materials:

PCMs with the appropriate melting temperatures are widely

employed as ‘latent’ heat storage materials. The chemical bond within the

PCM breaks up due to a rise in the source temperature as the material changes

its phase from solid to liquid. The phase change materials used in LHS

devices should fulfil a number of requirements. Basically, a good PCM

should have a melting point in the desired operating temperature range, high

latent heat, congruent melting and absence of super cooling during freezing.

The various criteria that govern the selection of the phase change storage

materials are given below.

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Thermo-Physical Properties:

Melting temperature in the desired operating temperature range

High latent heat of fusion per unit volume, so that the

required volume of the container to store a given amount of

energy is less

High specific heat to provide for additional significant

sensible heat storage

High thermal conductivity of both the solid and liquid

phases to assist the charging and discharging of energy in

the storage systems

Small volume changes in the phase transformation and small

vapour pressure at operating temperature to reduce the

containment problem

Congruent melting of the PCM for a constant storage

capacity of the material with each freezing/melting cycle

Kinetic Properties

High nucleation rate to avoid super cooling of the liquid

phase

High rate of crystal growth, so that the system can meet the

demands of heat recovery from the storage system

Chemical Properties

Chemical stability

Complete reversible freeze/melt cycle

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No degradation after a larger number of freeze/melt cycles

Non-corrosiveness of the construction materials

Non-toxic, non-flammable and non- explosive materials for

safety

Economic Properties

Low cost

Large scale availabilities

The selection of the PCM depends on the temperature range in

which it is to be used, its compatibility with the encapsulation material and

cost, and the prime factor which determines the thermo economic feasibility

of the latent heat storage system.

PCMs are broadly classified under two groups, namely, organic and

inorganic; they are shown along with their subgroups in Figure 1.5 and their

relative merits and demerits are given in Appendix 1 (Table A1.3). The

thermo physical properties of commercial paraffin and the various PCMs

studied for thermal storage applications by various researchers are given in

the Appendix 1 (Tables A 1.4 to A 1.6).

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PCM

Organic

Eutectic (Singletemperature)

Mixtures(TemperatureInterval)

Eutectic (Singletemperature)

Mixtures(TemperatureInterval)

Paraffin Fatty Acids Hydrated Salts

Commercial Grade Analytical Grade

Inorganic

Figure 1.5 Classification of the phase change materials

1.4.3 Applications of TES systems

Phase change materials have been used for various heat storage

applications since 1980s, and they have recently been used as a storage media

for air conditioning applications with economic benefit. The various

applications of PCM based thermal energy storage found in literature are

given in Table 1.1.

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Table 1.1 Applications of PCM based TES

TES applications References

Solar energy: Space

heating, cooker, heat

storage, solar collector,

and absorption cooling

and water heater.

Lane (1986), kurklu (1998a), Li et al (1991),

Sokolov and Keizman(1991), Domansai et

al (1995),Fath (1995),Buddhi and Sahoo

(1997), Bruno and Saman (2002), Buddhi et

al (2003), Esen M(2004), Nallusamy et al

(2006) , Cheralathan et al (2006),Nallusamy

et al (2009), Zhai et al (2010),Muthu

sivagami et al (2010)

Waste heat recovery

system.

Vasiliev et al (2000), Desai and Bannur

(2001), Talbi and Agnew(2002),

Subramanian et al (2004), Pandiyarajan et al

(2011)

Passive storage using

PCM in building:

curtains, glass system,

solar shading system,

ceiling and wall.

Feldman et al (1986), Peippo et al (1991),

Inaba and Tu (1997), Ismail and Henriquez

(2000), Merker et al (2002), Ismail and

Henriquez (2002), Velraj et al (2002),

Marin et al (2005), Pasupathy and Velraj

(2006),Shanmuga sundaram and Velraj

(2010)

Cooling : Building air

conditioning and

industrial refrigeration

Hasnain et al (1997), Lorsch et al (1997),

Hasnain (1998), Kurklu (1998b), Manske et

al (2001), Omer et al (2001), Riffat et al

(1995), Vakilaltojjar and Saman (2001),

Abdullatif et al (2002), Egolf and Kauffeld

(2005), Cheralathan et al (2007), Antony

Arulraj and Velraj (2010)

Thermal protection of

electronic devices.

Bellettre et al (1997), Pal and Joshi (1997),

Jaworski and Domanski (2006), Shanmuga

sundaram and Velraj (2008)

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1.5 THERMODYNAMIC EXERGY ANALYSIS

The traditional method of assessing the energy disposition of an

operation involving the physical or chemical processing of materials and

products, with the accompanying transfer and transformation of energy, is by

the analysis of the energy balance. This balance is apparently based on the

first law of thermodynamics. In this energy balance, information about the

system is employed in an attempt to reduce heat losses or to enhance heat

recovery. However, from such a balance, no information is available on the

degradation of energy occurring in the process and to quantify the usefulness

or quality of the heat content.

The exergy method of analysis overcomes the above said limitations

of the first law of thermodynamics. The concept of exergy is based on both

the first and second law of thermodynamics. Exergy is defined as the

maximum amount of work that can be produced by a stream of matter, heat or

work as it comes to equilibrium with a reference environment. In fact, it is a

measure of the potential of a stream to cause change, as a consequence of not

being completely stable, relative to the reference environment. For the exergy

analysis, the state of the reference environment or the reference state must be

specified completely. This is commonly done by specifying the temperature,

pressure and chemical composition of the reference environment. Exergy is

not subject to a conservation law. Rather, exergy is consumed or destroyed,

due to irreversibilities in any process.

The exergy analysis is a method that uses the conservation of the

mass and energy principles, and is suitable for furthering the goal of more

efficient energy-resource use. It enables us to determine the locations, types,

and true magnitudes of wastes and losses. Therefore, the exergy analysis can

reveal whether or not, and by how much, it is possible to design more

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efficient energy systems by reducing the sources of inefficiency in existing

systems. In the past, exergy was called essergy, availability, and free energy.

From the energy and exergy application point of view, it is

important to note that if a fossil fuel-based energy source is used for low-

temperature thermal applications like space heating or cooling, there would be

a great difference between the energy and exergy efficiencies, such as 50-70 % as

energy efficiency and 5% as exergy efficiency. This is due to the fact that

high quality (or high temperature) energy sources such as fossil fuels, are

being used for relatively low temperature processes, like water and space

heating or cooling, etc.

1.6 PROFILE OF THE PRESENT WORK

In the present work, the performance of a PCM encapsulated,

combined sensible and latent heat storage system arranged in a cascaded

mode is experimentally investigated in detail. This storage system is

integrated with an IC engine setup through a shell and finned tube heat

exchanger. The thermal performance of the Heat Recovery Heat Exchanger

(HRHE) and the storage system is evaluated for the charging process of the

PCM in the thermal energy storage tank in various engine operating

conditions. Thermodynamic energy and exergy analyses have also been

carried out to measure the quantity and quality of the energy extracted from a

diesel engine.

A survey of the literature pertinent to the present investigation is

carried out and it is presented in Chapter 2. Studies on the waste heat recovery

heat exchanger and thermal energy storage system, phase change materials,

cascaded storage system with multiple PCM, energy and exergy analyses,

thermal analysis and applications are reviewed in the survey.

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Chapter 3 describes the construction details of the experimental set-

up, instrumentation and experimental procedure in detail. Castor oil is used as

the HTF and also as the SHS medium. The HTF flows through the tube side

and the exhaust gas flows through the shell side of the heat exchanger.

The results obtained from the experimental investigation, such as

the temperature distribution in the HRHE, and the TES tank, and the

performance parameters like the heat extraction rate from HRHE, the

charging rate and energy stored in the TES tank are given in Chapter 4.

The importance of the exergy analysis, and the parameters used for

the energy and exergy evaluation are presented in Chapter 5. In addition, the

energy and exergy balance for the overall system is quantified and illustrated,

using the energy and exergy flow diagrams. The efficiency of the cascaded

storage system is explained with an illustrative example for three different

cases during the discharge process.

Chapter 6 summarizes the key results and the conclusions of the

present work. The present work has also relevance in other engineering

applications, such as process industries, food preservation transportation, and

air conditioning. The scope for future work is also presented in this Chapter.