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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.