study of solar water heaters based on exergy analysis - YMCA ...

Proceedings <strong>of</strong> the National Conference on
Trends and Advances in Mechanical Engineering,
YMCA University <strong>of</strong> Science & Technology, Faridabad, Haryana, Oct 19-20, 2012
STUDY OF SOLAR WATER HEATERS BASED ON EXERGY ANALYSIS
Dilip Johari 1 , Ashok Yadav 2 , Ravi Verma 3
1
M.Tech Student, 2 Asst. Pr<strong>of</strong>., Department <strong>of</strong> Mechanical Engineering, Dayalbagh Educational University,
Agra, U.P., India
3 Asst. Pr<strong>of</strong>., Department <strong>of</strong> Mechanical Engineering, Graphic Era University, Dehradun, Uttrakhand, India
Email: 1 dilipjohari@gmail.com, 2 ashokyadavaca@gmail.com, 3 raviverma2020@gmail.com
Abstract
Energy application from the sun to heat <strong>water</strong> is well known. Solar <strong>water</strong> heater is a device which is used for
heating the <strong>water</strong> for domestic and industrial purposes by utilizing the <strong>solar</strong> energy. Solar energy is the energy
which is coming from sun in the form <strong>of</strong> <strong>solar</strong> radiations in infinite amount, when these <strong>solar</strong> radiations falls on
absorbing surface, then they gets converted into the heat, this heat is used for heating the <strong>water</strong>.This paper presents
the <strong>study</strong> <strong>based</strong> on three procedure theory. Exergy analysis is conducted with the aim <strong>of</strong> providing some methods to
save cost and keep the efficiency <strong>of</strong> <strong>solar</strong> <strong>water</strong> heater to desired extent and at the same time figuring out related
exergy losses. In the Exergy analysis <strong>of</strong> <strong>solar</strong> <strong>water</strong> heater systems, the conversion <strong>of</strong> <strong>solar</strong> radiation is typically
included within the analysis. Exergy analysis has been widely used for the optimisation and allocation <strong>of</strong> losses in
energy systems. Exergy is the expression for loss <strong>of</strong> available energy due to the creation <strong>of</strong> entropy in irreversible
systems or processes. The exergy loss in a system or component is determined by multiplying the absolute
temperature <strong>of</strong> the surroundings by the entropy increase. Exergy is also a measure <strong>of</strong> the maximum useful work that
can be done by a system interacting with an environment. It has been widely used in the design, simulation and
performance evaluation <strong>of</strong> energy systems.
Keywords: Solar <strong>water</strong> heater, Laws <strong>of</strong> Thermodynamics, Exergy Analysis, Three procedure theory
1. Introduction
The <strong>solar</strong> energy is the most capable <strong>of</strong> the alternative energy sources. Despite the characteristic <strong>of</strong> low density and
unsteady in nature, the research <strong>of</strong> <strong>solar</strong> energy has received significant attention in recent years. Due to increasing
demand for energy and rising cost <strong>of</strong> fossil type fuels (i.e., gas or oil) <strong>solar</strong> energy is considered an attractive source
<strong>of</strong> renewable energy that can be used for <strong>water</strong> hearing in both homes and industry. Heating <strong>water</strong> consumes nearly
20% <strong>of</strong> total energy consumption for an average family. Solar <strong>water</strong> heating systems are the cheapest and most
easily affordable clean energy available to homeowners that may provide most <strong>of</strong> hot <strong>water</strong> required by a family.
Solar heater is a device which is used for heating the <strong>water</strong>, for producing the steam for domestic and industrial
purposes by utilizing the <strong>solar</strong> energy. Solar energy is the energy which is coming from sun in the form <strong>of</strong> <strong>solar</strong>
radiations in infinite amount, when these <strong>solar</strong> radiations falls on absorbing surface, then they gets converted into the
heat, this heat is used for heating the <strong>water</strong>. This type <strong>of</strong> thermal collector suffers from heat losses due to radiation
and convection. Such losses increase rapidly as the temperature <strong>of</strong> the working fluid increases.
Exergy is a measure <strong>of</strong> the maximum useful work that can be done by a system interacting with an environment
which is at a constant pressure and temperature. Exergy is the expression for loss <strong>of</strong> available energy due to the
creation <strong>of</strong> entropy in irreversible processes. The analysis is <strong>based</strong> on the three procedure theory given by Pr<strong>of</strong>essor
Hua Ben. The three different procedures <strong>of</strong> this theory are: Conversion procedure, utilization procedure, and
recycling procedure.
2. Problem Statement
In today’s modern world, where new technologies are introduced every day, the use <strong>of</strong> non renewable energy is
increasing quickly particularly petroleum fuel. Quickly depleting reserve <strong>of</strong> petroleum and decreasing air quality
raise question about the future. The fact that non renewable energy resources will be available at the present usage
level only for a limited period has been accepted worldwide. As a consequence, the need for renewable energy
resources becomes very urgent. As an absolutely clean energy, <strong>solar</strong> energy is <strong>of</strong> most importance and has been
most emphasized on so far.
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Proceedings <strong>of</strong> the National Conference on
Trends and Advances in Mechanical Engineering,
YMCA University <strong>of</strong> Science & Technology, Faridabad, Haryana, Oct 19-20, 2012
Solar <strong>water</strong> heater technology is a method <strong>of</strong> <strong>solar</strong> energy utilization. It has been well developed and can be easily
implemented at a low cost. But, the use <strong>of</strong> <strong>solar</strong> <strong>water</strong> heating system not familiar in India and the people in India
still not realize about the practical <strong>of</strong> using <strong>solar</strong> <strong>water</strong> heating systems.
Earlier studies on <strong>solar</strong> <strong>water</strong> <strong>heaters</strong> were <strong>based</strong> on the First law <strong>of</strong> Thermodynamics which tells us that energy is a
conserved quantity; “Energy can neither be created nor destroyed, only transformed from one form to another”.
Perhaps then we should stop worrying about “energy saving” and “energy conservation” and instead focus on
recycling all this energy which will always be here Unfortunately, Thermodynamics has a Second law which states
that processes occur in a certain direction, and that energy has quality as well as quantity. So, it is necessary to
evaluate <strong>solar</strong> <strong>water</strong> <strong>heaters</strong> from the point <strong>of</strong> view <strong>of</strong> the Second law <strong>of</strong> thermodynamics because, as we know, it is
the quality <strong>of</strong> energy that is important not the quantity <strong>of</strong> energy.
3. Solar Water Heating System
SWH systems are generally very simple using only sunlight to heat <strong>water</strong>. A working fluid is brought into contact
with a dark surface exposed to sunlight which causes the temperature <strong>of</strong> the fluid to rise. This fluid may be the <strong>water</strong>
being heated directly, also called a direct system, or it may be a heat transfer fluid such as a glycol/<strong>water</strong> mixture
that is passed through some form <strong>of</strong> heat exchanger called an indirect system. These systems can be classified into
three main categories:
(a) Active systems
(b) Passive systems
(c) Batch systems
3.1 Active Systems
Active systems use electric pumps, valves, and controllers to circulate <strong>water</strong> or other heat-transfer fluids through the
collectors. So, the Active systems are also called forced circulation systems and can be direct or indirect. The active
system is further divided into two categories:
(a) Open-loop (Direct) Active System
(b) Closed-loop (Indirect) Active System
3.1.1 Open-Loop Active Systems
Open-loop active systems use pumps to circulate <strong>water</strong> through the collectors. This design is efficient and lowers
operating costs but is not appropriate if the <strong>water</strong> is hard or acidic because scale and corrosion quickly disable the
system. These open-loop systems are popular in nonfreezing climates.
Fig1. Open-Loop Active Systems
3.1.2 Closed-Loop Active Systems
These systems pump heat-transfer fluids (usually a glycol-<strong>water</strong> antifreeze mixture) through collectors. Heat
exchangers transfer the heat from the fluid to the household <strong>water</strong> stored in the tanks. Closed-loop glycol systems
are popular in areas subject to extended freezing temperatures because they <strong>of</strong>fer good freeze protection.
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Proceedings <strong>of</strong> the National Conference on
Trends and Advances in Mechanical Engineering,
YMCA University <strong>of</strong> Science & Technology, Faridabad, Haryana, Oct 19-20, 2012
Fig2. Closed loop Active System
3.2 Passive Systems
Passive systems simply circulate <strong>water</strong> or a heat transfer fluid by natural convection between a collector and an
elevated storage tank (above the collector). The principle is simple, as the fluid heats up its density decreases. The
fluid becomes lighter and rises to the top <strong>of</strong> the collector where it is drawn to the storage tank. The fluid which has
cooled down at the foot <strong>of</strong> the storage tank then flows back to the collector. Passive systems can be less expensive
than active systems, but they can also be less efficient. Thermosiphon system is the best example <strong>of</strong> passive systems.
3.2.1 Thermosiphon Systems
In the thermosyphon system, <strong>water</strong> comes from the over head tank to bottom <strong>of</strong> <strong>solar</strong> collector by natural circulation
and <strong>water</strong> circulates from the collector to storage tank as long as the absorber keeps absorbing heat from the sun and
<strong>water</strong> gets heated in the collector. The cold <strong>water</strong> at the bottom <strong>of</strong> storage tank run into the collector and replaces the
hot <strong>water</strong>, which is then forced inside the insulated hot <strong>water</strong> storage tank. The process <strong>of</strong> the circulation stops when
there is no <strong>solar</strong> radiation on the collector. Thermosiphon system is simple and requires less maintenance due to
absence <strong>of</strong> controls and instrumentation. Efficiency <strong>of</strong> a collector depends on the difference between collector
temperature and ambient temperature and inversely proportional to the intensity <strong>of</strong> <strong>solar</strong> radiation.
Fig3. Thermosiphon System
3.3 Batch systems
Batch System (also known as integral collector storage systems) are simple passive systems consisting <strong>of</strong> one or
more storage tanks placed in an insulated box that has a glazed side facing the sun. Batch systems have combined
collection and storage functions. Depending on the system, there is no requirement for pumps or moving parts, so
they are inexpensive and have few components in other words, less maintenance and fewer failures.
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Proceedings <strong>of</strong> the National Conference on
Trends and Advances in Mechanical Engineering,
YMCA University <strong>of</strong> Science & Technology, Faridabad, Haryana, Oct 19-20, 2012
Fig4. Batch System
4. Components <strong>of</strong> Solar Water Heater
SWH generally consists <strong>of</strong> a <strong>solar</strong> radiation collector panel, a storage tank, a pump, a heat exchanger, piping units,
and auxiliary heating unit. Some <strong>of</strong> important components are described in the next sections.
4.1 Solar Collectors
The choice <strong>of</strong> collector is determined by the heating requirements and the environmental conditions in which it is
employed. There are mainly three types <strong>of</strong> <strong>solar</strong> collectors like flat plate <strong>solar</strong> collector, evacuated tube <strong>solar</strong>
collector, concentrated <strong>solar</strong> collector.
4.1.1 Flat Plate Collectors
Flat-plate collectors are used extensively for domestic <strong>water</strong> heating applications. It is simple in design and has no
moving parts so requires little maintenance. It is an insulated, weatherpro<strong>of</strong>ed box containing a dark absorber plate
under one or more transparent covers. They collect both direct and diffuse radiation. Their simplicity in construction
reduces initial cost and maintenance <strong>of</strong> the system. A more detailed picture <strong>of</strong> these systems is <strong>of</strong> interest and is
presented in the following section.
.
Fig5. Flat plate collector along with heat loss mechanism.
4.1.2 Evacuated-Tube Collectors
Evacuated-Tube Collectors are made up <strong>of</strong> rows <strong>of</strong> parallel, transparent glass tubes. Each tube consists <strong>of</strong> a glass
outer tube and an inner tube, or absorber, covered with a selective coating that absorbs <strong>solar</strong> energy well but inhibits
radiative heat loss. The air is withdrawn (“evacuated”) from the space between the tubes to form a vacuum, which
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Proceedings <strong>of</strong> the National Conference on
Trends and Advances in Mechanical Engineering,
YMCA University <strong>of</strong> Science & Technology, Faridabad, Haryana, Oct 19-20, 2012
eliminates conductive and convective heat loss. They are most suited to extremely cold ambient temperatures or in
situations <strong>of</strong> consistently low-light. They are also used in industrial applications, where high <strong>water</strong> temperatures or
steam need to be generated where they become more cost effective.
Fig6. Evacuated-Tube Collectors
4.1.3 Concentrating Collectors
Concentrating collectors use mirrored surfaces to concentrate the sun's energy on an absorber called a receiver. A
heat-transfer fluid flows through the receiver and absorbs heat. These collectors reach much higher temperatures
than flat-plate collectors and evacuated-tube collectors, but they can do so only when direct sunlight is available.
However, concentrators can only focus direct <strong>solar</strong> radiation, with the result being that their performance is poor on
hazy or cloudy days.
Fig7. Concentrating Collectors
4.2 Storage Tank
Most commercially available <strong>solar</strong> <strong>water</strong> <strong>heaters</strong> require a well-insulated storage tank. Thermal storage tank is made
<strong>of</strong> high pressure resisted stainless steel covered with the insulated fiber and aluminum foil. Some <strong>solar</strong> <strong>water</strong> <strong>heaters</strong>
use pumps to recirculate warm <strong>water</strong> from storage tanks through collectors and exposed piping. This is generally to
protect the pipes from freezing when outside temperatures drop to freezing or below.
4.3 Heat Transfer Fluid
A heat transfer fluid is used to collect the heat from collector and transfer to the storage tank either directly or with
the help <strong>of</strong> heat exchanger. In order to have an efficient SHW configuration, the fluid should have high specific heat
capacity, high thermal conductivity, low viscosity, and low thermal expansion coefficient, anti-corrosive property
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Proceedings <strong>of</strong> the National Conference on
Trends and Advances in Mechanical Engineering,
YMCA University <strong>of</strong> Science & Technology, Faridabad, Haryana, Oct 19-20, 2012
and above all low cost. Among the common heat transfer fluids such as <strong>water</strong>, glycol, silicon oils and hydrocarbon
oils, the <strong>water</strong> turns out to be the best among the fluids. Water is the cheapest, most readily available and thermally
efficient fluid but does freeze and can cause corrosion.
5. Exergy Analysis
To provide an efficient and effective use <strong>of</strong> fuels, it is essential to consider the quality and quantity <strong>of</strong> the energy
used to achieve a given objective. In this regard, the first law <strong>of</strong> thermodynamics deals with the quantity <strong>of</strong> energy
which states that energy cannot be created or destroyed, whereas the second law <strong>of</strong> thermodynamics deals with the
quality <strong>of</strong> energy, i.e., it is concerned with the quality <strong>of</strong> energy to cause change, degradation <strong>of</strong> energy during a
process. First and second law efficiencies are <strong>of</strong>ten called energy and exergy efficiencies, respectively. It is expected
that exergy efficiencies are usually lower than the energy efficiencies, because the irreversibilities <strong>of</strong> the process
destroy some <strong>of</strong> the input exergy.
Exergy analysis method is employed to detect and evaluate quantitatively the causes <strong>of</strong> the thermodynamic
imperfection <strong>of</strong> the process. Exergy is also a measure <strong>of</strong> the maximum useful work that can be done by a system
interacting with an environment which is at a constant pressure and temperature. Exergy is the expression for loss <strong>of</strong>
available energy due to the creation <strong>of</strong> entropy in irreversible processes. The exergy loss in a system or component
is determined by multiplying the absolute temperature <strong>of</strong> the surroundings by the entropy increase. The concepts <strong>of</strong>
exergy, available energy, and availability are essentially similar. The concepts <strong>of</strong> exergy destruction, exergy
consumption, irreversibility, and lost work are also essentially similar.
5.1 Three Procedure Theory
An energy analysis entitled ‘Three Procedure Theory’ can be conveniently conducted as presented by Pr<strong>of</strong>essor Hua
Ben. Compared with other theories <strong>of</strong> energy analysis, three procedure theory furnishes us a good platform to
perform energy analysis. The three different procedures <strong>of</strong> this theory are: Conversion procedure, utilization
procedure, and recycling procedure. A schematic diagram <strong>of</strong> three procedure theory for the <strong>solar</strong> <strong>water</strong> heater is
shown in Fig 8. In Three procedure theory energy conversion procedure takes places at the sun. The nuclear reaction
in the sun makes it possible for the sun to emit a great quantity <strong>of</strong> power, which is transmitted in the form <strong>of</strong>
electromagnetic waves. Energy utilization is carried out in the collector. Solar radiation penetrates the cover and is
incident on the black-color plate where it heats <strong>water</strong> flowing through the pipe. Energy recycling procedure takes
places between the collector and the storage tank which corresponds to the storage tank keep hot <strong>water</strong> is pumped to
users and cold <strong>water</strong> fills the storage tank from the bottom pipe simultaneously.
Fig8. Three procedure theory for the <strong>solar</strong> <strong>water</strong> heater
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Proceedings <strong>of</strong> the National Conference on
Trends and Advances in Mechanical Engineering,
YMCA University <strong>of</strong> Science & Technology, Faridabad, Haryana, Oct 19-20, 2012
Energy balance equations:
At Collector:
(1)
Where,
= Energy from Sun (input Energy) (W)
= Energy from storage tank to collector associated with <strong>water</strong> recycle (W)
= Energy losses due to imperfectly thermal insulation in collector (W)
= Energy from collector to storage tank (W)
At Storage Tank:
(2)
Where,
= Energy losses due to imperfectly thermal insulation in storage tank (W)
= Energy from storage tank to user (output Energy) (W)
Exergy balance equations:
At Collector:
(3)
Where,
= Exergy from sun (input power) (W)
= Exergy from storage tank to collector associated with <strong>water</strong> recycle (W)
= Exergy losses due to imperfectly thermal insulation in collector (W)
= Exergy from collector to storage tank (W)
At Storage Tank:
(4)
Where,
= Exergy losses due to imperfectly thermal insulation in storage tank (W)
= Exergy from storage tank to user (output exergy) (W)
= Exergy losses due to irreversibility in storage tank
In utilization procedure, we assume the change in kinetic energy are very small since the <strong>solar</strong> <strong>water</strong> heater is driven
by the difference <strong>of</strong> density <strong>of</strong> <strong>water</strong>, namely no great decrease in pressure is involved, so we can calculate exergy
from collector to storage tank ( ) by use the following equation. [23]
(5)
Where,
= Mass flow rate <strong>of</strong> <strong>water</strong> (kg/s)
= Outlet temperature <strong>of</strong> <strong>water</strong> from collector to storage tank (K)
= Ambient temperature (K)
= Specific heat <strong>of</strong> <strong>water</strong> {J/(kg.K)}
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Proceedings <strong>of</strong> the National Conference on
Trends and Advances in Mechanical Engineering,
YMCA University <strong>of</strong> Science & Technology, Faridabad, Haryana, Oct 19-20, 2012
Assuming the temperature distribution in the storage tank is linear (∆T α ∆L), where L is the height <strong>of</strong> the storage
tank), we get;
Where T x , T L and T 0 are the temperature <strong>of</strong> <strong>water</strong> at position X, L and O from the bottom <strong>of</strong> the storage tank. Then
we obtain the exergy from storage tank to users ( ) by use the following equation:
(6)
6. Conclusion
The <strong>study</strong> <strong>of</strong> <strong>solar</strong> <strong>water</strong> heater <strong>based</strong> on exergy analysis is done in this paper. the conversion <strong>of</strong> <strong>solar</strong> radiation in
the evaluation <strong>of</strong> direct-<strong>solar</strong> systems leads to extremely high exergy losses in the direct <strong>solar</strong> systems.
Subsequently, the optimisation <strong>of</strong> these systems should be oriented so as to reduce the magnitude <strong>of</strong> exergy losses in
the conversion device. Exergy efficiency <strong>of</strong> <strong>solar</strong> systems is highly dependent on the daily <strong>solar</strong> radiation and
radiation intensity. To improve the exergy efficiency, we must select the material and design the number layer <strong>of</strong>
transparent cover and a judicious choice <strong>of</strong> the length <strong>of</strong> pipe is necessary. It is a good way to find out to design a
new style <strong>of</strong> storage tank, because large exergy losses in the storage tank.
7. Suggestions for Future work
• In this <strong>study</strong>, the flat plate collector is considered for analysis and it would be a good initiative to explore the
impact <strong>of</strong> other types <strong>of</strong> <strong>solar</strong> collector such as an evacuated tube or a concentrated collector.
• In this paper, only two components <strong>of</strong> the <strong>solar</strong> <strong>water</strong> heater are analyzed. Other components such as pump and
piping system could also be considered. Analysis <strong>of</strong> other components would help in calculation <strong>of</strong> pressure
drop across the system in order to select the pump and optimize the piping size.
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