Experimental investigation of Flat Plate Collector with and without PCM
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Transcript of Experimental investigation of Flat Plate Collector with and without PCM
National Conference on Recent Trends in Mechanical Engineering (March 20-21, 2015)
Beant College of Engineering and Technology, Gurdaspur, Punjab-143521((India)
EXPERIMENTAL INVESTIGATION OF SOLAR THERMAL FLAT
PLATE COLLECTOR WITH AND WITHOUT PCM
Rajesh Kumar*, Parminder Singh, Brij Bhushan
Department of Mechanical Engineering, BCET, Gurdaspur, Punjab, India * corresponding author e-mail: [email protected]
ABSTRACT
The intermittent, variable and unpredictable nature of solar energy makes the use of energy storage system for solar
water heating system (SWHS) indispensable. Utilization of phase change material (PCM) in SWHS is a prevalent
technique to bridge the gap between supply and demand of energy. An experimental investigation has been made to
find out the possibilities of utilizing PCM inside the solar thermal flat plate collector to act as short term energy
storage media and thus to improve the performance and utility of SWHS. Two significantly different configurations
of flat plate collector (FPC) are developed, one is conventional FPC i.e. without PCM another is a novel kind of
FPC having tube in tube type risers, the outer tubes contains water and the inner tubes which are sealed from both
end contains PCM. Two PCMs both organic non- paraffin’s with different melting temperature and latent heat of
fusion are used in the experimental investigation. The instantaneous efficiency of SWHS containing PCM has been
evaluated and compared with that of conventional SWHS.
Keywords: Solar thermal flat plate collector, Phase change material (PCM), Solar water heating system (SWHS).
1. Introduction
Sun is gigantic nuclear fusion reactor, supplying its
inexhaustible energy to almost every part of earth, in
abundance. Out of all the renewable energy resources
well explored till today, solar thermal energy is the most
ample one. With its immense vigour, sun emits energy at
a rate of 3.8×1023 KW of which about 1.08×104 KW
reaches at earth surface. The total annual solar radiation
falling on earth is 7500 times more than that of world’s
primary energy consumption of 450 EJ [1]. The most
easiest and used practical application of solar energy is
to convert it into thermal energy. Solar thermal
conversion efficiencies are around 70% which is far
more than the solar electrical direct conversion
efficiency of about only 17% [2]. In the world solar
thermal market solar water heating systems (SWHS)
dominates, with their contribution of 80% towards the
market [3]. It is a well known fact that solar energy is
available only in day time, but the usage of hot water is
not limited to day time only. Peak solar radiation occurs
at noon; contrary to this peak hot water demand occurs
either in morning or evening. Thus to bridge the gap
between the supply and demand of energy the use of
energy storage systems for SWHS is indispensable.
There are two ways to store solar thermal energy, the
one which is presently used in almost every
commercially available SWHS; sensible heat storage. In
sensible heat storage, the heat is stored by raising the
temperature of the heat storage material. The amount of
heat stored depends upon the mass of heat storage
media, specific heat of storage media, degrees by which
temperature is raised. Another way to store solar thermal
energy is by latent heat storage. In latent heat storage,
actually the heat is stored as blend of both kinds of heat
storages i.e. sensible heat storage and latent heat storage.
Firstly heat is stored in the material by raising its
temperature closely up to the melting temperature of
storage material, this is sensible heat storage, after that
phase transition of material occurs at almost constant
temperature, this is latent heat storage and subsequently
at last there is once more the temperature of material
rises on further heat absorption, this too is sensible heat
storage. The latent heat storage materials are also called
as phase change materials (PCMs). Use of PCM for
solar thermal energy storage provides higher energy
storage densities. PCMs store 5-14 times more heat per
unit volume than sensible heat storage materials such as
water [4]. Utilization of PCM in SWHS for thermal
energy storage is not a new technique, it is pioneered by
Dr. Telkes in 1940 [5]. Since then various methods are
developed to utilize the PCM in SWHS to improve the
energy storage performance and bridge the gap between
supply and demand of energy. The most used method
for utilizing PCM in SWHS is to put PCM into the water
storage tank. Another method is to put PCM inside the
solar thermal collector. In the last two or three decades
several new type of configurations of solar thermal
collectors containing PCM are build and experimentally
investigated by various researchers. Kürklü et al. [6]
developed a solar collector which consists of two
adjoining sections on filled with water and other with
PCM, paraffin wax was used as PCM with melting
temperature of about 50°C. In experimental investigation
National Conference on Recent Trends in Mechanical Engineering (March 20-21, 2015)
Beant College of Engineering and Technology, Gurdaspur, Punjab-143521((India)
it was found that the lower and upper limits of
collector’s instantaneous efficiency are 20% and 80%
respectively. The temperature of water was recorded
never below than 36°C till next morning, by covering
the collector with glass wool blanket. Thaib et al. [7]
incorporated PCM inside a solar flat plate collector of a
thermosyphon type SWHS. The experiment showed that
the use of PCM in solar collector can improve the
performance of system by maintaining the water hotter
for longer period of time. The maximum temperature of
water and efficiency of solar collector recorded were
70°C and 36.6% respectively. Lin et al. [8] compared the
performance of conventional collector and collector
containing 28 Kg paraffin wax as PCM, it was reported
that by using collector containing PCM the duration of
availability of hot water can be increased by 3 hours at
night. Gond et al. [9] compared the performance of a
solar collector which uses phase change material as
short term heat storage media with conventional solar
flat plate collector. The results of experiment revealed
that the maximum temperature of outlet water has
considerably higher for system with PCM filled in flat
plate collector than the conventional flat plate collector
system. When the temperature of water was at its
maximum, a temperature difference of 35°C is recorded
in outlet water temperature of both collectors for the 1st
day and temperature difference of 30°C on the next day.
From all these researches it is evident that adding PCM
into the solar thermal collector causes heat retention
capabilities of the SWHS get elevated. Taking
consideration of the experimental research done by
Kürklü et al. [6], Thaib et al. [7] and Gond et al. [9] it
can be observed that PCM is utilized inside solar
thermal collector in such a way that the heat flows from
absorber plate to PCM first, which is in contact with
absorber plate and then ultimately from PCM to water. It
is well recognized fact that the thermal conductivity of
PCM’s is comparatively low than that of water. So, in
this way PCM may act as thermal resistance to the heat,
flowing from absorber plate to water. Hence it is
proposed that it will be more favourable to use PCM
inside the solar thermal flat plate collector in a manner
such that the heat flowing from absorber plate will first
come in contact with water and then subsequently with
PCM. An experimental investigation to find the
possibilities of utilizing PCM inside the solar thermal
flat plate collector to act as short term energy storage
media and thus to improve the performance and utility
of SWHS is carried out with a novel kind of FPC having
tube in tube type risers, the outer tubes contains water
and the inner tubes which are sealed from both end
contains PCM.
Fig. 1 shows the basic concept behind this new type of
collector developed.
Fig. 1. Tube in tube type riser.
2. Experimentation
Two independent separately working thermosyphon type
SWHS, one fitted with conventional FPC another fitted
with FPC containing PCM are installed at a roof top
with unrestricted sun shine. Photographic view of
suitably instrumented experimental set-up is shown in
Fig.2. Both the SWHS resembles with each other in all
aspects other than one, which is flat plate collector
configuration.
Fig. 2. Experimental setup.
Although the solar thermal FPC used for both
independent domestic solar water heating system
(DSWHS) in experimental setups looks alike externally
National Conference on Recent Trends in Mechanical Engineering (March 20-21, 2015)
Beant College of Engineering and Technology, Gurdaspur, Punjab-143521((India)
and are constituted of similar ingredients but they are
quite different in their configuration. The difference
exists in the risers (water carrying copper tubes) which
are bonded underneath the black painted copper absorber
plate. Fig. 3 depicts the geometric cross sectional view of
conventional solar thermal FPC i.e. without PCM
collector.
Fig. 3. Cross sectional view of conventional FPC. (Dimensions
in cm)
The outer box which contains all other components of
FPC in it is made up of wooden board. A black painted
copper absorber plate of exposed area nearly 1m2 is
installed to collect maximum possible heat from incident
solar radiation. Copper tubes which act as riser are
bonded to absorber plate. The outer and inner diameter
of the riser is 15 and 12 mm respectively. A single
glazing of 6 mm thickness is used to produce green
house effect. Extruded polystyrene (XPS) of thickness
3.5 cm is used to minimize the bottom heat loss from the
collector. Fig. 4 depicts the geometric cross sectional
view of solar thermal FPC incorporating PCM in it.
Mostly all the specifications of this newly developed
collector are similar to the conventional FPC with a
difference which lies in riser tubes. A tube in tube type
riser system is developed.
Fig. 4. Cross sectional view of FPC incorporating PCM.
(Dimensions in cm)
The outer tube of copper which are in sudden contact
with absorber plate contains water, the inner copper
tubes containing PCM and sealed from both ends are
placed inside the outer tubes. Two PCMs both organic
non- paraffin’s with phase transition (solid- liquid)
temperature of 48°C and 55°C and latent heat of fusion
275 KJ/Kg and 210KJ/Kg respectively are used to fill
into the collector alternatively. Storage tank of 30 litre
storage capacity is placed well above the height of the
FPC in the SWHS. Inlet and outlet plumbing fitting is
provided on the storage tank at appropriate position. It is
worth to mention here that to avoid any air bubble
occurrence in the flow circuit of this kind of
thermosyphon SWHS the inlet of water to the storage
tank must be provided on the vertical cylindrical surface
of the tank rather than on the horizontal upper flat
surface of the tank. And the water level inside the
storage tank should be maintained sufficiently above the
water inlet fitting. Flexible transparent synthetic polymer
pipes are used to transport water which is Heat transfer
fluid (HTF) from the collector to storage tank and then
back. The diameter of the pipes is selected in accordance
with the headers of the FPC of both experimental setups.
Temperature of water at the inlet and outlet of solar
thermal FPC is a crucial data required for this
experimental investigation. Thus four thermocouples are
installed at the inlet and outlet of both solar thermal
FPC’s. An active temperature display panel which reads
the temperature from the thermocouple is installed in the
setup. Fig. 5 shows how thermocouple is installed at the
inlet or outlet of FPC. The value of solar radiation
intensity falling on the inclined surface of solar thermal
FPC is another important data required for this
experimental investigation. To achieve the above
mentioned task digital pyranometer is used.
Fig. 5. Thermocouple installed at outlet of FPC.
As it is a well known fact that the rate of water flow is
not significantly high in a thermosyphon type SWHS. It
is not feasible to measure the value of flow rate in such
systems with conventionally available flow rate
measuring instruments. Thus an alternative method to
calculate mass flow rate of thermosyphon type SWHS as
proposed by Ong [10] is used here. The method is called
as die trace injection test. In the flexible transparent
synthetic polymer pipe connecting the outlet of collector
to the inlet of storage tank, a commercially available
injection syringe is inserted near the outlet of solar
National Conference on Recent Trends in Mechanical Engineering (March 20-21, 2015)
Beant College of Engineering and Technology, Gurdaspur, Punjab-143521((India)
thermal FPC. An adequate amount of colouring agent
(die) is injected into the water with the help of syringe.
The velocity of flow is calculated by measuring the time
taken by the die to travel the fixed length of transparent
synthetic polymer pipe. To calculate the mass flow rate
of (HTF) water below mentioned equations are used.
�̇� = 𝜌𝐴𝑉
A same criterion is performed on both the experimental
setups to calculate the mass flow rate. Experimental
investigation is carried out for two consecutive days
while using PCM-OM48 in one of the solar thermal
FPC. Experimental data is recorded for both the setups
and then conveniently utilized to compute the
instantaneous efficiency of both SWHS. To calculate the
instantaneous efficiency of the solar thermal FPC below
mentioned equation is used.
η =ṁcPΔT
IA
In the similar fashion experimental investigation is
carried out for another two consecutive days while using
PCM-OM55 instead of PCM-OM48 which is previously
used. Experimental data is recorded for both the setups
and then conveniently utilized to compute the
instantaneous efficiency of both SWHS.
3. Results and discussion
Experimental data; water temperature at the inlet and
outlet of collector, mass flow rate of water and solar
radiation intensity were recorded at regular interval of
time and subsequently the instantaneous efficiency of
both the experimental set-up is calculated.
3.1. Inlet/outlet water temperature profile
Firstly by incorporating PCM-OM48 in one of the solar
thermal FPC, the thermal performance of both SWHS
i.e. with and without PCM is investigated for two
consecutive days. Fig.6 and Fig.7 depicts the plot of
recorded temperature at the inlet and outlet for both with
and without PCM system versus time. It can be observed
from both the given plots that the outlet water
temperature of both the setups remains quite close to
each other almost throughout the day time except
evening hours. It can also be seen from the graphs that at
the evening hours near to the sunset time, temperature of
water at the outlet of conventional solar thermal FPC
falls more rapidly than with PCM system. When in the
evening hours solar incident radiation intensity
decreases the outlet water temperature of both the setups
gradually decreases altogether until the temperature of
water reaches to the phase transition temperature of
PCM. Afterwards when the phase transition temperature
of PCM is reached the temperature of outlet water in
with PCM systems becomes sort of stable for short
while and contrary to this the temperature of outlet water
in without PCM system goes on decreasing. The credit
of maintaining the outlet water temperature for with
PCM system considerably higher than that of
conventional system goes to the PCM.
Fig. 6. Temperature vs. Time while using PCM-OM48 for 1st
day.
Fig. 7. Temperature vs. Time while using PCM-OM48 for 2nd
day.
The PCM changes its phase from liquid to solid and thus
releases latent heat of fusion which is stored in PCM
priory while changing phase from solid to liquid. This
latent heat is now utilised to maintain the temperature of
water in with PCM SWHS sufficiently above the
temperature of water in conventional SWHS. Thus for
nearly one hour or so the outlet water temperature of
with PCM SWHS can be maintained about 10°C higher
than that of the without PCM SWHS at evening time by
incorporating PCM inside the solar thermal FPC. The
maximum temperature of water recorded at the outlet of
collector containing PCM and conventional collector is
84.7°C and 83.6°C respectively for 1st day and the same
is recorded as 88.2°C and 87.2°C respectively for 2nd
day. It can also be observed from the above graphs that
National Conference on Recent Trends in Mechanical Engineering (March 20-21, 2015)
Beant College of Engineering and Technology, Gurdaspur, Punjab-143521((India)
the inlet water temperature of both the experimental
setups remains almost equal throughout the day.
Now by incorporating PCM-OM55 inside the one of
solar thermal FPC, the thermal performance of both
SWHS i.e. with and without PCM is investigated for
another two consecutive days in the similar way as
discussed previously. Fig.8 and Fig.9 depicts the plot of
recorded temperature at the inlet and outlet for both with
and without PCM system versus time. Similar set of
experimental results are observed by incorporating
PCM-OM55 inside the solar thermal FPC as observed
while incorporating PCM-OM48. It can be observed
from both the given plots that the outlet water
temperature of both the setups remains quite close to
each other almost throughout the day time except
evening hours.
Fig. 8. Temperature vs. Time while using PCM-OM55 for 1st
day.
Fig. 9. Temperature vs. Time while using PCM-OM55 for 2nd
day.
And in the evening hours the outlet water temperature of
with PCM systems remained nearly 10°C above than
that of without PCM system for both of the days. The
maximum temperature of water recorded at the outlet of
collector containing PCM and conventional collector is
78.2°C and 77.4°C respectively for 1st day and the same
is recorded as 82.2°C and 81.3°C respectively for 2nd
day. The effects of using two different PCMs with
different melting temperature are not considerably
explicit from the results. This may be due to the fact that
difference in the phase transition temperature of two
PCMs in not that much large to give an adequately
distinguishable result and secondly may be the quantity
of PCM incorporated inside the solar thermal FPC is
also not sufficiently enough to produce such a
distinguishable effects.
3.2. Efficiency of SWHS with and without PCM
Fig. 10 and Fig. 11 depict the plot of instantaneous
efficiency for both conventional and with PCM SWHS
while incorporating PCM-OM48 versus time for two
consecutive days.
Fig. 10. Efficiency vs. Time while using PCM-OM48 for 1st
day.
Fig. 11. Efficiency vs. Time while using PCM-OM48 for 2nd
day.
Fig. 12 and Fig. 13 depict the plot of instantaneous
efficiency for both conventional and with PCM SWHS
while incorporating PCM-OM55 versus time for two
consecutive days.
National Conference on Recent Trends in Mechanical Engineering (March 20-21, 2015)
Beant College of Engineering and Technology, Gurdaspur, Punjab-143521((India)
Fig. 12. Efficiency vs. Time while using PCM-OM55 for 1st
day.
Fig. 13. Efficiency vs. Time while using PCM-OM55 for 2nd
day.
From all the four figures given above it can be observed
that instantaneous efficiency of both SWHS remains
almost equal till noon and just after the noon. But the
instantaneous efficiency of with PCM system remains
considerably higher than that of the conventional system
in the late afternoon hours (Nearly after 3:40 p.m.).
Table 1 Maximum improvement achieved in the instantaneous
efficiency.
Day Ƞ with PCM
(%)
Ƞ without PCM
(%)
ȠIncrease,Max
(%)
1 36.4 21.7 14.7
2 41.3 26.1 15.2
3 27.5 11.1 16.4
4 27.1 15.1 12.0
This improvement in the instantaneous efficiency in late
afternoon hours is possibly due to the fact that the
temperature of water at the outlet of FPC of with PCM
SWHS is higher than that of conventional SWHS at
these times, which in turn makes temperature difference
across the FPC for with PCM SWHS higher than that of
conventional SWHS. The maximum improvement
observed in the instantaneous efficiency of SWHS
containing PCM while using PCM-OM48 for first two
days and PCM-OM55 for next two days in comparison
to that of conventional SWHS for late afternoon hours is
given in the Table 1. It is also observed from the
efficiency vs. time plots that the maximum value of
instantaneous efficiency for both SWHS remains almost
equal for all the four days. There is not any significant
effect of using PCM inside the collector is observed on
the maximum value of instantaneous efficiency
achieved.
4. Conclusion
In the pursuit of improving the performance and utility
of SWHS by incorporating PCM inside the FPC,
experimental investigation is carried out with a novel
kind of FPC having tube and tube type risers containing
PCM in it and a conventional FPC. It is inferred from
the results of experimental investigation that to
incorporate PCM inside a solar thermal FPC to act as
short term thermal energy storage media is quite
satisfactory and useful in improving the Instantaneous
efficiency of the SWHS at evening hours. Maximum
improvement achieved in the instantaneous efficiency of
SWHS containing PCM in comparison to that of
conventional SWHS for evening hours is 14.7% and
15.2% for first two days while using PCM-OM48 and
16.4% and 12.0% for another two days while using
PCM-OM55. By using PCM inside the collector the
duration of availability of hot water can be extended,
which further depends upon the quantity of PCM used,
phase transition temperature of PCM and latent heat of
fusion PCM possesses. Thus further research is required
to optimize this technique of utilizing PCM inside the
FPC to maximize its benefits in SWHS.
5. Nomenclature
ṁ = Mass flow rate.
ρ = Density of water
A = Area of cross section of pipe carrying water.
V = Velocity of water.
ƞ = Instantaneous efficiency.
cp = Specific heat of water.
ΔT = Temperature difference of water across the FPC.
I = Solar radiation intensity.
A = Collector Area.
National Conference on Recent Trends in Mechanical Engineering (March 20-21, 2015)
Beant College of Engineering and Technology, Gurdaspur, Punjab-143521((India)
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