experimental investigation of flat plate collector with and without pcm

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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 3.8×10 23 KW of which about 1.08×10 4 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

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This article is a original research paper, which reveals the results of using phase change material (PCM) inside a (FPC) to enhance the performance of a solar water heating system (SWHS), presented at National conference on Recent Trends in Mechanical engineering, 20-21 march, at Beant college of engineering at technology, Gurdaspur, Punjab, India.

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Page 1: 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

Page 2: 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)

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

Page 3: 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)

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

Page 4: 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)

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

Page 5: 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)

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.

Page 6: 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)

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.

Page 7: 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)

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