chapter - 3 exergy analysis of solar heating...

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CHAPTER - 3

Exergy Analysis of Solar Heating Devices

3.1 Introduction

It is well known that thermal comfort plays a very important role on the health,

growth, working efficiency and feeling of human beings. All living beings including

humans are very much concerned about the suitable climate and thermal comfort,

especially, temperature and humidity. Due to the limited resources of energy and the

increasing demand, there is a concern in the scientific community to rethink and to

redevelop the energy efficient system which is not only economical but also

environment friendly. The increasing pressures of energy demand, the degradation

of environment, global warming and depletion of ozone layer etc., has created the

need of efficient energy utilization and waste heat recovery options for commercial

and industrial installations [1].

The thermal energy storage is defined as the temporary storage of the thermal

energy at high or low temperatures. The energy storage can reduce the gap between

energy supply and energy demand, and it plays an important role in energy

conservation, and hence, maximizes the use of available energy. Kovarik and Lesse

[2] studied the optimal flow for low temperature solar heat collector while Farries et

al. [3] studied the energy conservation by adaptive control for a solar heated

building. The optimal and semi optimal control strategies and sensitivity for the mass

flow rate and other parameters for liquid solar collector system were carried out by

Orbach et al. [4] and Winn and Winn [5]. Bejan [6, 7] studied the second law analysis

and exergy extraction from solar collector under time varying conditions.

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Use of solar energy depends on the type of application, one of the most common

application of solar energy at low temperatures is heating of water for useful

purposes. The most common types of collector used in solar water heating systems

(SWH) are flat plate collectors (FPCs), evacuated tube collectors (ETCs) and

compound parabolic collectors (CPCs). Out of these three, FPCs and ETCs are the

most widely used collectors for small-scale water heating applications. Lot of studies

have been carried out on the improvement and advancement of FPCs [8,9] and

ETCs [10,11] . Mason and Davidson [12] had worked on the selective absorbing

surface of vacuum tubes and the use of single or double glazing. The use of

transparent insulating materials over the absorber surface has been suggested by

Goetzberger and Schmidt [13].

The present chapter deals with the exergy analysis of solar heating systems

which has two sections first one is the evacuated tube collector (ETC) based solar

air heater with and without thermal energy storage and the second one is the exergy

analysis of ETC based direct flow solar water heater (SWH). The evacuated tube

collector based solar air heater collector with and without thermal energy storage has

been studied using the experimental data measured at a typical location in North

India. The measured data includes solar radiation, temperature of the air/working

fluid at different state points for different mass flow rate of air. Using the air

properties like enthalpy and entropy etc. energy, exergy, first and second law

efficiencies were calculated. Based on findings, conclusions were made about the

most probable time in a day at which the first law and second law efficiencies were

found to be the maximum for all the three cases. In this analysis it was observed that

both the efficiencies are significantly higher in the case where thermal energy

storage materials viz. hytherm oil and paraffin wax have been used than that of

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without any thermal energy storage. However, both the efficiencies were found to be

slightly better in the case of paraffin wax than that of hytherm oil, filled within the

tubes of solar collector. For solar water heater, the experiments have been carried

out at different volume flow rate such as 10, 15, 20 25 and 30 LPH and has been

found that the present system shows the better performance at 15 LPH flow rate and

also this SWH shows the better efficiency than those of other ETC based SWHs

investigated by earlier authors [20-21].

3.2 Solar Air Heater

This section presents the exergy analysis of ETC based solar air heater with and

without thermal energy storage for which following experimental set up and

procedure has been used.

3.2.1 Experimental Set-up and Description

In the present experimental study, the solar air heater collector with and without

thermal energy storage (TES) have been fabricated using evacuated tubes (ETC).

Total 12 numbers of tubes viz. 4 for PCM, 4 for hytherm oil and 4 without TES

individually, have been arranged in the series. The copper tube of 12mm diameter

has been inserted inside the evacuated tubes for air circulation. Out of three

arrangements, mentioned above, one arrangement was filled with Paraffin wax

inside ETC tubes and outside the copper tube, in second arrangement ETC tubes

were filled with hytherm oil, in a way that the temporary heat energy storage material

is coated around the copper tube and heat is stored in the material and transferred to

the copper tube and finally to the blowing air inside it. It also overcomes the sudden

drop in the outlet temperature to the hot air, due to fluctuation in the solar radiation

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arises due to cloud and/or other reasons. On the other hand, there was no temporary

heat storage material in the third arrangement.

Asbestos cloth was used for covering the copper tubes exposed into the open air

viz. outside the ETC tubes for insulation purpose to reduce/minimize the heat loss to

the ambient air. Calibrated J-type thermocouples made of Copper-Constantine with

temperature range -200oC to 1350oC were used to measure air temperature at

different state points and total 13 sensors have been used in the experimental set-

up. The cross-sectional view of a tube along with the temporary heat energy storage

in Fig.3.1 (a) and the schematic and photographic of the experimental setup are

shown in Figs.3.1 (b-c) respectively.

Fig.3.1(a): Cross-sectional view of ETC tube with TES

In this arrangement, one thermocouple has been used for measuring the input air

temperature and other four were used for measuring the outlet temperature of air at

different state points. The outlet air temperature of the first tube is the inlet of the

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second tube and the outlet of the second tube is inlet to the third tube and so on. In

this arrangement where TES is used, one thermocouple has been inserted inside the

collector tube for measuring the temperature of storage material, besides, the same

number of thermocouples have been used at different state points mentioned above.

To measure air flow rate, a Rotameter of up-to 200 LPM capacity was used, which

has been placed between the compressor and inlet of ETC [Fig.3.1 (b)].

Fig.3.1 (b): Schematic of the experimental set up

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Fig.3.1 (c): The photographic view of the experimental set-up

Air was forced circulated through the system using half HP air compressor. For

measuring solar radiation pyranometer with multiplication factor 8.52x10-6 V/(W/m2)

has been used in the experiment. The pyranometer is kept on the horizontal surface

nearby the experimental set-up in the open air, so that no shadow and/or reflection

of solar radiation from any other surface/object falls on it. For collection of data, HP

data acquisition unit attached with a computer has been used in this study. The

specifications of the evacuated tube collector are given in Table-3.1. As shown in

Fig.3.1 (a) solar collector consists of a double walled evacuated glass tubes. Forced

air flow was used as the working fluid in the system and PCM/hytherm oil as a heat

storage material/fluid so that this stored heat can be used when solar radiation is not

available and/or suddenly fluctuates due to any reason in the day time and/or late

evening hours.

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There were four vacuum tubes in each arrangement having a black absorbing

coating on the outer surface of the inner tube. The tubes were made of glass and the

specification is given in Table-3.1, while the length exposed to sunlight is 172cm and

inclined at 450. The volume flow rate of the circulating fluid is measured by volume

flow meter before it enters the first tube. There is a vacuum between the annular

spaces of double walled glass tubes to reduce the heat loss by conduction and

convection. Whenever fluid enter in the first tube its temperature rises, which can be

identified by measuring temperature with the help of thermocouple provided at inlet

and outlet of each tube.

Table3.1: Details about the solar air heater collector and storage materials

Specification of collector tubes Values

Total length 179.5 cm

Inner length 176 cm

Coating length 172 cm

Inner diameter 44 mm

Outer diameter 57.5 mm

Properties of the Paraffin wax as a PCM

Melting point 53.04C *

Specific heat 2.05 kJ/kg°C

Latent heat of fusion 183.1 kJ/kg *

Thermal conductivity 0.21 (solid) (W/m K)

Density at 70 °C 0.769 kg/m3

Properties of HP Hytherm 500 Oil

Kinematic viscosity @ 40 c, cst 27-35

Flash point coc, c, min 194

Viscosity index 95

Power point c max 0.0

Copper strip corrosion 3hr @ 100 c (astm), max 1.0

Neutralisation number mg koh/gm, max 0.15

260c 0.731

280c 0.751

300c 0.772

260c 0.097

280c 0.096

300c 0.095 *Measured by Differential Scanning Calorimeter (DSC)

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Data was collected from the system for few days in different months by varying

the volume flow rate of air. Energy and exergy analyses have been carried out to

evaluate the first and second law efficiencies of solar air heater collector system with

and without thermal energy storage. Volume flow rate of the fluid in the system is

specified and the schematic description is shown in Fig.3.1 (b). The energy analysis

is based on the first law of thermodynamics viz. the law of conservation of energy,

while the exergy analysis is based on the second law of thermodynamics, i.e. using

the concept of entropy generation and/or the law of degradation of the quality of

energy, as given in the next section.

3.2.2 Energy Analysis

The energy analysis is based on the first law of thermodynamics and the

corresponding first law efficiency has been calculated. The energy analysis is based

on the fact that it is an upper limit of efficiency with which the solar radiation can be

converted into heat and the heat can be transferred for useful applications at a given

frequency spectrum and intensity. Energy incident on the evacuated tube is given by:

sc AIQ (3.1)

where Qc is the energy incident on the collector tube, A is the projected area of

collector tube exposed to the sun light, and Is is the intensity of solar radiation at any

particular site. Useful energy gained from the collector can be written as:

AIQ su (3.2)

where is the absorptance of inner surface of evacuated tube collector, is the

transmittance of outer surface of the collector. Useful energy transmitted into the

evacuated tubes is absorbed by fluid, and can be calculated using the first law of

thermodynamics, viz. the law of conservation of energy:

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TCmQQ pfu (3.3)

where Qf is the energy absorbed by air, Cp the specific heat of air and T is the

temperature difference and m is the mass flow rate of air, the first law efficiency of

the collector system is given by:

spcf AITCmQQ // (3.4)

where is the abbreviation used for first law efficiency of the system

3.2 3 Exergy Analysis

The rate at which exergy is collected by the solar collector can be increased by

increasing the mass flow rate of the working fluid. Since, the collector tubes are the

most expensive component of any solar thermal system which needs advanced

material and associated technology to build and therefore, requires large investment.

In order to reduce the capital cost, we need to optimize the dryer area, as the fuel

(sunlight) is free. Again, for large mass flow rates, the fluid outlet temperature is very

low and requires more power to pump/blow air/fluid through it. On the other hand,

low flow rate results in high outlet temperature of the working fluid with high specific

work potential. But due to the nature of entropy generation, exergy losses increase

due to the temperature differences and hence, the optimum mass flow rate is

required. The exergy analysis has been performed based on the configuration of

solar air heater collector shown in Fig.3.1(b). The exergy received by collector is

given by [6-7, 16-18]:

)/(1( SaCc TTQEx (3.5)

where aT is ambient temperature, and ST is the temperature of the source while,

the exergy received by fluid is written as [6-7, 16-18].

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)]()[()( iOaiOiOf ssThhmEEmEx

(3.6)

where oh is output specific enthalpy, ih is input specific enthalpy, os is output

entropy, is is input entropy, and m is mass flow rate of air blowing through the

collector tubes. The output specific enthalpy of the fluid is given by [16-18]:

oh = oPC OT (3.7)

where OT is outlet temperature, and oPC is the specific heat of air at outlet. The

specific enthalpy of inlet air is given by [16-18]:

ih = iiP TC (3.8)

where iPC is input specific heat, iT is inlet temperature. While the entropy difference

has been calculated using the following set of equations [16-18]:

iP TkaCi

(3.9)

OP TkaCO

(3.10)

kdTTdTaTdTbTaTdTCTdqds p /)/()(// (3.11)

Using Eqs.(3.9-3.10), the values of constants a and k can be calculated and

hence, the entropy difference thereafter using Eq.(3.11). The second law efficiency

i.e. exergy efficiency of the system can be written as [6-7, 16-18]:

)/1(/)]()[(/ SOCiOOiOCf TTQssThhmExEx (3.12)

and first law efficiency can be written as [18]:

CiO Qhhm /)( (3.13)

oO hmKJH sec)/( (3.14)

ii hmKJH sec)/( (3.15)

45cos4 sC AIQ (3.16)

where CEx is the exergy received by collector, fEx is the exergy received by fluid, a

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and k are constants, is the first law efficiency and is the second law efficiency of

the solar collector-dryer system. Based on the above mentioned equations, a sample

calculation has been made and the results are shown in Table-3.2, for a typical set of

operating conditions.

3.2.4 Uncertainty Analysis

The data given on solar radiation, thermal energy storage materials, solar

collector tubes, digital temperature displayer and sensors, air flow meter, air

compressor, rotameter has been measured/calculated using different instruments.

For example, solar radiation data has been compared with the weather monitoring

systems available at Solar Energy Centre, Gurgaon (India) and found to be in good

agreement with those taken by the pyranometer with an error of 0.1-0.2% Properties

of the thermal energy storage material such as latent heat, melting temperature etc.

have been measured with the differential scanning calorimeter (DSC) and it is found

to be almost similar as given by the supplier with an error of 0.2-0.5%. The error in

the efficiency of the compressor is found to be 0.5-1% depending on the ambient air

conditions, whereas the error in the rotameter found to be between 2.0-3.3%. The

error in the properties of air was found to be around 0.5-1% which is mainly due to

the presence of moisture in the circulating air over the period. The temperature

sensors were calibrated before and after use with the standard scale available in the

laboratory with an error of about 0.05%.

As it is well known that the wind speed also affects heat transfer rates to and

from the solar collector and hence, affects the energy and exergy efficiencies of the

system. Besides, the deposited dust particles on the collector, moisture content in

the ambient air, location, orientation, and so on, only slightly affect the overall

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performance of the experimental system. Since, there is an error of around 0.5-0.8%

in the exergy and the energy efficiencies due to various parameters and the

performance accuracy of the instruments. But as mentioned above, the wind speed

and deposition of dust particle were not taken into account in the present

experimental study. Therefore, the overall influences of these input errors on the

total results can also be very small and hence, can be neglected in the present

study. However, to make an error free system, all these parameters mentioned

above must be taken into account for the better accuracy and performance of such

systems for real life applications.

3.2.5 Results and Discussion

Comparative study on first and second law analyses of a typical solar air heater

collector system with and without thermal energy (TES) storage (viz. hytherm oil and

paraffin wax) have been carried out at different mass flow rates using hourly solar

radiation. Three different arrangements viz. solar air heater without TES, with PCM

and with hytherm has been taken for performance evaluation of solar air heater. The

inlet, outlet temperatures and solar radiation with respect to time for different mass

flow rates (i.e. 10, 20, 30, 40, 50LPM) with and without TES recorded and the

performance has been illustrated in Figs. (3.2-3.6). It is noted that in case of without

TES the outlet temperature increases as time increases attains its peak at near

about 13:00, 14:20, 12:00, 13:40, and 13:00 hrs for different mass flow rates viz. 10,

20, 30, 40, and 50 LPM respectively and then decreases gradually after 16:30 hrs.

The similar trend has been observed for solar radiation but with fluctuations

throughout the daytime.

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Fig.3.2. Inlet, outlet temperatures and solar radiation vs Time for 10 LPM

Fig.3.3: Inlet, outlet temperatures and solar radiation vs Time for 20 LPM

100.0

325.0

550.0

775.0

1,000.0

20

45

70

95

120

10:00 13:00 16:00 19:00

Tem

pera

ture

s(°

C)

Time (hr)

Tin Thtm Tpcm T Is(W/m2)

0.0

250.0

500.0

750.0

1,000.0

20

45

70

95

120

10:00 13:00 16:00 19:00

Tem

pera

ture

s(°

C)

Time (hr)

Tin Thytherm Tpcm T Is(W/m2)

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0.0

250.0

500.0

750.0

1,000.0

10

40

70

100

130

10:00 13:00 16:00 19:00

Tem

pera

ture

s(°

C)

Time (hr)


200.0

400.0

600.0

800.0

1,000.0

10

40

70

100

130

10:00 13:00 16:00 19:00

Tem

pera

ture

s(°

C)

Time (hr)


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In, arrangements with TES, the outlet temperature increases as time

increases attains its peak at 14:50, 15:40, 14:30, 13:30 and 13:30 hrs for different

mass flow rates viz. 10, 20, 30, 40, and 50 LPM respectively and decreases

afterwards slowly. But as heat is stored in the PCM during the day time it is also

supplied to the working fluid during low insolation and/or in the absence of solar

radiation, generally after 16:30 hrs during winter time. The heat released by TES to

the working fluid is nearly up to 20:30 hrs, which gives back up of approximately four

hrs every day. As observed from Figs. (3.2-3.6), outlet temperature is good enough

even after 16:30 hrs, it is very useful for many low and medium temperature heating

applications. This shows the usefulness of thermal energy systems for such devices.

Also, as the mass flow rate increases the optimum temperatures also increases in all

0.0

250.0

500.0

750.0

1,000.0

10

40

70

100

130

10:00 13:00 16:00 19:00

Tem

pera

ture

s(°

C)

Time (hr)


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the three cases as can be seen from Figs. (3.2-3.6). It is also found that the optimum

outlet temperature of the working fluid at different mass flow rates is slightly higher in

the case of PCM than those of with hytherm oil, but, in general the optimum outlet

temperature with TES is better than those of without TES.

The solar radiation, first and second law efficiencies against time are shown in

Figs. (3.7-3.11). From the graphs, it is found that both the efficiencies increase as

the time increases in all the three cases (with and without TES). But there is some

fluctuations in the efficiencies as can be seen from these graphs, which is obvious

because of the fluctuation in solar radiation throughout the day. In case of temporary

storage material, both the efficiencies have peak at different times, than those

obtained without TES, If we see graphs with phase change material and hytherm oil,

have their peak at about 16:30, while it is in the first half in general, for those without

TES, besides, they are fluctuating in nature as can be seen from Figs. (3.7-3.11).

The peak with TES occurs at around 16:30hrs, which is due to the fact that solar

radiation goes down sharply, while circulating air gets heated by temporary storage

material at almost constant temperature for some time. As a result, the output to

energy input ratio with temporary storage increases sharply, and hence, the first and

second law efficiencies attain their peak during the late afternoon with some shifting

due to different mass flow rate.

But due to finite heat storage capacity and mass of the storing material, the

stored energy of TES decreases afterwards and hence, both the efficiencies in most

of the cases decrease at the same pattern, as can be seen from the graphs [Fig(3.7-

3.11)]. As mentioned above that the temporary storage material has finite heat

capacity and limited mass due the space available in the evacuated tubes, the

instantaneous fluctuation in solar radiation is compensated by the storage material

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which supplies heat at almost constant temperature. However, if the solar radiation

fluctuates more often and/or for longer time due to weather constraints, the

fluctuation is also found in the efficiencies of collector with TES. But in the case,

where there is no temporary storage, the fluctuation in both the efficiencies is found

to be more frequent and significant, as can be seen from Figs. (3.7-3.11). It is also

important to note that the solar radiation is given in kW/m2 in Figs. (3.7-3.11), which

is done to fit the efficiency and radiation curve on the same axis.

It is also noted that the fluctuation in the efficiencies is in phase where there is no

temporary storage material, while the fluctuation in both the efficiencies is in the

opposite phase where temporary storage is being used, as can be seen from Figs.

(3.7-3.11).

Fig. 3.7(a): Solar radiation and efficiencies vs time for 10 LPM flow rate with PCM

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Fig. 3.7(b): Solar radiation and efficiencies vs time for 10 LPM with hytherm oil

Fig.3.7(c): Solar radiation and efficiencies vs time for 10 LPM without TES

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Fig. 3.8(a): Solar radiation and efficiencies vs time for 20 LPM with PCM


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Fig.3.8(c): Solar radiation and efficiencies vs time for 20 LPM without TES


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Fig. 3.9(c): Solar radiation and efficiencies vs time for 30 LPM without TES

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83



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Fig .3.11(b): Solar radiation and efficiencies vs time for 50 LPM with hytherm oil


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It can be explained with the same physical significance that the thermal energy

storage material behaves as temperature regulator by supplying additional heat

during the fluctuation in solar radiation. From the observations, it is found that first

law efficiency is much higher than that of the second law efficiency, because exergy

represents the quality of energy which is obviously enhances with the increase in the

temperature unlike the quantity of energy. Also as explained by several authors [14-

18], exergy once lost is lost forever and cannot be recovered, unlike energy. Also the

exergy loss is more in the collector receiver-assembly and not in the low temperature

utility unlike energy. This results in more losses in exergy than that of energy and

hence, we found that the second law efficiency is much less than that of the first law

efficiency. The curves for second law efficiency are found to be smoother than that of

first law efficiency which may be due to the fact that exergy losses are less sensitive

to the input energy viz. solar radiation, while it is reverse in the case of energy

losses.

However as the mass flow rate of the working fluid increases, the efficiencies in

all the three cases increases which is due to the fact that the heat gain by the

moving fluid (viz. by the air) in the receiver tube increases, as a result, we get higher

output, resulting in the higher efficiency for all the cases. Also whatever the mass

flow rate, first law efficiency is always greater than that of the second law efficiency.

In case where temporary storage material has no been used, only one peak is

observed and it shifts towards origin for both the efficiencies as the mass flow rate of

working fluid is increased. This is due to the fact that role of temporary storage

material is significant in this way.

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Table 3.2: Sample calculation for different parameters for a typical set of operating conditions

Time

(hrs)

T1,in

(K)

Ts

(K)

To

(K)

m

(gm/s)

CP,i

(J/kg-K)

CPo

(J/kg-K)

k

(J/kg-K2)

a

(J/kg-K)

∆ s

(J/kg-K)

Qc

(W)

Ec

(W)

Ef

(W)

ηII

(%)

ηI

(%)

10:00 308 395 353 0.580 1005.8 1008.99 0.071 984.07 137.37 129.74 29.23 2.52 8.63 20.74

10:30 309 401 362 0.582 1005.9 1009.85 0.075 982.63 159.54 155.39 36.42 3.36 9.23 20.51

11:00 310 413 371 0.584 1005.9 1010.77 0.081 981.21 181.12 159.49 40.54 4.31 10.63 23.13

11:30 311 424 381 0.586 1006.1 1011.88 0.085 979.68 204.81 178.18 48.33 5.50 11.38 23.90

12:00 312 426 390 0.587 1006.2 1012.95 0.089 978.31 225.23 187.64 51.09 6.67 13.06 25.43

12:30 313 429 391 0.590 1006.1 1013.07 0.091 977.99 224.61 178.81 49.18 6.70 13.62 26.78

13:00 314 427 393 0.592 1006.1 1013.31 0.091 977.55 226.57 188.71 50.82 6.87 13.53 25.80

13:30 313 428 392 0.590 1006.1 1013.19 0.091 977.86 227.21 208.66 57.04 6.85 12.03 23.25

14:00 312 421 391 0.588 1006.0 1013.07 0.089 978.18 227.83 191.94 50.61 6.82 13.48 25.18

14:30 311 419 390 0.586 1006.2 1012.95 0.088 978.49 228.46 165.15 43.36 6.80 15.67 29.16

15:00 311 413 388 0.586 1006.1 1012.70 0.088 978.75 223.25 143.94 36.25 6.50 17.92 32.59

15:30 311 407 385 0.586 1006.2 1012.35 0.086 979.15 215.38 140.02 33.71 6.06 17.98 32.18

16:00 310 398 380 0.586 1005.9 1011.77 0.084 979.99 205.38 109.32 24.72 5.48 22.16 38.80

16:30 309 389 376 0.584 1005.9 1011.32 0.081 980.71 197.92 55.34 11.67 5.06 43.33 73.04

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In other words the peak observed around 13:00hrs (Fig.3.7c). In case of 10LPM,

while it is found to be 11:05hrs and 12:40hrs in case of medium mass flow rates.

However in cases where temporary storage have been used, both the efficiencies

attains their highest peak at near about 4:30hrs [Figs. (3.7-3.11)] for all mass flow

rates because of the fact that at this time, solar radiation goes down sharply and

heat is supplied by TES up to some reasonable duration.

Both the efficiencies of solar air heater collector using PCM are slightly better

than those in the case of hytherm oil. But in general, both the efficiencies with TES

have been observed to be better than those of without TES. Besides, as the

temporary storage material regulate the supply of heat at almost constant

temperature, both the efficiencies, are found to be smoother than that of without TES

as can be seen from Figs. (3.7-3.11). However both the efficiencies increases slowly

and attains peak at nearly 16:30hrs, and then decreases and again it increases

which is due to the fact that the heat stored in the PCM/hytherm is supplied by TES

which does not happen in the case of without TES. In case of empty collector tubes

the efficiencies once attain their peak in the first half, in general and then goes down

further as time increases because of the fact that solar radiation also decreases,

while it is found to be different in the case with TES.

3.3 Direct Flow Evacuated Tube Collector Solar Water Heater

Present section represents the exergy analysis of direct flow ETC based solar

water heater for which following materials and methods has been used.

3.3.1 Experimental Set-up and Procedure

In the present experimental study the direct flow evacuated tube collector

(ETC) solar water heater using energy and exergy analysis has been investigated.

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Total 9 numbers of ETC tubes have been arranged in the series and the

photographic view of the experimental set-up is shown in Fig.3.12 (a-b). The

evacuated tube collector and copper tubes have been used of the same dimensions

as mentioned in table-1. The glass wool was used to cover the copper tubes

exposed into the open air viz. outside the ETC tubes for insulation purpose to

reduce/minimize the heat loss to the ambient air. Calibrated J-type thermocouples

made of Copper-Constantine with temperature range -200°C to 1350°C were used to

measure air temperature at different state points.

In this arrangement, one thermocouple has been used for measuring the inlet

water temperature and other for measuring the outlet water temperature at the

defined state point. The outlet water temperature of the first tube is the inlet of the

second tube and the outlet of the second tube is inlet to the third tube and so on. To

measure water flow rate, a rotameter of 200 LPM capacity is used, which has been

placed before the inlet of ETC. Water was circulated through

Fig.3.12 (a): ETC based Solar water heater

Fig.3.12 (b): Hot water storage tank

Fig.3.12: Photographic view of experimental set-up

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the system directly from the water tank situated at the roof of the building. For

measuring solar radiation Suryamapi has been used in the experiment. The

Suryamapi is kept on the horizontal surface nearby the experimental set-up in the

open air, so that no shadow and/or reflection of solar radiation from any other

surface/object falls on it. Data of temperatures at different state points has been

collected by using digital temperature displayer. The volume flow rate of the

circulating fluid was measured by volume flow meter before it enters the first tube.

Whenever fluid enter in the first tube its temperature rises, which can be identified by

measuring temperature with the help of thermocouple provided at inlet and outlet of

each tube. Data was collected from the system for few days in different months by

varying the volume flow rate of water and has been evaluated to find out the

efficiency of solar water heater.

3.3.2 Energy and Exergy Analysis

Same set of equations as mentioned in sections 3.2.2 and 3.2.3 have been used for

energy and exergy analysis of the present solar water heater. Based on the above

mentioned equations, calculations have been made for a typical set of operating

conditions and the sample calculation is given in Table-3.3.

3.3.3 Results and Discussion

Present experimental study is an attempt to investigate the performance

evaluation of direct flow evacuated tube collector (ETC) based solar water heater

using energy and exergy analyses. Flat plate collectors based solar water heaters

(SWH) were very famous initially but in the modern scenario solar water heaters

based on evacuated tube collectors (ETC) are of more interest because of low cost

90

and increased efficiency of the system. In conventional ETC based SWHs, storage

tank is at the top of the collectors and hot and cold water remains in the same tank.

Hot water always remains at the top of the storage tank and cold water at the bottom

level this is because of the density difference between hot and cold water. Solar

water heaters based on ETC used in the present study is different from conventional

ETC based SWH. In this system water at the inlet is taken from the storage tank

which is already situated at the roof of the building and hot water at the outlet is

stored in properly insulated water tank so that to avoid heat losses.

The experiments were performed for typical climatic conditions of Katra, India.

The experiments have been carried out in the winter season i.e. in the month of

January and February at different volume flow rates viz. 10, 15, 20, 25 and 30 litres

per hour (LPH). Data has been collected manually for a clear sky day of the months

January and February. The measured data includes solar radiation and

temperatures at different state of points (inlet, outlet and ambient). Based on the

collected data calculations for energetic and exergetic efficiencies have been made

and analysed. The discussion of results is given as below:

Figure 3.13, illustrates the variation in solar radiation and efficiencies (energy

and exergy) with respect to time for 10 LPH volume flow rate. From the figure it can

be seen that experiment starts at 11:30 AM when irradiation is quite good i.e. about

500 W/m2. From the figure it is also observed that solar radiation first increases with

time then remains almost constant for some time then again decreases which is an

obvious case in practice. It also seen from the figure that initially both the efficiencies

are low which is due to the fact that initially solar radiation is also low and also

temperature difference is less initially because it takes some time to warm the water.

91

Table 3.3- Energetic and exergetic efficiencies at 15 LPH volume flow rate for a typical set of parameters

Time (Hrs) Intensity (W/m2) Qin (W) Qo (W) Exin (W) Exo (W) η (%) ψ (%)

11:30 660 1,841.40 1,027.11 943.69 87.95 55.78 9.32 11:35 645 1,799.55 962.13 922.07 79.44 53.47 8.62 11:40 640 1,785.60 1,059.25 915.09 90.70 59.32 9.91 11:45 645 1,799.55 1,027.11 922.24 87.95 57.08 9.54 11:50 640 1,785.60 1,027.11 915.09 87.95 57.52 9.61 11:55 640 1,785.60 994.73 915.09 85.18 55.71 9.31 00:00 650 1,813.50 995.90 929.05 81.98 54.92 8.82 00:05 660 1,841.40 997.07 943.51 87.89 54.15 9.31 00:10 670 1,869.30 1,063.15 957.63 93.43 56.87 9.76 00:15 670 1,869.30 1,162.24 957.81 108.85 62.18 11.36 00:20 670 1,869.30 1,129.83 957.81 105.82 60.44 11.05 00:25 670 1,869.30 1,196.02 957.99 118.88 63.98 12.41 00:30 670 1,869.30 1,229.81 957.99 125.57 65.79 13.11 00:35 680 1,897.20 1,197.38 971.92 115.06 63.11 11.84 00:40 690 1,925.10 1,263.59 986.40 128.63 65.64 13.04 00:45 695 1,939.05 1,265.21 993.36 128.41 65.25 12.93 00:50 720 2,008.80 1,298.98 1,029.29 139.18 64.66 13.52 00:55 720 2,008.80 1,298.98 1,029.29 139.18 64.66 13.52 13:00 740 2,064.60 1,333.04 1,057.68 142.40 64.57 13.46 13:05 725 2,022.75 1,399.56 1,036.43 157.35 69.19 15.18 13:10 720 2,008.80 1,368.45 1,028.91 149.36 68.12 14.52 13:15 720 2,008.80 1,335.77 1,029.10 149.73 66.50 14.55 13:20 705 1,966.95 1,435.50 1,007.66 164.66 72.98 16.34 13:25 700 1,953.00 1,502.05 1,000.70 180.60 76.91 18.05 13:30 700 1,953.00 1,469.56 1,000.70 176.69 75.25 17.66 13:35 700 1,953.00 1,536.41 1,000.70 188.68 78.67 18.86 13:40 690 1,925.10 1,537.15 986.40 196.61 79.85 19.93 13:45 720 2,008.80 1,571.51 1,029.10 200.42 78.23 19.48 13:50 725 2,022.75 1,572.32 1,036.24 204.49 77.73 19.73 13:55 715 1,994.85 1,642.69 1,021.95 221.86 82.35 21.71 14:00 715 1,994.85 1,543.03 1,022.14 201.26 77.35 19.69 14:05 705 1,966.95 1,612.17 1,007.84 218.36 81.96 21.67 14:10 720 2,008.80 1,649.53 1,029.29 227.52 82.12 22.10 14:15 705 1,966.95 1,686.18 1,007.84 236.74 85.73 23.49 14:20 710 1,980.90 1,724.96 1,015.37 256.28 87.08 25.24 14:25 705 1,966.95 1,755.60 1,007.84 255.09 89.25 25.31 14:30 700 1,953.00 1,786.51 1,000.70 263.92 91.48 26.37 14:35 690 1,925.10 1,410.52 986.40 158.58 73.27 16.08 14:40 570 1,590.30 994.73 814.85 82.13 62.55 10.08 14:45 530 1,478.70 893.68 757.67 66.20 60.44 8.74 14:50 510 1,422.90 793.15 729.08 51.89 55.74 7.12 14:55 470 1,311.30 825.48 671.77 51.43 62.95 7.66 15:00 460 1,283.40 824.04 657.60 51.50 64.21 7.83 15:05 450 1,255.50 756.98 643.42 45.23 60.29 7.03 15:10 430 1,199.70 690.18 614.83 37.13 57.53 6.04 15:15 410 1,143.90 687.51 586.34 35.03 60.10 5.97 15:20 400 1,116.00 588.84 571.83 22.65 52.76 3.96 15:25 390 1,088.10 489.20 557.84 19.00 44.96 3.41 15:30 280 781.20 324.15 400.50 7.46 41.49 1.86

92

Input energy is low initially and the corresponding outlet temperature is also low

and therefore temperature difference (∆T) is low and hence corresponding energy

and exergy output is also low. As solar radiation increases both the efficiencies i.e.

energetic and exergetic increases with time while, solar radiation starts decreasing

after some point of time but both the efficiencies are still increasing in nature. These

efficiencies takes peak at a particular point of time then these also decreases slowly

afterwards. This variation in efficiencies with solar radiation can be explained in

terms of increased outlet temperature due to copper tube inserted inside ETC tubes.

As explained above, in this system copper tubes are inserted inside the ETC tubes

due to which temperature at the outlet doesn’t drop suddenly, it takes time and

hence we got better efficiency even if solar radiation decreases.

Fig.3.13: Solar radiation and efficiencies Vs time for 10 LPH volume flow rate

Variation in solar radiation and efficiencies with time for 15 LPH volume flow

rate has been shown in Fig.3.14. From the figure it is seen that initially irradiance

level is good i.e. approximately 660 W/m2 increases slowly and maintains good level

0

23

46

69

92

11:30 00:30 13:30 14:30 15:30

Eff

icie

ncie

s (

%)

Time (hr)

η ψ Is

93

of irradiance for a long period of time and then decreases suddenly. As explained

above, in this case also both the efficiencies increases as solar radiation increases

but drops suddenly as the corresponding solar radiation drops which is unlike the

case of 10 LPH volume flow rate. At 10 LPH after some time solar radiation

decreases but in this case as the solar radiation decreases both the efficiencies

suddenly decreases.

Fig.3.14: Solar radiation and efficiencies Vs time for 15 LPH volume flow rate

Figure 3.15 shows the variation in energy and exergy efficiencies and solar

radiation with time at 20 LPH volume flow rate. From the figure it is observed that for

this particular day solar radiation is very much fluctuating in nature. A mixed

response in the variation of efficiencies with solar radiation has been found in this

case. Sometimes as the solar radiation drops due to clouds corresponding

efficiencies also decreases suddenly. However, sometimes solar radiation is high

and the corresponding efficiencies are low. This mixed response in the variation of

0

185

370

555

740

0

23

46

69

92

11:30 00:50 14:10 15:30

Eff

icie

ncie

s(%

)

Time(hr)

η

ψ

Is

94

efficiencies is due to fluctuation in solar radiation which is intermittent in nature.

Variation in solar radiation and efficiencies with time at 25 LPH and 30 LPH has

been shown in Figs. 3.16 and 3.17 respectively. Same nature of variation in

efficiencies with solar radiation has been found as that of 10 LPH volume flow rate.

Fig.3.15: Solar radiation and efficiencies vs time 20 LPH volume flow rate

Fig.3.16: Solar radiation and Efficiencies vs time 25 LPH volume flow rate

0

185

370

555

740

0

17

34

51

68

10:50 11:50 12:50 13:50 14:50

Eff

icie

ncie

s(%

)

Time(hr)

η

ψ

Is

0

173

346

519

692

0

14

28

42

56

11:30 00:45 14:00 15:15

Eff

icie

ncie

s(%

)

Time(hr)

η

ψ

Is

95

Fig.3.17. Solar Radiation and efficiencies Vs time for 30 LPH volume flow rate

Comparative variation in energetic and exergetic efficiencies at different flow

rates is shown in Fig.3.18. From the figure it has been found that both the

efficiencies for 15 LPH volume flow rate is highest among all the flow rates at which

the present ETC based SWH has been experimented and analysed. The average

energetic and exergetic efficiencies at 15 LPH flow rate are 66.57 % and 13.38 %

respectively. As can be seen from the figure, as the volume flow rate increases 15

LPH onwards both the efficiencies decreases. Both the efficiencies for 10 LPH flow

rate are second best for the system and average energy and exergy efficiency has

been found to be 62.17% and 11.14 % respectively. Energy and exergy efficiencies

for 30 LPH flow rate has been found to least which are 30.36% and 2.22%

respectively. Energy and exergy efficiencies for 20 LPH were 45.45% and 5.24%

and for 25 LPH flow rate has were 35.49% and 3.30% respectively.

0

180

360

540

720

0.00

13.00

26.00

39.00

52.00

11:00 00:15 13:30 14:45 16:00

Eff

icie

nc

ies(%

)

Time(hr)

η

ψ

Is

96

Fig.3.18: Comparative variation in efficiencies at different volume flow rates

Therefore performance of direct flow ETC based solar water heater with nine

tubes has been found to be best at 15 LPH flow rate i.e. 15 LPH is the optimum flow

rate for nine tube ETC based solar water heating system. This is due to the fact that

at this flow rate output energy/exergy is better comparative to other flow rates

presented in this study therefore losses are low and ultimately SWH works better at

15 LPH volume flow rate. Also performance of this SWH has been found to be better

than that of other SWHs investigated by earlier authors [20-21]. This better

performance of the present system is due to the fact as explained above. Energy

efficiency has been found to be always higher than those of exergy efficiency. This is

due to the fact that energy efficiency is based on first law of thermodynamics and

doesn’t consider losses associated with the system. However, exergy efficiency is

based on second law of thermodynamics and considers the losses/irreversibilities

associated with the system. Therefore exergy represents quality of energy rather

than quantity.

0.00

17.00

34.00

51.00

68.00

10 15 20 25 30

Eff

icie

ncie

s (

%)

Volume flow rate (LPH)

η

ψ

97

The solar air dryer has been designed and fabricated in the central workshop of

the university to study the performance of such system with different modifications

for various applications. The site built was not brought from the market because of

the novelty of the idea and its design features. In general, the flat plate solar collector

are available in the market however, ETC based solar dryer are occasionally

available and also here copper tubes were inserted inside the tubes to study its

quality over those available in the market. The outlet temperature was measure to be

more than 150ºC which not only a uniqueness of this system but also a very useful

for many applications in real life. Further, to check the novelty of idea the same

system was converted into solar water heater with direct flow from the roof top water

tank and direct use/storage of hot water in a separate tank which may be filled with

PCM for later use. However, based on the requirements and site specification, the

optimization of design parameters can be carried out and the technology may be

transferred to the willing party for marketing the product.

3.4 Conclusions

The comparative study based on the first and second law analyses of a typical

solar air heater collector system with and without temporary thermal energy storage

has been carried out at different mass flow rates using hourly solar radiation. From

the present experimental study, some interesting results are found and can be

summarized as below:

I. The optimal outlet temperatures in the case of with thermal energy storage

are found to be higher than those without thermal energy storage. Also in

general, the optimal outlet temperature of arrangement with paraffin wax is

found to be higher than those of with hytherm oil for all the mass flow rates

with thermal energy storage.

98

II. It is found that there is fluctuation in both the efficiencies which is mainly due

to the fact that solar radiation also fluctuates throughout the day as can be

seen clearly from the figures given in this paper. Also as time increases, both

the efficiencies first increase and then decrease in case without temporary

storage material and the similar trend is found for the solar radiation.

III. In case of without temporary heat energy storage material, the efficiency

increases with time, attains its peak in the first half in general (Figs.2-6) and

then decreases after that. However in cases where temporary heat storage

material is used, both the efficiencies increase with time, attain their peak at

near about 16:30hrs with a small fluctuation with flow rate and then decreases

smoothly. This is due to the fact that the solar radiation sharply decreases in

the late afternoon and heat is supplied only by TES for some time. But due to

the limited mass and capacity of the storage material, this supply also

decreases slowly after a certain time. As a result, the stored heat energy

decreases smoothly, and hence, both the efficiencies again decrease towards

the late evening hours.

IV. As the mass flow rate increases peak of both efficiencies slightly shifts

towards origin in case without storage material/fluid. However, in case with

TES, there is a small shift in the peak due to due to different mass flow rate of

the working fluid except in case of 40LPM which is because of shift in the

peak of solar radiation during that particular day.

V. It is also noted that the fluctuation in the efficiencies is in phase where there is

no temporary storage material, while the fluctuation in both the efficiencies is

in the opposite phase where temporary storage is being used, as can be seen

from the graphs. It can be explained with the same physical significance that

99

the temporary heat energy storage material behaves as temperature regulator

by supplying additional heat during the fluctuation of solar radiation.

VI. From the observations, it is also found that first law efficiency is much higher

than that of the second law efficiency, because exergy represents the quality

of energy which is obviously enhances with the increase in the temperature

unlike the quantity of energy. Also as explained by several authors mentioned

in this paper. Thus exergy, which is the quality of energy, once lost is lost

forever and cannot be recovered, unlike energy. Also exergy loss is more in

the collector receiver-assembly and not in the low temperature utility unlike

energy. This results in more losses in exergy than that of the energy and

hence, we found the second law efficiency much less than that of the first law

efficiency.

VII. The curves for second law efficiency are found to be smoother than that of

first law efficiency which may be due to the fact that exergy losses are less

sensitive to the input energy viz. solar radiation, while it is reverse in the case

of energy losses.

Thus the results obtained in this study will be very useful and informative for real

life applications using the temporary storage in both the solar collector and in the

thermal energy utilities for better performance. Most importantly, where there is a

need of thermal energy without much fluctuation in the outlet temperature and heat

content out of the solar collector system for various applications of physical

importance.

Further analysis of direct flow ETC based SWH which was designed and

fabricated with copper tubes inserted inside ETC revealed that:

100

I. After some point of time solar radiation starts decreasing which is an obvious

nature of solar radiation, ideal case is that as solar radiation decreases the

corresponding energy and exergy efficiency should also decrease as there is

no thermal energy storage in the system. But with copper tubes inserted

inside the ETC efficiencies increases for some time which ultimately increases

the overall system efficiency.

II. Optimum flow rate of the present SWH has been found to 15 LPH i.e. this

SWH performs better at 15 LPH among the other flow rates analysed and

presented in this study.

III. The energetic and exergetic efficiencies at 15 LPH flow rate has been found

to be 66.57 % and 13.38 % respectively.

IV. In the present SWH hot water at the outlet is directly collected to well

insulated storage tank and water at inlet is taken from the storage tank

already available at the roof of the building due to which no mixing of water

occurs and we got enhanced efficiency of the SWH.

V. Also heat exchanger is made inside the hot water storage tank so that water

remains hot for a longer duration of time and can be used in the night time

when there is no availability of solar radiation.

VI. This SWH is very useful for domestic applications due to high efficiency also if

this SWH is applied to larger scale may be useful for industrial applications as

well.

Other researchers can use the outcomes of this study for designing of different

solar thermal devices for real life applications depending on the site and

requirements of the end users. Further, The concept of energy and exergy analysis

101

can also be applied to other solar thermal systems to study the realistic performance

of such devices and other energy conversion systems as well.

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chapter - 3 exergy analysis of solar heating...

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