CHAPTER - 3. Exergy Analysis of Solar Heating Devices

Transcription

1 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. 61

2 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 62

3 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, 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 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 63

4 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 setup. 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 64

5 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 65

6 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/m 2 ) 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. 66

7 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 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 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 C * Specific heat 2.05 kj/kg C Latent heat of fusion kj/kg * Thermal conductivity 0.21 (solid) (W/m K) Density at 70 C kg/m3 Properties of HP Hytherm 500 Oil Kinematic 40 c, cst Flash point coc, c, min 194 Viscosity index 95 Power point c max 0.0 Copper strip corrosion 100 c (astm), max 1.0 Neutralisation number mg koh/gm, max c c c c c c *Measured by Differential Scanning Calorimeter (DSC) 67

8 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 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: Qc AI s (3.1) where Q c is the energy incident on the collector tube, A is the projected area of collector tube exposed to the sun light, and I s is the intensity of solar radiation at any particular site. Useful energy gained from the collector can be written as: Q u I A (3.2) s 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: 68

9 Q Q mc T (3.3) u f p where Q f is the energy absorbed by air, C p 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: Q / Q mc T / AI (3.4) f c p s where is the abbreviation used for first law efficiency of the system 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]: Ex Q 1 ( T / T ) (3.5) c C ( a S where T a is ambient temperature, and T S is the temperature of the source while, the exergy received by fluid is written as [6-7, 16-18]. 69

10 Ex f m E E ) m [( h h ) T ( s s )] (3.6) ( O i O i a O i where h o is output specific enthalpy, h i is input specific enthalpy, s o is output entropy, s i 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]: h o = CPo T O (3.7) where T O is outlet temperature, and C P is the specific heat of air at outlet. The o specific enthalpy of inlet air is given by [16-18]: h i = where C (3.8) P T i i C Pi is input specific heat, T i is inlet temperature. While the entropy difference has been calculated using the following set of equations [16-18]: CP i a k T i (3.9) CP O a k T O (3.10) ds dq / T Cp dt / T ( a bt)( dt / T) adt / T kdt (3.11) Using Eqs.( ), 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]: Ex Ex m [( h h ) T ( s s )]/ Q (1 T / T ) (3.12) f / C O i O O i C O S and first law efficiency can be written as [18]: m ( h h ) / Q (3.13) O i C H ( KJ /sec) m (3.14) O h o H ( KJ /sec) m (3.15) i h i Q AI cos 45 (3.16) C 4 s where Ex C is the exergy received by collector, Ex f is the exergy received by fluid, a 70

11 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 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 % 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 %. 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 %. 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 71

12 performance of the experimental system. Since, there is an error of around % 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 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. ( ). 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. 72

13 Temperatures( C) Temperatures( C) 120 1, Tin Thtm Tpcm T Is(W/m2) 20 10:00 13:00 16:00 19:00 Time (hr) Fig.3.2. Inlet, outlet temperatures and solar radiation vs Time for 10 LPM 120 1, Tin Thytherm Tpcm T Is(W/m2) 20 10:00 13:00 16:00 19:00 Time (hr) 0.0 Fig.3.3: Inlet, outlet temperatures and solar radiation vs Time for 20 LPM 73

14 Temperatures( C) Temperatures( C) 130 1, Tin Thtm Tpcm T Is(W/m2) 10 10:00 13:00 16:00 19:00 Time (hr) 0.0 Fig.3.4: Inlet, outlet temperatures and solar radiation vs Time for 30 LPM 130 1, Tin Thtm Tpcm T Is(W/m2) 10 10:00 13:00 16:00 19:00 Time (hr) Fig.3.5: Inlet, outlet temperatures and solar radiation vs Time for 40 LPM 74

15 Temperatures( C) 130 1, Tin Thtm Tpcm T Is(W/m2) 10 10:00 13:00 16:00 19:00 Time (hr) 0.0 Fig.3.6: Inlet, outlet temperatures and solar radiation vs Time for 50 LPM 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. ( ), 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 75

16 the three cases as can be seen from Figs. ( ). 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. ( ). 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. ( ). 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( )]. 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 76

17 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. ( ). It is also important to note that the solar radiation is given in kw/m 2 in Figs. ( ), 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. ( ). Fig. 3.7(a): Solar radiation and efficiencies vs time for 10 LPM flow rate with PCM 77

18 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 78

19 Fig. 3.8(a): Solar radiation and efficiencies vs time for 20 LPM with PCM Fig. 3.8(b): Solar radiation and efficiencies vs time for 20 LPM with hytherm oil 79

20 Fig.3.8(c): Solar radiation and efficiencies vs time for 20 LPM without TES Fig. 3.9(a): Solar radiation and efficiencies vs time for 30 LPM with PCM 80

21 Fig. 3.9(b): Solar radiation and efficiencies vs time for 30 LPM with hytherm oil Fig. 3.9(c): Solar radiation and efficiencies vs time for 30 LPM without TES 81

22 Fig. 3.10(a): Solar radiation and efficiencies vs time for 40 LPM with PCM Fig. 3.10(b): Solar radiation and efficiencies vs time for 40 LPM with hytherm oil 82

23 Fig. 3.10(c): Solar radiation and efficiencies vs time for 40 LPM without TES Fig. 3.11(a): Solar radiation and efficiencies vs time for 50 LPM with PCM 83

24 Fig.3.11(b): Solar radiation and efficiencies vs time for 50 LPM with hytherm oil Fig. 3.11(c): Solar radiation and efficiencies vs time for 50 LPM without TES 84

25 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. 85

26 Table 3.2: Sample calculation for different parameters for a typical set of operating conditions Time (hrs) T 1,in T s T o m C P,i C Po k a s Q c E c E f η II η I (K) (K) (K) (gm/s) (J/kg-K) (J/kg-K) (J/kg-K 2 ) (J/kg-K) (J/kg-K) (W) (W) (W) (%) (%) 10: : : : : : : : : : : : : :

27 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. ( )] 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. ( ). 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 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. 87

28 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 88

29 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 Energy and Exergy Analysis Same set of equations as mentioned in sections and 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 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 89

30 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/m 2. 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. 90

31 Table 3.3- Energetic and exergetic efficiencies at 15 LPH volume flow rate for a typical set of parameters Time (Hrs) Intensity (W/m 2 ) Q in (W) Q o (W) Ex in (W) Ex o (W) η (%) ψ (%) 11: , , : , : , , : , , : , , : , : , : , : , , : , , : , , : , , : , , : , , : , , : , , : , , , : , , , : , , , : , , , : , , , : , , , : , , , : , , , : , , , : , , , : , , : , , , : , , , : , , , : , , , : , , , : , , , : , , , : , , , : , , , : , , , : , , : , : , : , : , : , : , : , : , : , : , :

32 Efficiencies (%) 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 η ψ Is 0 11:30 00:30 13:30 14:30 15:30 Time (hr) 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 From the figure it is seen that initially irradiance level is good i.e. approximately 660 W/m 2 increases slowly and maintains good level 92

33 Efficiencies(%) 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 η ψ Is :30 00:50 14:10 15:30 Time(hr) 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 93

34 Efficiencies(%) Efficiencies(%) 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 and 3.17 respectively. Same nature of variation in efficiencies with solar radiation has been found as that of 10 LPH volume flow rate η ψ Is :50 11:50 12:50 13:50 14:50 Time(hr) 0 Fig.3.15: Solar radiation and efficiencies vs time 20 LPH volume flow rate η ψ Is :30 00:45 14:00 15:15 Time(hr) 0 Fig.3.16: Solar radiation and Efficiencies vs time 25 LPH volume flow rate 94

35 Efficiencies(%) η ψ Is :00 00:15 13:30 14:45 16:00 Time(hr) Fig 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 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 % and % 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 % 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. 95

36 Efficiencies (%) η ψ Volume flow rate (LPH) 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. 96

37 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. 97

38 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 98

39 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: 99

40 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 % and % 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 100

41 can also be applied to other solar thermal systems to study the realistic performance of such devices and other energy conversion systems as well. References 1. Tyagi S.K., Park S.R., Tyagi V.V., Anand S. (2010). Second law based performance evaluation and parametric study of a sea water source cascade heat pump. Int. Journal of Exergy, 7(3): Kovarik M., Lesse P.F. (1976). Optimal control of flow in low temperature solar heat collectors. Solar Energy, 18: Farries D.R., Melsa J.L., Murray H.S. (1977). Energy conservation by adaptive control for a solar heated building. Proceedings of IEEE International Conference on Cybernetics and Society 4. Orbach A., Fischl R., Herczfeld P.R., Konyk J.S. (1979). Optimal and suboptimal control strategies and sensitivity study for solar liquid collector systems. Proceedings of ISES Atlanta: USA 5. Winn R.C., Winn C.B. (1981). Optimal control of mass flow rate in flat plate solar collectors. ASME Journal of Solar Energy Engineering, 103: Bejan A., Kearney D.W., Kreith F. (1981). Second law analysis and synthesis of solar collector systems. ASME Journal of Solar Energy Engineering, 103: Bejan A. (1982). Extraction of exergy from solar collectors under time-varying conditions. International Journal of Heat and Fluid Flow, 3: Pangavhane D.R., Sawhney R.L. (2002). Review of research and development work on solar driers for grape drying. Energy Conversion & Management, 43: