Exergy Analysis of Solar Photovoltaic Systems

Transcription

1 CHAPTER - 4 Exergy Analysis of Solar Photovoltaic Systems 4.1 Introduction Due to the growing concern and awareness of environmental issues among the scientific community, power generation from renewable energy sources, particularly solar energy has become significantly important for the last few decades. Solar energy due to its intermittent nature is not available for a long time during the day. Its durability varies from country to country and place to place. Solar energy reaching to the surface of the earth can be utilised directly in two ways viz. directly converting the solar radiation to the electricity for useful purposes by the means of solar photovoltaic (SPV) modules or by heating the medium source for low temperature heating applications. Photovoltaic module is not only an expensive but also an essential component of any PV system and therefore its thermal assessment based on the exergy analysis is very important. Researchers from all over the world have been working for the last few decades towards the enhancement of efficiency, low cost materials and fabrication of solar cells using several approaches. One of the options might be the reduction in the amount of material for the fabrication of solar cells. Wafer slicing technology including kerf loss reduction for a thin having width less than 2 μm and the manufacturing technology for the same are under investigation [1]. By using the thin film technology for solar cells shortage of silicon material can be managed. Thinner active layers having width less than 1 μm are the efforts in the direction of achieving high-efficiency and low-cost solar cells [2], however before proceeding to mass production there is need of further efforts to reduce the process cost. The 14

2 enhancement in the efficiency of solar cells means obtaining more power from the same solar cells which in turn, reduces the amount of material required to fabricate them and hence, reduced cost of cells. Recently, a lot of work all over the world has been carried out in the direction for achieving the low cost and high efficiency solar cells. So far, multi-crystalline silicon (mc-si) has been found to be a good option as compared to mono-crystalline silicon (c-si). However, as far as the efficiency is concerned, many problems has been found in achieving the high efficiency ( around 2%) with a practical size. The problems are crystal growth technology to control the grain quality uniformly, passivation technology for surface, light-trapping technology, crystal grains itself and grain boundary. These technologies have been systematically and actively investigated at the laboratory level [3]. For obtaining both low cost and high efficient solar cells, a different approach has been applied to develop a new a-si/c-si heterojunction structure, called hetero-junction with intrinsic thin layer (HIT) [4-8]. This structure features a very thin intrinsic a-si layer inserted between a doped a-si layer and a c-si substrate. This structure has several advantages such as, an excellent surface passivation and p n junction which results in high efficiency, lowtemperature processes (<2 ºC) that can prevent any degradation of bulk quality which happens with the high-temperature cycling processes in low-quality silicon materials like Czochralski Si and compared with It has much better temperature coefficient as compared to conventional diffused cells therefore high-voc of the cells can be obtained. Taguchi et al reported the high conversion efficiency of 21.5% in HIT cells with a size of 1.3 cm 2 [9]. In the present chapter, the performance evaluation and parametric study of three different solar photovoltaic (SPV) modules viz. thin film, multi-crystalline and 15

3 HIT (hetero junction with intrinsic thin layer) have been carried out using the energetic and exergetic analysis under typical climatic zone in North India for different months of a year. 4.2 Materials and methods In this experimental study, the comparative energetic and exergetic analysis of different solar photovoltaic (SPV) modules has been carried out for a typical year under the Indian climatic conditions for which following materials were used Weather Station The weather station available at the site i.e. Solar Energy Centre, Gurgaon (India) includes a meteorological measurement system and data acquisition system. The meteorological measurement system composed of following equipments. a) Pyranometers The weather station has two pyranometers one at horizontal i.e. parallel to earth s surface and the second at the tilted positions equal to the latitude of the experimental location for the measurement of global solar radiation having the wavelength range of each pyranometer is 35 nm to 28 nm. The photographic view of pyranometres at horizontal and tilted positions has been shown in Figs.4.1 (c). As the silicon pyranometer is a very sensitive having the execution time a one second. Also an UV pyranometer has been used for the measurement of ultraviolet radiation only which is having the wavelength in the range of 28 nm to 4 nm. 16

4 b) Spectroradiometers The Spectroradiometers for this particular system has been designed to measure the spectral power distributions of illuminants. Also two different Spectoradiometers such as visible Spectroradiometer (MS 71) and infrared Spectroradiometer (MS 712) having the range of nm and 9-17 nm respectively, have been used in this experimental study and the photographic view of Spectroradiometers is shown in Fig. 4.1(d). Figure.4.1(a) Figure.4.1(b) Figure.4.1(c) Figure.4.1(d) Fig.4.1: Photographic view of the weather monitoring system 17

5 c) Wind monitoring: The wind monitoring is being done through a high performance wind sensor which measures both the wind speed and the wind direction in the range -1 m/s and -36 respectively and photographic view of wind monitor is shown in Fig. 4.1(b). d) Air Temperature and humidity sensor: Finally the temperature and humidity were measured using air temperature and humidity sensors, having the measurement range of -4 C to 6 C for air temperature and.8% to 1% RH for humidity, respectively and the photographic view of the complete SPV power plant is shown in Fig Fig. 4.2: Photographic view of the SPV power plant Methods The data was collected using data logger then this data loggers is connected to a personal computer (PC) by using software LoggerNet. The daily data was 18

6 collected in a separate folder and the execution time for the data logger connected to silicon pyranometer was set to be one second while for other two data loggers, it was set to one minute and the data loggers were programmed to save the data in separate folders in. 4.3 Energy and Exergy Analyses After the data was recorded and collected in a PC on day to day basis, the enrgy and exergy analysis was carried out separately, using the following expressions: The input energy i.e. energy of solar radiation is given by:. Q in I s A (4.1) where I s is intensity of solar radiation and A is area of SPV module The actual output of the SPV module may be defined as below: Q o V oc I sc FF (4.2) where Vocis open circuit voltage, Isc is short circuit current and FF is fill factor. The fill factor (FF) of the SPV system can be defined as the ratio of the product of voltage corresponding to maximum power ( V m ) and the current corresponding to maximum power ( I m ) to the product of open circuit voltage and short circuit current and can be expressed as below: V FF V m oc I I m sc (4.3) Using the above definition, Eq.(4.2) can also be expressed as below: Q V o m I m (4.4) 19

7 The input exergy i.e. exergy of solar radiation is given by:. Ex solar. Ta Exin I s A T 1 s (4.5) where T s is the temperature of sun which is taken as 5777 K. The exergy output of the SPV systems can be given as follows: Ex out Ex elec Ex therm. Ex d Ex. elec I' (4.6) where. I ' Ex Ex d, elect Exd, therm which includes internal as well as external losses Internal losses are electrical exergy destruction i.e Ex, d elect and external losses are heat loss, Ex, which is numerically equal to Ex. therm for PV system. For the d therm calculation of electrical exergy of the PV system i.e. Ex elec it has been assumed that exergy content received by photovoltaic surface is fully utilized to generate maximum electrical exergy ( V ). I oc sc. Ex elec E elec I' V oc I sc ( V oc I sc V m I m ) (4.7) where, V represents the electrical energy and V I V I ) represents the I oc sc ( oc sc m m electrical exergy destruction. Therefore from the above equation we find the electrical exergy as below:. Ex V elec m I m (4.8) The thermal exergy of the system ( Ex. therm ) which is defined as the heat loss from the photovoltaic surface to the ambient can be represented as below: Ex. therm T. a 1 Q T cell (4.9) 11

8 . where Q h A T T ca cell a and h ca v where hca the convective (radiative) heat transfer coefficient and v is the wind speed. Using the above equations exergy of SPV system can be written as below:. Ex PV T VmI m 1 T a cell h ca A T cell T a (4.1) The solar cell power conversion efficiency ( electrical output to the input solar radiation ( as below: pce I s ) can be defined as the ratio of actual A ) on the PV surface and can be given VmI m pce I A s (4.11) The power conversion efficiency can also be written in the terms of FF using the above equation as follows: FFVocI pce I A s sc (4.12) In general the exergy efficiency ( ) is defined as the ratio of output exergy to that of the input exergy and given as follows; Output Exergy Input Exergy (4.13) follows: Using the above equations, the exergy efficiency ( ) can be expressed as V m I m T a 1 hca A T T cell T a 1 I s A T s cell T a (4.14) However, the exergy efficiency can also be calculated using photonic energy as given by Markwart et al. [1] and Joshi et al. [11], due to the fact that the solar 111

9 energy reaching on the surface of the earth can also be explained in the term of photonic energy which travels in the form of packets ( h ) called photons [12]. The energy of a photon ( E ) ) can be calculated as: ph ( E ph ( ) h hc (4.15) The chemical potential or chemical exergy for the PV system can be given [13] as below: E. chem T E ph( ) 1 T cell s hc AN ph T 1 T where A is the surface area of PV module, cell s (4.16) N ph is the number of photons falling on the surface of PV module. Exergy is given by: Ex. chem pce. E (4.17) chem where pce is the power conversion efficiency. In the present thesis, two different approaches have been presented for the performance analysis of different SPV modules based on exergy analysis. The first method is based on the thermodynamic fundamentals and second is based on the chemical potential of the solar radiation. Either method can be used for the performance evaluation of the SPV module as both are realistic. 112

10 4.4 Results and Discussion The experimental study of SPV modules has been carried out for around 1 months at the typical climatic zone in North India which is located at 28º latitude and 77 º 1 16 longitudes. The experiments have been carried out for each sunny day of the different months of a year in the real outdoor conditions from 9 AM to 5 PM. The calculations were made for clear sky days of each month of the year except the month March and November because data of these two months could not be collected due to technical problem and maintenance in the system. The measured parameters includes, the wind speed, solar radiation, open circuit voltage, short circuit current, maximum voltage, maximum current, fill factor (FF), ambient temperature, average temperatures at the top, middle and the bottom of the module, minimum temperature at the top, middle and bottom of the module, maximum temperature at the top, middle and bottom of the module. The specifications of SPV modules as given by the manufacturer at standard test conditions (STC) viz. solar radiation of 1 W/m 2, air mass of 1.5 and ambient temperature of 25 ºC, are as below: Table 4.1: Parameters of SPV modules at standard test conditions (STC) as given by manufactures. Type of SPV module P max V oc (V) I sc (A) V m (V) I m (A) Thin Film Multicrystalline HIT

11 The specifications of the SPV modules at STC as obtained through the sun simulator in the laboratory are given as below: Table 4.2: Specifications of SPV modules at standard test conditions (STC) as tested in the laboratory. Type of SPV P max V oc (V) I sc (A) V m (V) I m (A) η cell (%) η module (%) module Thin Film Multi-crystalline HIT Based on the experimental data recorded through the data loggers and weather monitoring system as mentioned earlier, at the site, the energy, power conversion and exergy efficiencies have been calculated and plotted against the operating time from 9 AM to 5 PM for a typical set of operations at a particular climate in North India and the discussion of results for different SPV modules is given below: Thin film SPV module Figures 4.3 and 4.4 shows the variations of solar radiation, energy and power conversion efficiencies against time for the month of January and February respectively at a typical set of operating and designed conditioned as mentioned above. It is seen from Fig.4.3 that the solar radiation first increases, attains its peak near the middle of the day and goes down sharply towards the end of the day, while 114

12 fluctuating throughout the entire day which is an obvious case in practice. It is also observed that all the efficiencies viz. energy, power conversion and exergy efficiencies fluctuate with time which may be due to the fact that solar radiation also fluctuates. Initially, all the three efficiencies are high and as the time increases they decrease slowly then remain almost constant for over a long time period i.e. approximately between 1 AM to 4 PM, and finally decreases which can be explained in terms of lower input i.e. solar radiation. The fluctuation in the exergy efficiency is found to be more than that of energy and power conversion efficiencies which is may be due to the fact that the exergy efficiency is strongly dependent on the wind speed, ambient and module temperatures which are also fluctuating in nature throughout the day η(%) I (W/m2) :AM 11: AM 1: PM 3: PM 5: PM Fig.4.3. Efficiencies and Solar Radiation vs Time for thin film SPV module during the month of January 115

13 η I (W/m2) :AM 11: AM 1: PM 3: PM 5: PM Fig.4.4. Efficiencies and Solar Radiation vs Time for thin film SPV module during the month of February It is also found that the energy efficiency is always higher than that of exergy efficiency throughout the day. This is due to the fact that the former is based on the first law of thermodynamics and represents the quantity of energy while the later based on the second law of thermodynamics and represents the quality of energy which incorporates the losses/irreversibilities due to various parameters. Similarly, energy efficiency is also found to be higher than that of the power conversion efficiency which is the real measure of a module in terms of conversion ratio from solar radiation to electrical output. Therefore output for power conversion efficiency is less than that of energy efficiency and hence, the energy efficiency being a theoretical ratio based on the measured parameters is always higher than that of power conversion efficiency as well as the exergy efficiency. It is also found that the 116

14 average energy, power conversion and exergy efficiencies are found to be 9.63%, 5.61% and 2.97% respectively in the month of January. It is seen from Fig.4.4 that there is almost the same pattern of variation in all the parameters i.e. energy, power conversion and exergy efficiencies and solar radiation against time as that of Fig.4.3. From Fig. 4.4 it is found that for the month of February, the fluctuation in the exergy efficiency is less than that of month of January. This can be explained in terms of variation in wind speed, which varies month to month and day to day. Also average energy, power conversion and exergy efficiencies for the month of February are found to be 9.94%, 5.85% and 4.81% respectively. Also from Figs. 4.3 and 4.4, it can be seen that the average exergetic efficiency for the month of February is found to be higher than that of month of January. Variation of efficiencies and solar radiation against time of thin film SPV module in the months of April and May can be seen from Figs.4.5 and η(%) I (W/m2) :AM 11: AM 1: PM 3: PM 5: PM Fig.4.5 Efficiencies and Solar Radiation vs Time for thin film SPV module during the month of April 117

15 η(%) I (W/m2) :AM 11: AM 1: PM 3: PM 5: PM Fig.4.6. Efficiencies and Solar Radiation vs Time for thin film SPV module during the month of May From Fig.4.5 it is seen that the energy and power conversion efficiencies are almost constant in nature throughout the day which may be due to the fact that the fluctuation in solar radiation is not much for this particular data. However, the exergy efficiency fluctuates due to the fact that it depends on various parameters other than input solar radiation. The average energy, power conversion and exergy efficiencies are found to be 9.75%, 5.91% and 4.26% respectively, for the month of April. Fig.4.6 also shows almost the same pattern of variation as that of figures and due to the same reason as explained above. The average energy, power conversion and exergy efficiencies are found to be 8.64%, 5.28% and 3.24% respectively for the month of May. The energy, power conversion and exergy efficiencies and the solar radiation are plotted against time for the months of June and July as can be seen in Figs

16 Efficiencies (%) and 4.8 respectively. From Fig. 4.7, it is found that there is a sharp dip in all the three efficiencies at around 11.2 AM :AM 11: AM 1: PM 3: PM 5: PM Fig.4.7. Efficiencies and Solar Radiation vs Time for thin film SPV module during the month of June η(%) I (W/m2) :AM 11: AM 1: PM 3: PM 5: PM Fig.4.8. Efficiencies and Solar Radiation vs Time for thin film SPV module during the month of July 119

17 This dip in the efficiencies are due to sharp fall in the solar radiation at that particular instant and therefore, the output to input ratio decreases sharply as a result we got the lower instantaneous efficiency. The average energy, power conversion and exergy efficiencies are found to be 9.48%, 5.68% and 4.77% respectively for the month of June. From Fig.4.8, it is found that the initially all the three efficiencies are very high then decreases sharply. This is due to the fact that the initially solar radiation is very low and then increases sharply and also in the morning time module is cool and as the time increases module gets heated and the corresponding temperature increases. Therefore in the morning time corresponding voltage is high and as the time increases voltage of the module decreases and we got the higher efficiencies in the morning time. Average energy, power conversion and exergy efficiencies are found to be 1.33%, 6.19% and 5.85% respectively for the month of July. Sample calculation for the month July has been given in Table 4.3. Figures 4.9 and 4.1 show the variations of solar radiation, energy and power conversion efficiencies against time of SPV module for the month of August and September respectively. From Fig.4.9, it is found that the solar radiation for the month August is found to be more fluctuating in nature due to cloudy season in this part of the country therefore, the corresponding efficiencies also found to be fluctuating. The average energy, power conversion and exergy efficiencies are found to be 1.23%, 6.9% and 3.96% respectively, for the month of August. Similarly, from Fig.4.1 it is found that the solar radiation for the month September is very good as compared to the month of August and the average solar intensity is found to be 5 W/m 2 besides, the wind speed is not very much varying in this particular month. 12

18 Table 4.3: Sample calculation for typical set of operating conditions for thin film SPV module Time (hrs) Is (W/m 2 ) v (m/sec.) T a (ºC) T cell (ºC) P max (W) V oc (V) I sc (A) V m (V) I m (A) FF ψ (%) η pce (%) η (%) 9:AM :1AM :2AM :3AM :4 AM :5 AM : AM :1 AM :2 AM :3 AM :4 AM :5 AM : AM :1 AM :2 AM :3 AM :4 AM :5 AM : PM :1 PM :2 PM :3 PM :4 PM :5 PM Cont...

19 1: PM :1 PM :2 PM :3 PM :4 PM :5 PM : PM :1 PM :2 PM :3 PM :4 PM :5 PM : PM :1 PM :2 PM :3 PM :4 PM :5 PM : PM :1 PM :2 PM :3 PM :4 PM :5 PM : PM

20 Efficiencies (%) Therefore, the efficiencies were found to be less fluctuating during the month of September. The average energy, power conversion and exergy efficiencies are found to be 1.9%, 5.9% and 4.68% respectively for the month of September. The exergy efficiency is found to be more fluctuating while the energy efficiency is the least fluctuating followed by power conversion efficiency as can be seen from Fig η(%) 9:AM 11: AM 1: PM 3: PM 5: PM Fig.4.9. Efficiencies and Solar Radiation vs Time for thin film SPV module during the month of August η(%) I (W/m2) 9:AM 11: AM 1: PM 3: PM 5: PM Fig.4.1. Efficiencies and Solar Radiation vs Time for thin film SPV module during the month of September 123

21 Efficiencies (%) The energy, power conversion and exergy efficiencies along with the solar radiation are plotted against time for the months of October and December respectively as can be seen in Figs.4.11 and η(%) I (W/m2) 9:AM 11: AM 1: PM 3: PM 5: PM Fig Efficiencies and Solar Radiation vs Time for thin film SPV module during the month of October η(%) I (W/m2) :AM 11: AM 1: PM 3: PM 5: PM Fig Efficiencies and Solar Radiation vs Time for thin film SPV module during the month of December 124

22 From Figs , it is found that the average intensity of solar radiation is high enough and therefore, the efficiencies were found to be less fluctuating in nature during months. The average energy, power conversion and exergy efficiencies are found to be 1.26%, 6.% and 4.83% respectively, for the month of October while they are found to be 8.32%, 4.67% and 3.91% respectively, for the month of December Multi-crystalline SPV Module The variation of energy, power conversion and exergy efficiencies of multicrystalline SPV module along with solar radiation against hourly day light time is shown in Figs for the month of January and February respectively, at a given set of operating and designed conditions mentioned above. It is observed from these figures that the solar radiation is increasing initially, attains its peak near the middle of the day and goes down sharply towards the end of the day. It is also found that the solar radiation fluctuates throughout the entire day which is an obvious case in practice and so as the efficiencies of the multi-crystalline SPV module as can be seen from these figures. Also all the efficiencies are more fluctuating in nature with sharp peaks during the morning hours which may be explained in the terms of the fluctuations in the solar radiation and temperature of module in the morning time which can be seen from Fig It also seen from Fig.4.13, that all the three efficiencies viz. energy, power conversion and exergy are initially increase and attain sharp peaks and then as the time increases decrease slowly and remains almost in limited range while fluctuating for over a long time period and again increases towards the end of the day. This can be explained in terms of variation in solar radiation, module and ambient air temperatures. 125

23 Efficiencies (%) ηpce (%) η Is (W/m2) 9:AM 11: AM 1: PM 3: PM 5: PM Fig. 4.13: Variation of different efficiencies and solar radiation against time for multi-crystalline SPV module during the month of January η(%) Is (W/m2) 23 9:AM 11: AM 1: PM 3: PM 5: PM Fig. 4.14: Variation of different efficiencies and solar radiation against time for multi-crystalline SPV module during the month of February. 126

24 In other words, the solar radiation, module and ambient air temperatures are higher during the noon time as compared to those of the morning and evening times. In fact the module temperature is lesser during the morning and evening times as compared to the noon time, as a result the losses enhances due to the higher module and ambient air temperatures and hence, we get the results shown in Figs. ( ). Also, all the efficiencies, in general are found to be decreasing function for the month of February while solar radiation is found to be in the general trend as can be seen from Fig, The average energy, power conversion and exergy efficiencies are found to be %, 13.2 % and 1.87 % respectively, in the month of January while for the month of February they are found to be %, % and % respectively. The physical significance of these results can be explained in different ways. For example, during noon hours, as the temperature of the module increases, the voltage decreases due to negative temperature coefficient of module and also current increases but not in the comparative ratio as that of the voltage, thus the net product of the voltage and the corresponding current decreases. However, during the morning and evening hours the module and ambient air temperatures are lower as compared to noon hours, thus the voltage is high therefore, the output to input ratio is high leading to a better efficiency during morning and evening hours. Similarly, the energy efficiency is found to be higher than that of the exergy efficiency throughout the day. This is due to the fact that the former represents the quantity of energy while the later deals with the quality of energy. Sample calculation for the month of February has been given in Table 4.4 as below: 127

25 Table 4.4: Sample calculation for typical set of design parameters for multi-crystalline SPV module Time (hrs) Is (W/m 2 ) v (m/sec.) T a (ºC) T cell (ºC) P max (W) V oc (V) I sc (A) V m (V) I m (A) FF η pce (%) η(%) 9:AM :1AM :2AM :3AM :4 AM :5 AM : AM :1 AM :2 AM :3 AM :4 AM :5 AM : AM :1 AM :2 AM :3 AM :4 AM :5 AM : PM :1 PM :2 PM :3 PM :4 PM :5 PM : PM

26 1:1 PM :2 PM :3 PM :4 PM :5 PM : PM :1 PM :2 PM :3 PM :4 PM :5 PM : PM :1 PM :2 PM :3 PM :4 PM :5 PM : PM :1 PM :2 PM :3 PM :4 PM :5 PM : PM Cont

27 The energy, power conversion and exergy efficiencies and the solar radiation for multi-crystalline module are plotted against time for the months of April and May as can be seen in Figs and 4.16 respectively. It is seen from these figures that the energy and power conversion efficiencies are almost constant in nature throughout the day which is due to the fact that the fluctuations in solar radiation are lesser as compared to the months of January and February. However, the exergy efficiency fluctuates due to the fact that the fluctuation in the wind speed and increasing of module temperature during the day time which has been explained in the previous section also η(%) Is (W/m2) 22 9:AM 11: AM 1: PM 3: PM 5: PM Fig. 4.15: Variation of different efficiencies and solar radiation against time for multi-crystalline SPV module during the month of April. Again the energy and power conversion efficiencies for the month April are found to be higher than those of the month of May, however, the exergy efficiency for the month of May is found to be higher than that of month of April so as the solar radiation. Also the fluctuation in the exergy efficiency as well as the energy efficiency 13

28 during 1:3-3: hrs are found to be more as compared during 11:3-1:3 hrs during the month of may which can be explained in terms of ambient air temperature, module temperature and wind speed as given earlier. The average energy, power conversion and exergy efficiencies are found to be 17.7 %, % and 1.2 % respectively, for the month of April, while they are found to be %, % and 1.13 % respectively, for the month of May η(%) Is (W/m2) 9:AM 11: AM 1: PM 3: PM 5: PM Fig. 4.16: Variation of different efficiencies and solar radiation against time for multi-crystalline SPV module during the month of May. The variation of efficiencies and solar radiation against time for multicrystalline SPV module during the months of June and July are shown in Figs.4.17 and 4.18 respectively. From these figures it is observed that there is a sharp dip in all the three efficiencies at 11.2 AM for the month of June and at around 9.3 AM for the month of July and again at around 3. PM. This dip in the efficiencies are due to the sharp fall in the solar radiation at that particular instant therefore, the output to input ratios decrease sharply, as a result we got the lower instantaneous efficiencies 131

29 Efficiencies (%) as can be seen from Figs The average energy, power conversion and exergy efficiencies are found to be 16.22%, 11.31% and 1.7% respectively, for the month of June while, they are found to be 16.3%, 11.12% and 9.3% respectively, for the month of July η(%) Is (W/m2) :AM 11: AM 1: PM 3: PM 5: PM Fig. 4.17: Variation of different efficiencies and solar radiation against time for multicrystalline SPV module during the month of June η(%) Is (W/m2) 9:AM 11: AM 1: PM 3: PM 5: PM Fig. 4.18: Variation of different efficiencies and solar radiation against time for multi-crystalline SPV module during the month of July. 132

30 The solar radiation, energy and power conversion efficiencies are plotted against time in Figs and 4.2 for the multi-crystalline SPV module during the month of August and September, respectively. It is seen from these figures that all the efficiencies for the month of August is fluctuating very frequently which is due to the fact that the corresponding solar radiation also fluctuates during this particular month which is an obvious case in this part of country. The average energy, power conversion and exergy efficiencies are found to be 17.41%, 12.% and 9.68% respectively, for the month of August. Since, the solar intensity is much higher, days are more shining and the variation in wind speed is lesser during the month of September as a result, the better performance was observed with lesser fluctuations as can be seen from Fig Also the average energy, power conversion and exergy efficiencies are found to be 17.8%, 12.6% and 1.91% respectively, for the month of September which is higher than the month of August, mentioned above η(%) Is (W/m2) 9:AM 11: AM 1: PM 3: PM 5: PM Fig.4.19: Variation of different efficiencies and solar radiation against time for multicrystalline SPV module during the month of August. 133

31 From the above results it is concluded that the all the efficiencies in the month of September is higher than that of month August however, it is also seen that there is not much variation in energy and power conversion and exergy efficiencies. This is due to the fact that during the month of September, the insolation is high, the day is clear with higher sun shining and at the same time the wind speed is not varying much therefore, the losses are also less and hence, we got the better in the month of September than those for the month of August η(%) Is (W/m2) 245 9:AM 11: AM 1: PM 3: PM 5: PM Fig.4.2: Variation of different efficiencies and solar radiation against time for multi-crystalline SPV module during the month of September. The energy, power conversion and exergy efficiencies and solar radiation against time are shown in Figs respectively, for the months of October and December. From these figures, it is found that during these two months the intensity of solar radiation is much better and the fluctuations are very less as compared to other months of the year as can be seen from these figures, mentioned 134

32 above. Therefore, the efficiencies are found to be in better range with lesser fluctuations especially, during the morning and evening times. However, due to higher solar intensity, the module temperature rises and hence, the losses also enhances, this has been explained earlier, as a result, all the efficiencies especially, the exergy efficiency goes down significantly during the noon hours as can be seen from these figures η(%) Is (W/m2) 9:AM 11: AM 1: PM 3: PM 5: PM Fig. 4.21: Variation of different efficiencies and solar radiation against time for multi-crystalline SPV module during the month of October. Also the average energy, power conversion and exergy efficiencies are found to be 18.9%, 12.26% and 11.17% respectively, for the month of October while they are found to be 15.15%, 11.3% and 1.5% respectively, for the month of December. Also all the efficiencies are found to be less fluctuation and in higher range for the month of October as compared to those for the month of December, which can be explained in terms of solar radiation, module and ambient air temperature as explained earlier. 135

33 η(%) Is (W/m2) :AM 11: AM 1: PM 3: PM 5: PM Fig. 4.22: Variation of different efficiencies and solar radiation against time for multi-crystalline SPV module during the month of December Hetro-junction with Intrinsic Thin Layer (HIT) SPV Module The variation in energy, power conversion, exergy efficiencies and solar radiation against time of HIT SPV module for the month of January and February respectively has been illustrated in Figs and 4.24 at a typical set of operating and designed conditions as mentioned above. From Fig it is observed that the solar radiation increases initially, attains its peak near the middle of the day and goes down sharply towards the end of the day, while fluctuating throughout the entire day which is an obvious case in practice. It has also been found that all the efficiencies such as energy, power conversion and exergy fluctuate with time which is due to the intermittent nature of solar radiation which also fluctuates with time as mentioned above. Also all the efficiencies are fluctuating in nature which means that a small change in radiation causes a sharp change in efficiencies which may be explained in the terms of temperature of module in the morning and evening times. It is also observed from Fig that all the three efficiencies are initially high and increases 136

34 as the time increases and then decreases slowly then remains almost constant for over a long time period i.e. approximately between 1 AM to 4 PM, and again increases. This variation in efficiencies is due to variation in solar radiation which is also given in the figure and variation in temperature of module throughout the day η Is (W/m2) 25 9:AM 11: AM 1: PM 3: PM 5: PM Fig Variation in efficiencies and solar radiation against time for HIT SPV module during the month of January It can also be observed from the Fig that all the three efficiencies are high in the morning and evening time as compared to noon time which is due to the fact that during the morning time the module temperature is lower and as the time increases temperature of the module also increases and finally during the evening time the temperature of the module goes down as compared to that of the noon time. As the temperature of the module increases the voltage decreases due to negative temperature coefficient of module while the current increases but not in the ratio of voltage so the product of voltage and current i.e. output energy decreases. 137

35 η 23 Is (W/m2) 9:AM 11: AM 1: PM 3: PM 5: PM Fig Variation in efficiencies and solar radiation w.r.t. time of HIT SPV module in the month of February In other words, the product of voltage and current is high In the morning and evening time as compared to noon time and hence, the output to input ratio i.e. efficiencies are high in the morning and evening time as compared to the noon time. The exergy efficiency fluctuates more frequently than that of energy efficiency which is due to the variation in the wind speed throughout the day because exergy efficiency is strongly dependent on the wind speed as well as the ambient and module temperatures. The energy efficiency is found to be always higher than that of the exergy efficiency which has been explained above. The average energy, power conversion and exergy efficiencies are found to be %, % and % respectively, during the month of January. The sample calculation for the month of February has been given in Table 4.4 as below: 138

36 Table 4.4: Sample calculation for typical set of design parameters for HIT SPV module Time (hrs) Is (W/m 2 ) v (m/sec.) T a (ºC) T cell (ºC) P max (W) V oc (V) I sc (A) V m (V) I m (A) FF η pce (%) η(%) 9:AM :1AM :2AM :3AM :4 AM :5 AM : AM :1 AM :2 AM :3 AM :4 AM :5 AM : AM :1 AM :2 AM :3 AM :4 AM :5 AM : PM :1 PM :2 PM :3 PM :4 PM :5 PM : PM Cont... 1:1 PM

37 1:2 PM :3 PM :4 PM :5 PM : PM :1 PM :2 PM :3 PM :4 PM :5 PM : PM :1 PM :2 PM :3 PM :4 PM :5 PM : PM :1 PM :2 PM :3 PM :4 PM :5 PM : PM

38 Fig shows almost the same pattern of variation in all the performance parameters i.e. energy, power conversion and exergy efficiencies and solar radiation against time as that of Fig However, Fig shows that the average exergetic efficiency is higher than that of power conversion or actual efficiency which is reverse in the case of crystalline technology based modules. The higher exergy efficiency than that of power conversion efficiency for HIT based modules shows that the losses are less as compared to that of the crystalline based technology due to its internal structure which combination of both amorphous silicon (a-si) and crystalline silicon (c-si). It is also found that the fluctuation in all the three efficiencies for the month of February are less than that of month of January which is due to the variation in solar radiation, ambient air temperature and wind speed etc. All the efficiencies have been found to be high in the morning and evening time as compared to noon time same as in the month of January but all the efficiencies in the morning time are higher than that of evening time. Also the average energy, power conversion and exergy efficiencies for the month of February are found to be %, 18.3 % and % respectively. Also found that all the efficiencies for the month of February are found to be higher than those of month of January. The energy, power conversion and exergy efficiencies and the solar radiation for HIT module are plotted against time for the months of April and May as can be seen in Figs.4.25 and 4.26 respectively. From Fig it is seen that the energy and power conversion efficiencies are almost constant in nature throughout the day this is due to the fact that the fluctuation in solar radiation is not much. However the exergy efficiency fluctuates due to the reason as explained above. The average energy, power conversion and exergy efficiencies are found to be 2.52 %, 16.4 % and % respectively for the month of April. 141

39 Efficiencies (%) η(%) Is (W/m2) :AM 11: AM 1: PM 3: PM 5: PM Fig Variation in efficiencies and solar radiation against time for HIT SPV module during the month of April η(%) Is (W/m2) :AM 11: AM 1: PM 3: PM 5: PM Fig Variation in efficiencies and solar radiation against time for HIT SPV module during the month of May Again Fig shows almost the same pattern of variation of all performance parameters as that of figures which can be explained in similar way as mentioned above. The average energy, power conversion and exergy efficiencies 142