We now apply the model presented above to some actual data (Joshi et al., 2008) sets as obtained through experiments in New Delhi, India, which is located at
77°12' E longitude and 28°35' N latitude. The test was performed from 9:00 a.m. to 4:00 p.m. on March 27, 2006, and the data measured included total solar irradiation, voltage, open-circuit voltage, current, short-circuit current, cell temperature, ambient temperature, and velocity of the air just above the photovoltaic surface. The data for hourly total solar radiation and the wind velocity are measured for different places on the photovoltaic surface and an average value for both is used to calculate energy and exergy of the photovoltaic system. The uncertainty analysis of measured global radiation is done and the internal estimate of uncertainty is evaluated following (Joshi, 2006) and it is found that the value for uncertainty for the measured global radiation is 2.23% (Joshi, 2006). The system includes two modules in series, and the area of one solar cell is 0.0139 m2. Number of solar cells in the two modules was 72. Therefore, the efficiency analysis of a PV system for its performance assessment is done here based on some experimental data as explained above.
Exergy Efficiency (PV/T) Exergy Efficiency (PV) Energy Efficiency
Fig. 3.4 Energy and exergy efficiencies of PV and PV/T systems.
Using Eqs. (3.1-3.6) and experimental data from Joshi et al. (2008), energy and exergy efficiencies are calculated and shown in Fig. 3.4. It is clear from the figure that the energy efficiency (33-45%) is higher than that of the exergy efficiency (11-16%) of PV/T system and (7.8-13.8%) of PV system. Maximum exergy efficiency for PV/T (16%) and PV(13.8%) can also be seen at 4 p.m whereas a minimum exergy efficiency for PV/T (11%) and PV (7.8%) is at 12 p.m. In the present study, natural air is used to derive the heat from photovoltaic surface. However, if air is supplied beneath the photovoltaic surface by a forced mode, e.g., by putting a fan beneath the photovoltaic panel, as done by Joshi (2006), more thermal energy can be removed in a better as well as convenient way. In that case a higher energy and exergy efficiency can be achieved.
Fig. 3.5 Comparison of exergy efficiency of PV and PV/T systems.
Figure 3.5 shows the comparison of exergy efficiencies of both PV and PV/T systems. Comparing both the curves one can see that the exergy efficiency of PV/T is on an average 20% more than that of PV. Carbon dioxide, a major greenhouse gas, is responsible for the global warming hence there is a need to understand the ways by which we can reduce the greenhouse gas emission. One solution to this problem can be adopting non-conventional energy sources wherever applicable, for example, for water heating one can use solar energy which is more eco-friendly as compared to using an electrical water heater that runs on electricity produced by conventional sources. Another example could be solar pumping, farmers irrigate their field in day time and they can use solar water pumping instead of using an oil-based generator to produce electricity and use it to run the water pump. The non-conventional energy sources often called as renewable energy sources are environmentally benign as they emit less greenhouse gases into the atmosphere as compared to conventional ones. Though the non-renewable or conventional sources of energy like coal, oil, and natural gas are more economical than the renewable sources, they pollute the environment at a much faster rate. Coal-based electricity generation causes highest greenhouse gas emission amongst all conventional and non-conventional sources during the operation and the installation of the power plant.
In renewable energy sources solar energy sometimes is more attractive than the others as it can be used not only for small-scale applications like stand-alone systems but also for large-scale applications like grid connections. The hybrid technology can be a good alternative to reduce the greenhouse gas emissions and improve the electricity generation by coupling two technologies. This not only can give better reliability and sustainability of the system performance but also can help reduce the greenhouse gas emission into the atmosphere. Solar photovoltaic can be beneficial not only for electricity generation but also for thermal applications as it can be used for a variety of applications ranging from stand-alone systems to grid-connected systems.
The sustainability index for the PV system has been calculated using Eqs. (3.7-3.9) and is shown in Fig. 3.6. A theoretical calculation is also done to show the variation of the sustainability index with exergy efficiency and shown in the same figure. The exergy efficiency of the PV system varies between 7.8 and 12.5% for the typical day of March 27, 2006, at New Delhi and the sustainability index (SI) varies between 1.08 and 1.14. It is clear from the graph that sustainabil-ity index (SI) increases with increase in exergy efficiency.
Sustainability index for PV/T system is also calculated and it varies between 1.13 and 1.19 for exergy efficiency ranging from 11.3 to 15.4%, respectively.
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