Solar PV performance is affected by so many factors which include irradiance, Temperature, Solar PV quality, Humidity etc. The influences of temperature and irradiance variations on the different solar cell parameters are studied. It is useful to understand the effect of temperature and irradiance on the solar cell and module performance, in order to estimate their performance under various conditions. Conventional PV panels convert up to 20 percent of solar energy into electricity. Most of the energy that is not transformed into electricity is converted into heat, and because PV modules are semiconductor devices, they become less efficient as their temperatures rises.1
Like all other semiconductor devices, solar cells are sensitive to temperature. Increases in temperature reduce the band gap of a semiconductor, thereby effecting most of the semiconductor material parameters. The decrease in the band gap of a semiconductor with increasing temperature can be viewed as increasing the energy of the electrons in the material. Lower energy is therefore needed to break the bond. In the bond model of a semiconductor band gap, reduction in the bond energy also reduces the band gap. Therefore increasing the temperature reduces the band gap.2
So no doubt in the fact that increase of temperature causes degrading energy output from Solar PV thus temperature control is an important criteria to ensure an optimum operating condition is achieved. Several method has been used to control the temperature of Solar PV, so this study want to look at hybrid passive cooling as a temperature control method.
2.0 LITERATURE REVIEW
Increasing energy demand in recent year because of fast economic growth of the thickly populated nations increases the utilization of sustainable energy sources and energy conservation methods.3 Shortage of conventional energy sources and high energy cost has caused the alternate methods for cooling down the increasing temperature which cause degrading energy output from Solar PV.4
2.1 Operation of Photovoltaic Solar Cell
A typical solar cell is made of a p-n junction. This junction is formed by joining n-type material with a p-type material. At equilibrium, the junction has a band gap energy Eg that is equal to the difference between the lower edge of the conduction band and the upper edge of the valence band. Solar radiation enters the p-n junction as a beam of photons with energy E=?V.5 If this energy equals to or more than the semiconductor’s bandgap Eg, the photon is absorbed by the cell and an electron-hole pair is generated. This process of creating free electrons and holes carriers is referred to as photogeneration. The built in potential of the p-n junction causes the generated electrons to drift to the n-type region to wards the external contact. Similarly, the generated holes move to the p-type region towards the p-side external contact. When electrons and holes accumulate at their prospective external contacts, a potential difference appears between the two external metal contacts and current can flow if they’re
connected to an external load6
Fig 1: SCHEMATIC OF A SOLAR CELL 7
2.2 Thermal Managements of Solar Cells
Only a fraction of absorbed solar radiation energy is turned into useful output. The rest is converted to heat inside the cell, the heat need to be removed because of maximum efficiency of Solar PV. This heat is removed by two main approaches, Passive Cooling and Active Cooling8.
2.2.1 Passive Cooling
Passive cooling is the removal of heat by flowing medium like air or water. Most solar cells uses passive cooling because of its reliability and low cost. Examples of passive cooling include heat sinks, heat pipes, and microchannels. A Heat sink is a metal device that absorbs the heat from the solar cell and dissipates it into the surrounding air by natural or forced heat convection.9 Heat sinks are made of materials with high thermal conductivity like aluminum or copper. They can be shaped as a flat panel below a solar cells array with fin arrays in one side, or many sides if the design allows or using single solar cell. Heat sinks are optimized by changing the geometrical shape of the fins. Water can also be used to cool heat sinks. In this case, more heat exchange takes place due to the higher heat capacity of water10.Output power of a forced air cooling PV cell increased by 10% over natural air cooling and 66% with water cooling 11.
Heat pipes are another method of passively cooling solar cells. Heat pipes are metal pipes filled with small amount of water that transfers heat by evaporating and condensing in continuous cycles. They require no input power and can effectively transfer heat over a long distance. When heat is absorbed at one end of the pipe, it causes water to evaporate. The increasing vapor pressure results in a difference in pressure which drives the vapor to the other end of the heat pipe, the condenser section, which is placed in a cooler environment. The section between the hot and cold ends is the transfer section where two states of the liquid flow in opposite directions. Heat pipe cooling is a suitable option for solar cells under concentrated conditions due to its excellent thermal conductivity and constant temperature uniformity. 12.
Also, Present study and researches discusses performance of Earth Air Pipe (EAP) and Solar Chimney as passive ventilation and cooling systems as well as performance of some integrated passive cooling systems 13 Solar chimney mainly uses the technology which working on buoyancy principle, solar radiation passes through the glazed part of Solar Chimney part and walls of Solar Chimney gets heated. Inside air temperature of the Solar Chimney (SC) channel rises and if the temperature difference is high enough, then the stack effect drives the air from interior of the building and the exhaust air is replaced by fresh air this increases ventilation rate inside room and reduces inside temperature of the room and the heat is carried out through convective cooling principle 14.The Earth Air Pipe (EAP) heat exchanger usage geothermal energy for the cooling effect inside the building. Mechanism of heat exchangers basically controls the temperature of the system by adding or extracting heat.
Fig 2 Schematic diagram of a coupled system with EAP and Solar Chimney15
Thermal energy can be exchanged from one medium to another medium only by means of temperature difference without using any power source. It uses underground soil as heat source. Several researchers have studied the different parameters that affect the performance of Solar Chimney (SC). Computational Fluid Dynamics (CFD) analysis done by Chung et al. 16 shows optimum values of parameters which affects performance of Solar Chimney. Researcher found that optimum air width gap ranges from 0.6m to 1.0m, length of chimney varies from 1.5m to 2m and induced air speed from .04m/s to 0.22m/s. Previous experimental study shows that Ventilation rate increases by 24% and it also shows that at air gap 10cm when the angle increases from 15 to 45° 17. Alzaed et al. 18 shows by experimental study that air gap 5cm achieves better ventilation compared with 10cm air gap. Tongbai et al. 19 shows by CFD model that at 6° channel expansion, flow ventilation increases by 90% 20. Solar chimney gives better cooling performance when integrated with evaporative cooling cavity which maintains the air temperature of 27.31°C to 31.1°C and optimum evaporative cooling cavity length found 2m 21.