PV technology has come through leaps and bound in the last two decades. PV Installations have cropped up exponentially and the technology has been instrumental in electrifying far corners of the earth.
It is one of the most reliable methods of producing electricity and now is becoming one of the cheapest.
However one of the drawbacks of PV technology is its drop in performance with rising temperatures. It has to be understood that just like all other electrical /electronic devices, the internal (Ohmic) resistance inside PV , increases with increase in temperature. There is a general thumb rule among engineers, that suggests that for every 10 degree rise in temperature, the resistance doubles. Similar is the case in PV. A drop of 0.1 % efficiency per degree centigrade rise in temperature is common in silicon cells. Other types of PV panels such as thin-film panels are less sensitive to heat. Values of 0.04%/ºC have been reported for thin-film panels.
This drop in efficiency can be noticed in the chart (Right)
To elaborate the point further, let us consider that a PV panel is being exposed to solar irradiance of 1000 W/m2. Let’s assume that the panel area is 1 meter square. If the cell efficiency is 20% than almost 200Watts would be converted into electricity. Some portion would be reflected. The reflected portion depends upon the reflectivity of glass cover. This is normally quite low, 10% at most (low iron glass). Meaning 100 Watts would be lost in reflection. This means that almost 700 Watts will be absorbed by the panel that would be converted into heat. This is a huge portion of energy that gets not only wasted but can also lower the efficiency of the panels (because of raised temperature). Furthermore the life of the panel is also reduced. Note that researchers have found that effective PV life can be extended to almost 50 years if the temperature is controlled and high temperature is avoided.
Even in concentrated multi-junction PV systems, the heat has to be continuously removed.
In high latitude colder climate countries, high PV temperatures are rarely a problem. This is because of lower ambient temperatures. For instance in Scotland, even during peak summers, the temperature rarely crosses 23 ºC .
However in hotter climates PV temperatures can reach exceptionally high. For example in Kufra, Libya, PV temperatures in access of 70 C have been reported. Cooling in these conditions becomes necessary. Find below some of the effective methods of cooling PV systems.
This type of cooling relies of forced convection through wind. If the climate is windy, than light breeze is enough to wick the heat away. If PV systems are mounted on frames than leaving at least 1 inch space (25 mm) between the back of the panel and mounting surface is recommended. This allows air to flow freely behind the panels not only heat cab be removed from the front but also from the back of the panel. Therefore PV panels systems should not be added flush against the roof. For harsher climates, fins on the backside of the panel can be added that act as heat sink.
More robust active cooling systems can be created using water jackets on which the panels are mounted. These cooling jackets are nothing but thin metallic shells with coolant inside and exits in the diagonal direction. The cooling jackets require pumping systems which can consume energy. However, the amount of energy invested can be significantly less compared to the dividends gained because of higher efficiency of panels. Before investing in cooling jackets, one should ensure that in a particular conditions they would be feasible or not. Pumping requirements are not extremely huge and can make up only 1% of the total system output.
Peltier-PV in Tandem
Peltier device is a thermo-electric device that can be used for heating or cooling. The device just like PV is essentially solid state diode. It releases heat from one face while absorbs heat from the other face if electricity is applied across it. Research has been carried out in which Peltier cool (heat absorbing ) side has been attached to the back side of a PV panel, showing promising results in extending PV life.
Just like the cooling jacket, where a portion of PV electricity went to the pump, in this system, a portion of electricity goes towards Peltier device. It has to be noted that Peltier device are not cheap and the system cost can double if this system is used.
Phase Change Material Pouch
Plastic cases containing Phase Change Material are used to cool batteries and other electronic equipment in extremely hot climates. These packs absorb heat during the scorching day and release them during night. PCM pouches similarly are available that are cool to touch. In case of excessive heat build up, these pouches can absorb heat from the backside of the panel. They are a relatively cheaper and robust solution that does not require engineering design consideration.
Through the above mentioned methods PV systems can be cooled in hotter climates. There have been some work carried out in placing a thin-film of water instead of glass-cover which would cool the panel.
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