Picture provided by SolarFitUK.com
Conversion of Solar Energy to Electrical Energy
The scientific principle upon which a solar cell operates is the photovoltaic effect. When light impacts the surface of a material, some electrons become "excited" by the light and receive enough energy to move from the valence band into the conduction band. Once in the conduction band, the electrons are capable of moving through the material and thus carrying an electrical current. Most commonly, silicon (a semiconducting material) is used in the construction of solar cells.
In our attempt to improve the efficiency of the solar cell given to us, we examined two basic concepts: a mirror booster system and a cooling system. We determined that the most effective use of our time and resources would be to construct only a mirror booster system for our solar cell.
In our attempt to improve the efficiency of the solar cell given to us, we examined two basic concepts: a mirror booster system and a cooling system. We determined that the most effective use of our time and resources would be to construct only a mirror booster system for our solar cell.
Calculating the Efficiency of a Solar Cell
In order to find the efficiency of a solar cell, a number of quantities in the following equations need to be determined:
efficiency = (FF*Isc*Voc) / Pin = (Im*Vm) / Pin = Pmax / Pin
FF = (Im*Vm) / (Isc*Voc) = Pmax / (Isc*Voc)
where Im = max current, Vm = max voltage, Pmax = max power generated by the solar cell, Pin = total power input, FF = fill factor, Isc = short circuit current, and Voc = open circuit voltage.
Four quantities (Isc, Voc, Im, Vm) can be derived from an I-V (current-voltage) curve constructed from data when the solar cell is illuminated. To do this, it is necessary to have a variable power supply, computer data logging software, a decade resistor, and some other equipment. Pin is the power of the sunlight that is incident on the solar cell and can be roughly estimated by using a photo detector.
We elected to not carry out these calculations since we were lacking access to the necessary equipment to calculate such data. Instead, we had a much simpler and equally useful calculations at hand:
relative efficiency = [ (Vdata - Vbaseline) / (Vbaseline) ] * 100
where Vdata represents any data value obtained when using the mirror booster and Vbaseline represents any data obtained without the mirror booster. With this equation, we are able to determine on a relative scale if the solar cell becomes more or less efficient when the mirror booster system is employed.
efficiency = (FF*Isc*Voc) / Pin = (Im*Vm) / Pin = Pmax / Pin
FF = (Im*Vm) / (Isc*Voc) = Pmax / (Isc*Voc)
where Im = max current, Vm = max voltage, Pmax = max power generated by the solar cell, Pin = total power input, FF = fill factor, Isc = short circuit current, and Voc = open circuit voltage.
Four quantities (Isc, Voc, Im, Vm) can be derived from an I-V (current-voltage) curve constructed from data when the solar cell is illuminated. To do this, it is necessary to have a variable power supply, computer data logging software, a decade resistor, and some other equipment. Pin is the power of the sunlight that is incident on the solar cell and can be roughly estimated by using a photo detector.
We elected to not carry out these calculations since we were lacking access to the necessary equipment to calculate such data. Instead, we had a much simpler and equally useful calculations at hand:
relative efficiency = [ (Vdata - Vbaseline) / (Vbaseline) ] * 100
where Vdata represents any data value obtained when using the mirror booster and Vbaseline represents any data obtained without the mirror booster. With this equation, we are able to determine on a relative scale if the solar cell becomes more or less efficient when the mirror booster system is employed.
Mirror Booster System
Since the amount of incident sunlight is the driving force for the amount of electrical current produced by the solar cell, we looked for methods of maximizing the amount of incident sunlight. We hypothesized that by placing a mirror near the solar panel, the mirror will reflect
additional solar radiation onto the solar cell throughout the course of the
day and consequently the efficiency of the solar
panel will increase. In the course of our research, we found that mirror systems have been known to increase the efficiency of
solar panels by up to thirty percent.
Certain incident angle of the mirror worked better than other at different times of the day. This is because of the Earth's rotation creates a different angle between the horizon and the sun at different times of the day. Therefore, the most effective design for a mirror booster system would incorporate a means of varying the mirror's angle. The most sophisticated way to do this would be to use sensors that track the position of the sun that control motors that move the mirror accordingly. However, if such a system is considered too expensive or impractical for a particular application, a few measurements throughout the day would provide results as to the most efficient angle of the mirror, which would depend on the angle of the solar panel and geographic location.
There is a drawback of using the mirror booster system. Although it normally is not an issue with small solar cell systems or in temperate climates, a mirror booster system could cause too much sunlight to be absorbed by the photovoltaic cell, which in turn could cause overheating. In order to prevent such an event from occurring, a cooling system could be implemented to regulate cell temperature.
Certain incident angle of the mirror worked better than other at different times of the day. This is because of the Earth's rotation creates a different angle between the horizon and the sun at different times of the day. Therefore, the most effective design for a mirror booster system would incorporate a means of varying the mirror's angle. The most sophisticated way to do this would be to use sensors that track the position of the sun that control motors that move the mirror accordingly. However, if such a system is considered too expensive or impractical for a particular application, a few measurements throughout the day would provide results as to the most efficient angle of the mirror, which would depend on the angle of the solar panel and geographic location.
There is a drawback of using the mirror booster system. Although it normally is not an issue with small solar cell systems or in temperate climates, a mirror booster system could cause too much sunlight to be absorbed by the photovoltaic cell, which in turn could cause overheating. In order to prevent such an event from occurring, a cooling system could be implemented to regulate cell temperature.
Cooling System
The material properties of photovoltaics show
a trend of increasing efficiency with decreasing operating temperature. This is due to the fact that as a
solar panel heats up due to the incoming sunlight, the conductivity of the semiconductor inside the solar
cell increases. As a result, charges separate across the solar cell causing the operating
voltage and transferred current to decrease. Experiments have
shown that once a solar panel reaches 42 degrees Celsius, a 1.1% drop in efficiency occurs for every additional
degree Celsius. Therefore, it is beneficial to keep the operating temperature of the solar cell as low as possible.
This is why, in our original concept, we examined the possibility of constructing and adding a cooling system to the solar panel in order to decrease solar cell's temperature and thus increase its overall efficiency. We investigated three possible cooling systems: a liquid based system and two different air based systems.
In a liquid based system, a working fluid would draw heat away from the solar cell by coming into contact with it. The liquid would then be pumped away to dissipate the heat into the environment, and then be returned back towards the solar cell to repeat the process. We determined this method to be too time consuming, complex, and that the risk of damaging the solar cell due to potential leakage was too high.
In the air based systems, we contemplated either using fins or small, computer heat sink fans. The concept behind the fins was that by increasing the surface area of the solar cell, it would be easier for heat to dissipate since there would a great amount of area in contact with the air. We were unable to think of a feasible way in which to employ such fins without blocking any of the photovoltaic components of the solar cell. The idea of using the small fans was to create a continuous flow of air over the top of the solar cell that would would draw heat away from the solar cell faster. Though the principles behind these systems have merit, the practicality of our situation insisted that we ignore the option of air based cooling systems. Since we are gathering our experimental data in Champaign, Illinois during the late fall months, we concluded that since the outside temperature is typically low, the ambient air would be roughly equivalent of constructing and implementing a cooling system. Thus, we believed we would have great difficulty in identifying the benefits of a cooling system in our data.
This is why, in our original concept, we examined the possibility of constructing and adding a cooling system to the solar panel in order to decrease solar cell's temperature and thus increase its overall efficiency. We investigated three possible cooling systems: a liquid based system and two different air based systems.
In a liquid based system, a working fluid would draw heat away from the solar cell by coming into contact with it. The liquid would then be pumped away to dissipate the heat into the environment, and then be returned back towards the solar cell to repeat the process. We determined this method to be too time consuming, complex, and that the risk of damaging the solar cell due to potential leakage was too high.
In the air based systems, we contemplated either using fins or small, computer heat sink fans. The concept behind the fins was that by increasing the surface area of the solar cell, it would be easier for heat to dissipate since there would a great amount of area in contact with the air. We were unable to think of a feasible way in which to employ such fins without blocking any of the photovoltaic components of the solar cell. The idea of using the small fans was to create a continuous flow of air over the top of the solar cell that would would draw heat away from the solar cell faster. Though the principles behind these systems have merit, the practicality of our situation insisted that we ignore the option of air based cooling systems. Since we are gathering our experimental data in Champaign, Illinois during the late fall months, we concluded that since the outside temperature is typically low, the ambient air would be roughly equivalent of constructing and implementing a cooling system. Thus, we believed we would have great difficulty in identifying the benefits of a cooling system in our data.