Solar cell efficiency

The efficiency of a solar cell depends on many factors. It is therefore possible that a single solar cells performance varies widely depending on its location. This presents the industry with a problem: how do you express the power of a solar cell? The solution was found in the wattpeak (Wp). The power of a solar cell is almost always expressed in this unit, which represents its efficiency under laboratory conditions. These conditions are set at a temperature of 25°C, a light travel distance of 1.5 air mass and a light intensity of 1 kw/m2. As discussed in de article on solar panels, this theoretical limit is almost never reached. Here, we discuss some of the most important effects influencing solar cell (and thus: solar panel) efficiency.

The sun’s intensity

The first factor is probably the most obvious. The brighter the sunlight, the more there is for the solar cell to convert. It is for this reason that a solar cell performs best during spring and summer; in fall and winter the sunlight is less intense and thus less able to ‘kick loose’ the electrons from their parent atoms (see solar cells). This mainly reduces the flow of current; the voltage is usually not that much affected. It is also due to this factor, that a solar cell will be able to deliver more energy in the sunnier areas. The map below is the so-called insolation map for the United States. It displays the average amount of kilowatthours received per day. Since a solar cell’s performance is measured at an intensity of 1 kw/m2, you can also read the insolation as the average amount of daily hours of sunshine. So, for example, from the diagram we can infer that a person in Florida receives from 5 to 6 hours of sunshine per day. Note that the definition of “one hour of sunshine” is chosen to match the laboratory conditions of the solar cell specifications (1 kW/m2).

PV solar radiation per square meter of solar panel per day

Temperature

Contrary to popular belief, the efficiency of a solar cell decreases with increasing temperature. The reason for this, is that a higher temperature increases the conductivity of the semiconductor. This balances out the charge within the material, reducing the magnitude of the electric field at the junction. This in turn inhibits charge separation, which lowers the voltage across the cell. It should be noted that a higher temperature increases the mobility of electrons, which causes the flow of current to increase slightly. This increase is however minor and insignificant compared to the decrease in voltage.

IVT-diagram of a solar cell
This figure displays the response of a solar cell to varying temperature. If you look carefully, you will see that the current increases slightly, whilst the voltage decreases rapidly. The result is a lower overall power yield (P=V*I).

The listed power of a solar cell is the power measured under ideal laboratory conditions, which prescribe a temperature of 25 °C (77 °F). However, on a typical hot summer day, it is not uncommon for a solar cell to reach a temperature of 70 °C (158 °F). A general rule of thumb is that the efficiency of a solar cell decreases with 0.5% for every 1 °C (1.8 °F) above 25 °C (77 °F). This means that on a hot summer day, the efficiency of a solar cell could drop as much as 25%. It is therefore extremely important to keep your solar panels well ventilated. Make sure the wind is able to cool on all sides, including the underside. Another very clever option might be to implement liquid cooling, using the heat captured by the liquid for household heating purposes.

Series resistance

When tying solar cells together, it is important to keep series resistance of the circuit to a minimum. Resistance directly influences both voltage and current, and an increasing resistance will cause the voltage-current curve of the solar cell to move away from the so-called maximum power point (MPP). At this point, a solar cell produces maximum output (through the equation P=V*I) and it is thus advantageous to maintain this point. Since the material in a solar cell acts as a resistor to current flow, it is often advisable to limit the amount of serially connected solar cells. By wiring individual ‘serial batches’ of solar cells in parallel, one can overcome this limitation. Pveducation has an excellent online series resistance calculator, which will graphically display this effect.

Effect of series resistance on the characteristic of the solar cell
This figure displays the effect of series resistance on a solar cell’s output voltage and current. Note that by increasing series resistance, the solar cell moves away from the maximum power point.

Shading

Since batches of solar cells are connected in series, the entire batch will operate at the current level of the weakest cell. By (partly) shading a single cell, one can thus adversely influence the output of all other cells! Note that the same goes for solar panels as a whole: since solar panels are generally wired in series, the (partly) shading of a single solar panel will adversely affect the entire array! It is often impossible to wire solar panels in parallel, since this will cause voltage compatibility problems with the inverter. Therefore, make sure your solar array is as little possible affected by shadows cast by trees, other buildings or other element of your solar array. When installing the system, remember that the angle of the sun changes throughout the day (and the year)!

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