From n-type to p-type and monocrystalline to monocrystalline, there are many different kinds of solar panels and each type of solar panel responds differently to various amounts of light intensity.
While solar panels are often tested using a standardized level of irradiation, the outdoor application of solar panels never involves a consistent light level. Outdoor-installed solar panels are often in low-light conditions and research has shown the performance of solar panels in these conditions is a primary driver of variation in a photovoltaic system.
Therefore, the performance of various types of solar panels under low-light conditions is an important differentiator. Furthermore, there are also solar panels designed to work under high-intensity lighting conditions.
Solar Panel Behaviour as Light Decreases
Generally speaking, current from a solar panel decreases linearly with decreasing irradiance, while the voltage drops logarithmically. However, there is significant variation among various types of solar panel with respect to these declines.
The low-light functionality of a solar cell is primarily reliant on the shunt resistance and series resistance of the cells, which are the resistance related to contacts at the top and the bottom of the cell and the resistance related to the current that circulated the emitter.
At low light levels, the impact of shunt resistance becomes increasingly relevant. As intensity decreases, the bias point and current also decrease, with the equivalent resistance of the solar cell starting to approach the shunt resistance. When these two resistances are nearly equal, the fraction of the overall current flowing through the shunt resistance rises, in so doing, it increases the fractional power loss as a result of shunt resistance. Therefore, under cloudy conditions, a solar cell with a high shunt resistance keeps a greater portion of its original power compared to solar cell with a low shunt resistance.
Low light behaviour may differ based on cell type and from manufacturer to manufacturer, even on the same kind of cell. Experts have reports as much as 10 percent difference in yearly energy yields. Clearly, the challenge for a manufacturer is to ensure consistency, generally through the proper usage of materials.
Concentrator Solar Cells
The light intensity on a solar cell is measured in units known as ‘suns’, where 1 sun relates to standard illumination at AM1.5, or 1 kW/m2. A concentrator is a solar cell intended to function under illumination more than 1 sun.
Under this type of system, sunlight is focused or directed by optical elements so that a high-strength light beam shines on a small solar cell. Concentrators have multiple positive aspects, including a greater efficiency potential than a one-sun solar cell and the potential for lower cost. The short-circuit current from a solar cell is related linearly to light intensity, so that a device functioning under 10 suns would have 10 times the short-circuit current as it would under one sun. However, this effect does not supply a boost in efficiency because the incident power also rises linearly with concentration. Rather, increased efficiency comes from the logarithmic dependence of the open-circuit voltage on short circuit. As a result, under concentration, voltage rises logarithmically with light intensity.
The price tag on a concentrating photovoltaic system may be less than that for a corresponding flat-plate system because only a small area of solar cells is required. However, the efficiency benefits of concentration may be decreased by higher losses in series resistance. Series resistance has a greater impact on performance at high intensity, and, as previously noted, shunt resistance has a greater impact on cell performance in low light.