Modules
This solar-powered water-pumping station is on the Martin Ketterling Ranch north of Heil, North Dakota. This system has two Solarjack 50-watt flat-plate panels and a Solarjack water pump and controller. Because the water tanks are relatively small, two Interstate marine batteries provide backup on cloudy days and at night. The system pumps about 4 gallons per minute, enough to water about 40 head of cattle.
Let's look at a typical crystalline silicon PV module. It consists of a transparent top surface, an encapsulant, a rear layer, and a frame around the outer edge. In most modules, the top surface is glass, the encapsulant is ethyl vinyl acetate (EVA), and the rear layer is Tedlar.
Front Surface Materials — The front surface of a PV module must have a high transmission in the wavelengths that can used by the solar cells in the PV module. For example, for silicon solar cells, the top surface must have high transmission of light having wavelengths in the 350 to 1200 nm range.
Also, the reflection from the front surface should be minimal. An antireflection coating to the top surface can greatly reduce the reflection of sunlight, and texturing of the surface can cause light that strikes the surface to stay within the cells. Unfortunately, these textured modules are not "self-cleaning," and the advantage of reduced reflection is usually outweighed by losses due to dust sticking to the surface.
The top surface should also be impervious to water, be able to resist damage from hail impact, be stable under long-term exposure to ultraviolet radiation, and have low thermal resistivity. If water as liquid or a vapor is able to get inside a PV module, it will cause corrosion of metal contacts and interconnects, which will greatly shorten the lifetime of the PV module. Also, the front surface often provides rigidity for the module.
Of several choices for a top surface material, a common choice is tempered, low-iron glass, which is low cost, strong, stable, highly transparent, impervious to water and gases, and has good self-cleaning properties.
Encapsulant — An encapsulant helps to hold together the top surface, PV cells, and rear surface of the PV module. The encapsulant must be stable at high temperatures and high levels of ultraviolet radiation. It must also be optically transparent and have a low thermal resistance. Ethyl vinyl acetate—or EVA—is the most commonly used encapsulant. Thin sheets of EVA are inserted between the solar cells and the top and rear surfaces. Heating this "sandwich" causes the EVA to polymerize, thus bonding the module into one piece.
Rear Surface — The material used as the rear surface of the PV module must have low thermal resistance and must prevent the ingress of water and gases. In many modules, the rear surface material is a thin polymer sheet, typically Tedlar.
Frame — A final structural component of the module is the frame, which is typically made of aluminum.
Flat-plate collectors, which typically contain a large number of solar cells mounted on a rigid, flat surface, can make use of both direct sunlight and the diffuse sunlight reflected from clouds, the ground, and nearby objects.
We add one final point. Because the amount of power produced by a single cell is relatively small, designers group solar cells together to form electrical modules that supply a more useful level of voltage, current, and power. Solar cells may be connected in series to produce higher voltages. This is accomplished by connecting the positive terminal of one cell to the negative terminal of the next cell. Cells may also be connected in parallel to produce more current. This is accomplished by connecting the positive terminal of the first cell to the positive terminal of the next cell, and the negative terminal of the first cell to the negative terminal of the second cell.