Identification of Critical Parameters and Design Aspects of a Silicon Solar Cell
Silicon-based PV semiconductor solar cells have been used to demonstrate the most practical and reliable application of the photovoltaic effect. A simple, rugged semiconductor junction can be produced from single-crystal silicon. Low resistance contacts are added to tap the electrical energy produced when the cell is exposed
to sunlight. Approximately a DC voltage of 0.45 volts is generated across each cell regardless of the dimensions for this particular cell architecture. The DC current and thus the power available are strictly dependent on the cell area exposed to the sun and the absorption capability of the silicon wafer, which is located between the two contacts, as illustrated in Figure 1.3. It is important to mention that the higher the absorption capability of the semiconductor material, the higher the PV voltage will be across the cell terminals. In case of a material with weak absorption capability like silicon, most carriers are generated near the surface. Based on the preliminary calculations, one can expect DC output power ranging from roughly 250 mW from a 57 mm cell to about 1000 mW or 1 kW/m2 from a 100 mm cell under standard conditions. Standard conditions are defined by NASA as 100 mW/ cm2 solar intensity (I) with cell temperature of 28掳C at sea level. Higher voltage is
possible by connecting the cells in series, while higher output power is possible by connecting the cells in parallel. Solar cells can be installed on glass-filled polyester substrate. The net cell output is the product of solar intensity (I = 100 mW/cm2) and the conversion efficiency of the device, which is typically now about 16 percent for a silicon cell. A solar module may contain several cells connected in series and parallel using the most advanced interconnect and encapsulation techniques. The module design must offer the cost-effective approach to meet specific solar output power requirements with no compromise in reliability. These modules can be mounted on solar panels in series and parallel configuration to meet specific power generating capability. Solar cells must be packaged in a variety of modules to optimize the electrical performance of the solar power system best suited for a specific application. It is important to mention that basic solar cells are available with diameters of 57, 90, and 100 mm. Furthermore, half cells, quarter cells, or other configurations can be used where a specific application is warranted. Modules could use different packaging and encapsulating materials to achieve maximum economy and to meet different environmental requirements. For many applications, silicone is used to hermetically seal and to protect the cell integrity under harsh operating conditions. Polycarbonate or glass cases must be used to meet high impact resistance and maximum protection under severe mechanical conditions. All modules should be designed or constructed with minimum onsite maintenance and with self-cleaning capability for optimum shelf life. Other accessories such as voltage regulators, inverters, and mounting hardware are required to complete the solar generating system.
As mentioned earlier, a large number of solar cells and modules will be required to meet specific output power requirements. The base of the panel provides the mechanical integrity, while the glass cover offers maximum protection from environmental factors such dust, rain, wind, humidity, and suspended foreign particles in the atmosphere. This cover must provide the maximum transmission to the solar radiation, but with minimum reflection and minimum absorption losses. Typical panels commercially available have a length of 48 inches, width of 18 inches, and depth not exceeding 2 inches, and the assembled panel weighs less than 20 pounds. Computer analysis is necessary to achieve the most cost-effective panel design comprising the solar modules, also known as solar arrays, and the inverter. The analysis must specify the proper angle of tilt capable of optimizing both the location and solar system performance. Panel configuration, location, and installation requirements will be discussed in greater detail in a separate chapter.