Photovoltaic (PV) Cells
A solar cell, or photovoltaic (PV) cell, is a device that converts light into electric current using the photoelectric effect. Currently, there are 14 types of photovoltaic cells, the most prevalent being thin film, monocrystalline silicon, polycrystalline silicon and amorphous.
Concentrating photovoltaics (CPVs) are a newer method for generating electricity from the sun. CPV systems employ sunlight concentrated onto photovoltaic surfaces for the purpose of electrical power production. Solar concentrators of any variety may be used and these are often mounted on a solar tracker in order to keep the focal point upon the cell as the Sun moves across the sky.
Concentrating Solar Power
Concentrating Solar Power (CSP) systems use lenses or mirrors, and tracking systems to focus a large area of sunlight into a small beam. The concentrated heat is then used as a heat source for a conventional power plant.
A number concentrating technologies currently exist, however, the most developed are the parabolic trough, the concentrating linear fresnel reflector, the Stirling dish, and the solar power tower. While technology uses its own unique methods to track the Sun and focus light, the basis for each system it is ability to heat fluid via concentrated sunlight, and then us this fluid to generate power or store energy.
Solar energy is not available at night, making energy storage an important issue in order to provide the continuous availability of energy. With solar, all available power output must be captured when it is available and either stored for when it can be used, or transported to where it can be used.
Solar energy can be stored at high temperatures using molten salts. Salts are an effective storage medium because they are low-cost, have a high specific heat capacity and can deliver heat at temperatures compatible with conventional power systems.
Off-grid PV systems traditionally have used rechargeable batteries to store excess electricity. With grid-tied PV systems, excess electricity can be sent to the grid once it is captured. Net metering programs, set up by the electric company and other entities give grid-based PV systems credit for the electricity they deliver to the grid. This credit offsets the electricity provided from the grid when the PV system is unable to meet demand. This is the equivalent of effectively using the grid as a storage mechanism. Credits are typically rolled over each month, and any remaining surplus settled annually.