Solar Energy and Solar Power

Solar energy is the radiant light and heat from the Sun that has been harnessed by humans. Solar power is the generation of electricity using solar energy.

Solar Power Technology

Sunlight can be converted directly into electricity using photovoltaics, or indirectly with concentrating solar power, which normally focuses the sun's energy to boil water which is then used to provide power.

Solar Cells and Solar Panels

A solar cell, or photovoltaic cell (PV), is a device that converts light into electric current. Solar cells produce direct current electricity from light, which can be used to power equipment or to recharge a battery. Most solar cells available today require protection from the environment and are usually packaged tightly behind a glass sheet.

When more power is required than a single cell can deliver, cells are electrically connected together to form photovoltaic modules, or solar panels. A single module is enough to power an emergency telephone, but for a house or a power plant the modules must be arranged in multiples as arrays.

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 light is then used as a heat source for a conventional power plant or is concentrated onto photovoltaic surfaces.

Image: The PS10 solar power tower near Seville concentrates sunlight from a field of heliostats on a central tower.

Concentrating solar power systems are mainly divided into three categories- concentrating solar thermal, concentrating photovoltaics, and concentrating photovoltaics and thermal.

Concentrating Solar Thermal (CST) is used to produce renewable heat or electricity (generally, in the latter case, through steam). CST systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. The concentrated light is then used as heat or as a heat source for a conventional power plant (solar thermoelectricity). A wide range of concentrating technologies exist, including the parabolic trough, Dish Stirling, Concentrating Linear Fresnel Reflector, Solar chimney and solar power tower.

Concentrating photovoltaics (CPV) systems employ sunlight concentrated onto photovoltaic surfaces for the purpose of electrical power production. Solar concentrators of all varieties 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 Photovoltaics and Thermal (CPT) technology produces both electricity and thermal heat in the same module. Thermal heat that can be employed for hot tap water, heating and heat-powered air conditioning (solar cooling), desalination or solar process heat.

Energy storage methods

Solar energy is not available at night, making energy storage an important issue in order to provide the continuous availability of energy. Solar power is an intermittent energy source, meaning that all available output must be taken when it is available and either stored for when it can be used, or transported, over transmission lines, 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 have traditionally used rechargeable batteries to store excess electricity. With grid-tied systems, excess electricity can be sent to the transmission grid. Net metering programs give these systems a credit for the electricity they deliver to the grid.

Pumped-storage hydroelectricity stores energy in the form of water pumped when surplus electricity is available, from a lower elevation reservoir to a higher elevation one. The energy is recovered when demand is high by releasing the water: the pump becomes a turbine, and the motor a hydroelectric power generator.

This solar concentrator is 109.4 M2 with a peak power of 15 kW, has a fixed-focus faceted dish with a concentration of ~250 suns. The reflective surface is flat, single-strength back-silvered glass tiles with 88% reflectivity. The power conversation units is a dense-array concentrating receiver. It is single crystal silicon solar cells that reach a temperature of < 65 Celsius. They are actively cooled with radiation-fan and pumped water loop. The electrical connection is a DC output from solar cells converted to AC by solid state inverters. The system can be used for large fields connected to the utility grid, hydrogen generation and water pumping. Credit: SAIC Source:

NREL