Solar
Solar Energy
Basics
There are two ways to harvest solar energy for electricity.
Photovoltaic (PV devices) or “solar cells” change sunlight directly into electricity. When photons from sunlight strike a photovoltaic cell, they are absorbed, providing energy to generate electricity. Individual PV cells are made of semiconductors, such as crystalline silicon or various thin-film materials. PV cells are grouped into panels and arrays of panels. Thousands of houses and buildings around the world have PV systems on their roofs.
Solar Thermal/Electric Power Plants generate electricity by concentrating solar energy to heat a fluid and produce steam that is used to power a generator. Concentrating solar power technologies use mirrors to reflect and concentrate sunlight onto receivers that collect the solar energy and convert it to heat. This thermal energy can then be used to produce electricity via a steam turbine or heat engine driving a generator.
The four main types of solar thermal power systems are:
- Parabolic trough – Uses a long parabolic-shaped reflector to focuses the sun’s rays on a receiver pipe located at the focus of the parabola.
- Solar Field – Uses many parallel rows of solar parabolic trough collectors aligned on a north-south horizontal axis. At a central location, the fluid circulates through pipes so it can transfer its heat to water to generate high-pressure, superheated steam. The steam is then fed to a conventional steam turbine and generator to produce electricity.
- Solar Dish – Uses concentrating solar collectors that track the sun, so they always point straight at the sun and concentrate the solar energy at the focal point of the dish. Mechanical power is created by compressing the working fluid when it is cold, heating the compressed working fluid, and then expanding the fluid through a turbine or with a piston to produce energy.
- Solar power tower – Uses hundreds to thousands of flat sun-tracking mirrors called heliostats to reflect and concentrate the sun’s energy onto a central receiver tower. The energy can be concentrated as much as 1,500 times that of the energy coming in from the sun.
Opportunities
- Solar energy systems do not produce air pollutants or carbon dioxide.
- When located on buildings, they have minimal impact on the environment.
- Conversion from sunlight to electricity is direct, so that bulky mechanical generator systems are unnecessary.
- PV arrays can be installed quickly and in any size.
- The environmental impact is minimal, requiring no water for system cooling and generating no by-products.
Challenges
- The performance of a photovoltaic array is dependent upon sunlight.
- The efficiency of most commercially available photovoltaic modules in converting sunlight to electricity ranges from 5% to 15%.
- The amount of sunlight that arrives at the Earth’s surface is not constant. It varies depending on location, time of day, time of year, and weather conditions. Climate conditions (such as clouds or fog) have a significant effect on the amount of solar energy received.
- Because the sun doesn’t deliver that much energy to any one place at any one time, a large surface area is required to collect the energy at a useful rate.
- Some toxic materials and chemicals, and various solvents and alcohols are used in the manufacturing process of photovoltaic cells. Small amounts of these waste materials are produced.
- Although sunlight is free, solar cells and the equipment needed to convert their direct-current output to alternating current for use in a house is expensive.
Future Trends
Covering 4% of the world’s desert area with photovoltaic devices could supply the equivalent of all of the world’s electricity. The Gobi Desert alone could supply almost all of the world’s total electricity demand.
Despite sunlight’s significant potential for supplying energy, solar power provides less than 1% of U.S. energy needs. This percentage is expected to increase with the development of new and more efficient solar technologies. Researchers around the world are trying to achieve efficiencies up to 30%.