What Technologies are currently in place for solar?

Photovoltaics (PV)

Photovoltaic energy is the conversion of sunlight directly into electrical power. Commonly referred to as solar cells or PV cells, photovoltaic cells were initially created to capture the sun's energy and turn it into electricity to power satellites in space. These PV cells are non-mechanical devices that are most commonly made of the semiconducting material, crystalline silicon. When the semiconducting material absorbs sunlight, the solar energy knocks electrons loose from its atoms, allowing the electrons to flow through the material into a wire to produce electricity. This process is called the photovoltaic effect.

PV cells can vary in size from about 1 centimeter (1/2 inch) to about 10 centimeter (4 inches) across. One cell can produce 1 or 2 watts, which is not sufficient power for most applications. To increase power output, PV cells are used as building blocks, electrically connected into a packaged, weather-tight module, which is typically composed of glass or other materials to protect the cells.

Modules are first mounted in a framework on the ground or on a building. They are then wired, in series, into strings. These strings are, in turn, wired in parallel to form an array. The number of modules connected in the array determines the amount of power generated, the greater number of modules per array the greater the output of power. A PV system that is to be connected to a power grid requires an inverter to convert the DC output of the array to the AC power required by the electric power grid.[1]

Photovoltaic System Schematic

Source: DOE, 2008[2]

It is important to note that, photovoltaic array performance is dependent upon the amount of available sunlight. Climate conditions, such as clouds and fog, can have a significant impact on the amount of solar energy gathered by a photovoltaic array and, in turn, its performance.

Types of PV technology:[3]

• Wafer-Based Crystalline Silicon - solar technology that involves the fabrication of discrete solar cells from silicon wafers. The cells are then electrically interconnected to form a module.
• Thin film - solar cells comprised of layers of semiconductor materials only a few micrometers thick, which are often deposited on glass via processes such as vacuum deposition. Some manufacturers, however, deposit thin-films on flexible substrates, which are then incorporated into building materials, such as shingles.
• Concentrating PV – technologies that use lenses or mirrors to concentrate sunlight 25-1000 times onto a high-efficiency silicon or multi-junction solar cell. Only direct sunlight can be concentrated, so CPV systems usually have large two-axis tracking structures. Utility-scale systems are the target market for most CPV technologies.

Concentrating Solar Power (CSP)

Concentrating Solar Power (CSP) plants produce electric power by first focusing the sun’s energy, using various mirror configurations, and converting it into high-temperature heat used to boil a fluid. Next, the resultant steam drives a turbine creating mechanical power for a conventional generator, which produces electricity. Concentrating Solar Power technologies are well suited for centralized power production. Utilities are becoming increasingly aware of the potential economic benefits of CSP, although power from CSP currently costs more than other renewable options such as wind. Problems of solar inconsistency can be overcome with thermal storage, or hybridization with natural gas, allowing electricity to be dispatched when needed. In Sunbelt regions, such as the Southwest, the widespread availability of solar energy provides flexibility in locating CSP power plants near existing or planned transmission lines.

Types of CSP Technology:[4]

• Trough-electric systems - concentrate the sun’s energy using a highly curved parabolic mirror shaped like a trough to focus the sunlight onto a receiver pipe running down a central point above the curve of the mirror. Fluid flowing through the receiver pipe is heated. The steam from the heated fluid is collected and used to generate electricity in a conventional steam cycle. To meet utility peak load requirements, trough systems can use thermal storage or can be hybridized to dispatch power. So far, Parabolic Trough technologies have achieved the most commercial success in the CSP market.

Trough System

Source: DOE[5]

CSP Trough Plant near Tucson, AZ

Source: DOE, 2008[6]

• Dish/Stirling Systems - concentrates the sun’s energy at the focal point of a parabolically shaped dish. The dish structure tracks the sun over the course of the day to continuously reflect the sunlight’s beam onto the thermal receiver. The thermal receiver is the interface between the dish and the engine/generator. The engine/generator is located at the focal point of the dish and converts the absorbed heat energy into electricity.

Dish Engine

Source: DOE, 2008[7]

150kW Dish/Stirling power plant (National Solar Thermal Test Facility)

Source: DOE, 2008[8]

• Solar Power Towers – utilize a field of heliostats, flat sun-tracking mirrors, which reflect the sun’s rays to a receiver located on top of a tall, centrally located tower. The solar energy is absorbed by the molten-salt working fluid flowing through the receiver. That fluid can be used to boil water to make steam, which in turn, is used in a conventional turbine-generator to produce electricity. While early power towers (e.g., the Solar One plant) utilized water/steam as the heat transfer fluid, current designs (e.g., Solar Two plant) utilize molten nitrate salt due to its superior heat transfer and energy storage capabilities. The Solar One plant in California's desert was re-built using newer technologies than when it was first built in the early 1980s. The Solar II plant was a retrofit of Solar One to demonstrate the advantages of molten salt for heat transfer and thermal storage.

10 MW Power Tower Pilot Project, Barstow, CA

Source: DOE, 2008[9]

References:

DOE: Solar Energy Technologies Program: Multi Year Program Plan 2008-2012, 04/15/08, Online: http://www1.eere.energy.gov/solar/pdfs/solar_program_mypp_2008-2012.pdf

DOE: Energy Efficiency and Renewable Energy Network (EREN): CSP Technologies Overview, Online: http://www.energylan.sandia.gov/sunlab/overview.htm [n.d.]

The Energy Story: Chapter 15: Solar Energy webpage: http://www.energyquest.ca.gov/story/chapter15.html [n.d.]

DOE: Energy Information Administration (EIA): Energy Kid’s Page: http://www.eia.doe.gov/kids/energyfacts/sources/renewable/solar.html

Union of Concerned Scientists: Clean Energy: How Solar Energy Works webpage: http://www.ucsusa.org/clean_energy/technology_and_impacts/energy_technologies/how-solar-energy-works.html [n.d.]

Concentrating Solar Power: http://www.onlineuniversities.net/solar-power

Research by Diane Meade