Solar

Posted: 27 January 2012

There are a variety of technologies which have been developed to take advantage of solar energy. These include concentrating solar power systems, passive solar heating and daylighting, photovoltaic systems, solar hot water, and solar process heat and space heating and cooling.

Photovoltaic (PV)

Semiconductors which convert solar energy directly into electricity are called photovoltaic (PV) devices or solar cells. Although there are about 30 different types of PV devices under development, there are three main technologies in commercial production - monocrystalline cells, polycrystalline cells and thin-film cells.

Solar PV array on building
Solar PV array on building
Photovoltaic system on the EPA's Research Triangle Park facilityCredit: Environmental Protection Agency

The amount of power from a PV array is directly proportional to the intensity of the light hitting the array. Photovoltaic arrays produce direct-current (DC) electricity. They can be configured to produce any required combination of voltage and current - including conventional residential alternating current (AC) voltages.

There is currently an installed cumulative capacity (at the end of 2010) of over 40 GW; applications range from consumer products and small-scale stand-alone units for rural use to grid-connected roof-top systems and large central-grid power stations.

Typical system size varies from 50W to 1 kW for stand-alone systems with battery storage, from 500 W to 5 kW for roof-top grid-connected systems and from 10 kW to several megawatts for grid-connected ground-based systems and larger building-integrated systems.

Annual market growth for PV solar power applications has averaged more than 30 per cent in many countries over recent years. Increasing mass production of PV technology continues to reduce the cost. Costs have fallen by 20 per cent for each doubling of cumulative sales.

Concentrating solar

Solar power plant
Solar power plant
This solar thermal power plant in Kramer Junction, California, uses parabolic troughs to collect the sun's energy. Photo © NREL

 A new generation of power plants uses the sun by concentrating its rays as a heat source. There are three main types of concentrating solar power systems: parabolic-trough, dish/engine, and power tower.

Parabolic-trough systems concentrate the sun's energy through long rectangular, curved mirrors. The mirrors focus sunlight on a pipe that runs down the center of the trough. This heats the oil flowing through the pipe. The hot oil then is used to boil water in a conventional steam generator to produce electricity.

A dish/engine system uses a mirrored dish (similar to a very large satellite dish). The dish-shaped surface collects and concentrates the sun's heat onto a receiver, which absorbs the heat and transfers it to fluid within the engine. The heat causes the fluid to expand against a piston or turbine to produce mechanical power. The mechanical power is then used to run a generator or alternator to produce electricity.

solar power tower
solar power tower
EU's first commercial concentrating solar power tower near Seville, Spain (Photo courtesy Abengoa)

A power tower system uses a large field of mirrors to concentrate sunlight onto the top of a tower, where a receiver sits. This heats molten salt flowing through the receiver. Then, the salt's heat is used to generate electricity through a conventional steam generator. Molten salt retains heat efficiently, so it can be stored for days before being converted into electricity.

Solar hot water

Solar energy is also used for direct heating of domestic water and swimming pools. Most solar water heating systems for buildings have two main parts: a solar collector and a storage tank. The most common collector is called a flat-plate collector. Mounted on the roof, it consists of a thin, flat, rectangular box with a transparent cover that faces the sun. Small tubes run through the box and carry the fluid - either water or other fluid, such as an antifreeze solution - to be heated. The tubes are attached to an absorber plate, which is painted black to absorb the heat. As heat builds up in the collector, it heats the fluid passing through the tubes. The storage tank then holds the hot liquid.