Parabolic trough collectors

Parabolic trough collectors belong to the category of solar concentrators. They concentrate direct solar radiation from a large area onto a focal line along which the absorber tube runs. In conventional systems, the heat-transfer medium can reach temperatures of around 400 °C, making these collectors suitable for electricity generation in large solar thermal power plants. They can also supply heat for industrial processes.

In terms of design, a parabolic trough collector is essentially a long parabolic mirror with an absorber tube running along its focal line. Sunlight reflected from the mirror is concentrated onto the absorber, where it heats the heat-transfer fluid, traditionally a synthetic oil. To minimise heat loss, the metal absorber tube is typically enclosed within an evacuated glass envelope. The efficiency of parabolic trough collectors depends on the optical quality of the mirrors, the operating temperature and the thermal losses from the receiver.

Parabolic trough collectors are interconnected to form rows, which are the basic units of a solar field. The rows are most commonly aligned along a north-south axis, with the collectors rotating about their longitudinal axis to track the Sun from east to west. Alternatively, the rows can be aligned along an east-west axis and operated at a nearly fixed inclination. Such systems have a lower annual energy yield but do not require continuous daily tracking; only occasional adjustments are needed to account for seasonal changes in the Sun's position.

The Solar Energy Generating Systems (SEGS) in California's Mojave Desert were a pioneering complex of nine parabolic trough solar thermal power plants with a combined capacity of 354 MW. Built between 1984 and 1990 at Daggett, Kramer Junction and Harper Lake, they demonstrated the commercial viability of parabolic trough technology. Some SEGS plants could also use natural gas to supplement solar heat when solar radiation was insufficient. All nine plants have since been decommissioned.

Parabolic trough solar thermal power plants have since been built in many countries with favourable solar conditions, including Spain, Egypt, Algeria, Morocco and the United Arab Emirates. One important European project is Andasol 1 in Andalusia, Spain (map), a 50 MW parabolic trough power plant equipped with thermal energy storage.

The overall solar-to-electric efficiency of parabolic trough solar thermal power plants is typically around 15—20%.

Linear Fresnel Reflectors

A parabolic reflector can be replaced by several rows of linear reflectors that exploit the Fresnel lens principle. Each row of mirrors has a slightly different inclination so that all of them cast the reflected sunlight on the bottom part of the absorber. The irradiation of the absorber is about 30 times higher than natural. The heat transfer medium is usually water which is heated to 300 °C. The resulting steam directly powers a turbine.

The manufacturing costs of flat mirrors are significantly lower than the cost of parabolic mirrors. Since they are close to the ground, there is little risk of wind damage and the output is about the same as with parabolic mirrors. A Fresnel solar power plant requires lower investment than a comparable parabolic trough thermal solar power plant. Reflector rows follow a north-south axis and track the Sun. Compact linear Fresnel reflectors use two absorbers for one batch of mirrors. Each absorber runs along one side of a reflector row. Thus, sunlight is used more effectively and the plant requires less space than a single, central absorber line Fresnel solar power plant.

History

The linear Fresnel reflector was designed by Giorgio Francia in 1961 in Italy but the technology stayed undeveloped for a long time. The first Fresnel solar power plant was the Puerto Errado 1 Thermosolar Power Plant in Spain (map). It was commissioned in 2009, with an installed capacity of 1.4 MW. Another plant, the Kimberlina (map) was built in California and has an installed capacity of 5 MW. Further power plants with installed capacities over 100 MW are being planned.

Fresnel lenses will probably be used in the field of concentrated photovoltaics where they will concentrate sunlight onto a new generation of powerful solar cells.