Parabolic Trough Solar Collectors
Large-scale solar thermal power plants use parabolic trough collectors. There are curved mirrored surfaces, usually coated with silver or polished aluminium, in a parabolic shape with a receiver tube running its length at the focal point. Sunlight is reflected by the mirror and concentrated on the tube. The trough is usually aligned on a north-south axis, and rotated to track the sun as it moves across the sky each day.
Alternatively the trough can be aligned on an east-west axis, which reduces the overall efficiency of the collector. However, it only requires the trough to be aligned with the change in seasons, avoiding the need for tracking motors. This tracking method works correctly at the spring and fall equinoxes with errors in the focusing of the light at other times during the year. The magnitude of this error varies throughout the day, taking a minimum value at solar noon. There is also an error introduced due to the daily motion of the sun across the sky, this error also reaches a minimum at solar noon. Due to these sources of error, seasonally adjusted parabolic troughs are generally designed with a lower solar concentration ratio. In order to increase the level of alignment, some measuring devices have also been invented.
The usual heat transfer fluid is oil which runs through the tube to absorb the concentrated sunlight. This increases the temperature of the fluid to some 400°C. The heat transfer fluid is then used to heat steam in a standard turbine generator. The process is economical and, for heating the pipe, thermal efficiency ranges from 60–80%. The overall efficiency from collector to grid, i.e. Electrical Output Power / Total Impinging Solar Power, is around 15%, similar to PV technology but less than Stirling dish concentrators.
Current commercial plants utilising parabolic troughs are hybrids; fossil fuels are used during night hours, but the amount of fossil fuels used is limited to a maximum 27% of electricity production, allowing the plant to qualify as a renewable energy source. Because they are hybrids and include cooling stations, condensers, accumulators and other things besides the actual solar collectors, the power generated per square meter of space ranges enormously.
Types of mirrors
Usually, the mirrors are are single piece parabolic forms although V-type parabolic troughs exist which are made from 2 mirrors and placed at an angle towards each other.
Energy storage
As this renewable source of energy is only provided during the day, methods for energy storage have been studied, e.g. the single-tank (thermocline) storage technology for large-scale solar thermal power plants. The thermocline tank approach uses a mixture of silica sand and quartzite rock to displace a significant portion of the volume in the tank. Then it is filled with the heat transfer fluid, typically a molten nitrate salt.
Existing Plants
The largest operational solar power system at present is one of the SEGS plants and is located at Kramer Junction in California, USA, with five fields of 33MW generation capacity each, i.e. the total capacity is 165MW.
The 64MW Nevada Solar One also uses this technology. In the new Spanish plant, Andasol 1 solar power station, the so-called “Eurotrough” collector is used. This plant went online in November 2008 and has a nominal output of 49.9MW.
