that aligns the trough with the sun throughout the day, maximising the solar heat gain.
The collector generally has a single rotation axis along the length of the trough which can be orientated in an east-to-west direction, tracking the sun from north to south, or orientated in a north-to-south direction and tracking the sun from east to west.
Parabolic troughs are generally aligned on a north-to-south axis, and are rotated to track the sun as it moves across the sky each day from morning to night.
The advantages of this type of tracking mode is that very little collector adjustment is required during the day resulting in the solar trough always facing the sun at noon time, but the collector performance early in the morning or late in the afternoon is greatly reduced due to the large incidence angles of the trough.
Even though solar trough collectors use tracking systems to keep them facing the sun, they are most effective in sunnier climates where there are good solar resources. Like many other solar collectors, parabolic trough reflectors are modular, that is individual troughs can be connected together to give a larger surface area of absorber producing large amounts of solar hot water than can be created by an individual trough. Many single troughs connected together form a collector field were they are connected together in series and parallel rows.
As line-focused concentrating collectors, parabolic trough reflectors are more efficient for industrial and commercial applications that require large amounts of hot water around the clock. In these types of installations, the solar energy trapped by the solar troughs heats a special type of thermal oil to very high temperatures.
The oil circulating around a closed loop active system is used to heat high volumes of water or to generate steam at very high temperatures of up to 400oC which can then be used to generate electricity. Also connecting together parabolic troughs to form collector fields requires large areas of land for the installation, yet they offset the need for conventional energy and provide energy savings and environmental benefits.
Solar concentrators such as parabolic troughs, have a small absorber area and therefore smaller heat losses and provide high efficiencies of around 12% at much higher working temperatures compared to standard flat panel collectors. However, they have the disadvantage of having a smaller angle of view, and therefore, require some form of tracking system or manual adjustment to keep them at the correct focal point.
Also parabolic trough reflectors can not collect most of the diffused solar radiation. Parabolic collectors are not recommended for domestic household use due to their size and high water temperatures, but are desirable for certain industrial and commercial needs to provide large amounts of hot water and/or for producing electricity by running steam turbines.
In the next tutorial about Solar Heating we will look at another type of solar collector which is designed to concentrate the received solar radiation even more into a single focal point while at the same time accept most of the diffuse radiation improving their efficiency. Furthermore, these concentrators can be stationary or only need small seasonal tilt adjustments to keep their heat pipe within the correct focal point. These types of solar thermal collectors are called