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Parabolic trough
A parabolic trough is a type of solar thermal collector that is straight in one dimension and curved as a parabola in the other two, lined with a polished metal mirror. The energy of sunlight which enters the mirror parallel to its plane of symmetry is focused along the focal line, where objects are positioned that are intended to be heated. For example, food may be placed at the focal line of a trough, which causes the food to be cooked when the trough is aimed so the Sun is in its plane of symmetry. Further information on the use of parabolic troughs for cooking can be found in the article about solar cookers.
For other purposes, there is often a tube, frequently a Dewar tube, which runs the length of the trough at its focal line. The mirror is oriented so that sunlight which it reflects is concentrated on the tube, which contains a fluid which is heated to a high temperature by the energy of the sunlight. The hot fluid can be used for many purposes. Often, it is piped to a heat engine, which uses the heat energy to drive machinery or to generate electricity. This solar energy collector is the most common and best known type of parabolic trough. The paragraphs below therefore concentrate on this type.
== Efficiency ==
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; this reduces the overall efficiency of the collector due to cosine loss but only requires the trough to be aligned with the change in seasons, avoiding the need for tracking motors. This tracking method approaches theoretical efficiencies at the spring and fall equinoxes with less accurate focusing of the light at other times during the year. The daily motion of the sun across the sky also introduces errors, greatest at the sunrise and sunset and smallest at solar noon. Due to these sources of error, seasonally adjusted parabolic troughs are generally designed with a lower concentration acceptance product.
Parabolic trough concentrators have a simple geometry, but their concentration is about 1/3 of the theoretical maximum for the same acceptance angle, that is, for the same overall tolerances of the system to all kinds of errors, including those referenced above. The theoretical maximum is better achieved with more elaborate concentrators based on primary-secondary designs using nonimaging optics〔Julio Chaves, ''Introduction to Nonimaging Optics'', CRC Press, 2008 ISBN 978-1-4200-5429-3〕〔Roland Winston et al.,, ''Nonimaging Optics'', Academic Press, 2004 ISBN 978-0-12-759751-5〕 which may nearly double the concentration of conventional parabolic troughs〔Diogo Canavarro et al., ''New second-stage concentrators (XX SMS) for parabolic primaries; Comparison with conventional parabolic trough concentrators'', Solar Energy 92 (2013) 98–105〕 and are used to improve practical designs such as those with fixed receivers.〔Diogo Canavarro et al., ''Infinitesimal etendue and Simultaneous Multiple Surface (SMS) concentrators for fixed receiver troughs'', Solar Energy 97 (2013) 493–504〕
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