Editing Parabolic trough (section)

== Efficiency ==

[[File:Parabolic trough.svg|thumb|upright=0.75|A diagram of a parabolic trough solar farm (top), and an end view of how a parabolic collector focuses sunlight onto its focal point.]]

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 the sunlight striking the collectors at an angle but only requires the trough to be aligned with the change in [[season]]s, avoiding the need for tracking motors.  This tracking method approaches theoretical efficiencies at the spring and fall [[equinox]]es 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 noon. Due to these sources of error, seasonally adjusted parabolic troughs are generally designed with a lower [[Acceptance angle (solar concentrator)#Concentration acceptance product (CAP)|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 (solar concentrator)|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]]<ref name="IntroNio2e">{{cite book | first = Julio | last = Chaves | title = Introduction to Nonimaging Optics, Second Edition |url=https://books.google.com/books?id=e11ECgAAQBAJ | publisher = [[CRC Press]] |  year = 2015 | isbn = 978-1-4822-0673-9}}</ref><ref>Roland Winston et al.,, ''Nonimaging Optics'', Academic Press, 2004 {{ISBN|978-0-12-759751-5}}</ref> which may nearly double the concentration of conventional parabolic troughs<ref name="XXSMS">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</ref> and are used to improve practical designs such as those with fixed receivers.<ref name="XXcSMS">Diogo Canavarro et al., ''Infinitesimal etendue and Simultaneous Multiple Surface (SMS) concentrators for fixed receiver troughs'', Solar Energy 97 (2013) 493–504</ref>

[[Heat-transfer oil|Heat transfer fluid]] (usually [[thermal oil]]) runs through the tube to [[absorption (electromagnetic radiation)|absorb]] the concentrated sunlight. This increases the temperature of the fluid to some 400&nbsp;°C.<ref>{{cite web|url=http://www.abengoasolar.es/sites/solar/en/technologies/concentrated_solar_power/parabolic_trough/index.html|title=Absorber tube temperature|website=abengoasolar.es|url-status=dead|archive-url=https://web.archive.org/web/20090801143959/http://www.abengoasolar.es/sites/solar/en/technologies/concentrated_solar_power/parabolic_trough/index.html|archive-date=2009-08-01}}</ref> 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 to 80%.
The overall efficiency from collector to grid, i.e. (Electrical Output Power)/(Total Impinging Solar Power) is about 15%, similar to PV (Photovoltaic Cells) but less than [[Stirling engine|Stirling]] [[Solar thermal energy#Dish designs|dish concentrators]].<ref>Patel99 Ch.9</ref>