byChris Laughton, The Solar Design CompanyPublished: 2010-07-01
An increasing problem in today’s crowded buildings is finding the best position to fit solar collectors. The two main forms of collectors found on roofs are for water heating (solar thermal) and photovoltaic (solar electricity). If space is short, how can you decide which of these two technologies gets priority?
The “fight” for roof space grows more significant every year, not only as equipment becomes more affordable and hence larger but also because optimum locations are occupied first by other equipment. These include roof skylights, flues, extract ducts, air conditioning and aerials. Such equipment not only reduces the available roof area but can also cast shadows that reduce the performance of solar equipment. Shading has a particularly strong effect on solar collectors, with even small shadows causing significant losses on photovoltaic modules.
Shading can occur from other roof objects or nearby buildings, trees or hills. These are particularly prominent when the sun is low in the sky, such as morning and evening or during winter. In order to correctly position solar collectors, consideration must be given to the daily sun-path between sunrise and sunset. This sun-path also alters seasonally according to the latitude of the location. There are also local climatic conditions to consider, where perhaps morning mists or afternoon thunderstorms occur.
It is worth noting that particular collector types react differently to the sun’s constantly changing position. In some cases, automatic tracking devices are used to improve performance by altering the collectors to always face in the optimum direction. This affects what is known as the ‘angle of incidence’ of the beam solar radiation on to the collector. Solar radiation in fact falls on to collectors by one of three routes: ‘beam’ radiation in a straight line from the sun; ‘diffuse’ from all points in the atmosphere; and a proportion that is reflected from other surfaces (called the ‘albedo’). During a typical day, not only does the total radiation change but so does the proportion of these three types. Some collectors use mirrors and tubular absorbers to improve the performance of a collector through a longer arc of the sun’s path.
Professional solar engineers now use computer simulation software, such as T
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