Solar Energy Fights for Roof Space

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*SOL for solar thermal or PV*SOL for solar photovoltaic, to work out the best use of a given roof area. This allows rapid calculation of the total annual energy output from each hour of the year, as well as easy comparisons of all options. Good quality programs allow for comparison between different brands of equipment and user load profiles.

Separate programs are used for solar water heating and photovoltaics because they are two fundamentally different technologies. For example, the conversion efficiency of solar radiation to thermal occurs at a much higher rate than conversion to electricity. Also, heat is usually stored in water whereas electricity is either sold straight to the utility grid or stored in batteries.   It is important not to underestimate the effect of changing loads on domestic hot water heating or the existence of a surplus feed-in tariff for electricity. The user may indicate a desire to alter their patterns of use to optimise solar contribution; however, it is unlikely that this will be retained through staff or ownership changes. Professional simulation software can visualise each of these situations and present a clear interpretation to clients.

While we can never fully anticipate conditions for a given day or week ahead, we can be increasingly confident of long-term monthly and annual results. Even if there is an acceptable margin of error in calculating future values, comparisons of equipment configurations and user load profiles are still accurate provided the same climate data set is used in the simulation. It is always best to reveal any assumptions for scrutiny and to use certified collector values when performing computer simulations for clients.

Where a roof becomes crowded, the more efficient solar products are likely to be prioritised in order to give the smallest footprint. Computer simulations will anticipate the best overall total energy production to displace conventional energy sources. An on-site tool, such as a solar site selector, is used to anticipate shading as an ideal accompaniment for computer simulations. These allow for real-time analysis of any site using a template and viewfinder, the results of which can then be recorded digitally. Generally speaking, if both solar water heating and photovoltaic collectors are being used, the priority goes to the latter if this permits avoiding shading.

Chris Laughton is Managing Director of The Solar Design Company. He is an experienced heating engineer, author and lecturer, and a regular columnist in magazines, journals and on-line media. His latest book Solar Domestic Water Heating: The Earthscan Expert Handbook for Planning, Design and Installation was published by Earthscan last month.

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