Optimizing Solar Contribution
By Chris Laughton
Solar radiation, the earth’s most abundant energy supply, falls for us free from the sky every day. There are many ways to make use of this energy: for example, we already use it to light our buildings, grow our plants and even obtain a suntan. But solar radiation also can be used to provide both hot water and electricity to supplement more traditional energy sources.
The equipment designed to collect solar radiation is fixed onto the outside of our buildings, either flat (horizontal) or more often pitched up to best face towards the sun. The two main forms of collectors found on roofs are for water heating (solar thermal) and photovoltaic (solar electricity). The increasing problem in today’s crowded buildings is firstly finding the best position to fit solar collectors and secondly deciding 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 first occupied by other equipment. This can include roof skylights but also flues, extract ducts, air conditioning and aerials. Such equipment not only reduces the available roof area but also can cast shadows that reduce the performance of solar equipment. Shading has a particularly strong effect on photovoltaic modules with even small shadows causing significant losses. Shading also occurs from objects such as trees, other buildings and hills. These are particularly prominent when the sun is low in the sky such as during winter or at high latitudes.
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 predominate. 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 onto the collector. Solar radiation in fact falls onto 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 is the total radiation changing but so is 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 to work out the best use of a given roof area. This allows for rapid calculation of the total annual energy output from each hour of the year, as well as easy comparisons of all the 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 it does for 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 optimize solar contribution; however, it is unlikely that this will be retained through staff or ownership changes. Professional simulation software can visualize each of these situations and present a clear interpretation to clients.
The accuracy of any simulation model depends strongly on the accuracy of the data entered into the program. To increase confidence in simulation results, any engineer should be prepared to reveal their assumptions for scrutiny to allow double-checking. The assumed climate data has particular importance for both solar radiation and ambient temperatures, and it is always best to use certified collector values when using computer simulations for clients. Most engineers use a long-term average data set from the last 20 years from a trusted and published source.
Where a roof becomes crowded, the more efficient solar products are likely to be prioritized in order to give the smallest foot-print. 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 any shading — 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 photovoltaics collectors are being used, the priority goes to the latter if this permits avoiding shading.
The advent of 3D animated shading in photovoltaic computer simulations now greatly enhances the design experience and sets a new benchmark in the industry. With professional simulation software, the solar industry can move forward with technological advances in equipment and provide high quality support to design engineers. Training packages are available to assist familiarization with these new developments.
Chris Laughton is the managing director of UK-based The Solar Design Company, The Solar Design Company is offering free trials of T*SOL and PV*SOL solar simulation software available to download from its website: www.solardesign.co.uk.