The Low Carbon Partnership

Site considerations

Is your site south-facing?
A solar thermal system can be installed on a pitched roof or ground mounting facing between 0° and 90° of south. The closer to south the better as this means more daylight will reach the collectors leading to higher system efficiency. Many collectors can be fitted to a tracking device to follow the movement of the sun during the day.

The pitch of the collectors is also important for maximising solar gain and should be between 20° and 50°.

Is your site free from shading?
Solar thermal collectors work in diffuse light as well as direct sunlight, but if part or all of a collector is shaded during daylight hours, e.g. by trees or buildings, heat transfer is diminished. As a guide, there shouldn’t be much more than two hours of shading on a spring/autumn day.

Is your site suitable for installation groundworks or a mounting system?
The structural integrity of an existing roof space or any other mounting location must be carefully assessed before installation. Not only must the area be able to accommodate the number and size (m2) of solar collector required for your water heating demand, it must also be strong enough to take the weight of the equipment.

In most cases, the proposed installation space must also be suitable for fixing collectors at the right pitch for daylight capture, though some tube collectors can be flat mounted.

How much hot water do you use, and when?
Demand for hot water varies according to what it is to be used for. You will need to have a rough idea of how much is required, and when, in order to size your system. The larger the demand, the more collectors can be usefully installed, but the balance of cost should be taken into account. As a guide, taking individual households in a Housing Association scheme for instance, roughly 1m2 of collector would be needed per person.

In the UK, it is typical for a solar thermal system to deliver hot water to complement an existing hot water store and boiler serving washing appliances and faucets (though some types of combi-boilers are not compatible with solar thermal). It can also be used to provide warm water for swimming pools and, in some cases where a building is highly insulated, for radiators/space heaters.

The output of a solar thermal system is best during the summer months when water temperatures of 80°C and more are possible. During the UK wintertime, another heat source would be required for water, though some solar gain is possible.

On average, across the year a solar thermal system can provide around 50% of water heating demand.

Could you use solar thermal equipment in place of existing building materials?
The two main types of solar collector – flat plate and evacuated tube – come in a range of forms. Some flat plate collectors are manufactured to sit flush to a pitched roof, like a window or skylight, and others can be fitted to balcony balustrades. Also, some tube collectors can be mounted flat and the internal heat absorbers rotated to the optimum angle for heat absorption.

How far is your proposed solar collector location from the hot water store?
The efficiency of collectors in solar thermal systems is affected by several in-use variables, including temperature of the ambient air, temperature of the heat absorber fluid in the collectors, insulation of the collector, and angle and intensity of direct sunlight. In addition, as with any plumbing system, the pipes carrying the heat transfer fluid should be well lagged and the heat store properly insulated.

Some systems use a twin cylinder heat transfer and store, in which case the 'solar cylinder' should be as near as possible to the hot water cylinder. Where there is just one water cylinder housing a twin coil, it should be located close to the collectors.

Will you need planning permission?
With the Low Carbon Buildings: Phase 2 grant programme the government is actively promoting renewable energy installations and permissions for new systems is becoming easier to obtain.

However, some local authorities will insist on planning consent being gained prior to installation, particularly if your potential site is in a conservation area or involves a listed building. Always check before going ahead as retrospective permissions can be difficult to acquire.

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