The solar installation must receive as much light as possible. Shadows cast by tall trees and neighbouring buildings must be kept in mind during the design process. The best location for solar PV is on the south-facing roof or side of a building.
2. Calculate energy outputs
The following energy outputs can be used as a rough rule of thumb for the UK, assuming a reasonable tilt, orientation and system efficiency.
3. Select optimum angle of inclination
The maximum total annual solar radiation is usually at an orientation due south and at a tilt from the horizontal equal to the latitude of the site minus roughly 20°. For example, 30° is an optimal tilt in southern England, increasing to almost 40 ° in northern Scotland. If the optimum angle is not achievable, more than 90% of the maximum can still be achieved at 10° and 50° tilts. South-facing vertical facades generate about 70% of the maximum.
4. Fixing details
Ensure that the fixing does not cover or shade any part of the PV cells. The fixing must allow for thermal expansion without breaking the glass. Temperatures at the back of the modules can rise to 80ºC if they are poorly ventilated – or higher if they are directly insulated.
The mounting must allow for the safe maintenance and possible replacement of individual modules. The expected lifespan of the support structure must be at least equivalent to that of the PV array. The preferred materials are aluminium, stainless steel or glass-fibre. Protection from corrosion is important, especially as residual currents may be present.
Other considerations are wind loading, the location of junction boxes and the position of electrical wiring, as this could penetrate the waterproof skin.
5. Minimising shade
Minor shading can result in significant loss of energy, because the cell with the lowest illumination determines the operating current of the series string to which it is connected. Many modern modules use bypass diodes to minimise shade effects, but shading must still be considered, preferably early in the design. Watch out for landscaping, trees, and even handrails.
If shading is unavoidable, or poor light is expected on a regular basis, the best types of PV to use are amorphous thin-film products, such as Kaneka or Unisolar products, or hybrid arrays which mix monocrystalline and amorphous technology, such as Sanyo HIT modules.
Maximising the energy benefits of photovoltaic cells.
Some PV products have now been designed to provide thermal insulation as well as electricity: it is possible to specify flat roof systems and solar metal roofing systems that include insulation, and solar glass laminates with low-emissivity glass.
6. Health and safety
The main issues to be aware of when appraising PV for CDM purposes are weight and toxicity. Modules may be heavy, with some exceeding 50 kg. No PV module releases sufficient toxic materials to cause harm during installation or maintenance, but there could be disposal issues. A small proportion of some makers' systems (not Solar Century's) contain cadmium, the oxides of which are toxic in very small doses. Some of these may release toxic dust if crushed during disposal. For a full appraisal, see the DTI guide Photovoltaics in Buildings: Safety and the CDM Regulations.
7. Glass laminates and thin film PV
PV glass laminates are well suited to facades and transparent roof tops. They can be fitted to standard curtain walling structures and are suitable for any application where glass is used, as long as there is a reasonable level of light. A PVB laminate can be used for the extra strength required by a roof top. Thin-film PV is durable and flexible and is encased in waterproof, self-cleaning polymer. It can be used in shingle form for roofing or in designs that exploit its flexibility.
The degree of soiling will depend on the location, but usually dust accumulation and self-cleaning reach a steady state after a few weeks if the array tilt is at least 15°. In extreme cases, dust may cause a power reduction of about 10%. At low tilts, horizontal glazing bars can trap debris that could lead to shading of part of the array.
9. Lifetimes of PV products
Most solar products have a lifetime of around 30 years. Modules of all types usually have a 20-year warranty, as do most thin-film integrated products. Crystalline PV slates and PV glass laminates usually have a 10-year warranty. These are only a rough guide and should be checked for each specific product.
- BS EN 61215
- BS EN 61646
- The Electric Supply Regulations 1988
- The Building Regulations 1991 (and amendments)
- The Construction (Design and Management) Regulations 1994