Photovoltaics may help reduce energy costs,
as well as cutting CO2.
Peter Mayer of Building LifePlans looks at the
options and whole-life costs of PV systems
The simplest photovoltaic systems are designed to provide electricity to the building directly. This may be as direct current (DC) or in combination with an inverter to convert the DC to alternating current (AC).
More sophisticated systems include batteries to store energy or are connected to the grid. In the latter case, excess electricity fed into the grid may generate an income. Equipment can also include power controllers, grid protection devices and meters.
The basis of commercially available PV systems is the silicon cell which converts daylight into electricity. Cells vary in efficiency, appearance and cost.
Silicone cell types
Monocrystalline silicon are cells sliced from a single silicon crystal. These have a uniform blue–black appearance; 13–17% efficiency; 25 – 30 year expected life. Polycrystalline silicon describes cells sliced from a cast block comprising many silicon crystals; sparkly surface, 12 – 15% efficiency and a 20 – 25 year expected life.
Monocrystalline and polycrystalline silicon cells are usually constructed as modules, sandwiched between a low-iron glass and a backing layer of metal, plastics or glass.
The silicon cells are encapsulated with ethylene vinyl acetate (EVA) or a transparent resin. Larger modules may be less costly as wiring and framing costs are lower but the extra weight may increase installation costs.
Thin–film amorphous silicon, also known as triple-junction thin film silicon, is made by depositing 3 different layers of amorphous silicon; the silicon is laminated between metal and glass or plastics for a flexible finish, suitable for roofing membranes or shingles- 5 – 8% efficiency and a 15 – 20 year expected life.
Typically, silicon cells are blue–black in colour. Other colours are available but cost more and are less efficient.
Module efficiency is about 1% less than cell efficiency. The energy output of PV systems tend to decrease over time due to the effects of weathering. Typically manufacturers quote a reduction in output no more than 10% over 10 – 12 years.
PVs to IEC 61215 give assurance of performance under severe conditions, including temperature variation from –40°C to 85°C, hail impact, high temperatures and relative humidity, local shadowing, static loading to 2400 Pa and wind loads up to 200km/h.
Application and location
Factors that influence the costs and benefits of PV systems:
• Location: southerly locations generally receive more solar radiation
• Weather: areas with less cloud cover and lower temperatures give higher outputs
• Shading from adjacent trees or building will reduce output
• Aspect: south facing is the most effective
• Tilt: Optimum cell inclination is dependant on latitude. In London, the optimum tilt is 30°, in Scotland it is about 40°
Where photovoltaic components are used for wall or roof cladding, the PV system costs can be offset against the cost of traditional claddings. Larger modules give a lower cost for a given output usually measured as the peak wattage (Wp).
Framing, mounting and fixing components should be of a similar durability to the PV units – typically, aluminium with EPDM gaskets. Design should ensure the fixings and framing can withstand expected wind loads and provide for maintenance. To clean the PV surfaces, access may be important, as accumulation of dust results in decreased output.
The design of the system should include ventilation to prevent the cells overheating. Above 25°C output decreases by about 1% for every 2°C rise in cell temperature.
PV systems are most effective where daytime demand uses solar-generated electricity directly and a whole-building integrated approach is applied to the design.
Net present value for 60 years £/kWp
Monocrystalline silicon based modules to IEC 61215, 150W nominal rating.
Polycrystalline silicon based modules to IEC 61215, 130W nominal rating. 10m2 roof area
Polycrystalline silicon based modules to IEC 61215, 65W nominal rating
Polycrystalline silicon panels integrated with roofing tiles or slates. PV tiled area 8m2.
Thin-film amorphous silicon photovoltaic cells to IEC 61646 integrated single ply membrane roofing system. Roof area 35m2.
• Costs are based on an installation with a nominal output of 5kWp. Aluminium mounting and frame, tempered low–iron glass, EVA encapuslant unless otherwise stated. System linked to the grid.
• Life cycle costs include for cleaning, inspection, electrical checks, minor repairs and component replacement at the average of the range.
• A discount rate of 3.5% is used to calculate net present values.