What it costs: photovoltaics

Specification options

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 for commercially available PV systems is the silicon cell, which converts daylight into electricity. Cells vary in efficiency, appearance and cost.

Silicon cell types

Monocrystalline silicon are cells sliced from a single silicon crystal. These have a uniform blue-black appearance – 13-17% efficiency and a 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 three 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 tends to decrease over time due to the effects of weathering. Typically, manufacturers quote a reduction in output of 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 200 km/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 trees or buildings will reduce output
• Aspect: south-facing is the most effective
• Tilt: Optimum cell inclination is dependent on latitude. In London, the optimum tilt is 30°, in Scotland it is about 40°.

Where PV 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).

Design

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 the cell.

PV systems are most effective where daytime demand uses solar-generated electricity directly and a whole-building, integrated approach is applied to the design.