Solar Photovoltaics (PV) Factsheet

Definition

Solar photovoltaics (PV) are a semiconductor-based technology that converts the energy in sunlight into electricity.

The technology

When a small amount of light (a photon) lands on a PV cell it gives energy to an electron. The electron moves away from the cell into an electrical circuit, creating direct current (DC) electricity. This can be used to charge batteries or power DC devices. The world has been using the technology for decades to power calculators and satellites. However, for PV panels in the UK the electricity would normally be converted to alternating current (AC) via an inverter to help meet the electrical demands of the site. It is also linked into the grid, with any surplus after on-site use generating an income.

The PV cells are normally silicon-based semiconductors and can be split into four categories which are outlined below –

Mono-crystalline: High quality and most expensive, these are manufactured from thin slices of silicon wafer and work best in good light conditions. They have conversion rates of 13-15% (energy from daylight turned into electrical energy).

SOLAR PV config chart Poly or multi-crystalline: Manufactured from off-cuts of silicon wafer, these are slightly cheaper than mono-crystalline and have conversion rates of 12-14%.

Amorphous or thin film silicon: Cheaper to buy and more flexible in application but, at 5-7%, less efficient at converting light into electricity, although good performance in low light conditions when compared to categories above.

Hybrid cells: A combination of mono-crystalline and thin-film technologies that have a high peak output coupled with excellent performance in poor light conditions, giving conversion rates of 16-17%.

PV systems, known as arrays, are highly flexible and can be building-integrated, building-mounted or free-standing. Free-standing PV arrays can have purpose-built structures or cassettes, ideal for ground-mounting but also for flat roofs. Ideally they will be pointing within 60° of due south and have an angle of elevation of between 30-50°. However, PV will work to differing degrees in a wide range of scenarios as shown in the chart, above right.

The PV cells that convert daylight into electricity are traditionally formed into panels, but can also be placed between two sheets of toughened glass, or coated onto a myriad of backings, such as metal or bitumous type products for use in areas such as roofing – see images below.

solar pv home mounted solar pv panels flashed in
1 kWp home-mounted, panel-based PV system Standard PV panels flashed into roof (image courtesy of PV Systems)
solar pv roof tiles solar pv cells integrated
PV roof tiles PV cells integrated in glass canopy

Operation, grid connection & maintenance

A typical grid-connected, building-mounted PV system operates in the following manner; the PV array feeds electricity into the building, offsetting that imported from the national grid. When electricity demand on site is more than the output from the PV array, the difference is drawn from the grid. Should electricity demand be less than the output from the PV array, electricity can be exported back into the grid. Depending on negotiations with the electricity supplier or a 'consolidator' (dedicated agent), this export of electricity should generate a credit on the electricity bill, or a one off annual payment, and may also include payment for the Renewable Energy Obligation Certificates (ROCs).

All generators of renewable electricity are entitled to claim ROCs for every 1,000 kWh (kilowatt-hours) or units generated. This is currently a flat rate across all the technologies, but will soon be differentiated and PV installations will be entitles to two ROCs per 1,000 kWh generated. ROCs have a changeable value established through trading on dedicated markets (£48.06 per ROC on 27th January 2011). They are an additional source of income, separate from the actual price of electricity consumed/exported.

There are no maintenance costs, although inverters would probably need replacing at some stage within the panel guarantee period and a replacement factor of once every fifteen years would be a reasonable assumption.

Costs, energy generation and other key facts

As a rule of thumb, bespoke systems from installers will start from around £4,000 per kWp (the maximum rated output) ex VAT installed capacity, depending on product, size of array and ease of installation. The economies of scale start to apply as the systems become larger.

It should be noted that energy generated from a 1 kWp system will depend on angle of orientation, tilt and product selected, but as a general figure if southerly facing at 45º tilt, the output will be between 800 – 850 kWhs or units per annum.

PV panels can also have CO2 benefits. For example a 3kWp will offset around 1.08 tonnes of CO2 per year while a 10kWp system will offset around 4.8 tonnes of CO2

The information above and additional facts of area required for panels and carbon offset are included in the table below for two sizes of system.

System size Area req. (m2) Elec units/yr (kWh/yr) Approx. CO2 saving (kg) Approx. Installed price (before grant)
1kWp 6 - 8 8-850 400 £6,500 inc. VAT @ 5%
5kWp 30 - 40 4-4,250 1,900 £22,500 inc. VAT @ 5%

Payback

There is an assumption that PV cannot pay for itself within its lifetime. This is not correct. Payback should be measured not solely on today's fuel prices, but under the assumption that fuel costs are likely to continue to rise. Given a cost per unit of electricity of just 7p per kWh, if we assume an annual rise of 10% per annum, then 50% of a PV system would pay for itself in around 18 - 20 years (this assumes a government grant of 50%). Whilst this is a long payback period, it should be borne in mind that PV panels are often guaranteed for 25-30 years and are expected to work for considerably longer with only a slight reduction in efficiency. They are also part of the building fabric and thus offset other construction costs.

Planning

New guidance for this technology in domestic settings is expected soon. However, planning permission is usually required only when a building is listed and/or in a conservation zone. Some local planning authorities have stipulations as to how much of a roof area that can be covered with PV modules without requiring a planning application, so guidance from local planning officers should be sought. Examples of installations, even on listed buildings, can be supplied if required.

solar pv free-standing panels solar pv pergola
4.8kWp free-standing panel PV array 0.6kWp PV pergola, comprising glass-laminated PV cells

Free-standing PV structures (such as those above) may also require planning permission, depending on size and location.

Key links

  1. The Solar Energy Society - www.uk-ises.org - A non-profit organisation created as a forum for all those interested in the advancement of the utilisation of the sun's energy.
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