MYC RIGGULSFORD continues his series on renewable energy by looking at systems which turn sunlight into power
If you are particularly unromantic, you can think of sunlight as simply a stream of photons, each bringing some of the sun’s immense power to earth. The energy trapped in different colours or wavelengths of light varies, with ultraviolet having the highest frequency and highest energy and photons in infrared light carrying less energy. Solar photovoltaic, or PV materials, usually made from a combination of two types of semiconducting silicon, use this energy from the sun to create electricity. The stronger the intensity of the light, the greater the flow of electricity.Modern PV panels just need daylight; they do not have to be in direct sunlight, so you can still generate electricity on a cloudy day, though in the UK they should face south, south west or south east. You can now get a wide variety of different types of PV fittings ranging from the familiar shiny panels which look a bit like rooflight windows, to solar tiles that can be used instead of slates for roofs, and special glass for conservatories and windows that reduce glare while generating electricity. Solar panels are quite heavy, so you need to check that your roof is strong enough if you’re putting in a sizeable PV array.
SustainabilityThe sustainability, or embedded carbon, of solar PV cells is surprisingly good, even with quite energy intensive manufacturing costs. Dr Patricia Thornley, from the Tyndall Centre for Climate Change, tells me that solar PV panels save 85% of the carbon dioxide emissions of grid electricity over the lifetime of the cells. A typical domestic system could save over a tonne of carbon dioxide every year, and should run for about 25 years.However, according to the European Commission Joint Research Centre, solar PV is most viable south west of a line drawn across from North Wales to Norfolk – so PV panels fitted in Scotland may never generate enough electricity to pay back their embedded carbon, although solar panels in Somerset probably should.“PV costs have fallen rapidly over the last few years, but it remains one of the most expensive ways to generate electricity – costs can work out at 30p/kWh,” says Patricia Thornley. If photovoltaic panels are built into the fabric of new buildings they can replace other materials such as roof slates, which reduces their extra costs. However, most smallholders would be fitting them onto existing farmhouses or barns, so solar PV is not as economically attractive as many other forms of electricity generation such as wind or water turbines.According to the Low Carbon Buildings Programme, costs for photovoltaics should be around £5,000-£7,500 per kW of generating capacity, and a domestic system will usually be from 1.5-3kW. They reckon that a 2.5kW array will provide enough electricity to meet around half a typical household’s needs, saving around £250 a year. Panels that sit on top of an existing roof are cheaper than ones fitted in as part of the roof surface. Solar tiles and slates cost more than conventional panels. If you are going to join the PV system to your normal electricity connection, which allows you to export any surplus electricity to the grid, you will need an inverter. Otherwise, for a stand alone system such as on a remote barn, you’re going to need an expensive array of batteries to store your power, and these will need checking and testing regularly and replacing every few years.Solar slates and tiles provide a simple alternative to conventional slates or roofing tiles and can be laid in patches or as a complete roof. Solar Century’s C21e solar slates (600mm x 300mm or 500mm x 250mm) and tiles each replace four conventional tiles or slates and are attached straight to the roof battens, generating 52W per tile or about 1kW per 8m2. Solar Century has a network of approved installers but is also training ordinary roofers who can purchase and fit the slates and tiles through builders merchants. Installation and commissioning should be £300-£600, with costs for a 24 tile or slate solar electric 1.25kW system generating 1,036kWh per year of £8,510 and a 36 slate or tile 1.86kW system costing £12,060 (see www.solarcentury.com).Photovoltaic glass is being developed by several companies around the world. Some types have crystals in the glass which divert part of the light at right angles to the edges of the pane where it is harvested by solar PV cells in the frame space between the double glazing panels. Another glass, made by Romag in Consett, County Durham, called PowerGlaz, has groups of 5ins or 6ins square PV wafers incorporated into each single or double glazed panel, providing shade control and electrical power. This type of glass has been used on the new education building ‘The Core’ at the Eden Project in Cornwall, producing 80W per panel. Romag is currently working on a PV glass car parking canopy which could recharge an electric vehicle.“If you’re building a conservatory, and your budget stretches to it, and you want a high concept product, then choose solar glass,” says Hugo House of renewable energy electricity supplier Good Energy. I also asked him about thin film and PV membranes. “At the moment they are more military applications than commercialised products – the US army has tents which can generate electricity, so their troops can operate off-grid,” says Hugo House. “These new types of PV materials are one of the products with the widest possible application today, and could be used by smallholders since you can stick it on properties in places where you cannot fit other renewable technologies. With greater efficiency and lower costs they have the potential to boom in the future.”Photovoltaic grants, equipment and installersRumours that the government’s solar PV grants have run out are untrue – the Energy Saving Trust assures me that private householders are still eligible for up to £2,500 towards the costs of accredited PV panels fitted by approved installers; see their website: www.energysavingtrust.org.uk.The BERR website www.lowcarbon buildings.org.uk lists grant approved installers for each region and, as usual, the South West one names 88 approved companies in the West Country … from Norfolk to Aberdeenshire. It also lists a whole range of approved PV products from 52 different manufacturers, some with a dozen different types of module.The British Photovoltaic Association is housed in the National Energy Centre, Milton Keynes, which is also the base for the solar thermal trade body, the Solar Trade Association. Their website, at www.solar-trade.org.uk, has weblinks to the British Photovoltaic Association, which unfortunately don’t currently work, and neither does the European Commission’s link at www.pv-uk.org.uk, which bounces you to the National Energy Foundation.
How PV materials workSolar photovoltaic materials were discovered in 1839 by Edmund Becquerel. The sun’s light energy is harnessed by joining a semiconductor, such as modified silicon crystals, which has lots of free electrons to a semiconductor with lots of electrons missing (think of it as a set of holes ready to be filled by electrons). The spare electrons would like to move from one semiconductor material to the other, but they need energy to do it. Electrons only exist in definite energy states, according to quantum mechanics so, if exactly the right wavelength (or frequency or energy state) photon comes along, it can help to nudge an electron from one side to the other where it fills a hole. The moving electron creates an electric current if a circuit is completed.Photovoltaic panels are made from materials where visible light will provide the right energy nudge, and provide the most electricity as an electric current. Solar PV cells are usually made from a combination of two types of silicon – one with extra electrons, one needing some.
Solar TowersThe Co-operative Insurance Solar Tower in Manchester is Europe’s largest commercial solar facade and was installed by Solar Century in 2006, costing £5.5 million. The 400ft high tower was covered in 7,244 solar panels, replacing the 40 year old mosaic tiles on the listed building, which should generate 180,000kW or enough energy to power 55 homes for a year, see www.solarcentury.comAlthough not strictly solar photovoltaic, more a specialised version of solar hot water, the world’s first commercial solar tower plant was built in March 2007 in Sanlucar la Mayor, not far from Seville, in Andalusia, Spain. It produces 23 gigawatt hours a year, replacing 16,000 tonnes worth of CO2 from fossil fuels every year, according to Devon Association for Renewable Energy. The solar plant has 624 reflectors, each measuring 124m2, which focus the sun’s rays onto a 115m tall tower which absorbs the energy into sealed water filled panels that heat under pressure to 250£C, converting the water into steam which powers a turbine. This Spanish power plant is planned to have nine towers by 2013, producing enough energy to power 180,000 homes, or all of Seville. Similar towers built in North African deserts could theoretically power most of Europe (or indeed, Africa) in the future, according to our politicians and policy wonks.n To comment on Country Smallholding’s renewable energy series contact Myc Riggulsford at firstname.lastname@example.org