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The main two types of solar energy technology used in the UK are solar thermal hot water (STHW) and photovoltaic (PV). The former provides hot water primarily used hot water production, rather than space heating in the UK, and the latter generates electricity. STHW systems can provide up to 70% of your hot water demands during the year. They use either flat plate or evacuated tube collectors to harness solar energy and convert it to useful hot water which is circulated by a pump to a hot water storage cylinder. Most STHW systems are coupled to a twin coil cylinder where the lower coil is connected to the solar collectors and the second coil, above it, to the boiler circuit. Therefore the system will always call for free solar hot water first before using the boiler to top up the system. Photovoltaic (PV) array's can use monocrystalline, polycrystalline, thin film or hybrid panels which, by chemical reaction with solar energy, create a current which can be inverted from 'direct current' (DC) to alternating current (AC) for use in homes and businesses.
For more information see the Energy Saving Trust Factsheets.
From June 2011 renewable energy technologies producing heat rather than electricity will be eligible for RHI payments from the energy companies. Unlike Feed In Tariff payments for electricity generation which are based on how much is generated all heat technologies will be 'deemed'. This means the MCS company will certificate to the energy company how much heat will be produced and you will be paid for that amount of renewable heat. Whilst actual figures for each technology haven't been confirmed it is likely that a solar thermal hot water system will provide about £250 per year as a RHI payment which in addition to about £100 fuel saving is certainly worth considering and could bring payback periods to about 10 years.
The sun emits electromagnetic radiation which is received by the earth as light and heat. Solar energy can be harnessed to produce usable heat and generate electricity. The solar potential in a given location is sometimes referred to as the available solar resource this is the amount of available sunlight (solar radiation) in the area where you intend to place solar heaters or solar panels. There are other factors, however, which need to be considered when determining the viability of solar energy in any given location. These are;
A specific location on the earth will always receive the maximum amount of solar energy when the sun’s energy hits the surface directly. The further you move away from the equator the greater the angle of incidence and therefore the less solar energy. This occurs because of the greater distance the electromagnetic radiation has to travel and therefore is subject to scatter.
During the summer in a given location the elliptical orbit of the earth around the sun means that the location will be closer and therefore will receive more concentrated solar energy. Generally speaking the northern hemisphere is closer to the sun for half the year and the southern hemisphere for the other. The tilt of the earth means that whilst the northern hemisphere will receive longer the days the southern hemisphere will receive shorter days for half the year and vice versa.
Areas such as the United States and Europe receive more solar energy between May and September not only because days are longer, but also because the sun is almost directly overhead during this season. The UK still gets over 60% of the irradiation compared to the equator. Irradiance measured in kilowatt hours per metre squared (kWh/m2) is between 900 kWh in Scotland to 1,200 kWh in southern England.
It is possible for the magnitude of solar energy to be three times stronger in June compared to December. The sun's rays are far more slanted during the shorter days of the winter months.
In addition to location sunlight passing through the Earth's atmosphere, some of which is absorbed, scattered or reflected, also can affect the magnitude of solar energy. When this happens the light is called diffuse. The following materials can cause the sunlight to become diffused;
Sunlight affected in this way is referred to as diffuse solar radiation or diffuse sunlight. Sunlight that reaches the Earth's surface without being diffused is called direct sunlight. The sum total of all diffuse and direct solar radiation in a given location is called global solar radiation which is the sum of both diffuse and direct sunlight values. Pollution and other atmospheric conditions (such as weather patterns) can reduce direct sunlight by 10% on clear dry days. They can reduce direct sunlight radiation by 100% on heavy, cloudy days. Note that the absence of direct sunlight does not imply total darkness, as some diffuse light will still get through.
Scientists measure the amount of sunlight available in specific locations during the different times of year. They are then able to estimate the amount of sunlight which falls on similar regions at the same latitude with similar climates and conditions. Measurements of solar energy are normally expressed as "total radiation on a horizontal surface", or as "total amount of radiation on a surface tracking the sun". In this last case, the assumption is that one is using a solar panel that automatically tracks the sun. In other words, the solar panel would be mounted on a tracking device so that the panel would remain at right angles to the sun throughout the day. This system is primarily used for industrial setups, when it is used at all.
Radiation data (the amount of solar energy available at a given location) for solar electric (photovoltaic) systems is often represented as kilowatt-hours per square meter (kWh/m2). Direct estimates of solar energy may be expressed as watts per square meter (W/m2). Radiation data for solar water heating and space heating systems is usually represented in British thermal units per square foot (Btu/ft2).