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Solar thermal systems use the direct heat of the sun to warm up water or similar - unlike Solar Photovoltaic (PV) which generates electricity from photons.
Solar heating systems are very simple and so are the most widely used renewable energy systems. It is compulsory for new home installations in places such as Israel and Spain.
Well over 32 million households in China are using some type of solar heating. Systems can be as simple as tubes with water in which are heated by the sun, to be fed into a hot water system. This is an easy job for an amateur builder.
The UK currently has around 120 MW of solar heating capacity installed.
Solar heating systems consist of three main parts: solar thermal collectors, a fluid based system to move the heat around and a storage tank / reservoir to store the heated water for later usage.
Home based systems rely on placing collector panels onto the roof of the building. In its most basic form the system works by pumping cold water into the collector which is then heated by the sun. The hot or warmed water is then moved into a storage tank for use throughout the building.
There are two main types of panels used to collect solar radiation.
Formed Plastic Collectors and Evacuated Tube Collectors. Formed Plastic Collectors consist of tubing made from polypropylene or PET through which the water to be heated passes.
These systems are only really effective for hot climates or for non-domestic heating such as swimming pools as the exposed tubing can quickly lose heat if the ambient temperature drops too much.
In Evacuated Tube Collection systems the panels are made from flat panes of glass with a series of tubes laid underneath. These tubes are created like vacuum or Thermos flasks with a double layer with a vacuum in between to ensure maximum insulation. Through these pipes passes a liquid solution, normally water and anti freeze. This is what heats up passing through a heat exchanger to provide heat to the water. This has the major advantage of not freezing during cold winter nights or requiring maintenance based on seasonal temperature changes.
In colder climates a heating coil may also be fitted, this is to allow the solar collectors to warm up the water to its maximum available level and then "top up" the heat with electricity during the cold winter months.
There are different systems for moving the water from the roof to the storage tank, Passive and Pumped.
The most common type of system used in Northern Europe is pumped systems. These require a small electric pump in order to move the hot water from the roof into a storage tank within the building instead of on top of it.
This allows systems to be put onto existing buildings without the need to build supports for external tanks. Newer systems include solar powered pumps to offset the increased carbon footprint of using electrical pumps.
Passive systems rely on the storage tank being placed at a higher level then the collector panels. When the water is heated in the collector it moves upward through the pipes and forces cold water back down into the collector panels. These systems are not suited to colder climates with the storage tank and pipework exposed to colder outside temperatures and the risk of freezing which can cause damage.
A lesser used but still viable form of solar heating is Passive Solar Air Heating. This system is popular in Scandinavia and is useful for space heating. The system works by using external boxes designed to absorb the heat from the sun, which causes the hot air to rise, forcing cold air down. This warmer air can then be channelled into a building where, as an added bonus, it has a dehumidifying effect because the air has been dried by the heating process. These systems require no power to operate so the heating gained is completely free!
Contrary to popular belief solar heating will continue to work during cloudy days and even winter months. This is because solar heating relies on the radiation from the sunlight, which still penetrates cloud cover. A standard boiler is still required as the systems work together and the normal boiler will still provide the bulk of heating during winter months.
The savings a solar hot water system makes depend on the climate and the location of the building. A typical system in the United Kingdom can save between £40-£80 a year and shave off 350kg of CO2 emissions.
A domestic system will require about 2-4 square meters on a south-east to south west facing roof and a average domestic system can be installed for between £3,200 - £4,500.
Grants may be available from the government to reduce the cost of installation, if anyone can figure out what the latest wheeze is. The UK government has made it easier by requiring that the only condition for planning permission to fit solar heating systems is that it does not negatively affect neighbouring properties. Once installed systems have a life span of 25 years and more with little maintenance.
While solar powered heating systems cannot replace conventional methods of heating, they can provide additional 'free' heat, which reduces the cost of heating a house. They also pay for themselves through heat generated within a much smaller time frame than the more widely used double glazied windowpanes. Of course all these systems should be used together, to give really big savings.
"Renewable heating and cooling systems should be part of the fabric of a building and are not solutions you can just tack onto a building without knowing the context, demands on it and proper design" - Edel Walsh of Invisible Heating Systems.
Photos and diagram from Invisible Solar Heating www.invisibleheating.co.uk
See also Solar UK www.solaruk.net - thermal installations using vacuum tubes, and can also advise on solar PV.