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Solar thermal water heating technology

For a family of four, a well designed solar thermal heating system should contribute between 1,500kWh and 2,000kWh annually, which will be equivalent to 40 to 50% of the household's water heating energy needs.

How it works

A solar thermal water heating system comprises three main components:

  • solar panels fitted to the roof which retain heat from the sun's rays and transfer the heat to a fluid.
  • hot water cylinder to store the water that is heated during the day for use when required.
  • plumbing system of piping and a pump to move the fluid round the collector

The illustration shows a typical Solar Heating system layout. An electronic controller constantly compares the temperature of the solar collectors with the temperature of the water in the cylinder. Whenever the collectors are hotter than the cylinder, the controller switches on the system's circulating pump. A mixture of antifreeze and water is then circulated through the collectors and the cylinder's heat exchanger, heating the cylinder in just the same way as a central heating boiler.

Types of solar collectors

There are two main types of solar collector, which can be used in solar thermal water heating systems. These are:

Flat Plate Collector

Flat plate collectors

Flat plate systems comprise a copper grid absorber plate painted with a dark coating to absorb and retain heat, with a transparent solar glass cover. The sun’s radiation is transferred to fluid passing through pipes attached to the absorber plate. Insulation at the sides and bottom of the collector minimises heat loss.

Evacuated tubes

The vacuum tubes comprise a row of heat-resistant glass tubes, each containing an absorber plate feeding into a manifold, which transports fluid heated by the absorber. Each tube consists of a double-walled glass tube. The sun's radiation is absorbed by the selective coating on the inner glass surface, but prevented from re-radiating by the silvered innermost lining, like a one-way mirror. Most of the energy, some 93%, reaching the tube's surface is absorbed and only 7% is lost through reflection and re-emission. The vacuum wall prevents any losses by conduction or convection - evacuated tubejust like a thermos flask. Because of this, the system will work even in very low temperatures.

A heat-pipe collector transmits absorbed heat to fluid in the heating circuit via the copper header, or manifold. It must be mounted at an angle to the horizontal. Image courtesy of Apricus Solar Co. Ltd. (

There are two types of evacuated tube collector, depending on how the heat is transferred to the hot-water cylinder. In direct flow collectors the fluid heated in the copper absorber is pumped around the system through the heat exchanger in the cylinder. In heat-pipe collectors, the fluid in the copper absorber, or heat pipe, heats fluid in a second circuit running through the manifold, and it is this manifold fluid that heats the hot-water cylinder. The latter system is more expensive, but is claimed to function more efficiently at lower levels of solar radiation.

Evacuated tubes are smaller and more efficient, but tend to be more expensive (although they are now competitive with flat-plate collectors). Their higher efficiency means they are better able to take advantage of any winter sunshine, and are therefore well suited to the UK climate. However, over the course of a year they may provide only 10-15% more energy than flat plate collectors.

Advantages and disadvantages of flat plate collectors and evacuated tubes

Flat Plate Collectors

Evacuated Tubes

Cheap and simple construction

Generally more expensive but costs coming down

Collect about 450kWh/m2/year Collect up to 550kWh/m2/year

Peak energy output at mid-day when sun is perpendicular to the collector

Cylindrical shape means they absorb sun’s energy throughout the day

Loses heat more rapidly in wind and cool temperatures

The vacuum reduces heat loss

Often requires antifreeze to protect from cold weather

Can be used in low temperatures without sustaining damage

If damaged the whole plate needs to be replaced

If damaged it is easy to replace one tube

Very efficient in hot sunny conditions

Can heat water all year round

Larger collector area

Smaller collector area

Large collector can make installation more difficult.

Easy installation

Flat glazing more aesthetically pleasing and easier to blend into building design

Roof top collector more visible

    Thermodynamic and PVT hybrid solar panels

    These are two relatively recent developments in solar thermal, at least in the UK. Thermodynamic panels are a cross between conventional solar thermal and an air source heat pump. They work like a fridge in reverse and can absorb heat from the atmosphere day and night in virtually all weather conditions. The heat is transferred via a special fluid to a hot water tank or heating system via a heat exchanger.

    Solar PVT, or solar photovoltaic-thermal panels, combine the functions of a conventional solar thermal panel and a solar PV panel, thereby producing both heat and electricity. They maximize the yield of energy, and are worth considering when space is restricted.

    Payments from the Renewable Heat Incentive (RHI)

    Certified solar hot water systems qualify for incentive payments through the Renewable Heat Incentive (RHI) scheme. The Energy Saving Trust estimates that, for example, these could be around £345 per year for a four-person household with a 4 m2 collector.

    You can find out how much money you are likely to receive by using the government's own Renewable Heat Incentive Calculator.

    Field trial of solar water heating technology

    Solar water heating systems have the potential to work well in the UK when installed properly and controlled adequately by the user. This was the conclusion of a year-long trial carried out by the Energy Saving Trust and published in 2011, which looked at 88 systems throughout the UK and the Republic of Ireland. The main findings can be summarized as follows:

    • A properly installed system can provide around 60% of a typical household's hot water, although this figure varied widely in practice, from a little as 9% up to 98%. This equates to savings on heating bills of £55/year when replacing gas, and £80/year when replacing an electric immersion heater.
    • How a system is set up, controlled and used is crucial to its overall efficiency. Key factors include the volume of the hot-water tank, the timing of back-up heating in relation to main hot-water use, temperature settings, and insulation.
    • There was no difference in the amount of solar energy captured between the two main types of collector, i.e. flat plate and evacuated tube.
    • Amounts of electricity need to run the pumps were generally modest compared to the heat delivered by the system, but faulty timing of pump operation (e.g. at night) will waste electricity.
    • Typical carbon savings are 230kg/year when replacing gas, and 510kg/year when replacing electricity - the same savings that you would make by draughtproofing all doors, windows and skirting boards!