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Thermal Energy


Solar water heating (known as solar thermal) systems capture the free heat from the sun and use it to heat up water for use in the home. It’s a simple process:

  • panels on your roof absorb heat from the sun – they are known as the collector

  • the water in the panels heats up

  • this hot water is pumped through a coil in your cylinder

  • which transfers the heat to the water in the cylinder


Is solar thermal suitable for my home?

The ideal situation for solar panels is facing due south, although they are effective facing anywhere between south east and south west. As a rule of thumb you need between 1 and 2 m2 of collector (solar panels) per person living in the house.Most panels are mounted on a roof, but they can also be mounted at ground level. It is important that they get direct sunlight. To get the best results they should be at an angle between 20 and 50 degrees from horizontal (most pitched roofs fall within this bracket).

If you have an electric shower it won’t use your solar hot water. Similarly cold-fill dishwashers and / or washing machines heat the water they use. In this situation, solar water heating is not suitable unless you use the bath for most of your washing and bathing, as you won't be able to use much of the solar hot water you generate.

Solar panels are compatible with most existing hot water systems, although if you may well need a new cylinder that is tall and thin, with two coils, and ideally big enough to hold two days worth of hot water.  Solar hot water with combi boilers is more difficult, but still possible. If you have a combi boiler it is important to check with the manufacturer that it will accept pre-heated water.If your present system is gravity fed, it will need a control (such as a valve and pump) for the hot water circuit, if the panels are to work effectively in winter when the boiler is running for central heating.


How much hot water from solar thermal panels?

Solar thermal panels should provide most of your hot water from April to September, and make a worthwhile contribution in the months on either side of that period. Outside of that estimates vary depending on who you ask.

Solar panels will provide about 60 per cent of a household's hot water needs, if well-installed and properly used according to Energy Saving Trust field trials.

The reality will depend on a variety of factors:

  • How much interest you take in how the system works and adapt to make the most of the free hot water (ie having showers in the evening rather than the morning). The sun isn’t as reliable as a timer clock.

  • The size of your cylinder. Many cylinders only hold enough water for a day’s supply of hot water, so a day or two of cloud and rain will mean you have to turn on the boiler or immersion heater.

  • How you programme your back up heating. If your control panel does not allow you to programme the hot water and central heating separately, you may not get the maximum benefit from the solar panels when the heating is turned on. By only boosting the hot water once the sun has gone down, you maximise opportunity for solar heating.

  • Adequate insulation of both cylinder and pipes carrying hot water.

  • Allowing hot water temperature to vary. If you do not need high temperatures all the time, you will have less need for back-up heating. You will also reduce heat loss. However, it is important to make sure your cylinder reaches more than 60 degrees centrigrade at least once a week to avoid risk of Legionella.

The EST identified a huge range of performance, with the best system producing 98 per cent of the household's hot water, and the worst just 9 per cent. The median across all systems was 39 per cent, so it is important to take action to maximise your solar gain.


What type of solar thermal panel is best?

There are two types of solar thermal panel: flat plate panels and evacuated tubes.Flat plate panels consist of an absorber plate in an insulated metal box. The top of the box is glass or plastic, to let the sun’s energy through, while the insulation minimises heat loss. Lots of thin tubes carry water through the absorber plate heating it up as it passes through.Instead of a plate, evacuated tube collectors have glass tubes containing metal absorber tubes, through which water is pumped. Each tube is a vacuum (the air is ‘evacuated’ hence the name), which minimises heat loss.  The Energy Saving Trust field trial found little difference in performance between the two. For many people the decision is a matter of aesthetics.Research from Swiss-based Solartechnik Prufung Forschung shows that the best performing collectors are more than twice as efficient as the worst ones. It found that the most effective  flat plate collectors are made in Austria and Germany and the best evacuated tubes in Switzerland and Northern Ireland. Six of the 10 worst performing evacuated tube collectors are made in China.


What does solar thermal cost?

Domestic solar thermal panels installations are subject to a huge range of prices. Researchers at Oxford University's Environmental Change Institute analysed the prices of systems getting low carbon buildings programme grants between 2006 and 2008. They found prices ranging from £1,000 to £8,000. The cost varies a good deal from one contractor to another, and will depend on the type and quality of the panels, whether you need scaffolding, and how easy it is to integrate into your existing plumbing system.

As a guide, average systems are likely to cost between £3,000 and £5,000.

Phase 1 of the Renewable Heat Incentive (RHI) is for non-domestic installations. It is available for installations with a capacity of less than 200kWth. The incentive will pay 8.5p per kWh of heat generated. The tariff will be index linked, and paid for 20 years. The heat generated must be measured by an approved meter.

Phase 2 of the Renewable Heat Incentive will be for domestic installations. It is expected to start in summer 2013, with a consultation document due in September 2012. In the meantime one off capital grants of £300 are available from the Renewable Heat Premium Payments scheme. Apply through the Energy Saving Trust.

Eligible solar thermal systems installed since 15 July 2009 are expected to receive the incentive, but until the consultation is announced we don't know how 'eligible' will be defined.

Well installed, and properly-used, solar hot water systems will provide around 60 per cent of the hot water a home needs. Typical savings from a well-installed and properly used system in the EST field trial (published October 2011) were £55/year when replacing gas and £80/year when replacing electric immersion heating


Information sourced from 


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