Solar Thermal

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Solar Thermal Water Heating

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.

There is little difference in performance between the two. For many people the decision is a matter of aesthetics.

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Installing Solar Thermal

Solar panels are compatible with most existing hot water systems. However, you will need a new cylinder with two coils. Ideally it should be 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 so the panels can 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. The Energy Saving Trust field trials found that solar thermal panels will provide about 60 per cent of a household’s hot water needs, if well-installed and properly used.

How much you benefit will depend on a variety of factors:

  • How much hot water your household or business uses. The higher the usage, the more benefit you get from a solar thermal system.
  • How much interest you take in how the system works and adapt to make the most of the free hot water (i.e. 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 centigrade at least once a week to avoid risk of Legionella.

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What does solar thermal cost?

 The cost of installing a solar thermal system 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.

The cost of scaffolding and the new cylinder are both significant, so a cost- effective time to install would be when you need a new hot water cylinder, or when you are having repairs done on the roof (or at the same time as a solar PV installation).

Suitability Checks

  1.  Do you have a suitable roof for the collectors (panels)? South-facing is ideal, but anywhere between south east and south west is ok.
  1.  Is the roof is strong enough?
  1.  How will the pipe work be routed to where the cylinder is?
  1. Is there space for the larger cylinder needed by a solar thermal system (or space for a cylinder, if there’s a combi boiler)?
  1.  The cylinder may possibly be sited in the loft – it’s important to check, however, that the floor is strong enough to hold it, and that the roof is high enough for it to fit.
  1.  Is it a suitable household – will the occupants use enough solar-heated water to make it worthwhile? If the house has electric showers and a dishwasher, it often has low hot water consumption and it’s not suitable.
  1. Holiday homes, or homes where there are long periods when residents are absent, are not suitable.

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