Choosing your Solar Thermal System

Solar thermal systems collect free energy from the Sun and use it to directly heat water.

There are two types of rooftop solar collectors in use in the UK. Both systems are suitable for installation throughout most of the UK as they use sunlight and outside temperature to heat the water.

Solar Flat Plate Panels

A flat plate panel is a highly insulated box containing a grid of copper pipes bonded to a flat black copper absorber plate. Special glass enhances absorption of heat energy from the sun. Flat plate solar panels are often a cheaper alternative than evacuated tubes. They are excellent for domestic hot water use and are the type we most commonly install in homes. They can either be fixed to roof tiles or integrated into the roof.

Solar Evacuated Tubes

An evacuated tube system comprises a set of individually sealed tubes covered in a heat absorbing material. Each tube has had the air removed so they retain heat (just like a thermos flask), inside the tube is a pipe containing a liquid that is heated by the sun. The heat rises to the top of the tube where it heats water that is fed into your hot water system.

Solar evacuated tubes are more efficient in converting sunlight into heat than flat plate panels. However, care has to be taken in installing evacuated tubes so as not to oversize them. Accounting for the heat required in winter could result in overheating in the Summer.

An added advantage of solar evacuated tubes is that it’s possible to mount certain types on flat roofs and vertical walls.

Combined Systems

Flat plate and solar evacuated tubes can even be used together. First the flat plates warm the water, then the higher temperatures of the solar evacuated tubes boost the temperature before sending it to your storage tank.

Choosing your System

You can mount a solar collector on a south-facing (or south-east or south-west) roof. However, your choice of solar collector might be affected by any of the following considerations:

Size - A flat plate system will typically need to be 20% larger than one containing evacuated tubes in order to heat an equivalent quantity of water. However, as the name implies, a flat plate collector generally has a lower profile than the tubes. Tubes are also significantly heavier than flat plate collectors. Knowing whether your roof is structurally up to the job is an important consideration when deciding on flat plate or evacuated tubes.

Efficiency - Evacuated tube systems are typically more efficient at heating water (hence they can be smaller as noted earlier). Flat plate collectors deliver water at lower temperatures. However this is not always a disadvantage. Some parts of evacuated tube systems can reach several hundred degrees centigrade and much of that energy may dissipate and be wasted if the pipe bore, pump speed and distance to holding tank are not properly considered. Oversized evacuated tubes can deliver too much hot water which is in itself wasteful.

Cost - Flat plate systems are typically cheaper and easier to install. Ensure your installers do the mathematics to work out whether short term savings or long term gains are best for both the environment and your pocket. The ultimate cost of flat plate or evacuated tubes will vary according to the size, position and heat efficiency of the house itself.