While Solar PV system turn the sun’s energy directly into electricity, solar thermal panels harness the sun’s energy by turning the solar radiation into heat. This heat is normally then used to heat water for use in the home.
At the heart of every solar thermal system is the collector and broadly speaking there are three types of collector to choose from – flat panelled collectors, plastic collectors and evacuated tube solar collectors.
The evacuated tube solar collectors are the most expensive, but importantly they are also the most efficient with a conversion efficiency of over 90%. This means that they can produce more heat, so while a simple plastic collector might be fine to help keep a swimming pool warm – if you want to produce hot water to use in your radiators then ideally you are going to need to go with one of these.
The glass / glass evacuated tube solar collector
The evacuated tube comprises of a smaller glass tube suspended within a larger glass tube. The air is then pumped out of the space between the small inner tube and the larger outer tube creating a vacuum thermal insulation layer. This vacuum layer is absolutely key since it reduces heat loss from the solar collector.
The inside of the inner glass tube is coated with a selective light absorber such as aluminium nitrate or titanium nitride oxide, which helps maximise the absorption of solar radiation over a large range of wavelengths.
An absorber plate (normally made from copper) then runs the length of the inner glass tube, which absorbs the heat and transfers it to a heat transfer fluid. In passive systems, convection drives the movement of the heating fluid around the solar collector, when the transfer liquid gets heated it evaporates and turns to steam. This rises to the top of the evacuated tube solar collector the heat is transferred via a heat exchanger to another liquid – typically the potable water that is then stored in the hot water storage tank. The transfer liquid, having given up its heat to the heat exchanger then condenses and falls back down the evacuated tube where the process can start again.
The Cost and Potential Returns for an evacuated Tube solar system
A typical evacuated tube solar collector system will cost about £3,000 – £5,000 to get installed on your property, and will typically produce about 1,000 – 2,500 kWh of useful heat – or about 50% of your hot water requirements.
As long as you get it installed by a MCS approved installer you will then be entitled to the renewable heat premium payment grant which for solar thermal installations is about £600. In addition, in March 2013 the Renewable heat incentive is launching which will pay you 17.3p / kWh of heat produced.
Therefore you should be looking at an annual return of between £170 and £430 depending on the exact output you evacuated tube solar collector and payment is guaranteed for 7 years, but obviously the key thing is that you are producing free hot water for use in your home.
It is worth remembering that temperature of the water produced by your system is dependant on the weather – you obviously produce more hot water in hot sunny weather – and also the season, with the majority of the hot water produced during the summer months.
As a result we would not suggest solely relying on a evacuated tube solar collector for all your hot water. Instead it should work in conjunction with your existing hot water system (normally gas boiler or immersion heater). In addition it is recommended that water is heated to 650C to ensure that the temperature of the water is sufficient to kill Legionella bacteria (that cause Legionnaires disease), therefore if your solar thermal system only heated it up to 600C for example, then the boiler can just provide the ‘top-up’.
Finally, if you have a combi boiler installed in your home – to take advantage of the hot water you produce you will need to install a hot water storage tank.