Solar thermal - cost £1000/m2; passive

Solar thermal (or solar hot water/heating) is a tried and tested technology that uses free energy from the sun to heat your hot water. It remains the least expensive and most effective form of renewable energy. It generally plugs into your heating system, with a conventional boiler providing hot water when the solar energy is unavailable.
 
The cost of installing a typical solar thermal system is around £1,000 per m2 – as a general rule 1m2 is required per occupant.  Savings aren’t huge, perhaps up to £100 per year, although solar thermal is covered by the Green deal and RHI which could earn you about £100 per person per year – see our funding section for more information.
 
Solar thermal panels do not need sunlight to work, and will provide some energy all year round – in the summer you will be able to turn your boiler off, although in the winter it will still be doing the lion’s share of the work. As a general benchmark, solar thermal can provide at least 50% of your annual hot water requirements – for free!
 
This means that once you've paid for the initial installation, your hot water costs will be reduced by 50%, as will the carbon dioxide emissions from heating your water.
 
Solar thermal systems use solar collectors, which are generally fitted to your roof. They collect the heat from the sun and use it to heat up water stored in a hot water cylinder. A back-up boiler can then be used as required
 
Very large solar thermal collectors can provide some contribution to heating your home as well, although this requires some inter-seasonal heat storage, which is expensive and space-consuming and generally not considered worthwhile. 

Types of solar thermal collector:

There are two main types:
- Flat Plate Collectors
- Evacuated Tubes
 
Both of these share the common component of an absorber plate – a coated metal plate that absorbs the sun’s radiation and transfers the heat to fluid, which in turn feeds the hot water system.
 
flat-plate collector consists of a large flat absorber, a transparent cover, a frame, and insulation - only very little of the heat emitted by the absorber escapes the cover. The heating fluid runs through pockets in the absorber plate and directly down through the coil in your thermal store. Flat plate collectors can be fixed onto the roof or frame, or incorporated into the roof, thus saving money on tiles in new-builds.
 
In evacuated tube collectors, the absorber strip is located in an evacuated and pressure-proof glass tube. The heat transfer fluid flows through the absorber either directly or indirectly. Several tubes connected to each other make up the solar collector. Heat pipe collectors incorporate a special fluid which begins to vaporize at low temperatures - the steam rises in the individual heat pipes and heats the heating fluid by means of a heat exchanger. The condensed liquid then flows back into the base of the heat pipe.
 
Evacuated tubes offer the advantage that they work efficiently with high absorber temperatures and with low radiation, and each tube is able to be twisted toward the South, making them more versatile and able to be mounted on vertical surfaces. It is also possible to change individual tubes if they are faulty.
   
In general for a given absorber area, evacuated tubes are more likely to maintain their efficiency over a wide range of ambient temperatures and weather types, making them more suited to the UK. In constantly sunny climates flat plate collectors are more efficient, but in cloudy conditions their energy output drops off rapidly in comparison. 

Positioning

If you think solar thermal is for you, then first and foremost you need a good sunny spot in which to install the collectors, about 1m2 per occupant. Being directly south-facing and at the optimum angle isn’t as important for Solar Thermal as for PV, especially if you go for evacuated tubes (see below). Collectors installed on both east and west slopes will do the same job as those installed on a south slope – although obviously you’ll need twice as many.
 
Generally, solar thermal collectors will be positioned on a roof, with the thermal store somewhere below in the house - the closer the better. This means that a pump is required to move the heating fluid around the system. In passive systems however, the collectors are mounted below the thermal store, relying on gravity and the tendency for water to naturally circulate as it is heated to move the heating fluid around without pumps. Because they contain no electric components, passive systems are generally more reliable, easier to maintain, and possibly longer-lasting than active systems.
 
Most conventional boiler and thermal store systems are compatible with solar water heating - the key is to have a dedicated coil. However, if your boiler is a combination boiler (combi) and you don't currently have a hot water tank, a solar hot water system may not be compatible, or you may need to replace the boiler.

Maintenance

Ongoing maintenance costs of a solar thermal system are very low, if any, since a well-installed system should run for years without problem.
 
However, keep an eye on your system to check that it is doing what it is supposed to do, and from time to time it is worth checking for leaks – solar thermal systems use anti-freeze and this will have a strong smell. If you are not getting hot water or the solar pipework is cold when the pump is running on sunny days then again you should contact your installer. It’s also a good idea to keep the collectors clean.
 
Every 5 years (or as specified by your installer) you should have your system checked thoroughly. It is likely that the anti-freeze that is used to protect your system in the winter months will need to be topped up or replaced as it breaks down over time, reducing the performance of your system. The cost of replacing the anti-freezer is around £100. Your installer will also check the pump – it should last for at least ten years and costs around £90 to replace.

Planning restrictions

Solar Thermal systems come under permitted development in the UK, with some guidelines:
· The array must not project in excess of 200mm from the plane of the roof
· The array must not extend higher than the highest part of the roof or chimney
· The array will require listed building consent if the building is listed
 
As with PV, if you do decide to install solar thermal collectors, it is imperative that you have a structural engineer check that your roof can handle the extra load first – an MCS accredited installer should sort this for you.
 
To estimate exactly how much electricity a Solar Thermal array could generate for you, use the Encraft tools here – you will have to register first.
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