Solar Thermal/Passive Solar

This method of utilizing solar power is designed solely to heat your water. This could be rigged for liquid in your heating system as well as the water in your taps. The most important feature in this system will be an accumulator tank allowing you to maintain a store of warmed water.

Under the new RHI Premium Payments Scheme (RHI = Renewable Heat Incentive Scheme), the Energy Savings Trust is to run an initial scheme to be put in place from 1 August 2011 to March 2012. In this £12million scheme any householder can receive the following for their main home, providing that they have 250mm of insulation and cavity wall insulation if appropriate:

  • a £300 voucher towards solar thermal technology,
  • a £950 voucher for a biomass boiler for homes not heated by gas from the grid
  • a £850 voucher for an air source heat pump or
  • a £1250 voucher for a ground source heat pump

NBThis scheme is cash limited, so it is a case of first-come, first-served. However, it is anticipated that from next year the scheme will be extended. The first link will take you to the site from which you can apply for the RHI premium vouchers. and the next link takes you to a list of sites from which you can delve deeper into DECC's schemes.

Please mention TigerGreen whenever you contact any of our linked Top Providers, Suppliers and Contractors.

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How might it work for you

Normally Solar Panels are attached to your roof through the tiles and plumbed into your water heating system. The whole is made weatherproof.

Although this is not so critical as for PV, since the heating can be effected by the surrounding air temperature; the installers currently use south facing roofs with a slope of approx. 35° - 37° to the horizontal, that is not overshadowed by trees, buildings etc. Think specifically of the situation in the winter when, in most parts of the UK you will be lucky to get a full 180° of sun at an high enough elevation to cause real heating. It is very important that the panels can intercept as much of the heat as possible. A report by Prof. Anne Jacobson of Humboldt State University states that the best results are obtained if the panels at latitudes above 20°N are tilted at 35° to the south, and those above 40°N are tilted at 55° to the south. In the UK we are between latitudes 50 and 60°N, so the optimal angle of tilt will be over 55°. Bear in mind that:

  • the best returns will be got by the sun hitting the panels at right angles
  • you will require maximum heat in the winter
  • the elevation of the sun in the winter is low, the maximum height attained being dependent on latitude

If the aspect and slope is right, then measure the total area of your roof, to give an idea of how many panels you could fit over it.

Next you will need to know how much electricity or electrical equivalent you have used on water and space heating in the past year i.e. kWh of electricity and kWh equivalents of gas/oil. This gives you a rough idea of how much you need to generate in order to cover a reasonable percentage of your annual usage - preferably 100%.

The panels supplied in the UK are likely to be 1.82m long by 1.2m wide and the depth is ca.10cms. You will now be able to state the approximate max. number of panels that you would need. It is highly advisable to install a well lagged Accumulator tank with this system, and in new builds an underfloor tank would be optimal. This allows very hot water to be stored for later circulation, either at night or during low sun periods in the winter. Additionally it will also help to keep the house warm.

Since from a distance the panels look just like ordinary sky lights or velux windows, planning permission is rarely needed; although there may be problems with putting the system on a listed house.

It is also possible to erect these panels on a Stand Alone structure. This could be in an unobstructed position in a garden or field. The advantage of this is that it will be easier to alter the inclination of the cells to the sun, to maximize heating in the winter months when the sun is at a lower elevation across the UK.

The next stage is to get an idea what other methods of heat generation would be appropriate for your property.

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Facts - Energy Produced and Costs of Production

The solar radiation hitting the earth's atmosphere is 1.360kW/m² (New Scientist 24/11/2007 P45), of this a lot gets absorbed and reflected in the upper atmosphere and the clouds. However, each year the surface of the earth receives an average of 1068kW/m²/yr, and the UK gets an average of 150W/m² on the soil surface. This anticipates a surface that is 100% absorptive - panels are not! There is a lot of space between the pipes in the panels. These panels have a Solar Radiation Absorption of 95%. The Jacobson study mentioned above suggests that Solar Heating in the northern latitudes has an efficiency of 50 - 80%/ann. over the whole area of a static panel.

The panel size quoted above has an area of 2.18sq.m and an absorber area of 1.96sq.m. It is normal to use approx 5sq.m of roof space, at a total cost of £3,000 - £5,000 for the installation. Currently there is grant aid available of £400 or 30%, whichever is least; and from June 2011, even renewable heat will attract RHI (Renewable Heat Incentive) payments for any installations completed after 15th July 2009. These are to be like the FiTs for production of electricity, only they will apply to Renewable/Sustainable Energy Installations that produce heat directly and are not connected to the Grid. Their installation does of course mean that fossil fuel is not being used for heating and there is therefore a saving of Grid electricity and fossil hydrocarbons.

Once installed maintenance is close to zero, an occasional wash down been the only probable necessity. The installation is ruggedly built and the lifespan is likely to be far greater than 20 years.

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Pros and Cons


  • Relatively cheap and easy to install.
  • No annual maintenance costs.
  • Water in the Accumulator tank will be at an high temperature compared to that of the incoming mains water; hence there will be a great cost saving, even if electricity is used as a top up.


  • A boost to the heating may be required from the electrical supply, from time to time, even with a well lagged Accumulator tank.
  • All heating systems require electricity to pump hot water through the piping to radiators/floors.

  • The Solar thermal panels (Passive solar panels) will need to be washed/hosed down from time to time to stop dust and algae/moss accumulating over the years.
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Variations on the theme

Passive or Thermal Solar heating can be as simple as a thin-line radiator in a box on your roof; of which the top of the box is clear glass and the back and radiator is coated with matt black paint. However, systems installed now-a-days use specially designed panels to achieve slightly higher heat returns than did the first series. The cost is considerably more, but then the returns and the longevity of the design are also much better.

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How and Why it works

This system is very simple. The sun's heat is absorbed through the safety glass/toughened glass cover. It is transferred to water from the tank in the house via Copper absorber tubes - copper being a very good conductor of heat.

The 35% of the Infrared Radiation that reaches the earth causes the behaviour of our Winds, Tides, Water Cycle (Rain and Waterfalls) by the process of heating and cooling; as well as powering all the Flora and Fauna on the planet. A 1sq.m area of plant leaf intercepting 0.8kw is able to make use of ca. 0.1% of this to produce the chemical energy needed to power photosynthesis. A lot of the infrared radiation is reflected and much of the 'stray' energy causes the soil to heat up from the surface down.

The centre of our gaseous Sun is around 20,000,000°K (0°K = -273.16°C = -459.69°F). However, by the time that it has reached the visible 'edge' of the Sun, the temperature has cooled to 6,000°K; and emits ca. 5.44 x 1033J of energy. By the time that this heat has travelled the 149,600,000km to earth, by Infrared Radiation (Electromagnetic radiation of wavelength 780nm - 1mm) across the near vacuum of space, it has cooled a lot and only ca. 2.7 x 1024J (½ a billionth of the initial energy) reaches earth. Our atmosphere reflects 65% of this; and the energy reaching the ground is an average of 0.8kW/sq.m and a max. of 1.2kW/sq.m. It is this 35% of the Infrared Radiation, from the Sun, that is responsible for powering all the processes here on the surface of Earth.

Top Tips new and used

  • This is especially for those of you who have installed or are about to install passive solar, otherwise known as solar thermal water heating panels. Many modern Washing Machines. are plumbed in with only one hose that is connected to the cold water mains. This is a good idea if you would otherwise have to turn on your water heating using electricity or gas in order to do your wash. But if using solar power to heat your water anyway, it makes no sense at all to use electricity when you already have free hot water in the tank! The washing machine may of course have to increase the temperature slightly, or even cool it, so it is very important to buy a washing machine that has two pipes and is designed to take from the hot water input first, only using the cold water input to cool the water if the sun is too hot!
  • A passive solar wall is a wonderfully simple, low cost, resource friendly way of getting the sun's heat into your house, whilst still maintaining your insulation status. Basically this requires a south-facing wall without a window, in which a narrow slot (6 - 10cm max. depth) or 2 lines of circular holes can be made; one set at floor level and the other just below the ceiling NB the holes must effectively be pipes completely separated from the cavity wall. On the outside of this a single thickness of window glass is erected leaving a gap of 10 - 20cms. This now looks like a false window that is firmly sealed to the wall. One huge sheet of glass would do; but a wooden framwork of panes works just as well. The winter sun now heats the air between the glass and the wall. So on a sunny day the bottom and top holes are opened allowing the hot air to pass in at the top and the cold air to roll out of the house at the bottom. Finally, one needs to arrange blocks of boxed insulating material in such a way that the holes can be made heat-tight when the sun is not shining! For ideas on operation of the covers see some of the companies in Energy Saving/Building related and other.

We can help you to decide on the best combination of Renewable Energy Technologies for your home or business.

Contact us