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Department for Business, Energy and Industrial Strategy

Solar Thermal Collectors

Solar thermal systems use heat from the sun to heat water. This replaces other energy sources such as natural gas and electricity as a means of providing hot water to buildings. A conventional boiler can be paired with the solar thermal system to make the water even hotter, or to provide hot water when solar energy is unavailable

The most important part of a solar thermal system is the ‘collector’. The collector’s role is to absorb the sun’s energy and efficiently convert it to heat for transfer to the hot water system. There are a number of different types of solar thermal system; a typical system is shown in Figure 1.

solar thermal collectors image 1.png

Figure 1. Typical twin coil solar thermal system

Hot water is supplied to the building by transferring the heat (energy) absorbed by the collector and pumping it down to the cylinder tank. The solar energy reduces the heat demand on the boiler, and therefore energy consumption and cost.

Solar Thermal collectors can be hugely beneficial in residential buildings where the hot water demand is high and in non-residential buildings such as leisure centres and swimming pools.


There is a common misconception that solar thermal systems do not operate when it is cloudy. In the UK there is a relatively high percentage of cloudy days compared to clear days. When the sun’s radiation passes through the clouds, it is dispersed and is known as diffused radiation. On clear days, with no dispersion effect, it is known as direct radiation. Solar thermal systems are able to operate in both conditions.

However, solar thermal systems will provide more hot water during the summer than during the winter due to the lower levels of sunlight available in winter months, among other factors. This means that an additional heat source will still be required to heat the water during the winter months. However, solar thermal systems are still able to save between 40% and 60% of the energy that would have been required annually to heat up the hot water using conventional energy sources, such as gas.

Types of solar thermal collectors listed on the ETL

The ETL covers solar thermal systems incorporating two main types of solar collector:

  • Glazed flat plate collectors
  • Evacuated tube collectors
    • Direct flow collectors
    • Heat pipe collectors

Glazed flat plate collectors

Glazed flat plates are constructed with insulation on their back and sides and are covered by a transparent cover. This reduces the heat loss to the surrounding area and therefore improves the performance of the collector.

A commercial installation of 43m2 ETL listed glazed flat plate collector with an annual yield of 350kWh/m2 could result in the following annual savings when compared to conventional water heating using a gas, oil or electrically heated boiler.


Solar thermal replacing gas boiler

Solar thermal replacing oil boiler

Solar thermal replacing electric water heating

Fuel Savings (£/yr)




Energy Saving (kWh/yr)




Carbon Dioxide Savings (kgCO2e/yr)




Payback period

20 years

11 years

6 years


Evacuated tube solar collectors

Evacuated tube collectors use glass tubes in which a vacuum is created. This acts as a source of insulation, reducing heat losses from the collector and making the product very efficient. There are two types of evacuated tube collectors: direct flow and heat pipe collectors.

Direct flow collectors

Direct flow evacuated tube collectors use an evacuated tube inside a U-shaped tube.

The fluid in the solar system is used as the heat transfer medium and the fluid runs through a concentric tube-in-tube or a U-shaped tube to the base of the glass bulb and then returns to the header.

Heat pipe collectors

A heat pipe evacuated tube collector uses alcohol or water in a vacuum which is used to absorb the sun’s energy.

Due to the vacuum, the alcohol or water will evaporate at a low temperature of 25°C to form a vapour. This vapour then rises up the collector tube to the heat exchanger where heat transfer to the solar fluid takes place. Following heat transfer, the vapour condenses back to a liquid and flows back down the collector tube.