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Solar Hot Water (SHW)

Solar Hot Water Panels
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Solar radiation from the Sun totals 5.4 million exajoules (EJ) per year. This solar radiation can be converted either into thermal energy (heat) or into electrical energy. This page looks at Solar Thermal Energy (STE) or Solar Hot Water (SHW).

Solar thermal systems use the sun’s energy to heat water for use in a property for washing and other domestic uses; they do not usually heat the property itself. In the UK solar water heating can deliver around 50-60% of year round water heating.

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Benefit of SHW in the UK

The two standard collector panel types are "flat plate" and "evacuated tube". Flat plate systems use a dark plate in an insulated box to transfer energy into the water system. Evacuated tube systems are more expensive and sophisticated, using metal strip collectors in vacuum tubes but have the advantage that smaller panels are needed.

Example: Large dwelling with a 300 litre cylinder served by 6.0m2 (three 2.0m2) solar collector panels. Installed cost estimate for panels, cylinder and pipework is £8,000. This would yield roughly 4,200kWh per annum, save 840kgCO2 per year and provide a RHI payment of around £756 per annum. Payback is 10.5 years.

Solar Hot Water Rules of Thumb

The following are rule of thumb approximations for solar thermal (hot water) installations which can be used for preliminary assessment:

  • 1m2 of evacuated tube per person
  • 1.5m2 of flat plate per person
  • 1m2 = 500-800 kWh saving per year
  • 1m2 = 120-160 kgCO2 saving per year (based upon replacing gas)
  • 1m2 = £20-30 saving per year (on gas at 3.9p per kWh)
  • 1m2 = 50 litres of hot water per day (relates to cylinder size)

Note savings depend upon which fuel the comparison is made against.

SHW Design Notes

Ideally solar panels for generating hot water or electricity should be installed at an inclination of 45° on a south facing roof clear of shading. There is little reduction on performance if the inclination is varied between 20° and 50° or the panels face up to 20° either side of south. Installing in a horizontal or vertical plane will add 20-30% to the required absorber area.

Other Terms

SDHW: Solar Domestic Hot Water

STE: Solar Thermal Energy

Further Information

Solar Energy

Solar radiation totals 5.4 million exajoules (EJ) per year, of which 30% is reflected back into space leaving 3.8 million EJ for use on earth, >10,000 times greater than the 230 EJ consumption of fossil and nuclear fuels in 2002 (Boyle).

SHW Potential

In the UK solar water heating can deliver around 50-60% of year round water heating.

Summer to Winter

In the summer months, 90% of the hot water needs of a typical home may be met by these systems. However, this can fall to less than 20% in winter.

Suggested Reading

CIBSE: Solar Heating Design and Installation Guide

Energy Saving Trust: CE131 Solar water heating systems – guidance for professionals, conventional indirect models

Boyle: Boyle, G (2004), Renewable Energy, Power For a Sustainable Future, Oxford: Oxford University Press

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