Solar hot water: introduction

What is solar hot water?

Also known as solar thermal, a domestic solar hot water system is one which absorbs the sun’s energy and transfers it to a storage cylinder. It is different from photovoltaics; solar hot water panels do not produce electricity, they heat water directly.

Two solar hot water panels on a domestic roof; they are thicker than solar electric panels, but only protrude about 10cm from the roof

Two solar hot water panels on a domestic roof; they are thicker than solar electric panels, but only protrude about 10cm from the roof.

In the UK it will not be the sole provider of hot water; it will complement a conventional system using gas, oil, electricity or solid fuel, but it will pre-heat water so that bills are drastically reduced. During summer months the system can provide all the hot water needed by a household.

Panels can either transfer heat to a separate pre-heat cylinder, or heat a twin-coil cylinder via the bottom coil.

In a direct system, the water that passes through the panels is the water that eventually comes out of the hot tap. In this type of system, there are issues around the water in the panels freezing in winter (so they need to be drained) and lime-scale build-up; in an indirect system, the water in the panels passes through a heat exchanger (coil) in the cylinder and then back to the panels in a continuous loop. Anti-freeze can be added, and there is no problem with lime-scale build-up.

a typical indirect solar hot water system: the gas boiler will kick in if the solar coil in the cylinder doesn’t raise the temperature of the water enough

A typical indirect solar hot water system: the gas boiler will kick in if the solar coil in the cylinder doesn’t raise the temperature of the water enough.

The two main types of collectors are flat-plate and evacuated tube. Flat-plate collectors heat the water directly, evacuated tubes contain a fluid which evaporates at low temperatures, and the resulting gas rises and condenses on a manifold, transferring its heat as it does so.

What are the benefits of solar hot water?

Solar hot water, along with photovoltaics, wind power, hydro, wave and tidal power and geothermal energy are renewable energy sources which don’t involve the burning of fossil fuels, and its associated problems.

Burning fossil fuels releases nitric oxides, nitrogen dioxide and sulphur dioxide into the atmosphere. This causes acid rain which damages forests, wildlife and human health; it also releases carbon monoxide, nitrous oxides, lead, particulates and hydrocarbons, which pollute the atmosphere, and cause damage to plants and ecosystems, and human health, especially respiratory problems.

an entire solar hot water kit, comprising panels, twin-coil cylinder, pump & control set, expansion vessel, air release set, filling bottle, anti-freeze, high-temperature pipe insulation and manual

An entire solar hot water kit, comprising panels, twin-coil cylinder, pump & control set, expansion vessel, air release set, filling bottle, anti-freeze, high-temperature pipe insulation and manual.

The burning of fossil fuels adds an extra 5 billion tonnes of carbon dioxide into the atmosphere each year. CO2 is an important greenhouse gas. In pre-industrial times there were 290ppm (parts per million) of CO2 in the atmosphere; it’s now over 380ppm, and rising by 2ppm every year.

Most scientists agree that the increase in the amount of greenhouse gases in the atmosphere is raising the earth’s temperature, and that it could rise between 1-4°C in the next 100 years (there is only a 5°C difference between now and the last ice age); this would mean lower overall global rainfall, global biomass reduction and extinction of many species, and for humans it would mean desertification, famine, forest fires, increase in tropical diseases, and flooding due to the melting of polar ice.

What can I do?

Solar hot water is probably the most cost-effective renewable energy technology that you can install in a domestic situation in this country, with the shortest payback time. A DTI investigation into solar hot water systems in the UK from 1970-2000 found that a typical system will provide 72% of a household’s hot water over the course of a year (c. 15% in winter and 100% in summer). This is assuming that the roof is south-facing – although if it faces south-east or south-west there will only be a 5% loss of efficiency.

The first thing to do is to choose either evacuated tubes or flat-plate collectors. Installed prices for both are typically in the range £2500 to £5000. If you choose flat-plate, make sure that the collectors have a selective surface – a special coating that maximizes the absorption of solar radiation and minimizes re-emission; in the UK’s climate, selective surface flat-plate collectors are only slightly less efficient than evacuated tubes. However, there is more to go wrong with evacuated tubes, and there have been quality issues. Try asking the salesman for a copy of the product’s EN12975 test report, and remember that cheap doesn’t always mean good value.

fitting solar hot water panels to a roof; a scaffold tower is essential

Fitting solar hot water panels to a roof; a scaffold tower is essential.

You can buy a system installed, self-build and install, or self-build and then have your system installed by a professional. There are often government incentives to install renewables. For solar thermal, the latest is the Renewable Heat Incentive.

There are (cheaper) special systems for swimming pools, consisting of a large area of black tubing.

NB: if you’re thinking of getting solar hot water at some point in the future, then don’t install a combination (combi) boiler – it’s not impossible, but it’s very difficult and expensive to combine solar hot water with a combi. Condensing boilers are fine.

See here for more detailed info on getting and using a solar hot water system.

 


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David Thorpe of One Planet Life is the author of The Solar Energy Pocket Reference Book, The Earthscan Expert Guide to Solar Technology, The One Planet Life, and is a former manager of the publications department at CAT. He runs consultancy and workshops on aspects of solar thermal system design, commissioning and cost.


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