Solar hot water: introduction

“The sunlight that strikes Earth’s land surface in two hours is equivalent to total human energy use in a year. Only a small portion of that enormous daily, renewable flux of energy will ever be needed by humanity.” – Christopher Flavin

What is solar hot water?

Also known as solar thermal, a domestic solar hot water system is one that absorbs the sun’s energy and transfers it to the water in a well-insulated storage cylinder, so that you have hot water when you need it. Solar thermal is different from solar photovoltaics, as solar hot water panels (usually called ‘collectors’) don’t produce electricity – they have water flowing through them, which they heat directly.

In temperate regions, it won’t 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 your hot water, and it should provide more than half the annual hot water needs for an average household.

Two flat-plate panels on a domestic roof; they’re thicker than solar electric panels, but only protrude about 10cm from the roof.

Collectors are usually installed on a sloping roof, but can also be installed on a flat roof, or even on the ground.

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

Insulated pipes deliver the heated water from the collectors to a hot water cylinder.

There are two kinds of system – direct and indirect. 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.

Plumber demonstrates they mysteries of heat transfer coils in hot water cylinders by cutting away the side of the cylinder. Coils can transfer heat to or from the cylinder. So for example, hot water from solar panels can transfer heat to the cylinder via a coil; but also, hot water in the cylinder can transfer heat to water in a coil for the central heating. A cylinder can have one, two or more coils.

In an indirect system, the liquid in the panels doesn’t come out of the tap – it transfers its heat to the water in the cyclinder via a heat exchanger (a coil of pipe in the cylinder) and then returns to the panels in a continuous loop, to be re-heated. In this kind of system, the heat transfer liquid can contain anti-freeze, and there’s no problem with lime-scale build-up. Almost all solar hot water systems are indirect.

Panels can either transfer heat to a separate pre-heat cylinder, or heat a twin-coil cylinder via the bottom coil (see image and video). There will be another coil in the cylinder that usually goes to a conventional boiler, but could be used with a wood stove (see below).

History: in the 1880s a French engineer, Charles Tellier, built flat-plate collectors to heat water that produced stream to run a mechanical water pump. He went on to use the collectors to heat water for his home. Unfortunately, fossil fuels then took over for most water heating – but renewables are on the rise again nowadays.

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.

What are the benefits of solar hot water?

Solar thermal, and other renewable energy sources, doesn’t involve the burning of fossil fuels, which releases nitric oxides, nitrogen dioxide and sulphur dioxide into the atmosphere. This causes acid rain which damages forests, wildlife and human health, and acidifies the oceans; it also releases carbon monoxide, nitrous oxides, lead, particulates and hydrocarbons, which pollute the atmosphere, and cause damage to plants, ecosystems, and human respiratory health.

The burning of fossil fuels adds over 40 billion tonnes of carbon dioxide into the atmosphere each year, and rising. CO2 is an important greenhouse gas. In pre-industrial times there were 290ppm (parts per million) of CO2 in the atmosphere; it’s now well over 400ppm. The last time there was such a concentration of carbon in the atmosphere was 3 million years ago.

Solar hot water

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 increase in greenhouse gases in the atmosphere is raising the earth’s temperature, which will accelerate biodiversity loss, and for humans will mean desertification, famine, forest fires, increase in tropical diseases, and flooding due to the melting of polar ice.

In short, burning fossil fuels is a really bad idea, and we should relegate it to history as soon as possible.

Solar thermal reduces your energy bills, and is possibly the most cost-effective renewable energy technology that you can install in a domestic situation, 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.

A look at a solar thermal (evacuated tube) self-installation, with a wood-burner with back boiler connected as well (n.b. you can use a cheap plunger-type filling bottle to pump your heat-transfer liquid into the loop of pipe from the panels to the cylinder).

What can I do?

Maybe talk to other system owners, read our book, Solar How Water: Choosing, Fitting & Using a System, attend courses, ask questions – get as much basic information as you can. Make sure your home is well-insulated, in good repair and dry before thinking about renewable energy systems.

A solar hot water system is for delivering hot water to your taps, shower and washing machine / dishwasher. It could be used for supporting space heating, but those types of systems are significantly more expensive and make specific requirements of your house – for example, it needs to be super-insulated and you need more panels.

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

Make sure your roof is facing the right way (see ‘benefits’, above), and that either your cylinder has an additional coil to add solar, or that an extra coil can be fitted – but often, you’ll need to make sure you have the space to install a larger, twin-coil cylinder. If you can manage to position your collectors below your cylinder, then because hot liquid rises, you won’t even need a pump to deliver the transfer fluid from your panels to your cylinder.

You can buy a system installed, self-build and install, or self-build and then have your system installed by a professional. You have to know what you’re doing if you intend to self-install. You absolutely need plumbing experience, and to understand that although it may look like conventional plumbing, solar thermal installation has some unique characteristics that are more demanding and potentially more dangerous.

Evacuated tubes.

There are often government incentives to install renewables. Search online for grants, or see the Energy Saving Trust website for the latest news.

You probably won’t need planning permission, unless you live in a listed building or a conservation area. Check with your local planners, or your installer can help.

Decide whether you want evacuated tubes or flat-plate collectors. Installed prices for both are typically in the £3-5000 range. If you choose flat-plate, the collectors will have a ‘selective surface’ – a special coating that maximizes the absorption of solar radiation and minimizes re-emission; in temperate climates, selective surface flat-plate collectors are only slightly less efficient than evacuated tubes. But tubes are a bit more efficient, so you’ll need a smaller area for the same energy output – which may be important if you don’t have much roof space. Flat-plate systems historically have been more reliable over longer periods of time – there’s less to go wrong, and they tend to operate at lower temperatures.

Ingenious and cheap home-made solar swimming pool heater.

Solar hot water works well in combination with a wood stove with a back boiler. This way the solar panels will provide all your hot water in the summer, and the wood stove all your hot water in the winter. Then you’ll have year-round hot water from renewables. This will involve installing a triple-coil cylinder (or having another coil installed in your twin-coil cylinder) if you intend to keep your existing boiler, or a twin-coil cylinder if you don’t. More information about this can be found in our Heating with Wood book.

Modern systems are automated. You don’t have to decide when to switch them on and off – it does it for you. You don’t have to re-set any clocks after power cuts; and you don’t have to change how and when you use hot water (although if you do, you may get more benefits from your system – for example, if you change from having a shower in the morning to the evening).

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

Solar hot water

In hotter countries, you’ll often see the tank on the roof too – above the panels, so that the tank can be heated via a thermosyphon, without the need for a pump.

NB: if you’re thinking of getting solar hot water at some point in the future, then maybe think twice about installing 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.

Your installer will leave you with a little checklist to go through every couple of months, just to make sure everything is working properly (if you install yourself, you’ll know what these are). It might be a good idea to have a full maintenance check every 5 years or so.

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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