Solar thermal energy is freely available, easy to store, works whenever the sun is out, and avoids greenhouse gas emissions. After its initial installation cost, hot water bills are cut significantly, and it replaces fossil-fuelled or electric water heating. No surprise then, that solar thermal energy is increasingly used across the globe.
- 41 million solar water heaters in use in 2008, and market growing rapidly.
- 58% of global capacity is installed in China.
- A 4m² solar collector provides about eight showers per day in the tropics, four on average in the UK.
How solar thermal systems work
A solar thermal system needs a collector to capture heat from the sun, and usually some form of heat storage as well. To capture as much heat as possible, the collector is designed to maximise the absorption of solar energy, minimise heat loss and transfer heat efficiently to where it will be used. Heat is often transferred by circulating a fluid through pipes but sometimes the collector is part of the system that needs the heat.
Collectors for solar water heaters
- Glazed, flat-plate collectors have a 2 to 4 m² plate to collect the solar heat , with a flat sheet of high-transmission glass in front of it to reduce both radiant and convective heat loss. Insulation cuts heat loss from the back of the plate and a ‘wavelength selective’ surface on the front cuts radiant heat loss while maximising the absorption of solar radiation.
- Evacuated tube systems use an array of narrow (50 to 60 mm wide) collector plates inside glass tubes. The tubes are evacuated to minimise convective heat loss and thus increase efficiency, particularly when the outside temperature and solar radiation are low.
- Unglazed flat plates are cheap. In situations where only a small temperature lift is needed they are efficient, so they are generally used for heating swimming pools.
Uses and benefits of solar thermal systems
There are a variety of uses for solar heat.
Solar water heaters
The most common use for solar heat is to provide hot water, usually at relatively low temperatures (less than 100oC) for domestic use. Such systems are popular because hot water is needed all year round and is cheap and easy to store in a well-insulated tank, so that heat can be collected at one time of day and used at another. The solar heat often replaces fossil-fuelled or electrical water heating, cutting utility bills and greenhouse gas emissions. Access to hot water promotes hygiene and health, and solar water heaters are sometimes mandatory in social housing – for instance, in several programmes in Brazil.
Many solar water heaters use an electric pump to circulate a heat-transfer fluid between the collector and the tank. The water in the tank heats up during the day, and will still be hot the following morning, provided that the tank is well insulated. The alternative is to use the buoyancy of the heated fluid in the collector to drive the circulation which removes the need for electric power, so that solar hot water can be available away from the electricity grid or during power cuts. However, non-pumped systems work only if the hot water tank is mounted higher than the collector, and if lengths of connecting pipe are short.
If the hot water tank is designed to take two heat sources, then a pumped solar water heater can be used with another heat source, for example a gas boiler. This type of tank does not cost much more than a standard one, but it is costly to have to replace an existing tank when installing a solar system. Ashden Award-winner Willis Renewables developed the ‘Solasyphon’ – an external heat exchanger which can be used to connect a pumped solar water heater to an existing tank – avoiding the cost and inconvenience of replacing the tank.
Solar water heating can be used in different climates. Around the tropics, where the amount of solar radiation does not vary much throughout the year, a solar water heater can provide nearly all the hot water needed for daily showers and other uses throughout the year. (see box) Ashden Award winner ECAMI has installed solar water heaters in both homes and hotels in Nicaragua. Even in the UK, a solar water heater which is sized to provide nearly all the hot water in the three sunniest months will continue to make a contribution during other seasons, and provide about half the hot water over a year. Kirklees Council installed solar water heaters in both existing and newly built social housing to do this.
How much water is heated?
A typical 4 m² solar water heater supplies approximately 10 kWh of heat on an average day in the tropics. This could increase the temperature of 280 litres of water (enough for about eight showers) by 30°C. The same system would supply about half as much heat on an average day in the UK, with much more variation between summer and winter.
Solar heat can be used to dry crops, by increasing the temperature of the crop and also increasing the rate of air flow through it. The simplest solar driers combine the solar collector with the drying cabinet. More complex driers have a separate collector to pre-heat air which then rises due to buoyancy and flows through the crop.
The food drying company Fruits of the Nile in Uganda, works with farmers who dry sliced bananas and pineapple using solar driers each about 6 m² in area. An individual drier can take about 25 kg of freshly sliced fruit, which dries in about three days.
This need for a large area makes solar drying difficult for large scale food processing. But it fits ideally with on-farm processing. A solar drier allows a farmer to make an income from fruit which would otherwise have gone to waste, either because it was grown too far from the market, or because of seasonal gluts of produce.
A well-known, simple use of solar thermal energy is in greenhouses, which raise the temperature of the air around crops by capturing solar heat, but a conventional greenhouse, without supplementary heating, does not keep very warm in winter.
By looking carefully at solar design, GERES and its partners in Ladakh have developed simple greenhouses which keep the inside temperature above freezing even when the air temperature outside falls to -25oC. This allows vegetable production to continue through the winter, and improves the nutritional value and variety of the diet. These greenhouses are built with a south-facing polythene wall and massive walls on the north made from local stone, which are painted black inside to absorb the solar heat and store it during the day, and are insulated with straw on the outside. The polythene is covered by a cloth at night to reduce heat loss.
Solar thermal power generation
When sunlight is focussed onto a solar collector using mirrors or lenses, the temperatures reached can produce high-pressure steam to run a steam turbine that generates electricity. Concentrated solar power (CSP) has been used in this way since the 1980s. It is now close to commercially viable in places with high levels of sunlight and high daytime electricity demand. If heat storage is included, then CSP systems can generate power whenever it is needed.
Solar thermal systems can be used for many other purposes, including:
- Cooking – where a reflector is used to focus the sun’s rays onto the cooking pot;
- Desalination and water purification – where solar energy is used to evaporate water from a contaminated or saline source. The water vapour condenses on a cooler surface, providing clean drinking water.
The cost of solar water heaters varies greatly between countries. In the UK and USA, the installed cost of a 4 m² solar water heater and tank is about US$6,000 to US$8,000 (£4,000 to £5,000), with evacuated tube systems at the upper end of the range. By contrast, a standard 2 m² evacuated tube water heater, ready to install, costs only about US$250 in China.
The International Energy Agency (IEA) estimated that there were 41 million solar water heaters in operation at the end of 2008, with a total area of 220 million m², an average of about 5 m² per system. Evacuated tube systems make up about 54% of this area, glazed flat plates 33%, and unglazed collectors 13%.
China has the largest installed area, about 58% of the global total (mostly evacuated tubes), followed by the USA with 10% (mostly unglazed) and Turkey and Germany with 5% each.
The estimated global capacity of solar thermal power generation is nearly 1 GW, using about 6 million m² of solar reflectors.
Globally, solar water heaters supplied about 110 TWh of heat in 2009, with around 2 TWh of electricity from solar thermal power stations. To put these numbers in context, wind turbines generated about 340 TWh of electricity and solar photovoltaics about 24 TWh of electricity during that year.
There is much less use of other solar thermal systems, and no systematic international data collection.
Renewable heat attracts much less publicity than renewable electricity, but despite this the use of solar thermal energy is increasingly rapidly. 42 million m² of solar water heaters were installed during 2008, and an estimated 55 million m² during 2009. If the cost reductions achieved in China start to impact on the rest of the world, then this growth will accelerate.
There is growing interest in solar thermal power generation. Around 15 GW of capacity is currently in development or under construction, and scenarios produced by the IEA anticipate around 1,000 GW installed by 2050
Author: Dr Anne Wheldon