THERE IS GREATER POTENTIAL FROM SOLAR ENERGY AT HIGHER LATITUDES
This article on solar thermal systems is the last in the AJ's renewables series to appear in the magazine, although a piece on labyrinths and buried tubes will appear on www. ajplus. co. uk. An extended version, including more on the technology and its longevity, is also available on the website, along with related information on data sources, contacts, grants and installers.
We don't need to heat our houses in the summer because the sun shines longer and more directly on our hemisphere; we need heat in winter when the sun is low and the skies are clouded with snow and rain. Since we use hot water throughout the year, solar water heating is an effective way of saving energy year-round. Solar energy can also contribute directly to heating in winter, though this can make the technologies more complicated and the gains less effective. Passive solar gain, which is integral to the design of our buildings, is likely to be a more effective way of using the sun's energy for space heating.
There is a presumption that using solar energy is more effective in southern climates. In the UK, the further north you go the colder it gets - and thus the longer is the heating season and the less the solar gain. However, the reduction of solar gain is less than the increase in heating demand, so there are greater potential savings from using solar thermal energy in higher latitudes.
Solar gain is also dependent on solar intensity; the east coast of Scotland, for example, fares better than the cloudier west coast; higher areas slightly better than low-lying areas. During the heating season, about three quarters of the solar radiation is diffuse due to cloud cover, so a rooight will then collect more energy than a window.
Like many other sources of renewable energy, the supply of solar radiation is intermittent, so some way of storing this energy is essential if it is to be used. Thermal storage, say in hot water, rock stores or the building fabric, needs to be matched to the pattern of need.
DOMESTIC SOLAR WATER HEATING In the UK, solar panels can provide on average of about 30-45 per cent of the hot-water requirements of domestic properties.
This percentage can be higher depending on the design and storage, but we can estimate that savings of 1,000-1,500kWh per year can be obtained for a household (annual use is about 3,000kWh). As a rough guide, a typical household would have anything between 3 and 5m 2 of solar panels for hot water, the area varying with panel efficiency and demand. Such a domestic system could cost £2,500-£4,000 installed, so the take-up has been slow.
However, they still give the highest return on investment in free energy of all the solar systems.
There are three main collector options:
at plate collectors - at their simplest, solar water heating panels are made from a sheet of metal painted black, which absorbs the Sun's energy. Water is fed through the pipework embedded in/on the metal sheet, thus picking up the solar energy. The metal sheet is set in an insulated box and covered with glass or clear plastic;
evacuated tubes - this system is more advanced because it uses glass tubes within which a metal absorber collects the heat and transfers it to a manifold at the end of each tube. The tubes are highly insulated, due to a vacuum in the glass. While more efficient than at plate collectors, they are also more costly; and
unglazed collectors - often used for heating swimming pools or where a lower output temperature is required. The panels use an antifreeze mixture. They can be made from plastic or stainless steel and integrated into the roof. They are cheaper than glazed systems.
INTEGRATED SYSTEMS Since solar hot water is easily stored and useful throughout the year, there are most efficiencies in using panels for this. However, some boilers are designed to accept preheated water from solar water panels, allowing solar energy to contribute to space-heating.
Projects where the space heating is provided by a heat pump (eg. a ground-source system) will usually have their own thermal store and distribute heat to a low-temperature underoor system. In these cases it can make sense to connect solar water panels to that thermal store, thus providing additional free heat throughout the year. However, generally it is more efficient to keep the solar thermal store independent of other heating devices.
PERFORMANCE Panels vary in efficiency. Some have 'selective surfaces' - these are films which absorb a higher proportion of solar radiation, and are about 10 to 15 per cent more effective than 'non-selective' films.
Double-skinned surfaces are also used and work well in cold conditions. Polycarbonate, acrylic and polyester sheet, Teon and Tedlar films are all used as well as glass. Thin membranes like Tedlar and Teon have improved shortwave transmission. The lessefficient panels can be made bigger to provide similar annual gains.
Most simple at-plate collectors will provide domestic hot water in summer. However, if solar energy is also contributing to heating (in winter, and often cloudy weather) better results are likely to be obtained from double-glazed panels with selective surfaces or from evacuated tubes.
A DTI comparison of eight solar panels has shown that it is also important to know the parasitic electrical losses that accompany some panel systems for powering pumps, etc.
An interesting alternative is the Solartwin panel, which has a small photovoltaic collector attached that drives the pump, thus matching the ow of water to the solar supply.
Selection of suitable storage and designing the system to address the timing of hot-water requirements can be critical.
Hotels, for example, which use significant amounts of hot water, can benefit from solar heating. However demand for hot water is high in the mornings and evenings, so any design has to match the solar gain to the period of use through thermal storage.
Swimming pools require large volumes of warm water so unglazed collectors can be most appropriate for use here, with the pool itself acting as the thermal store for this system.
However, where the pool also has a heating system, glazed collectors (which are more efficient) can contribute to the system by way of a heat exchanger.
There are advantages in implementing solar thermal as communal systems, for example by providing a single thermal store which contributes heat to each flat, but keeping all the solar panels and control systems close together.
PLACING AND POSITIONING The best orientation at our UK latitude is slightly west of south, with a tilt of 30infinity-35infinity to the horizontal. But panels can also be orientated anywhere from south east to south west with a tilt varying between 10infinity to 60infinity and still perform well. The steeper angles will increase solar collection during spring and autumn at the expense of summer surplus. British Standard BS5918 provides information on procedures for estimating performance.
Panels are often fixed on top of existing roof coverings.
Normally planning consent will not be required, unless the building is listed or in a designated area. But in all cases it is best to consult your local planning department to check on specific conditions.
SOLAR AIR COLLECTORS Solar air collectors consist of an absorber panel of black-painted metal encased with a glazed face. Warm air accumulates within and circulates from the collector to the adjacent room by natural convection. Simple wall-mounted collectors can be added to blank south facing walls (vertical is not the ideal mounting angle).
The same principle works to provide solar preheated ventilation; some window designs incorporate a base panel which draws preheated air into the trickle ventilation system.
Solar air collection is perhaps most effective when a positive-pressure ventilation system can take preheated fresh air from a solar collector. It is also possible to use the roof tiles or slates as solar air collectors, as they can be at least 10infinityC above the incoming air temperature when it is collected through the space under roof tiles (best when south facing).
SUNSPACES AND CONSERVATORIES Sunspaces are basically habitable solar air collectors. They work best when the wall and floor they are attached to have a high thermal mass and can be used to store the heat gained throughout the day. Ventilation to internal rooms is essential to allow the transfer of heat inside.
Since sun spaces are only meant to be heated by the sun, they should be kept small, otherwise there is a tendency for people to use the space in winter and heat it, wasting energy.
In flatted properties with small open balconies, these can be enclosed as sunspaces. Simple single-glazed systems are available where the glazing is frameless and the windows can be slid back to provide open balconies in hot weather.