Solar Thermal Systems

1. Solar Heating

Solar collectors convert sunlight into heat and produce hot water, and, in larger systems, assist the space-heating system. Solar-thermal systems can save significant quantities of energy and reduce

CO2 emissions. Most operate in combination with another heating source (oil, gas, electricity, wood) – the non-solar component only operates when heat demand is too high for the solar system alone to satisfy. Heat storage and distribution systems need to be optimised.

In a solar water heating system the solar collectors are usually installed on the roof of the building. The circulating fluid in the system, which delivers the solar heat from the collectors to the storage tank, must be both frost and heat-resistant. The heat is transferred to the water in the storage tank via a heat-exchanger. Another source of heat (gas, electricity, oil, wood) is used to heat the water during periods of low solar energy. Other system components are the pump and pump controller, temperature display, expansion vessel and valves.

If, as well as providing hot water, solar energy is to be used to top up the central heating system, the size of the collector surfaces is increased by a factor of about 2 to 2.5. The saving on fuel is somewhere between 10 % and 30 %, depending on the insulation levels of the building. With low energy buildings, savings of up to 50 % are achievable.

Where solar heat is used to assist a heating system, either a second storage tank (buffer store) or a combination storage can be used. Stratified tanks or cylinders are also available.

 Solar-thermal systems for hot water production and space heating assistance are suitable for one-family houses and multi-storey multi-occupancy buildings – high rates of growth can be expected in both these sectors – in both retrofitting and new-build. Solar thermal systems are also found in hospitals, hotels and sport facilities. Most heating systems can benefit from solar-thermal assistance. Unit costs decrease with systems size. Subsidies and other financial incentives are accelerating market growth.

 Solar collectors can provide hot water for both open-air and indoor swimming pools. Significant savings in energy costs can be achieved. In Southern latitudes thermosyphon systems which do not require pumps are used – the insulated storage tanks is situated above the collectors, often as part of a single unit. Solar-assisted industrial process heating is still in its early stages but the potential is enormous. Solar-thermally driven cooling systems – so-called solar air-conditioning – is also a potentially enormous market.

Most heating systems, including heat pumps, can be effectively combined with solar-thermal systems. Ready-made solutions are available for most applications.

Off-the-shelf systems considerably reduce installation times. These pre-assembled systems allow for quick and safe mounting. High quality control and industry standards ensure reliability and energy savings for decades.

2. The Components

EHI-member companies produce a range of collector types of varying characteristics and sizes. They are high quality and designed to last. The choice of collector depends on the application and architectural considerations. Heat transfer fluids are designed to withstand temperatures of -30 °C and are non-toxic. The efficient circulation pumps and their controllers are very economical to run. All fittings and pipes are suitable for operation at high temperatures.

Flat-plate collectors are the most frequently used type of collector. Selective-surface coated high-performance absorbers ensure optimum yields. Collectors of a large range of appearances and systems for in-roof, roof top or fl at roof mounting are available.

Vacuum tube collectors – a heat pipe in an evacuated glass tube (vacuum) – can achieve high yields and temperatures. Because of their higher efficiencies they require less surface area than fl at-plate collectors.

A range of storage are available (immersion cylinders, buffer tanks, combi-tanks). General indicators of their quality are their slim, tall construction and thick insulation.

3. Solar Cooling

The demand for air-conditioning has been rising steadily for years, not only in Southern Europe, be it for offices, homes or public buildings. At the same time there is a multitude of technical equipment that needs additional cooling in summer, such as computers and industrial installations, as well as foodstuffs.

The greatest need for cooling, therefore, goes hand-in-hand with the highest incidence of sunshine. This is precisely where solar-driven air-conditioning comes into its own: the sun’s energy can be used as an energy source for cooling buildings – using today’s level of technological development. This recommends itself, because it is specifically on those days when there is the greatest need for cooling, that the greatest gains are possible from the sun’s energy. In practice, this means that intermediate storage of energy over long periods is no longer necessary.

The great advantage of solar refrigeration machines lies in the fact that the cooling requirement and the sunshine occur concurrently: so these machines produce the greatest output when it is especially hot.

Traditional air-conditioning systems operate using electrically-driven compressors. The hotter the air becomes, the more the consumption of energy rockets. This results in massive loads on the national grid, particularly at lunch time. If more solar-driven air-conditioning systems were used, the peak demand for electricity would be effectively reduced. As a result the use of solar-driven air-conditioning could make an essential contribution to relieving the pressure on the national grid and to increasing the reliability of supply.

Solar cooling has long since left its pilot phase behind. Yet, although the technology has been in use for a number of years now, widespread application is still only in the early stages. In Europe, in recent years, new refrigeration units which run on the sun’s energy have been developed for smaller scale operation, so that the technology is now becoming of interest for domestic buildings.

The design of a solar-driven cooling system does not differ in practical terms from that of a conventional system. First of all, the cooling performance and load profile of the building must be ascertained. On this basis the output and type of refrigeration unit can be established. Most often it is single-effect chillers that are used for solar-assisted cooling.

In an absorption chiller, compression is achieved by the temperature-dependent solution of the refrigerant in another medium. This is also sometimes called thermal compression. The refrigerant is absorbed into a second substance in the solvent circuit at low temperatures and desorbed at higher temperatures. For air-conditioning units, in particular, the usual pair of substances is lithium bromide and water. In this case it is the water which acts as the refrigerant, so that the lowest output temperature for the cold water is limited to about 5 °C. Input temperatures of the heating medium (hot water, steam) at the generator are about 90°C, depending on the type and application of the chiller.

These units are, therefore, often used where there is waste heat in the region of 80–120 °C or where a solar heat source is available. As well as those generators that are heated indirectly with hot water or steam, LiBr (lithium bromide) absorption chillers, that are directly heated by oil or gas, are also available. The heat ratio for a single-effect absorption chiller under nominal conditions (heat-source temperature: 120 °C, cooling water temperature: 29 °C) is between 0.6 and 0.7. For double-effect absorption chillers it is between 1.0 and 1.3.

Directly heated LiBr absorption systems have cooling outputs of between 10 kW and 2,300 kW. Large single-effect systems are available with cooling outputs from 180 kW to 5,300 kW.

The advantage of the LiBr absorption chiller lies in the low generator temperatures together with the fact that the temperature range which this technology needs to operate can easily be guaranteed with solar units. Moreover water as a refrigerant is completely safe for use in the home. Since the cooling effect occurs at negative pressures, the possibility of any bursts resulting from excessive pressure is excluded, providing that the heat source is protected.

The output of the collectors at these operating temperatures is assumed for design purposes to be approx 500 W/m2. If the unit will permit it, the heat exchanger in the primary circuit should be dispensed with and the heat transfer medium piped directly to the absorber in the chiller. The refrigeration process is designed for very low heat-source temperatures, so that the chiller operates with a relatively poor COP (coefficient of performance). Hence, solar coverage > 50 % needs to be achieved, in order to convert less conventionally generated heat with limited efficiency into a cooling effect. Solar energy for this process is provided using either vacuum tubes or fl at plate collectors.

Solar cooling saves electricity: thermally driven refrigeration processes need only around 25 % to 50% of the electrical power. And with this technology, in contrast to that of solar heating, there are no storage problems: the cooling requirement rises and falls almost simultaneously with the availability of solar energy.

The principle of solar cooling works very efficiently; a great deal of energy can be saved using conventional vacuum tube collectors as well as with a design that requires only low heat-source temperatures. The energy requirement is in direct proportion to the amount of sunshine. Conventional airconditioning systems are coming in for more and more criticism, not only because of questionable refrigerants (CFCs and HFCs), but also with regard to the CO2 emissions that are involved.