Solar Thermal Technologies

Solar thermal technologies are harnessing solar energy for thermal energy (heat). Solar thermal technologies comprise flat collectors for low and medium temperatures and high temperature collectors concentrating sunlight using mirrors and lenses. Solar heating is the utilisation of solar energy to provide process heat, water heating, cooking or space heating.

Solar Thermal Collectors

A flat plate is the most common type of solar thermal collector, and is usually used as a solar hot water panel to generate hot water. A weatherproofed, insulated box containing a black metal absorber sheet with built in pipes is placed in the path of sunlight. Solar energy heats up water in the pipes causing it to circulate through the system by natural convection. The water is usually passed to a storage tank located above the collector.

There are many flat-plate collector designs but generally all consist of

1.  a flat-plate absorber, which intercepts and absorbs the solar energy,
2. a transparent cover that allows solar energy to pass through but reduces heat loss from the absorber,
3. a heat-transport fluid (air, antifreeze or water) flowing through tubes to remove heat from the absorber and
4. a heat insulating backing.

One flat plate collector is designed to be evacuated, to prevent heat loss. The absorber may be made from one of a wide range of materials, including copper, stainless steel, galvanised steel, aluminium and plastics. When choosing an absorber material, it is important to ensure that it is compatible, from the point of view of corrosion, with the other components in the system and with the heat transfer fluid used. The absorber must also be able to withstand the highest temperature that it might reach on a sunny day when no fluid is flowing in the collector (known as the stagnation temperature).

The fluid passageways of the absorber may consist of tubes bonded to an absorbing plate, or may form an integral part of the absorber. Experience has shown that simple mechanical clamping of tubes to an absorber plate is likely to result in an absorber with a poor efficiency. A good thermal bond, such as a braze, weld or high temperature solder is required for tube and plate designs, in order to ensure good heat transfer from the absorbing surface into the fluid.

Matt black paints are commonly used for absorber surfaces because they are relatively cheap, simple to apply and may be easily repaired. Paints, however, have the disadvantage that they are usually strong emitters of thermal radiation (infrared), and at high temperature this results in significant heat losses from the front of the collector. Heat losses from the collector can be substantially reduced by the use of absorber coatings known as 'selective surfaces'. These surfaces may be applied by electroplating or by dipping a metal absorber in appropriate chemicals to produce a thin semi-conducting film over the surface. The thin film will be transparent to solar radiation but at the same time appear opaque to thermal radiation. However, these surfaces cannot be produced or applied easily.

Flat-plate collectors usually have a transparent cover made of glass or plastic. The cover is required to reduce heat losses from the front of the collector and to protect the absorber and the insulation from the weather. Most covers behave like a greenhouse. They permit solar radiation to pass into the collector, but they absorb the thermal radiation emitted by the hot absorber.

At night it is possible for the collector to lose heat by radiation and the circulation will be in the opposite direction, so the water will cool. This can be overcome by use of a suitable non-return valve. However, there is a danger with solar collectors when used under clear night conditions (e.g. in arid and semi arid regions) that they can actually freeze even when the ambient temperature is above freezing point. In such conditions it may be necessary to have a primary circuit through the collector filled with antifreeze and a separate indirect hot water cylinder where the water from the collector passes through a copper coil to heat the main water supply. This problem will only apply in certain desert regions in the cold season or at high altitudes in the tropics and sub-tropics.

Costs

Low temperature flat-plate solar collectors typically cost 21 US $ per square metre (0,0021 US $ /cm²). Medium to high temperature collectors generally cost around 200 US $ per square metre. Flat plate collectors are sized at approximately 0,1 square metre (929 cm²) per gallon (3,79 l ) of daily hot water use or 245 cm² per l of hot water. A complete system installed costs around 14 US $/l or 2000 US $ per 150 l.

Maintenance

Solar thermal systems are relatively maintenance free and involve on an occasional base the checking of the piping for leaks and the cleaning of the collectors. In some regions it may also be necessary to inspect the transfer fluid for freeze protection and to remove the build up of lime scale that chokes the collector and tank recirculating pipes over a period of time.

Concentrating Solar Thermal Power

Concentrating solar thermal power systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. The concentrated heat is then used as a heat source for a conventional power plant. A wide range of concentrating technologies exists; the most developed are the parabolic trough, the concentrating linear fresnel reflector, the Stirling dish and the solar power tower. Various techniques are used to track the Sun and focus light. In all of these systems a working fluid is heated by the concentrated sunlight, and is then used for power generation or energy storage.

Solar Water Heaters (SWH)

The Technology

Solar water heating (SWH) systems are typically composed of solar thermal collectors, a storage tank and a circulation loop. The two basic classifications of solar water heaters are:

• Active systems which use pumps to circulate water or a heat transfer fluid. There are the two types of active solar water-heating systems:

1. Direct-circulation systems use pumps to circulate pressurized potable water directly through the collectors. These systems are appropriate in areas that do not freeze for long periods and do not have hard or acidic water.
2. Indirect-circulation systems pump heat-transfer fluids through collectors. Heat exchangers transfer the heat from the fluid to the potable water. Some indirect systems have "overheat protection," which is a means to protect the collector and the glycol fluid from becoming super-heated when the load is low and the intensity of incoming solar radiation is high.

• Passive systems transfer and circulate heat naturally. Passive solar water heaters rely on gravity and the tendency for water to naturally circulate as it is heated. Because they contain no electrical components, passive systems are generally more reliable, easier to maintain, and possibly have a longer work life than active systems. The two common types of passive systems are:

1. Integral-collector storage systems or batch systems consist of a tank that is directly heated by sunlight. These are the oldest and simplest solar water heater designs. They are good for households with significant daytime and evening hot-water needs; but they do not work well in households with predominantly morning draws because they lose most of the collected energy overnight. These solar collectors are suited for areas where temperatures rarely go below freezing.
2. Thermosyphon systems are an economical and reliable choice. These systems rely on the natural convection of warm water rising to circulate water through the collectors and to a storage tank located above the collector. As water in the solar collector heats, it becomes lighter and rises naturally into the tank above. Meanwhile, the cooler water flows down the pipes to the bottom of the collector, enhancing the circulation.  Indirect Thermosyphon systems use a glycol fluid in the collector loop as a heating medium.  

To design, size and select a solar water heating system, the following data is required: daily hot water requirement (litres/day), average insolation (kWh/m2 day), water quality and storage requirements.

Applications and Efficiency

SWHs are employed in residential, commercial, industrial and public buildings for the provision of hot water as well as heating of swimming pools. The current commercial market for SWH in the region is predominantly households (high income), hospitals, commercial establishments and tourist facilities.
State of the art solar water heaters incorporate features such as selective surface absorbers, anti-reflective glazing, well-designed collector arrays, efficient storage systems achieving operation efficiencies of the order of 35 to 40%.
A 300-liter system typically suited for family of 4-6 persons will displace up to 1000 kWh of electricity annually.

Capability and Limitations

•  Water quality - Solar water heaters require clean, non-hard water for long term operation.  Hard or dirty water leads to blockage and corrosion of pipes and storage tanks.  Closed circuit systems are recommended where water is hard.
• Installation, Commissioning and Maintenance - Improper installation and commissioning and maintenance of SWHs are the leading causes of system failures.
• High cost of SWH is a major limitation in their uptake. Typical prices  for small units range between US$ 1,500 (180 litres) to US$ 2,500 ( 300 litres).

Links to further reading

http://en.wikipedia.org/wiki/Solar_thermal_energy
http://www.therenewableenergycentre.co.uk/solar-heating
http://www.isfh.de/institut_solarforschung/solarthermie.php