VoxSolaris: The Voice of the Sun
  Solar Thermal Systems

Solar thermal systems provide mostly hot water and space heating. There are two main design types, the cheaper but less efficient flat solar thermal panels and the more expensive but more efficient vacuum solar thermal panels. Both designs provide good performance for the money and can in some circumstances, serve in place of roof tiles. Thus in a new build or where a roof is in need of replacement, building regulations permiting, costs can be further mitigated.

Flat solar thermal panels:

These consist of a back supporting panel typically made of plastic, a layer of insulating material and a blackened sheet of metal called the absorber and a sheet of glass. There is a small gap of perhaps 2 or 3mm between the absorber and the glass through which a fluid of high specific heat capacity is pumped. The sun's radiation passes through the glass and is absorbed by the absorber which becomes hot. This heats up the fluid which is pumped off to a heat water in a tank via a heat exchanger. In spite of being the less effective of the two designs the performance is still impressive. Such devices can extract up to 60 percent of available energy and the fluid can reach temperatures of up to 90C or more, even in the UK. In some countries the temperature can hit 100C. The fluid can be water which can be pumped directly to and from the tank but it is more common to use water mixed with antifreeze or a specially formulated oil as these will not freeze in winter or boil in summer.

Vacuum solar thermal panels:

These consist of an array of evacuated glass tubes, each with an absorber consisting of a blackned metal inner tube carrying the pumped fluid. The inner wall of the glass tubes is coated with a material which transmits visible light but reflects infrared radiation from the absorber. The part of the tubes that are not exposed to the sun are coated on the outside with a reflective material such as aluminum. This is to capture sunlight that would otherwise miss the absorber. The array of tubes could be constructed as two corregated sheets, one of glass and the other of aluminum, although this construction is less common. The vacuum significantly cuts conductive heat loss from the absorber while the inner glass coating cuts absorber re-radiation losses. As a consequence this design is more efficient than the flat panel and can thus produce higher temperatures. In summer in hot climates they can produce maximum temperatures in excess of 150°C and even produce better than 60°C in the UK in winter.


Temperature vurses efficiency

In both types the absorber could, with the pumps switched off, reach a maximum temperature where the absorber is radiating as much energy as it recives. This would be of no use to us because without pumping we don't get to heat the hot water tank. But the more we pump the more we lower the absorber's operatingtemperature. The lower the absorber operating temperature the lower the loss of energy by radiation and the higher the energy retention and thus the more efficient the process is. Efficiency is a function of the difference between themaximum temperature and the loweroperating temperature. The vacuum design reaches a higher maximum temperature than the flat panel design and so it is more efficient and will provide a greater flow rate at any given operating temperature than the flat panel design.

As an example, if we want our hot water to be about 60°C (333°K) we set flow rates to whatever level they need to be to provide this temperature. With sunlight conditions such that a flat panel system can reach a maximum temperature of 90°C (363°K), a lot of the 1,000 watts per square meter are going to be lost. Radiation is proportional to the absolute temperature raised to the power of 4 so in this example we will lose (333/363>) to the power of 4. This is 70 percent of the incoming sunlight. Only 30 percent is retained. But with the same sunlight conditions a vacuum panel could reach a maximum temperature of 150°C (423°K) and the loses will be cut to (423/333) to the power of 4 or 39 percent with 61 percent retained. Of course in both cases other losses will apply such as conduction in the panel itself and the pipes connecting to the tank etc.

Although in this example the vacuum panel is twice as efficient as the flat panel this is not linear. At an operating temperature of 40°C (313K) the flat panel would retain 45 percent and the vacuum panel 70 percent. Very impressive but no longer twice as efficient.

What size installation do I need?

Such is the efficiency and low cost of solar thermal panels that it is easy to make a dent in your hot water and heating bill, with a very small and cheap instalation. The only problem with this technology is the mismatch between supply of thermal energy and demand. The sun's radiation at sea level averages about 1,000 watts per square meter. More at the equator and less at the more northen or southern locations. More in summer and less in winter. While the demand for hot water tends to be constant throughout the year, demand for space heating is higher in the winter which is when the availale solar energy is lower. It is not practical to save up enough heat during summer to last the winter as this would need a truly massive and very well lagged tank. Because of this the general approach to solar thermal systems in northern and southern locations is to aim for a small system that will cope with the summer demand and make a useful contribution in the winter, accepting that much of the winter demand will be met by other means. There is little benefit in the overkill approach of having enough solar thermal capacity to cope with winter demand even though the vacuum solar thermal panels could in many cases achieve this. The overkill approach does have merit if you have a swiming pool though! Covering the whole roof of a modest house can actually heat a reasonable sized swiming pool during the summer. Foregoing the use of the pool during winter could mean being completely self sufficient. The other thing to consider is the extent to which having solar thermal panels detract from having solar voltaic panels.


And there are ways to make any shortfal much for efficient than simply burning gas. Obviously you improve insulation because that lessens demand both for heating in the winter and air conditioning in the summer. But where heat is still short, use a heat pump. This is an air conditioner operating in reverse and has a heat output many times that of its electrical input. We have a particularly smart design of heat pump that refrigerates the fluid in solar thermal panels during the day, driving up both the efficiency of the solar thermal panels and the heat pump. The simple measure of a small solar thermal installation and a heat pump can cut greenhouse gases associated with hot water and heat by better than 80%. People almost always blame cars, particularly American ones, for the global warming crisis but actually houses are a major contributor to the problem. Houses are also a lot less excusable than cars because of the ease by which the use of fossil fuels to heat houses can be massively reduced. There have been real technological barriers to running cars on renewable energy sources but one could hardly put solar thermal panels and heat pumps or even heat pumps that refrigerate solar thermal panels, beyond the wit of man or even beyond the budget.