Evacuated-tube Solar Collectors
Evacuated tubes can be seen as the newer of solar collectors. They are made of a copper “manifold” where the water flows through and a number of evacuated tube heat pipes connected to it. Figure 1 shows multiple heat-pipe Evacuated-tube solar collectors in series.
Evacuated tube solar collectors convert direct and diffused solar radiation into heat. Infra-red rays, which can pass through clouds, are also absorbed and converted into usable heat.
The collectors efficiently collect and transfer this energy through a special collector plate and a rapid heat transfer channel ‘the heat-pipe’, situated in an evacuated glass tube, to a highly insulated manifold heat exchanger –see Fig 1 and Fig 2. The collector plate has a special wavelength ‘selective’ coating using a semi-conductor layer. This special absorber plate converts the maximum amount of solar radiation into heat whilst having very low radiation losses.
The heat-pipe has a very low heat capacity but an exceptionally rapid conductivity and therefore is a very efficient and speedy heat conductor.
It also provides the system with a diode function i.e. heat transfer is always in one direction – from the absorber to the water and never the reverse. The special fluid inside the ‘heat-pipe’ evaporates when heated, transferring heat energy to its top. The vapour condenses inside a special heat exchanger (which is located within a water manifold) and the fluid then returns to its original position, at the bottom of the ‘heat-pipe’, due to gravity and the cycle is repeated continuously.
The vacuum in the glass tube, being the best possible insulation for a solar collector, suppresses heat losses and also protects the absorber plate and the ‘heat-pipe’ from external adverse conditions. This results in exceptional performance. Due to the huge growth in the solar heating market over the last number of years, this cutting-edge technology has come down drastically in price.