In a project supported by Greenpeace International, 3 types of solar desalination plants using multi-layer heat recovery were investigated:

• 4 m2 flat plate collector with a 1 m2 evaporation surface

• 2.4 m2 parabolic reflector with a 0.66 m2 evaporation surface

• 2 m2 evacuated tube collectors with a 1 m2 evaporation surface

Figure 2 shows a diagram of the desalination module with the collector. The lowest level of the unit serves as the first level of the desalination tower and as a water tank to catch the desalinated water. The heat produced by the surface collector is introduced here. The condensate runs directly through the collectors acting as a heat transfer medium. As the collectors are situated under the condensation unit, the circulation through the solar collector is driven by normal thermal convection. The two collectors with 2 m2 of surface area each have their own circulation system. An additional reflector at the top and bottom of the collectors enlarges the surface of the aperture. Figure 3 shows the results of measurements taken using this system on a summer’s day. The temperature of the unit and the upper level is shown, as well as the production of desalinated water and the irradiation over a period of 24 hours. The ratio of condensed water to salt water was set to around 1:1.8 . The unit produced a total of 44 kg of desalinated water over the 24 hours. The total amount of solar energy collected in one day stands at around 7.23 kWh/ m2d. This means that the effectiveness of the solar energy in relation to the energy actually used (Q= Mdest*h) can be calculated to be at 98%. A simple greenhouse-type distillation unit was set up as a reference, and produced in the same time frame a total of 4.0 litres per square meter aperture. When the two units are compared, it can be seen that it is possible to obtain a production which is 2.75 times more efficient with the same energy input than in the simple system. Alternative system designs, which use evacuated tube collectors and parabolic reflectors are depicted in Figures 4 and 5.

The parabolic reflector model is always placed in an east-west alignment. The Aperture width is 1,5 m. The reflector has to be tracked over the day to keep the focal line always on the receiver. This must be done with as little effort as possible. For that reason the focal point is allowed to move over a width of 20 cm over the absorber surface during the course

of the day. This means that, depending on the geographical latitude, only one daily adjustment is required to keep the focal line in position. The absorber also holds the salt water and so serves as the first stage of the desalination tower. Heat is delivered directly and without delay to the system. In windy conditions, however, large heat losses occur as the absorber is only protected by a shallow cavity. Additionally, during night-time intense heat losses occur at the bottom surface and cool down the system fast. In particularly windy areas, the idea of protecting the absorber by a glass cover may be considered. But due to reflection on the glass, energy losses have to be taken into account. When the sun’s rays are at a low angle as many units as possible must be connected together to minimise the loss of concentrated radiation at the edges of the parabolic reflector (see Figure 4). Measurement results have shown that on the one hand the parabolic reflector system heats up very fast due to the little heat capacity and on the other hand loses heat as fast again in the night time. Overall, with a reflector aperture of 2,4 m2 the same daily output can be achieved as with a flat plate collector with 4 m2. Since a concentrating system uses only direct insolation, it depends on the climatic region if a concentrating system or a flat plate system is to be preferred.

Another solution is the utilisation of evacuated tube collectors. For a prototype 20 Sydney — type tubes were connected directly to the evaporation stage. The distillate flows driven by natural convection, directly through the tubes, carrying the heat to the evaporator. Evacuated tube collectors are currently available at low cost from Chinese manufacturers and offer an interesting economical alternative. These collectors are very insensitive to wind and work at a high efficiency even at temperatures of 100°C. However, it must be noted that with large spacing between the tubes the effective area is significantly reduced and only a limited performance is achieved. Regarding the complete collector area the optical efficiency amounts to only 40% for a system without CPC.