The idea of using heat pipes and two-phase thermosyphons in systems that convert solar energy into thermal energy has been known already for quite a long time [3-5]. These devices are here parts of collectors and concentrators of solar radiation. Loop heat pipes
can also fulfill this function and, at the same time, serve for transferring thermal energy with insignificant losses and without any additional means of pumping.

It is possible to consider, in particular, two variants of using LHPs in systems of sun heat supply. The first of them is presented schematically in Fig. 6. It envisages the use of a sun collector in the form of a flat panel joined to several parallel evaporators. When it is considered that the density of solar-radiation energy typical for Western Europe at the summer period of the year is approximately 300 W/m2, then for the collection of 1 kW it is necessary to have such a collector with an area of about

3.3 m2. Fig. 6. System of sun heat supply with the

In principle, for transferring a heat flow of 1 use of an LHP

KW it will suffice to have one LHP with an evaporator 200-250 mm long from 25 to 28 mm in diameter. However, in this case, when one evaporator is joined to a collector with a larger surface, the problem of concentration of a heat flow arises. This problem can be solved by increasing the effective thermal conductivity of the panel, for instance, by using conventional heat pipes or thermosiphons, or by increasing the number of evaporators. In the latter case one should bear in mind that the efficient operation of an LHP is possible at a heat-load density per evaporator not lower than 0.5X10-4 W/m2. Therefore, more optimum here may become a combined variant, when several LHP evaporators are joined to a sun collector equipped with conventional heat pipes.

For more powerful heat-supply systems it is expedient to use parabolic sun concentrators combined with an LHP evaporator (Fig. 7).

Characteristic values of the heat-flow maximum density at the surface of a cylindrical evaporator with an area of (250-300)x10-4 m2 in water LHPs are about 10*10-4 W/m2. It means that on one such evaporator situated in focus with regard to a small parabolic concentrator a heat flow of 2.5-3 KW can be focused. To increase the system capacity, one can use two evaporators combined by a common thermal interface or one evaporator of a larger diameter. It should be mentioned that with decreasing LHP dimensions and evaporator heating surface the heat-load density may be considerably increased. Thus, for instance, in an

References:

1. "Heat Transfer Apparatus”, US Patent, 4. 515. 209, 1984.

2. P. D. Dunn, D. A. Reay, "Heat Pipes”, Pergamon Press, Oxford, 1976.

3. "Solar Thermal Energy Collection Systems”, US Patent, 3. 875. 296, 1975.

4. D. R. Adkius. "Design Consideration for Heat Pipe Solar Receivers”, J. of Solar Eng., Vol.112, 1990, pp. 168-176.

5. J. Danilewicz, B. Nowak, M. A. Sayegh. Two-Phase Thermosyphon Solar Collector Absorber. Proc. of the 9th Int. Heat Pipe Conference, Albuquerque, NM, 1995, pp. 364­368.