This system uses reflecting panels which have a parabolic shape and track the Sun by rotating around two orthogonal axes. These panels also concentrate solar radiation towards a receiver which is installed at the focal point. High tempera­ture heat (>650°C) is normally transferred to a fluid (helium or sodium vapour) and is then used in a motor, which is located above the receiver (see Fig. 95), where mechanical or electrical energy is directly produced. For economic reasons, concentrator dimensions do not exceed a diameter of 15 m, limiting its power to about 25-30 kWe. With a row of these collectors, it is possible to realize sys­tems of any size and power. An interesting application of parabolic dish collec­tors is the one which regards electrical energy production for small communities which are decentralized and distant. These systems have a conversion efficiency which is more than 30% (the highest efficiency among the currently existent solar technologies) [45, 50,51]. This technology has now reached the industrial phase, mostly due to the research which has been developed in Europe, in the USA and in Australia.

Among the described technologies, this system is the one which has the highest electrical energy production cost (in 2004 costs were about 1 €/kW h); neverthe­less, it is interesting for the prospects it offers concerning the drop in this cost [50]. The cost for the construction of a solar thermal electrical system which uses para­bolic dish collectors in 2004 was about 7100-3700 €/kWe with a forecast for the medium term of about 2000-1200 €/kWe.

In the parabolic dish collectors, the thermal vector fluid can reach temperatures which can be even higher than 1000°C, and at such high temperatures it is also possible to produce hydrogen by the dissociation of water. In prospect, this is the most important reason for the interest shown in this technology: in Europe, since 2002 the hydrogen economy has become one of the mainstays of the EU sustain­able energy policy, acknowledging the uniqueness of hydrogen both as a clean fuel and as a high efficiency energy vector [45, 53, 56].