Parabolic dish systems
A parabolic dish concentrates solar energy onto a receiver at its focal point. The receiver absorbs the energy and converts it into thermal energy. This can be used directly as heat or supply for power generation. The thermal energy can either be transported to a central generator for conversion, or it can be converted directly into electricity at a local generator coupled to the receiver.
Dishes track the sun on two axes, and thus they are the most efficient collector systems because they are always focussed. Concentration ratios usually range from 600 to 2000, and they can achieve temperatures in excess of 1500 °C. Rankine-cycle engines, Brayton-cycle engines, and sodium-heat engines have been considered for systems using dish-mounted engines the greatest attention though was given to Stirling-engine systems.
Current developments in the USA and Europe are focussed on 7.5 kWe systems for remote applications. In Europe, three dish/Stirling systems are demonstrated at PSA in Spain, whereas in the USA a program has been set to demonstrate water pumping and village power applications . Stretched-membrane concentrators are currently the focus of considerable attention because they are most likely to achieve the goals of low production cost and adequate performance. Both multifaceted and single-facet designs are being pursued. Recently, a 7-meter single-facet dish was developed, which demonstrated excellent performance in tests.
The greatest challenge facing distributed-dish systems is developing a power-conversion unit, which would have low capital and maintenance costs, long life, high conversion efficiency, and the ability to operate automatically. Several different engines, such as gas turbines, reciprocating steam engines, and organic Rankine engines, have been explored, but in recent years, most attention has been focused on Stirling-cycle engines. These are externally heated piston engines in which heat is continuously added to a gas (normally hydrogen or helium at high pressure) that is contained in a closed system. The gas cycles between hot and cold spaces in the engine stores and releases the heat that is added during expansion and rejected during compression.