The invention relates to central solar receivers with volumetric solar absorber. 
A central solar receiver absorbs concentrated sunlight at a high temperature, commonly about 700.degree.-1300.degree.  C. and transfers the heat from the solar absorber therein to a working fluid which either serves as heat carrier fluid or else is designed to perform a heat induced, possibly catalyzed, endothermic chemical reaction between components of the working fluid.  For various industrial applications such as the operation of gas turbines for electricity generation or the performance of endothermic reactions of the kind specified, it is necessary that the working fluid circulates through the system at an elevated pressure of say 10-30 atmospheres.  At such a pressure the density of the circulating gaseous working fluid is much higher than in the unpressurized state and consequently pressure losses during circulation are lower. 
A directly irradiated central solar receiver with volumetric absorber comprises a housing having a window for the admission of incident concentrated solar radiation and one of the most critical problems associated with pressurized central solar receivers is the mechanical strength of the receiver's window.  Materials that have the required optical and thermal properties tend to be brittle, which means that while they can withstand large compression stresses they tend to crack or shatter under even relatively small tension stresses.  Stresses in the window are created by the gas pressure in the interior of the receiver and also by uneven thermal expansion of the window and other receiver components as they heat up during operation.  Non-uniform stresses exist, as a rule, at different points in any window of a central solar receiver and consequently hitherto central solar receivers could not be operated at elevated pressures. 
In IL 97091 and the corresponding U.S. application Ser.  No.  07/820,418, EP 495395 A1 and patent applications in other jurisdictions, there is described an improved central solar receiver comprising a housing with a window for the admission of incident concentrated solar radiation accommodating a volumetric solar absorber having a base body with an array of absorber members spaced from each other and projecting from one face thereof with their free ends facing the window, the working fluid being injected into the volumetric solar absorber in a flow which intersects the absorber members. 
It is the object of the present invention to provide a central solar receiver based on the teachings of our IL 97091 and designed for operation at elevated pressure. 
It is a further object of the present invention to provide a window unit adapted for incorporation in the housing of a central solar receiver operating at elevated pressure. 
It is yet another object of the present invention to provide a volumetric solar absorber for incorporation in a central solar receiver of the kind specified operating at elevated pressure.