Development of a Measurement Technique to Determine the Air Return Ratio of Open Volumetric Air Receivers

Kurzfassung

In solar tower power plants with open volumetric air receivers, the heat transfer medium air is sucked through the receiver and after passing the heat exchanger or heat storage is returned to the receiver front. To improve the efficiency of the power plant the fraction of recirculated air has to be maxim-ized. This Air Return Ratio (ARR) is dependent on geometry and design, environmental conditions and operational modes. The ARR can be increased by a geometrically optimized receiver, wind pro-tection measures and through improved operational modes. In order to validate these optimizations the ARR has to be measureable. To determine the ARR of this open system an energy balance of the receiver cannot be used, since heat transfer between absorber and recirculated air is substantial [1]. A tracer gas method, where an easily detectable gas is added to the air flow and measured later on, has been chosen. The state of the art tracer gas methods however cannot operate under the extreme conditions of a solar receiver. The most commonly used tracer gases (SF6, CO2, forming gas) are either not stable under the occurring surface temperatures of the receiver of up to 1000°C or have to be added in too large quantities to be measureable against their high natural background concentrations. SF6 should furthermore be avoided due to climate protection reasons. Helium has been chosen as tracergas on the basis of its inert nature and low natural concentration. An experimental setup to develop the measurement method has been constructed. Two methods of measurement, a stationary and a dynamic one, have been developed, compared and validated at a model scale. Additionally, a qualitative measuring method for the visualisation of large scale air flows has been developed on a laboratory scale and field tested at a solar dish in Almería, Spain. The examined return air flow is injected with an infrared active gas. This allows the air flow to be visualised with an infrared camera. The promising results show that the newly developed qualitative measurement tech-nique can be applied in the difficult measurement environment of concentrated solar power plants.