Development of a Broadband Antireflection Coated Transparent Silica Window for a Solar-Hybrid Microturbine System

Kurzfassung

A promising way of a solar-thermal power plant with high overall efficiency is the coupling of a Braytontype microturbine with a solar receiver. Within the SOLHYCO project, a 100kWe solar-hybrid microturbine demo system is currently under development. The compressed air is preheated up to 800°C with solar radiation using a metallic tube receiver. The tube receiver is located in a cavity with a circular opening Thermal analysis of the receiver showed that the aimed overall receiver efficiency of 80% cannot be reached with an economically sized receiver cavity. Reasons for this are high losses by infrared radiation and convection caused by high temperatures of the tubes of up to 950°C. Covering the cavity opening with a transparent window could reduce these losses. Fused silica has ideal properties to fulfill the technical needs. It is highly transparent in the solar spectrum and it can be used at high operating temperatures. Further a very low thermal expansion coefficient allows high temperature gradients. Although there are a few disadvantages: Glass is a brittle material which can not withstand high tensile stresses. Further a flat plate as a receiver window would lead to additional losses by reflected solar radiation. Within the ARTRANS project an antireflective coated fused silica window was developed. The prototype, consisting of ten pipe segments is held by a frame of insulating material. A broadband antireflective coating was developed to reduce the losses by reflected solar radiation. The segments of the prototype were coated by dip-coating from a silica sol containing solution and tempered at 900°C. Optical measurements of the resulting 110 nm porous silica coating showed that the transmission could be increased as expected from 93% to more then 97% in the solar spectrum. The overall performance of the receiver without and with transparent window was supported by numerical simulations. The simulations showed that a receiver with coating will have a 13.1% points higher receiver efficiency. Using a window without coating, the receiver efficiency will still be 9.5% points higher than without window.