Piecewise-Focusing Collectors Reduce the Mirror Area in Concentrating Solar Thermal Power Plants

Abstract

Piecewise-focusing (PWF) collectors replace the heliostat field and central tower receiver of Gen 2 concentrating solar thermal (CST) power plants. The PWF collector consists of a large base frame supporting a tiered array of reflectors that concentrate sunlight into a cavity receiver. The base frame rotates about a central pivot to track the sun in azimuth, while the reflectors rotate about individual nearly horizontal inclined axes to track the sun in elevation. Compressed gas such as CO2 is used as the heat transfer fluid (HTF) from the receiver to a conventional molten-salt thermal reservoir, which supplies heat for a steam turbine power block. Physical and numerical modeling of an approximately paraboloidal PWF collector shows that about 250 separate reflectors will be required for adequate focus on the entrance to a cavity receiver at a concentration ratio of 2000 with little spillage at all sun elevations. With an area of 10–20 times larger than the cavity entrance, the absorber functions at peak temperatures that are within range of standard stainless steel tubing, despite using a gaseous HTF. Reflectors of PWF collectors have significantly better cosine factors than heliostats, and cavity receivers lose much less heat than open cylindrical receivers, so that reflector area is reduced to about 62% of heliostat area. Thus, the capital cost of a CST plant can be reduced substantially. By continuing to use Gen 2 technology for thermal storage and the power block, PWF-based CST can be developed much sooner than “Gen 3” CST.