The Role of “Allowable Flux Density” in the Design and Operation of Molten-Salt Solar Central Receivers

In the 1980s, the Utility Study [Hilesland, T., Jr., and Weber, E. R., 1988, “Utilities’ Study of Solar Central Receivers,” Fourth Int. Symp. on Research, Development, and Applications of Solar Thermal Technology, Santa Fe, NM] identified the external cylindrical molten-salt-in-tube receiver with a surround heliostat field as the most cost effective and practical design for commercial applications. Such designs typically require 50–1000 MW of design-point thermal power at outlet temperatures around 1050°F (565°C). Using computer codes such as RCELL [Lipps, F. W., and Vant-Hull, L. L., 1978, “A Cellwise Method for the Optimization of Large Central Receiver Systems,” Solar Energy, 20(6) pp. 505–516.] or DELSOL [Kistler, B. L., “A Users Manual for Delsol 3,” Sandia National Laboratories Livermore, SAND86-8018, 1987.] it is straightforward to design an optical system to meet these requirements, defining the smallest receiver (lower cost and thermal losses) and the most cost effective heliostat field. As the performance of heliostats in the anti-sun locations is better, such fields tend to be biased (in the northern hemisphere) to the north side of the receiver, and produce very high flux densities there; typically 2–5 MW/m². However, the receiver is typicaly limited to a salt velocity and temperature dependent allowable flux density (AFD) of about 1 MW/m². Design methods to reduce this peak flux to a nominally acceptable value in a cost effective manner are presented. Residual excess flux events under non-nominal conditions are handled by a real-time processor which selects specific heliostats for removal from track. This same processor is used to preheat the receiver, using a special algorithm to define the required flux density.