A preliminary analysis of various performance trade-offs involved in designing a two-stage central receiver plant secondary to achieve ultrahigh concentrations using a nonimaging CPC type has been carried out. The approach was based on simple geometry and the optical characteristics of CPCs.
We find that the highest possible concentrations can only be achieved with an axially symmetric circular field surrounding a central tower with a CPC looking vertically downward. In this configuration, the achievable concentration increases asymptotically toward the ideal limit as the CPC acceptance angle is reduced and the tower height increases. Although there is no well-defined optimum configuration, 80% of the ideal limit can be achieved in this configuration with a tower height-to-field diameter ratio of about 1.0. As one tilts the CPC view cone away from straight down, the intercepted area for heliostats becomes an ellipse of increasing area and eccentricity, and the maximum achievable geometric concentration decreases rapidly. (This result is independent of scale.) If the intercept area for heliostats is held constant, the corresponding tower height decreases gradually. Obliquity effects (cosine losses) are not prohibitively large in a circularly symmetric surround field geometry, mainly because the tower heights are comparable with the field dimensions. Effects of blocking and shading and ground cover constraints have not yet been calculated in detail but could be expected to be less in a surround field geometry than in a north field configuration. Furthermore, our approach guarantees that the calculated limit already includes the first-order correction for the effects of blocking and shading losses.
