As the longitude has a minor influence on heliostat annual average efficiency, the influence of latitude is only discussed in this paper. Fig.3 is the scattergram of heliostat annual average efficiency in the field on 25 degrees north considering the influence of tower but neglecting the interaction of heliostats. From Fig.3, we can know that the location which lies due north of the tower with a distance of 1.48Ht is maximum efficiency point. Efficiency get lower with gradually outward from this spot, furthermore the efficiency is rapidly declined when the distance is less than 0.4Ht. The distributing regulation of heliostat efficiency in the field lying on other latitude is

about same. Thus, it can be defined that the line connecting the maximum efficiency point in the field and the center of receiver aperture is the normal of the center of receiver aperture. And the angle between this line and horizon is receiver depression angle aR.

The relationship between receiver depression angle aR and latitude L is shown as Fig.4, and the relationship curve can be numerically expressed as the follows by fitting. aR = -0.0027L2 + 1.0452L + 31.4456 ( 8 )