Optical performance

In principle, there is some loss in optical performance with larger heliostats for given receiver size. This is primarily due to direct spillage from the larger reflected beams with a given tracking/beam error and by having greater off-axis aberration losses. This effect results in greater total field reflector area with larger heliostats, compared to smaller heliostats, for the same power incident on the receiver. This reduction in heliostat area directly improves the system cost of the plant with a given receiver because the increase in optical performance increases the total annual energy. Second­ary improvements can also occur with smaller heliostats. For example, having a receiver with a lower mass and area, and thus lower cost, coupled with a lower mass and cost tower can result from this change. There would also be some slight performance improvement in the receiver, since there would be less radiation and convective loss for the smaller area, assuming the same surface temperature. Since the receiver and tower costs are of the order of about 8% of the total plant cost (or about 14% of the hardware installed cost), some slight improvement in cost can occur from these effects. However, these effects are not considered here.

Direct optical performance effects are treated by Kolb et al. (2007) for a representative field. DELSOL was used to determine the total field reflec­tive area for the same annual absorbed power as for the base line case of a 150 m2 heliostat, at a cost of $150/m2. A linear fit to the data for field area was determined as:

Field area = 75.7 * (heliostat area in m2) + 229,359 m2. [17.13]

Expressing this as a ratio, or correction factor, F, the field area for a given heliostat area, Aheliostat, compared to that for the base line heliostat is:

For very small heliostats, Eq. [17.14] shows that the total field area is about 5% less than for the base line heliostat case. This small optical cor­rection factor will be used in the parametric estimate for the total heliostat cost as a function of area.

Updated: August 22, 2015 — 3:29 pm