Probabilistic Analysis to Quantify Optical Performance and Error Budgets for Next Generation Heliostats

Abstract

Current heliostats cost ∼$200/m2 of reflective area and are estimated to contribute up to 50% of the total solar power tower plant costs. A drastic overall cost reduction is required in order for concentrated solar thermal power to be economically viable. The Department of Energy has set forth the SunShot initiative targeting a levelized cost of energy (LCOE) of $0.06/kWh by the year 2020. The cost of each heliostat must be brought down to an estimated $75/m2 to achieve this rigorous goal. One of the driving aspects of heliostat design and cost are the heliostat optical errors. At the moment, it is relatively unclear about the amount of error that can be present in the system while still maintaining low cost and high optical accuracy. The optical errors present on heliostat mirror surfaces directly influence the plant LCOE by causing beam spillage. This can result in an increase in the number of heliostats, an increased receiver size, and decreased thermal efficiency. Assuming a fixed heliostat cost of $75/m2, the effects of optical errors on LCOE are evaluated within the software DELSOL. From a probabilistic analysis, beam quality errors (i.e. slope error, alignment errors, etc.) are shown to have more importance on the LCOE than tracking errors. This determination results in a realization that the tracking errors and beam quality errors can be combined into a “bundled” root-sum-square (RSS) error value. A “bundled” error value of 2 mrad results in an LCOE of $0.06/kWh. This “bundled” value is the basis for a new optical error budget and is decomposed into five individual errors. These five errors can be used as design specifications for new generation heliostats.