Optical performance considerations for analysis and simulation of power tower plants

Abstract

South Africa has implemented a ‘time of day’ tariff structure for concentrating solar power plants in the Renewable Energy Independent Power Producer Procurement Programme. It is hypothesised that payment allocation factors for the ‘time of day’ and the ‘time of use’ dispatch schedule influence the optimal heliostat field layout. SolarPILOT software is used to generate and optimize the heliostat field layout of a 100 MWe power tower plant in Upington, South Africa with 8 hours of thermal energy storage in the SunShot scenario with a high receiver thermal efficiency of 90%. A large size heliostat with a total area of 115.56 m2 and an external cylindrical receiver are considered for the heliostat field layout. A subset of 12 days is simulated on an hourly basis to achieve convergence and to take seasonal, daily and hourly weather variability into account. During the optimization of a heliostat field layout, the heliostats are ranked and selected according to a performance metric. In this study, two field layouts are compared based on two different performance metrics, namely: power delivered to the receiver and the time of use weighted power. The optical performance is simulated using both the Hermite (analytical) and the Monte-Carlo Ray-Tracing methods. By accounting for the TOU weighted power, it is found that the LCOE increases from 0.1831 $/kWh to 0.1870 $/kWh using the Hermite (analytical) method. Similarly, when MCRT techniques are used for the optical characterization, the LCOE value increases from 0.1781 $/kWh to 0.1832 $/kWh. It is recommended that payment allocation factors and the tariff structure for the time of day be included when comparing field layouts with other layout generation and optimization strategies. This study will be useful for power tower developers in identifying practices to be included in the optical characterization of their heliostat field layouts for better simulation results.