Extremum-seeking control of a supercritical carbon-dioxide closed Brayton cycle in a direct-heated solar thermal power plant

Abstract

One promising avenue for the development of next generation CST (Concentrating Solar Thermal) technology focuses on the use of a direct-heated sCO2 (supercritical-CO2) CBC (closed Brayton cycle) as the generator power cycle. Initial investigations into such a CST plant, while promising, have found its power output and efficiency to be sensitive to fluctuations in solar heat input and ambient temperature over a day and between seasons. Given the difficulty in developing complete models across all operating conditions due to non-linearities in CO2 properties, an extremum-seeking controller is proposed to maximise the power output of the CBC as the solar heat input and cooling-air temperatures change. This controller achieves this effect by manipulating the CO2 mass inventory in the CBC. Slack variables are introduced into the extremum-seeking control performance metric to impose constraints on turbine inlet temperature and pressure to protect the CBC from damage. The performance of the proposed scheme is tested through simulations on representative summer and winter days. Simulations indicate that the performance of the CBC under ESC (extremum-seeking control) based inventory-control compares favourably to operation with a fixed-CO2 inventory in both summer and winter and does not require retuning between seasons.