OPERATIONAL OPTIMISATION OF THE MAIN HEAT REJECTION UNIT OF CSP PLANTS BASED ON CARBON DIOXIDE MIXTURES

Abstract

This research, developed in the framework of the SCARABEUS project, studies the off-design performance of transcritical power cycles running on CO2-SO2 mixtures in Concentrated Solar Power applications. The objective of this work is to identify optimum operational strategies that maximise net energy production when exposed to variable ambient temperature, with special focus on the operation of the Heat Rejection Unit (Air-Cooled Condenser). The power cycle is simulated in Thermoflex, modified with user-defined scripts to account for the specific off-design performance characteristics of key components. The Air-Cooled Condenser is modelled by means of an in-house Matlab tool, already validated in previous publications, able to accurately simulate the heat transfer process between working fluid and cooling medium (air) and to calculate auxiliary power consumption. Four different strategies are identified, depending on ambient temperature: variable or constant condensation pressure for ambient temperatures lower than the design value, and constant turbine inlet temperature or constant return temperature of the heat transfer fluid for ambient temperature higher than design value. The results show that a combination of variable and constant minimum cycle pressure is the best alternative for low ambient temperatures, enabling net system efficiencies higher than 41%. On the other hand, constant turbine inlet temperature enables higher net performance than constant return temperature of the heat transfer fluid, even if at the expense of a reduction in energy storage capacity for the same inventory of molten salts.