On the off-design performance of supercritical carbon dioxide power cycles

On the off-design performance of supercritical carbon dioxide power cycles

Cycle modelling is a critical task for the selection of the optimal power cycle configuration for a given nominal operating point and application. With the current trend of increased penetration of renewables in the electricity market, there is increasing importance for a cycle to operate with a high turn down whilst maintaining acceptable thermal efficiency. Thus there is increased need for accurate models for the on and off-design analysis of cycles. Through evaluating cycle performance for changing output requirements and changing ambient conditions it is possible to identify configurations to optimise overall power generation.

This paper presents the results of an investigation into the off-design and part-load operation of the supercritical carbon dioxide (sCO2) recuperated Brayton and recompression Brayton cycle. The cycle model takes full account of the operating and performance characteristics of the turbomachinery and heat exchanger components and how their performance is affected by changes in cycle operating conditions. This is achieved by modelling the components using physics based approaches or using experimentally determined performance maps scaled to the current applications.

The analysis shows that adjusting the compressor speed is an effective way to modulate the cycle output power. Without additional modifications of the compressors power can be reduced down to 55%, while at the same time cycle efficiency reduces by less than 2 percentage points. This demonstrates the suitability of the sCO2 cycles for applications that require substantial operation away from the design point, for example in applications that require load following or extended part load operation. In contrast reducing turbine inlet temperature is very detrimental to cycle operation, leading to reduction in thermal efficiency of approximately 1 percentage point per 20K.

Analysis of the off-design operating points of the turbomachinery components also reveals new insight towards setting their nominal conditions and the desired shape of performance maps.