Modeling Off-Design and Part-Load Performance of Supercritical Carbon Dioxide Power Cycles

Continuing efforts to increase the efficiency of utility-scale electricity generation has resulted in considerable interest in Brayton cycles operating with supercritical carbon dioxide (S-CO2). One of the advantages of S-CO2 Brayton cycles, compared to the more traditional steam Rankine cycle, is that equal or greater thermal efficiencies can be realized using significantly smaller turbomachinery. Another advantage is that heat rejection is not limited by the saturation temperature of the working fluid, facilitating dry cooling of the cycle (i.e., the use of ambient air as the sole heat rejection medium). While dry cooling is especially advantageous for power generation in arid climates, the reduction in water consumption at any location is of growing interest due to likely tighter environmental regulations being enacted in the future. Daily and seasonal weather variations coupled with electric load variations means the plant will operate away from its design point the majority of the year. Models capable of predicting the off-design and part-load performance of S-CO2 power cycles are necessary for evaluating cycle configurations and turbomachinery designs. This paper presents a flexible modeling methodology capable of predicting the steady state performance of various S-CO2 cycle configurations for both design and off-design ambient conditions, including part-load plant operation. The models assume supercritical CO2 as the working fluid for both a simple recuperated Brayton cycle and a more complex recompression Brayton cycle. INTRODUCTION This paper describes the development of models capable of predicting both the design and off-design (including part-load) performance of supercritical carbon dioxide (S-CO2) Brayton power cycles for utility-scale electricity generation. There are a number of potential advantages to using an S-CO2 Brayton cycle compared to a traditional steam Rankine cycle; these have generated significant interest across a range of industries [1-4]. For example, relatively small turbomachinery can be used due to the high density of carbon dioxide (464 kg/m at its critical point) in the region above the vapor dome. Also, heat can be rejected from the S-CO2 cycle at a comparatively higher temperature than traditional Rankine cycles; thereby, offering the potential for dry cooling. Dry cooling is a means of heat rejection, in whole or part, to the ambient air directly without water and is expected to become increasingly important in the future as water conservation efforts expand. The models developed in this work are used to predict the design point, off-design point, and part-load performance of two cycle configurations: the simple recuperated Brayton cycle, shown in Figure 1(a), and the recompression Brayton cycle, shown in Figure 1(b). The recompression Brayton cycle, which adds an additional compressor and heat exchanger, has been suggested as a viable alternative to the simple recuperated cycle due to its increased efficiency at the design point [1]. Figure 1. Diagram of a simple Brayton cycle (a) and a recompression Brayton cycle (b). Precooler Primary Heat Exchanger Recuperator Main Comp Buffer Volume Tmc,in Pmc,in