Publications
Advanced Nuclear Power Program
CO2 Brayton Cycle Design and Optimization
V. Dostal, M.J. Driscoll, P. Hejzlar, and N.E. Todreas
MIT-ANP-TR-090 (November 2002)
Abstract
This report is focused on the development of an advanced power conversion cycle for next generation reactors. The supercritical CO2 recompression Brayton cycle has great potential to fulfill all the requirements of an attractive power conversion cycle. It achieves cycle efficiencies on the order of ~45% for the turbine inlet temperature of 550ºC and it is extremely compact and simple – factors key to good economy. The simple supercritical CO2 Brayton cycle without intercooling is capable of achieving a cycle efficiency of ~40% at 550ºC and 20 MPa and more than 45% at 650ºC and 30 MPa. Intercooling is not attractive for this type of cycle. To further increase the cycle efficiency a recompression cycle is introduced and analyzed. An optimization scheme for operating conditions and recuperator sizes was developed and employed to define a reference cycle that achieves thermal efficiency of 46.1% at 550ºC and 20 MPa, which corresponds to net efficiency of about 43.6%. Potentially, thermal efficiencies close to 50% are attainable with this cycle at 650ºC and 30 MPa. A preliminary component design was accomplished. Cycle heat exchangers (recuperators and precooler) that use compact heat exchanger printed circuit technology were evaluated. The target specific volume of recuperators, 0.25 m3/MWe, was satisfied. For the 300 MWe reference cycle this leads to a 75 m3 recuperator volume. The precooler was optimized to minimize cooling water pumping power while having a reasonable volume. Preliminary assessment of the turbomachinery shows that it is very compact: a 450 MW turbine (300 MWe) has about 5 stages and the hub diameter is less than 1 m. A correspondingly small size was found for the compressors. The main compressor has 2 stages, the recompressing compressor 3 stages. These features combined yield a cycle that is very attractive for gas cooled reactors. The current reference cycle for a Gas Cooled Fast Reactor (GCFR) and its operating conditions are described and the cycle is compared to its principal competitors: the steam Rankine cycle (in its superheated and supercritical versions) and the helium Brayton cycle. The superiority of the supercritical CO2 recompression Brayton cycle in the turbine inlet temperature range of 500 – 700ºC is demonstrated.
