Supercritical CO₂ Brayton cycles for solar-thermal energy

Of the mechanisms to improve efficiency for solar-­‐thermal power plants, one of the most effective ways to improve overall efficiency is through power cycle improvements. As increases in operating temperature continue to be pursued, supercritical CO2 Brayton cycles begin to look more attractive despite the development costs of this technology. Further, supercritical CO2 Brayton has application in many areas of power generation beyond that for solar energy alone.One challenge particular to solar-­‐thermal power generation is the transient nature of the solar resource. This work illustrates the behavior of developmental Brayton turbomachinery in response to a fluctuating thermal input, much like the short-­‐term transients experienced in solar environments. Thermal input to the cycle was cut by 50% and 100% for short durations while the system power and conditions were monitored. It has been shown that despite these fluctuations, the thermal mass in the system effectively enables the Brayton cycle to continue to run for short periods until the thermal input can recover. For systems where significant thermal energy storage is included in the plant design, these transients can be mitigated by storage; a comparison of short-­‐ and long-­‐term storage approaches on system efficiency is provided. Also, included in this work is a data set for stable supercritical CO2 Brayton cycle operation that is used to benchmark computer modeling. With a benchmarked model, specific improvements to the cycle are interrogated to identify the resulting impact on cycle efficiency and loss mechanisms. Status of key issues remaining to be addressed for adoption of supercritical CO2 Brayton cycles in solar-­‐thermal systems is provided in an effort to expose areas of necessary research.