The paper entitled, “Supercritical CO2 Brayton Cycles for Solar-Thermal Energy” presents the first experimental data of a supercritical CO2 Brayton cycle for use in solar-thermal energy where a transient solar resource is the norm. Supercritical CO2 Brayton cycles are, in general, a new technology with only a few examples of turbomachinery in operation. While there are several references that address Brayton cycle operation with a supercritical CO2 fluid, very few experimental works are available in the literature. To the authors’ knowledge, none of these experimental works have addressed the specific niche of CO2 Brayton for solar. Momentum is gaining behind this power cycle, especially as consideration is being given to higher temperature ranges where supercritical steam may not be as attractive.
This work presents transient operational data for a supercritical CO2 Brayton cycle where the thermal input power is reduced for short-terms much like a solar-thermal plant may experience due to weather effects. Losses are carefully outlined and uncertainty in the data is provided for the experimental system and compared with a Fortran model used to extrapolate to future improvements. Given that a supercritical CO2 Brayton cycle does not currently have a commercial demonstration, we also outline the areas of necessary research and development required to make this cycle successful for a solar-thermal application. This becomes especially important when considering the tradeoffs with other cycles that may already be commercially available. With the data presented in this work, we envision enabling others to benchmark their modeling efforts as well as drive the conversation behind what is needed for adoption of this novel cycle.
Figure legend: Reprinted from Applied Energy, Vol. 111, Iverson, B. D., Conboy, T. M., Pasch, J. J., and Kruizenga, A. M., “Supercritical CO2 Brayton cycles for solar-thermal energy,” pp. 957-970, 2013, with permission from Elsevier.
Applied Energy, Volume 111, November 2013, Pages 957-970.
Brian D. Iverson, Thomas M. Conboy, James J. Pasch, Alan M. Kruizenga
Brigham Young University, UT 84602, United States and Sandia National Laboratories, Albuquerque, NM 87185, United States
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.