Performance study of solar power plants with CO₂ as working fluid. A promising design window

Highlights: • High efficiency regenerative Brayton cycle with carbon dioxide working close to its critical point. • Efficient coupling between multi-tube solar receivers and close-to-critical carbon dioxide Brayton cycle. • Concentrated radiation intensity showing a threshold at 25 kW/m² for achieving high efficiency in multi-tube solar receivers. - Abstract: In this paper a systematic analysis is reported on the use of CO₂ as heat carrier fluid in solar thermal receivers and as thermodynamic working fluid. It includes the performance of close-to-critical regenerative Brayton cycles, which opens a broad field of cycle possibilities with low pressure ratios (very simple turbines) complemented with large but standard heat exchangers as regenerators. Radiation intensities needed to reach relevant efficiencies are in the range above 25 kW/m², but receiver efficiencies do not increase significantly beyond that value, featured as a threshold. Receivers are made of multi-tube bundles enclosed in glass-windowed collectors with compensated pressure and dilatation, which eliminates the problem of gas leakage through rotating joints and other non-hermetic fits. This leads to needing concentrators compatible with those collectors, which can be either finely optimized Linear Fresnel Reflectors or central minitowers. CO₂ was chosen for this study because its critical temperature (31 °C) is very close to environmental temperature, which conveys very positive features for the efficiency of the cycle. The overall result of the theoretical study is the identification of a set of different types of efficient, flexible and robust CSP plants with CO₂ as the only fluid which deserves further research at experimental level and in the design and construction of new plant components