Thermodynamics analysis of the supercritical CO₂ binary mixtures for Brayton power cycles, P. Tafur-Escanta, I. López-Paniagua, J. Muñoz-Antón, Energy, 270, 126838, 2023, Online version, https://doi.org/10.1016/j.energy.2023.126838

Abstract

Supercritical CO₂ Brayton-based cycles are a promising technology for concentrated solar power plants. Their high efficiency can be improved by using CO₂ mixtures as working fluid instead of pure CO₂. This work studies 3 cycle topologies: simple recompression (RCC), recompression with main compressor intercooling (RCMCI) and partial cooling recompression (PCRC), across a range of regenerator conductance (UA), compressor and turbine inlet temperatures. Each topology is simulated with different working fluids: CO₂, CO₂–NH₃, CO₂–SO₂, CO₂–COS and CO₂–H₂S. The influence of the working fluid, operating conditions and cycle irreversibility on the overall efficiency has been analyzed in depth, finding two interesting, opposing trends. The mixtures with higher critical pressure improve the heat absorption and rejection temperatures of the cycles, but also increase irreversibility. The combination of topology and mixture that achieves the best compromise between both trends will have the higher efficiency. In general, using RCMCI instead of PCRC or RCC can improve efficiency by up to 4% depending on the working fluid and operating conditions. Using a mixture instead of pure CO₂ by up to near 3%.

Experimental assessment of Fe-doped CaMnO3 porous pellets in a bench-scale packed-bed reactor for thermochemical energy storage, E. Mastronardo, M. Sánchez, J. González-Aguilar, J. M. Coronado, Journal of Energy Storage, 57, 106226, 2023, Online version, https://doi.org/10.1016/j.est.2022.106226

Abstract

Thermodynamic cycles requiring high operating temperatures (≥750 °C up to 1200 °C) are currently being explored to improve the sun-to-electricity conversion efficiency of Concentrating Solar Power (CSP) plants. This is calling for the design of new efficient high-temperature (≥750 °C) Thermochemical Energy Storage (TCES) systems, which are fundamental for supplying power on demand during off-sun periods. Recently, Fe-doped CaMnO₃ oxides have been proposed as TCES candidate materials, and the determination of their thermodynamics properties via thermogravimetric (TG) analysis allowed evaluation of their heat storage capacity at a very small scale (mg scale). A 10 % Fe-doped CaMnO₃ composition (CaMn_0.9Fe_0.1O_3-δ – CMF91) emerged as optimum candidate material for TCES application due to its large heat storage capacity complemented by enhanced thermal stability over multiple oxidation/reduction cycles. To advance in the thermal characterization of these materials at a multigram scale, here we carried out bench-scale reactor tests using CMF91 under conditions considered representative of future CSP plants. The redox-active material has been extruded in the form of porous pellets through a simple production method that required the use of carboxymethylcellulose as a removable binder and water. With the bench-scale reactor tests, the CMF91 pellets showed fully reversible reduction-oxidation in cycles between 500 and 1100 °C under relevant operating pO₂ conditions without any deterioration of the pellet’s structural integrity. Remarkably, the material exhibited the same δ(T, pO₂) profile at this significantly larger scale (~40 g) than the one derived from thermodynamics. Nevertheless, slight differences in oxygen release/uptake profiles between cooling and heating branches can be tracked down to an excess heat generation in the perovskite bed not efficiently extracted by the carrier gas. These results demonstrate that CMF91 oxide is ideally suited for thermal energy storage applications with a large total (thermochemical and sensible) heat storage capacity (~ 916 kJ/kg_ABO3 or ~ 400 kWh/m³) and good scalability.

Energy and exergy analysis of microchannel central solar receivers for pressurised fluids, D. D’Souza, M. J. Montes, M. Romero, J. González-Aguilar, Applied Thermal Engineering, 219, 119638, 2023, Online version, https://doi.org/10.1016/j.applthermaleng.2022.119638

Abstract

Within the new generation of advanced central solar receivers, microchannel pressurised gas receivers are emerging as reliable and efficient alternatives to operate at high temperatures and pressures. This paper presents an optimisation and comparative analysis of different compact plate-fin type structures, constituting the receiver’s absorber panels, classified according to the type of fin arrangement inside: plain rectangular, plain triangular, wavy, offset strip, perforated, and louvred fin. A versatile thermo-fluid receiver model is implemented, allowing simple variation of characteristic geometric parameters of each structure. Exergy efficiency is chosen as the optimisation function, as it considers both heat and pressure losses.

The framework of the analysis is set by the receiver’s boundary conditions, operating at the design point conditions of a solar thermal power plant. For each compact structure, the optimal configuration is determined, providing interesting findings that have not been reported in the state-of-the-art to date. Although all geometries show good thermal performance, the perforated and plain rectangular configurations demonstrate the best exergy efficiencies of 59.21% and 58.80%, respectively, favouring taller and narrower channels. This analysis methodology could be seamlessly extrapolated to other gases and working conditions, owing to the thermo-fluid model’s versatility, to reveal the optimal configuration for each case.