Studies on Pressurized Solar Thermal Receiver and Thermal Energy Storage System

Abstract

High-efficiency power cycles for concentrating solar power (CSP) technology such as air or supercritical carbon-dioxide (s-CO2) based Brayton cycle require high-pressure high-temperature conditions at the gas turbine inlet. This requires heating of the heat transfer fluid (HTF) to those conditions (~3-35 bar, 900-1600 K for air and ~200 bar, 1000 K for s-CO2) using a solar thermal receiver. Thus, the design and analysis of the pressurized receiver system form an important part towards the development of such solarized power plants. A coupled optical and thermal model is developed for analyzing a cavity-based pressurized receiver, with dynamic variation of solar radiation input. The optical part involves the focal region flux characterization of a fixed-focus Scheffler reflector that provides a spatially resolved heat flux to the receiver cavity surface. This is achieved using a combination of on-sun experiments and ray-tracing simulation. On the other hand, the transience in heat input to the receiver is captured by curve-fitting the measured DNI variation with time corresponding to the experimental testing period. This spatially and temporally varying heat flux is coupled to the thermal analysis of the receiver to predict the flow field and the enthalpy gain by the heat transfer fluid (HTF) along with the thermal losses from the receiver cavity. This numerical model is subsequently validated with on-sun experimental testing of a hybrid tubular and cavity receiver using a 32 m2 Scheffler dish for heat input and compressed air at 20 bar as the HTF. The numerical and experimental results are found to be in good agreement under comparable conditions, thus proving the effectiveness of the coupled optical and thermal model. To account for the transient nature of the receiver heating during the on-sun experiments, the receiver efficiency definition is modified to include the thermal inertia of the receiver material. It is observed that natural convection is the dominant heat loss mechanism that significantly reduces the overall thermal efficiency of the receiver.

In the context of cavity receivers, the rate of heat transfer to the pressurized HTF is limited by the forced convection mode. For the enhancement of heat transfer in such systems, a passive method using metallic wire meshes in the HTF flow path is explored. Firstly, a pore-scale analysis is performed on the inline stacked wire mesh geometry for determining the hydraulic and thermal characteristics of the medium. The heat transfer taking place between the wire struts and the airstream at the local level is captured by thermal analysis on a representative elementary volume (REV) defined for this mesh geometry. This yields an interstitial Nusselt number for capturing the local heat transfer between the two phases. Subsequently, a homogeneous equivalent porous medium is defined using the properties obtained from the pore-scale analysis. For modelling the heat transfer within the two phases, the local thermal non-equilibrium (LTNE) model is implemented using the obtained Nusselt number correlation. The numerical model is subsequently validated with laboratory-scale experiments performed on a channel stacked with wire mesh layers and thermal load provided using an electric heater. The model prediction shows good agreement with the experimental results. However, the LTNE effect is not that pronounced under the present thermal conditions.

Among the storage options available for such applications, sensible heat storage using ceramic material with honeycomb structure having gas flow passages has been used for the present study, because of its cost advantage and stability at high temperatures and pressures. To enhance the performance of such systems, the effect of block arrangement is analysed with respect to the rate of charging and discharging. Towards this end, two configurations are explored for the same storage material volume; a bigger cross-sectional area system with smaller HTF flow length and a smaller cross-sectional area system with a longer flow length. These systems are thermally cycled between 443 K and 300 K using compressed air at 10 bar pressure. Analytical and numerical models are developed that are validated using laboratory experiments, and the results are in good agreement with both the modelling approaches. This study reveals that the block arrangement that allows for higher flow velocity through the honeycomb channels of the ceramic block charges and discharges the system at ~1.5 times faster than the other configuration with slower air stream velocity under identical thermal conditions.