A preliminary evaluation of a molten metal helical coil solar receiver for heating pressurised air

Abstract

Heating pressurised air to high temperatures with concentrated solar thermal energy is technically challenging mainly due to the heat transfer properties of the pressurised air and the intermittent nature of the solar energy. To address this challenge, a novel molten metal helical coil (M2HC) solar receiver for heating pressurised air or other gases is presented. This system comprises a cavity and a helical coil, both of which are submerged in a molten bath, e.g. a molten metal, which serves as a heat transfer fluid (HTF) between the submerged cavity and the helical coil. The absorbed concentrated solar thermal heat in the cavity is transferred to the molten bath via convection. Pressurised air is heated within the submerged helical coil, without any contact with the molten bath. The molten bath is agitated via the bubbles of an injected inert gas enabling a high rate of heat transfer between the submerged cavity and the helical coil. The purpose of this paper is to introduce the concept of the M2HC solar receiver and provide a preliminary evaluation to identify its potential benefits and challenges. In doing so, a one-dimensional model of the proposed M2HC solar receiver is developed, predicting the system performance considering gallium (Ga) and argon (Ar) as the HTF and dispersed inert gas into the molten Ga bath, respectively. The heat transfer model of the cavity, gallium bath, Ar bubbles and the helical coil is implemented with the consideration of convective and radiative losses in the cavity. The mixture of Ga and Ar bubbles is considered as one phase and modelled with a one-dimensional drift-flux correlations. A 1-D model has been also considered for the heating of the pressurised air within the coil. The reliability of the model was assessed through comparison with the available relevant experimental data in literature. The model is then employed to perform a sensitivity analysis to the variations in the temperature of the outlet air, length of helical coil and the ratio of the mass flow rates of the injected Ar into the molten bath and the inlet air. The predicted absorption and exergy efficiencies of the proposed M2HC solar receiver are ∼64% and 73%, respectively, at an output temperature of 900 °C and 5% Ar to air mass flow ratio. It has been also found that the variation of Ar to air mass ratio from 0% to 25% decreases the fraction of the absorbed energy that goes to air from 100% to ∼90%. Nonetheless, it increases the convective heat transfer between the molten bath, submerged cavity and the helical coil.