A novel approach to high temperature solar receivers with an absorbing gas as heat transfer fluid and reduced radiative losses

Abstract

We report on a new high temperature solar receiver concept which exploits the ability of some molecular gases such as water vapor or carbon dioxide to absorb a significant fraction of longer wavelength thermal radiation while being mostly transparent to terrestrial solar radiation. The receiver operation principle is similar to the “greenhouse effect”: It relies on a gas volume between an aperture and a black surface heated by solar radiation to absorb the thermal re-radiation of the surface and shield the aperture from it. At the same time, the gas is heated by such absorption and acts as heat transfer fluid (HTF). The receiver works with radiation only as heat transfer mechanism and does not require, a priori, any convective contribution. To demonstrate the potential of the approach we modelled the system with the most accurate method available, spectral line-by-line (LBL) photon Monte Carlo raytracing with the absorption coefficients derived from the HITEMP 2010 spectroscopic database. We applied the model to a cylindrical cavity geometry with an axial gas flow. We simulated the performance for a large 16 m diameter, 16 m-long near-ambient pressure receiver with and without window at the 100 m2-aperture. The gases considered as HTF were steam, CO2, and mixtures of them. The gas outlet temperature was varied between 1000 K and 2000 K. We also simulated a small 1.6 m diameter, 1.6 m-long cylindrical cavity receiver with a 1 m2 windowed aperture and pressurized steam at 10 bar. With a solar irradiation flux of 1200 kW/m2 at the aperture, the receiver efficiency remained above 80% for gas outlet temperatures up to 1800 K for all receivers operated with steam. Carbon dioxide led to lower efficiencies, but water-rich mixtures of H2O and CO2 performed closely as good as pure steam. The proposed receiver concept represents a new opportunity towards low-cost high temperature solar technologies for processes beyond 1000 K.