Experimental investigation on start-up and heat transfer performance of the high-temperature heat pipe receiver with an ellipse-concave plate absorber

A novel ellipse-concave plate heat pipe (ECPHP) receiver using sodium as the working fluid was developed for using on the solar thermochemical reactor. The start-up and heat transfer performance of the ECPHP under different cross-section heat fluxes were experimentally investigated. By introducing the temperature difference index parameters, the start-up regime of the ECPHP was specified as the melting stage, vapor flow transfer stage, and the effective start-up stage, respectively. The effective start-up time decreased to 21.7 min when the cross-section heating flux increase to 279.3 kW/m², the temperature of evaporator at effective start-up was about 680–770 °C. In addition, the ECPHP demonstrated good heat transfer performance in the quasi-steady stage. The temperature difference on the condensing section was less than 93.8 °C, the thermal resistance and the heat transfer coefficient were in the range of 0.054–0.083 °C/W and 670.4–1032.3W/(m²·°C), respectively. Moreover, the temperature image on the evaporator illustrated that the ellipse-concave design of the absorber could effectively relieve the hot spot on the heating surface, which would reduce the thermal stress, thereout enhancing the heat transfer stability of the receiver when applied to concentrate the solar energy.