Heat loss of a trapezoidal cavity absorber for a linear Fresnel reflecting solar concentrator

The present paper studies the heat loss of a linear absorber with a trapezoidal cavity and a set of pipes used for a linear Fresnel reflecting solar concentrator. The study includes the measurements on a 1.4 m long prototype installed in a laboratory, and its thermal simulation in steady-state using EnergyPlus software. Results of the measured vertical temperature variation inside the cavity, the surface interior and exterior wall and window temperatures, the global heat loss at steady-state and the heat loss coefficients, are presented for six different temperatures of the pipes. Measurements revealed a stable thermal gradient in the upper portion of the cavity and a convective zone below it. Around 91% of the heat transferred to outdoors occurs at the bottom transparent window, for a pipe temperature of 200 °C. The heat loss coefficient per area of absorbing pipes ranged from 3.39 W/m2K to 6.35 W/m2K (for 110 °C < Tpipe < 285 °C), and it increased with the increase of Tpipe. Simpler and less time-consuming available free software originally designed for heat transfer in buildings was tested to be a possible replacement of the highly complex CFD software commonly used to simulate the steady-state heat loss of the absorber. The experimental and predicted data sets were found to be in good agreement.

► A linear absorber with a trapezoidal cavity was measured and simulated.
► Stably stratified and convective zones were found inside the cavity.
► Heat loss is predominantly by radiation of the bottom transparent window.
► Heat loss coefficient ranged from 3.39 to 6.35 W/m2K (for 110 °C < Tpipe < 285 °C).
► The experimental and predicted data sets were found to be in good agreement.