Heat loss analysis from a trapezoidal cavity receiver in LFR system using conduction-radiation model

Linear Fresnel Reflector systems are medium temperature (100-400 °C) application systems where heat flux from the sun is concentrated on absorber tubes housed in a trapezoidal cavity by an array of mirrors. The absorber tubes carry working fluid inside them. Several earlier works have considered convective and radiative heat transfer from these trapezoidal cavities in LFR systems. It has also been shown that the convective heat transfer constitutes up to 15% of the total heat losses which is significant. On the other hand, it is seen that the flow velocities are negligible due to stratification of isotherms with hot air trapped on top of the cavity which suggests that convection should be negligible. In the present work, it is shown that the heat transfer which is considered as convective is actually, only conduction through (almost) static air inside the cavity. Due to the abovementioned reason, in the present work, only conduction-radiation problem is considered in the cavity which is far easier to solve due to the absence of complex Navier-Stokes equations. The comparison of heat transfer results obtained using conduction-radiation model and those obtained using convection-radiation model show that the difference between the two results is negligible. Moreover, new correlations are developed with fewer parameters to capture the underlying physics of the heat loss mechanism in such cavities.