A solar cylindrical cavity receiver is a type of solar thermal energy storage system that can be used to store and discharge solar energy for use in a variety of applications, including electricity generation, space heating and cooling, and industrial process heat. In a solar cylindrical cavity receiver, solar energy is collected and concentrated by a solar concentrator and directed into a cylindrical cavity. The cavity is lined with a high-absorptivity material, such as blackened stainless steel or graphite, which absorbs the solar energy and heats up. The heat is then transferred to a working fluid, such as water or oil, which is circulated through the cavity. The hot working fluid can be used to generate steam for electricity generation, or it can be used directly for space heating or industrial process heat. To evaluate the convection and radiation heat losses through the cylindrical cavity, we developed an analytical model based on different Nusselt number correlations. Depending on the concentration ratio and mass flow rate of the work fluids, the model can then be used to predict the total heat lost and predict the outlet temperature of the working fluid.