Development of a solar collector with a stationary spherical reflector/tracking absorber for industrial process heat

• An experimental system based on a stationary reflector and tracking absorber was designed and tested.
• More suitable technology for mass production has been considered.
• The design involved a detailed analysis of the optical and thermal behavior.
• A tracking system was employed using an algorithm that predicts the sun movement.
• A detailed economic analysis was performed.

A system for collecting solar energy at intermediate temperatures was developed and built in this research project. The system consists of a stationary, 120° included angle, 2.8 m diameter stationary spherical reflector with a tubular, tracking absorber which moves automatically into the focus following the sun’s movement. The system is capable of heating water or other fluids to temperatures above 250 °C, thus making it possible to obtain process heat for domestic and industrial use and to store solar energy in a compact and economical form.An analysis of the system’s optical and thermal characteristics was performed to aid in the design of the reflector and absorber. The overall performance of the system has been analyzed in detail by means of a mathematical model. Results of the study show that the efficiency of the collector is almost constant up to working temperatures of 300 °C. The analysis indicates that the optical properties of the mirror, glass envelope and absorber are the most important of the principal governing parameters in determining system performance.The particular feature of the new system, as compared to other concentrating collectors, is that the reflector is stationary and can hence be produced by cheaper and simpler technology. It can in fact be made part of the roof structure of a building in which the process heat that it produces is utilized. The reflector was built from 20 mm-thick curved steel sheet and then machined to its accurate spherical shape. After producing this spherical bowl it was lined with a reflective film. The absorber is a cylindrical coil painted with flat-black stove paint that can resist 600 °C. An evacuated glass envelope covers the absorber in order to protect it from heat losses by convection. The absorber follows the sun by means of four cables driven by small electric motors controlled by an astronomic code that predicts the sun’s position.The performances of the spherical collector were tested under different weather conditions by measuring the flow rate and the temperature of the pressurized water. Additional tests were performed using thermal oil as HTF to enable operation at higher temperatures. Total efficiencies (solar to thermal) of ∼50% were obtained for a wide range of temperatures up to 200 °C. The simulations predict higher efficiencies of approximately 70–80% up to 300 °C depending on the optical properties. The results of the present study demonstrate the possibility to use the spherical collector in cooling and heating systems and make possible extensive utilization of solar energy at considerable savings relative to fossil energy in the sunny countries of the world.