Helically coiled solar cavity receiver for micro-scale direct steam generation

Abstract

Concentrated solar systems have a large potential for power generation and water pumping in rural off-grid settlements with a high solar irradiance resource. With the use of a solar dish concentrator, superheated steam can be directly generated in a solar cavity receiver. Volumetric expanders show promise for micro-scale power production (below 1 kW). However, naturalistic experimental investigations of micro-scale direct steam generation using a solar dish is limited. In this work, a low-cost helically coiled cavity receiver and a novel solar concentrator manufactured from off-the-shelf components are experimentally investigated for steam generation at water pressures of above 3 bar. Working fluid temperatures were measured throughout the coil length and pressures were measured at the inlet and outlet of the receiver coil. Using a concentrator with a total incident area of 2.70 m2 and a water flow rate of 0.294 g/s, the working fluid captured 861 W of heat with an outlet temperature of 343 °C at an average solar irradiance of 757 W/m2. The average thermal efficiencies of the collector and receiver were determined to be 42% and 50%, respectively, for the testing period. Furthermore, the average second-law efficiency was 12%. Flow patterns within the coil were investigated by considering the temperature differences between the coil surface and the working fluid. A heat loss model was developed that could predict the steady-state heat loss rate with an accuracy of 97%. The observations made during the study and the results obtained highlight important design aspects that need to be considered in future work.