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

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.The National Research Foundation (NRF) of South Africa, the Technology Innovation Agency (TIA) of South Africa and the Department of Science and Innovation (DSI).https://www.elsevier.com/locate/apthermeng2021-12-06hj2021Mechanical and Aeronautical Engineerin