Features of Fabrication Technology and Properties of Wicks of Heat Pipe Receivers for Solar Dish/Stirling Systems

Elaboration of robust and reliable capillary systems for solar energy heat pipe receivers is the important step for future application of this product for systems with thermal power of 30–80 kW. The paper considers a new approach to fabrication of capillary structure of heat pipe receivers on the basis of discrete metal fibers with diameter of 30 microns made of stainless steel 316L, and describes some of methods of wicks characterization as well. This technology has been demonstrated by fabrication of porous 4 mm thick wicks with bulk porosity about 0.82 applied to convex surfaces of dome-shaped receivers with radius 178 mm/height 119 mm (thermal power 36 kW) and radius 247 mm/height 173 mm (thermal power 68 kW) and for inner surface of tube with length 450 mm and diameter 73 mm (thermal power 14 kW). The distinction of the proposed technology is in the use of discrete fibers, which are felted on the curved surface in a special way and in the combination of procedures of the felt formation and their sintering to the surface (substrate material is Haynes Alloy 230). Execution of an extensive program of experimental characterization of a wick layer attached to the substrate has been developed and completed. The characterization of applied wicks determines a definition of their structural (local porosity, thickness of porous layer), mechanical (quality of wick bonding to substrate) and hydrodynamic properties (pumping diameter, one-phase and two-phase permeability). Initial estimation of wick performance was performed on the basis of methods developed at the National Technical University of Ukraine for two main modes of receiver operation — with return of sodium to a point on the dome (reflux) and without it. Prediction of receiver thermal performance, when they operate as a part of solar concentration assembly, was determined by specialized heat pipe performance software /Sandia National Laboratories, C. Andraka, 1999/.