Receiver shape optimization for maximizing medium temperature CPC collector efficiency

Abstract

Low optical-concentration solar thermal CPC collectors for process heat at 150-300°C generally use thermal oil as the collector fluid. Thermal oils have low thermal conductivity and high viscosity, which leads to significant thermal resistance and hence reduced collector thermal efficiency. One way to minimize the thermal resistance is by having turbulent flow of the thermal oil within the receiver. For a given receiver area and mass flow rate of the fluid, this can be achieved by narrowing the flow passage but keeping the receiver area constant by adding external flat fins. In this paper a new receiver design for a compound parabolic concentrator is proposed which is a hybrid of a U-shaped tubular receiver and a bifacially irradiated flat receiver. To keep the receiver area constant, the fins are increased in width as the tube diameter is decreased. Its performance when enclosed in a glass vacuum tube and a CPC has been modelled. The transmission and absorption of solar energy, optical losses due to the receiver-reflector gap, heat transfer within the receiver, and the thermal losses have been modelled. Keeping the receiver area and fluid flow rate constant, the thermal resistance of the thermal oil flow within the receiver reduces when the flow passage is narrowed leading to increased thermal efficiency. On the other hand, the hybrid receiver has lower optical efficiency as compared to a tubular receiver due to its higher gap loss. Overall, the hybrid receiver has similar or better thermal efficiency than the tubular receiver. Thermal efficiency and effective thermal efficiency, which accounts for the pumping power penalty, shows that the performance improvement with thermal oil due to receiver shape optimization depends on the receiver area, concentration ratio, absorptivity and emissivity of the selective surface, the mass flow rate through the receiver and fluid temperature. Highest effective thermal efficiency is generally achieved in the laminar-turbulent transitional regime. For temperatures below 150°C, water has been found to give better performance than thermal oil at all mass flow rates with no significant improvement in collector performance achieved by reducing the tube diameter.

A non‐tracking semi‐circular trough solar concentrator

“…Since the 1970s there have been hundreds of papers on the design, performance and application of very many concentrators based upon the CPC principle. To quote only a few examples, there are papers reporting the performance and efficiency of CPC troughs with various absorber shapes, [8][9][10][11] applications to hot water heating, [12][13][14][15][16][17] to refrigeration, [18][19][20] to chemical processes 21 and for water de-salination. 22 In a recent paper 23 it is suggested to mount an assembly of standard double-wall CPC on the side of buildings for water heating applications.…”

Topology Optimization Method of a Cavity Receiver and Built-In Net-Based Flow Channels for a Solar Parabolic Dish Collector

“…In the early stages, most studies focused on the influence of the cavity shape of the receiver, such as cylindrical, cuboidal, hemispherical, conical, and other irregular shapes [ 11 , 12 , 13 ]. For instance, Karwa et al [ 14 ] studied receiver shape optimization and proposed a new receiver design for a compound parabolic concentrator. For a given cavity shape, many scholars [ 15 , 16 , 17 , 18 ] have studied the influence of various geometric parameters such as cavity diameter and cavity height on the thermal performance of the receiver.…”

Experimental Investigation of a Medium Temperature Single-Phase Thermosyphon in an Evacuated Tube Receiver Coupled With Compound Parabolic Concentrator

“…In another study, Karwa et al (2015) studied the thermohydraulic performance of a CPC collector incorporating a U-tube absorber with fins. The receiver's dimensions (tube diameter and fin size) were optimized to maximize the effective efficiency of the CPC system.…”