Receiver Geometries for Solar Dish Concentrators
Creators: name: Dunn, Rebecca
Publisher: name: The Australian National University
Year: 2016
Subjects: solar
solar thermal
concentrating solar
concentrated solar
receiver geometry
reactor geometry
solar reactor
dish concentrator
dish receiver
energy storage
thermochemical energy storage
ammonia

description: This thesis studies receiver/reactor geometry for the ammonia thermochemical storage system and dish concentrators, using the case study of a masked 9 m2 dish. Experimental results proved that altering the geometric arrangement – in this case the cone angle – of reactor tubes within a cavity receiver significantly affects the heat transfers within and heat losses from the receiver. By consequence both the receiver efficiency and the solar-to-chemical efficiency of the dish-receiver system can be significantly increased. This is note-worthy as improvements in efficiency due to reactor tube cone angle do not increase the cost of manufacture of the receiver. Moreover, any increase in receiver efficiency allows an equivalent decrease in mirror area, and in a commercial plant, the solar collector field (mirror area) can account for around 40% of total project costs. Gains of up to 7.0% absolute in receiver efficiency and 6.4% absolute in solar-to-chemical efficiency were demonstrated experimentally in this thesis at a given solar altitude. Experimental data was examined for three receiver configurations with reactor tube half cone angles of 17.5º, 7.5º and 3.7º, with the 3.7º configuration producing the highest receiver efficiencies, the 7.5º receiver a close second, and the 17.5º configuration producing the lowest efficiencies. Solar-to-chemical efficiencies were however slightly higher for the 7.5º configuration than the 3.7º, indicating that system-level operation, especially operation of the heat exchanger, was slightly more optimal for the 7.5º case. Preliminary results integrating the receiver losses over the course of a year indicate that the 3.7º receiver geometry would reduce annual receiver losses by 25% compared to the 17.5º receiver geometry. The experimental data comprised 178 hours of “on sun” operation of the 9 m2 dish concentrator by the author, providing 311 steady state intervals of 10-15 minutes. The experimental results were compared with simulations using an existing computational receiver/reactor model, but gross disagreement was observed. Simulated reaction extents were 2-4 times those achieved experimentally. Therefore a new energy balance was developed for each of the three receiver geometries. This included 3D modelling of radiation losses, along with calculation of convection and conduction losses, and used the knowledge gained experimentally of reactor tube and cavity wall temperature profiles. These new energy balances were verified against a substantial body of experimental data – out of 86 steady-state data points at optimal flow, calculated receiver losses fell within the experimental uncertainties for all but seven points. The energy balances allowed curves of predicted receiver losses to be generated for each of the three receiver geometries over a wider range of temperatures and solar altitudes. These curves suggest that the 3.7º half cone receiver would still have the lowest receiver losses under all operating conditions, closely followed by the 7.5º half cone receiver. At mid and low solar altitudes, natural convection losses dominated over conduction and radiation losses for all three receiver geometries. Finally, conclusions on favourable flux and temperature profiles for the reactor tubes studied in this project were drawn. A flux of at least 2 W/cm2 within 100 mm of the reactor tube inlet resulted in higher conversion efficiencies, due to higher temperatures and reaction rates immediately at the tube inlet.