Design and development of a thermal rock bed storageexperimental facility

[摘要] ENGLISH ABSTRACT: The value of concentrating solar power (CSP) plants lies in dispatchability,which is provided through an integrated cost-effective thermal energy storagesystem (TESS). Compared to current state of the art molten salt thermalenergy storage systems, a rock bed thermal energy storage system has thepotential to reduce both the capital costs and Levelized Cost of Electricity(LCOE) significantly. The Stellenbosch University (SU) first-generation rockbed thermal energy storage design served as a proof of concept while thesecond-generation rock bed design was designed for significant cost reduction.This work presents the third-generation rock bed TESS at SU, through partialre-design, predominantly aiming at maximizing the usable rock mass.The rock bed thermal energy storage system is charged by air at a temperatureof 650C. An existing experimental facility, based on the second-generationdesign, has recently been constructed. To modify the facility, a concept wasdeveloped with knowledge gathered from both the first and second-generationconcepts. The new concept charges the rock bed from the top downwards,with a predicted near-linear thermocline progression, where the thermocline isdefined as the transition layer from the high temperature to the low temperaturewithin the rock bed. Although the concept has a higher capital cost, animproved performance is predicted for the entire system.After development, the concept was adapted to the existing facility. Threeexperimental test campaigns were conducted, concluding with a multiple cycletest. This test consisted of three charge-discharge cycles, where the rock bedwas discharged to a minimum outlet temperature of 327C. Determining anaccurate discharge mass flow rate was a challenge throughout testing, withflow leakages detected within the system. A flow loss assumption of 40 % wasmade after several cold air flow rates were tested. The second cycle within themultiple cycle test yielded a heating capacity of 336.67 kWhth, a volumetricefficiency of 60.30 % and a thermal efficiency of 92.40 %. An overall efficiencyof 94.24 % was achieved over the three cycles.An analytical model was developed to be validated by the experimental results.From the validation, a possible prediction can be made on the performanceof such a rock bed thermal energy storage system on an industrial scale.The thermal efficiency comparison yielded a maximum difference between theexperimental and analytical results of +8.00 % for the first two cycles and+19.36 % for the third cycle. It is clear from this comparison that the modelover-predicts the performance of the facility. Considering that the model isone dimensional and that it disregards both radiation and convection as heattransfer elements, as well as thermal losses, the model appears to be acceptable.However, it is recommended that further improvements be made to themodel for a more accurate comparison.The overall results show that there has been an improvement in performance ofthe rock bed after the design changes that were made. These design changesinclude the addition of insulation and introducing the high temperature airinto the top of the rock bed, rather than at the bottom. Room for improvementon the design to achieve higher overall performance has been identifiedand possible solutions are presented within this project.

[发布机构] Stellenbosch University