Browsing by Author "Erasmus, Stephanus Johannes"

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    Design and development of a thermal rock bed storage experimental facility
    (2019-04) Erasmus, Stephanus Johannes; Von Backstrom, Theo; Lubkoll, Matti; Dinter, Frank; Laubscher, Henk; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering (CRSES)
    ENGLISH ABSTRACT: The value of concentrating solar power (CSP) plants lies in dispatchability, which is provided through an integrated cost-effective thermal energy storage system (TESS). Compared to current state of the art molten salt thermal energy storage systems, a rock bed thermal energy storage system has the potential to reduce both the capital costs and Levelized Cost of Electricity (LCOE) significantly. The Stellenbosch University (SU) first-generation rock bed thermal energy storage design served as a proof of concept while the second-generation rock bed design was designed for significant cost reduction. This work presents the third-generation rock bed TESS at SU, through partial re-design, predominantly aiming at maximizing the usable rock mass. The rock bed thermal energy storage system is charged by air at a temperature of 650 C. An existing experimental facility, based on the second-generation design, has recently been constructed. To modify the facility, a concept was developed with knowledge gathered from both the first and second-generation concepts. The new concept charges the rock bed from the top downwards, with a predicted near-linear thermocline progression, where the thermocline is defined as the transition layer from the high temperature to the low temperature within the rock bed. Although the concept has a higher capital cost, an improved performance is predicted for the entire system. After development, the concept was adapted to the existing facility. Three experimental test campaigns were conducted, concluding with a multiple cycle test. This test consisted of three charge-discharge cycles, where the rock bed was discharged to a minimum outlet temperature of 327 C. Determining an accurate discharge mass flow rate was a challenge throughout testing, with flow leakages detected within the system. A flow loss assumption of 40 % was made after several cold air flow rates were tested. The second cycle within the multiple cycle test yielded a heating capacity of 336.67 kWhth, a volumetric efficiency of 60.30 % and a thermal efficiency of 92.40 %. An overall efficiency of 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 performance of such a rock bed thermal energy storage system on an industrial scale. The thermal efficiency comparison yielded a maximum difference between the experimental 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 model over-predicts the performance of the facility. Considering that the model is one dimensional and that it disregards both radiation and convection as heat transfer elements, as well as thermal losses, the model appears to be acceptable. However, it is recommended that further improvements be made to the model for a more accurate comparison. The overall results show that there has been an improvement in performance of the rock bed after the design changes that were made. These design changes include the addition of insulation and introducing the high temperature air into the top of the rock bed, rather than at the bottom. Room for improvement on the design to achieve higher overall performance has been identified and possible solutions are presented within this project.