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Session: Session 6A: Novel/advanced architectures (1)
Session starts: Wednesday 11 September, 09:00
Presentation starts: 10:00
Room: Olympia

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Goran Durakovic (SINTEF Energy Research)
Monika Nikolaisen (SINTEF Energy Research)

Abstract:
Energy intensive industries generate vast amounts of low-grade surplus heat, and potential for direct re-use can be limited due to insufficient local demand. Conversion of heat into electric power thus becomes an attractive option. However, heat-to-power conversion is limited by poor efficiencies for heat sources at low and medium temperatures. To optimally utilize the available energy, the choice of technology could be important. In this work, single-stage subcritical, dual-stage subcritical and single-stage transcritical Rankine cycle technologies were compared. The investigated heat sources are an air equivalent off-gas at 150 and 200°C and a liquid heat source at 150°C. These were all evaluated with the same available duty. The Rankine cycles were compared with equal total heat exchanger areas to enable fair comparisons of different cycles, instead of comparing cycles based on equal pinch point temperature differences as is frequently reported in literature. Six working fluids were investigated for both heat source temperatures, and cycle optimization was performed to determine the best Rankine cycle for each working fluid. This means that the optimizer could choose between single-stage and dual-stage cycle during one optimization, and determine which gave the highest net power output. The results were benchmarked against single-stage subcritical Rankine cycles. The results showed that the cycle yielding the highest net power varied depending on the allowable total heat exchanger area. When the total heat exchanger area was large, the dual-stage and transcritical cycles performed better than the benchmark, and the improvement became larger as total area increased. For the total heat exchanger areas investigated for the gaseous heat sources, the maximum performance increase was 3 %. Below a certain total area threshold, the benchmark performed the best. For the liquid heat source, the transcritical and dual-stage cycles increased performance by 10 % and 5 % compared to the benchmark, respectively. This larger performance increase is primarily owed to an improved heat transfer in the heat recovery heat exchanger. In fact, using a liquid heat source could improve the net power production by up to 75 % for the same total heat exchanger area compared to when using a gas heat source. Dual-stage and transcritical Rankine cycles seem to offer only a slight improvement over the benchmark for air-equivalent gas heat sources, but only for large total heat exchanger areas. Performance improvement is higher for liquid heat sources. The results indicate that the performance increase for dual-stage and transcritical Rankine cycles compared to the subcritical benchmark depends on the total heat exchanger area and heat source composition.