Design and analysis of a natural draft direct dry cooling system for a supercritical carbon dioxide power cycle for CSP application
Natural draft direct dry cooling systems (NDDDCSs) offer a modern alternative to conventional dry cooling systems such as mechanical draft direct air coolers or natural draft indirect dry cooling systems. This technology combines the benefits of both, promising reduced auxiliary power consumption, decreased system complexity and better efficiency from direct cooling. In this work, such an NDDDCS with vertically arranged heat exchangers around the tower circumference is designed and evaluated for a supercritical carbon dioxide (sCO2) power cycle for a concentrated solar power application. The system is configured to house both the precooler (PC) and intercooler (IC) heat exchangers within the same tower. Three numerical models are developed to simulate NDDDCS performance, each with an increasing order of fidelity aimed at addressing the limitations of the preceding approach. Initially a lumped parameter model is used to explore a larger design space, intended to obtain a best-performing system geometry by minimising the total system material cost and lost work. The results indicate that cost-effective NDDDCSs for sCO2 applications favour smaller cooling towers with taller heat exchangers. Further, employing complex multi-pass counterflow heat exchanger geometries allow for enhanced heat rejection capacities along with reduced cooling tower footprints. The second model extends the analysis to a 1-D discretised heat exchanger network to capture parameter distributions resulting from the highly variable property gradients of sCO2 near the critical point. These results confirm that lumped parameter models overpredict performance as property gradients are not accounted for accurately. The 1-D model shows a 4.66 % reduction in overall heat rejection at design point, split between decreases of 2.35 % at the PC and 7.35 % at the IC. The discretised 1-D model also investigates the effects of off-design ambient temperatures and approximated crosswinds on system performance characteristics. Finally, a co-simulation model is developed, integrating the 1-D discretised sCO2-side heat exchanger model with a 3-D computational fluid dynamics (CFD) air-side model. Compared to the preceding models, the co-simulation model captures additional complexities such as recirculation, temperature maldistribution and flow separation. Under no wind conditions, co-simulation results indicate reductions of 13.85 % in heat rejection and 6.25 % in air mass flow rate, compared to the predictions of the stand-alone 1-D model. The co-simulation model is also employed to evaluate the effect of grouped or alternating PC and IC heat exchanger tube bank configurations. Both windless and windy conditions are investigated, with results showing a 1.02 % increase in heat rejection for the alternating configuration during no wind conditions. For windy conditions, the results reveal significant performance deterioration due to flow separation and temperature recirculation, hence external and internal wind walls are recommended to mitigate against the negative effects of wind. The results also indicate the sensitivity of the system performance to heat exchanger location relative to the crosswind direction. The potential exists to position the most sensitive heat exchangers in the prevailing crosswind direction to ensure that cycle performance losses can be minimised.
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