A methodology to design air-cooled condensers for supercritical power cycles using carbon dioxide and carbon dioxide mixtures

Abstract

The SCARABEUS project investigates the use of CO₂–based mixtures as working fluid in power cycles for nextgeneration Concentrated Solar Power plants. These fluids exhibit a critical temperature higher than pure CO₂, enabling dry condensation of the working fluid even at the high ambient temperatures typical of sites with a high solar radiation. As a consequence, the SCARABEUS power cycle achieves higher thermal efficiency than standard sCO₂ cycles, whose performance deteriorates significantly with ambient temperature. In any case, the actual feasibility of this concept is still to be confirmed by a complete techno-economic assessment. To that purpose, it is critical to accurately estimate the power consumption of the Heat Rejection Unit (HRU), which is one of the most important parasitic loads of the system.

Bearing all this in mind, this manuscript presents the design of a horizontal, direct air-cooled condenser (ACC). The bundle geometry proposed is comprised of seven tubes in three passes, with a staggered arrangement. The complete thermal model, developed in MatLab, has been already disclosed by the SCARABEUS consortium in a previous paper, and validated both experimentally in a dedicated test rig and against results obtained by the commercial software Xace®. The novelty in the present manuscript lies in the integration of this thermal model of the tubes with a complete design and integration tool of the whole heat rejection sub-system, including the design of a rotoronly axial fan and supporting frame. The impact of several design parameters (i.e., air temperature rise, acceptable hot pressure drops, tube length) is studied, taking into account auxiliary power consumption, footprint and cycle efficiency as main figures of merit. Two candidate mixtures are taken into account, identified in previous works by the same authors (85%CO₂-15%C6F6 and 80%CO₂-20%SO₂), and a pure sCO₂ case is also considered for the sake of comparison. The results show that, for a given gross cycle output, using pure sCO₂ yields the smallest ACC with the lowest fan power consumption. Moreover, tube length and air face velocity are found to be the key-parameters driving the design process of an ACC, for which increasing tube length is always beneficial as far as the ACC design is concerned. Finally, various considerations regarding the role played by the optimum design of the ACC within the global optimisation of the power plant are made. It is found that the rationale employed for the design of the ACC may be in conflict with that used from an overall plant optimisation standpoint. It is hence concluded that the definition of the optimal design space of an Air-cooled Heat Exchanger (ACHE) must be included in the global optimisation of the power plant.