Supercritical CO2 cycles (sCO2) are recognized as a promising solution for the exploitation of different energy sources: from fossil fuel combustion and nuclear energy to solar energy and waste heat recovery. The large range of possible applications and the possible scarcity of water for sCO2 systems makes the use of direct air-cooled heat rejection units (HRU) of great interest.
This paper deals with the numerical modelling, supported by experimental data provided by an HRU manufacturer, of a sCO2 system, equipped with a direct air-cooled HRU, exploiting a stream of 53 kg/s of flue gases at 550°C. The selected sCO2 cycle is a simple recuperative cycle, whose maximum/minimum pressure and maximum
temperature are optimized in design conditions obtaining an overall recovery efficiency of 21.71%. The analysis of the system in off-design conditions is carried out for different ambient temperatures and the optimal HRU operational strategy is investigated considering the possibility of varying the HRU fan rotational speed with no variation of the working fluid inventory. Overall results are presented as the trend of minimum cycle pressure and minimum cycle temperature required to maximize the system power output at every considered ambient temperature. It is found that the plant performance is strongly penalized by high ambient temperatures for two main reasons: (i) the sharp increase of the compressor/turbine power ratio because of the loss of real gas effects in compression and (ii) the severe penalization of the compressor efficiency, which moves its operating point towards regions characterized by poor performance. For these reasons, more effective HRU solutions should be adopted in locations characterized by relatively high seasonal temperatures
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Reference: Alfani, Dario & Astolfi, Marco & Binotti, Marco & Romano, Matteo & Macchi, Ennio & Filippini, Stefano & Merlo, Umberto. (2019). SIZING CRITERIA AND PERFORMANCE EVALUATION OF DIRECT AIR COOLED HEAT REJECTION UNITS FOR SUPERCRITICAL CO2 POWER PLANTS.