ON AIR-COOLED CONDENSERS FOR ORC SYSTEMS OPERATING WITH ZEOTROPIC MIXTURES

Abstract

The use of mixtures as working fluids of ORC systems is being intensively investigated because of the better temperature profile matching achievable in the heat exchangers, resulting in lower thermodynamic irreversibilities and increased efficiency. The gains are expected to be higher for lowtemperature air-cooled power plants, where the ratio between the auxiliary power consumption associated to the cooling of the working fluid and the net power output of the ORC plant is higher. For instance, a temperature glide in the condenser may enable a reduction in the fan consumption at the cost of an increased heat transfer area, and possibly a decrease in the minimum temperature of the thermodynamic cycle. This solution is expected to be attractive for geothermal applications: since the drilling of the geothermal well is by far the dominant cost, the additional investment for the condenser can be more easily compensated by the increased revenues related to the greater electrical power output. This study focuses on the modelling and sizing of an air-cooled condenser for geothermal ORC power plants operating with working fluid binary mixtures. A detailed fin and tube air cooled condenser model is developed and integrated with an in-house tool for the simulation of ORC systems. Working fluid thermodynamic properties are computed with the PCP-SAFT equation of state (EoS). The tool is used to investigate the effect of the condenser design assumptions on the geothermal plant maximum power output for an optimal working fluid mixture, whose composition is determined by optimizing the PCPSAFT parameters. The outcome is a pseudo-fluid mixture that represents the ideal working fluid for the given thermal source. The results indicate that the adoption of mixtures allows the air-cooler consumption and generally the minimum cycle temperature to be decreased, leading to an increased plant efficiency. In addition, design guidelines for the condenser are derived, based on the tradeoff between component size and plant efficiency. Finally, the optimization results show that if the onset of the mixture condensation occurs in the recuperator, it might be possible to reduce the fan consumption and size of the condenser simultaneously, albeit at the expense of an increased complexity of the regenerator design.