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ON AIR-COOLED CONDENSERS FOR ORC SYSTEMS OPERATING WITH ZEOTROPIC MIXTURESGalieti, Lorenzo; De Servi, Carlo; Alfani, Dario; Silva, Paolo; Bombarda, Paola; Colonna, PieroThe 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.- POTENTIAL OF TRIGENERATIVE WASTE HEAT RECOVERY CO2- MIXTURE TRANSCRITICAL POWER PLANTS FOR INCREASING THE SUSTAINABILITY OF DISTRICT HEATING AND COOLING NETWORKSBaiguini, Mattia; Doninelli, Michele; Morosini, Ettore; Alfani, Dario; Di Marcoberardino, Gioele; Giulio Iora, Paolo; Manzolini, Giampaolo; Invernizzi, Costante Mario; Astolfi, Marco
- Pareto front analysis for the design and the working fluid selection in ORC-based pumped thermal energy storage technology in both pure electric and cogenerative applicationsAstolfi, Marco; Alfani, Dario; Giostri, AndreaCarnot Batteries are a sub-technology of Pumped Thermal Energy Storage concept where closed cycles are used in both charging and discharging phase. In charging mode, cheap off-peak electricity from the grid is used to store heat at a temperature different than the ambient one (generally higher) while in discharging mode the stored heat is exploited for power production. Carnot Batteries can be designed with different thermodynamic cycles (Brayton cycle, Steam Rankine cycle, Organic Rankine cycle) and can adopt different types of thermal energy storage technologies. For low duration storage (daily or weekly cycling) sensible or latent energy storage with phase change materials can be adopted, while for long duration storage (months, seasonal) the use of thermochemical reactions could be a valid option, as proposed by H2020 RESTORE project. Performance of Carnot Batteries is highly affected by the thermal storage temperature, the condensation temperature in discharging mode and the availability of residual heat at a temperature higher than the ambient one for boosting the heat pump coefficient of performance in charging cycle. This paper focuses on the optimal design of a reversible thermodynamic system working as a heat pump (HP) cycle in charging mode and as an organic Rankine cycle (ORC) in discharging mode. A dedicated numerical model developed in Python is employed to compare the techno-economic performances of different working fluids and select the most promising candidates. Main difficulty is related to the use of reversible heat exchangers which design impacts both on the charging and discharging operation, strongly affecting the system round trip efficiency, and requiring a discretized off-design modelling. A Pareto front of optimal round trip efficiency against total area of heat exchangers is obtained by varying the main design parameters, such as heat transfer temperature differences and heat exchangers pressure drops. Results show that for a given overall heat transfer area low critical temperature fluid are mainly penalized by poor HP performance due to high compressor specific work, while high critical temperature fluids are mostly penalized by high pressure drops caused by the low fluid density. The most eaching a RTE close to 35% for the pure electric configuration at its maximum RTE over total heat transfer area parameter
- Investigating the application range of ORC power plants for the exploitation of two-phase geothermal resourcesMerbecks, Tristan; Pietra, Claudio; Bombarda Martin, Paola; Saar, O.; Silva, Paolo; Alfani, DarioThe design of a geothermal power plant exploiting a two-phase geothermal resource is a demanding task, due to the fact that different technologies and plant configurations must be considered, as well as the lack of flexible and reliable models for computing the thermophysical properties of geofluids, whose composition and chemistry is highly site-specific. This study presents a novel thermophysical property modelling framework aimed at modelling any arbitrary single- or two-phase geothermal brine. Recognising the potential synergy of two historically separate fields of research, fluid partition and property modelling, existing partitioning simulators, like Reaktoro, are coupled with high-accuracy thermophysical fluid property computation engines, like CoolProp and ThermoFun. The resultant Python-based framework, GeoProp, is validated against a range of primary and secondary data and will be made available to the public. Considering a two-phase geothermal resource, the design of direct steam cycles with flash as well as binary geothermal power plants is investigated from a thermodynamic perspective. A parametric study on geofluid composition, i.e. mineral and non-condensable gases (NCG) content, and inlet conditions, i.e. temperature and steam quality, is used to compare the performance and define the application envelope of these two technologies.
