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Session: Parallel Session: Heat exchangers
Session starts: Friday 23 September, 16:00
Presentation starts: 16:20
Room: Auditorium

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Stefano Filippini (LU-VE Contardo)
Umberto Merlo (LU-VE Contardo)
Matteo Romano (Politecnico di Milano)
Giovanni Lozza (Politecnico di Milano)

Abstract:
Heat rejection is a crucial issue in the design and operation of an Organic Rankine cycle, since more than 75% of inlet thermal power must be typically released to the ambient. Dry air condensers are often the preferred option, since no water is required for their operations. For this reason, they are selected for those ORC plants which operate in areas where large amounts of cooling water are not available. One limit of dry condensers is their performance decay when ambient temperature increases above the design one, especially in locations where relevant daily and seasonal temperature variations occur. The effect is a reduced net power output of the ORC plant during hot hours, either due to the enhanced condensing temperature (and the consequently lower turbine expansion ratio) or the increased fans consumption (when regulated to keep a condensing pressure closer to the design one). In order to reduce the negative effects of high ambient temperatures, LU-VE Contardo has developed the DRY and SPRAY® series, where condenser can work both in conventional “dry operation” with dry fins, or in “wet operation” mode with water sprayed onto the fins. The evaporation of the water dramatically increases the capacity of the unit, allowing to increase the net power production. It must be highlighted that most of the water sprayed onto the fins evaporates off. This means that it is not necessary to fit a drain tray beneath the unit to collect and recycle the sprayed water, excluding the possibility of any build-up of impurities in the water and the proliferation of legionella. The ambient transition temperature from dry to spray operation is a design option which must be optimized on the basis of technical and economical considerations, taking into account the cost of the water, the price of the electricity in each condition and the maximum operating hours limit under spray operation. The aim of the present work is the assessment of the potential of a water-sprayed condenser in ORC applications. Starting from the project of an ORC condenser for geothermal application operating with R134a, the benefits of the water-spray solution are discussed. The increased thermodynamic performance of the ORC cycle at different ambient conditions are calculated by means of an accurate numerical model, using thermodynamic properties from Refprop® code. The economic benefits at different ambient temperatures for different water cost and electricity price, which are very site dependent, are also calculated to provide a rather complete picture of the potential of this innovative option for heat rejection.