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16:00   Parallel Session: Heat exchangers
Chair: Jos van Buijtenen
20 mins
Dariusz Mikielewicz, Jaroslaw Mikielewicz, Jan Wajs, Tomasz Muszynski, Eugeniusz Ihnatowicz
Abstract: In the paper the idea of a novel microjet heat exchanger is shown together with the flow and thermal experimental results of the prototype. Two flow cases are studied, namely water-water and air-air. In case of measurements of such complex geometries recording of wall temperatures is impossible and hence determination of heat transfer coefficient difficult. Therefore the Wilson technique was used for determination of the heat transfer coefficient. The results of accomplished measurements are satisfactory with the view of developing effective heat exchangers.
20 mins
Stefano Filippini, Umberto Merlo, Matteo Romano, Giovanni Lozza
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.
20 mins
Carsten Trapp, Piero Colonna
Abstract: In the transition period leading to electricity generation fully based on renewable energy resources, fossil fuel power plants need to be equipped with CO2 removal units. An ongoing research project involving Nuon-Vattenfall, TU Delft, and ECN, is aimed at the development of pre-combustion CO2 removal technology from an integrated coal gasification combined cycle (IGCC) plant in the Netherlands [1]. A fully instrumented pilot plant of the CO2 capture unit has been built at the Buggenum IGCC power station and is being commissioned for further studies and experiments. The removal of CO2 from the syngas is very energy demanding and therefore technical solutions to reduce the efficiency penalty in thermal power plants must be investigated. This study concerns the feasibility of recovering low-grade thermal energy from the CO2-capture process by means of an Organic Rankine Cycle power plant. In comparison to geothermal applications, which are widely documented in the literature [2,3], the heat source in this case is a syngas water mixture which is cooled from a temperature of 130-140 °C and condenses due to the heat transfer to the ORC evaporator. First, the application of commercially available ORC units is explored by means of steady-state simulations. The plant composed of commercially available ORC units is simulated and taken as a benchmark for a tailor-made ORC power plant. The working fluid has an important influence on system performance and therefore the effect of selecting a fluid from the hydrocarbons and refrigerants families are investigated, targeting the maximum net power output. In addition to pure fluids, also two component mixtures are investigated in a subcritical ORC cycle configuration. The use of mixtures as working fluids in ORC systems allows for a better match of the temperature profiles in the evaporator and the condenser [4], due to the temperature glide associated with phase-transition, leading to lower irreversibilities within the heat exchanging equipment. In order to further improve the thermal coupling of the cooling heat source to the heating of the working fluid, a supercritical ORC configuration is also studied. The three ORC configurations (commercial ORC, customized subcritical & supercritical ORC) are analyzed in terms of net power output, second law efficiency and component based exergy destruction.