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Session: New applications: Solar
Session starts: Monday 07 October, 11:10
Presentation starts: 11:50
Room: Willem Burger Zaal

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Sotirios Karellas (National Technical University of Athens)
Konstantinos Braimakis (National Technical University of Athens)

Abstract:
In the recent years, there has been a growing interest in the implementation of the Organic Rankine Cycle (ORC) for energy conversion in low grade heat recovery applications. The most commonly considered energy sources include industrial waste heat streams, solar and geothermal plants, while an additional perspective extensively discussed is the use of the process for Combined Heat and Power (CHP) production from biomass combustion. The utilization of the above-mentioned heat sources has lately become attractive as a promising way to reduce fossil fuel consumption and consequently diminish CO2 emissions. In the current study, a novel micro-scale co-generation and tri-generation system of refrigeration and combined heat and power production based on parallel operation of an ORC and a Vapor Compression Cycle (VCC) is presented. The two cycles are interconnected, both operating with the same organic fluid. The power required for the VCC compressor is provided by the ORC expander, while any surplus power is converted to electricity by a generator connected to the same spindle. A single condenser is used for both cycles, since the condensation of the organic medium takes place under the same pressure, generating the output heat of the system. The ORC also incorporates a regenerative pre-heater (recuperator) in order to make better use of the high energy content of the expander outlet vapor by preheating the sub-cooled organic liquid exiting the pump. The heat input to the ORC originates partly from biomass combustion and partly from solar energy utilization. Two separate intermediate pressurized water circuits are used for the thermal transfer from each heat source to the organic medium. The first circuit is heated by a biomass boiler and the second by parabolic-through solar (PTC) collectors. The compressed working fluid is initially preheated, evaporated and superheated by the biomass boiler circuit and subsequently further superheated by the solar collectors’ circuit. N-S as well as W-E solar collector orientation was considered. The organic mediums reviewed for the process are R134a and R152a for various evaporation pressures, corresponding to subcritical as well as supercritical operating conditions. A key operating parameter of the system is the maximum superheating temperature of each of the two organic fluids by the solar collectors, which was set to 180 oC (R134a) and 210 oC (R152a). The evaporation step of the VCC takes place at 6oC, while the condensation of both cycles (ORC and VCC) is performed at 50oC. Both scenarios of co-generation (heat and power) and tri-generation (heat, cooling and power) are examined, assuming the solar power potential of a typical winter and summer day (December 21st and June 21st) respectively for the case of a Greek island. In the case of the winter operation scenario, the VCC is disconnected from the rest of the system, since no refrigeration is necessary. The analysis of the described installment is carried out from a thermodynamic point of view in three steps. Firstly, given the maximum superheating temperature of each organic medium under a specified cooling load equal to 8 kW (for the summer time scenario) and for a fixed biomass combustion-derived heat input of 50 kW to the ORC, the required solar collector area is estimated for varying values of the recuperator cold stream outlet temperature. The second part of the analysis is conducted considering the solar collector area determined in the previous step, while assuming the highest possible organic fluid recuperator pre-heating temperature and pump discharge pressure for subcritical operation. A sensitivity analysis on system performance and characteristics is carried out for various temperature values of the hot water stream exiting the biomass boiler, which is directly related to biomass fuel consumption. Finally, the dynamic system operation is evaluated under variable solar heat input, corresponding to typical daily winter and summer radiation data. In this part of the analysis, the flue gases produced by the combustion of biomass provide a base, fixed rate heat input to the ORC, while the solar collectors function as an auxiliary heat source of variable intensity during each hour of the day. [...]