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11:10   New applications: Solar
Chair: Dr. Christos Markides
20 mins
Piero Colonna, Sebastian Bahamonde
Abstract: The presentation illustrates the plan of a project recently started at TU Delft, in partnership with a multinational company, and aimed at developing validated methodologies for the design of mini Organic Rankine Cycle (ORC) turbogenerators. The solar-powered total energy system of so-called green buildings is the envisaged application. De-localized generation of utilities for commercial and residential buildings (electricity, heating and cooling) from solar energy can have an enormous impact on the path to a renewable energy future. Constraints on cost, efficiency, energy storage, reliability, materials, scalability and complexity have so-far prevented widespread adoption of technologies for solar energy conversion, with the partial exception of photo-voltaic panels. In principle a small CSP plant based on the ORC concept features unique advantages: it can efficiently generate electricity for local utilization, and in addition it allows for thermal energy storage, together with cogeneration of heating and cooling (by integrating an absorption chiller). This total energy system boasts an extremely high electric efficiency (up to 20\%), unparalleled total conversion efficiency (up to 95\%), and a high utilization factor throughout the year, which would put it well above competing technologies. Miniaturization of ORC technology involves a number of scientific challenges. In addition, design for volume production requires a paradigmatic change in the approach and in the base technology, starting from the adoption of a new working fluid. Studies of the TU Delft group highlighted possible solutions to the problem of realizing a small ORC turbogenerator for this application. A new turbine architecture would benefit from a new cycle configuration integrating thermal storage. Siloxanes, new perfluorocarbons and their mixtures can be used as working fluids, a key optimization parameter of the system. In this project, the group wants to further study and experiment on a new type of expander, the centrifugal turbine. Preliminary assessments indicate that it is particularly suitable for mini-ORC turbogenerators for solar applications, because it can handle the highly supersonic flow and ultra-high expansion ratio. In addition the group wants to investigate new working fluids that can also be used for thermal storage, if the recently proposed flash-cycle configuration is adopted. System studies, together with simulations on the envisaged turbine will lead to the design and implementation of a test bench that can be used to experiment with mini-turbines. We will use and further develop proprietary software for system simulations, CFD design of turbomachinery, and fluid thermophysical property estimation. One of the main deliverables will therefore be a proof-of-concept turbine prototype and all the characterizing measurements.
20 mins
George Kosmadakis, Dimitris Manolakos, Kostas Bouzianas, George Papadakis
Abstract: In low-concentrating PV/Thermal units the heat produced is of low-temperature and should be effectively removed. The motivation of the present work is to recover this low-grade heat and then feed it to a supercritical organic Rankine cycle (SCORC) for additional electricity production. Such bottoming engine can operate with acceptable efficiency at temperature of around 80-90 oC, showing an efficiency higher than around 10% in comparison to a similar subcritical cycle, due to the better thermal match and specific thermodynamic properties. The key parameter of such system design is the temperature of the heat transfer fluid in the CPV/T circuit, transferring heat from the solar collectors to the SCORC unit, through the supercritical heat exchanger. This temperature has a major effect on the performance of both the PV cells and the SCORC engine. For higher temperature, the PV cells’ efficiency decreases linearly, according to their temperature coefficient, while the SCORC efficiency is radically improved [1]. This process can be optimized, resulting to a total higher electric efficiency, and a significant higher productivity. The system includes a CPV/T field of 10 kWp producing 41 kWth, feeding the SCORC engine, which shows a thermal efficiency of 6-7%, and producing almost 3 kW. The total maximum power production is 10 kW, making such system appropriate for installation on buildings and for decentralised applications. The combined system is currently designed through extensive simulation and optimization studies. The so far calculated results are very promising, showing that the proposed system can be competitive to both PV and CPV systems in terms of productivity and specific cost (€/kWh), while the supercritical cycle shows some interesting features, as already mentioned. This integrated system is to be constructed and tested under real conditions by the end of 2013, and then evaluated. Acknowledgement: The research leading to these results has received funding from the European Union's Seventh Framework Programme managed by REA-Research Executive Agency, http://ec.europa.eu/research/rea ([FP7/2007-2013] [FP7/2007-2011]) under grant agreement n° 315049 [CPV/RANKINE], FP7-SME-2012. REFERENCES [1] G. Kosmadakis, D. Manolakos and G. Papadakis, “Simulation and economic analysis of a CPV/thermal system coupled with an organic Rankine cycle for increased power generation”, Solar Energy, Vol. 85, pp. 308–24, (2011).
20 mins
Sotirios Karellas, Konstantinos Braimakis
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. [...]