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09:00   Parallel Session: Systems Design, Optimization and Applications II
Chair: Francesco Casella
20 mins
Claudio Spadacini, Lorenzo Centemeri, Luca Giancarlo Xodo, Marco Astolfi, Matteo Carmelo Romano, Ennio Macchi
Abstract: In the large spectrum of organic fluids suitable for Rankine cycles, a fluid that is already well-known and available on industrial scale but currently excluded from this kind of application has been selected. This choice is due to the remarkable characteristics of the fluid, such as its high molecular weight, good thermal stability, non-flammability, and atoxicity. Compared to those fluids nowadays common in the ORC market, its thermodynamic properties and fluid dynamic behavior lead to a peculiar configuration of the cycle: • Supercritical cycle, when heat input is at medium-high temperature; • Massive regeneration, to obtain higher efficiency; • Low specific work of the turbine; • Relatively high volumetric expansion ratio and relatively low absolute inlet volumetric flow; Accordingly, an innovative cycle design has been developed, including a once-through Hairpin primary heat exchanger and a multi-stage radial outflow expander. This last innovative component has been designed to get the best performance with the chosen fluid: • The high inlet/outlet volumetric flow ratio is well combined with the change in cross section across the radius; • Compared to an axial turbine, the lower inlet volumetric flow is compensated by higher blades at the first stage. It is feasible thanks to the change in section available along the radius, so that there is no need for partial admission; • The prismatic blade leads to constant velocity diagrams across the blade span; • It minimizes tip leakages and disk friction losses, due to the single disk / multi-stage configuration; • The intrinsical limit of a radial outflow expander to develop high enthalpy drop is not relevant for this cycle, presenting itself a very low enthalpy drop. Moreover the tip speed is limited by the low speed of sound and consequently this kind of expander suits well with this cycle arrangement. The results of this study, conducted through thermodynamic simulations, CFD, stress analysis and economic optimization show an ORC system that reaches high efficiencies, comparable to those typical of existing systems.
20 mins
George Kosmadakis, Dimitris Manolakos, George Papadakis
Abstract: A small-scale, low-temperature Organic Rankine Cycle (ORC) is developed at the moment, within the framework of the project 09SYN-32-982, partly financed by the greek government. The heat input will be provided from evacuated tube solar collectors (maximum heat input: 100 kWth at around 130 °C), while the power produced will feed a Reverse Osmosis unit for desalinating seawater. Due to the seasonal, and mainly daily, solar radiation variation, critical issues should be addressed, in order to improve the performance of the system. Some important issues involve the control of the pump and the expander, and the fundamental design principles of the ORC (e.g. heat exchanger sizing, evaporation temperature etc.). One design principle that has not been extensively investigated so far is the number of expansion stages of the organic fluid, where two major configurations are identified for small-scale ORCs (except for the single-expansion). The first configuration concerns the use of a cascade ORC (two circuits), where the condensation heat of the upper-stage (with higher temperature) is actually the evaporation heat of the lower-stage [1]. The second configuration consists of a single circuit, with two expanders connected in-series, where the first expander is by-passed at low heat input [2]. Both these configurations can improve the performance of the system at part load in comparison to a single-expansion ORC, if they are properly controlled. The first goal of this study is to theoretically investigate the performance of these two alternative configurations under intermittent heat input (and feed temperature). For both configurations the same parameters have been used (e.g. organic fluid’s subcooling/superheating, expanders’ efficiency etc.) and the organic fluids have been carefully selected (HFC-245fa/HFC-134a for the upper/lower-stage respectively [3] and HFC-245fa for the two in-series expanders ORC). The thermal efficiency is calculated for each configuration and for the whole range of heat input (0-100%). An interesting observation is the increased efficiency of the ORC with two in-series expanders for the entire range of heat supply. Especially at low heat input, the efficiency gain is significant (4% instead of 2%), while it is limited at the maximum heat input (10.4% instead of 9.8%). Additionally, in the cascade ORC design the installation cost increases, due to the use of additional components and the higher complexity of the control unit of the system. The final design chosen is the one with two in-series expanders. Next steps will be the development of a smart control logic, the design of the system and at a later stage the construction of a prototype unit. REFERENCES [1] M. Kane, D. Larrain, D. Favrat and Y. Allani, “Small hybrid solar power system”, Energy, Vol. 28, pp. 14271442, (2003). [2] S. Quoilin, M. Orosz, H. Hemond and V. Lemort, “Performance and design optimization of a low-cost solar organic Rankine cycle for remote power generation”, Solar Energy, Vol. 85, pp. 955–966, (2011). [3] G. Kosmadakis, D. Manolakos, S. Kyritsis and G. Papadakis, “Design of an autonomous, two stage solar organic Rankine cycle system for reverse osmosis desalination”, Desalin Water Treat, Vol. 1, pp. 114–127, (2009).
20 mins
Carlo De Servi, Alessio Tizzanini, Roberto Bini, Claudio Pietra, Stefano Campanari
Abstract: Fuel cells (FC) are a promising technology for distributed electricity production, especially for power applications in the few hundred kW to 10 MW size range, but they have not yet achieved significant penetration into energy market, mainly due to their high specific costs compared to other conventional technologies. They can be applied to combined heat and power, recovering heat dissipated by stack exhaust gases, when the power plant can be installed in presence of a heat demand, and they can use natural gas as primary fuel as well as biogas (for instance from wastewater treatment) or fuel blends. A possible way to improve a FC power plant economics consists in enhancing the electrical efficiency of the overall system as much as possible, exploiting the waste heat to generate additional electricity by means of an Organic Rankine Cycle (ORC) used as a heat recovery bottom cycle [1]. In this paper, the potential benefits of the integration between a fuel cell (topping cycle) and an ORC (bottoming cycle) are assessed in relation to a specific case study, related to a fuel cell unit which is in an early commercialization stage: the molten carbonate fuel cell (MCFC) unit recently proposed by Fuel Cell Energy [2]. This kind of fuel cell has been selected due to its well established performances, its increasingly competitive cost of electricity and its availability on the market. On the other hand, the relatively low temperature of the exhaust heat generated by the fuel cells (the exhaust gases are released from the MCFC at about 370°C) is particularly suitable for recovery through a bottoming cycle based on the ORC technology. ORC are nowadays more and more applied in many fields, for example in the exploitation of low entalphy geothermal sources [3] or waste heat from biomass. In this study, to enhance the performances of the fuel cell+ORC combined cycle, both subcritical and supercritical technology for the ORC are considered and optimized for the chosen working fluid. Thanks to the modular features of the fuel cell system, it is possible to analyze two different power sizes, with the ORC resulting at about 500 kWel and 1 MWel power output, evidencing scale effects on the integration. Simulation of the integrated plant of the bottoming cycle are performed in Aspen Plus® environment and optimized by means of a Matlab® code. Results show that recovering waste heat from a MCFC unit could increase the electrical power and efficiency of the plant by more than 10%, well exceeding a 50% overall electrical efficiency. A preliminary economic analysis investigates the feasibility of the proposed solution, showing a reduction of about 8% of the levelized cost of electricity (LCOE) of the MCFC plant. On the other hand, an environmental comparison shows the extremely low pollutant emissions which could be achieved by this power plant with respect to competitive conventional technologies.
20 mins
Ignace Vankeirsbilck, Bruno Vanslambrouck, Sergei Gusev, Michel De Paepe
Abstract: To generate electricity from biomass combustion heat, geothermal wells, recovered waste heat from internal combustion engines, gas turbines or industrial processes, both the steam cycle and the Organic Rankine Cycle (ORC) are widely in use. Both technologies are well established and can be found on comparable applications. This paper presents a thermodynamic analysis and a comparative study of the cycle efficiency for a simplified steam cycle versus an ORC. The most commonly used organic fluids have been considered : R245fa, Toluene, (cyclo)-pentane, Solkatherm and 2 silicone-oils (MM and MDM). Working fluid selection and its application area is being discussed based on fluid characteristics. The thermal efficiency is mainly determined by the temperature level of the heat source and the condenser conditions. The influence of several process parameters such as turbine inlet and condenser temperature, turbine isentropic efficiency, vapour quality and pressure, use of a regenerator (ORC), is derived from numerous computer simulations. The temperature profile of the heat source is the main restricting factor for the evaporation temperature and pressure. Finally, some general and economic considerations related to the choice between a steam vs. ORC are discussed.
20 mins
Marco Astolfi, Matteo Romano, Paola Bombarda
Abstract: In the last ten years, the increasing attention to pollutants and greenhouse gases emission from the power generation sector and the concerns about fossil fuel supply and price have led to a massive growth of those technologies that can produce electric energy from renewable sources or from waste heat recovery. In this context, the exploitation of heat from a wide variety of sources, like hot geothermal brines, sun and exhaust gases from engines and industrial processes using an Organic Rankine Cycle (ORC) is certainly one of the most promising solution. The basic idea is to exploit low and medium enthalpy energy sources by a Rankine cycle using an organic fluids instead of water as working fluid. This choice is confirmed by several feasibility studies and industrial applications which clearly show that, in a range from 500kW to 5MW, ORC power plants can reach higher efficiencies than common Rankine steam cycles. Moreover, ORC power plants guarantee a compact design of turbines, primarily thanks to the properties of organic fluids, and permit to overcome the drawbacks related to the challenging design and the high specific cost of steam turbines in the considered power range. Fluids normally used in ORC plants cover a large variety of different compounds like siloxanes and perflourated organic molecules, whose thermodynamic properties notably differ from each other in term of critical parameters, maximum allowable temperatures and chemical stability. The wide option in the available working fluids and the various types of cycles that can be adopted entail a non univocal choice for the exploitation of a given heat source. An Excel®-VBA code was created in order to define the most efficient combination of fluid and cycle thermodynamic parameters. Thermodynamic properties of fluids are taken from Refprop® database, which allows to carry out the study with huge number of fluids including most of the commercial refrigerants, hydrocarbons and siloxanes. To achieve the purpose of this study, different heat sources at variable temperature are considered, in order to model the exploitation of different primary energy sources like medium-low enthalpy geothermal brine, solar energy and biomass or waste heat from industrial processes and endothermic engine exhaust gasses. For all the above-mentioned cases, an extensive thermodynamic analysis is carried out by investigating the potential of a number of fluids with different cycle configurations, starting from the basic non-recuperative saturated cycle up to supercritical and two pressure level cycles that allows the achievement of the highest efficiencies. The effects of fluid choice and cycle parameters on the main component design, e.g. heat exchangers surface and turbine size, are also discussed to provide a further term of comparison between the different options. All the plant assumptions for the calculation of the plant components, in particular related to heat exchangers and turbo-machinery, are set on the basis of data from literature, real power plants data sheets and preliminary design.
20 mins
Trond Andresen, Yves Ladam, Petter Nekså
Abstract: Aluminium production generates vast amount of heat. In Norway, smelting plants are installed in remote places were there is no market for the surplus heat as such. Conversion to electricity is an attractive alternative. However, power production from medium to low temperature heat sources is today impeded by high investment cost and poor efficiency. Cycle optimization is critical. One interesting heat source in an aluminium smelter is the pot gas. This is a sensible heat source, in which temperature decrease as heat is recovered. In order to recover more heat, the area of the Heat Recovery Heat Exchanger (HRHE) has to be increased. As the conversion efficiency from heat to power decreases with the heat source temperature, increase of power production will come to the expense of a large increase of HRHE area and therefore will lead to a non linear increase of plant cost. An in-house power cycle simulator based on physical heat exchanger geometry has been implemented. It allows for simultaneous optimization of power cycle parameters (heat uptake pressure and working fluid mass flow) and HRHE geometry, such that for a given maximum area of the HRHE, a maximum net power output is found. External constraints such as maximum cooling of the heat source can be taken into account in the optimization procedure. The case of a typical aluminium smelter was investigated. Typical power production potential and heat exchanger size and optimal layout were obtained. The performance of subcritical R134a cycles and supercritical carbon dioxide cycles are compared.