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14:00   Parallel Session: Small-capacity systems
Chair: Joost Brasz
20 mins
Antti Uusitalo, Juha Honkatukia, Teemu Turunen-Saaresti, Jaakko Larjola, Piero Colonna
Abstract: ABSTRACT Most of the Organic Rankine Cycle (ORC) power systems currently on the market features an electrical power output in the range 100 kWe – 5 MWe. However, there are many energy conversion applications where an ORC energy converter with an electrical power capacity of ca. 10 kWe is attractive. Examples are heat recovery from prime movers, concentrated solar power, small-scale cogeneration of heat and power (e.g. for domestic use). In all these applications the thermal source is at moderate or high temperature. One of the key issues in designing a small-scale ORC turbogenerator is the selection of a suitable working fluid. Siloxanes have been successfully adopted in high-temperature ORC power plants for larger power capacities (400 kWe – 2 MWe). The main focus of this study is the evaluation of eight siloxanes as working fluid for a small-capacity ORC turbogenerator based on a high-speed technology. High-speed technology refers to the concept of a compact hermetic component containing the turbine, the generator, and the feed pump coupled to the same shaft, rotating at high speed (typically more than 20 000 rpm) and using the working fluid in the liquid phase to lubricate the shaft bearings. The siloxanes considered in the study are D4, D5, D6, MM, MDM, MD2M, MD3M, and MD4M. Toluene is included in the analysis as a reference working fluid. The effects of adopting different siloxanes on the thermodynamic cycle configuration, conversion efficiency, and on the turbine and component design are studied by means of computations. The working fluid parameters which are most influential are the critical temperature and pressure, and the molecular complexity, and, related to them, the condensation pressure, density and specific enthalpy over the expansion which affects the optimal design of the turbine. The fluid thermal stability is also extremely relevant in the considered applications. The results of this study provide valuable information for the design of efficient ORC systems in the tens-of-kW power range utilizing siloxanes as working fluids. Further research will be centered on technological issues, such as material requirements, process component design, as well as safety and reliability issues. REFERENCES [1] J.P. van Buijtenen, J. Larjola, T. Turunen-Saaresti, J. Honkatukia, H. Esa, J. Backman and A. Reunanen: “Design and validation of a new high expansion ratio radial turbine for ORC application” 5th European conference on Turbomachinery, Prague, March 17-22, 2003 [2] P. Colonna, N. R. Nannan, A. Guardone, “Multiparameter equations of state for siloxanes: [(CH3)3-Si-O1/2]2-[O-Si-(CH3)2]i=1,...,3, and [O-Si-(CH3)2]6”, Fluid Phase Equilibria vol. 263, no. 2, pp. 115–130, 2008.
20 mins
Wolfgang Lang, Piero Colonna, Raimund Almbauer
Abstract: This presentation documents recent work performed within a cooperative project funded by the Austrian funding agency involving TU Graz, TU Delft and two European OEMs targeted at investigating options for converting the thermal power discharged by automotive engines via flue gases and the cooling system into mechanical/electrical power. The efficiency of reciprocating engines of cars and trucks has arguably reached its maximum limit and only marginal gains can be obtained by improving on currently adopted technologies. These engines discharge to the environment approximately 66% of the fuel energy content as thermal energy. The energy is available at different temperature levels depending on the type of engine: in car engines the temperatures range from 300 °C to 900 °C for the exhaust gases, and from 90 °C to 110 °C for the cooling system; in truck engines the two heat sources with the highest potential are the exhaust gases with temperatures ranging from 200 °C to 400 °C but thermal energy is available at even higher temperatures (280 °C to 580 °C), if also the heat from the exhaust gas recirculation (EGR) system is recovered. It is apparent that there is still a large fraction of the primary energy that is still untapped and the potential overall energy efficiency gain offered by effectively recovering wasted thermal power is very large [1]. The principle is already widely exploited in stationary power plants, while application on board of vehicles is very challenging and no commercial application exists. The current energy scenario has resumed strong interest into automotive heat recovery systems, much like it happened in the 70’s as a consequence of the first oil crisis [2]. This work is focused on one of the possible solutions to the technical problem of heat recovery for car and truck engines, namely a compact ORC turbogenerator using a siloxane as the working fluid. Two paradigmatic examples of operating conditions taken from existing automotive propellers are considered, one for a truck engine and one for a car engine. The design envelope is explored in terms of working fluid selection, thermodynamic cycle configuration, preliminary turbine and heat exchangers design, taking into consideration all the stringent requirements imposed by the automotive application. A Rankine power system using water as the working fluid is taken as a benchmark and the challenges related to adopting water as the working fluid are discussed. The results of simulations are analysed in order to provide initial guidelines and the most promising routes to successful implementation are outlined.
20 mins
Emiliano Casati, Matteo Pini, Giacomo Persico, Andrea Spinelli, Vincenzo Dossena
Abstract: Nowadays there is a general agreement that a substantial increase in the share of energy produced in a decentralized way is a desirable perspective for future sustainable societies. As a consequence, small-scale power plants are candidates to play a relevant role in the future distributed energy scenario [1]. Among the technologies that are suitable for high-efficiency conversion of thermal power into electricity in the small to medium power range, Organic Rankine Cycle turbogenerators stand out in terms of reliability and cost-effectiveness. It is well known that, as the size of a power plant reduces, the cost of the conversion engine (typically a turbo-expander) represents an increasing part of the total investment. The use of organic compounds as working fluids leads to relatively simple plant configurations and to design compact and reliable turbines. In common ORC applications the turbine is inherently characterized by a low enthalpy drop that is usually disposed in a low number of stages (even a single stage for radial turbines, or a few stages for axial turbines). These two peculiar aspects result in a very high stage expansion ratios, that, combined with the typical low speed of sound of organic fluids, induces strong supersonic phenomena. As a consequence the turbine maximum efficiencies are in the range of 80-85 % for the largest units [2]. The development of turbines with better performances in terms of efficiency and controllability is therefore a key-theme in the field of ORC: aim of the present work is the critical evaluation of a Ljungstrom-like multi-stage centrifugal turbine whose architecture could have substantial advantages over traditional solutions [3]. Due to the absence of experience and specific literature, a wide-spectrum design procedure has been conceived. A lumped parameter code has been initially developed, on the basis of initial project assumptions, machine design criteria, such as the Mach number at outlet section, and available models for losses evaluation. The results of the 0-D code are then verified with the new throughflow solver of zFlow, a quasi 3-D CFD-based code for turbomachinery applications coupled with the FluidProp package for accurate properties calculation [4]. The calculation method is proven to be a valuable tool to efficiently select a small number of preliminary machine’s designs for the proposed architecture as well as for axial machines, where consolidated experience and experimental data are not available. Furthermore this represent an effective way to determine a baseline configuration for subsequent more detailed optimizations. The overall procedure is finally applied to a 100 kW multi-stage turbine using the siloxane D4 as working fluid. The results are extensively discussed by a comparison with available data from existing machines. REFERENCES [1] U.S. Dept. of Energy, “The Potential Benefits of Distributed Generation and Rate-Related Issues That May Impede its Expansion”, Sect. 1817 - Energy Policy Act 2005. [2] E. Macchi, “Design criteria for turbines operating with fluids having a low speed of sound”, lecture series n. 100 on closed-cycle gas turbines, Von Karman Inst. For Fluid Dynamics,1977. [3] E.p. Coomes, D.G.Wilson et al., “Design of a high-power-density Ljungstrom turbine using potassium as a working fluid”, Proc. Intersociety Energy Conv. Eng. - 1986. [4] G. Persico, S. Rebay, C. Osnaghi, “A novel package for turbomachinery throughflow analysis”, European Turbomachinery Conference 2011
20 mins
Malick Kane
Abstract: District heating distributes heat and hot water to residential, commercial and public buildings over a large area. In many cities in Europe, the heat distribution system is designed to delivers hot water at a different temperature to meet the different thermal requirements of the customers served. A high-temperature loop delivers water at around 160-200°C and A-low temperature loop at around 90-120°C. For a particular need of steam for example in hospital, the high-temperature (HT) water is flashed into steam at the customer's site by heat exchangers and the low-pressure steam is then distributed. In many cases, there are many others end-use consumers which are connected to the HT heating system to only satisfy the need for low temperature heating and hot water (e.g. below 55°C). For example, for the typical situation of the Mon-Repos Swimming pool of Lausanne, Switzerland, the HT heating system (at around 170°C) operated by the “Services Industriels de Lausanne – SIL”, the Heat & Power utility company, is used for generating low temperature hot water and heating the pool at around 26°C. In this context, the use of ®ENEFCOGENGREEN provided by Eneftech Innovation SA of Nyon, Switzerland and based on ORC micro cogeneration system (producing electricity locally from the heating network at the customer’s site while serving their needs for low temperature heating) can generate substantial benefits and savings. A modular 15KWe power output is installed at the swimming pool in Lausanne, running 7500 hours a year and saving approx 23,000CHF of electricity grid previously purchased from the grid.