11:20
Parallel Session: Systems Design, Optimization and Applications III
Chair: Steven Wright
11:20
20 mins
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DESIGN, SIMULATION AND CONSTRUCTION OF A TEST RIG FOR ORGANIC VAPOURS
Andrea Spinelli, Matteo Pini, Vincenzo Dossena, Paolo Gaetani, Francesco Casella
Abstract: A blow-down wind tunnel for real-gas applications has been designed, validated by means of dynamic simulation and finally constructed. The facility is aimed at characterizing an organic vapour stream, representative of expansions taking place in Organic Rankine Cycles (ORC) turbines, by independent measurements of pressure, temperature and velocity. ORC turbine performances are expected to strongly benefit from characterization of such flows and validation of design tools using experimental data, which still lack in scientific literature.
The flow field investigation within industrial ORC turbine passages has been considered strongly limited by the unavailability of calibration tunnels for real-gas operating probes, by poor plant availabilities and by restricted accesses for instrumentation. As a consequence, the opportunity of building a test rig has been exploited and a dedicated facility has been implemented.
The paper thoroughly discusses the design and the dynamic simulation of the apparatus, presents its final layout and shows the facility “as built”.
A straight-axis planar convergent-divergent nozzle represents the test section for early tests, but the test rig can also accommodate linear blade cascades.
The facility implements a blow down operating scheme, due to high fluid densities and temperatures of operation, which result in an unaffordable thermal power to be provided in case of continuous operation. A wide variety of working fluids can be tested with adjustable operating conditions up to maximum temperature and pressure of 400 °C and 50 bar respectively. Despite the fact that the test rig’s operational mode is unsteady, the inlet nozzle pressure can be kept constant by a control valve.
In order to estimate the duration of both set-up and experiments and to describe the time evolution of the main cycle processes (namely the fluid heating/evaporation, the vapour expansion and the vapour condensation) the dynamic plant operation, including the control system, has been simulated.
Design and simulation have been performed with either a lumped parameter or a 1D approach using siloxane MDM and hydrofluorocarbon R245fa as the reference compounds and by adopting state-of-the-art thermodynamic models of the selected fluids.
The above calculations shown how experiments may last from 12 seconds to several minutes (depending on the fluid and test pressure) while their set-up requires a few hours. These durations have been considered consistent with those required to perform the desired experiments. Moreover, the economic constraints have been met by the technical solution adopted for the plant, allowing the construction of the facility.
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11:40
20 mins
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EXERGOECONOMIC ANALYSIS OF A GEOTHERMAL ORGANIC RANKINE CYCLE WITH ZEOTROPIC FLUID MIXTURES
Florian Heberle, Dieter Brüggemann
Abstract: Optimal operating parameters for geothermal applications of the Organic Rankine Cycle (ORC) are identified under exergoeconomic criteria. For typical geothermal conditions in Germany the use of the zeotropic mixture isobutane/isopentane as a working fluid is evaluated compared to the pure components. Therefore the minimum temperature difference in the evaporator and condenser is varied to figure out the minimal specific costs of electricity generation. In addition, exergetic variables, like second law efficiency and irreversibilities of each component, are calculated. The purchase equipment costs are determined as a function of heat surface area and power of the turbine or pump. The costs of the fuel consist of costs associated with the exploration of the geothermal resource and operating and maintenance costs of the borehole pump.
The results of the exergetic analysis show that second law efficiency using the zeotropic mixture increases up to 15 % compared to the pure components. Due to a better glide matching the irreversibilities in the condenser decrease significantly for those mixture compositions, where temperature glide at phase change and temperature difference of the cooling water are equal. In general the condenser shows the highest required surface area in consequence of a low logarithmic mean temperature difference and a high amount of transferred heat. In case of fluid mixtures a reduction of heat transfer coefficients due to additional mass transfer effects leads to a higher surface area of the condenser and evaporator compared to pure fluids. Therefore the total purchase equipment costs increase in the range of 5 % to 30 %. In case of pure working fluids isobutane leads to slightly lower specific costs of electricity than isopentane. In both considered geothermal case studies, the most suitable concept under exergoeconomic criteria is the choice of the fluid mixture as a working fluid.
The investigations show that the use of an isobutane/isopentane mixture is a promising optimization strategy for low-temperature geothermal applications. Related with high exploration costs, the efficiency increase overcompensates the additional heat transfer areas. The calculations point out that different fluids should be selected, if a variable minimum temperature difference in the condenser and evaporator is assumed.
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12:00
20 mins
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INFLUENCE OF CONDENSER CONDITIONS ON ORC LOAD CHARACTERISTICS
Tobias Erhart, Ursula Eicker, David Infield
Abstract:
Introduction:
Within the EU-project POLYCITY over six years of supply and demand in a city quarter with app. 7000 people have been monitored. The cornerstone of the energy supply is a combined heat and power plant (CHP) with 5.3 MWth and 1 MWel nominal output based on an OR-Cycle. A district heating system is serves as sink for the power plant and a thermal cooling device (single effect absorption chiller) is connected to it.
All applied systems such as biomass furnace, thermal oil system, district heating and heat rejection unit have been monitored long-term, to show the constraints of the heat guided operational mode for the grid feed-in. The measured states have been analysed to elaborate the influence of components in the process.
Relevance:
Many ORC systems fuelled by biomass are heat-guided. In order to increase the annual or seasonal overall performance, the interaction of heat losses, electric efficiencies and performance of thermal cooling systems have to be taken into account. Questions regarding solar thermal support of district heating in summer can only be clarified in this way.
Method:
Measured data from the control system have been collected and transferred via an OPC UA interface (OLE for process control – unified architecture). With custom software the values could be acquired and saved in a database. Combining and unifying the data from the internal control and the external metering systems a comprehensive view on the conditions of the system is given. Thermodynamic states of the silicone oil are computed by using the formulations in the Refprop library based on Colonna, Nannan and Guardone. The observed characteristic is compared to the expected results from a condenser model.
Results:
Various mass flows, temperature levels and temperature spreads result in varying pressure levels in the condenser. The pressure level ranges from 90 mbar to 140 mbar at feeding temperatures between 72 °C and 85 °C. High mass flows on the secondary side, respectively low temperature spreads, result in lower pressures. These lead to higher electric efficiencies due to higher pressure differences across the turbine. The findings give several conclusions for an economic operation of ORCs. Under normal operation conditions the overall influence of the condenser conditions sum up to one percentage point in electric efficiency. The question of higher efficiencies versus increasing pump power in the district heating network is discussed.
REFERENCES
[1] Colonna P., Nannan N.R., Guardone A., Lemmon E.W., Multiparameter equations of state for selected siloxanes, Fluid Phase Equilibria 244, 2006
[2] Colonna P., Guardone A., Nannan N.R., Siloxane: A new class of candidate Bethe-Zel’dovich-Thompson fluids, Physics of Fluids 19, 2007
[3] Colonna P., Nannan N.R., Guardone A., Multiparameter equations of state for siloxanes, Fluid Phase Equilibria 263, pages 115-130, 2008
[4] Flynn D., “Thermal Power Plant Simulation and Control”, IEE power Series, The Institution of Electrical Engineers, London, 2003
[5] Swamee P.K., Jain A.K., "Explicit equations for pipe-flow problems". Journal of the Hydraulics Division (ASCE) 102 (5): 657–664, 1976
[6] Churchill S. W., Friction factor spans all flow regimes, Chemical Engineering 84, p. 91, 1977
[7] Duvia A., Technical and economic aspects of Biomass fuelled CHP plants based on ORC turbogenerators feeding existing district heating networks, 2009
[8] REFPROP, NIST Standard Reference Database 23, Version 9.0, 2010
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12:20
20 mins
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OPTIMIZATION OF SUBCRITICAL AND TRANSCRITICAL ORC'S FOR LOW TEMPERATURE HEAT SOURCES
Daniël Walraven, Ben Laenen, William D'haeseleer
Abstract: Many types of low temperature (100-150°C) heat sources exist: waste, geothermal, solar, ... Classical, water-based power cycles cannot convert this heat into electricity efficiently. For these heat sources an Organic Rankine Cycle (ORC) is the better choice. In this paper, the thermodynamic optimization of ORC's for low-temperature heat sources is discussed and transcritical cycles are compared to subcritical cycles.
In the literature, different efficiencies (energetic, exergetic, ...) are defined. For economic reason, it is concluded that the total plant efficiency should be maximized and not the cycle efficiency as is often done in the literature. This is because the latter efficiency does not take into account the heat source cooling.
On our search for optimum plant designs, we investigated both subcritical and transcritical ORC's. The advantage of the latter cycle is that the fluid does not pass through the two-phase region during heating, so a better fit between the working-fluid heating curve and heat-source cooling curve is achieved. Less irreversibilities are generated in the heat exchange and higher efficiencies can be obtained.
The simplest ORC configuration is compared to cycles with a recuperation heat exchanger and to cycles with turbine bleeding. It is concluded that these two extended cycles are only useful when a limit on the heat-source-outlet temperature exists.
The optimum working fluid depends strongly on the heat source inlet temperature and the optimum cycle is often of the transcritical type. With a careful choice of the working fluid exergetic plant efficiencies of 50-60% can be achieved.
Due to the low temperature of the heat source, both the plant and cycle efficiency decrease strongly with increasing condenser temperature and pinch-point-temperature difference. So for low-temperature heat sources, a low condenser temperature and low pinch-point temperatures are even more important than in classical power plants.
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