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Thermodynamic performance of selected HCFS for geothermal applications

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Abstract

The thermodynamic efficiencies of selected HFCs (hydrofluorocarbons) have been investigated for applications in binary geothermal conversion systems. Following examination of thermodynamic cycles, interactions with sensible heat sources were considered. For every working fluid, there is a temperature range for optimal use. Each fluid is best applied with a heat source at a temperature somewhat above the critical fluid temperature.

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... The ORC module uses a geothermal source in cascade application to supply thermal demands for space heating and cooling and domestic hot water demands of the district in a polygeneration application; otherwise, the electric output energy is partly used to supply the energy request of thermal network and the remaining part is sold to electric national grid. The selection of organic fluids is based on a literature analysis and depends on the thermophysical, economic, and environmental properties [55,56]. An appropriate selection of the working fluid of the cycle is crucial for optimizing the efficiency of the binary plant, to maximize the conversion efficiency or to determine the best configuration for a given plant capacity. ...
Article
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This paper presents a thermodynamic, economic, and environmental analysis of a renewable polygeneration system connected to a district heating and cooling network. The system, fed by geothermal energy, provides thermal energy for heating and cooling, and domestic hot water for a residential district located in the metropolitan city of Naples (South of Italy). The produced electricity is partly used for auxiliaries of the thermal district and partly sold to the power grid. A calibration control strategy was implemented by considering manufacturer data matching the appropriate operating temperature levels in each component. The cooling and thermal demands of the connected users were calculated using suitable building dynamic simulation models. An energy network dedicated to heating and cooling loads was designed and simulated by considering the variable ground temperature throughout the year, as well as the accurate heat transfer coefficients and pressure losses of the network pipes. The results were based on a 1-year dynamic simulation and were analyzed on a daily, monthly, and yearly basis. The performance was evaluated by means of the main economic and environmental aspects. Two parametric analyses were performed by varying geothermal well depth, to consider the uncertainty in the geofluid temperature as a function of the depth, and by varying the time of operation of the district heating and cooling network. Additionally, the economic analysis was performed by considering two different scenarios with and without feed-in tariffs. Based on the assumptions made, the system is economically feasible only if feed-in tariffs are considered: the minimum Simple Pay Back period is 7.00 years, corresponding to a Discounted Pay Back period of 8.84 years, and the maximum Net Present Value is 6.11 M€, corresponding to a Profit Index of 77.9% and a maximum Internal Rate of Return of 13.0%. The system allows avoiding exploitation of 27.2 GWh of primary energy yearly, corresponding to 5.49∙103 tons of CO2 avoided emissions. The increase of the time of the operation increases the economic profitability.
... Dealing with heat sources with finite heat capacity, the total efficiency in Equation (1) can be optimized adopting an ideal "triangular" cycle. These conditions can be approximated by resorting to working fluids with a critical temperature close to that of the maximum heat source [27,28]. On the other hand, when the critical temperature of the working fluid is much lower than the maximum temperature of the heat source, the ideal thermodynamic cycle approaches the transcritical conditions. ...
Article
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This study aims to provide a thermodynamic comparison between supercritical CO2 cycles and ORC cycles utilizing flue gases as waste heat source. Moreover, the possibility of using CO2 mixtures as working fluids in transcritical cycles to enhance the performance of the thermodynamic cycle is explored. ORCs operating with pure working fluids show higher cyclic thermal and total efficiencies compared to supercritical CO2 cycles; thus, they represent a better option for high-temperature waste heat recovery provided that the thermal stability at a higher temperature has been assessed. Based on the improved global thermodynamic performance and good thermal stability of R134a, CO2-R134a is investigated as an illustrative, promising working fluid mixture for transcritical power cycles. The results show that a total efficiency of 0.1476 is obtained for the CO2-R134a mixture (0.3 mole fraction of R134a) at a maximum cycle pressure of 200 bars, which is 15.86% higher than the supercritical carbon dioxide cycle efficiency of 0.1274, obtained at the comparatively high maximum pressure of 300 bars. Steam cycles, owing to their larger number of required turbine stages and lower power output, did not prove to be a suitable option in this application.
... As a general rule [26,43], fixed the maximum temperature T A of a finite thermal capacity heat source, the best thermodynamic cycle is for a working fluid with a critical temperature roughly equal to T A . However, in our case, the source thermal power is fixed and we imposed instead the maximum cycle temperature T max , so better is the thermodynamic efficiency, greater will be the useful mechanical power. ...
Article
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This study investigates the use of pure and hydrocarbons binary mixtures as potential alternatives working fluids in a usual biomass powered organic Rankine cycle (ORC). A typical biomass combined heat and power plant installed in Cremona (Italy) is considered as the benchmark. Eight pure hydrocarbons (linear and cyclic) and four binary mixtures of linear hydrocarbons were selected. The critical points of the binary mixtures at different composition were calculated using an in-house code developed in MATLAB c (R2018b) environment. Based on the critical point of a working fluid, supercritical and subcritical cycle configurations of ORC were analysed. A detailed thermodynamic comparison with benchmark cycle was carried out in view of cycle efficiency, maximum operating pressure, size of the turbine and heat exchangers. The supercritical cycles showed 0.02 to 0.03 points lower efficiency, whereas, subcritical cycles showed comparable efficiencies than that of the benchmark cycle. The cycles operating with hydrocarbons (pure and mixtures) exhibited considerably lower volume flow ratios in turbine which indicates lower turbine size. Also, size parameter of regenerator is comparatively lower due to the lower molecular complexity of the hydrocarbons. A noticeable increase in turbine power output was observed with change in composition of the iso-octane/n-octane binary mixture at the same thermodynamic efficiency.
... Desideri and Bidini [7] studied on the optimization of binary plants and have presented that it is possible to optimize these cycles by modifying the main parameters such as turbine inlet pressure and fluid type. Invernizzi and Bombarda [8], in their study about the thermodynamic performance of hydrofluorocarbons (HFCs) in geothermal binary plants, have indicated that there is a temperature range for optimal use and each fluid is best applied with a heat source at a temperature somewhat above the critical fluid temperature. Gu and Sato [9,10] evaluated the supercritical cycles for geothermal binary power plants and have optimized the cycle parameters such as condensing temperature and pressure, they have also investigated the different working fluids for a given liquid dominated geothermal resource and determined that propane and R-134a are more suitable in their studies. ...
Article
Artificial neural network is a new tool, which works rapidly for decision making and modeling of the processes within the expertise. Therefore, ANN can be a solution for the design and optimization of complex power cycles, such as ORC-Binary. In the present study, the back-propagation learning algorithm with three different variants, namely Levenberg–Marguardt (LM), Pola-Ribiere Conjugate Gradient (CGP), and Scaled Conjugate Gradient (SCG) were used in the network to find the best approach. The most suitable algorithms found were LM 16 for s1 type cycle and LM 14 for s2 type cycle. The Organic Rankine Cycle (ORC) uses organic fluids as a working fluids and this process allows the use of low temperature heat sources and offers an advantageous efficiency in small-scale concepts. The most profitable cycle is obtained with a benefit of 124.88 million US$ from s1 type supercritical ORC-Binary plant with an installed capacity of 64.2 MW when the working fluid is R744 and the design parameters of T1b, T2a and P2a are set to 80 °C, 130 °C and 12 MPa, respectively.
... A great number of scientific studies has been released on the adequate selection of the working fluid and on the optimization of the corresponding cycle parameters for a number of applications in waste heat recovery (Dai et al., 2009), biomass combustion (Drescher and Brüggemann, 2007), solar heat (Tchanche et al., 2009), geothermal sources (Invernizzi and Bombarda, 1997) and geothermalsolar hybrid concepts (Marco et al., 2011). The advanced cycle configurations such as supercritical cycles (Zhang and Jiang, 2012) and multi-level cycles (Walraven et al., 2013), the use of the mixtures (Chen et al., 2011) and predictive theoretical methods (Papadopoulos et al., 2010) to define the optimal working fluids are also being explored. ...
... As we have already observed (see Sect. 3.5), if the heat source can be considered for the most part isothermal, optimisation of the thermodynamics is invariably favoured by a fluid with high critical temperature and a simple molecular structure. By contrast, when the heat source has a variable temperature, the maximum power is a combination of the engine fluid capacity to cool the source and, at the same time, the capacity of the thermodynamic cycle to best use the heat extracted from the source [28]. Figure 3.27 shows the performance of the second-law efficiency as a function of the maximum temperature of the geothermal water source for binary cycles using different working fluids. ...
Chapter
Chap. 3 is dedicated to Rankine cycles with organic fluids: the so-called organic Rankine cycles (ORC), which in recent years have had a large success on the market. After a brief historical review we discuss the characteristics that must have the potential working fluids to be used in an ORC engine. A large discussion is about the interactions between thermodynamics, the plant engineering and the design of turbomachines. In Sect. 3.5 the thermodynamics of heat recovery (an area in which the ORC are now widely used) is discussed. In Sect. 3.6 some significant examples of application of the ORC technology are presented and Sect. 3.7 is dedicated to a discussion of the use of multicomponent working fluids in Rankine cycles.
... In this context, the Organic Rankine Cycle (ORC) is mainly applied as energy conversion system. Regarding the optimisation of the subcritical ORC, a selection of pure media as working fluids is performed by numerous authors in respect to the heat source characteristics [3][4][5][6][7][8]. A promising optimisation approach for ORC systems is the use of mixtures as working fluids. ...
Article
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We present a thermo-economic evaluation of binary power plants based on the Organic Rankine Cycle (ORC) for geothermal power generation. The focus of this study is to analyse if an efficiency increase by using zeotropic mixtures as working fluid overcompensates additional requirements regarding the major power plant components. The optimization approach is compared to systems with pure media. Based on process simulations, heat exchange equipment is designed and cost estimations are performed. For heat source temperatures between 100 and 180 °C selected zeotropic mixtures lead to an increase in second law efficiency of up to 20.6% compared to pure fluids. Especially for temperatures about 160 °C, mixtures like propane/isobutane, isobutane/isopentane, or R227ea/R245fa show lower electricity generation costs compared to the most efficient pure fluid. In case of a geothermal fluid temperature of 120 °C, R227ea and propane/isobutane are cost-efficient working fluids. The uncertainties regarding fluid properties of zeotropic mixtures, mainly affect the heat exchange surface. However, the influence on the determined economic parameter is marginal. In general, zeotropic mixtures are a promising approach to improve the economics of geothermal ORC systems. Additionally, the use of mixtures increases the spectrum of potential working fluids, which is important in context of present and future legal requirements considering fluorinated refrigerants.
... 4, ottimizzazioni di natura economica e/o termodinamica possono portare a risultati discordanti(Marco Astolfi, 2013). Sulla base delle succitate considerazioni e della specifica applicazione ORC in esame, i possibili fluidi di lavoro non sono tanto numerosi quanto i candidati riportati in (Marco Astolfi, 2013) o in (CostanteInvernizzi, 1997). Infatti, la stabilità termica del fluido qui costituisce una questione seria che orienterebbe la possibile scelta del fluido verso alcuni silossani, i perfluorocarburi, il toluene e un alogenuro, precisamente il tetracloruro di titanio (TiCl 4 ), sebbene non trattasi di fluido organico propriamente detto. ...
Technical Report
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l presente documento descrive le attività di ricerca svolte all’interno dell’Accordo di collaborazione “Definizione e studio di fluidi organici o loro miscele in grado di operare a temperature massime superiori a quelle attualmente in uso e valutazione dei cicli termodinamici” Responsabile scientifico ENEA: Roberta Roberto Responsabile scientifico Politecnico di Milano: Ennio Macchi
... ORC working fluid selection The working fluid must be selected according to its critical temperature, which must be suitable for the temperature level of the geothermal source (see for example Angelino et al.(1995) and Invernizzi and Bombarda (1997) for more details). The choice is restricted by the well known harmful effects of CFCs, which demand the adoption of either hydrocarbons or new fluids recently developed. ...
Article
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The optimum cycle for a geothermal power plant depends on the geothermal brine/steam characteristics and the site features. Selecting the most suitable configuration must therefore be based on the geothermal source and heat reject conditions (i.e. ambient conditions). This paper investigates the suitable plant configurations for four different kinds of geothermal sources, with increasing enthalpy, going from relatively low temperature (115 °C) liquid brine to high temperature (250 °C) superheated steam, and points out the best solution for each case. In order to do this, a sophisticated calculation model, which can manage different geothermal fluids, different working fluids and various plant configurations, is employed. The code is capable of optimizing the cycle thermodynamic parameters in order to get the highest economic return. The power conversion cycles considered include the "traditional" flash and binary cycles and the "advanced" combined and mixed cycles, obtained respectively by: • coupling a flash cycle (topping cycle) with a binary cycle (bottoming cycle) • splitting the liquid and gaseous content of the source in two separate flows and then feeding a binary cycle with the liquid fraction, and a flash cycle with the gaseous fraction. The results show the optimum solution for each geothermal resource, the consequent economic return and power produced.
... Desideri and Bidini (1997) have studied the optimization of binary plants and have presented that it is possible to optimize these cycles by modifying the main parameters, such as turbine inlet pressure and fluid type. Invernizzi and Bombarda (1997), in their study about the thermodynamic performance of hydrofluorocarbons (HFCs) in geothermal binary plants, have indicated that there is a temperature range for optimal use and each fluid is best applied with a heat source at a temperature somewhat above the critical fluid temperature. Sato (2001, 2002) have evaluated the supercritical cycles for geothermal binary power plants and have optimized the cycle parameters, such as condensing temperature and pressure; they have also investigated the different working fluids for a given liquid dominated geothermal resource and determined that propane and R-134a are more suitable in their studies. ...
Article
Geothermal energy is the most attractive energy source from an electricity generation point of view, since it is the most stable one among the renewables. Although the geothermal resources are local, it is also a solution for the global warming problem, so the climate changes, since its usage depends on environment-friendly technologies. In this study, the electricity generation ability of the Simav geothermal field has been investigated. In this study, two different cycle types, including ORC-binary and combined cycles, have been handled. In addition, seven different cycle types of ORC-binary have been taken into consideration. Finally, the designed plants have been evaluated in an economical point of view using Life Cycle Cost analysis. As a conclusion, it has been determined that it is available to generate electricity at a rate of 50–53 MW with a benefit of 288 million US$ during the plant lifetime.
Article
The present work documents extensive experimental campaigns involving the first ever L-shaped Pitot tube measurements in non-ideal subsonic and supersonic flows of siloxane MM (hexamethyldisiloxane, C6H18OSi2), a fluid commonly employed in high-temperature Organic Rankine Cycles (ORCs). The objective is to establish reliable methodologies for pressure probes usage in flows relevant to ORCs, contributing to power generation efficiency through the improvement of current components design and plant regulation capabilities. Experimental campaigns were carried out on the Test-Rig for Organic Vapors (TROVA) at Politecnico di Milano, with a total-static Pitot tube designed according to ISO 3966, in non-ideal subsonic flows at Mach numbers M=0.2,0.5 with total pressure and temperature in the range PT=1−7bar, TT=195−205°C and a corresponding compressibility factor ZT≥0.8. Adequate performance of the complete system was verified and Pitot tube behaviour was found to be unaffected by flow non-ideality, requiring no calibration in the investigated conditions. A simple Pitot tube was then employed for direct total pressure loss measurements across normal shock waves in non-ideal supersonic flows at M≃1.5, with total conditions of PT=1.5−12.8bar, TT=212−233°C and ZT≥0.66. The good agreement between measured losses and theoretical ones calculated from conservation equations attests the validity of the developed methodologies even with supersonic flows.
Article
Compared with the pure fluids, the zeotropic mixtures can balance the requirements of environmental protection, heat source matching and system safety, and exhibit excellent thermodynamic performance. However, compared to the widespread applications of pure fluids, zeotropic mixtures are rarely exploited in thermodynamic cycles, and there is a lack of targeted summary on refrigeration systems, organic Rankine cycle systems and combined power and refrigeration systems. In the recent years, zeotropic mixtures are developing at an unprecedented pace, while the working fluids components are inevitably explored in the process. In this paper, the research progress of zeotropic mixtures in the field of refrigeration systems, organic Rankine cycle systems and combined power and refrigeration systems are reviewed. Based on the review of zeotropic working mixtures, the reasonable predictions can be proposed. In the future, environmental problems will still be one of the most important concerned issues. Therefore, the zeotropic mixtures consisting of natural hydrocarbons and carbon dioxide, which are environmentally friendly, have great potential for development. Furthermore, zeotropic mixtures of natural working fluids can improve comprehensive energy efficiency of combined systems and will play an important role in future carbon emission reduction technologies.
Article
Artificial neural network is a new tool, which works rapidly for decision making and modeling of the processes within the expertise. Therefore, ANN can be a solution for the design and optimization of complex power cycles, such as ORC-Binary. In the present study, the back-propagation learning algorithm with three different variants, namely Levenberg–Marguardt (LM), Pola-Ribiere Conjugate Gradient (CGP), and Scaled Conjugate Gradient (SCG) were used in the network to find the best approach. The most suitable algorithms found were LM 16 for s1 type cycle and LM 14 for s2 type cycle. The Organic Rankine Cycle (ORC) uses organic fluids as a working fluids and this process allows the use of low temperature heat sources and offers an advantageous efficiency in small-scale concepts. The most profitable cycle is obtained with a benefit of 124.88 million US$ from s1 type supercritical ORC-Binary plant with an installed capacity of 64.2 MW when the working fluid is R744 and the design parameters of T1b, T2a and P2a are set to 80 °C, 130 °C and 12 MPa, respectively.
Article
This paper presents the approach for selection of optimal working fluids and cycle performance prediction for organic Rankine cycle (ORC) based on a theoretical exergy efficiency model (EEM) under reduced temperature. 18 working fluids with critical temperature from 100 to 200 °C are under evaluation. When the condensing temperature is 40 °C and reduced evaporating temperature is 0.85, the maximum overall exergy efficiency will be obtained when the heat source temperature approaches to the critical temperature of working fluid. Then, the optimal working fluids are selected by overall exergy efficiency. When the heat source inlet temperatures are 130 °C, 150 °C, 170 °C and 190 °C, the optimal working fluids are R236ea, R245fa, R245ca and R365mfc, respectively. In addition, under the same entire reduced temperatures the exergy efficiencies of different working fluids tend to be approximately equal and obey the circular distributions. The quantitative correlation between the optimal reduced evaporating temperature and reduced heat source temperature is given. The correlation provides a non-dimensional method to calculate the optimal evaporating temperature for different working fluids on the target of exergy efficiency.
Article
By the growing usage of geothermal energy as an alternative approach to produce useful work such as electricity, the emission of global greenhouse gases could be reduced because of its environmentally friendly. In this paper, the thermodynamic and economic performances of three systems which contain a basic Organic Rankine Cycle (ORC), a Regenerative Organic Rankine Cycle (RORC) and a Two-Stage Evaporation Organic Rankine Cycle (TSEORC) are investigated in order to generate electrical power from geothermal sources. For operating the considered cycles, three types of pure organic working fluids including dry (R600a, R601a), wet (R152a and R134a) and isentropic (R11 and R123) ones are selected. Firstly, according to thermodynamic aspect, Peng Robinson (PR) and Soave-Redlich-Kwong (SRK) equations of state are used to determine thermodynamic properties of mentioned working fluids and geothermal water, respectively. Furthermore, the operating parameters involving evaporator and regenerative temperatures, degree of superheat and pinch point temperature difference in evaporator are optimized with three optimization methods. Objective functions are exergy efficiency, Specific Investment Cost (SIC) and a combination of exergy and SIC for thermodynamic, economic and exergoeconomic optimizations. The amount of boundary conditions constituting of heat source inlet temperature, heat sink inlet temperature, heat source inlet pressure, heat sink inlet pressure temperature of condenser, pinch point temperature in condenser and heat source mass flow rate are 423.14 (K), 293.15 (K), 5 (bar), 2 (bar), 308 (K), 5 (K) and 50 (kg/s) respectively. Optimizations results show that among all considered operating parameters, degree of superheat ranged between 0 and 20 is the most effective parameter which is almost obtained at lower, upper and in the middle range of optimization bounds in the thermodynamic, economic and exergoeconomic investigations respectively. Secondly, from economic view point, three economic indicators: Levelized Cost Of Electricity (LCOE), Return On Investment (ROI) and Payback Period (PBP) are utilized so as to focus on the economic performance of three mentioned ORC configurations based on exergoeconomic results for twenty countries with geothermal resources as well as different cost of electricity production and tax rates. The results indicate that Australia has the maximum amount of ROI making up a bit more than 100% and minimum amount of PBP accounting for lower than four years when R123 is applied as the working fluid in TSEORC system. Also, the maximum and minimum values of LCOE are obtained in basic ORC- R134a and RORC- R123 (0.1474 and 0.0493 respectively). In addition, the investigation of impact of operating parameters on economic indicators for Iran illustrate that the ROI value dramatically rise by increasing the evaporator temperature and degree of superheat. In contrast, pinch point temperature difference leads to a decline in the amount of ROI. This note should be taken in to account that ROI and PBP show the reverse results.
Article
In this paper we investigate the potential replacement of HFC-134a in ORC applications by two low-GWP refrigerant fluids, namely HFO-1234yf and HFO-1234ze(E). After revising and discussing their main thermo-physical properties, we adopted in our calculations the Peng Robinson EOS available in Aspen Plus v7.3, integrated with literature data. By assuming as reference a geothermal plant operated with HFC-134a, we first consider the direct replacement of the original fluid by the two refrigerants. Results of such off-design simulations show a decrease of the net power of about 13% in case of HFO-1234yf and 1% in case of HFO-1234ze(E). Then, in case of heat recovery from a hot water source, from a comparison among the three refrigerants, with the hypothesis of a completely new design simulation, it turns out that the turbine power results lower than HFC-134a of about 20% and 28% for the cycles using HFO-1234yf and HFO-1234ze(E) respectively. We also show that in case of HFO-1234ze(E) the recuperative heat exchanger could be removed without tangible effects on the useful power and on the cycle efficiency. Finally we assess through an experimental thermal stability analysis that 200–250 °C could be a feasible working temperature limit for HFO-1234yf.
Chapter
Die Wahl eines geeigneten Kraftwerkssystems in Abhängigkeit der Charakteristika der Ressource ist ein Schlüssel für eine effiziente geothermische Stromerzeugung. Im Fall von Hochenthalpie‐Lagerstätten ist es möglich, das Thermalwasser direkt als Arbeitsmedium zu nutzen. So kann entweder gesättigter Dampf unmittelbar in der Turbine entspannt und genutzt werden oder im Fall eines geförderten Zwei‐Phasen‐Gemisches durch den Einsatz von Flash‐Prozessen. Für Niederenthalpie‐Lagerstätten mit Thermalwassertemperaturen unter 200 °C, wie sie in Deutschland vorliegen, bedarf es binärer Kraftwerke. Hierbei handelt es sich um geschlossene Sekundärprozesse, auf die die thermische Energie des Thermalwassers übertragen wird. Als Kraftwerkstechnologien stehen der Organic Rankine Cycle (ORC) und der Kalina Cycle (KC) zur Verfügung. Diese Prozesse unterscheiden sich sowohl in der Prozessführung als auch in der Wahl des Arbeitsmediums. Neben den bereits umgesetzten Standardkonzepten existiert eine Vielzahl von Optimierungsansätzen, welche unter Berücksichtigung der geologischen Randbedingungen, zu einer signifikanten Effizienzsteigerung führen können.
Article
Full-text available
In the view point of recent technological developments, it is available to generate electricity from geothermal resources with low and medium enthalpy One of these technologies is binary cycle system, a kind of Organic Rankine Cycle (ORC), in which geothermal flow energy is used as an energy source. However, the design of these technologies requires more proficiency and longer times within complex calculations. Artificial Neural Network (ANN) is a new tool to make a decision and modeling of the processes within the expertise. In this way, ANN can be a solution for the design of complex power cycles such as ORC-Binary, since ANN is an information technology inspired by human brain's information processing technology. In this study, the back-propagation learning algorithm with three different variants, namely Levenberg-Marguardt (LM), Pola-Ribiere Conjugate Gradient (CGP), and Scaled Conjugate Gradient (SCG) were used in the network so that the best approach could be found. The most suitable algorithm was found as LM with 12 neurons in single hidden layer for b2 type cycle. For b3 type cycle, the most suitable algorithm was found as LM with 8 neurons in the first hidden layer and 10 neurons in the second hidden layer.
Article
A performance comparison of two types of bottoming cycles, including a Kalina cycle and a transcritical organic Rankine cycle (ORC) using working fluids with sliding-temperature boiling characteristics, is conducted in order to analyze energy saving of the sensible exhaust waste heat recovery (WHR) under various internal combustion engine (ICE) working conditions. Through quantitatively analyzing the relation between exhaust waste-heat behaviors and the ICE load of a commercial ICE, two bottoming subsystems models, including a transcritical ORC using some several Alkanes and a Kalina cycle using NH3-H2O as working fluids, are build under the same ICE various-temperature exhaust heat-source and air heat-sink conditions. Compared to Kalina cycle, the transcritical ORC shows prominent advantages on the overall thermal efficiency, low operation pressure and simple components configuration at the ICE load with exhaust temperature over 491 K. The optimal thermal performance of the transcritical ORC appears at the ICE load with the certain exhaust temperature of 569-618 K. However, thermodynamic performance of the bottoming transcritical ORC is worsened considerably at the ICE load with the exhaust temperature over or under the certain value. Moreover, the extremely high turbine expansion ratio requires a complex multi-stage turbine design and big turbine dimensions for the bottoming transcritical ORC using Alkanes-based working fluid.
Conference Paper
A blow-down wind tunnel for real gas applications has been designed to characterize an organic vapour stream by independent measurements of pressure, temperature and velocity. Experiments are meant to investigate flow fields representative of expansions taking place in Organic Rankine Cycles (ORC) turbines. Strong real gas effects, high Mach numbers and approximations affecting the calculated properties of novel compounds, make the knowledge of ORC turbine blade passage flow field still rather limited. A significant enhancement of turbines efficiency is expected from detailed investigations on expansion streams. Despite Organic Cycles have attracted large research efforts in recent years, present days design tools still suffer from a lack of relevant experimental data. So far, ideal gas test cases and equilibrium measurements have supported separately CFD and thermodynamic model validations. These considerations prove the relevance of such a test rig. This paper discusses the design and the final layout of the facility, whose construction is currently in progress. A straight axis supersonic nozzle has been chosen as test section for early tests; investigations on blade cascades are foreseen in the future. Due to high stream densities and temperatures, a throat size compatible with probes intrusion made a continuous cycle plant unaffordable, requiring an input thermal power of around 2.5 MW. A reduction to 30 kW has been achieved by adopting a blow-down tunnel: the fluid, slowly vaporized in a high pressure vessel, feeds the nozzle at a lower pressure. The vapour is then collected in a low pressure tank and condensed. The loop is closed by liquid compression through a pump. Such a batch operating system also offers the option to select test/condensation pressures and temperatures, allowing experimentation of a wide variety of working fluids, even though new ORC compounds (e.g. Siloxanes, Fluorocarbons) remain of major interest. Maximum temperature and pressure are 400 °C and 50 bar. Despite the unsteady operational mode, the inlet nozzle pressure can be kept constant by a control valve. Depending on the fluid and test pressure, experiments may last from 20 seconds to several minutes, while their set-up requires a few hours. Fast response pressure transducers, pressure probes and thermocouples have been selected for thermodynamic measurements; Laser Doppler Velocimetry (LDV) and Schlieren techniques allow direct measurements of velocity and flow visualization. The design has been carried out with a lumped parameter approach, using Siloxane MDM and Hydrofluorocarbon R245fa as reference compounds and FluidProp® for properties calculation.
Article
This two-part paper investigates the potential of ORC (Organic Rankine Cycles) for the exploitation of low-medium enthalpy geothermal brines. Part A deals with thermodynamic analysis and optimization, while Part B focuses on economic optimization. In this part, an economic model was defined and implemented in the Matlab® code previously developed. A routine was also implemented to estimate the design of the turbine (number of stages, rotational speed, mean diameter), allowing to estimate turbine efficiency and cost. The tool developed allowed performing an extensive techno-economic analysis of many cycles exploiting geothermal brines with temperatures between 120 °C and 180 °C. By means of an optimization routine, the cycles and the fluids leading to the minimum cost of the electricity are found for each geothermal source considered. Cycle parameters found from the techno-economic optimization are compared with those assumed and found from the thermodynamic optimization. Quite relevant differences show the necessity to perform optimization on the basis of specific plant cost. As a general trend, it is however confirmed that configurations based on supercritical cycles, employing fluids with a critical temperature slightly lower than the temperature of the geothermal source, lead to the lowest electricity cost for most of the investigated cases.
Chapter
Utilization OptionsEGS Plant DesignCase StudiesReferences
Article
The advantages of organic fluid space power cycles are studied and compared with alternate options. The principal characteristics of organic power cycles are shown to be predictable with a good level of accuracy through a general methodology, which requires the knowledge of limited information concerning the fluid properties, i.e., specific heat in the ideal gas state, the critical parameters, and a portion of the saturation curve. Based on this theory, the adoption of fluids with a relatively complex molecular structure and condensation at the lowest practically admissable reduced temperature allow a better efficiency than attainable with the use of toluene, which is employed as a reference fluid. It is shown that only the combined optimization of fluid and thermal dynamic variables leads to the definition of an optimum working fluid and power cycle.
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An accurate and efficient numerical method for computing compressible internal flows in 2-D is presented. The method has been designed to deal with subsonic-to-supersonic problems inside nozzles without shocks; it is seen to be competitive as well in the subsonic range and can be easily adapted to compute transonic flows by means of any suitable shock-fitting procedure. For the subsonic-to-supersonic flow case an alternating direction block-line-Jacobi method is employed to obtain the steady state solution in the subsonic region, up to the first computational column for which the longitudinal velocity component is supersonic at all gridpoints. The validity of the technique is demonstrated for a well-documented nozzle-flow problem for both subsonic-to-supersonic and subsonic flow conditions.
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The atmospheric lifetimes of the fluorinated gases CF4, C2F6, c-C4F8, (CF3)2c-C4F6, C5F12, C6F14, C2F5Cl, C2F4C12, CF3Cl, and SF6 are of concern because of the effects that these long-lived compounds acting as greenhouse gases can have on global climate. The possible atmospheric loss processes of these gases were assessed by determining the rate coefficients for the reactions of these gases with O(1D), H, and OH and the absorption cross sections at 121.6 nanometers in the laboratory and using these data as input to a two-dimensional atmospheric model. The lifetimes of all the studied perfluoro compounds are >2000 years, and those of CF3Cl, CF3CF2Cl, and CF2ClCF2Cl are >300 years. If released into the atmosphere, these molecules will accumulate and their effects will persist for centuries or millennia.