14:00
Session 2B: Working Fluids - Mixtures
Chair: Spyros Voutetakis
14:00
20 mins
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Experimental Comparative Study of an ORC Using Pure Fluid and Zeotropic Mixture for Waste Heat Recovery
Quetin Blondel, Nicolas Tauveron, Nadia Caney, Nicolas Voeltzel
Abstract: The energy sector is facing major challenges in the upcoming century as energy demand is rising and its major impact on the global warming issue needs to be addressed. Among the solutions to overcome these challenges, renewable energies and process energy efficiency could be partially fulfilled by the use of the Organic Rankine Cycle (ORC) technology [1].
A previous study [2] with a compact ORC (0.25m3) has shown good performances of the system and the pure fluid used. This working fluid (NovecTM649) is confirmed as an interesting replacement fluid.
In this study, the ORC performances with a zeotropic mixture (NovecTM649/HFE-7000) are investigated. The heat source of the system simulates a low grade waste heat recovery at 110°C. The experimental results are compared with the precedent pure fluid results for the same ORC. Thermodynamic analysis issued from the experimental data are carried out with EES software [3] coupled with REFPROP software [4] for the pure fluid and mixture thermophysical properties.
As a preliminary investigation of this zeotropic mixture, the global performances of the system and the expansion component are screened and compared. The aim is to understand how the ORC operation is impacted by this “non-design fluid”. Indeed, the installation and all the components were originally designed to work with the pure fluid (NovecTM649). Such as the expander, a high speed micro turbine, which was specifically designed with this pure fluid.
During experiments, the expander behaviours and performances from a qualitative point of view are similar for the mixture or the pure fluid as working fluids. At equivalent power production, the use of this zeotropic mixture leads to a significant increasing of the energetic and exergetic efficiencies. These results show that a zeotropic mixture could be an interesting adaptation parameter for an existing ORC to make an optimal utilisation of the available heat source.
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14:20
20 mins
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An Improved Method for the Investigation of Thermal Stability of Organic Fluids
Simone Gallarini, Andrea Spinelli, Luca Lietti, Alberto Guardone
Abstract: The thermal stability of the working fluid is a key aspect for the design of an efficient organic Rankine cycle. Different methods for assessing thermal stability are available in literature, based either on pressure deviation during an isothermal stress test or on the comparison of the vapor pressure curve measured before and after the fluid underwent thermal stress. Both these methods present strong and weak points. On this basis, an improved methodology for thermal stability analysis is proposed here. During a single
test, a sample of the fluid under scrutiny is placed in a closed vessel and stressed at constant temperature for 80 hours. The vapor pressure curve of virgin and stressed fluids are measured and compared. If present, the deviation between the curves provides an indication of the decomposition extent. Further, the virgin and stressed fluid composition, measured, for both liquid and vapor phase, by means of gas chromatography and mass spectrometry are compared. This permits to obtain a trend of the sample composition for varying stress temperature, whereas by using the vapor pressure method the quantitative
relation between deviation and decomposition amount cannot be easily retrieved. An experimental campaign is being carried out at CREA Laboratory of Politecnico di Milano (Italy) on linear siloxanes MM and MDM and on equimolar mixtures of MM/MDM. Linear siloxanes were chosen due to their wide employment as pure working fluids in high temperature organic Rankine cycles. Early results obtained employing the improved methodology are reported here and compared to those obtained using the comparison of vapor pressure only.
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14:40
20 mins
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Technical Evaluation of Zeotropic Fluid Mixtures in Geothermal ORC Applications
Davide Toselli, Florian Heberle, Dieter Brüggemann
Abstract: The ORC technology demonstrated to be a sustainable and reliable technology to exploit low-temperature sources, such as low-enthalpy geothermal reservoirs. In the last years, numerous studies focused on fluid mixtures in ORC applications. Heberle and Brüggemann (2015) investigated fluid mixtures as working fluid in geothermal ORC applications according to boundary conditions available in Molasse Basin, Southern Germany. The combination isobutane/isopentane appeared as the most promising one. The corresponding thermodynamic analysis revealed how the 90/10 (mole-fraction isobutane/mole-fraction isopentane) composition provides the highest turbine power output. In this work, an extended technical evaluation is provided with regard to the use of isobutane/isopentane mixtures in geothermal ORC power systems. On-design simulations are firstly performed in order to maximize the turbine power output for several mole fraction compositions. The 95/5 composition provides the highest turbine power output, 6.78 % more than pure isobutane. Later, yearly simulations considering the ORC behaviour are performed according to real ambient temperature data in Southern Germany. A comparison between on-design and average annual results is proposed. For the 95/5 composition, the annual average net power is 103 kWel lower than the on-design value. Next to technical criteria also selected economic parameters are calculated: the 70/30 composition provides +7.66 % more in net cash flow than the 95/5. In principal, even though the 95/5 mixture provides the highest annual power production, the 70/30 appears more economically feasible under consideration of yearly ambient temperature profile and the corresponding ORC off-design performance.
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15:00
20 mins
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Potential of Zeotropic Mixtures as Working Fluids in Partial Evaporating Organic Rankine Cycles (PEORC) for Low Temperature Heat Recovery
Anandu Surendran, Satyanarayanan Seshadri
Abstract: Recent studies on new organic Rankine cycle (ORC) architectures shows partial evaporating ORC (PEORC) architecture to be of improved exergetic efficiency than the simple ORC (SORC)[1]. Previous studies on the use of zeotropic working fluids have shown to improve exergetic efficiency of SORCs owing to their temperature glide characteristics[2]. So far only a few studies have focussed on the use of zeotropes in partial evaporating ORCs operating with finite capacity heat sources, where maximum heat extraction is desirable [3]. Using a zeotrope of high-temperature glide and latent heat in PEORC could lead to increased exergetic efficiency due to the combined effect of improved temperature matching and increased heat extraction. The performance of partial evaporating mixtures should be analyzed from a thermodynamic and economic standpoint. In this study, a zeotropic mixture of R245fa/cyclopentane is analyzed with three cycle architectures: SORC, PEORC and Trilateral cycle (TLC).
The effect of vapour fraction and mixture mass fraction on cycle performance is analyzed by optimizing the evaporation temperature for a fixed condenser dew point temperature. PEORC with mixtures achieves maximum heat source utilization at lower vapour fractions. Also, the optimum evaporation temperatures corresponding to maximum power outputs are higher at lower vapour fractions. Thermal efficiency and net power output of mixtures in PEORC is higher than that of pure fluids. Compared to pure fluids, PEORC with mixtures exhibit intermediate volumetric flow ratios, increased heat exchanger UA requirements and expander sizes. For heat source temperatures ranging from 150°C-90°C, the cycles are optimized for maximum power output using Genetic Algorithm. PEORC with R245fa/cyclopentane mixture outperforms SORC with R245fa in terms of net power output by 37-62%, and TLC with R245fa by 2-19%. However, this performance gain results in 2.3-2.8 times higher heat exchanger UA requirements and 1.9-5 times increase in expander volume coefficients than SORC with R245fa. A compound function considering net work output, expander volume coefficient and heat exchanger UA requirements is used to compare various cycles thermo-economically. PEORC with pure R245fa shows the highest thermo-economic performance.
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15:20
20 mins
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Ultra-low GWP Refrigerant Mixtures as Working Fluids in ORC for Waste Heat Recovery
Konstantinos Braimakis, Angelos Mikelis, Antonios Charalampidis, Sotirios Karellas
Abstract: The F-gases regulations of the European Union have introduced restrictions and bans on refrigerants with global warming potential (GWP) above 150 in order to reduce the use of fluorinated gases (such as HFCs) by two-thirds compared to 2014 levels. As a result, significant research efforts have been lately oriented towards ultra-low GWP refrigerants in Vapor Compression and Organic Rankine Cycles (ORC). Meanwhile, the use of zeotropic mixtures in ORCs has been suggested as a promising second law efficiency improvement strategy in ORC applications. Considering the above, in the present study binary zeotropic mixtures of R32 and 6 ultra-low GWP fluids (R1234yf, R1234ze(E), n-pentane, propylene, isobutane and CO2) at variable molar concentration ratios, corresponding to a total of 21 working fluid combinations, are evaluated as working fluids in ORCs for waste heat recovery at heat source temperatures ranging from 100 to 300 °C. In each case, the cycles are optimized with respect to the molar concentration ratio of the mixture components and the evaporation pressure. The relative exergetic efficiency improvement attained by the use of mixture fluid ORCs (ZORCs) compared to the optimized pure fluid ORCs (PORCs) running with their constituent components strongly depends on the mixture and the heat source temperature and typically ranges from a minimum of 0.56 % to a maximum of 62.92 %. For each binary mixture, ZORCs are mostly favourable compared to the PORCs of its components within a region between the critical high critical temperature component (HTC) and LTC critical temperatures. The application of ZORCs running with ultra-low GWP fluids is primarily appealing at very low heat source temperatures (100 and 120 °C), for which the relative exergetic efficiency benefits are maximized. Among the mixtures examined, n-pentane/isobutane exhibits the highest exergetic efficiency within a broad range of heat source temperatures.
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