11:00
Session 1: Working fluids
Chair: Matthias Lampe
11:00
20 mins
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THERMAL STABILITY OF HEXAMETHYLDISILOXANE (MM) FOR HIGH TEMPERATURE APPLICATIONS
Markus Preißinger, Dieter Brüggemann
Abstract: The design of efficient ORC units for the usage of industrial waste heat at high temperatures requires direct evaporating systems without an intermediate thermal oil circuit. Therefore, the thermal stability of high temperature working fluids gains importance. In this study, the thermal degradation of hexamethyldisiloxane (MM) is investigated in an electrically heated tube. The results include qualitative remarks on degradation products as well as the annual degradation rate as quantitative parameter. It is shown that MM is stable up to a temperature of 300 °C with annual degradation rates of less than 3 %. Furthermore, the break of a silicon-carbon bond can be a main chemical reaction that influences the thermal degradation. Finally, the impact of the results on the future design of ORC units is discussed.
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11:20
20 mins
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MULTI-OBJECTIVE OPTIMIZATION OF ORGANIC RANKINE CYCLE POWER PLANTS USING PURE AND MIXED WORKING FLUIDS
Jesper Graa Andreasen, Martin Ryhl Kærn, Leonardo Pierobon, Ulrik Larsen, Fredrik Haglind
Abstract: For zeotropic mixtures, the temperature varies during phase change, which is opposed to the isothermal phase change of pure fluids. The use of such mixtures as working fluids in organic Rankine cycle power plants enables a minimization of the mean temperature difference of the heat exchangers when the minimum pinch point temperature difference is kept fixed. A low mean temperature difference means low heat transfer irreversibilities, which is beneficial for cycle performance, but it also results in larger heat transfer surface areas. Moreover, the two-phase heat transfer coefficients for zeotropic mixtures are usually degraded compared to an ideal mixture heat transfer coefficient linearly interpolated between the pure fluid values. This entails a need for larger and more expensive heat exchangers. Previous studies primarily focus on the thermodynamic benefits of zeotropic mixtures by employing first and second law analyses. In order to assess the feasibility of using zeotropic mixtures, it is, however, important to consider the additional costs of the heat exchangers. In this study, we aim at evaluating the economic feasibility of zeotropic mixtures compared to pure fluids. We carry out a multi-objective optimization of the net power output and the component costs for organic Rankine cycle power plants using low-temperature heat at 90 C to produce electrical power at around 500 kW. The primary outcomes of the study are Pareto fronts, illustrating the power/cost relations for R32, R134a and R32/R134a (0.65/0.35mole). The results indicate that R32/134a is the best of these fluids, with 3.4 % higher net power than R32 at the same total cost of 1200 k$.
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11:40
20 mins
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EFFECT OF WORKING-FLUID MIXTURES ON ORGANIC RANKINE CYCLE SYSTEM: HEAT TRANSFER AND COST ANALYSIS
Oyeniyi Oyewunmi, Christos Markides
Abstract: The present paper considers the employment of working-fluid mixtures in organic Rankine cycle (ORC) systems with respect to heat transfer performance, component sizing and costs, using two sets of fluid mixtures: n-pentane + n-hexane and R-245fa + R-227ea. Due to their non-isothermal phase-change behaviour, these zeotropic working-fluid mixtures promise reduced exergy losses, and thus improved cycle efficiencies and power outputs over their respective pure-fluid components. Although the fluid-mixture cycles do indeed show a thermodynamic improvement over the pure-fluid cycles, the heat transfer and cost analyses reveal that they require larger evaporators, condensers and expanders; thus, the resulting ORC systems are also associated with higher costs, leading to possible compromises. In particular, 70 mol% n-pentane + 30 mol% n-hexane and equimolar R-245fa + R-227ea mixtures lead to the thermodynamically optimal cycles, whereas pure n-pentane and pure R-227ea have lower costs amounting to 14% and 5% per unit power output over the thermodynamically optimal mixtures, respectively.
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12:00
20 mins
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INTEGRATED DESIGN OF WORKING FLUID MIXTURES AND ORGANIC RANKINE CYCLES (ORC) IN THE CONTINUOUS-MOLECULAR TARGETING (COMT) FRAMEWORK
Matthias Lampe, Peter Edel, Johannes Schilling, Joachim Gross, André Bardow
Abstract: Organic Rankine Cycles (ORCs) provide power by exploiting low-temperature heat of renewable sources or waste heat. To enhance the efficiency of ORCs, binary mixtures have been proposed as working fluids. Using a working fluid mixture leads to a temperature glide during evaporation and condensation and thus to a better match between the temperature profile of the heat source and the working fluid. We present a method for the integrated optimization the working fluid mixture, i.e., its components and its composition, and the ORC process parameters. Mixture properties are calculated by the PC-SAFT equation of state. In our design framework, the so-called continuous-molecular targeting (CoMT), the pure component parameters are relaxed in the optimization to allow for a simultaneous optimization of the working fluid mixture and the process. However, the resulting optimal mixture components do in general not coincide with any real fluid. Real fluids are identified in the second step of the CoMT framework, the structure-mapping. In this paper, only the CoMT optimization is employed to quantify the potential benefit of working fluid mixtures. The results show that mixtures are not always beneficial and that their benefit depends on the conditions under which the ORC system is finally installed.
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12:20
20 mins
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A REVIEW OF POTENTIAL WORKING FLUIDS FOR LOW TEMPERATURE ORGANIC RANKINE CYCLES IN WASTE HEAT RECOVERY
Konstantinos Kontomaris, Jason Juhasz, Luke D. Simoni, Claus-Peter Keller
Abstract: The focus of this paper will be specific to working fluids for use in various technologies for waste heat recovery (WHR) of exhaust heat including internal combustion engines (ICE) and in the use of Organic Rankine Cycles (ORC). Several novel fluids have been developed (DR-2 or HFO-1336mzz(Z) and DR-12) which have a good potential fit for these low temperature heat recovery applications (up to 250oC) and they have been characterized as having desirable working fluid properties such as good safety classification and environmental footprint. Additional properties from an ORC system, where mechanical systems are incorporated, are good thermal stability, chemical compatibility, material compatibility and thermodynamic performance. These systems must be reliable and therefore the interactions with the working fluids are paramount as design basis becomes an important attribute in the development of ORC components. The aforementioned HFO fluids will be assessed on the criteria mentioned to help identify their candidacy in using them in heat recovery technology platform, where interest is specifically ORC based. These novel HFO fluids provide a good alternative to existing working fluids currently under consideration with an added advantage of meeting low GWP regulations.
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