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

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Abstract

For zeotropic mixtures, the temperature varies during phase change, which is opposed to the isothermalphase change of pure fluids. The use of such mixtures as working fluids in organic Rankine cyclepower plants enables a minimization of the mean temperature difference of the heat exchangers whenthe minimum pinch point temperature difference is kept fixed. A low mean temperature differencemeans low heat transfer irreversibilities, which is beneficial for cycle performance, but it also results inlarger heat transfer surface areas. Moreover, the two-phase heat transfer coefficients for zeotropic mixturesare usually degraded compared to an ideal mixture heat transfer coefficient linearly interpolatedbetween the pure fluid values. This entails a need for larger and more expensive heat exchangers. Previousstudies primarily focus on the thermodynamic benefits of zeotropic mixtures by employing firstand 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 evaluatingthe economic feasibility of zeotropic mixtures compared to pure fluids. We carry out a multi-objectiveoptimization of the net power output and the component costs for organic Rankine cycle power plantsusing low-temperature heat at 90 C to produce electrical power at around 500 kW. The primary outcomesof 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 powerthan R32 at the same total cost of 1200 k$.
Original languageEnglish
Title of host publicationProceedings of the 3rd International Seminar on ORC Power Systems
EditorsV. Lemort, S. Quoilin, M. De Paepe, M. van den Broek
Number of pages11
Publication date2015
Article number32
Publication statusPublished - 2015
Event3rd International Seminar on ORC Power Systems - Brussels, Belgium
Duration: 12 Oct 201514 Oct 2015
http://www.asme-orc2015.be/

Seminar

Seminar3rd International Seminar on ORC Power Systems
CountryBelgium
CityBrussels
Period12/10/201514/10/2015
Internet address

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