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Design and optimisation of organic Rankine cycles for waste heat recovery in marine applications using the principles of natural selection Ulrik Larsena,∗, Leonardo Pierobona, Fredrik Haglinda, Cecilia Gabrieliib aDepartment of Mechanical Engineering, Technical University of Denmark, Building 403, Nils Koppels All ́e, 2800 Kgs. Lyngby, Denmark bChalmers University of Technology, Maritime Operations, SE-412 96 Gothenburg, Sweden Abstract Power cycles using alternative working fluids are currently receiving significant attention. Selection of working fluid among many candidates is a key topic and guidelines have been presented. A general problem is that the selection is based on numerous criteria, such as thermodynamic performance, boundary conditions, hazard levels and environmental concerns. A generally applicable methodology, based on the principles of natural selection, is presented and used to determine the optimum working fluid, boiler pressure and Rankine cycle process layout for scenarios related to marine engine heat recovery. Included in the solution domain are 109 fluids in sub- and supercritical processes, and the process is adapted to the properties of the individual fluid. The efficiency losses caused by imposing process constraints are investigated to help propose a suitable process layout. Hydrocarbon dry type fluids in recuperated processes produced the highest efficiencies, while wet and isentropic fluids were superior in non-recuperated processes. The results suggested that at design point, the requirements of process simplicity, low operating pressure and low hazard resulted in cumulative reductions in cycle efficiency. Furthermore, the results indicated that non-flammable fluids were able to produce near optimum efficiency in recuperated high pressure processes. Keywords: Process optimization, organic Rankine cycle, Exhaust heat recovery, Large ships, Genetic algorithm 1. Introduction There is a strong motivation in the marine sector for increasing the propulsion system energy efficiency, mainly because of increasing fuel prices and stricter upcoming reg- ulations. Therefore technologies suitable for converting low grade heat into power are currently being studied. One of the most promising technologies is the organic Rankine cycle (ORC), which is a relatively simple power cycle with good flexibility, in terms of efficient utilization of various heat sources. The main reason is that the working fluid can be selected to suit given temperature conditions of the heat source(s) and sink(s). Selecting the optimum working fluid is a complex task and the topic has received signifi- cant attention in the scientific literature. Recently, Wang et al. [1] presented a method for selection among 13 fluids based on a multi-objective optimisation model. In 2012 Wang et al. [2] presented a study on fluid selection for a small scale ORC plant applied for waste heat recovery from a combustion engine. Seemingly no single fluid can fully meet the numer- ous requirements for the ideal working fluid in an ORC process [3, 4]. Foremost, the fluid should be thermody- namically suitable, such as having appropriate evapora- ∗Principal corresponding author. Tel.: +45 532-503-03 Email address: ular@mek.dtu.dk (Ulrik Larsen) tion and condensation properties. Heat transfer proper- ties, such as viscosity and thermal conductivity, are also highly relevant. Among non-thermodynamic concerns are environmental measures such as Global Warming Poten- tial (GWP), corrosiveness, chemical stability over the rel- evant temperature range, toxicity, flammability, explosive- ness, general industrial acceptance, lubrication properties and cost. Therefore the fluid evaluation process is a mat- ter of finding the candidate that best meets multiple re- quirements, weighted according to their (subjective) im- portance in the application. In the literature, guidelines on fluid selection based on thermodynamic properties have been proposed. A recur- ring focus is the slope of the saturated vapour line in a temperature-entropy fluid property plot, which categorises the fluids as wet, isentropic or dry. In order to avoid low vapour quality in the expander, wet fluids require super- heating in the process, whereas isentropic and dry fluids do not [5]. Dry fluids, however, require an internal heat exchanger (recuperator) in order to avoid wasting the in- herent fluid energy at the outlet of the expander [4]. Also frequently discussed in the literature is the critical point of the fluids. The main advantage of operating at super- critical pressure is that the heat uptake is non-isothermal, thus potentially raising the average temperature during heat uptake and resulting in a higher thermal efficiency [6]. Recently Kuo et al. presented promising results con- 1PDF Image | organic Rankine cycles for waste heat recovery in marine settings
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