Abstract
In the present paper, we consider the employment of working-fluid mixtures in organic Rankine cycle (ORC) systems with respect to thermodynamic and heat-transfer performance, component sizing and capital costs. The selected working-fluid mixtures promise reduced exergy losses due to their non-isothermal phase-change behaviour, and thus improved cycle efficiencies and power outputs over their respective pure-fluid components. A multi-objective cost-power optimization of a specific low-temperature ORC system (operating with geothermal water at 98 °C) reveals that the use of working-fluid-mixtures does indeed show a thermodynamic improvement over the pure-fluids. At the same time, heat transfer and cost analyses, however, suggest that it also requires larger evaporators, condensers and expanders; thus, the resulting ORC systems are also associated with higher costs. In particular, 50% n-pentane + 50% n-hexane and 60% R-245fa + 40% R-227ea mixtures lead to the thermodynamically optimal cycles, whereas pure n-pentane and pure R-245fa have lower plant costs, both estimated as having ∼14% lower costs per unit power output compared to the thermodynamically optimal mixtures. These conclusions highlight the importance of using system cost minimization as a design objective for ORC plants.
Highlights
Organic Rankine cycle (ORC) power systems are a relatively mature, commercially available technology, that is highly appropriate for the conversion of heat at temperatures below ∼300–400 ◦ C to useful work at power-output scales from a kW to tens of MW [1]
By presenting a method for evaluating the heat-transfer coefficients (HTCs) of working-fluid mixtures, this work aims to explore the effects of using such mixtures on the overall heat transfer processes and component sizing in organic Rankine cycle (ORC) engines, which are important in understanding the role that these fluids play on the overall system performance and cost
We begin with the thermodynamic optimization of an ORC system in the specified application with two sets of working-fluid mixtures: the n-hexane + n-pentane alkane system; and the
Summary
Organic Rankine cycle (ORC) power systems are a relatively mature, commercially available technology, that is highly appropriate for the conversion of heat at temperatures below ∼300–400 ◦ C to useful work at power-output scales from a kW to tens of MW [1]. Both experimental and theoretical studies have been performed into the benefits of employing refrigerant [7,8,9,10], hydrocarbon [11,12] and siloxane [13,14] fluid mixtures, over a range of heat-source temperatures. Excellent second law analyses have shown significant potential benefits [15,16,17] These benefits are especially magnified in applications with limited cooling-water supply, e.g., combined heat and power (CHP) systems where the non-isothermal temperature profiles of the condensing fluid mixtures provide a good thermal match to the temperature profile of the heated cooling stream [5]. Some investigators have begun to develop and apply advanced computer-aided molecular design (CAMD) methodologies [19,20,21,22] with a view towards identifying or designing optimal fluids for ORC applications
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