Thermo-Economic and Heat Transfer Optimization of Working-Fluid Mixtures in a Low-Temperature Organic Rankine Cycle System
File(s)
Published version
Accepted version
Author(s)
Oyewunmi, OA
Markides, C
Type
Journal Article
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.
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.
Date Issued
2016-06-09
Date Acceptance
2016-05-27
Citation
Energies, 2016, 9
ISSN
1996-1073
Publisher
MDPI
Journal / Book Title
Energies
Volume
9
Copyright Statement
© 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
article distributed under the terms and conditions of the Creative Commons Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
License URL
Sponsor
Engineering & Physical Science Research Council (EPSRC)
Grant Number
EP/J006041/1
Subjects
09 Engineering
02 Physical Sciences
Publication Status
Published
Article Number
448