Performance of working-fluid mixtures in ORC-CHP systems for different heat-demand segments and heat-recovery temperature levels
File(s)
Published version
Author(s)
Oyewunmi, OA
Kirmse, CJW
Pantaleo, AM
Markides, CN
Type
Journal Article
Abstract
In this paper, we investigate the adoption of working-fluid mixtures in ORC systems operating in combined heat and power (CHP) mode, with a power output provided by the expanding working fluid in the ORC turbine and a thermal energy output provided by the cooling water exiting (as a hot-water supply) the ORC condenser. We present a methodology for selecting optimal working-fluids in ORC systems with optimal CHP heat-to-electricity ratio and heat-supply temperature settings to match the seasonal variation in heat demand (temperature and intermittency of the load) of different end-users. A number of representative industrial waste-heat sources are considered by varying the ORC heat-source temperature over the range 150–330 °C. It is found that, a higher hot-water outlet temperature increases the exergy of the heat-sink stream but decreases the power output of the expander. Conversely, a low outlet temperature (~30 °C) allows for a high power-output, but a low cooling-stream exergy and hence a low potential to heat buildings or to cover other industrial thermal-energy demands. The results demonstrate that the optimal ORC shaft-power outputs vary considerably, from 9 MW up to 26 MW, while up to 10 MW of heating exergy is provided, with fuel savings in excess of 10%. It also emerges that single-component working fluids such as n-pentane appear to be optimal for fulfilling low-temperature heat demands, while working-fluid mixtures become optimal at higher heat-demand temperatures. In particular, the working-fluid mixture of 70% n-octane + 30% n-pentane results in an ORC-CHP system with the highest ORC exergy efficiency of 63% when utilizing 330 °C waste heat and delivering 90 °C hot water. The results of this research indicate that, when optimizing the global performance of ORC-CHP systems fed by industrial waste-heat sources, the temperature and load pattern of the cogenerated heat demand are crucial factors affecting the selection of the working fluid.
Date Issued
2017-07-21
Date Acceptance
2017-05-31
Citation
Energy Conversion and Management, 2017, 148, pp.1508-1524
ISSN
0196-8904
Publisher
Elsevier
Start Page
1508
End Page
1524
Journal / Book Title
Energy Conversion and Management
Volume
148
Copyright Statement
© 2017 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Sponsor
Engineering & Physical Science Research Council (EPSRC)
Identifier
PII: S0196-8904(17)30542-3
Grant Number
EP/P004709/1
Subjects
organic Rankine cycle (ORC)
combined heat and power (CHP)
heat recovery
working-fluid mixtures
thermal energy demand
Publication Status
Published