A Review of Working Fluids for Organic Rankine Cycle (ORC) Applications

Babatunde, A. F and Sunday, O. O (2018) A Review of Working Fluids for Organic Rankine Cycle (ORC) Applications. IOP conference series :materials science and engineering, 4 (13). pp. 1-13.

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The organic Rankine cycle (ORC) systems are commercially employed for small scale and large scale thermal conversion to electricity of a large variety of abundant heat sources such as exhaust waste heat and some renewable energy sources where conventional steam Rankine and open-gas cycle turbine cycles cannot provide any viable, sustainable, techno-economic solution for power generation. The ORC operates the conventional steam Rankine cycle in subcritical level but can as well operate trans critical cycle while employing heavy molecular organic fluids in place of steam which makes the ORC suitable for medium and low grade heat conversion into electricity. No singular ideal fluid exists for any application and hence over 600 pure and zeotropic mixtures have been investigated by various researches for their best suitability for different applications and operating conditions based on performance characteristics such as efficiency, cost and environmental impacts. In this study a review of working fluids selection for different applications has been conducted. This study helps in identifying the possible most suitable organic fluids for various ORC applications depending on the operating conditions. Keywords: risk assessment; failure mode 1. Introduction Organic Rankine Cycles (ORCs) are the most prevalent low-grade waste heat recovery cycles largely because of their simplicity and readily available components [1]. ORC characteristically performs better than the conventional steam Rankine cycle at the low operating temperatures due to the five factors explained by [2] as highlighted below: • favourable thermodynamic cycle modification/architecture • practical enthalpy drop and volume flow rate in the turbine • favourable operating condition of the turbine • possibility of lower maximum operating pressure and consequently, reduced costs of associated components • the possibility of selecting positive gauge condensing pressure, hence, avoiding air infiltration. Designing the best system configuration alongside selecting the best appropriate choice of working fluid for the viable operating condition is therefore the most essential requirement for any ORC system design for any application [3, 4]. An essential advantage of ORC systems is however, the utilization of organic working fluids that can simply employ a single stage expander hence offering advantages of simple design, low cost capital cost and low maintenance [3]. Because of relatively low specific enthalpy drop of organic vapours in turbine compared to water vapour, efficient ORC systems as small as 25 kW utilizing single stage turbo-expanders with reasonable tip speed have been possible. Whereas, water vapour process in most cases require three or four stage turbine with practical minimum size limited to 2 MW [5]. On the whole, the Organic Rankine Cycle units are most appropriate and most widely used for medium and low grade heat sources with wider flexibilities of reconfiguring and modifying the primary cycle to adapt it to the peculiarities of the heat sources [6]. However, the optimal cycle performance and system architecture primarily depends on the selection of the best appropriate working fluid [7]. The use of optimally mixed zeotropic fluids in place of customary pure fluids is a remarkable development to further improve the cycle performance but rather increases the complexity of the system. Pure organic fluids have fixed boiling temperatures and this gives rise to an incompatibility between the temperature profile of the working fluid and that of the heat source [3]. Two pure organic fluids with sufficient difference in their boiling points when optimally mixed produces a temperature glide at phase change which provides a better temperature compatibilities in aimed at either

Item Type: Article
Subjects: T Technology > TJ Mechanical engineering and machinery
Depositing User: Mr Adedamola Bameke
Date Deposited: 30 Jul 2020 18:34
Last Modified: 30 Jul 2020 18:34
URI: http://eprints.federalpolyilaro.edu.ng/id/eprint/981

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