Abstract
The wider adoption of organic Rankine cycle (ORC) technology for power generation or cogeneration from renewable or recovered waste-heat in many applications can be facilitated by improved thermodynamic performance, but also reduced investment costs. In this context, it is suggested that the further development of ORC power systems should be guided by combined thermoeconomic assessments that can capture directly the trade-offs between performace and cost with the aim of proposing solutions with high resource-use efficiency and, importantly, improved economic viability. This paper couples, for the first time, the computer-aided molecular design (CAMD) of the ORC working-fluid based on the statistical associating fluid theory (SAFT)-γ Mie equation of state with thermodynamic modelling and optimisation, in addition to heat-exchanger sizing models, component cost correlations and thermoeconomic assessments. The resulting CAMD-ORC framework presents a novel and powerful approach with extended capabilities that allows the thermodynamic optimisation of the ORC system and working fluid to be performed in a single step, thus removing subjective and pre-emptive screening criteria that exist in conventional approaches, while also extending to include cost considerations relating to the resulting optimal systems. Following validation, the proposed framework is used to identify optimal cycles and working fluids over a wide range of conditions characterised by three different heat-source cases with temperatures of 150 °C, 250 °C and 350 °C, corresponding to small- to medium-scale applications. In each case, the optimal combination of ORC system design and working fluid is identified, and the corresponding capital costs are evaluated. It is found that fluids with low specific-investment costs (SIC) are different to those that maximise the power output. The fluids with the lowest SIC are isoheptane, 2-pentene and 2-heptene, with SICs of £5620, £2760 and £2070 per kW respectively, and corresponding power outputs of 32.9 kW, 136.6 kW and 213.9 kW.
Highlights
Despite the growing interest in the utilisation of renewable and sustainable thermal-energy sources, as well as in improving energy efficiency and reducing fossil-fuel consumption and our impact on the environment, there remains a lack of widespread deployment of relevant technologies and a significant amount of waste heat that is currently rejected to the atmosphere
From the perspective of an end-user, technical solutions are required that are both environmentally friendly and economically feasible. This demands the identification of both novel fluids that meet all legislative requirements, and organic Rankine cycle (ORC) systems that are optimised in terms of economic performance indicators such as the net-present value (NPV) or the levelised cost of energy (LCOE)
The identification of new working fluids that can improve thermodynamic performance and reduce system costs while meeting increasingly restrictive environmental legislation, along with the determination of novel and optimal ORC system architectures and designs based on combined thermoeconomic performance indicators are key steps toward improving the economic viability of ORC technology and enabling its widespread uptake for power generation or cogeneration from renewable or recovered waste-heat in many applications
Summary
Despite the growing interest in the utilisation of renewable and sustainable thermal-energy sources, as well as in improving energy efficiency and reducing fossil-fuel consumption and our impact on the environment, there remains a lack of widespread deployment of relevant technologies and a significant amount of waste heat that is currently rejected to the atmosphere. If an equation of state is available that can predict thermodynamic properties based on the functional groups from which it is composed, the molecular structure of the working fluid can be simultaneously optimised alongside the ORC system In this sense, CAMD-ORC models have the potential to identify novel working-fluids which may otherwise be overlooked, whilst removing pre-emptive and subjective screening criteria. The authors of the current paper have previously developed a CAMD-ORC framework, based on the SAFT-g Mie groupcontribution equation of state [49] In this previous work, empirical group-contribution transport property prediction methods for hydrocarbon working fluids were validated against NIST REFPROP [50].
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