Heat recovery and conversion technologies with organic fluid cycles: optimal working fluid and system design
Creators: name: Oyewunmi, Oyeniyi Alabi
Publisher: name: Imperial College London
Year: 2018

description: Heat recovery and conversion technologies have become increasingly important for a variety of reasons relating to growing global concerns on energy security, rising electricity costs, CO2 emissions and global warming. Although many energy intensive industries could benefit significantly from the integration of these technologies, the current adoption rate is limited due to the high investment costs involved and the difficulty of prospective investors and end-users to recognise and, ultimately, realise the potential energy savings from such deployment. Thus, the wider adoption of these technologies can be facilitated by improved performance and reduced investment costs. In this context, integrated thermoeconomic optimisation of such power systems through the optimal design of working fluids and operating parameters is invaluable in improving economic viability. The performance of single-component working fluids is limited by the large exergy destruction associated with evaporation and condensation. Thus, working-fluid mixtures promise reduced exergy losses due to their non-isothermal phase-change behaviour, and improved cycle efficiencies and power outputs over their constituent pure fluids. The optimisation of organic Rankine cycle (ORC) systems revealed that mixtures do indeed show a thermodynamic improvement (in terms of the system power output and efficiency) over the pure-fluids. Although systems with fluid mixtures could see up to a 30% increase in power output over those with pure ones, they require larger expansion devices and heat exchangers (evaporators and condensers) due to their deterioration in heat transfer during the phase-change processes; thus, the resulting ORC systems are also associated with higher costs. Hence, ORC systems with pure working fluids have lower plant costs per unit power output, up to 14% lower than those with mixtures, highlighting the importance of considering system cost minimisation in designing ORC plants. The Up-THERM heat converter is a two-phase unsteady heat engine; it contains fewer moving parts than conventional ones and represents an attractive alternative for remote or off-grid power generation. With the aid of a validated first-order lumped dynamic model, its performance with respect to working-fluid selection for its prospective applications is investigated. With the engine's power output and efficiency being conflicting objectives, fluids with low critical temperatures (and high critical pressures, reduced pressures and temperatures) resulted in designs with high power outputs and correspondingly low efficiencies. For a nominal Up-THERM design corresponding to a target application with a heat-source temperature of 360 °C, R113 was identified as the optimal fluid, followed by i-hexane in maximising the power output. The Up-THERM heat converter was also seen to be effective over a range of heat-source temperatures delivering in excess of 10 kW (about four times higher than with water as working fluid in the nominal design) when utilising thermal energy at temperatures above 200 °C. Of all the working fluids considered, ammonia, R245ca, R32, propene and butane feature prominently as optimal and versatile fluids on the developed optimal working-fluid selection maps, delivering high power over a wide range of heat-source temperatures. To facilitate simultaneous optimal working fluid and process design of waste-heat recovery systems, a mixed-integer non-linear programming (MINLP) optimisation framework for the computer aided molecular and process design of heat engines was developed. The components of the framework, consisting of thermodynamic process models, heat-exchanger sizing models, component cost correlations, economic evaluation models, transport property group-contribution correlations and the SAFT-γ Mie group-contribution equation of state are individually and collectively validated to acceptable degrees of accuracy against experimental and available commercial data. Following validation, the framework is used to identify optimal working-fluids for ORC systems with three different industrial waste-heat sources (150, 250 and 350 °C), with different objective functions. Although slightly related, maximising the power output and minimising the specific investment costs (SIC) are not equivalent objectives. From NLP optimisations, n-propane, 2-pentene and 2-hexene were the optimal working fluids when maximising power output, while n-propane, 2-butane and 2-heptene were optimal when SIC is minimised. With MINLP optimisations minimising the SIC, 1,3-butadiene and 4-methyl-2-pentene were the best performing working fluids for the 150 °C and 250 °C heat sources respectively; these novel working fluids do not belong to the common hydrocarbon families assessed in the NLP optimisations. Similarly, with multi-objective cost-power optimisation, the same molecules are identified, striking a delicate balance between investment costs and system performance. Ultimately, the results demonstrate the potential of this framework to drive the search for the next generation of organic Rankine cycle and waste-heat recovery systems, and to provide meaningful insights into identifying the working fluids that represent the optimal choices for targeted applications.