EFFECT OF WORKING-FLUID MIXTURES ON ORGANIC RANKINE CYCLE SYSTEMS: HEAT TRANSFER AND COST ANALYSIS

The present paper considers the employment of working-fluid mixtures in organic Rankine cycle (ORC) systems with respect to heat transfer performance, component sizing and costs, using two sets of fluid mixtures: n-pentane + n-hexane and R-245fa + R-227ea. Due to their non-isothermal phase-change behaviour, these zeotropic working-fluid mixtures promise reduced exergy losses, and thus improved cycle efficiencies and power outputs over their respective pure-fluid components. Although the fluid-mixture cycles do indeed show a thermodynamic improvement over the pure-fluid cycles, the heat transfer and cost analyses reveal that they require larger evaporators, condensers and expanders; thus, the resulting ORC systems are also associated with higher costs, leading to possible compromises. In particular, 70 mol% n-pentane + 30 mol% n-hexane and equimolar R-245fa + R-227ea mixtures lead to the thermodynamically optimal cycles, whereas pure n-pentane and pure R-227ea have lower costs amounting to 14% and 5% per unit power output over the thermodynamically optimal mixtures, respectively

Towards the computer-aided molecular design of organic rankine cycle systems with advanced fluid rheories

Organic Rankine cycle (ORC) power-generation systems are increasingly being deployed for heat recovery and conversion from geothermal reservoirs and in several industrial settings. Using a case study of an exhaust flue-gas stream, an ORC power output in excess of 20 MW is predicted at thermal efficiencies ranging between 5% and 15%. The considerable influence on cycle performance of the choice of the working fluid is illustrated with alkane and perfluoroalkane systems modelled using the SAFT-VR Mie equation of state (EoS); in general, the more-volatile pure components (n-butane or n-perfluorobutane) are preferred although some mixtures perform better at restricted cycle conditions. The development of computer-aided molecular design (CAMD) platforms for ORC systems requires both cycle and working-fluid models to be incorporated into a single framework, for the purposes of whole-system design and optimization. Using pure alkanes and their mixtures as a case study, we test the suitability of the recent group-contribution SAFT- Mie EoS method for describing the thermodynamic properties of working fluids relevant to the analysis of ORC systems. The theory is shown to predict accurately the relevant properties of these fluids, thereby suggesting that this SAFT-based CAMD approach is a promising approach towards working-fluid design of ORC power systems

Working-fluid selection and performance investigation of a two-phase single-reciprocating-piston heat-conversion engine

We employ a validated first-order lumped dynamic model of the Up-THERM converter, a two-phase unsteady heat-engine that belongs to a class of innovative devices known as thermofluidic oscillators, which contain fewer moving parts than conventional engines and represent an attractive alternative for remote or off-grid power generation as well as waste-heat recovery. We investigate the performance the Up-THERM with respect to working-fluid selection for its prospective applications. An examination of relevant working-fluid thermodynamic properties reveals that the saturation pressure and vapour-phase density of the fluid play important roles in determining the performance of the Up-THERM – the device delivers a higher power output at high saturation pressures and has higher exergy efficiencies at low vapour-phase densities. Furthermore, working fluids with low critical temperatures, high critical pressures and exhibiting high values of reduced pressures and temperatures result in designs with high power outputs. For a nominal Up-THERM design corresponding to a target application with a heat-source temperature of 360 ◦C, water is compared with forty-five other pure working fluids. When maximizing the power output, R113 is identified as the optimal fluid, followed by i-hexane. Fluids such as siloxanes and heavier hydrocarbons are found to maximize the exergy and thermal efficiencies. The ability of the Up-THERM to convert heat over a range of heat-source temperatures is also investigated, and it is found that the device can deliver in excess of 10 kW when utilizing thermal energy at temperatures above 200 ◦C. Of all the working fluids considered here, ammonia, R245ca, R32, propene and butane feature prominently as optimal and versatile fluids delivering high power over a wide range of heat-source temperatures

Thermo-Economic and Heat Transfer Optimization of Working-Fluid Mixtures in a Low-Temperature Organic Rankine Cycle System

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

Comparison of a novel organic-fluid thermofluidic heat converter and an organic Rankine cycle heat engine

The Up-THERM heat converter is an unsteady, two-phase thermofluidic oscillator that employs an organic working fluid, which is currently being considered as a prime-mover in small- to medium-scale combined heat and power (CHP) applications. In this paper, the Up-THERM heat converter is compared to a basic (sub-critical, non-regenerative) organic Rankine cycle (ORC) heat engine with respect to their power outputs, thermal efficiencies and exergy efficiencies, as well as their capital and specific costs. The study focuses on a pre-specified Up-THERM design in a selected application, a heat-source temperature range from 210 °C to 500 °C and five different working fluids (three n-alkanes and two refrigerants). A modeling methodology is developed that allows the above thermo-economic performance indicators to be estimated for the two power-generation systems. For the chosen applications, the power output of the ORC engine is generally higher than that of the Up-THERM heat converter. However, the capital costs of the Up-THERM heat converter are lower than those of the ORC engine. Although the specific costs (£/kW) of the ORC engine are lower than those of the Up-THERM converter at low heat-source temperatures, the two systems become progressively comparable at higher temperatures, with the Up-THERM heat converter attaining a considerably lower specific cost at the highest heat-source temperatures considered

Performance comparison of a novel thermofluidic organic-fluid heat converter and an organic rankine cycle heat engine

The Up-THERM engine is a novel two-phase heat engine with a single moving part–a vertical solid piston–that relies on the phase change of a suitable working fluid to produce a reciprocating displacement and sustained thermodynamic oscillations of pressure and flow rate that can be converted to useful work. A model of the Up-THERM engine is developed via lumped dynamic descriptions of the various engine sub-components and electrical analogies founded on previously developed thermoacoustic principles. These are extended here to include a description of phase change and non-linear descriptions of selected processes. The predicted first and second law efficiencies and the power output of a particular Up- THERM engine design aimed for operation in a specified CHP application with heat source and sink temperatures of 360 ○C and 10 ○C, are compared theoretically to those of equivalent sub-critical, nonregenerative organic Rankine cycle (ORC) engines. Five alkanes (from n-pentane to n-nonane) are being considered as possible working fluids for the aforementioned Up-THERM application, and these are also used for the accompanying ORC thermodynamic analyses. Owing to its mode of operation, lack of moving parts and dynamic seals, the Up-THERM engine promises a simpler and more cost-effective solution than an ORC engine, although the Up-THERM is expected to be less efficient than its ORC counterpart. These expectations are confirmed in the present work, with the Up-THERM engine showing lower efficiencies and power outputs than equivalent ORC engines, but which actually approach ORC performance at low temperatures. Therefore, it is suggested that the Up-THERM can be a competitive alternative in terms of cost per unit power in low-power/temperature applications, especially in remote, off-grid settings, such as in developing countries where minimising upfront costs is crucial

Performance of working-fluid mixtures in an ORC-CHP system for different heat demand segments

Organic Rankine cycle (ORC) power systems are being increasingly deployed for waste heat recovery and conversion to power in several industrial settings. In the present paper, we investigate the use of working-fluid mixtures in ORC systems operating in combined heat and power mode (ORC-CHP) with shaft power provided by the expander/turbine and heating provided by the cooling-water exiting the condenser. The waste-heat source is a flue gas stream from a refinery boiler with a mass flow rate of 560 kg/s and an inlet temperature of 330 °C. When using working fluids comprising normal alkanes, refrigerants and their subsequent mixtures, the ORC-CHP system is demonstrated as being capable of delivering over 20 MW of net shaft power and up to 15 MW of heating, leading to a fuel energy savings ratio (FESR) in excess of 20%. Single-component working fluids such as pentane appear optimal at low hot-water supply temperatures, and fluid mixtures become optimal at higher temperatures, with the combination of octane and pentane giving an ORC-CHP system design with the highest efficiency. The influence of heat demand intensity on the global system conversion efficiency and optimal working fluid selection is also explored

Thermodynamic Optimization of Recuperative Sub- and Transcritical Organic Rankine Cycle Systems

There is significant interest in the deployment of organic Rankine cycle (ORC) technology for waste-heat recovery and power generation in industrial settings. This study considers ORC systems optimized for maximum power generation using a case study of an exhaust flue-gas stream at a temperature of 380 °C as the heat source, covering over 30 working fluids and also considering the option of featuring a recuperator. Systems based on transcritical cycles are found to deliver higher power outputs than subcritical ones, with optimal evaporation pressures that are 4-5 times the critical pressures of refrigerants and light hydrocarbons, and 1-2 times those of siloxanes and heavy hydrocarbons. For maximum power production, a recuperator is necessary for ORC systems with constraints imposed on their evaporation and condensation pressures. This includes, for example, limiting the minimum condensation pressure to atmospheric pressure to prevent subatmospheric operation of this component, as is the case when employing heavy hydrocarbon and siloxane working fluids. For scenarios where such operating constraints are relaxed, the optimal cycles do not feature a recuperator, providing some capital cost savings, with some cycles showing more than three times the generated power than with this component, making investments in sub-atmospheric components worthwhile

Integrated Computer-Aided Working-Fluid Design and Power System Optimisation: Beyond Thermodynamic Modelling

Improvements in the thermal and economic performance of organic Rankine cycle (ORC) systems are required before the technology can be successfully implemented across a range of applications. The integration of computer-aided molecular design (CAMD) with a process model of the ORC facilitates the combined optimisation of the working-fluid and the power system in a single modelling framework, which should enable significant improvements in the thermodynamic performance of the system. However, to investigate the economic performance of ORC systems it is necessary to develop component sizing models. Currently, the group-contribution equations of state used within CAMD, which determine the thermodynamic properties of a working-fluid based on the functional groups from which it is composed, only derive the thermodynamic properties of the working-fluid. Therefore, these do not allow critical components such as the evaporator and condenser to be sized. This paper extends existing CAMD-ORC thermodynamic models by implementing group-contribution methods for the transport properties of hydrocarbon working-fluids into the CAMD-ORC methodology. Not only does this facilitate the sizing of the heat exchangers, but also allows estimates of system costs by using suitable cost correlations. After introducing the CAMD-ORC model, based on the SAFT-γ Mie equation of state, the group-contribution methods for determining transport properties are presented alongside suitable heat exchanger sizing models. Finally, the full CAMD-ORC model incorporating the component models is applied to a relevant case study. Initially a thermodynamic optimisation is completed to optimise the working-fluid and thermodynamic cycle, and then the component models provide meaningful insights into the effect of the working-fluid on the system components