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

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

A two-phase single-reciprocating-piston heat conversion engine: Non-linear dynamic modelling

A non-linear dynamic framework is presented for the modelling of a novel two-phase heat engine termed ‘Up-THERM’, which features a single solid moving-part (piston). When applied across the device, a constant temperature difference between an external (low- to medium-grade) heat source and an external heat sink is converted into sustained and persistent oscillations of pressure and volumetric fluid displacement. These oscillations are transformed in a load arrangement into a unidirectional flow from which power is extracted by a hydraulic motor. The Up-THERM engine is modelled using a system of first-order differential equations that describe the dominant thermal/fluid processes in each component of the device. For certain components where the deviations from a linear approximation are non-negligible (gas spring in the displacer cylinder, check valves and piston valve, and heat exchangers), a non-linear description is employed. A comparison between the linear and non-linear descriptions of the gas spring at the top of the displacer cylinder reveals that the non-linear description results in more realistic predictions of the oscillation frequency compared to experimental data from a similar device. Furthermore, the shape of the temperature profile over the heat-exchanger surfaces is modelled as following a hyperbolic tangent function, based on findings from an experimental investigation. Following the validation of these important device components, a parametric study is performed on the Up-THERM engine model with the aforementioned non-linear component descriptions, aimed at investigating the effects of important geometric parameters and of the heat-source temperature on key performance indicators, namely the oscillation frequency, power output and exergy efficiency of the engine. The results indicate that the geometric design of the displacer cylinder, including the height of the gas spring at the top of the cylinder, and the heat-source temperature have the most significant influence on the performance of the engine. A maximum exergy efficiency of 2.8% and a maximum power output of 175 W are observed at the proposed operating temperature of 450 °C for a nominal Up-THERM design (based on the physical dimensions of a device prototype and water as the working fluid; the role of the working fluid is explored in follow-up paper Ref. [1]) but with shorter displacer cylinder gas-spring lengths relative to a nominal design. The results and insight can assist the further development of this technology, in particular as a prime mover in combined heat and power applications

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

A thermo-economic assessment and comparison of the Up-THERM heat converter and an organic Rankine cycle engine

In this paper we present a thermodynamic and economic comparison of a recently proposed two-phase thermofluidic oscillator known as the Up-THERM heat converter and the more established organic Rankine cycle (ORC) engine, when converting heat at temperatures below 150 °C using the refrigerant R-227ea as the working fluid. The Up-THERM heat converter is being considered as a possible prime mover for small- to medium-scale combined heat and power (CHP) applications. Using suitable thermodynamic models of both systems, it is found that the power output and thermal efficiencies of a pre-specified Up-THERM design are generally lower than those of an equivalent ORC engine. The Up-THERM, however, also demonstrates higher exergy efficiencies and is associated with lower capital costs, as expected owing to its simple construction and use of fewer and more basic components. Interestingly, the specific costs (per rated kW) of the ORC engine are lower than those of the Up-THERM converter at lower heat source temperatures, specifically below 130 °C, whereas the Up-THERM becomes a more cost effective alternative (in terms of the specific cost) to the ORC engine at higher temperatures

A THERMO-ECONOMIC COMPARISON OF THE UP-THERM HEAT CONVERTER AND AN ORGANIC RANKINE CYCLE HEAT ENGINE

In this paper we compare a recently proposed two-phase thermofluidic oscillator device termed ‘Up-THERM’ to a basic (sub-critical, non-regenerative) equivalent organic Rankine cycle (ORC) engine. In the Up-THERM heat converter, a constant temperature difference imposed by an external heat source and sink leads to periodic evaporation and condensation of the working fluid, which gives rise to sustained oscillations of pressure and volumetric displacement. These oscillations are converted in a load arrangement into a unidirectional flow, which passes through a hydraulic motor that extracts useful work from the device. A pre-specified Up-THERM design is being considered in a selected application with two n-alkanes, n-hexane and n-heptane, as potential working fluids. One aim of this work is to evaluate the potential of this proposed design. The thermodynamic comparison shows that the ORC engine outperforms the Up-THERM heat converter in terms of power output and thermal efficiency, as expected. An economic comparison, however, reveals that the capital costs of the Up-THERM are lower than those of the ORC engine. Nevertheless, the specific costs (per unit power) favour the ORC engine due to its higher power output. Some aspects of the proposed Up-THERM design are identified for improvemen

Performance of working-fluid mixtures in ORC-CHP systems for different heat-demand segments and heat-recovery temperature levels

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