Low Temperature Differential Research Articles

Operation of a low-temperature differential heat engine for power generation via hybrid nanogenerators

This work aims for the exploitation of low-grade thermal energy (<100 °C) in conjunction with the operation of nanogenerators run by a highly responsive low-temperature differential (LTD) heat engine. Two different types of nanogenerators were fabricated and tested in four different schemes: triboelectric in non-contact sliding mode (TENG), piezoelectric in contact-separation mode (PENG), triboelectric in contact-separation mode (TENG-2), and coupled triboelectric and piezoelectric in contact-separation mode (TENG-PENG). A series of tests were performed in generating power from the coupled action of triboelectric and piezoelectric nanogenerators with the operation of a LTD Stirling engine to harness low-grade thermal energy. This stands out as compared to previous studies from the perspective of operating two different types of nanogenerators in two different modes at the same time and the exploitation of low-grade thermal energy rather than the ambient mechanical energy, which is witnessed in most accomplishments in the relevant area. Running the triboelectric nanogenerator (non-contact sliding mode) with a small LTD heat engine (MM-7 Stirling engine) delivered a maximum output voltage of 35 V for a temperature difference of 73.2 °C. Meanwhile, the piezoelectric, triboelectric, and hybridized triboelectric-piezoelectric (contact-separation mode) nanogenerator produced output voltages of 4 V, 20.1 V, and 40 V, respectively. A maximum combined voltage of 74 V was also measured when the output of the triboelectric generator in noncontact sliding mode was combined with the hybrid (triboelectric-piezoelectric) nanogenerator operating in contact-separation mode. Operating the nanogenerators in conjunction with an electromagnetic generator (EMG) was also tested as appropriate, which clearly demonstrates the potential of their application in a hybrid manner if needed.

Quasilinear irreversible thermodynamics of a low-temperature-differential kinematic Stirling heat engine.

Low-temperature-differential (LTD) Stirling heat engines are able to operate with a small temperature difference between low-temperature heat reservoirs that exist in our daily lives, and thus they are considered to be an important sustainable energy technology. The author recently proposed a nonlinear dynamics model of an LTD kinematic Stirling heat engine to study the rotational mechanism of the engine [Y. Izumida, Europhys. Lett. 121, 50004 (2018)EULEEJ0295-507510.1209/0295-5075/121/50004]. This paper presents our study of the nonequilibrium thermodynamics analysis of this engine model, where a load torque against which the engine does work is introduced. We demonstrate that the engine's rotational state is in a quasilinear response regime where the thermodynamic fluxes show a linear dependence on the thermodynamic forces. Significantly, it is found that the response coefficients of the quasilinear relations are symmetric, which is similar to Onsager symmetry in linear irreversible thermodynamics. Based on these relations, we formulate the maximum efficiency of the engine. We also elucidate that the symmetry of the quasilinear response coefficients emerges by reflecting the (anti-)reciprocity of the Onsager kinetic coefficients identified for the relaxation dynamics of the engine in the vicinity of an equilibrium state. We expect that the present study will pave the way for developing nonequilibrium thermodynamics of autonomous heat engines described as a nonlinear dynamical system.

Power Generation from a Hybrid Generator (TENG-EMG) Run by a Thermomagnetic Engine Harnessing Low Temperature Waste Heat

This work explored the scavenging of low temperature waste heat and conversion of it into electrical energy through the operation of a gadolinium (Gd) based thermomagnetic engine. Gd is one of the unique materials whose magnetic property changes from ferromagnetic to paramagnetic depending on the temperature (“the Curie temperature”), which is around 20 °C. In the present work, two different types of generators were designed and applied to the rotating shaft of a Gd-based thermomagnetic engine developed for low temperature differential (LTD) applications. Of these, one is the so-called triboelectric nanogenerator (TENG), and the other is the electromagnetic generator (EMG). These have been designed to produce electricity from the rotating shaft of the thermomagnetic engine, exploiting both the electromagnetic and triboelectric effects. When operated at a rotational speed of 251 rpm with a temperature difference of 45 °C between the hot and cold water jets, the hybrid (TENG-EMG) generator produced a combined pulsating DC open circuit voltage of 5 V and a short circuit current of 0.7 mA. The hybrid generator effectively produced a maximum output power of 0.75 mW at a loading resistance of 10 kΩ.

Theoretical Analysis of Hybrid Dense-Array Concentrator Photovoltaic and Stirling Engine System

Abstract In this paper, we analyse the performance of newly proposed hybrid dense-array concentrator photovoltaic (CPV) and gamma type low temperature differential (LTD) Stirling engine. Schmidt Analysis was applied to compute the performance of LTD Stirling engine while computational fluid dynamic was employed to simulate the heat transfer in the CPV receiver. The simulated result was subsequently used to determine the power conversion efficiency of the hybrid system. Three major parameters have been considered to optimize the overall system performance including water inlet temperature, water flow rate, and CPV surface temperature. In our theoretical analysis, the highest efficiency of the hybrid system ranging 37.8 - 38.0 % can be achieved at water inlet temperature of 90°C. It has shown significant improvement by producing 10.1 -10.7 % extra energy as compared to the efficiency of CPV system without waste heat recovery.

Development of Stirling Pump mechanism for Irrigation in remote rural farms

Inadequate supply of safe water, the requirement of food exports, Low GDP creates a scenario where 26% of the population in the Sub-Saharan Africa suffer from food insecurity. Cultivation of food locally is necessary to drive the cost of food down. This can be done using the abundant groundwater source in the region, however, the energy cost of harvesting the water supply is very high and hence not presently utilised. While solar PV technology is readily available, PV solutions are much too expensive for small scale rural farmers. This research aims at developing a solar thermal powered Stirling pump that can be produced locally, using local resources to reduce irrigation costs and provide income, food and energy security in the region, by promoting independent farming through small-scale farms. In order to develop the Stirling pump, a Low Temperature Differential (LTD) Stirling engine has been modelled in 3D, simulated and investigated with the help of Computational Fluid Dynamics with ANSYS 16.1 software. The Analysis is aimed at obtaining a true representation of the pressure, temperature and compression ratio that dictate the functioning of a Gamma type Stirling engine. This model developed can then be modified and improved upon to further boost the performance of the Stirling Engine to be integrated with a water pumping system to extract ground water for irrigation purposes.

Proof of concept to recover thermal wastes from aluminum electrolysis cells using Stirling engines

The production of aluminum is a low energy-efficient industrial process. In fact, almost half of its energy demand is lost under the form of heat along the production line. For this reason, aluminum producers are interested in improving energy efficiency of their smelters by recovering the thermal losses. It is well known that a significant loss of energy occurs across the sidewalls of the electrolysis cells. This paper proposes a proof of concepts where the sidewall thermal wastes are used to drive Low Temperature Differential (LTD) Stirling engines. To estimate the losses, a heat transfer study is implemented; heat conduction coupled with mixed convection and radiation is evaluated by using the OpenFOAM Computational Fluid Dynamic (CFD) software. Then, a parametric study is performed to analyse cell working conditions under which thermal wastes could be recovered. The heat flux and the temperature of the thermal wastes are used to judge the utilization of possible Stirling engine technologies. Two scenarios are studied: (i) a plane heat collector coupled to the radiative sidewall and (ii) a plane heat collector in direct thermal contact with the cell sidewall. It is demonstrated that the recovery of the wastes by radiation is technically feasible and it does not threaten the safety of the smelter; furthermore, the losses can power some Stirling engines given in the open literature. Therefore, the recovery of the radiative wastes and the further conversion by means of the Stirling engine is an interesting solution to improve the aluminium smelter energy efficiency.

Nonlinear dynamics analysis of a low-temperature-differential kinematic Stirling heat engine

The low-temperature-differential (LTD) Stirling heat engine technology constitutes one of the important sustainable energy technologies. The basic question of how the rotational motion of the LTD Stirling heat engine is maintained or lost based on the temperature difference is thus a practically and physically important problem that needs to be clearly understood. Here, we approach this problem by proposing and investigating a minimal nonlinear dynamic model of an LTD kinematic Stirling heat engine. Our model is described as a driven nonlinear pendulum where the motive force is the temperature difference. The rotational state and the stationary state of the engine are described as a stable limit cycle and a stable fixed point of the dynamical equations, respectively. These two states coexist under a sufficient temperature difference, whereas the stable limit cycle does not exist under a temperature difference that is too small. Using a nonlinear bifurcation analysis, we show that the disappearance of the stable limit cycle occurs via a homoclinic bifurcation, with the temperature difference being the bifurcation parameter.

Thermodynamics Analysis for Optimal Geometrical Parameters and Influence of Heat Sink Temperature of Gamma-configuration Stirling Engine

Abstract This paper provides theoretical predictions on the thermodynamic analysis of a low-temperature differential (LTD) Stirling engine. A twin power piston, gamma-configuration, LTD Stirling engine is considered with a heat source temperature of 353 K and heat sink temperature of 307 K. Second-order thermodynamic models are performed with MATLAB software. The geometrical parameters investigated include the power piston stroke, the displacer stroke, the displacer diameter, and the displacer thickness. Results from this study indicate that the indicated power density is achieved at a fixed 4 RPM engine. This engine has geometrical parameters with a power piston stroke of 0.29 m, displacer stroke of 0.21 m, displacer diameter of 1.44 m, and displacer thickness of 0.26 m. Additionally, the effects of the heat sink temperature on the indicated power output is performed and analyzed. The result shows that the indicated power output increased with a decreasing heat sink temperature.

Thermal-Flow Analysis of a Simple LTD (Low-Temperature-Differential) Heat Engine

A combined thermal and flow analysis was carried out to study the behavior and performance of a simple, commercial LTD (low-temperature-differential) heat engine. Laminar-flow solutions for annulus and channel flows were employed to estimate the viscous drags on the piston and the displacer, and the pressure difference across the displacer. Temperature correction factors were introduced in the thermal analysis to account for the departures from the ideal heat transfer processes. The flow analysis results indicate that the work required to overcome the viscous drags on engine moving parts is very small for engine speeds below 10 RPS (revolutions per second). The work required to move the displacer due to the pressure difference across the displacer is also one-to-two orders of magnitude smaller than the moving-boundary work of the piston for temperature differentials in the neighborhood of 20 °C and engine speeds below 10 RPS. A comparison with experimental data reveals large degradations from the ideal heat transfer processes inside the engine.

Thermal-flow analysis of a simple LTD (Low-Temperature-Differential) heat engine

A combined thermal and flow analysis was carried out to study the behavior and performance of a small, commercial LTD (Low-Temperature-Differential) heat engine. Laminar-flow solutions for annulus and channel flows were employed to estimate the viscous drags on the piston and the displacer and the pressure difference across the displacer. Temperature correction factors were introduced to account for the departure from the ideal heat transfer processes. The analysis results indicate that the work required to overcome the viscous drags on engine moving parts and to move the displacer is much smaller than the moving-boundary work produced by the power piston for temperature differentials in the neighborhood of <TEX>$20^{\circ}C$</TEX> and engine speeds below 10 RPS. A comparison with experimental data reveals large degradations from the ideal heat transfer processes. Thus, heat-transfer devices inside the displacer cylinder are recommended.

Neural network-based control of an intelligent solar Stirling pump

In this paper, an ANN (artificial neural network) control system is applied to a novel solar-powered active LTD (low temperature differential) Stirling pump. First, a mathematical description of the proposed Stirling pump is presented. Then, optimum operating frequencies of the converter corresponding to different operating conditions (i.e. different sink and source temperatures and water heads) are investigated using the proposed mathematical framework. It is found that the proposed complex mathematical scheme has a very slow convergence and thus, is not appropriate for real-time implementation of the model-based controller. Consequently, a NN (neural network) model with a lower complexity is proposed to learn the simulation data obtained from the mathematical model. The designed neural network controller is thus applied to a digital processor to effectively tune the converter frequency so that a maximum output power is acquired. Finally, the performance of the proposed mechatronic system is evaluated experimentally. The experimental results clearly demonstrate the feasibility of pumping water at low temperature difference under variable operating conditions using the proposed intelligent Stirling converter.

A CFD parametric study on the performance of a low-temperature-differential γ-type Stirling engine

An in-house CFD code has been applied to a low-temperature-differential (LTD) γ-type Stirling engine to understand the effects posed by several geometrical and operational parameters on engine performance. The results include variations of pressure, temperature, and heat transfer rates within an engine cycle as well as variations of engine’s power and efficiency versus these parameters. It is found that power piston stroke and radius influence engine performance very similarly, and power and efficiency both increase as these two parameters increase. In fact, the effects of the two parameters can be assimilated into those by the parameter of compression ratio. The stroke of displacer is observed to affect strongly on heat input but weakly on power, thus causing the efficiency to decrease as it increases. As expected, both power and efficiency increase as temperature difference between the hot and cold ends increases. Lastly, engine speed is observed to pose strong positive effects on power but exert weak effects on efficiency. This study reveals the effects produced by several important parameters on engine performance, and such information is very useful for the design of new LTD Stirling engines.

Preliminary Investigation of an Up-Scaled Gamma-Type Stirling Engine for Power Production

This paper presents the development of Gamma-type Stirling engine for High Temperature Differential (HTD) and self-pressurized mode of operation. The engine is the up-scaled version from the Low Temperature Differential (LTD) miniaturized gamma-type Stirling engine. The test engine is featured with 85cc power piston and 4357cc displacer piston swept volumes, respectively. The characterization of few critical engine parameters and components that includes heater head section, cooler section, displacer and power pistons material selection and heat source system had been conducted. Air is used as a working fluid and Liquefied Petroleum Gas (LPG) is utilized as the heat source in order to cater for the heater temperature up to 1000°C. The workability test of the engine revealed that the lightweight in mass of the displacer piston and the auxiliary cooling effect at the cooler section had contributed to a significant improvement on the engine rotational motion. The static load test determined that the engine is capable of producing the friction power of 1.2W for stainless steel mesh wire displacer and 0.3W for polystyrene displacer. Based on Beale formula, the estimated power of 4W can be produced by the engine using stainless steel mesh wire displacer and 2.4W of power using polystyrene displacer. Good agreement has been shown, where the potential net power production of 3.8W and 2.1 W for stainless mesh wire displacer and polystyrene displacer, respectively. Further investigation is needed to improve the heat regeneration in between hot and cold sections of the engine to realize the sustainable performance of the engine at higher range of temperature difference and output power.

Indicated diagrams of a low temperature differential Stirling engine using flat plates as heat exchangers

Indicated diagrams of a low temperature differential (LTD) Stirling engine (SE) were obtained. The evaluation of LTDSE performance was carried out using the results. The heat source temperatures were 75, 80, 85, 90 and 95° Celsius. The working fluid was air with a mean pressure of atmospheric pressure. The shape of the heat exchangers was flat. The stroke volume of the power piston was 3.9 cc, and the stroke volume of the displacer was 238 cc. The LTDSE did not have any regenerators. The maximum indicated power was 3.34 mW. The polytropic exponents becomes larger than 1.4 before the displacer reached dead center, and the working fluid temperatures fluctuated. These behaviors suggest that the heat exchanger did not work effectively. The evaluation of LTDSE performance was carried out using “the maximum fluctuation of ensemble averaged working fluid temperature”. The value is the fluctuation of the internal energy per the heat capacity, and has dimensions of temperature. The obtained values were from 3.2 to 4.7 °C. The comparison of these values with actual temperature differences suggests that the indicated work of conventional LTDSE was much lower than the thermodynamic upper limit.

Simulation, construction and testing of a two-cylinder solar Stirling engine powered by a flat-plate solar collector without regenerator

In this research, a gamma-type, low-temperature differential (LTD) solar Stirling engine with two cylinders was modeled, constructed and primarily tested. A flat-plate solar collector was employed as an in-built heat source, thus the system design was based on a temperature difference of 80 °C. The principles of thermodynamics as well as Schmidt theory were adapted to use for modeling the engine. To simulate the system some computer programs were written to analyze the models and the optimized parameters of the engine design were determined. The optimized compression ratio was computed to be 12.5 for solar application according to the mean collector temperature of 100 °C and sink temperature of 20 °C. The corresponding theoretical efficiency of the engine for the mentioned designed parameters was calculated to be 0.012 for zero regenerator efficiency. Proposed engine dimensions are as follows: power piston stroke 0.044 m, power piston diameter 0.13 m, displacer stroke 0.055 m and the displacer diameter 0.41 m. Finally, the engine was tested. The results indicated that at mean collector temperature of 110 °C and sink temperature of 25 °C, the engine produced a maximum brake power of 0.27 W at 14 rpm. The mean engine speed was about 30 rpm at solar radiation intensity of 900 W/m 2 and without load. The indicated power was computed to be 1.2 W at 30 rpm.