Stirling Convertor Research Articles

Development and performance measurements of a 2.5 kW-class free-piston stirling converter with detailed design and fabrication processes

This study presents the development and performance measurement of a 2.5 kW class free-piston Stirling engine with detailed design and fabrication processes, considering flexure spring stiffness effects on the engine dynamics and performance. The development of the test engine includes tube type heat exchangers and planar flexure springs. In particular, the flexure springs are designed via finite element analyses and verified through stiffness measurements and fatigue life tests. For the performance measurements, an engine test rig is constructed with an LNG combustion heat supply. The thermal efficiency is evaluated in terms of combustion gas components based on the reaction formula of methane. As a result, the electric output and efficiency of the test engine were measured to be 2.46 kW and 19.46%, respectively. Furthermore, the effects of displacer spring stiffness are experimented with a stack design of flexure springs by varying the number of springs. As a result, increases in displacer stiffness increased engine operating frequency and decreased both the piston amplitudes, while power piston velocity remained relatively constant as well as the engine power output. The experimental results are analyzed through a simple linear dynamic model, which showed reasonable agreements.

Results of the KRUSTY Nuclear System Test

The Kilowatt Reactor Using Stirling TechnologY (KRUSTY) was a prototypic nuclear-powered test of a 5-kW(thermal) Kilopower space reactor. This paper presents results from the KRUSTY nuclear system test, which operated the power system at various temperatures and power levels for 28 consecutive hours. The testing showed that the system operated as expected and that the reactor is highly tolerant of possible failure conditions and transients. The key feature demonstrated was the ability of the reactor to load-follow the demand of the power conversion system. The thermal power of the test ranged from 1.5 to 5.0 kW(thermal), with a fuel temperature up to 880°C. Each 80-W(electric)–rated Stirling converter produced ~90 W(electric) at a component efficiency of ~35% and an overall system efficiency of ~25%.

Titanium-Water Heat Pipe Radiators for Space Fission Power System Thermal Management

For future space transportation and planetary exploration mission power applications, NASA Glenn Research Center (GRC) is currently developing a small-scale nuclear fission system (i.e. Kilopower system), which has an operable range of 1 to 10 kWe and a design life of 8 to 15 years. The thermal management system of Kilopower system involves two types of heat pipes: high temperature alkali metal heat pipes that are used to transport thermal energy from the nuclear reactor to the Stirling convertors hot end and titanium water heat pipes that are used to remove the waste heat from the convertors cold end and transport it to the radiators for ultimate rejection. This paper presents the development of the titanium water heat pipes, which are featured with a special wick structure design: it has bi-porous screened evaporator and screen-groove hybrid wick in the adiabatic and condenser sections. This will allow the heat pipe to (1) operate in space with zero gravity (2) operate on planetary surface with gravity-aided orientation (3) be tested on ground with slight adverse gravity orientation and (4) to startup smoothly after being frozen. Under a research and development program, several freeze/thaw tolerant heat pipes were designed, fabricated and experimentally validated. Later, various heat pipe radiators were developed and tested in a thermal vacuum chamber (TVC). Test results successfully demonstrated that the titanium heat pipes with radiator attached are able to transfer the required power at the working temperature of 400 K under space-like testing conditions with a thermal resistance of 0.019 °C/W while the total heat pipe radiator weight is less than 0.73 kg.

Applying Particle Swarm Optimization to Study the Effect of Dominant Poles Places on Performance of a Free Piston Stirling Engine

In this paper, the effects of dominant closed-loop poles places of a free piston Stirling engine (FPSE) on the engine performance are investigated. First, linear and nonlinear formulations of an FPSE are presented. Then, based on the presented formulations, dynamic behavior of a prototype free piston Stirling engine, namely SUTECH-SR-1, is studied. Afterward, a part of the simulation results is compared with those of an experimental work to validate the mathematical models. Consequently, the obtained reasonable agreement between the experimental and simulation results affirms the validity of the proposed nonlinear theorem. Next, according to linear and nonlinear formulations, the necessary condition for startup state of the engine is investigated. In addition, it is demonstrated that existence of a stable limit cycle is the necessary condition for engine startup based on theory of nonlinear systems. Subsequently, the effects of two important factors including real and imaginary parts of the dominant closed-loop poles of the FPSE on the engine performance are investigated using particle swarm optimization. According to the obtained simulation outcomes, the free piston Stirling converter is more sensitive to variation of the real component of the dominant closed-loop poles than that of the imaginary part.

Computational analysis of external heat transfer for a tubular Stirling convertor

Stirling convertors are an ideal choice for use in numerous energy conversion applications owing to their simplistic design, fuel independence, and robustness. The use of additive manufacturing allows for significant improvements in the overall system efficiency by removing the design limitations of traditional manufacturing techniques. This allows for the design of a novel burner–heater head interface that can vastly improve the external heat transfer. A numerical analysis of the combustion flow over an additively manufactured tubular heater head was conducted. It was shown that the use of flow guides improves the velocity distribution of the combustion gases over the heat acceptor tubes. Additionally, the use of a diffuser plate can be employed to further improve the heat transfer. The developed model can be used further to optimize the overall design of the heat acceptor, regenerator, and heat rejecter.

Development of an Integrated Thermal Energy Storage and Free-Piston Stirling Generator for a Concentrating Solar Power System

Incorporating thermal energy storage (TES) into a concentrating solar power (CSP) system extends the power production hours, eliminating intermittency and reducing the Levelized Cost of the Energy (LCOE). The designed TES system was integrated with a 3 kW free-piston Stirling convertor. A NaF–NaCl eutectic salt was chosen as the phase change material (PCM) with a melting temperature of 680 °C. This eutectic salt has an energy density that is 5 to 10 times that of a typical molten salt PCM. In order to overcome the drawbacks of the material having a low thermal conductivity, heat pipes were embedded into the PCM to enhance the heat transfer rate within the system. Since the dish collector tracks the sun over the course of the day, two operational extremes were tested on the system; horizontal (zero solar elevation at sunrise/sunset) and vertical (solar noon). Although the system’s performance was below the expectations due to improperly sized wicks in the secondary heat pipes, the results indicated that the Stirling engine was able to produce 1.3 kWh of electricity by extracting latent heat energy from the PCM; thus, the concept of the design was validated.

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 novel solar-powered active low temperature differential Stirling pump

This paper presents a novel solar-powered active Stirling converter with liquid power piston and solid controllable displacer. First, the working principle of the proposed converter is described. Then, finite time thermodynamic approach incorporating heat transfer equations are employed to determine gas temperatures in hot and cold spaces based on the assumption of imperfect regeneration. Accordingly, pressure variation of the gas due to reciprocating motion of the displacer piston is investigated using the obtained gas temperatures and Schmidt theory. Next, total work done by the converter and thermal efficiency are evaluated. Kinematic and dynamic equations governing the pump system are presented and the water flow characteristics in suction and discharge states are investigated. A simulation study is carried out through coupling and simultaneously solving the obtained equations. An optimization scheme is thus conducted to find an optimum frequency of the active converter so that a maximum power is generated. The influences of regenerator efficiency, dead volumes, and water head on the optimum operating frequency and the generated power are investigated. Finally, the proposed converter is constructed and primarily tested. The experimental outcomes clearly reveal the feasibility of pumping water at low temperature difference through the proposed active Stirling pump.

New 5 kW free-piston Stirling space convertor developments

The NASA Vision for Exploration of the moon may someday require a nuclear reactor coupled with a free-piston Stirling convertor at a power level of 30–40kW. In the 1990s, Mechanical Technology Inc.'s Stirling Engine Systems Division (some of whose Stirling personnel are now at Foster-Miller, Inc.) developed a 25kW free-piston Stirling Space Power Demonstrator Engine under the SP-100 program. This system consisted of two 12.5kW engines connected at their hot ends and mounted in tandem to cancel vibration. Recently, NASA and DoE have been developing dual 55 and 80W Stirling convertor systems for potential use with radioisotope heat sources. Total test times of all convertors in this effort exceed 120,000h. Recently, NASA began a new project with Auburn University to develop a 5kW, single convertor for potential use in a lunar surface reactor power system. Goals of this development program include a specific power in excess of 140W/kg at the convertor level, lifetime in excess of five years and a control system that will safely manage the convertors in case of an emergency. Auburn University awarded a subcontract to Foster-Miller, Inc. to undertake development of the 5kW Stirling convertor assembly. The characteristics of the design along with progress in developing the system will be described.

Towards Fully Three-Dimensional Virtual Stirling Convertors For Multi-Physics Analysis and Optimization

The desire for fast, low-cost, and reliable Stirling convertor design decisions has motivated the development of a comprehensive in-house Stirling analysis capability. One of the predominant goals of this project is to enhance the Stirling industry’s suite of design tools beyond the currently employed one-dimensional analysis to the level of a virtual three-dimensional full fidelity convertor including structural, mechanical, electromagnetic, and thermal dynamics. This virtual Stirling engine/linear alternator coupling can be used to pre-test prototype hardware, examine internal part performance, and investigate design modifications before expensive convertors are fabricated. An approach is presented for incorporating the complete three-dimensional geometry of modern Stirling convertors and coupling this with multi-physics capability to examine the effects of magnetic, pressure, gravity, spring, and inertial forces on startup behavior, material creep, and overall system performance/efficiency/reliability.

A05 400W級フリーピストンスターリング発電機(スターリングエンジン及び関連要素と応用システム(2))

A test model of a free piston Stirling engine (FPSE) converter was developed as a heat-to-electricity power converter for a demonstration of solar heat energy utilization at the Japan Aerospace Exploration Agency, for future aerospace applications. Based on these achievements, prototype models of the FPSE for household use were designed and tested as part of a project supported by the New Energy and Industrial Technology Development Organization. The final goal of the project is to contribute to environmental protection by realizing a household cogeneration system suitable for Japanese residences. To achieve this, it is necessary to develop a compact Stirling converter with high efficiency and low emission. High system efficiency can be expected when compared with conventional engines because the FPSE does not have mechanisms such as a crank or cam. Therefore, the authors have been focusing on realizing a highly efficient FPSE converter for the cogeneration system. The development of 400 W class prototype model that can be installed in the cogeneration system is ongoing.

Prototype of Free Piston Stirling Converter for Household Use

A test model of a free piston Stirling engine (FPSE) converter was developed as a heat-to-electricity power converter for a demonstration of solar heat energy utilization at the Japan Aerospace Exploration Agency, for future aerospace applications. The target performance of the converter was 200 W electrical power output and 20% overall efficiency. A bench test of the converter was conducted to evaluate performance. The converter showed good performance as expected. In addition, the characteristics of the FPSE were investigated. Based on these achievements, prototype models of the FPSE for household use were designed and tested as part of a project supported by the New Energy and Industrial Technology Development Organization.