MPa Mean Pressure Research Articles

A 2 kW-class free-piston Stirling heat pump prototype suitable for cold regions for domestic heating

Compared with conventional vapour-compression heat pump technology, the free-piston Stirling heat pump technology attracts wide attention for its high efficiency, reliability, and environmental friendliness. In this work, a free-piston Stirling heat pump prototype with a two-kilowatt capacity, which can operate with an outside ambient temperature ranging from -20 to 10 °C, was designed, built and tested. First, numerical simulations were carried out to explore the distributions of the acoustic field and exergy loss. Second, the effects of several operating parameters on the heating performance were numerically investigated in detail. Finally, to validate the accuracy of the simulation results, verification experiments were conducted on the experimental platform of a free-piston Stirling heat pump driven by a dual-opposed linear compressor. Experimental data show that with a hot-end temperature of 45 °C and outside ambient temperature of 0 °C, the prototype achieved a heating capacity of 2253 W with a corresponding COPh of 2.76 (based on PV power input) and a relative Carnot efficiency of 39.1 %; in particular, the prototype was able to achieve a COPh of 2.5 when pumping heat from -20 °C to 45 °C for 3.25 MPa mean pressure. The simulation and experimental results demonstrated that the free-piston Stirling heat pump system has good potential to achieve high efficiency as a heating supply in a cold region.

New approach of decision support method for Stirling engine type choice towards a better exploitation of renewable energies

Stirling engines are constructed with different mechanical configurations namely Alpha, Beta, Gamma and all of them can be powered by any source of renewable energies including biomass, solar energy or even waste heat from industrial sectors. The question that arises is: how can we choose the right type of Stirling engine that best matches our renewable energy source? In the existing bibliography, there is a great lack of an in-depth approach that establishes an objective comparison between the different types of Stirling engine, which leads researchers to make an inappropriate choice of a given configuration for a specific application. Consequently, a decrease in engine output performance can be noted while increasing the engine operating price. In this context, this paper develops a solution to Stirling engine users that will help them choose the suitable configuration for a given energy source. To do this, a non-ideal adiabatic model which takes several thermal and mechanical losses into consideration has been proposed using MATLAB software. The results of the current model have been compared with experimental findings of NASA Lewis Research Center and previous models and the comparison results demonstrate the high accuracy of the present model. The same physical and geometrical parameters were used to analyze the different configurations of Stirling engine by using crank drive mechanism. The results of this work demonstrate that each Stirling engine type is more suitable for a particular application where it provides high performances. The findings show the ability of Alpha configuration to operate with high temperature difference which correspond to waste heat recovery in industrial sectors where the gas temperature reaches high values. Alpha engine can produce high power-volume ratio and efficiency thanks to its low dead volume in working spaces and the separation that exists between its hot and cold sources. Under the same operating conditions, the output power and efficiency of Alpha, Beta and Gamma were 4120 W, 2280 W, 2500 W and 40.19%, 32.13%, 32.95% respectively. Beta and Gamma types have been found to be more suitable for low and medium temperature difference such as biomass and solar energy. Within these temperature ranges, Beta and Gamma types can produce almost the same output power as Alpha machine with less engine pressure requirement since they contain only one power piston. Beta and Gamma types have efficiencies of 17.57% and 18.40% with only mean pressure of 2.6 MPa and 2.7 MPa respectively, while Alpha arrangement provides an efficiency of 20.95% at 3.5 MPa mean pressure. This contributes to a main advantage of Beta and Gamma Stirling engines in terms of sealing process which will decrease the engine construction cost of these types. In addition, the code developed in this work indicates, at its output, the appropriate type for any application based on the user’s input parameters including the temperature of the hot source. This contributes to a new decision support approach allowing the choice of the suitable configuration for a given energy source.

A novel looped low-temperature heat-driven thermoacoustic refrigerator operating in room temperature range

In this work, a novel looped low-temperature heat-driven thermoacoustic refrigerator is proposed, simulated and analyzed. Under the operating conditions of 10 MPa mean pressure, 50 °C ambient temperature and 290 °C heat source temperature, a total cooling power of 3102W at 10 °C is obtained, corresponding with a coefficient of performance (COP) of 0.41 and an overall relative Carnot efficiency of 13.4%. Then, energy conversion characteristics are illustrated by analyzing the distributions of key parameters including acoustic field, acoustic work and exergy loss. The results indicate that the travelling-wave acoustic field dominates the system with high acoustic impedance in the regenerators, which accounts for the high thermal efficiency. Exergy losses mainly exist in the regenerators and the resonant tubes, which gives a hint for further optimization. Results also reveal that a larger cooling power is obtained under a higher mean pressure and a higher heat source temperature for each cold-end temperature.

Numerical Investigation on a 70 Hz Pulse Tube Micro-cryocooler

In this paper, a 70Hz pulse tube micro cryocooler (PTMC) driven by two opposite linear generators has been studied using linear theory of thermoacoustic. In contrast to conventional thermodynamic calculation method the results of a small-scale PTC as a thermoacoustic system were obtained. Numerical simulation was conducted to design the system and optimum structure parameters were obtained. With operating condition of 2.5MPa mean pressure and 70Hz operating frequency, a cooling power of 8W at 77K and a relative Carnot efficiency of 21% were achieved. In order to prove the applicability of numerical simulation using linear theory of thermoacoustic to such small-scale cooling system and to better understand the energy conversion characteristics of the system, distributions of key parameters such as acoustic work, exergy were presented and discussed.

Numerical Investigation on a 300Hz Pulse Tube Cryocooler Driven by a Double-acting Thermoacoustic Heat Engine

Abstract In this paper, a 300 Hz pulse tube cryocooler (PTC) driven by a double-acting thermoacoustic heat engine (DATHE) has been studied. A double-acting thermoacoustic heat engine incorporates several travelling-wave heat engine units, which are serially connected in a loop. Compared with the conventional thermoacoustic heat engine which involves a travelling-wave loop and a resonator, it has advantages of compact size and potentially high thermal efficiency. In this paper, a DATHE-PTC system was designed, optimized and studied in detail. Firstly, numerical simulation was conducted to design the system and optimum structure parameters of the system were obtained. With the operating condition of 4 MPa mean pressure and 300 Hz working frequency, a cooling power of 7.75 W at 77 K and a relative Carnot efficiency of 11.75% were achieved. In order to better understand the energy conversion characteristics of the system, distributions of key parameters such as acoustic work, phase difference were presented and discussed. Then, the coupling mechanism of the system was investigated. In addition, the influence of mean pressure on the system performance was studied.

A 100 W-class traveling-wave thermoacoustic electricity generator

Experimental investigation on a traveling-wave thermoacoustic electricity generator is presented. In the experiment, more than 100 W electrical power was achieved under 2.5 MPa mean pressure, 64 Hz working frequency and 0.2 MPa pressure amplitude.

Investigation on a Stirling refrigerator with thermal buffer tube driven by an oil-lubricated compressor

In this article, a special configuration of Stirling refrigerator for domestic refrigeration purpose is introduced. A thermal buffer tube is installed between the refrigerator cold-end and the expansion piston to improve the system reliability by moving the expansion piston from low temperature to ambient temperature. Furthermore, a commercial oil-lubricated dual-piston compressor is modified to drive the refrigerator, inside which an elastic membrane is used to transfer acoustic work and separate the working gas of the refrigerator from that of the compressor. Experimental investigations on the refrigerator are performed using helium as the working fluid and a cooling power of 200 W at −78 °C is achieved at 15 Hz working frequency and 2.5 MPa mean pressure. Meanwhile, a rough estimation of the refrigerator COP in terms of cooling power divided by input acoustic power gives the value of 0.64. It gives the possibility of building a low-cost, high efficiency domestic refrigerator.

A high frequency cascade thermoacoustic engine

A miniature cascade thermoacoustic engine, which consisted of one standing-wave stage and one traveling-wave stage in series, was built and tested, which length was about 1.2 m, operating at 470 Hz using helium as working gas. The cascade modeling, the simulation and the primary experimental results are described in this paper. Four different configurations of the miniature cascade thermoacoustic engines had been designed and compared. According to the analysis, the diameter ratio of stages was designed to extend the traveling-wave region, which optimized value was about 1.69. The peak-to-peak value of the acoustic pressure was predicted to arrive to 3 bar at the 3 MPa mean pressure of helium when 300 W heating power was the input. The features of the engine were predicted delivering 68 W acoustic power with a thermal efficiency of up to 22.74% (the ratio of acoustic power to heater power). Due to careful designing, the engine self-excited the oscillation smoothly from the first experiment. An onset temperature gradient of about 4.5 K/mm was achieved, and the peak-to-peak acoustic pressure was 48 KPa at the 2 MPa mean pressure when 200 W heating power was the input. The design computation and experimental results showed a rather good agreement between the measured and calculated pressure phasor and temperatures distributions in the cascade thermoaoustic engine.

Thermoacoustically driven refrigerator with double thermoacoustic-Stirling cycles

Recently, considerable research efforts have been made to search substitution technologies for chlorofluorocarbon-based vapor compression cycles due to the concern over environmental issues. This letter introduces a helium-based thermoacoustic refrigeration system, which is a thermoacoustic-Stirling refrigerator driven by a thermoacoustic-Stirling heat engine, for domestic refrigeration purpose. In the regenerators of both the refrigerator and the prime mover, helium gas experiences near to reversible high efficiency Stirling process. At the operating point with 3.0MPa mean pressure, 57.7Hz frequency, and 2.2kW heat input, the experimental cooler provides a lowest temperature of −64.4°C and 250W cooling power at −22.1°C. These results show good potential of the system to be an alternative in near future for domestic refrigeration with advantages of environment-friendliness, no moving parts, and heat driven mechanism.