2nd eureca 2014 – Numerical Simulation of Stirling Engines for Solar Air Conditioning Unit

Numerical Simulation of Stirling Engines for Solar Air Conditioning Unit

Dominic Ang Ding Xiong1, Lim Chin Hong2* Department of Mechanical Engineering, School of Engineering, Taylor’s University, Malaysia

*Corresponding email: ChinHong.Lim@taylors.edu.my Abstract— This research attempts to use numerical simulation to maximize the power and torque output for a Gamma Stirling engine. The Stirling engine was modelled in a simplified 2D model, to see how the change in displacer design can improve the engine output. Experiment of an actual small scale Stirling engine will be used to calibrate the numerical model. The objective of this research is to show how the current gamma Stirling engines can be improved and use solar thermal to power air conditioning units. Keywords— numerical simulation, Gamma Stirling engine, air conditioning

1. Introduction In recent years, Stirling engine technology has progress to advanced level where it is used in space and submarines [1]. Stirling engines are external combustion engine that operates with various heat sources including recycled and solar heat. Over the last 20 years, dish stirling engine powered by solar has been used to produce electricity ranging from 2kW to 50 kW has been built in Japan, Russia, Germany and United States [2]. With the rise of energy cost, any alternative sources to fossil fuel are much welcomed. For the year 2011, about 99% electricity produced in Malaysia was from fossil fuels [3]. With cooling being one of the highest consumption of electricity, it is essential to find ways to reduce the cooling cost. One alternative that has been around is using absorption chiller. Since solar is the main source in this research, solar absorption chiller is in the interest of study. However, electrical driven compressor has a higher coefficient of perfromance (COP) compared to solar absorbtion chiller [4]. COP of electrical powered compressor are normally more than 1 while the COP of solar absorption chiller ranges from 0.35-0.6 [5]. Therefore the aim is to use the Stirling engine and connect it directly to the compressor of a commercial air-conditioning unit, thus lowering the cost needed for cooling. As the stirling engine is used to power the compressor directly, there will be no power loss through the wires as compared to the solar dish stirling system which produces electricity. The aim of this research is to use ANSYS to create a 2D numerical model of the gamma Stirling engine that is calibrated with experimental results. The model can then be configured for different displacer design to maximize power and torque output of the engine with the sun’s thermal heat to operate a commercial air-conditioning unit. The aim of changing the displacer design is to increase the temperature mixing in the cylinder. This area of research is new, as no other researcher has been investigating the change of piston or displacer design in a Stirling engine.

2. Research Methodology The research started out with extensive literature review on the basic of Stirling engines and it’s developments for the last few years. The review mainly focused on how various researchers used numerical simulation to simulate the Stirling engines output. The numerical simulation used in this research is ANSYS, using Fluent to simulate the gas flow, temperature and pressure in the engine. Next was to model the actual Gamma Stirling engine. However, the engine model was simplified to 2D to ease the computing power needed. No mechanical losses were considered in the model. Before any simulation can be done, the model had to be mesh. Mesh interface

feature was used to allow the displacer to move without affecting the adjacent mesh on the sides of the displacer. Dynamic meshing was used to control the speed and stroke of piston and displacer in the cylinder. User-defined function (UDF) is used to configure the dynamic meshing. This allows the researcher to control the movement of the piston with an equation that is derived from the horizontal movement of the piston with respect to the flywheel arrangement of the actual Stirling engine. The model will then be simulated with results from the experiment. Equation for displacer 𝑥 = 𝑅 − 𝑅𝑐𝑜𝑠(𝜃 + 90°) + 𝐿 − 𝐿! − (𝑅𝑠𝑖𝑛(𝜃 + 90°))! (1) Equation for power piston 𝑥 = 𝑅 − 𝑅𝑐𝑜𝑠𝜃 + 𝐿 − 𝐿! − (𝑅𝑠𝑖𝑛𝜃)! (2) Fig 2 shows the parameters of equation (1) and (2) derived from the Stirling engine motion.

Fig 2. Gamma Stirling configuration for UDF equation

Experiment was done with the actual Stirling engine to see the operating speed of the flywheel at a certain temperature. Once the model is calibrated with the experimental results, the Stirling engine model piston displacer and hot cylinder configuration can then be change to simulate the power and torque output of the engine. Fig.3 shows the different displacer configuration that was tested. The displacer and hot cylinder was changed to a concave shape design. The idea is to create better temperature mixing in the engine. All the results will then be compiled to show the best configuration.

Base Stirling engine Concave Stirling engine

Fig.3 Meshing of Base and Concave Stirling engine.

3. Results and discussion From the simulation, it was found out that the concave design gave a better force output from the same heat source. The heat source was set at 900w/𝑚!. Fig 4. shows the force output from both engines. The power piston produces power twice per cycle, during expansion and

Flywheel

L R Piston

x

𝜃

2nd eureca 2014 – Numerical Simulation of Stirling Engines for Solar Air Conditioning Unit

compression. It can be seen that the concave design produces much higher force during expansion compared to the base design. Compression force output difference was closer.

Fig.4 Force out put from both engines

The temperature contour of both engine at crank angle 288° is shown at Fig.5. It can be seen generally, that there is a lower temperature difference between the hot and cold cylinder in the base Stirling engine compared to concave Stirling engine. Chin-Hsiang Cheng et al. in their research shows that with higher temperature difference between the displacer and piston areas, higher engine output can be produced[6]. The concave displacer design creates swirl within the hot cylinder, thus allowing the working fluid to heat up faster and expand. This design also increases the displacer area, which Youssef Timoumi et al showed in their research increases the engine output[7]. With the increase in area, the swirling also creates turbulence within the regenerator, thus allowing the hot working fluid to mix faster with the cooled working fluid, hence the bigger temperature difference in Concave design. Chin-Hsiang Cheng et al. in their research use numerical simulation to show that by the temperature of the heat source, the work output and thermal efficiency increased [8]. Both engine were subjected to the heat source but due to better thermal efficiency, engine output is improved.

Base Stirling engine Concave Stirling engine

Fig.5 Temperature contour at 288° crank angle.

Fig.6 shows force output of piston and force needed to power the displacer. It can be seen that the base displace requires more force compared to the Concave design. The reason is that part of the force needed to power Concave displacer is supplement by the negative pressure around the displacer. Due to the concave of the displacer, more swirls are being made in the area above and below the displacer. These swirls create lower pressure in the middle, similar like a tornado. Hence before the displacer undergoes expansion, negative pressure occurs below the displacer, as shown in Fig. 7 at crank angle 0°. The same goes when just before the displacer undergoes compression at crank angle 144°, negative pressure occurs above the displacer. The negative pressure creates a suction effect, hence less force required for it during expansion and compression. The displacer of the base Stirling engine also produces swirls shown in Fig.7, crank angle 0°. However, it does not occurs directly blew the displacer, hence the suction effect is much less compared to concave Stirling engine

Fig.6 Force for Displacer and piston

Base Stirling engine Concave Stirling engine

0°

144°

Fig.7 Pressure Contours for both engines

4. Conclusion With the rising cost of cooling increasing as the population increased, it is essential to find ways to reduce the cost of cooling. The research aims to reduce the cooling cost by using solar thermal Stirling engine to power the commercial air-conditioning unit. By changing the displacer design within the Gamma Stirling engine, the research aims to optimize the output and efficiency of current Gamma Stirling engines.

References [1] Vineeth CS (2011). Stirling Engines: A Beginners Guide. India: Vineeth CS. pp11. [2] Mancini, T., & Heller, P. (2003). Dish-Stirling Systems: An Overview of Development and Status [J], J. Solar Energy Eng., 125 (2), pp135-151. [3] Haslenda Hashim, Wai Shin Ho. (2011). Renewable energy policies and initiatives for a sustainable energy future in Malaysia Reviews. Renewable and Sustainable Energy Reviews. 15 pp4780– 4787. [4] M. Shekarchiana M. Moghavvemi, F. Motasemi and T.M.I. Mahlia. (2012). Cost Benefit Analysis and Energy Savings of Using Compression and Absorption Chillers for Air Conditioners in Hot and Humid Climates. AIP Conf. Proc. 1440, pp521-523. [5] Rifat. A. Rouf. (2013). Solar Adsorption Cooling: A Case Study on the Climatic Condition of Dhaka. Journal of Computers. 8 (5), pp1101-1108. [6]Chin-Hsiang Cheng, Hang-Suin Yang. (2012). Optimization of geometrical parameters for Stirling engines based on theoretical analysis. Applied Energy. 92, pp395-405. [7]Chin-Hsiang Cheng, Hang-Suin Yang. (2012). Optimization of geometrical parameters for Stirling engines based on theoretical analysis. Applied Energy. 92, pp395-405. [8] Chin-Hsiang Cheng, Ying-Ju Yu (2010). Numerical model for predicting thermodynamic cycle and thermal efficiency of a beta-type Stirling engine with rhombic-drive mechanism. Renewable Energy. 35, pp2590-2601.

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Total  Force  output  

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