Investigation on a Free-Piston Stirling Engine and Pneumatic Output

Transcription

1 Investigation on a Free-Piston Stirling Engine an Pneumatic Output Kwankaomeng S, * an Promvonge P Department of Mechanical Engineering, Faculty of Engineering, King Mongkut's Institute of Technology Lakrabang, Bangkok, 52, Thailan *Corresponing Author: [email protected], Tel: (662) , Fax: (662) Abstract This paper presents numerical an experimental stuy of a free-piston Stirling engine (FPSE) with pneumatic output. A free-piston Stirling engine (FPSE) was esigne an manufacture which works at relatively low ifferential temperature. The FPSE is a beta type configuration. The FPSE couples with a pneumatic cyliner. Theoretical analysis was one over a wie range of conitions to etermine engine performance an characteristic with air as the working gas. In the proof-of-concept evice, the isplacer or the light piston was riven by pneumatic cyliner as gas spring that in turn, controls engine spee. The hot en of the isplacer cyliner was heate with electric heater at 25 C maximum temperature. The other en of the isplacer cyliner was coole with a water circulation having 4 C temperature. The power piston was connecte to the piston of pneumatic cyliner for lifting loa. The engine was operate at the initial pressure at approximately 6 bars. The testing results showe that the work an power was obtaine at.5 bars of pressure ifference an 2 rpm engine spee of 2.5 N.m an 4 W, respectively, while the work an power, from the simulation results were 5 N.m an 5 W, respectively, at the same operating conition an engine specification. Output power from numerical simulation was slightly higher than that of experiment accoring to theoretical assumptions. Keywors: free-piston, Stirling engine, pneumatic, Beta type. Introuction Global warming an environmental pollutions are continuing affect an become a serious problem. Emission an heat release from energy consumption is one of the major sources that prouce by energy conversion system from househols, transportations, an inustries. Searching renewable or sustainable energy an alternative engines is necessary to obtain efficient engine an compete conventional engine. Stirling engine, first patente by Robert Stirling in 86, is one of mechanical evice that convert heat from multi-fuels to be useful work. Many applications were investigate an integrate with the Stirling engine such as water pump, generator, linear alternator, hyraulic pump an etc. Beale W.T. [] purpose the esign concepts of a 25 watt free cyliner

2 Stirling engine for use as a solar water pump. However, the engines were inefficient for pumping. Loss in power occurre because the reuce pump frequency. West C.D.[2] esigne an focuse on liqui piston using fluiyne technology on Stirling engine to pump water in form of pulsating pressure. NASA Glenn Research Center presente ynamic moel of a linear alternator couple with Stirling engine proucing electricity [3], [4], an [5]. Moreover, NASA Glenn Research Center also performe research on Stirling engine with hyraulic output know as a kw (.33 hp) free-piston Stirling engine or RE-. The esign of a suitable hyraulic output mechanism of this engine was performe by a esign team from Foster-Milller, Inc. an Sunpower, Inc [6], [7], [8] an [9]. Stirling Energy Systems (SES), Inc., has been successful on a niche market of the solar power Stirling engine coupling electric generator []. Xi, C. et al. [] showe pneumatic connection of free piston an free isplacer impact on overall performance of Stirling cryocooler. The motion parameters such as isplacement, amplitue an phase shift of piston an isplacer were investigate both theoretical an experimental works. Thombare D.G. an Verma S.K. [2] presente technological evelopment in the Stirling cycle engines. They reveale that proper selection of rive mechanism an engine configuration is one of essential factor for a successful operation of engine system with goo efficiency. However, few research works have been conucte on pneumatic output of Stirling engine as a heat engine. In this stuy presents characteristic of the FPSE with pneumatic riving system an pneumatic output. The motion of the isplacer an the power piston coupling with the pneumatic piston such as isplacement, phase motion, velocity an pressure have been investigate respectively by means of simulation an experiment. 2. Methoology The FPSE consists of two moving pistons which are a light weight piston calle isplacer an a working piston as illustrate in Fig.. The Displacer an the working piston movements are controlle by flow of the working gas between hot an col spaces or gas expansion an compression zones, respectively. As the working gas expans at the hot space, the isplacer moves an compresses the working gas at the col space pushing the working piston to give power stroke from FPSE. The working piston, hence, can be extracte to be useful work or power. After the working piston gives the power stroke, the isplacer an the working piston are returne to restart the new cycle motion by gas spring an bounce chamber, respectively. x, x&,& x& p p x, x&,& x& Fig. Free-piston Stirling engine p

3 2. Design of the FPSE Moel The simulation moel of the FPSE coupling with pneumatic pump as a single-acting illustrate in Fig. 2. p T c p, T h, V e an motion. The prototype consists of two major ynamic components, the isplacer assembly an the piston assembly. The isplacer assembly consists of the isplacer an the pneumatic piston attache to the isplacer. The piston assembly inclues the working piston an the pump piston. The working piston an the pneumatic piston was connecte in-line attachment with spherical ro en an yoke in orer to ecrease alignment problem an eccentric allowance. Fig. 2 FPSE with single-acting pneumatic pump In theory assumption, the working piston connects rigily with the pump piston calle piston assembly. The bounce chamber treate as constant pressure tank. The isplacer is controlle by gas spring. When the working gas pressure (P w ) is higher than the bounce chamber pressure (P b ), the working piston moves an prouces work from this power stroke. The pump piston also pressurizes air in the pneumatic cyliner generating pressure high enough to open the check valve, letting the pressurize air flow from the pump cyliner to the loa or pneumatic accumulator Design of a FPSE Prototype The Schematic of the prototype with pneumatic connection is presente in Fig. 3. The initial esign focus is on a low temperature ifference of engine configuration [3]. An electric heater was selecte to heat the engine. The esign utilize a pneumatic cyliner to rive the isplacer that, in turn, controls engine spee Fig. 3 The schematic of a free-piston Stirling engine with pneumatic output 2.3. FPSE Specification The specifications of the FPSE are given in Table. Table Design specifications of free-piston Stirling Engine prototype Working piston cyliner iameter 38. mm Displacer cyliner iameter 82.6 mm Working Piston iameter/stroke 38. / 4 mm Displacer iameter/stroke 82/ mm Displacer ro/piston ro iameter 6.4 mm Pneumatic piston iameter 38.mm Hot / Col space temperature 25 C / 4 C Working flui/cooling flui Air/Water Power to heater 585 W 3. FPSEP Analysis an Numerical Results 3. Dynamic analysis of the FPSE The parameters an reference positions of FPSE prototype are illustrate Fig. 4.

4 Pneumatic piston area, Apneu TDC BDC Reference Line x Displacer stroke, S TDC Working piston stroke, Sp BDC Working pneumatic piston area, Ap Air in Th Tc m Pw,Pw mp Pb P pneu, :pneumatic pressure P pneu,2 :pneumatic pressure Initial Position x, x&,&& x Displacer ro area, Ar Displacer area, A Initial Position x, x&,&& x p p p Working piston area, Ap Working piston ro, Apr area Pneumatic Accumulator To Loa Ppump,pump pressure Fig. 4 Reference positions an variables of a free-piston Stirling engine with pneumatic output The free boy iagrams showing the pressures acting on the isplacer an piston assemblies are shown in Figs. 5-6, respectively. Fig. 5 Free boy iagram of isplacer assembly with pressure acting Fig. 6 Free boy iagram of the piston assembly with pressure acting The isplacer an the working piston acceleration can be erive from the equation of motion as shown respectively in Eqs. ()-(2). Where f is friction an subscript, p, i, c, an h represents isplacer, working piston, initial conition, col an hot respectively. ( P P PwAr f )( A pneu, pneu,2 pneu r & x = ( m ) () A ) x& p = P ( ) ( ) w b p b pr mp P A f pump pump P A + PA & (2) Equations () an (2) are functions of the working gas pressure (P w ). The working gas pressure equation as a function of the temperature (T) an volume (V) ratio erive from the ieal gas law with assume equality of pressure in the hot an col spaces an a fixe mass of the working gas mass [4] is: P P w wi, V c, ith, i + = Vh, itc, i ci hi hi V V c, T, V T h, + Vc, ivh, i Tc Vh, i Th (3) Hence, the ynamic action of the isplacer an working piston is obtaine by solving the Eqs. () - (3) simultaneously. The power of the FPSE can then be reaily etermine from the relationship between flow rate (Q) an pumping pressure (P pump or P p ): Power= (4) QP pump 3.2 Simulation results The simulation was one with air as the working gas at 6 bar working gas pressure an with an engine isplacement of 45 cm 3 (.75 in 3 ) running at 2 rpm can prouce 5 W. The performance of this operation conition an the results are shown in Figs Displacement an velocity Displacement, (cm) Displacer Piston Time(s) Fig. 7 Piston an isplacer isplacement

5 The working piston an isplacer isplacement, phase motion (relationship between piston an isplacer isplacement) an velocity are shown in Figs. 7-9, respectively. The motion of the isplacer leas the working piston with phase angle of 9 egree. Piston isplacement, x p (cm) Velocity, (m/s) Displacer isplacement, x (cm) Fig. 8 Relative motion of piston an isplacer isplacement Displacer Piston Time(s) Fig. 9 Piston an isplacer velocity Pressure an work In Fig., the working gas pressure starts at.6 MPa (87 psi) then increases to.2 MPa (74 psi) because the gas temperature rises ue to heat aition. The pressure then rops because of gas expansion at the power stroke of the engine an heat loss. The bounce chamber pressure is treate as an air spring forcing the working piston to push the working gas flow to the hot space which causes the working pressure to increase again. The pneumatic pressure can be set up epening on loa or check valve. The check valve was preliminary set up at.69 MPa ( psi) in the simulation. When the engine provies power as represente by the increasing of the working gas pressure, the air in pneumatic cyliner was also pressurize simultaneously with the power stroke of the Stirling cycle engine. Pressure, (MPa) Working pressure, P w (MPa).5.5 Working Pressure Pneumatic pump pressure Time (s) Fig. Pressure in the engine Total volume, (cm 3 ) Fig. Pressure-volume iagram Fig. shows pressure an volume iagram. The engine moel prouces work an power of 5 N.m an 5 W., respectively.

6 4. Experimental set up an Results 4. Experimental Set Up The schematic an FPSE prototype with experimental set up is presente in Fig. 3 an 2, respectively. In the proof of concept evice, a pneumatic cyliner use to actuate the isplacer. The isplacer controlling system inclues a function generator, solenoi valve, relay, pressure regulator, battery an pneumatic cyliner. The solenoi valve operates at 2 volts DC provie by a battery. This valve controls the pneumatic cyliner that actuates the isplacer. Air pressure to the valve is set by a pressure regulator. A linear potentiometer an a linear variable ifferential transformer (LVDT) are use to measure the isplacer an working piston isplacements, respectively. A yoke an spherical ro en connect the pneumatic cyliner ro to the linear potentiometer ro. The LVDT, linear potentiometer an pressure transucers are connecte to a ata acquisition computer to convert analog signals to igital which are isplaye on a computer monitor or written to a file. Fig. 2 The FPSE with pneumatic connection an experimental set up 4.2 Experimental Results The experiment was one with air as the working gas at a pressure of.62 MPa (9 psi). The testing results showe that the work an power was obtaine at.5 bars of pressure ifference an 2 rpm engine spee as 7 N.m an 4 W, respectively. The engine characteristics are shown in Figs Displacement Displacement (cm) Working Piston x x p Time (s) Displacer Fig. 3 Displacement of the isplacer an working piston Fig. 3 shows the measure isplacements of the isplacer an working piston uring 5 secons of operation. The motion of the isplacer was slightly ahea of the motion of the working piston as controlle by the function generator. The working piston follows the isplacer an respons to the change in pressure of the working gas Working gas pressure an pneumatic pump pressure Pumping pressure, P p, (MPa) P p Pump piston x Working Piston P w Time (s) Fig. 4 Pneumatic pump pressure an working gas pressure vs. time. Fig. 4 is a plot of pneumatic pump pressure an working gas pressure. The working gas pressure was higher than the pump P p Pp Pw Pw x p Working gas pressure, P w (MPa) Displacement (in)

7 pressure of the pneumatic cyliner. The pneumatic cyliner, however, can be varie in size in orer to obtain suitable pressure loa Pump Power Fig. 5 shows the instantaneous power along with average power (4W) varie with time. Power (W) Time(s) P ave = 4 Fig. 5 Power vs. time 5. Summary an Discussion A free-piston Stirling engine prototype has been simulate, built an teste. The engine was operate at the initial pressure at approximately 6 bars. The power piston was connecte to the piston of pneumatic cyliner for lifting loa. The testing results showe that the work an power were obtaine at.5 bars of pressure ifference an 2 rpm engine spee of 2.5 N.m an 4 W, respectively, while the work an power, from the simulation results were 5 N.m an 5 W, respectively. Output power from numerical simulation was higher than that of experiment accoring to theoretical assumptions. 6. References [] Beale, W. T. (979), A Free Cyliner Stirling Engine Solar Powere Water, International Solar Energy Society International Congress Atlanta, Georgia, June. [2] West, C. D. (983), Liqui Piston Stirling Engines, New York: Nostranb Reinhol. [3] Lewanowski, E. J., an Regan, T. F., Overview of the GRC Stirling Convertor System Dynamic Moel, Proceeings of the Secon International Energy Conversion Engineering Conference (IECEC 24), Provience, RI, 6-9 Aug. 24. [4] Regan, T. F., Lewanowski, E. J., Application of the GRC Stirling Convertor System Dynamic Moel, Proceeings of the Secon International Energy Conversion Engineering Conference (IECEC 24), Provience, RI, 6-9 Aug. 24. [5] Regan, T. F., an Lewanowski, E. J., Development of a Stirling System Dynamic Moel with Enhance Thermoynamics, Proceeings of the Space Technology an Applications International Forum (STAIF 25), Albuquerque, NM, 3-7 Feb. 25. [6] Toscano, W. M., Harvey, A.C., an Lee, K. (983) Design of Hyraulic Output Stirling Engine, NASA Contractor Report 67976, Jan [7] Schreiber, J. G., Geng, S. M. an Lorenz, G. V., RE- Free-Piston Stirling Engine Sensitivity Test Results, DOE/NASA/5-, NASA Technical Memoranum 88846, Oct [8] Schreiber, J. G., an Geng, S. M., Re- Free-Piston Stirling Engine Hyraulic Output System Description, NASA Technical Memoranum 85, Oct [9] Geng, S. M. (987), Calibration an Comparison of the NASA Lewis Free-Piston Stirling Engine Moel Preictions with RE- Test Data, NASA Technical Memoranum 89853, Aug.

8 [] SES Stirling energy systems, URL: energy.com, access on 24/4/29 [] Xi, C., Yi N.W., Hua Z., an Nan, C. (29). Stuy on the phase shift characteristic of the pneumatic Stirling cryocooler, Cryogenics, Vol. 49(3-4), March-April 29, pp [2] Thombare D.G. an Verma S.K. (28), Technological evelopment in the Stirling cycle engines, Renewable an Sustainable Energy Reviews, vol. 2(), January 28, pp [3] West, C. D. (986). Principles an applications of Stirling Engines, New York: Nostranb Reinhol. Chapter 3, pp. 58. [4] Beale, W. T. (969), Free Piston Stirling- Some Moel Tests an Simulation, Society Of Automotive Engineers Inc., International Automotive Engineering Congress, No. 6923, Detroit, MI, 3-7 Jan. The First TSME International Conference on Mechanical Engineering