Building of a Free PistonStirling Demonstrator EngineUniversity of Stellenbosch

Energy Postgraduate Conference 2013

Contents• What is a Stirling Engine?• What is Free Piston• FPSEs in a nutshell• The State of FPSEs• Solar Technology Comparisons• Objectives• Basic Design• Design Challenges• Modeling• Conclusions

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What is a Stirling Engine?

External Combustion

Regenerative Contained Working Fluid (gas)

Displacer Piston

Mechanical Work

Qin

Qout

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What is Free Piston?

Kinematic TypeSince 1816

Forced Relative Motion

Free Piston TypeSince 1964

Free Relative Motion

FPSEs in a Nutshell

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Displacer

Piston

MagnetsCoil

• Low maintenance• High power density• Frictionless operation• No lubricant requirement• Hermetically sealed• Silent operation• Self-starting• High efficiency

The State of FPSEs• Most studies are theoretical: Modeling and optimization• All CSP projects are demonstrational• Most knowledge propriety • Major companies:

– Sunpower Inc.: No commercial engine available– NASA and MTI: Discontinued – Infinia: PowerDish (claimed 24% conversion efficiency),

remote CHP for villages (still in development)– Microgen Engine Corporation: microCHP

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Solar Technology Comparisons• CSP vs PV:

– Heat storage: Supply can meet demand profile– Large scale deployment

• Stirling vs other CSP (Parabolic Trough, Solar Tower, Linear Fresnel)– Highest attainable efficiency of all CSP– Self contained: Distributed generation– Dry cooling: No water requirement– Location possibility on uneven terrain– No possibility of storage

• Stirling vs PV:– More complex– Multi-fuel capability– CHP: Combined Heat and Power

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Objectives• To build a FPSE that works and can be tested• To verify an one-dimensional thermofluid-dynamic model

with test results• Safe operation of a peak operating pressure of 100bar• To use local readily available materials• To identify major shortcomings for future research

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Basic Design

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Heater head

Regenerator

Cooling fins

Cylinder insert

Air bearings

Planar flexure bearings

Design Challenges• Heater head• Planar flexure bearings• Piston and displacer alignment• Frictionless operation: Air bearings• Thermal gradients• Seizure• Linear alternator• Dynamically stable operation

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Modeling• Fundamental equations:

– Mass: – Momentum:

– Energy:

• Discretization:– Mass:

– Momentum:

– Energy:

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Conclusions• The field of FPSE technology remains unknown• FPSEs provides competition in many different fields:

– CSP– PV– IC generators– Condensing boilers

• Multi-fuel capability allows use of biofuels and gas• FPSEs provide advantages over kinematic types i.t.o.

power density, low maintenance and self starting• At present efforts must be aimed at physical studies

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Thank you!!!!

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