1. Building of a Free Piston Stirling Demonstrator Engine University of Stellenbosch Energy Postgraduate Conference 2013

2. 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 iThemba LABS SAHPA ® South African Heat Pipe Association 2

3. iThemba LABS SAHPA ® South African Heat Pipe Association 2 What is a Stirling Engine? External Combustion Regenerative Contained Working Fluid (gas) Displacer Piston Mechanical Work Qin Qout

4. iThemba LABS SAHPA ® South African Heat Pipe Association 4 What is Free Piston? Kinematic Type Since 1816 Forced Relative Motion Free Piston Type Since 1964 Free Relative Motion

5. FPSEs in a Nutshell iThemba LABS SAHPA ® South African Heat Pipe Association 5 Displace r Piston Magnets Coil Low maintenance High power density Frictionless operation No lubricant requirement Hermetically sealed Silent operation Self-starting High efficiency

6. 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 iThemba LABS SAHPA ® South African Heat Pipe Association 6

7. 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 iThemba LABS SAHPA ® South African Heat Pipe Association 7

8. 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 iThemba LABS SAHPA ® South African Heat Pipe Association 8

9. Basic Design iThemba LABS SAHPA ® South African Heat Pipe Association 9 Heater head Regenerator Cooling fins Cylinder insert Air bearings Planar flexure bearings

10. Design Challenges Heater head Planar flexure bearings Piston and displacer alignment Frictionless operation: Air bearings Thermal gradients Seizure Linear alternator Dynamically stable operation iThemba LABS SAHPA ® South African Heat Pipe Association 10

11. Modeling Fundamental equations: – Mass: – Momentum: – Energy: Discretization: – Mass: – Momentum: – Energy: iThemba LABS SAHPA ® South African Heat Pipe Association 11

12. 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 iThemba LABS SAHPA ® South African Heat Pipe Association 12

13. Thank you!!!! iThemba LABS SAHPA ® South African Heat Pipe Association 13