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Cambridge | Jan-14 | Biomass-fuelled Stirling Engine for off-grid applications
This document discusses biomass-fueled Stirling engines for off-grid applications. It outlines that biomass and Stirling engines are well-suited for power generation in remote communities due to their ability to use low-quality fuels. The document then describes Oxford University's development of linear Stirling engine technology, which uses oil-free and wear-free components like flexures and clearance seals for long life and low maintenance. This technology is well-suited for biomass applications from 5-20kW. Finally, it summarizes Oxford's work developing new Stirling engine and solar concentrator designs to further improve biomass-powered systems.
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Cambridge | Jan-14 | Biomass-fuelled Stirling Engine for off-grid applications
- 1. Biomass- fuelled 5-20kW Stirling engine foroff-grid applications NGST/Honeywell Hymatic Mike Dadd, Nick Jelley, Paul Bailey, Richard Stone University of Oxford 1
- 2. Overview • Why Biomassand Stirling engines • Stirling engines and the advantages of linear oil free technology • The development of Oxford’s Linear technology • Stirling engine operation/configurations • Oxford Solar Cooker • Summary 2
- 3. Why Biomass? • Alongsidesolar power, biomass is a key renewable energy source in the developing world • Biomass: – Can contain useful amounts of energy – Includes various forms of waste e.g. agricultural waste, slurry, sewage etc – Easily generates combustible gaseous, liquid and solid materials but NOT clean fuels – Is often burnt to provide just heat but this is not the most effective use of the resource 3
- 4. Why Stirling Engines? •Stirling engines: – Can exploit any external heat source – Possible heat sources include solar power or biomass fuels – Fuel quality can be very low compared with IC engine requirements – Can give high efficiencies even at low power levels e.g. 1 to 10 kW with efficiencies in range 30 – 40% – Potential for power generation in the range 5 to 20+ kW – suited to isolated communities in the developing world – Can use thermal storage 4
- 5. Why Stirling Engines? Electricityoutput: Single or 3 Phase 30–40 % efficiency 1 to 20+ kW output Waste Heat: E.g. Hot Water Desalination Stirling Generator Thermal Storage Heat 5 Any Source of Heat: Fuel refinement is not necessary
- 6. Why Stirling Enginesand Biomass? • In principle Stirling engines could be used for solar power generation BUT: – There are other competing technologies that can be used to generate solar power – Costs of PV panels in particular have fallen to a level that discourages Stirling engine development for this application – SES Stirling and Infinia filed for bankruptcy (2011 and 2013 respectively) – both had invested heavily in Solar Stirling • Biomass/Stirling power generation in the small/medium power range (1 – 20 kW) has much less competition 6
- 7. Biomass - CompetingTechnologies • > 1 MW – Gas/steam turbines • 100-1000 kW – Gas engines/turbines • 1 - 100 kW – steam engines - inefficient • 1 - 100 kW – I.C. engines cope badly with unrefined fuel. • 1 - 20+ kW – Very well suited to Stirling engines provided operating costs are acceptable – a real opportunity for Stirling engine development • < 1 kW – Thermo-electric Generators? 7
- 8. Stirling Engines –The Problem(s) • Most Stirling engines (5kW+) use conventional oil lubricated crank mechanisms • These have proved expensive and not sufficiently reliable • Mechanically complex with seals that require frequent servicing ~ < 10,000hr intervals • Most critical seal keeps oil out of heat exchanger assemblies • In other respects - very elegant! 8
- 9. An Oil FreeRotary Stirling Engine • Stirling DK 35 kW generator – only generator currently marketed • Oil free but uses crank mechanism - Direct burning heater • Still requires maintenance – 8000 hr maintenance interval • Biomass: • Standardised pellets for biomass boilers Combustion area exposed to show heater head Stirling DK 35 Generator
- 10. Why Linear OilFree Stirling Engines? • Use of linear technology allows oil free/wear free operation with long-life ~ 15 years or more – Linear generators – Flexures to provide accurate linear movement – Non contacting wear free “clearance seals” • No maintenance required beyond: – Topping up charge pressure – Cleaning of heater assembly • Currently limited to power outputs up to 3 kW – larger outputs desirable 10
- 11. Oxford Linear Technology– Main Features • Technology: – Low cost linear motors/generators – Flexure bearings – Clearance seals • Main Features: – Oil free operation – No contact, no friction, no wear – High efficiency, long life, high reliability, no maintenance – Simple power control Flexure bearing (disc spring) 11
- 12. Development of LinearTechnology at Oxford – Original Split Stirling Cooler COLD HEAD COMPRESSOR PISTON CLEARANCE SEALREGENERATOR CLEARANCE SEAL SPRINGS MAGNET DRIVE COIL SPRINGS COLD TIP DISPLACER MAGNETIC CIRCUIT ORIGINAL SPLIT DESIGN FOR SPACE APPLICATIONS • 2 Motors operating 90° out of phase • Oil free operation • No wearing components • Infinite Life!! First Generation Space Cryocoolers c.1980 12
- 13. Oxford Linear Technology– Space Coolers – Ox/TRW/HH Collaboration HCC (26cm3) MINI (1.8cm3)HEC (6cm3) TRW: HEC Cryocooler with Pulse Tube Cold Head Not Shown: “Micro” (0.6 cm3) balanced Under Design: “Nano” (0.12 cm3) single Honeywell Hymatic NGST Reservoir Tank Cold Block Compressor 13
- 14. Oxford Linear Technology StirlingCycle Domestic Freezer 14
- 15. Oxford Linear Technology OilFree Refrigeration Compressor Compressor for Use with Compact Heat Exchangers 15
- 16. Oxford Linear Technology NovelLinear Motors/Generators Spring Pressure Containment Magnet Carrier Yokes and windings 16
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- 18. Schematic of anAlpha Configuration Stirling Engine Generator (power out) Gas Spring at either end (to increase operating frequency Heater (heat in) Cooler (heat out) Expansion Piston Flexure Bearings Regenerator Compression Piston Slave Motor
- 19. Oxford Linear Technology NewGamma Configuration for Space 19
- 20. Oxford Linear Technology NewMulti-cylinder Configuration • Outcome of UCSF project • Novel multi-cylinder configuration • Low moving mass allows extension of oil free linear technology to larger sizes – e.g. 3 to 30 kW units • Currently being developed for Biomass applications 20
- 21. Oxford Solar Cooker •New solar concentrator was initially intended for Solar Stirling power applications • Design uses novel combination of cone and parabolic reflectors to generate high concentrations • Reflectors can be formed from flat sheet – this allows easy transport in flat pack form • Currently not pursued for solar Stirling but is showing great promise for solar cooking applications in the developing world • There is a collaborative project with Dytecna, with field trials in Africa planned. 21
- 22. Oxford Solar Cooker •http://www.quberenewables.co.uk/downloads/solarqube-11-13.pdf 22
- 23. Oxford Renewable PowerTechnology Summary • Established track record for linear compressors/generators over 30 years – Stirling Cycle, Pulse Tubes, JT • Novel moving magnet motor/generator designs. • Novel Designs of Solar Concentrators – Suitable for Engines, CPV, Solar Cooking • Stirling Engine Development: – ESA space power project – Terrestrial applications – e.g. Biomass 23
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