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The external combustion of the Stirling engine makes it very attractive for small-scale Combined Heat and Power(CHP) plants using biomass fuels. Stirling Danmark has developed engines specically designed for powerproduction from biomass.

THE STIRLING PRINCIPLEStirling engines are based on a closed cycle, where the working gas is alternately compressed in a cold cylinder

 volume and expanded in a hot cylinder volume. Theheat input from the combustion of fuel is transferredto the working gas through a hot heat exchanger (theheater) at a high temperature typically between 950°Cand 1050°C. The heat drives the pistons, which drive thecrankshaft directly coupled to alternator shaft for powerproduction. The remaining heat is transferred to the water cooling system.

DESCRIPTIONThe engine is designed as a hermetically sealed unit withthe alternator incorporated in a pressurised crankcase.

Only static seals are necessary between the Helium working gas in the crank case and the surrounding air. All bearings are greased and the engine interior is oil-free. Piston rings and piston rod sealing rings have beenmade from PFTE-based materials for “dry-running”. Theinter-nal design of the engine eliminates the need forhigh pres-sure piston rod seals, which is known to giveproblems for majority of other Stirling engine designs.

The four cylinder Stirling engine, SD4E, has a nominalelectric power output of 35 kW. The cylinders are ar-ranged in a square, and Helium is used as the working

gas at a mean pressure of approximately 4,5 MPa. The

four heater panels form a square combustion chamber.

The asynchronous alternator, which is also used as astarter motor, has 6 poles corresponding to an enginespeed of approximately 1000 rpm when coupled directly to the power grid.

The heater is designed specically for direct combustionof biomass:

• Wide ue gas passages in the heater section• Careful design of ow paths through the heater• Large surface area per unit heat transferred• Easy cleaning access of heater panels either

manually or by shock blasts.

 APPLICATIONThe engine can by driven by almost any heat source which can deliver hot (ue) gas ow at approx. 1000°C with low or no particles and ashes. Currently the engine

has been optimised for gasication of low-quality woodchips.

Depending on the system design this could be fromsolid, liquid or gaseous fuels.

Examples:• Direct combustion of wood chips• Gasication (indirect combustion) of wood chips

and other solid biomass• Pyrolysis of difcult solid biomass fuels• Combustion of biogas or bio-oil.

• Combustion of waste products such as glycerine•  Add-on to large boilers on natural gas or biomass

Stirling Engines for Biomass ApplicationsEngine Model SD4-E

 Alternator integrated inhermetical design

Low maintenance, pressurised,

oil-free crankcase

Heater designed for biomass fuels

Optimized Stirling cycle design with high efciency regenerator

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  www.stirling.com

info@stirling.com

 

Stirling DK ApS

Phone +45 88 18 48 00Fax +45 88 18 48 80

RELIABILITY The engine has been optimised for low maintenanceand a long life-time. The design life of the engine is100,000 hours, with 4,000-hour service intervals.

RESEARCH & DEVELOPMENTThe current design is based on 15 years of researchand development at the Technical University of Denmark. More than 40,000 hours of operation have been accumulated on existing engines. Research anddevelopment is continuous, w.r.t. electrical efciency,new features and simplied maintenance.

ENGINE FEATURES• Sturdy design and hermetical sealing for

increased lifetime in dusty environments

• Oil-free mechanism with greased bearings.No risk of oil contamination of heat transfersurfaces

• Few movable parts• Patented yoke mechanism• Non-hazardous helium working gas• High electrical efciency • Heater designed for biomass combustion• Control system• Interface for combustion chamber• Proven design, thoroughly tested and veried

for long term trouble less operation

• Competitive life-cycle cost

ENGINE SPECIFICATION

 Engine type SD4E 

• No. of cylinders 4

• Bore, mm 142

• Stroke, mm 76

• Phase angle 90°

•  Working gas Helium

• Helium lling, Nm³ 9

• Mean pressure, MPa 4.5

• Min/max process pressure, MPa 3.3 / 5.7

•  Alternator poles for 50 / 60 Hz 6 / 8

• Engine speed at 50 / 60 Hz, rpm 1014 / 914

• Maximum Continuous Rating, KWe 35 / 32

• Heat output in water, kWt 88.5

• Heater temperature °C 700±50

• Heat input to heater at MCR, kW 125

• Max cooling water inlet temperature°C 60

• Mass in kg 2500

35 kW 4-cylinder engineClose-up of heater