Biomass-

fuelled 5-20kW

Stirling engine

for off-grid

applications

NGST/Honeywell Hymatic

Mike Dadd, Nick Jelley,

Paul Bailey, Richard Stone

University of Oxford 1

Overview

• Why Biomass and 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

Why Biomass?

• Alongside solar 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

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

Why Stirling Engines?

Electricity output:

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

Why Stirling Engines and 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

Biomass - Competing Technologies

• > 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

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

An Oil Free Rotary 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

Why Linear Oil Free 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

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

Development of Linear Technology at

Oxford – Original Split Stirling Cooler

COLD HEAD COMPRESSOR

PISTON CLEARANCE SEALREGENERATOR

CLEARANCESEAL

SPRINGS

MAGNET

DRIVE COIL

SPRINGS

COLD TIP

DISPLACER

MAGNETICCIRCUIT

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

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

Oxford Linear Technology

Stirling Cycle Domestic Freezer

14

Oxford Linear Technology

Oil Free Refrigeration Compressor

Compressor for Use with Compact Heat Exchangers 15

Oxford Linear Technology

Novel Linear Motors/Generators

Spring

Pressure

Containment

Magnet

Carrier

Yokes and

windings

16

Stirling Engine

Principles

17

Schematic of an Alpha

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

Oxford Linear Technology

New Gamma Configuration for Space

19

Oxford Linear Technology

New Multi-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

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

Oxford Solar Cooker

• http://www.quberenewables.co.uk/downloads/solarqube-11-13.pdf

22

Oxford Renewable Power Technology

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

Any Questions?

mike.dadd@eng.ox.ac.uk

n.jelley@physics.ox.ac.uk

24