Score Stove[1] Generating electricity in developing countries using thermo-acoustics powered by burning wood. Paul H. Riley, Score Project Director

Score Stove

Generating electricity in developing

countries using thermo-acoustics

powered by burning wood

Paul H. Riley, Score Project Director

Professor Mark Johnson

Background

In Poor Rural areas

•

_{>2 Billion are without}

electricity and cook on

an open fire

•

_{Smoke is a real hazard}

Score

•

_{Stove cooks, generates}

electricity and cooling

•

_{€3M project}

»

3 years research

»

2 years exploitation

•

_{Large volume}

manufacture after 2012

•

_{Extended partnerships}

Technical Challenges

Cost

• _{Low cost is the main driver} • _{Target = €30 per household}

delivered to capital city of country

• 2 billion units at €30

• 60 million units at €90

Weight

• _{In many areas hand carrying is the}

only option

• Target = 10 – 20 kg

Power output

• Electrical = 100W_e (from battery)

• Cooking = 1.6kW_th full power 0.75 kW for simmering

Fuel

• Consumption < 0.3 g/s (<2 logs per hour)

• _{Material initially wood. Looking at}

Options

Internal combustion engine with bio-gasifier

•

_{Expensive, high maintenance requirement}

Thermo-piles with wood burning stove

•

_{Expensive, low efficiency, lack of robustness}

Bio-Fuel fed Stirling engine

•

_{Expensive, maintenance may be an issue}

Thermo-acoustic engine

•

_{Travelling wave}

» Currently expensive, but options for cost reduction

» Units have been developed in power range

» Reasonable efficiency

•

_{Standing wave}

» Potentially the lowest cost

» Predicted efficiencies just acceptable.

» Lack of experimental data at the output power required.

Standing Wave TAE

Fractional wavelength design

• _{Frequency determined by alternator,}

not duct length.

• _{Complex acoustic - LA matching}

Combustor

• _{Initially wood burning}

• _{High efficiency}

• _{Low emissions}

• _{Waste heat used for cooking}

Hot Heat Exchanger (HHX)

• ₅₀₀OC gas temperature

Stack

• _{Heats and cools gas packets}

• _{Provides time lag at required}

frequency, eliminates displacer.

Ambient Heat Exchanger (AHX)

• ₈₀OC gas temperature

• _{Ambient heat exchanger Water}

System losses = 0.8kW_th

Power flow

(pre-optimisation)

Heat to cooking Hob = 1.6kW_th

TAE heat input (HHX) = 2kW_th

Heat to Water (AHX) = 1.7kW_th

Acoustic power = 300W_a

Alternator Loss = 150W_th

Storage Battery loss = 50W_th

Electrical Output to devices = 100W_e

• Laptop and light or • TV, Radio and lights •Charge mobile phones

Optimisation of the design

Optimisation: Cost

Paradox

•

_{Smoke free stove Nepalese}

manufacture ~ £25

» Low labour costs

» Excludes profit and transport

•

_{Gas stove (LPG) in UK}

» £14.99 includes:

» Local tax and transport

» Profit (manufacturer and retailer)

Low material content is key

•

_{Thin sections}

•

_{Strengthened by geometric shape}

Leads to low weight design

Optimising the TA Geometry

(Standing wave model)

L

x_h x xc

Lcb Hot

termination Hot h.e.

Stack Lhb Lh Cold h.e. Cold ‘outlet’ Hot bounce Cold matching zone Lc

Sensitivities in the TA model

(for single set of base conditions)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0 2 4 6 8 10 Pav / bar W/Wref Average pressure 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0 20 40 60 80 100 120 f / Hz W/Wref Frequency 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 L / m W/Wref Stack Length

Pressure

(at fixed drive ratio)

Frequency

Stack Length

Stack Passage Diameter

Optimisation examples

•

_{Increased frequency}

•

_{Increased pressure}

» Power output per volume

•

_{TAE topology}

» Standing wave less complex,

(Hence lighter for given efficiency)

» Travelling wave more efficient (Hence less weight per Watt)

•

_{Working gas}

» Air is cheapest

» Helium allows higher frequency

(hence lighter alternator and TAE)

Optimisation: Cost Issues

Power to thickness ratio

0 500 1000

1 2 3 4 5 6 7 8 9

Optimisation: Alternator

Power versus Frequency for different alternator model sizes, 20mm maximum coil movement

0 50 100 150 200 250 300 350 400 450 500 0 20 40 60 80 100 120 Hz W at ts Model F = £20, 50% efficiency Model A = £2.7 50% efficiency Model C = £4.2 50% efficiency Required output power

Model F = £20, 85% efficiency

Allowable range with simple electronics (Mains)

Operation at higher

frequency increases cost of electronics but dramatically reduces alternator cost. However, noise then becomes an issue.

Demo#0 DeltaEC Simulation

-based on the commercially available parts

0.00 0.04 0.08 0.12 0.16 0.20 0.24 0.28 -6 -4 -2 0 2 4 6 8 10 12 14 Im [ U ] (1 0 -2 m 3 /s ) R e[ p] ( kP a) X (m)

Segments in DeltaEC simulation

1 2 3 4

5

6

1. Bounce volume : 0.175m x 0.175m square, length=8cm 2. Hot heat exchange: HX, Porosity=0.4, Length=10 cm

3. Stack: Honeycomb, Pore D=3.2 mm, L=8 cm

4. Ambient heat exchange: AHX, Porosity=0.4, Length=2 cm

5. Transition Cone, S1=0.01 m2 _{S2=0.0167 m2, L=1.5 cm}

6. Loudspeaker

Diaphragm D: 15 cm; E-Resistance R: 5.5 ohms; BL: 23.8 T-m; Force factor K: 3850 N/m Mechanical resistance: 3.07N-s/m

This “design” is based on the commercially available parts: metal

honeycomb as stack, car radiator as CHX, loudspeaker as alternator.

0.00 0.05 0.10 0.15 0.20 0.25 0.30 -100 0 100 200 300 400 500 600 700 800 900 E d ot a nd H do t (W ) X (m) Edot Hdot

Distribution of Re[p] and Im[U] Distribution of Edot and Hdot

Optimisation Issues:

-

The volumetric velocity lU1l at the diaphragm of LA is decided by its stroke and the area. To transport sufficient acoustic power to the LA, a high pressure amplitude lp1l is required. Therefore, a smaller (but still big enough for HHXs) cross sectional area if required for the engine. As shown in the left figure: 0.01 m2 is chosen for this Demo #0.

In the resonator, the acoustic field is approximately a standing wave. The local acoustic impedance of stack (depends on the location) plays an important role in maximizing the energy conversion in the stack. The local impedance of the stack can be determined by the length of the bounce volume. 8 cm is given by the simulation.

Early Demonstrator#0

Off the shelf parts

Ambient pressure

Powered by Propane

6kW heat in

•

_{Tubular Hot Exchanger}

Stack

•

_{3.2mm hole size}

•

_{100mm square}

•

_{120mm long}

Car radiator ambient exchanger

Linear Alternator

•

_{17cm loudspeaker with}

Mass increasing to 500g

Demo#0 results

Self resonant

(power out condition)

Cold

Burner on

Speaker electrically driven to characterise T/A duct

Mechanical Q measured

• Thiele Q_ms well defined

• Easy to measure

• Separates T/A effect from rig and alternator losses

HHX

• _{Heat transfer mainly through}

radiation

• Significant loss through stack from radiation

• _{Large temperature profile}

Perceived noise increases above 40Hz, even when back of

speaker enclosed. Mechanical issues

• _{HHX cracking} • _Vibration

Acknowledgements

The Score project is funded by EPSRC, the UK

Engineering and Physical Research Council.

Thanks to

•

_{Professor Chris Lawn, Dr Catherine Gardner QMUL, Dr}

Zhibin Yu and Dr Ron Dennis for permission to use their

slides, Dr. Scott Backhaus of LANL and Dr Zhibin Yu at

Manchester for their support with the DeltaEC

calculations, my Nottingham colleagues Professor Mark

Johnson, Mr. Ian Dwyer and Mr. Chitta Saha and the Score

partners, Dr Artur Jaworski University of Manchester, Dr

Keith Pullen City University London, and Dr Teo Sanchez

from Practical Action.

References

[1] www.score.uk.com

[2] The World Bank 2005, “Rural energy and development for two billion people: Meeting the challenge for rural energy and development (September)”