Development of a wood-fired cooking stove to incorporate a thermo-acoustic engine-generator unit Ron Dennis Prof. Keith PullenCity University City UniversityLondon London

Introduction

• City University has been responsible mainly for stove development

• This includes development of stoves to test thermo-acoustic engines (TAE) developed by other members of the SCORE group and carrying out the testing

• This paper describes the development of the general concepts of the stove design and specifically of a stove in which the TAE is placed vertically above the stove combustion chamber (CC) with heat transfer principally by radiation

Aims of SCORE project

Develop a wood-fired cooking stove for domestic use that:

1 Produces 100W of electricity for up to 4 hours per day

2 Boils 3l of water in 15 minutes

3 Reaches 50% of full power in 20 minutes

4 Reduces wood consumption by 20%, compared to traditional stoves, to less than 1.4kg/hr

5 Has low emissions

Initial Design Concept

1 Heat initially to TAE then to cooking

2 Acceptable to users – use ‘Rocket Stove’ principle used widely in improved stoves. ‘L’- shaped combustion chamber –• Horizontal leg to feed wood and air• Vertical leg to draw in air and mix with wood gases for good

combustion

3 Radiation of 2kW to TAE from 300mm disc on top of combustion chamber (CC)

4 Exhaust gases from CC pass through cooking section then to chimney to minimise emissions

5 Two cooking pots, one cooking, one simmering

6 Low thermal mass

Initial Design

80% Vermiculite/20% CementOctagonal CC to fit 300mm disc

20mm fireclay on 20mm cast tile

Two recessed pots, 200 mm diameter; 10mm gap under pot , 20 mm gap around pot

Wood

Air

Instrumentation

Test Set-up

Test Results

Comparison of Temperatures for Combustion Chambers with Different Cross-section Areas

Wood burn rates: Large CC (290mm dia.) – 2.5 kg/hr Small CC (140mm dia.) – 1.5 kg/hr

Conical Combustion ChamberCombustion chamberCeramic or stainless steelTop ID – 300mmBottom ID – 200mmHeight – 135mm

40

130

200

40

FireboxVermiculite/cementCoated on inside with fire cement

Disc sits on top of conicalCombustionchamber

Exhaust

Test Results

Temperatures for Various Combustion Chamber Materials

Wood burn rate 1.5 kg/hr

Heat from Combustion of 1 kg of Wood

CO Emissions

Summary Of Results

Component of heat loss

% of Input Heat Loss Explanation

Test A Test B

Combustion 25.8 30.2 Heat from wood – gas heat at top flame temp

1 CC 26.0 21.5 Heat from wood – gas heat at CC exit – heat to drum

2 Pot 1 29.2 23.6 Gas heat before pot – heat after pot – heat to Pot 1

3 Pot 2 10.5 6.0 Gas heat before pot – heat after pot – heat to Pot 2

4 Flue gas 12.9 20.2 Gas heat to flue

Total loss 78.6 71.5 Losses 1 to 4

Useful heat 20.9 29.5 Heat to drum + heat to pots

Total check 99.5 101.0 Total loss + useful heat

Summary of Progress

Performance criterion Target Achieved to date

1 Heat to TAE 2 kW 1.08 kW

2 Boil 3l of water 15 minutes 27 minutes

3 Reach 50% power – 90% temperature 945 oK in 20 minutes

895 oK in 25 minutes

4 Wood consumption reduced 20% < 1.4kg/hr 1.5 kg/hr

4 Low emissions Minimise smoke Achieved with chimney

5 Stove efficiency 50% 29.5%

Conclusions1 Combustion•Probably approaching best that can be achieved for domestic stove with varied type and condition of fuel•One improvement in testing will be to adapt rig to use longer pieces of wood for more continuous combustion•Stainless steel CC probably not durable and need to find better material

2 Cooking•Improvement needed. Limit is area in contact with gases for convection. Can be considerably increased by ribbed hotplate which also has other significant advantages. Aluminium heat sink being tried

3 Stove design•Concept is for heat-resistant inner shell of low thermal mass well insulated with lightweight insulation (probably wool), contained in robust, low-cost case (probably concrete blocks)