Experiments with measurement of heat output from log

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LEAF Log stove project
Technical team
Jacinta MacDermot
Rob Gwillim
Chris Laughton
LEAF Log Stove Project
RHI and log stoves
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RHI =Renewable Heat Incentive
Introduced in phases; Non-households first
Household phase delayed till 2013
RHPP = Renewable Heat Premium Payment
Community groups and social landlords
‘competition’
• Biomass boiler - £950 grant (for homes without
mains gas heating)
• No log stoves !
LEAF Log Stove Project
Why are log stoves not RHI
eligible?
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Difficult to measure heat output
Some wood fuel not ‘renewable’
Multi-fuel stove market saturated
Air heating without using water – excluded
Intermittent vs primary use
Fossil fuel – user interaction
Fossil fuel (ignition process, pumps, fans)
Air emissions
Perceived low efficiency
LEAF Log Stove Project
Improving RHI compatibility
- Heat measurement
challenges
• Stoves emit most heat by convection and radiation
• Some stoves have ‘back’ boilers heating water
• Pump vs thermosyphon
• Surface temperatures at various temperatures
• Chimney losses
• Combustion air replacing room air
• Fuel temperature
• Combustion efficiency varies
• Calorific content and moisture content
LEAF Log Stove Project
Improving RHI compatibility
- Heat measurement
experiment
• New and old stove installations
• Machynlleth and Newtown
• Provide logs of known moisture content
• Users measure the mass burned each load
• Temperatures of stoves and flue recorded
• Flue gas velocity/mass flow calculated at full burn
• Theoretical efficiency looked up
• Infra-Red images for spread of heat
• Theoretical radiant emissivity and convection rates
LEAF Log Stove Project
Moisture content results
Sample
Start
weight
End
weight
Weight
differ
Moisture
content
wet basis
g
g
G
%
Moisture
content
dry basis
Electronic
moisture
meter
%
%
A
607
478
129
21.3
27.0
B
565
467
98
17.4
21.0
C
1005
832
173
17.2
20.8
D
589
471
118
20.0
25.1
E
697
563
134
19.2
23.8
19.0
23.5
LEAF Log Stove Project
23.4
Recording temperatures
LEAF Log Stove Project
Infra Red Camera
LEAF Log Stove Project
Temperature results
LEAF Log Stove Project
Temperature vs fuel input
Fuel Input and Temperature (Stove E)
250
30.0
25.0
20.0
150
15.0
100
10.0
50
0
12:00
5.0
18:00
0:00
Flue Temp (oC)
LEAF Log Stove Project
6:00
12:00
Case Top Temp (oC)
18:00
0:00
Fuel Added (kWh)
0.0
6:00
kWh
Temperature (oC)
200
Snapshot averages measured in operation
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Flue temperature 133 C
Top of stove casing 136 C
Mid stove casing 106 C
Bottom of stove casing 71 C
Stove glass 150 C
LEAF Log Stove Project
Average derived or manufacturer values at rated
output
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Stove efficiencies 75 % (probably gross)
Minimum flue draught 12 Pa
Mass flow-mass 7 grams/second
Flue speed 0.44 metres/second
290 C
LEAF Log Stove Project
Method A
• Erhi = Eligible heat for RHI
• Ein = Calorific value of wood
• Effic = Efficiency of combustion net (lower)
Method A
Ein * Effic = Erhi
Ein
LEAF Log Stove Project
* Effic
Erhi
Method B
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Ein = Calorific value of wood
Eflue = Lost energy up flue
Eair = Energy lost through combustion air change
Esundry = Energy lost from cold fuel, opening doors
Eelec = Pump or fan or ignition power
Erhi = Eligible heat for RHI
Method B
Ein – Eflue – Eair – Esundry – Eelec = Erhi
Ein
Ein
LEAF Log Stove Project
Eflue
Eair
Esundry
Eelec
Erhi
Method C
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Erhi = Eligible heat for RHI
Eemis = Energy emitted from stove as radiation
Econv = Energy convected from stove by hot air
Ewater = Energy circulated from the stove to water
Method C
Erhi = Eemis + Econv + Ewater
Eemis
Erhi
Econv
Ewater
LEAF Log Stove Project
Method D
• Erhi = Eligible heat for RHI
• Eair = Energy lost through combustion air change
• Esurfaces = Energy heating room surfaces
Method D
Erhi = Eair + Esurfaces
Eair
Erhi
Esurfaces
LEAF Log Stove Project
Proposals for improving RHI compatibility
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For new stoves and chimneys or re-lined
For stoves tested to EN 13240
Maintenance service agreement
Known wood supply to less than 25% MC
Primary heat source – not intermittent use
Not possible for appliance to be multi-fuel
Thermosyphon heat to be deemed
Recording method for proof of use
LEAF Log Stove Project
Margins of error
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Temp sensor calibration and insulation
Recording (logging) failures
Deviation of moisture content samples
Chimney calculation – Missing Appliance Data
Fuel not meeting manufacturer’s minimum
Non-compliant installations – Combustion air
Non-representative temperature locations
LEAF Log Stove Project
The Solar Design Company
0845 519 7914
www.solardesign.co.uk
LEAF Log Stove Project
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