LEAF Log Stove Project

LEAF Log stove project

Technical team

Jacinta MacDermotRob Gwillim

Chris Laughton

LEAF Log Stove Project

RHI and log stoves

• 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?

1. Difficult to measure heat output

2. Some wood fuel not ‘renewable’

3. Multi-fuel stove market saturated

4. Air heating without using water – excluded

5. Intermittent vs primary use

6. Fossil fuel – user interaction

7. Fossil fuel (ignition process, pumps, fans)

8. Air emissions

9. 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

SampleStart

weight End

weight Weight differ

Moisture content

wet basis

Moisture content

dry basis

Electronic moisture

meter

g g G % % %

607 478 129 21.3 27.0A

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 23.4

LEAF Log Stove Project

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)

0

50

100

150

200

250

12:00 18:00 0:00 6:00 12:00 18:00 0:00 6:00

Tem

pera

ture

(oC)

0.0

5.0

10.0

15.0

20.0

25.0

30.0

kWh

Flue Temp (oC) Case Top Temp (oC) Fuel Added (kWh)

LEAF Log Stove Project

Snapshot averages measured in operation

• 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

• 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 AEin * Effic = Erhi

Ein * Effic Erhi

LEAF Log Stove Project

Method B

• 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 BEin – Eflue – Eair – Esundry – Eelec = Erhi

EinEflue Esundry Eelec Erhi

EinEinEinEinEinEinEinEinEinEinEinEinEinEinEinEinEinEinEair

EinEin

LEAF Log Stove Project

Method C

• 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

Erhi

Method CErhi = Eemis + Econv + Ewater

Ewater

Econv

Eemis

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

EairErhi

Esurfaces

LEAF Log Stove Project

Proposals for improving RHI compatibility

• 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

• 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