What is the energy difference between green and seasoned fire wood?
I know from personal experience that greenwood (freshly cut) does not provide as much heat energy as seasoned wood (has been down for 6-12 months). When burning it spends a lot of it's energy boiling the water out of the wood. It takes more green wood to provide the same amount of heat as you would get from seasoned wood.
Assuming I let my firewood season for 12 months, if I burn seasoned fire wood and my neighbor burns green wood, how much more fire wood will he burn in season?
According to the Wikipedia article Heating value of firewood there are a lot of variables. I am just looking for general rule of thumb. I always burn seasoned wood, but my neighbor just moved into his homestead and will need to burn greenwood this winter. How much more wood should he expect to need, compared to my normal usage?
1 answer
TLDR
You need about 15 to 20% more green wood to achieve the same amount of heating as with seasoned wood.
Calculation
According to the linked Wikipedia article green wood weighs 70-100% more than seasoned wood and seasoned wood has an energy content of 4.5kWh/kg = 16.2MJ/kg. This obviously depends on the wood, but we are just looking for an estimate. Therefore any equal sign used here is meant as approximately equal.
As wood is usually measured and sold in volume (cord, stere, cubic meters, ...) we neglect the additional weight of the water and just take into account the energy lost due to "boiling" it. Water has a specific heat of 4181J/(kg K) and vaporization heat of 2257kJ/kg. Assuming an initial wood temperature of 0degC green wood thus has an energy content of (16.2 - (2.257 + 0.004181 * 100) * 0.7 to 1)MJ/kg = 14.3 to 14.9MJ/kg.
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