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date 20.Jan.2013

■ Flintstones firewood buying guide

Greece is going backwards in all sorts of departments thanks to the financial crisis and our friends. This winter those of us that aren't yet unemployed, were hit by a 30% price hike on heating petrol, so many people turned to more traditional methods for keeping warm, mostly firewood. We are thus faced with stone age conundrums like "do I buy freshly cut wood for 130€/ton or oven-dried wood at 170€/1.8m³"? As I used to be a scientist once upon a time, I took up the challenge for a logical comparison.

Burning wood sounds simple but if you want to make an objectively sound decision there are a lot of parameters to take into consideration. Some sell wood by weight and some by volume; there are problems in both methods. Wood has varying degrees of moisture (water) and this affects its weight. Buying by volume you solve the humidity problem but then you are not sure how much wood there is in a cubic metre — it depends on the wood size and how tidily it was packed. So which price would you prefer, 130€ for a ton (1000kg) of freshly cut wood (50% water by weight) or 170€ for 1.8 cubic metres of dry (<=20% water) wood?

Scientific wood price assessment

If you google you will find all sorts of guides on wood storing and tables of heat (energy) generated from burning wood with various levels of water content. But the real question for a budget conscious nouveau-poor is how much heating do I get per euro (or dollar)? Does it make sense to pay the extra 40€ to get the better quality dry firewood? We have to consider two issues, firstly how much wood (excluding water) we buy in both cases and then estimate the potential heating energy.

When you buy freshly cut wood with 50% water content, you are really bying a lot of water, which doesn't help with heating. The less humidity the better. So our 1000kg of wet wood is really 500kg of usable firewood. How much wood is there in our 1.8m³ pallet? This depends on the packing but the average density I found corresponds to 450kg/m³ for 20% humidity. With a simple calculation the dry wood in the pallet is 648kg (=1.8x450x0.8). Now you see why the dry pallet costs more: because it contains more wood. But 170/648 is slightly bigger than 130/500, so it would appear that the dry wood is 1% more expensive.

That's not the end of the story. When you use wet firewood for heating, some of the combustion energy is wasted for heating and boiling the water in the wood instead of heating your house. The formula is straightforward to derive. The average energy generated by burning wood is 5.3 KWh per kilo of dry wood (0% water). Each kilo of water in the wood must be heated up from its 20oC room temperature to boiling point (100oC), and then evaporated. The cost of this unwanted boiling is 0.72 KWh per kilo of water, not much but significant for our assessment.
wood or water

Given these numbers it is quite easy to plot the linear relationship between wood energy and its water content. The figure to the right regards 450 kg of wood at various moisture levels. The total weight is fixed, but as the humidity increases the combustion energy goes down. That's both because we have less wood to burn but also because of the energy wasted in boiling water. The green line takes into account just the dry wood; the heat content of 50% wet wood is exactly half of the equivalent 450 kg of dry wood as expected. The red line is the one we need for costing the firewood which takes water into account.

Our 20% oven-dry pallet with its 648kg dry wood would generate 3322 KWh heat for our €170, and the 50% wet ton of wood would give out 2327 KWh at €130. Under this light we get 9.15% more heat per euro spent for the dry wood. So the expensive pallet is better value for money — QED.
wood combustion energy as a function of its water content

Next week we continue our stone age series with tips for doing your wash in a nearby creek, like Aristotle used to do it <g>

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