15 05 2014

I have been asked by several people now to write about the efficient use of firewood as an energy source.  I’m a firm believer that as we approach the looming mother of all energy crises, anyone thinking ahead of the pack should be seriously considering their options regarding the ways they will keep warm (especially in cool climates), make hot water, and cook.  I regard firewood as being one of the most serious options out there.  Solar is still the best, because the sun is free…. but unless you have gone to as much trouble as I have to design a lifestyle around solar, you will find the limitations of renewables pretty quickly.

ERoEIchartSo, why firewood?  Just look at the chart at left….  Firewood’s ERoEI is better than imported oil, wind, gas, nuclear, solar PV, and bio fuels.

Firewood is a renewable resource.  It grows on trees!  It’s more renewable than solar as far as I’m concerned…  However, demand for this fuel can outpace its ability to regenerate on local and regional level.  For example in some places in the world and throughout history, the demand has led to desertification.  Good forestry practices, and I’m talking about growing your own here, is essential to having your own sustainable source of energy.  Once in Tasmania, it’s my intention to replace any tree I cut down with several new seedlings to replenish the source and absorb the greenhouse emissions thus generated.  Not being able to grow firewood is my main reason for refusing to ever live in a suburban setting again.

Furthermore, if you grow the stuff yourself, you won’t need to pay for it (read work!) or move it very far.  Remember that once the oil supply shuts off, ringing up and ordering a truckload of firewood will be either impossible or unaffordable.  Always think ‘worst case scenario’.  If you live near a sawmill, there is every chance they will sell you scraps for peanuts.

Heating value of firewood

The moisture content of firewood determines how it burns and how much heat is released.  Unseasoned (green) wood moisture content varies by the species; green wood may weigh 70 to 100% more than seasoned wood due to water content.  Typically, seasoned (dry) wood has 20% to 25% moisture content.  It takes more than one year of drying time to achieve this, and depending on where you live, it could take two or even three years……  Not only does green wood release less heat, it also smokes, smells,  and gums up flueways in stoves and cookers, requiring more frequent flue sweeping.  I learned this the hard way!

The energy content of a measure of wood depends on the tree species, ranging from 4.75 to 9.8 GJ (Giga Joules) per m³.  To put that into context (because only an energy nut like me can visualise what a GJ is!), that much energy is equivalent to 1320kWh to 2820kWh, or about 2 to 4 months of your average wasteful Australian household electricity consumption. As you can see, the energy content of firewood is not to be sneered at….

The higher the moisture content, the more energy that must be used to evaporate (boil) the water in the wood before it will burn. Dry wood delivers more energy for heating than green wood of the same species.  It’s as simple as that.

Firewood energy efficiency

Now you have your firewood cut, split, and stacked, you don’t want to waste all the energy that went into this effort, let alone the energy in the wood itself.  Surprisingly, the first thing to consider has nothing to do with the wood….  is your house adequately insulated?  If all the heat your device generates simply goes out the window, you will have done a lot of work for little gain, even if you like watching fires burn…  therefore, first do whatever it takes to ensure your ceiling space is well insulated, that you have curtains and pelmets on your windows, and that your house is as draftproof as possible.  I know first hand it’s very hard to get an existing house improved, but anything you do will help, and save you money, effort, and firewood…

Unfortunately, a lot of heat escapes straight up the flue.  The flue itself, however, can be the major source of space heating from your stove.  Heat reflectors that can be attached to flues to bounce heat back from the space near the wall (most stoves are installed in front of walls) and they also protect your wall from heat damage.

As most readers here would by now know, I am a great fan of AGA cookers because they are (as far as I know) the only ranges that inject cold air from the floor level into the flue to cool the flue down and reduce draft.  This ensures that much of the generated heat remains inside the cooker for as long as possible.

This works so well, our flue is rarely so hot that it will actually burn your skin on contact, something ‘ordinary’ heaters can do and anyone considering buying a stove should be aware of as a safety issue.  Even when ‘redlining’, the AGA’s flue is so cool where it exits the roof space, that one can (perched atop an eight foot step ladder!) put one’s hand on the flue and leave it there indefinitely…….  the best part of this is that we don’t even need ceiling exit protection to ensure the house won’t burn down!  Our flue simply goes straight out the cavity with just a few sheets of fibre board nailed to the studs for peace of mind.

My next AGA for the Tasmanian project will almost certainly be a fully recycled and overhauled four oven model, as I expect we will need a lot of younger hands to carry out the project and teach sustainability in return with loads of food to be cooked for the hungry hordes..!

4ovenagaThere are all sorts of clever devices on the market for burning wood, and this latest one really caught my eye..

Don’t discount the power of ‘sticks’….  we have fired pottery at over 1000ºC in a well insulated kiln using just 1 kg of scrap pine.  Pine burns much faster than hardwood and is great for achieving high temperatures very quickly.  I also use it to fire the AGA up and quickly get it to operating temperature whereupon I switch to hardwood for long burns and constant temperature.

I have never used rocket stoves – I think the bio-lite above may well be a version of one – but people I know who have swear by them.

They can be easily made from scrap material lying around or even turned into beautiful pieces of art such as the one at right.  Click on the photo and a great website about rocket stoves will be yours to peruse.

Also very efficient (but I have no idea of availability or cost in Australia) are so called scandinavian mass heaters.  After all, if ever there would be a people who know a thing or two about keeping warm, it’s the Scandinavians!

They rely on heavy thermal mass such as bricks or stones to store heat and can be very impressive looking to boot… though I don’t know how you’d build one inside your typical house without some major mods to the place! To me, they just look like a variation on the rocket stove theme, but having used the cast iron AGA, I can vouch for the usefulness of thermal mass or inertia in a stove.

Our latest wood burning addition is the cob oven which we will use in Summer when it’s too hot to fire up the AGA.  Apart from making pizzas in, we will use this oven like a regular one to roast meat and bake bread and cakes.

More of Alessandro's handiwork

More of Alessandro’s handiwork

In truth, your possibilities are only limited by your imagination and availability of the fuel.  The only rule to obey at all times is only burn aged firewood!  We don’t want to give firewood a bad name, and good luck….