Firewood

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…

varied heat reflectors

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….

 

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23 responses

15 05 2014
Fed up

Does that Aga burn wood. I thought the original one burnt peanut coke. Later ones seem to power by gas or other liquid fuel. Just asking.

15 05 2014
mikestasse

THAT AGA would not…….. but it will after I convert it!

https://damnthematrix.wordpress.com/2012/07/19/aga-wood-conversion/

15 05 2014
Fed up

If my memory of the Aga we had as a kid, you will find it hard to do. Not like other slow fuel burners, such as Raeburn.

Gas or oil, yeds would be easy. I am only stirring.

15 05 2014
mikestasse

I’m ALREADY doing it…. have been for three years now..

15 05 2014
Fed up

Still work as well as with peanut coke. Beautiful clean stove to run

15 05 2014
mikestasse

No……. it doesn’t run as well. The original design with the steel tube under the top ring that you feed coke to once a day is a brilliant idea. Running on wood, you have to continually feed it firewood…. maybe once an hour, but certainly not as convenient as a ‘proper AGA’…

But you can’t pass up the fully insulated jacket… uses way less wood than an ‘ordinary’ slow combustion wood stove.

15 05 2014
Fed up

It was two ton per year. Mum did not have hot water. Reckoned it would have detracted from the stove efficiency. We had a kerosene hot water service. Worked like gas does today. We also had kero fridges and 32 volt electricity. Yes, a engine and some couple dozens car batteries.

It was the time of the pound for pound wool prices. Yes, and those excessive or windfall profits that sent the rest of the economy ion to free fall. We live like kings for a short time.

I think the Coalition senate ladies had a liquid lunch. As nasty as only women can be. (am a woman). Such hatred for Jenny is being expressed.

15 05 2014
Fed up

Thought that would be the case. In fact it was fed night and morning. Very little ash. Also not much heat outside the stove. The one thing I wish I had from my childhood. Believe it used two tons peanut coke a year.

15 05 2014
mikestasse

Right….. I have wondered how often they have to be fuelled. I have to say I’m puzzled by your “peanut coke”….. I’ve heard of coke and anthracite, but not peanut coke.

This AGA also makes very little ash… I clean the ash out maybe once a week, and it all goes on the garden for minerals.

When I first fired it up, I meticulously weighed how much wood it consumed for a week and came up with an average 12kg/day, which is over 4 tonnes a year if used 24/7

Wikipedia states “The energy density of coal, i.e. its heating value, is roughly 24 megajoules per kilogram” and “The Sustainable Energy Development Office (SEDO), part of the Government of Western Australia states that the energy content of wood is 16.2 megajoules per kilogram”.

Therefore, coal is 50% more energy dense than wood, and the stove would therefore require 50% more wood than coal, or by your reckoning 3 tons per year as opposed to my four tons…. maybe coal burns more efficiently. Did your old AGA also heat water? That would also make a difference I suppose…

15 05 2014
Fed up

Personally. I like mallee roots, which was burnt in the open fire of my youth.

15 05 2014
lemmiwinks

Hi Mike,

No offence intended but given the time to harvest of your average hardwood (not all firewood is created equal) won’t you be dead before you cut down your first mature seedling?

15 05 2014
mikestasse

Ah…. this is why I am buying land with standing trees!

16 05 2014
lemmiwinks

You could be missing the point slightly WRT “sustainability” of firewood Mike 🙂

16 05 2014
mikestasse

How so…….??

15 05 2014
Lee

I’ve discovered when looking around my neighbourhood, there are a staggering number of wooden pallets and other wooden items left on footpaths and it’s all free. Over the last few months my partner and I gathered almost two years worth of firewood. No dedicated wood hunting trips with the car were required. We just looked for it when walking the dogs or out in the car for other reasons. A recent clean out of the garage yielded some more – some timber left over from various projects and some old furniture that I pulled apart and raided before throwing out the leftovers.

15 05 2014
mikestasse

I’ve been known to burn old furniture and floor boards…..!

15 05 2014
Fed up

That is what it was called in those days. Very small balls. Might have another name. As you found out, the cylinder where one puts the fire is not very wide. The coke burned from the bottom. Too young to recall all the technology. Had a grate, that one gave a gentle poke, twice a day. Big job relighting, if one let the fire go out. The convent in West Wyalong had a similar stove. The old nun, could not get used to a gentle prod. To used to old fuel stoves, My father, a non RC spent many a day there, putting the stove back together, Manage to collapse the grate regularly, causing all to fall to the bottom. Was over sixty years ago,

15 05 2014
mikestasse

relighting was a big job, taking 24 hours by all accounts! The best part of the wood conversion is that I can now get it up to temperature in under three hours…… which I did this afternoon to roast a chook.

Those rotating grates weren’t very reliable apparently… I suspect that coal burns very hot, if very slowly.

15 05 2014
Fed up

NO, the grates were very reliable. it was that many were too heavy handed., It was my job as a kid, NO problems whatever.

The old nun was used to stirring fires up, to keep them going. Could not understand one did not need a blazing fire.

16 05 2014
Don

Hi lemmiwinks
Acacia species particularly Acacia melanoxylon, which is one of the longer lived acacia varieties, will give usable firewood in about ten years and good furniture wood also if left to grow on. I am growing my own firewood. Started planting about fifteen years ago. The only problem with acacia is that they do not coppice, but a mixture of trees with some that do coppice is better than a monoculture. Another solution is New Zealand fodder willows harvested when still immature (good for rocket stoves) although you may have trouble sourcing them and with blinkered bureaucrats.

16 05 2014
lemmiwinks

Hi Don,

Reportedly Casuarina is also good and with the bonus of fixing nitrogen. Although if mine (casuarina cunninghamiana) is anything to go by I’ll be waiting a long time for any firewood 😦

16 05 2014
Don

Lemmiwinks,
Acacias also fix nitrogen. They are often used in permaculture for this purpose and as sacrificial trees.

16 05 2014
Linne

Reblogged this on A Random Harvest and commented:
I think many of you will find this very useful. Although many of you are already on this bandwagon. 🙂 ~ Linne

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