Energy Systems

Rockets That Don’t Fly

by Rob Avis

Living in Canada makes staying warm in winter an interesting challenge. In such a cold climate I have long wondered how to continue to keep humans warm (care of people) without bringing down forests or using fossil fuels (care of earth). Even the most energy efficient home with passive solar design will require some sort of external heat input during our winter.

Biomass is simply “ordered” carbon through the process of photosynthesis – ie. stored solar energy. Biomass comes in the form of straw, wood, stover, or generally any matter from living organisms. Wood is a premier choice for heating as it has a high carbon content and will burn hot. However, if there was a massive shift to heating with wood we would quickly deplete our forests and significanlty affect the climate. How do we heat ourselves without bringing down the lungs and life support system of the planet?

Recently we visited Nick and Kirsten of Milkwood Permaculture. I was immediately drawn to their shower block built from an old sheep dip and using rain water heated by what’s called a rocket stove.

Brilliant – this was the missing link! The heating component I have spent years contemplating. Combined with passive solar design I think that we can solve the above stated problem of heating in the cold Canadian winters. Before you get too excited and start jumping up and down screaming hot diggidy, I have two more great reasons to use rocket stoves: (i) anyone can build them and (ii) they are cheap as chips.

Rocket stoves are efficient, clean biomass burning appliances developed by Ianto Evans. The stoves are brilliant in their design as they look at biomass combustion in a totally different way than most biomass burners. The majority of wood stoves burn fuel in chambers that radiate heat away from the fire. This reduces the fires ability to properly combust the wood and creates incomplete combustion. The result is soot, creasotes, dirty smoke from the chimney and more wood is required for a given amount of heat.

So how does a rocket stove work? The rocket stove is built from refractive brick which keeps the thermal energy in the combustion chamber and thus in the combustion process. The burn chamber is designed to maintain the highest combustion temperature possible which ensures that all of the products of combustion are burned. Rocket stoves are designed to burn sticks and small woody biomass. As a handful of sticks have a higher surface area to volume ratio (more edge) than an equivalent-sized log, you get better oxygen mixing and better combustion. Essentially, the rocket stove is designed to provide the perfect ratio of oxygen to fuel to achieve what chemists call stoichiometric combustion.

After the combustion process is complete the combustion products rise up the flue. Because all of the fuel has been consumed the gases are clean and we can now remove the heat without being overly concerned about condensing nasty products such as creasotes, tar and soot. Creasotes, tar and soot are usually the result of incomplete combustion.

Best of all, the fuel source for a rocket stove can be quickly grown and re-grown and re-regrown in a coppice style woodland management. Willow, carrigana, poplar and alder are examples of wood species that would work well in our climate. And just when you thought that we had reached peak awesomness, there is more. All of those species are considered hard woods, grow fast, burn hot and alder and carrigana are nitrogen fixing. If you do not have your coppice system up and running you can also burn pine cones or forest litter – which many properties have in abundance.

What was especially interesting for me about the setup at Milkwood was how they used the rocket stove to heat water. I’ve only before heard of applications to use the flue gases to heat thermal mass (such as a cob bench) for space heating. My brain gears started turning and I realized that the next step would be to marry the two in one system.


(Art above courtesy of Milkwood Permaculture)

I’ll have to do a little analysis but my hunch is that a high efficiency passive house combined with a rocket stove combined with domestic hot water combined with a heated cob bench in a Canadian home (or any home for that matter) is going to work great and have the following benefits:

  1. increased thermal comfort from the radiant heat off the cob bench
  2. ability to heat sufficiently while using significantly less wood
  3. high thermal efficiency when compared to conventional wood stoves
  4. low fuel demand allowing home owner to be fuel self sufficient with coppice wood managment system
  5. easy and low cost set up with locally available and natural materials
  6. the ability to heat domestic hot water while heating the home
  7. the ability to use heated water in under floor hydronic systems

Watch out Canada – coming this May the rockets are going to land and when they do the only space they will ever deal with again is space heating.

14 Comments

  1. I think you will see that solar hot water is still the way to go even in northern latitudes. Using rocket stoves to heat up ovens and mass heaters is very efficient though. Especially if you use fast-growing stick, stalk or stake fuels instead of cutting up whole tree trunks. That way you can leave the trees be, except for the stuff that falls off them naturally or can be trimmed off without harming them.

  2. Pat,

    Solar hot water is great but in many northern climates we can get cold air temp and no sun for weeks on end. In these situations we are going to need some supplemental heat. Solar thermal combined with the system above would make a great combination. As always the question comes back to scale. Are we heating a 400m2 (4000 sqft) Mc Mansion or are we heating a well designed compact, energy efficient, high thermal mass home. If it is the later, the home will only need supplemental heat, which can be supplied via a rocket stove and coppice system.

    Thanks for the comment,

    Rob Avis

  3. This is really exciting, and gives even more to dream about!

    I wonder how long a stick fire would keep a house warm, assuming embedded water pipes in the floor and a heated cob bench (haven’t heard of that one before). I know there are variables such as insulation values and weather outside, etc., but this is interesting.

    So there are ZERO harmful emissions into the air?

    I wonder how easily an old house could be fitted with this technology (not embedded pipes, but a simple rocket stove to help heat the interior)?

  4. This is great! Will it work in humid continental climates as well? The winter of 2009-2010 was exceptionally cold for the northern half of the States.

  5. I don’t remember for sure, but I think I learned some salix bushes grow 3-4 times faster than Nprwegian Spruce, and should be ideal for “sticks”. And combining it with a hot water tank from solar energy is splendid, because still thermal solar energy gives 5 times more energy for the same area than solar cell panels. The only thing I miss here is a stirling engine which can transform the heat from the stove to electricity. In summertime you can move this fabolous engine outside, producing energy from a mirror parabol: https://en.wikipedia.org/wiki/Stirling_engine

  6. I think that if you put the hot side of the stirling engine at the hot water thank, this will stay so hot that you can make electrisity all day long, 24 hours. And in summer you don’t need to use the mirror parabol, because you harvest the solar energy with thermal energy, and then transform this to electricity by the hot water tank connected to the stirling engine.

  7. I think biomass is not available in abundance for this population and are people so conscious for the environment that they ready to use matter of living organism like cow dung.

  8. Worth a trip down to Portland to see rocket mass heaters in action. See Wikipedia for links. Ive been experimenting with Jean Pain composting for hot water and methane. Very promising also.

  9. hey leeroy, saw your compost heating videos, nice work.

    The rocket water heater makes for a lovely spot for students and interns to hang out after classes and is so bloody convenient. It only takes about a handfull of sticks and 10 minutes to get a shower going.

    I really can’t stress how much more efficient this thing is over the other wood fired water heaters we have built before.

    You should come up and have a look, since you are just down the hill now.

  10. Hi there,

    I have just discovered rocket stoves and am very excited. One concern we have is the effect it would have on the air inside the home. It is great that little to no smoke is created but I assume that, due to the strong draw, quite a lot of oxygen is required to fuel the fire. I wonder if this is an issue in a well insulated home?

  11. Kristen

    Your concerns are valid! In an air tight home this type of stove becomes and issue and so you would need to use a mass oven with direct air intake. Alternatively you could also do a little re-design and make the rocket stove direct vent.

    Thanks for the comment.

    Rob

  12. please dont seal up you houses air tight, nor run some kind of direct air feed. if your house has enough air for you to breath it most likely has enough air for you and the stove. the modern air tight trend is causing more problems than it solves. Mold, retained off gases, bacterial counts that are very high, this is called sick house syndrome. The tighter the house the worse it gets. A rocket stove is a great solution to heating but its not a magic pill for every situation. A rocket mass heater is easy to build however; we suggest you build a prototype outside to the exact dimensions in the book before you try to modify the design. to many time Owner builders call us with problems because they haven’t tested modifications before installing the stove in the home.

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