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?
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:
- increased thermal comfort from the radiant heat off the cob bench
- ability to heat sufficiently while using significantly less wood
- high thermal efficiency when compared to conventional wood stoves
- low fuel demand allowing home owner to be fuel self sufficient with coppice wood managment system
- easy and low cost set up with locally available and natural materials
- the ability to heat domestic hot water while heating the home
- 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.