Rocket Mass Heater in Raised Growbed

Hello,

I have seen several threads on building a rocket mass heater within a greenhouse. However, I'd like help brainstorming on how to build one within a covered raised growbed.

Please know I have an adequate understanding of growing but have no firsthand knowledge of RMHs.

I'm considering modifying a keyhole garden. These are typically round raised beds with a pie-shaped wedge cut from the edge to the near center, allowing easy access to the center which has been reserved for compost and watering.

I'd like to place the burn barrel in the center with the heated exhaust running under the bed. I would like, as much as is possible, for the majority of the energy to radiate up through the grow medium and not escape from the barrel.

Although the total height of the cover had not yet been determined, I don't have the luxury of a full height greenhouse. I fear heat from the barrel melting the cover. The obvious solution is to have a shorter stack within the barrel. (It seems a cooler barrel would improve draft.) Another option would be to install a thermal mass (such as a cylinder of gravel) enclosed in a wire mesh surrounding the barrel. The third option would be to install some form of heat deflector over the barrel to dissipate the heat.

I invision a round (or octagon?) bed with a 55-gallon barrel in it's center. A diameter of around seven feet should allow me to reach most of the growspace. A wedge from the circumference up to the barrel would allow access the stove. The stove would vent under the growled ideally into an exhaust with a thermal mass such as cinder blocks or clay pipe. Gravel could be placed over this to increase thermal mass as well as improve drainage. Grow media would be placed over this. In summer months the bed would remain open. In colder weather either a dome- or cylinder-shaped cover could trap both solar and radiant heat.

1) Are there any resources online where people document similar projects?
2) Is this project even feasible?
3) What improvements can be made?

Thank you.

Consider adding a flat piece of thin steel, slighter wider than the top of your barrel and suspended directly over your barrel about six inches. This will reflect most of the IR heat from the barrel back towards the soil, and would help to prevent your greenhouse plastic from melting.

Can't promise it'd work, though.

Chris A. : Have you been to rocketstoves.com to download Ianto Evans' great book 'Rocket Mass heaters' $15.oo U.S. ? This is where (page 36) I
got the information that the larger the barrel the lower the surface temp, think about it, for the rocket mass heater to push its hot exhaust gases horizontally
we have to create the temperature gradient between the heat riser and barrel bottom. A smaller barrel will have to radiate just as much heat off of a smaller
surface, the only way it can do this is to radiate at a higher temp!

This is why when a 3 yr old wants to climb up in your lap for a cuddle it is so pleasant, their mass to surface area is such that in order to maintain homeostasis
They have to radiate at a higher temp !

While a cylinder of gravel in contact with the barrel will work at start up, even before the gravel cylinder temp equalizes with the temp of the barrel your Rocket
Mass Heater will stop drawing ! If you increased the inside diameter of your keyhole you could move out about a foot before building a brick/pavers wall with
about as much air space as brick, think of a lattice wall !

Where do you live, and do have any experience with cob ?I only raise this as an issue because you may have to modify the way you construct your raised beds.
Local / Seasonal Greenhouse practices vary from region to region with the high humidity in the coastal N.E. and N.W. modifying practices of dryer areas !
Without depriving your plants, you may have to modify your watering technique, the clay/sand/straw cob mix is very Hydroscopic, and can soften and slump if
frequent watering and hi humidity is common and prolonged, where other people have used plastic or sheet metal between their raised benches and their grow
beds, you will have to be creative to protect your thermal mass from too much water- only time will tell what will work for you ! Y.M.M.V.

GOOD LUCK Be Safe, keep Warm, Pyro magically - Big Al

Nice idea, I had a similar one when I wanted to start aquaponics.

I was going to reserve the centre for the water tanks so the only place to put the RMH and the exhaust path would be on the outside perimeter of the grow beds.

Similar to the keyhole garden, as you approach the garden entrance the rocket stove is on the right of the doorway and the exhaust is on the left. The growbeds are all around the perimeter and as they are on top of the mass (old concrete) they are nice and high for cropping.

Needless to say the green house was going to be a geodesic dome.

So now I have to learn about RMH, got the book, just need to read it, waiting for the kickstarter DVDs to turn up to watch them. I have several books on aquaponics, that will be the easy part. I have several books on ferrocement (all from the 70s and 80s, nothing more recent). I have the book on earth sheltered greenhouses (I live on a gentle slope) I have several bookmarks for geodesic domes including the easy to make chicken tractor version.

My aim is to heat up circulating water which should help even out the fluctuations in temperature between the front and end of the exhaust tube as well as the initial fire up and die down temp variations. It will also act as a decent thermal mass.

I have a sketchup model at home, I will see if I can dig it out but I figured a 6m diameter keyhole would be about right, that way the partially buried dome would only be about 3m high, just below council regulations.

The only thing I want to make sure is that there will be enough draw to pull the exhaust along a 15m horizontal path, I am sure that is in the book somewhere. With a curved roof I don't have much support for a high vertical exit.

Chris A., Mark L. : Not to rain on anyones parade, but it sounds like you both might be thinking about using flexible stove pipe Which is a good product to use - just pricey !
Mark is hoping for a 15m system, ( 50' ) this is at the upper range of an 8 " system. When I was selling stove pipe in 30" sections we counted all 90 Elbows as equal to 1
section of that pipe to figure friction losses. Ernie Wisner counts a 90 degree Elbow as equal to 5', With adjustable elbows set to make 45 degree turns the loss is not as hi
but both of you need to do a real world physical layout of your stovepipe as well as laying it out on paper and carefully add up every piece, not forgetting to allow for plenty
of clean outs !

Again do to the fact that structural cob is made of clay slip, sand, and chopped straw it is hydroscopic and if allowed to be too wet, too often, it can slump, I would definitely
use a Floating cover on my aquaponics tank(s), and try to reduce spillage,especially in the hi humidity coastal areas of the N. E. and the N. W. . In these areas i would not
store my Dry wood supply for my Rocket Stove within the green house relying on a wood box outside that can be reached thru a well sealed door or hatch !


If this comes off as being negative, which I have been accused of being, I apologize, I just wanted to share this information so it wouldn't turn around and bite you !

GOOD LUCK, Be Safe, keep Warm, Pyro magically - Big Al

Not negative at all, just constructive! I have read your posts on the topic in the past and welcome your input.

I was actually thinking of using flexible vent pipe (used for tumble dryers and extraction fans) and putting a few layers of wire mesh and pouring a layer of concrete over it. Maybe even a slip made of refractory cement if ordinary wouldn't stand up to the temps. I figured it would not last very long before it burnt away and hopefully I would end up with a concrete exhaust.

I am still in the planning stage of course and looking at options, so far I have come up with building a brick tunnel and plastering as much of the inside as I can as I go along, to casting it in concrete sections and mortaring it together. I am not sold on the flexible vent tube as I beleive it is not very smooth and I haven't read enough to see if turbulence in the exhaust is a bad idea and whether it will lead to a blockage.

Always looking for advice and ideas!