First, let me congratulate you on your new kickstarter project, I can't wait to see the result. It will help a lot of people.
I've been interested in AGS (annualized geo-solar) for many years, I've exchanged e-mail with Don Stephens the originator of the concept
I'm considering using a modified version to help warm up the base of our earthship's walls this spring and we didn't install one when we built, but that's another story!
Our experience with the AGS system at Kinark is that it has reliably kept the ground under the building at ~15C since 2007 (one year after construction). Nobody ended up recording the data from the floor sensors there, but I've visited the building numerous times during winter, and the floor temp has been quite consistently in the 15C range.
This is an okay result for a low-tech system. To be able to keep a building well above freezing using stored summer heat is a decent accomplishment. However, 15C is not a comfortable living temperature for a full time residence. In most cases, a home owner will want to add some extra heat input to bump things up to the 18-22C range that is more typical for homes. The problem with an AGS-based system is that the high amount of mass in the floor will "swallow up" those extra inputs of heat, and it will do so fairly quickly as the homeowner is trying to warm such a vast chunk of earth.
At a place like Kinark, this is fine. The building is not continuously occupied and when it is occupied it's not for a long time. They can turn on the secondary heating system for a quick blast of heat to keep everybody comfortable for a few hours, and if that heat dissipates into the floor after a few hours, that's fine.
We repeated the system this past season for Circle Organic Farm's veggie processing facility. Again, this is a building that can easily function at 15C, as people are working in outdoor clothing and constantly in motion.
I wouldn't put the AGS system into a house in our climate. As with any ground-coupled building, the amount of heat input required to try to heat the planet is too big, and heat will need to be added constantly to keep things in the comfortable range.
Location: Chertsey, Qc
posted 5 years ago
Thank you Chris,
I understand what you're saying as I live in a ground connected Earthship in Quebec, with no AGS. The floor temps are around 11C and we still manage to burn less wood than all our neighbors thanks to our rocket mass heater.
I've been thinking of doing an insulated mass underneath a house and heating it with an AGS. Lots of experiments to do still
I am interested in building an earthbag home with a green roof and using AGS in north Alabama. I can't seem to find information on sizing the earth mass and the solar collector. Do you have any guidelines for sizing a system? I can see the possibility of adjusting/correcting the solar collector size after initial construction, but the earth mass under the house is going to be pretty well fixed. I would appreciate your advice on sources of sizing information.
What are your thoughts on using arched thin shell concrete for the ceilings. I'm thinking about multiple arched thin shell concrete rooms, with possibly duct work in the valleys between the arches with some type of lightweight fill for the remainder of the valleys to make a flat top (non level for waterrunoff) and then a 3-4" concrete slab with EPDM, then the green roof. How to finish the edges puzzles me too.
I am getting old and don't think I have the stamina to hand plaster the walls. What do you recommend for mortar sprayers?
posted 5 years ago
Before anybody builds anything, I always like to start with the question "Why?", and then encourage people to set out their criteria for the building before attaching any technologies or materials to the project. Then, when a person knows what they want to achieve (and why), it's much easier to decide how to go about achieving it.
That said, I'll assume you've thought this through and your proposed building is what will best suit your needs/wants.
There is no good and accurate way to size an AGS system. There are only a handful that have ever been built, and of those, even fewer that have accurate test results that would help to create some guidelines. Either yourself or with a suitable engineer, you'd have to look at the heat capacity of the particular soil you're working with, and the Btus you can expect in a typical year from whatever sort of collector you'd be using. You'd have to guess at the system's inefficiencies and losses, and you'd arrive somewhere in some ballpark that might hopefully be reasonable.
Chief among the losses to consider is migration of your stored heat away from underneath your building. The ideal AGS system would be fully excavated, lined with a really good insulation, and then have the dirt returned to its insulated home. Without full insulation, losses downward and to the sides are substantial, as you are essentially trying to warm the planet in every direction under your home, and the planet has an almost infinite appetite for your heat. That's a lot to try and figure out accurately...
Arched, thin shell concrete has some great structural properties and can look great from the inside. Again, I always try to return to goals and criteria. Why this kind of ceiling? If there are compelling reasons, great. The thin shell concrete will not offer any amount of insulation value, so you'd have to not only figure out the roof design and green roof layers, but be sure to be including adequate insulation for your climate. You may have read that green roofs provide a reasonable R-value, but this isn't really the case. They definitely help keep a roof from overheating in the summer, but in the winter they offer little to no insulation value. An undulating roof like the one you are proposing would be difficult to insulate in any way other than spray foam. Nothing wrong with choosing spray foam unless concern for the environment is anywhere in your criteria...
The edges of such a roof assembly will be difficult to finish cleanly, and it will be hard to keep runoff from coming down the walls and staining them. Straight edges can be troughed, but round roof edges make it hard to collect/direct water.
There are many companies with mortar sprayers that you can hire. You can even buy a decent mortar sprayer (we own a Tirolessa http://www.tirolessausa.com/), but you'll still have to hand trowel the sprayed plaster unless you like the random, fuzzy, bumpy look of sprayed plaster.
Your project sounds interesting, could be feasible, and also seems like a lot of work! I would suggest making sure that you are getting what you want out of this design before committing to it.
posted 5 years ago
Thanks for the reply Chris,
I have not been able to find much design data for the AGS system and it is at least somewhat reassuring to know that there really isn't much of it available. Your explanation of the best insulation techniques reinforces my present line of thinking.
I'm still struggling with the ceiling and roof design. Again your comments on insulation values was a good point. The trough concept on the edges has me thinking.
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Heat your home with the twigs that naturally fall of the trees in your yard