Gray House Specs - Climate Battery Greenhouse Version 1

The Concept

The concept of a climate battery greenhouse is to use the earth beneath as a sort of "thermal battery" (thermal mass) for storing excess heat generated by the structure. Greenhouses, in most climates, typically generate way more heat than they can immediately use. As a result, the excess heat is often vented to the outside to avoid injuring plants. In a climate battery system, excess heat is transferred to the floor beneath the greenhouse where it is "stored" for later use. The storage and transfer of this heat is typically accomplished by burying corrugated perforated tubing beneath the greenhouse structure and running a fan to push air through the tubing. During the day, warm (and typically moist) air is pushed underground, where it (ideally) condenses and releases heat, coming out the other end of the tube cooled and possibly at a lower humidity. At night, when heating may be needed, fans push colder air underground to pick up heat and release it into the structure.

The idea for this structure is not new and has been in development, I believe, since the 1980s or even before. Several individuals and companies have designed and deployed similar systems in climates much different than ours. We consulted with Eco Systems Design on our system, and Ceres Greenhouse Solutions also consults on climate battery projects. If you're unfamiliar with the concept of a climate battery, both sites are excellent in terms of information and Eco Systems Design features an FAQ utilizing many of the questions I asked them during our design phase. Other techniques for using the heat of the earth and excess captured heat from the sun can be found in structures like the walipini and solar greenhouses. Our desire was to build a system that could be used with an off-the-shelf high tunnel frame that could therefore be easily repeated and reproduced if proven successful.

Goals

Season Extension or Year-Round Growing

Our goal is to keep the greenhouse, at a minimum, 20F in a zone 6b/7a climate. This would allow us to overwinter subtropicals and grow most greens through the winter. In a structure this size, this will be accomplished through the climate batteries and a well-sealed structure. Additional energy-saving items could be employed to save on heating.

Utilize an Existing Greenhouse Frame

We've seen several examples of climate batteries installed in a highly insulated and custom-designed and engineered greenhouse structure. Since they're rather uncommon, these structures are costly both in terms of paying someone to design them, but also in the construction phase. Highly insulated structures are certainly more efficient energy-wise and may allow you to grow even tropical plants, but we couldn't justify the cost per unit of growing area versus a traditional greenhouse structure.

Low Ongoing Energy Consumption

When first looking at putting in our high tunnel, we considered using a propane heater to keep the greenhouse at or slightly above freezing. To do so would have necessitated a large heater and a few hundred gallons of propane a year. Our current system runs off of 3, 20" fans that together consume about 1.1kW when running at top speed. If run for an average of 16 hours per day for 8 months at $0.12 per kWH, would cost a little over $514/year to operate.

Design

We modeled our design off of several concepts we had encountered in research. One was a greenhouse in a zone 4 climate that was of similar size to our high tunnel (30'x96') that included four batteries. Our thought was that our climate wasn't nearly as severe, and we were only wanting to keep the system in the low 20s on the coldest nights of the year.

Climate Battery

Our design involved three separate "batteries" installed within the soil footprint of the greenhouse. Each of these batteries consisted of a 24" ADS pipe riser (a section cut to ~6' long) connected to a 20' long, 18" diameter horizontal manifold on one side of the greenhouse connected to another 24" riser with an 18" manifold on the other side. Connecting these two manifolds and spanning the width (rather than the length) of the house were 30' sections of 4" ADS drain pipe (corrugated, perforated). Each manifold held ~45 or 46 runs of the 30' long, 4" diameter tubing, meaning each climate battery contained approximately 1350-1380ft of tubing.

The tubing was run on a diagonal from side to side since our greenhouse width was only 30' and wouldn't have allowed for 30' side-to-side runs. Our goal was to bury all of the tubing starting at 4' below grade in 2-3 layers so that the tubing stopped 2' from the surface to allow us to plant in-ground. One climate battery would be installed even lower in the ground, ideally 6-8' underground, to allow us to capture some of the earth's latent heat for the coldest of nights.

Insulation

Based on some recommendations and research, we decided to insulate the perimeter of the structure with R5, 1" foam board insulation. The use of R5 was really determined by cost. We would have liked to have gone even heavier with insulation to perhaps a 2", R10 board, but the budget did not allow for it. We chose not to insulate underneath the climate battery due to some research we had seen saying that little heat is lost to beneath the structure, and indeed we even wanted some of the latent ground heat to come up into the climate battery during very cold periods of the year.

Greenhouse Structure

We determined to use a 30' wide x 96' long gothic arch style structure with extended ground-posts. This size is fairly typical and it's easy to find plastic for it. We chose double-layer inflated poly with polycarbonate end walls, a pretty typical setup, though the polycarbonate is a pricier option, both for the insulation benefit. We chose IR/AC (sometimes called IR/AD) film for our inner layer due to the ability of the film to reflect some IR heat back into the house rather than being lost. A more typical climate battery greenhouse will frame in and insulate the entire north wall (since no sunlight is gained through the north side of the house north of the equator), but we were unable to find commercially available designs (and kits) that would come close to the economics of a typical high tunnel kit. It may be that in future structures we will opt for a more heavily insulated structure, depending on what we're attempting to grow.

Construction

Site Selection

Our soils here are, in general, excellent agricultural silt loam limestone-based soils. We typically have very few small rocks in the soil but it's not uncommon to find boulders (more on that later). We chose a site due to its proximity to electricity access, closeness to our residence (so that we didn't mind going out to it), and relative lack of slope. Across the width the ground did not vary much in elevation, but down the length from west to east it dropped 11". We know we'd have to account for some of that in our foundation dirt work.

We began by marking corners and squaring up the site, then installing batter boards to ensure we could recreate those corners (again, more on that later). Short lengths of rebar were used as pins for the corners (more, again, on that later).

Foundation Work

Our excavation of the site for the climate battery installation initially went well, but we soon began to encounter boulders, lots of them, on the western side of the site. Our initial goal was not to excavate the entire site, but only those areas where the climate battery was to be installed. However it quickly became apparent that due to rocks, we'd have to excavate nearly the entire footprint of the site. Our excavator was able to remove all of the boulders (new expensive play toys for our goats), but one in the southeast corner of the site was particularly large (we never found the true edges of it), so we resolved to work around it. We changed the layout of climate batteries and were still able to proceed with the plan of three separate batteries. The western portion of the site was dug deeper than the remainder of the site, which meant that one of the batteries could be installed somewhat deeper (likely 6' through most of it, perhaps 8' in parts), though the aforementioned boulder did block a significant portion of this section.

I'll note here that we really should have setup our batter boards much further back from the site and pounded reference stakes far back as well. It was very easy for the excavator to bump into them and the original rebar corner pins were mostly ripped out or lost with the exception of one. One benefit of excavating the whole site was that in some ways we ensured a more smooth greenhouse post-pounding experience since the troublesome rocks were removed.

Climate Battery Prep Work