Climate Battery Greenhouse
The climate battery greenhouse was a project undertaken in late 2017 to provide a low-energy consumption means of controlling the climate in our 30x96 high tunnel. Construction was completed in November of 2017 and the tunnel will be used primarily for winter protection of in-ground grown fruit trees not typically grown in our climate.
The concept of a climate battery greenhouse is to use the earth beneath as a sort of "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 an 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 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 any heat stored there.
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 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 it proven successful.
Season Extension or Year-Round Growing
Our goal is to keep the greenhouse at or above freezing year-round in a zone 6b/7a climate. In a structure this size, this will be accomplished through the climate batteries and an internal row cover to cut down on the air volume needing to be conditioned.
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.
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 around the freezing mark on the coldest nights of the year.
Our design involved three separate "batteries" installed within the 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.
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 insulated 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.
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.
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).
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
Assembling the climate battery meant that we needed to first connect the 24" diameter risers to the 18" diameter manifolds. This meant cutting an 18" diameter hole in the 24" riser (which itself had been cut down to 6.5' from an original 20' length (that way we could get 3 riser sections out of each pipe). We then inserted the 18" pipe into the 24" riser at a 90 degree angle and attached it by bending back cut portions of the 18" single wall section.
Pictures here would be worth a thousand words so the picture to the right shows the assembled batteries. The risers with the connected manifolds were then moved to the edges of the site and positioned such that the diagonals would accommodate the 30' runs of 4" ADS. We purchased caps for the 18" pipes that we attached once the pipes were in placed, then tried as best we could to level the manifolds and the risers (though inevitably they shifted during backfill). Finally, we backfilled under each manifold to provide some support for it as we knew we'd be placing a significant amount of dirt (and therefore weight) on top of it. The pipes are rather strong but we didn't want to chance them buckling under the weight, plus we didn't want to create an area for dirt to settle into after backfill and grading.
Climate Battery Install
After the manifolds with risers were assembled, positioned, leveled, and backfilled, we began connecting opposite manifolds with 30' sections of pipe. 30' sections (plus a few inches) were cut from 250' rolls. The ends were wrapped with duct tape to hold the sock in place, suitable holes cut in the manifold with a 4.75" hole saw, the tubes inserted into the holes, then held in place with a single screw to prevent it from pulling out when the backfilling took place. Eight pipes were connected from one manifold to another, then backfilling took place with my compact tractor and the excavation soil using a ramp that we had the excavator create. Eight pipes were done at a time simply because that's all my tractor could reach. After the pipes were backfilled we hand-tamped each run except the topmost layer as we figured it would be tamped during the backfill and leveling process.
This was the most time-consuming and difficult portion of the process, and I've been thinking of ways to improve upon it ever since. The climate battery prep and install process probably took 2 weeks of solid work, though since this happened at the end of our season and was primarily done in short work sessions of 2-4 hours, it ended up stretching a month to a month and a half.
Another difficulty came in the way we oriented our piping. To achieve 30' runs, we had to angle the tubes across the width of the site. This meant that we created a narrow angle at one end of the run, which created difficulties in maneuvering the tractor in to deliver soil. Were I to do it again, I'd certainly find another means of backfilling so this wasn't an issue ordeal with the shorter tubing lengths from straight runs.
Since the backfill process took a while, a rain could delay the backfill process for a few days. The resulting backfill soil would be too muddy to work with and the site would take a couple to a few days to dry out well after rains. Backfilling in a single day or two days would be ideal but may not be possible.
Finally, the content of the backfill soil and the spacing concerned me somewhat. Per what I've read, the backfill soil shouldn't contain too much clay as the air moving through the tubing can cause it to create somewhat of a shell. A sandier soil may be ideal and in the future if we build other houses we may bring in sand for backfill. The pipe spacing was a concern because we only had a 20' manifold to work with and 20+ pipes to fit in each run. This meant that the pipes were closer than 1' on center. This didn't leave a lot of spacing for soil to absorb heat but hasn't (to date) seemed to impact the performance of the system.
Final Fill and Grading
After the last of the piping was covered, our excavator brought in his wheeled loader and skidsteer to complete the job. Due to the volume of boulders taken out along with the grading that needed to be done to level the site, we ended up having to take some soil from another portion of the site to bring the lower portion of the site up 11". We expected the site to settle somewhat but it really only settled around the risers where the equipment couldn't compact it very well. The equipment compaction alleviated the settling issue but would have to be addressed after the greenhouse was completed on top of the structure.
High Tunnel Installation
We assembled the high tunnel over the course of 4 days without major issues. We first needed to recreate the four corners of the house and also accommodate a hydrant and electrical run we had put in during excavation. Due to our excavation of almost the entire site, most of the ground posts went in without issue except those that encountered the boulder in the corner of the site. We opted for extended ground posts to create a taller building. We've heard anecdotally that a larger building is in some ways easier to heat and cool because of the larger air mass. This will also allow us to have equipment in the structure without issue, and taller plants should we choose to do so.
Electrical wiring to thermostatically control the fans was done over the course of a couple days. We may, in the future, invest in a greenhouse control system but to date this system has worked just fine.
Control Setup and Heating Structures
We've setup our greenhouse to have a two stage thermostat powering fans that draw 1.1kW at full speed. Following the advice of the other systems, we installed individual speed controls for each of the three fans. Currently we have one thermostat for heating and another for cooling, both of which simply turn on the fans. When the greenhouse reaches 70 degrees, thermostats turn on to pump the warm moist air underground to store the warmth and cool the house. The cooling thermostat is currently setup with a 5 degree differential (on a 70, off at 65).
Our heating thermostat is set to come on at 36 and off at 40. On a cold night, that stockpiled day warmth (along with some heat from the ground) is drawn out of the soil and into the house to warm it.
We've also setup an internal covering, much like an interior shade or energy curtain, that can be drawn over the greenhouse site on cold nights to essentially halve the area being heated. We're still experimenting when this is needed as it's not automated and involves some extra labor. In addition, the curtain seems to reflect sun during the day, meaning the house does not heat as much