Blue House Specs - Climate Battery Greenhouse Version 2

System Design

Our original system was designed around being driven by large, high CFM fans. In our case, these were 20” fans pushing out around 5,000 CFM. In practice, most of the CFM power of these fans were likely lost due to losses in the tubing, so I’m unsure what the actual CFM output of the system is. Building the system in this way was expensive due to the cost of large diameter drain conduit (24” risers and 18” manifolds) and the large amount of 4” drain conduit used per fan. Our new design utilizes twelve 6” ~400 CFM inline duct fans (increasing the fan cost), but inexpensive 6” DWV PVC pipe to decrease the tubing cost. Each 6” fan runs through ~250ft of drain conduit. Our original system had tubing somewhat “layered” at 2-4ft below the surface of the earth with tubing connected to manifolds in multiple layers, whereas the new system has a single layer of tubing at 6’ down, and another at 3’ down (each layer being driven by 6 fans). These systems are deeper to capture more of the latent heat of the earth and create a larger overall “battery” in which to store thermal heat. Each layer, due to our thermostat setup, can be driven independently, allowing, if desired, multi-stage heating and cooling. We also opted to use small gravel (1b stone, approximately 1/2” in size) as a backfill media around the drain tubing for ease and speed in backfilling, but it turned out to have an interesting performance habit (see below).

Cost of Installation

After undertaking last year’s construction, we hoped to be able to reduce the material cost of installation, reduce excavation time, and reduce the assembly and backfill time for the battery system. To reduce material cost, we opted for less expensive 6” DWV piping for the manifolds and risers, despite the fact that we’d need more fans as a result. To reduce excavation time, we only excavated a ~19’x86’ area despite an overall footprint of 34’x96’ for the greenhouse itself. We did this to ensure that our ground posts would have firm ground to pound into, and our earth battery itself would be insulated by 4-5’ (or more) of earth on each side until it reached the edge of the house. Because of this we opted not to install sheet insulation in the ground, saving additional money (though we’ll see in the performance if that ends up being a mistake).

Ease of Installation

We learned many lessons in our first excavation that we carried into the second. We saved our topsoil this time around, having learned that our subsoil is pretty heavy and contains a good bit of clay. We also learned how to make the excavation process easier in our ground so that much of it was performed with a wheel loader in order to get down to our 6’ depth quickly. Tubing was pre-cut and pre-assembled where it could be, with just the final connections made down in our pit. Backfilling around the tubing, to prevent crushing, was done with 1b stone (1/2” crushed limestone rock from a local quarry). Over that went our excavated material up to our 3’ depth, where we installed a second layer of tubing, more 1b stone, then more fill. Due to the extra stone (and not too many rocks encountered while digging), we had enough soil to level and raise the greenhouse footprint and somewhat crown it for water shedding purposes.

Performance

Air is moved in our greenhouse by 12, 6” inline duct fans capable of moving over 400 CFM each. Despite lower total CFM ratings than 3, 20” HAF fans, these fans seem to have more “oomph” (technical term) than our previous HAF fans and so we seem to be able to get more flow through the system this way. Interestingly, the backfill stone we used around the tubing seems to have created an interesting performance characteristic. Since the stone (relatively porous) is surrounding by heavier clay subsoil, the stone effectively creates a fairly large cavity of air when fans are directing air into it. This means that if even one fan is on at say, the 6’ depth, we get a little air flowing out of each of our risers. It will be interesting to determine if this increased exposed soil surface area means better heat transfer from and to the surrounding soils.

Controls

Each of our layers of tubing is driven by an independent 2-stage greenhouse thermostat. This was done somewhat for electrical reasons as each thermostat may not have been able to driven all 12 fans on its own, but it became an advantage in that we can have multi-stage heating and cooling. We will be tuning how this works over the coming winter months (it’s October as I write this) to determine how to best make use of this approach, both in terms of performance and saving money by only running half of our fans.

Cost of Operation

Our 6” inline duct fans use between 60-70 watts each (we used fans from two different manufacturers). When running all fans, this should pull around 700-800 watts. Our current fans run between 1,100 to 1,200 watts. This difference is fairly significant and additionally, the spacing we’re heating is larger due to the larger footprint of the greenhouse (34x96 versus 30x96 for the older greenhouse). We may utilize booster fans on the output risers to boost airflow, upping the total consumption to around 1,200 watts.

Formal Performance Analysis by PSU

Earlier in the fall of 2018 we approached mechanical engineering students at PSU Harrisburg regarding a formal study of both of our greenhouses. A group of students expressed interest and will be studying the greenhouse from late October 2018 into early April 2019. The group will attempt to determine how the system is working and suggest changes to improve its current operation as well as in construction of future climate battery greenhouses. Stay tuned for more information as we study the systems!