Beyond Earth Tubes - How Andrew’s Greenhouse Works
Updated: Aug 10, 2021
In our last blog posts we introduced Andrew and talked about the yields he’s been harvesting from his 4 season greenhouse. We also talked about the Earth Tubes he used and how they work to passively heat and cool the space. In this article we’ll dig into the other systems he implemented to get the most out of the space.
Andrew positioned the greenhouse to face true south, lining it up with the winter solstice sunrise. This simple measure maximizes the direct sunlight flooding into the greenhouse all winter long. The southern side of the greenhouse is angled towards the winter sun and it is clad with triple-layered polycarbonate.
The performance is great in the spring and fall, when there are sunny days and cold nights. On a hot spring day the greenhouse warms up and the earth tubes kick in once it gets above a certain temperature, keeping the plants cool without running the fan. When the temperature rises above 25 degrees the earth tubes pull the heat down to store it underground, and once it drops below 17 degrees they release the stored heat back into the greenhouse.
Water stores heat significantly better than any other substance. For this reason, Andrew buried water tanks under his raised beds in the greenhouses. He half filled the water tanks, so if it does get cold enough for them to freeze they won’t crack. Right now they’re being used entirely as a heat store, but they have the potential to be converted into an aquaponics system in the future.
One issue Andrew has been facing has been with pollinators getting sucked through the fan. While it would be fantastic to have pollinators in the greenhouse, they don’t fare so well when they come into contact with the fan’s blades. In order to solve that problem he’s used mesh coverings over the fans.
The greenhouse is ventilated using windows that are connected to a control system Andrew built for the greenhouse. He automated the ventilation system using a 12 volt car window actuator to run the vents and turn the fan on. The control system has a microprocessor connected to 7 temperature sensors that control when the vents need to be opened. There is also a humidity sensor connected to the fans. The entire system is connected to a bank of solar panels and batteries that charge up and run the actuator. Supplemental power can be generated at night using rocket stoves and mass heaters that help to warm up the water in the tanks, as well as any brick or concrete to keep the space heated during the night.
Andrew also has a human composting toilet. They compost on their property to manage waste, and if the waste is composted properly using hot composting techniques, all the bacteria dies within a year or two. They plan to use this compost at the bottom of their fruit trees, but as safe as it might be, they have no plans to grow root vegetables in it.
Andrew has proved his model and is reaping the benefits from it. Check out our next article to see how these systems can be implemented to retrofit existing homes into passive solar ships. See what else Earth Tubes can do!