So I am building a 16’ x 36’ greenhouse on the coastal plains of Virginia. It will have double layer, inflated, 6 mil PE glazing on the top and 6mm, twinwall polycarbonate glazing for both end walls. It will mainly serve as a seasonal propagation house for a 1 acre market garden type operation but I would like it to be able to do double duty and keep some potted citrus trees alive in the winter and possible have a few flats of loose leaf lettuce and greens going through our mild winters. I am very interested in using an existing, private storm drain system as an earth tube and going low-tech geothermal instead of installing a heater. I am also wondering if there is any way I can practically use such a system to help cool in the summer.
The storm drain is a 330’ run of 18” ADS pipe that is about 6’ below grade at one end of the run and maybe 2’ at the outlet. I would plan on tapping into the top most third of that pipe with 4-8 6” PVC drain pipes. A perpendicular run of 60’ would carry me over to the southern gable end of the greenhouse. The tubes would feed a box with an exhaust fan that would blow ground temperature air into the house. The tubes would also slope back towards the ADS pipe so that all condensate would drain back to the storm drain system.
The way I figure it, the 18” ADS pipe represents a possible 1237 CFM of ventilation at 700ft/min (this velocity is recommended by many extension services as an ideal velocity to ensure proper mixing of air without disturbing seedlings too much) if there is no water in the pipe. Each 6” pvc pipe I tap will pull about 137 CFM at the same velocity. 9 PVC runs would max out the dry pipe potential. With the 165’ pull through the ADS pipe plus the 60’ pull through the PVC there would be about a 215’ subterranean run for the air to warm or cool to the ground temperature, which seems adequate. So the big question remains is the project worth it?
I guess this all depends on what the ambient air temperature and what the ground temperature are at the time. From historical climate data, the coldest monthly average low is 29.6 degrees F in January. Most winters, the coldest night of the year will be in the upper teens but this usually only happens for a couple of nights and some winters we never fall below the low 20s. The historical average high occurs in July at 87.3 degrees F but we have many days throughout July and August where the temperature nears or surpasses the upper 90s. Thanks to data from the builditsolar.com website, I have a pretty good guess that the ground temp 4’ down will be about 52 degrees during peak warming season and about 72 degrees during peak warming season. Ideal winter temp for the greenhouse would be to keep it at or above 45 on winter nights. Ideal summer temp would be to keep the greenhouse at or below 75 on summer afternoons.
I was planning on installing 6 PVC pipes for a steady, 800 CFM supply of 4’ deep ground temperature air. Will 800 CFM of 52 degree air be enough to keep the 5,185 ft3 house from freezing in the winter? Will 800 CFM of 72 degree air do anything to help out in the summer when I’ve got the shade cloth on? I know I will have to run a regular exhaust fan to keep up with the solar gain but would it be possible to keep the house cool by just pushing 800CFM of cool air in while pulling 800 CFM of mixed air out of a gable fan? Does anybody know any formulas for the BTUs gained during a cloudless summer day with 1,336 ft2 of glazing covered by 50% shade cloth?
My gut is telling me that this system may be effective for winter warming and summer cooling on cloudy days but that sunny summer days would greatly overwhelm the system. I plan on having a traditional exhaust fan setup (minimum 1 exchange/minute) to keep the temps near ambient on those sunny summer days. Any thoughts on how I might set up a controller(s) to switch between the two systems?
Thanks for any help! |