Heat pump in an earth-banked greenhouse?
While I am waiting for a manual j calc along with another ductless quote I have been reading about the Chiltrix heat pumps. I am a bit leary as they don’t seem to be a common thing. Nonetheless I have a little idea I keep thinking about so here goes.
My build here in northern Ct. Zone 5 has a couple flat acres for the home site and then slopes sharply to the south about 25 away from the structure. Being that one draw back I see to the units water lines is the need for a glycol mix to prevent freezing , I wondered on the sense of burying the pex lines 4 ft down and installing the exterior unit in a walpini type structure, meaning a hole in the ground say 7 ft deep on the north wall 30 inches on the south and angled on the sides. Glaze with polycarbonate and keep some 55 gal drums of water down there for mass .
The idea being that the unit would have the warmth of the earth to draw from and freeze protection.
Poor man’s geothermal?
Not sure why I would feel the need to build yet another project, but I must have too much time on my hands on this rainy day..
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Replies
Scott,
Air-source heat pumps move tremendous volumes of air through their outdoor units. If you execute your plan, your outdoor unit will pull the entire volume of your greenhouse through the unit in a matter of seconds or minutes. At that point the air will have been cooled off. You'll either need to allow the greenhouse to pull huge volumes of outdoor air into the greenhouse, or your outdoor unit will quickly bring the temperature of the greenhouse down to very low levels -- colder than the outdoor temperature -- increasing your energy bills.
In short, this won't work. You can't put the outdoor unit of an air-source heat pump in a confined space.
-- Martin Holladay
It is of course possible to use a giant greenhouse to overcome Martin's objection that it can't be in a confined space. But that's very unlikely to be anywhere near cost effective. Unless it was truly huge, you'd want to have operable windows with automatic openers that would open it to the outside when the heat pump cooled it below the outdoor temperature.
I think it would be "poor man's geothermal" for a man who starts out rich, but becomes poor after spending way too much money on a greenhouse and getting only a small benefit.
I am building several large greenhouses in the Philippines (tropical, closer to Florida climate). We have six months dry season and 6 months heavy rains including 25 storms with high gusty winds and flooding every year. I am choosing to bury my greenhouses 10ft?, waterproof them and install above ground dams and canals to direct water around them.
I want to create the coldest environment naturally available year round.
would air source heat pumps be ideal? or other methods?
How cold do you want to go? What is your typical outdoor average temperature and humidity? Sometimes you can get enough cooling with evaporative coolers, which are cheaper to run even compared to heat pumps, but they don't work well in high average humidity levels. You could possibly use a cooling pond too to average out day/night temperature swings if you have much cooler nights, and that might be another option. It's hard to really make suggestions without some idea of the target temperatures you're looking for, your locatl conditions, and how "large" those "large greenhouses" actually are.
Bill
Bill,
The Philippines is typically 85-95 deg F all year. If I could achieve greenhouse temps of 50-60 deg F year round that would be awesome! I want to grow strawberries and lettuce. 6 months a year it is 60-75% humidity. There is not a large temperature or humidity change at night. 6 months a year it is near 100% (heavy rains, inches per day). I don't think evaporative coolers is an option. a large cooling pond would be easy to do also if I could obtain cooler temperatures deeper underground? I have to test the soil at different depths. I am thinking to build greenhouses 16 ft across by 30-50 ft long. The larger the better depending on logistics.