permaculture

["paul" <biz-remove-this-@soon.com>]

 I am in the midst of designing a dryland greenhouse for my land. I am hoping to catch water off the roof of the structure in sufficient quantity to run the greenhouse for 150 days. I’ll raise the year’s transplants by then, and the outside temps will be warm enough for growing the plants to maturity outdoors. If you’d like to help me think through all these issues, read on.

 

Climate: cold arid. I live at 7400 feet, with a growing season of 90 days and 12 inches of rain annually.

 

The greenhouse will be about 3000 sq ft, on the grid, with natural gas heat and electricity for running lights, fans, pumps, etc.

 

I first looked at doing a ferrocrete tank outside on the polewards end of the greenhouse to block winter winds. But since I need 20K gallons of water, caught over the entire year and stored for Feb – June use, I figured the tank would be too big to make cheaply. I became concerned about leaks or structural failure in something that size. I also need the water to be of a usable temperature for irrigation, so an outside aboveground tank didn’t seem to make much sense because it would be frozen when needed most.

 

I couldn’t very well put the tank above ground inside the greenhouse because it would take up too much space. I need all the room for plants I can get. All the concerns about structural issues remained. This led me to putting in a belowground cistern. The cistern would be built by digging a big hole, (say 10 feet by 6 feet by 45 feet), lining with pond liner, and building a floor on top to support container growing. Water from the roof of the greenhouse would channel into the pond. We would use a submersible pump to irrigate.

 

One smart friend noted that with such a huge heat sink (20K gallons of water weighs what, 80 tons?) that I might have problems heating the greenhouse. Is there any worry there? The tank will be covered and below ground, so I am unclear about why the water would be any more of a heat sink than the ground itself. I don’t have enough understanding about thermal mass, so maybe someone here knows more and can explain.

 

Am I headed in the right direction here? Anyone got a better idea how to design the relationship between the greenhouse and the water tank?