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GSHP and ice rink
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Posted by Tim on Tuesday, 30 June 2009, at 11:34 p.m.
File this under crazy ideas....
The problem: keeping a backyard ice rink frozen during warmer days in the winter.
The idea: I'm thinking it's theoretically possible to keep an ice rink frozen with help from my ground source heat pump.
First, here's some calculations:
1 cubic foot of water weighs approx 63 lbs.
It takes 144 Btu to change one lb of water to ice at 32F
It takes 1 Btu to lower the temp of water by 1F
I have a 4 ton unit (48,000Btu/hr)
A 30' x 60' x 3" ice rink works out to 450 ft3, or 28,093 lbs of water
Therefore, to lower the temperature of the slab by 1F, it should take 28.093 Btu, or 0.59 hours of run time at second stage (it will likely be more--this doesn't figure in any efficiency/losses). It would take 4,045,464Btu's to change the state of that much 32F water to 32F ice. Theoretically that would take 3.5 days to move that many Btu's of heat out of the slab--I don't know if it's reasonable to try to do that.
I'm mostly interested in the possibility of keeping the slab frozen once the outside air has done it's job of freezing it. Most nights it will freeze anyway, so really I'm looking to keep it frozen during the days where it gets above freezing.
All those calculations don't assume any heat loss/gain from the outside air, because I'm not sure how much heat gets transferred between water and air at a certain temperature.
My heat pump charts say I could scale the entering water temperature down to 25F. The ice should be about 28F maximum. I'm not sure how much difference there should be between the ice and the loop temp.
I have a small PLC with some analog and digital I/O--my ideas is to put a three way valve on my ground loop and monitor the water temperature, ice temperature, and indoor and outdoor temperature--then open or close the valves to find the balance between keeping my house warm, and the ice frozen. It would also be able to maintain a minimum supply water temp to the heat pump to prevent problems with it. I realize that the heat pump would not be efficient under those circumstances.
So like I said, it may be a crazy idea--please shoot my theories full of holes before I spend a lot of time figuring it out. Or, if you think it might work--any improvements?
Tim
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This site is all about building a cool, energy efficient house, that makes maximum use of earth sheltered design, passive solar heating and cooling, geothermal exchange energy management, and right sizing of the house for it's designated use. The home's placement is on a south-facing hillside in Deep Creek Lake, Maryland. This site describes the design process, the technologies used and the expected results. We also have a comprehensive Links Page for anyone who is also interested in designing a similar project.