Natural Creek to Power Ground Source Heat Pumps
I posted earlier on my current project of sizing/installing mini-split ductless units in our home. In a somewhat related but perhaps off the wall thought, I’m also starting to ponder a bit about a ground source heat pump. To be clear, I wouldn’t ordinarily consider it, because of the nature of our location, however, we do have one ‘resource’ that might make it not only feasible, but the preferred choice. BUT, because experience has taught me that my ‘good ideas’ often aren’t so good, I thought I’d ponder out loud here and see if anyone had any feedback. So, here goes.
We have a little creek that flows by the house over a slate bottom, and then disappears into the stream bed about 50 yards beyond the house where the creek bed turns to gravel. The dry creek bed itself continues on a half mile or so to a much larger creek that basically drains the whole southern part of the county.
The creek flows year-round, with an average minimum flow of about 300 to 400 gal/minute at the driest time of the year (which is Sept/Oct). I know because I set up a weir to measure it and have watched it for several year, with thoughts of perhaps one day putting in micro-hydro. The creek varies from 10 to 20 feet wide and runs from 1 to 3 inches deep. The one-time I measured summer time temp it was 62 degrees F. It’s spring fed, starting on our property from a multitude of tiny springs, so I honestly expected it to be colder, but I suppose that’s because it’s shallow and the dark slate bottom gets anywhere from full sun to dappled sunlight as it runs through the woods. I’ve never measured the temp in the winter, but I’ve never seen it frozen (though once when it got to around zero F there was ice along the banks. There’s not a great deal of head, but there is about 15 to 20 feet around 200 feet upstream from the house. I’m sure I could bury a pipe without too much difficulty and easily get 10 or 15 gpm to gravity flow to a heat exchanger at the house and then discharge back into the creekbed as an ‘open loop.’ All gravity, no pumping required.
So, it seems like a ‘natural’ for a GSHP, but I also know (or at least have read) that the efficiency of geo-thermal systems is VERY dependent upon design and installation. Also, and as discussed in the other thread on mini-splits, I’ve determined that my cooling load is pretty minimal (realistically about 1 ton, though I’m going to have to install more than that). I can get the mini-splits more or less as ‘canned’ units, where as a GSHP seems more involved and I don’t even know if can get one as small as I need. My thoughts are that even if I have the best, most easily exploited source of ground water in the world, if I still have to run a larger compressor than I need, it might turn out to be a wash (or worse). Also, the installation of the GSHP would (I think) be somewhat more involved (design wise) though I’d probably put the coil the existing ductwork of our propane furnace.
Frankly, I’m still leaning towards the mini-splits because it would be the easiest, but I keep looking at that creek and thinking I should be able to use it somehow. Obviously, given that we’re totally off grid and solar-powered, I would like to minimize power consumption. OTOH, I don’t want to dump extra resource into this for what might be only a marginal improvement.
Any thoughts appreciated.
P.S. We own both sides of the creek in its entirety, from the ‘headwaters’ until well below where it disappears into the earth. We don’t own the ‘mouth’ where the dry streambed connects to the larger creek, but water only flows that far during really torrential rains and then only for a few hours at most.
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