Re-thinking geothermal

[Rob-D]

A resistive heating element is 1:1, you put in one unit of electrical energy and you get one unit of heat. With a heat pump however you put in one unit of electrical energy and it can provide three or four units of heat. Air source heat pumps lose efficiency at low and high outside air temperatures. Ground source heat pumps (geothermal) take advantage of the relatively constant ground temperature and they operate efficiently regardless of the air temperature.

OK, you also mentioned you micro-hydro system, I want to hear more about that.

Right now my stream is running well enough to provide about 300 to 500 watts. I'm thinking of a modified Mitchell/Banki. The Banki is an old design but it has great potential for low head apps like mine. I'm working on a direct drive NdFeB magnet alternator for it that's pretty close to the ones I use in my windmills.

The turgo is a nicer application of the basic pelton design if you have decent head but a low flow rate.
The Stream Enigine is supposed to be relatively good for a small commercial microhydro product although I have no experience with it personally, Google it if you need to see some pics. I think they give you some idea of what kind of power to expect with specific flows and heads.
The first thing you need to do is build a weir and measure your flow rate and then figure your head. I used a laser level over about a 400 to 600 foot run for head but you can do it with pressure.
Rob

 

[Chilly807]

Thanks for the info Sean, how long are your trenches if you remember?

Rob

Ours were between 80 and 95 feet long, plus a connector trench to get us out away from the house to where we wanted to run the actual trenches. Looks a lot like a chicken's foot, with a 50 foot tail running under the frost wall and up through the floor into the utility room.

We spent about $1500 on the materials (geo specific pipe and electro-fusion couplings) for the ground loops, plus $1200 for excavation. I think we got off easy on the excavation price, it took longer than he expected, but he stuck by his quote.

Sean

 

[eepete]

My geo installer was a little embarrassed at the difference in performance of the pond vs. ground. My pond is about a half an acre, 8 feet deep. It was 40 degrees. My geo installer measured a 10 acre pond that was 14 feet deep, it was 42 degrees. So at least at this latitude, the size and depth of the pond didn't make much difference.

Also there are very few ponds around here that are more than 15 feet deep because all the rules for pond building, maintenance, and liability change if the pond is deeper than 15 feet. I suspect that the size of the pond is a very small factor (once you're over an acre or so) and that depth might be more, but only at really big depths. I remember scuba diving in rock quarries in the summer and spring and hitting a thermocline at around 35 feet or so- not a typical pond depth.

And Rob, abandoning the pond was a very expensive maneuver that moves the payback way out. I'll let you know how that worked for me in 2022, and it will come down to how much electricity cost by then.

Pete

 

[Rob-D]

Ours were between 80 and 95 feet long, plus a connector trench to get us out away from the house to where we wanted to run the actual trenches. Looks a lot like a chicken's foot, with a 50 foot tail running under the frost wall and up through the floor into the utility room.

We spent about $1500 on the materials (geo specific pipe and electro-fusion couplings) for the ground loops, plus $1200 for excavation. I think we got off easy on the excavation price, it took longer than he expected, but he stuck by his quote.

Sean

My excavator came in at $1250 which I think is a fair price but he knows me and we have worked together before. He did my fuondation when I built the house.

 

[Rob-D]

And Rob, abandoning the pond was a very expensive maneuver that moves the payback way out. I'll let you know how that worked for me in 2022, and it will come down to how much electricity cost by then.

Pete

I'll bet it was!
I'm curious about how the springs will effect my pond but I'm leary about using it as a heat source. I'm going to throw a thermistor in the deep end just to see what's there.

You know that story, you almost wish oil would go through the roof once you have everything in and running!

Rob

 

[Chilly807]

You know that story, you almost wish oil would go through the roof once you have everything in and running!

Rob

If it wasn't for everything else being affected by the oil price, I'd agree with the sentiment.

Sean

 

[Redneck in training]

I'll bet it was!
I'm curious about how the springs will effect my pond but I'm leery about using it as a heat source. I'm going to throw a thermistor in the deep end just to see what's there.

You know that story, you almost wish oil would go through the roof once you have everything in and running!

Rob

When we build our house there was not water in the pond to speak off. The coils were about half above the ice. I measured the water temperature in the inlet of the HP to be about 26F. Nevertheless the heat was working fine whole winter. The pond water temperature is not the whole story. The pipes between the pond and the house will also extract some heat from the ground.

 

[MacLawn]

I've read this entire thread, and am more confused than ever on geothermal! Whew, y'all boys must be engineers...

I did read somewhere online about geothermal about algae growing on the pipes in the pond based systems, and that can reduce the efficiency. Also, I read that the ground based pipes must be encased in some kind of conductor material - kind of a mortar, etc. stuff. Can't remember, but I did read about those issues. Anybody know?

 

[Redneck in training]

Algae growth might have some effect I suppose but it is easy and cheap to somewhat oversize the heat exchanger (fluffed coils) in the pond. As far as grout. It depends on the soil and the way the pipes are installed. When installing horizontal loops grout is seldom if at all used. It is used usually when vertical or horizontal drilling is used.

 

[nwbearcat]

I couldn't justify the cost to go geo-therm either. I looked at the excavation cost and it didn't make sense to me. We put in a air source heat pump. Looks about like a large AC unit, and has a reverse for making heat in the winter. During the dead of winter we burn corn/wood pellets.

Spend some time with this: www.eia.doe.gov/neic/experts/heatcalc.xls

It was going to take me a long time to recover the added costs of the geo unit vs. the heat pump, and I think in the end, the heat pump cost me less than a AC unit + LP furnace.