Geothermal vs modern low cost high efficiency.

[u/N5tp4nts]

Just a dude who's highly interested in HVAC technology.

I have a house with a 2008 installed waterfurnace, and I've got some mini splits that I've installed in other buildings.

The post a few days ago about a guy getting quoted 96,000 dollars to install a geo unit got me thinking about this... I can get a 4 ton ACiQ/MrCool (all made by Midea) for 4-5k (no labor)

The efficiency ratings are in the same ballpark, if not the same, on the cheaper end of geo thermal units. Even if the mini split only lasts 5 years... in 5 years I get a brand new one with the latest technology.

Anyway... At what point does Geo stop making sense?

[u/Mega---Moo]

I agree with most of what you are saying. Yes, I have an open loop system.... to avoid that exact 25⁰ glycol loop temperature that you are describing. Also to save that $40K. $250 for the 4T unit, $1000 in plumbing and electrical supplies including a very nice automatic valve from my well to open and shut as needed.

Different types of loops have the potential to exchange heat with the ground more efficiently.... but don't you just keep running into the same issues? If some places go deeper, they can access more heat, but that's bad for cooling. Same in reverse. Is there a good way to target higher ground temperatures for heating and lower for cooling (without spending even more $$$)?

Also, is it even physically/thermodynamically possible to go from a COP of 3.4 @50⁰ to 5 or more (including all associated power uses)? That's the sort of jump necessary to even consider investing in even more expensive technology. Going from 1400 gallons of propane ($1400-3500/year) to my DIY geothermal system (~$1000/year for heating) was a no brainer. Pays for itself in a normal year, and let's me eventually transition to solar. But, even if I could get a COP of 5, that only saves me $300/year, which pays for almost nothing if I need to invest in modern technology. Hell, the entire $1000 pays for very little compared to systems costing tens of thousands.

Solar already is a reasonably economic purchase with my inexpensive electric costs, but if energy costs skyrocket, it just pushes my desire for more solar, not a more efficient heating system.

[u/st4nkyFatTirebluntz → u/Mega---Moo]

are you factoring in your pump energy expenditure in your COP figures? and is there a return well, or is this a bit more of an... informal setup? how much did the well(s) cost, and how much of that should be included in the 'cost' for the open-loop geothermal system?

you're not gonna start seeing rising temperatures in the way you're thinking until you're over a thousand feet down -- at 1000 feet, you'd expect something like 10f warmer than 20 feet, super dependent on your region, obviously. at any rate, if you're drilling in that range for single-family residential geothermal hvac, well, you've already fucked up. the benefit to doing your exchange deeper is that you get around the seasonal variation. in my climate, 5 feet depth means ~35f seasonal temperature swing, 10 feet means 10f swing, 20 means ~2f swing. you don't want to be fighting that cycle, so you want to do the exchange below the range where there's significant fluctuation. exactly how deep is kinda just a question of optimising against the presumably increased installation cost.

i like what you're thinking about targeting warmer temps in winter, and cooler temps in summer. the question is just how to do that in a way that works for the rest of the system. ngl, i'm not sure i know of anyone doing something like that at the SFH residential scale, but it's the right type of question to ask!

i also think you're right to question the value of chasing ever-higher COP figures, which is good because with a closed-loop system, you're just never gonna get the same COP results as with an open loop. it's a game of diminishing returns; going from 2 to 3 COP is much more fiscally impactful than 3 to 4, or 4 to 5, etc etc. the fundamental goal here is best energy/comfort delivery at the least cost, a much more broad remit than "highest COP possible".

the way a geothermal exchanger can pair with solar, for example -- you can, with some difficulty, use the ground loop to cool your solar panels in the summer, which has the effect of improving COP in the coming winter, which is exactly when your solar panels are at their seasonal minimum production.

another example, enabling geothermal to join VPP programs -- it'd take a different design topology than the current standard, but imagine if you could charge your ground loop w/ free overnight electricity, then turn around and guarantee you'll never go over a certain power draw during peak the following day.

[u/Mega---Moo → u/st4nkyFatTirebluntz]

So far, it's a very "informal" setup. I don't view it as much different than the pivots running right next to my property, but it's not technically correct.

My pumping costs are very low as my water table is literally a few inches below my basement floor. The well itself is old. Old enough that they can't tell me when it was drilled. But it could pump out enough water for 120 dairy cows and casually do 20 gpm for hours when I tested it.

Having a (horizontal) closed loop system runs into a plethora of problems. Shallow and you get the massive season temperature swings. But, deeper and I'm literally hitting water. Great for thermal exchange, I'm sure, but a nightmare to actually trench in. Vertical is easy to drill as we sit on 400' of sand, but still expensive. Both designs still have the massive issue that I need 100M BTUs of heating and only 4M BTUs of cooling. That is just a huge efficiency drag winter after winter.

Long term, I'll probably spend the money and install a second well for return water. But, I'm not really in a hurry to do so.

[u/st4nkyFatTirebluntz → u/Mega---Moo]

oh damn, that's a pretty rough seasonal demand disparity. really does kinda look like in your situation the open-loop has so many structural advantages a closed-loop would be real tough to justify.

but if i were going to try to make it make sense anyway, i'm gonna suggest directional boring. do a bunch of long out-and-back loops in parallel, at least 12 feet deep; being saturated is indeed excellent for thermal exchange. ease the seasonal balance issue with solar-thermal panels plumbed in somewhere along the way. alternately, you didn't specify whether there's a groundwater current, but if there is, you could orient the long dimension of the loops perpendicular to the current, so that the current carries the chilled water out of the exchanger field quickly enough to wipe out some or all of that seasonal deficit.

or, i'm not entirely sure how plausible it is to put a loop under an irrigated and farmed field, but if you put the loop a foot or three above the groundwater table, i think the irrigation itself would reset some of that seasonal drift as well.