supplemental geo thermal heat/cooling source
We are building a home in Redmond, WA(climate zone 4C) and are digging a deep trench probably 200 feet long for a daylight basement drain that will average 10 feet below grade. Considering throwing in a loop of PEX to be able to circulate glycol to keep a glycol reservoir at a constant temperature of whatever the ground temperature is(probably around 55 F?). Was going to route heat pump return line from air handler through the reservoir with a simple heat exchanger. I would engage only during major cooling and heating season and probably by-pass during the shoulder seasons. Objective is to use the free energy of ground temperature to partially reduce coolant temperature in summer and partially heat in winter. Do you foresee any problems with this?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Cost would be my concern. Look into a desuperheater - it’s the same idea with less to go wrong. It would pump solar energy into a DHW tank.
"Do you foresee any problems with this?"
Yes. Systems like this need to be engineered. The problem I foresee is you spend a lot of money and get something that doesn't work or even makes your system work worse.
This is a little confusing... i never done any single family home project, but in commercial/industry project, Geothermal, heat pump, heat transfer loop, thermal exchangers, these types of energy efficiency strategy shines in shoulder seasons and mild cooling/heating season. In extreme weather, especially cold, they usually don't have enough capacity or too fragile, usually we have direct gas fire or electric heat as backup to handle extreme cold.
Also, not sure where exactly you are in 4C, but my gut feeling is 55 might be too optimistic for winter. I m in Chicago land and the winter soil temperature is below 40. It might be a good idea to find out exactly the temperature before acting, couple degrees difference could make huge difference. A simple indicator is to see if the lakes and pounds (flood ranivore) around you are frozen or not. If the pounds are frozen, the soil won't be any better.
And the safest way is keeping a boiler in the loop.
Here is a source i used but it's for illinois, just for reference. Hope you can find your local data.
https://warm.isws.illinois.edu/warm/soil/
thank you for comments. I will check out soil temp. Though the last home we built in Jackson Hole Wyoming I had a soil temperature probe down well below grade. It read a steady 57 degrees even when it was -35 outside!
I don't know Wyoming, but a 92 degree delta T... sounds like you have tapped into somewhere around volcano or hot spring or that sort, and we know these things does exist in Wyoming. If that is the case it would be a totally different story.
It's not just soil temperature, it's the ability of the soil to source provide and absorb heat.
In order for heat to flow, there has to be a temperature difference. So if you're trying to extract heat, you have to be pumping a fluid that is colder than the soil down into it. If you extract heat faster than the ability of the surrounding soil to provide it, the soil cools. So you have to lower the temperature of the fluid even further, which causes the soil to cool further.
A very common situation with ground-source installations is that the fluid temperature falls below 32F and the water in the soil surrounding the pipe freezes. Liquid water is a relatively good conductor of heat, ice is much less conductive, so the heat capacity of the soil collapses and the whole system shuts down.
There are similar effects when heat is pumped into the ground for cooling, but it's not as severe because you don't get the falling off the cliff effect of the phase change of the water.
The big, unsolved problem with ground-source is that it's hard to engineer the capacity of the soil. There are rules of thumb like "300 feet of pipe per ton," but they're just guesses. Drilling holes is expensive and soil varies considerably, in any particular spot there's no way to know except by drilling.
You know what else shines in shoulder seasons and mild weather? Air source heat pumps.
57° loop temp is the big lie from the industry.
When I read about people posting HP ground loop temps near the end of heating season they generally are at 32° or so. It makes sense they engineer the loop to be as small as possible in a feeble attempt get the cost low enough that it might save enough energy to recover its high cost.
Walta
Building Equinox CERV2 has a typical showing a ground loop going through its water coil. With the ground loop going round a basement foundation. Similar to the design for a Zehnder equipmemt for tempering incoming ERV air.
I would do the modeling around pre conditioning ERV inlet air for sizing that ground loop. What happens if you put a loop around your footers - the trench is a sunk cost, so you may be out the cost of PEX, a circulating pump and a fan coil.
Or maybe the cost of the electricity to operate the circulating pump and fan coil is greater than the heat you get out of it.
Heat flow is driven by temperature delta. Typical ground temperatures are pretty close to interior temperatures, to get usable amounts of heat flow you need to use a heat pump to increase the temperature delta. When you do that you very quickly run into the problem that the ground has a quite finite capacity to source and sink heat, you need to engineer the ground loops to be able to absorb or provide the needed amount of heat.
I would be very leery of taking heat with a heat pump from ground loops near a building's footings. Freezing the ground could cause the footings to heave.
Let's just focus on air conditioning as that is our biggest energy use. All I am trying to do is somewhat reduce the temperature of the coolant returning from the air handler en route back to the heat pump. Let's say that coolant exits the air handler at 120 degrees(I don't know if it gets that high but it certainly feels pretty hot on hot days). I'd run that coolant line through a bath of 57 degree glycol(or maybe water), probably with a couple of loops in the coolant line, and then it would continue on to heat pump. Let's say that the coolant temperature would be reduced by 30 degrees(total guess). Would that not lead to lower energy costs for the heat pump using the "free" cooling energy of the ground temp?
I think you're confusing the two pipes going to air handler. The one with refrigerant entering the air handler is warm, it's liquified refrigerant that has gone through the compressor and then been cooled by outside air blowing over it. The one coming back is cold, it's gaseous refrigerant that vaporized in the air handler.
R410A, the most common refrigerant, has a heat of vaporization of 117 BTU/lb and a specific heat of 0.39 BTU/lb/F. Reducing the temperature of the feed by 30F would remove 11 BTU/lb, which would theoretically increase cooling by 10%.
John,
We have done this on our last two new home builds. It is also a common solution for pre-heating one's ERV/HRV in cold climates to avoid freezing the unit. Zehnder makes units specifically for this-- the Comfofond-L.
It gets tricky in that one needs to pay attention to static pressure, filtration, high/low temp thermostatic controls, specific pumps needed, heat exchangers, condensation removal strategies, duct vapor control and insulation, space requirements and other issues. If done right it can deliver enough benefit that one can use a smaller heating/cooling unit that may not have the capacity, especially in the cooling seasons to keep up with the peak loads. It also makes the most sense in very high performance homes, in a 'legacy' type building the benefits will be so small as to not be noticeable.
We have set up our systems with various monitors and have seen a consistent 12 degree benefit on the peak days. We are also working on setting up the correct temperatures for our thermostatic on/off controls to turn the machine on. We are starting with 45 degree outdoor temp for heating conditions and 85 degrees for cooling. It will take a few years of monitoring to determine the optimal set points whereby the 'free' heating or cooling pump energy will be more efficient than just using the heat pump system.
I hope to publish an article on this forum in about a year's time about this system when all of the data comes in. In the meantime I would highly suggest finding a savvy engineer (Positive Energy comes to mind) that is willing to take this on. A super savvy installer would be needed, too. I would also recommend a robust energy model and manual J calculation.
"It also makes the most sense in very high performance homes, in a 'legacy' type building the benefits will be so small as to not be noticeable."
I'm always suspicious of comments like that.* If an idea makes sense, wouldn't the more energy you use mean the more it makes sense?
*(And comments like that are endemic in "alternative" energy discussions).
It makes sense when one is trying to blunt peak loads, which is the best use for this technology. In a legacy home, the BTU savings won't register. This isn't an energy argument as much as it is a performance consideration.
"We have set up our systems with various monitors and have seen a consistent 12 degree benefit on the peak days. "
Could you explain what you mean by "12 degree benefit"?
"We are starting with 45 degree outdoor temp for heating conditions and 85 degrees for cooling."
So do you run when it's above 45F or below 45F outside? Similarly for cooling, is 85F the turn-on point or the turn-off point?
Thanks.