Theoretical: Geothermal - Loop temps vs. balance point
Folks, as I am completing the design of my geothermal system with a vendor..... I keep coming back to fears that the units are undersized for my location, in terms of needing Auxiliary or backup heat more than I want.
From what I understand in the air-to-air world, most heat pumps are good until around 35-40F for reasonable efficiency if sized for teh heat/cooling requirements.... assuming that the sizing is correct, outdoor temp is important for both the compressor to collect heat as well as make sure the inside air temp is covered.
for example, is it true that if loop temps in winter sit at lower than 35 degrees, a geo theoretically won't do any better than a similar sized air-to-air in terms of needing aux/heat? correct?
So, sizing aside, doesn't it make sense to force loop sizes that would keep the temp of the water stable and in that range above 35 to the nominal ground water temp of 45-55 degrees?
Would there be more bang-for-the-buck with longer / deeper loops if the size suggested is on the low end?
Definitely NOT true, or geo systems wouldn't make sense in colder climates where the ground loop temp frequently drops below freezing. The reason that air-source heat pump efficiency drops off so significantly at or around freezing is that the coil begins to ice up and becomes next to useless as a heat exchanger. the coil must then be defrosted periodically which takes energy or shut down in favor of aux heat (someone correct me if I'm worng here, I know very little about air source systems).
Originally Posted by dzenzel
While the efficiency will certainly drop somewhat in a ground source system as the loop temp drops, it is nowhere near as dramatic since the efficiency of the heat exchange is more or less constant. For instance, the COP on my unit goes from about 4.5 at 40F loop temp to 4.0 at 30F loop temp and about 3.6 at 25F loop temp.
Oversizing the loop field makes some sense in terms of providing a safety factor, but it may not pay off in terms of efficiency. If I doubled my loop field, the incoming water temp might increase 4 or 5 degrees, but the resulting 10% or so efficiency gain wouldn't compensate for the added loop cost. Of course other climates or loop configurations might yield different results. If your installer is on the ball they should be able to show you how increasing the loop field by X% would affect system efficiency and running cost.
There are a couple of factors to consider. First the ground temp will vary seasonally. This affects horizontal closed loop systems most. The subterranean temperature will vary as a function of the depth of the hole/pipe. At 2' deep it will vary about +/- 20 deg, 5' +/- 10 deg, 10' +/- 5 deg and 30' little to no variation. This would imply that deeper is better, which is true, but often impractical. Cost of digging, bedrock, water and other factors will determine how deep you go. The time constant of this temperature change is 30-45 days, meaning that it lags the seasonal change by about a month, mostly due to the slow heat flow of dirt.
The second factor is that most soil, on average, will only support heat flow of about 1 BTU/foot/deg F. This means that if you pull/dump 40,000 BTUs and you have 2000' of pipe, the you will have a +/- 20 deg delta from the nominal ground temp to the pipe temp. In reality no system should operate 100% of the time, so a 40,000 BTU system may only average 20,000 BTUs over time. Again this temperature change is due to poor heat flow in dirt. Vertical bore holes are better in the temperature variation, but they still vary up to 20-30 deg. The amount of moisture or water can greatly affect the change, as well as the water flow rate through the aquifer if yuou have a "wet" well.
You have as a result perhaps 10-15 deg due to seasonal variations and 10-15 deg due to heat flow. This is why you can get into the 20's in your loop. Open loop systems are more efficient as pumped ground water tends to stay at 50-55 deg year round.
As far as efficiency, a couple of other things to consider. Air-air SEER ratings are tweaked so that the unit is optimized for the SEER tests. Geo have relatively fixed operating conditions and will operate more efficiently overall. Next the conditions for Geo are usually in your favor. The changes in ground temp and ground chilling occur over time, so the worst actually will be late winter/early spring. Anytime that the loop water is above (or below) the ambient, you are more efficient. This is most of the time. Sometimes in spring the water can be colder than the ambient, but the heating requirements are also lower at that time. In the summer you are always ahead of the game. My own home system will hit 70 deg loop water by summer's end. If the air temp is that cool, we don't use the A/C. Also you do have nicely warmed ground that helps in the first weeks or month of the heating season. My system will not get back to 50 deg water until mid to late November.
Overall a bigger and/or deeper loop will help. But I think that the additional cost will never pay back for the few percent in efficiency savings. BTW my electric AUX has never come on in the years I have has a geo system. I sized the loop as I would any customer. No AUX and no defrost help quite a bit with the overall savings.
Last edited by tecman; 08-04-2008 at 11:27 AM.
thanks, that makes sense.
I'm going slightly above what Climatemaster's Geodesigner came up with as a buffer for error..... approx 900ft on 5 ton and approx 450 feet on each their 2 ton units.
I haven't been able to find much, and probably don't want to do all the math, about our soil conductivity. We are in the "red shale" area of Bucks county for what its worth. Later this week I'll know if we hit any aquifer along the way on these and the content down to 300ft. I know that we are using 1in piping, dual pumps (so I'll likely have room to play with the GPM/per/ton flow rates as well).
Which leads to another question: Does flow rate play a significant factor in the closed loop heat transfer? contrasted with it being a little more expensive in run-time cost of more pumping power?
That's a tough one - I think it is highly dependent on the loop configuration, heat pump pressure drop, and pump efficiency curve. Increased flow will certainly help heat transfer but at the cost of pumping work as you said. I did this analysis for my system, but it was reasonably complex and I doubt it would be cost-effective for most residential systems unless there was a software package that would do most of the work for you.
Originally Posted by dzenzel
Actually flow rate is quite critical. You need a Reynolds number of greater than 2000. If your flow rate is too low, or the fluid too viscous (glycol antifreeze) you can end up with a low Reynolds number which will result in low below par transfer rate.
Higher flow is normally good, but will result is higher pumping requirements. The numbers you mention seem short if they are pipe lengths. Are you going bore hole or trench ? Are they pipe or trench lengths ? How many pipes in the trench ?
The lengths above were bore length. Actual pipe lengths will be 1800 ft in three bores for a 5-ton unit and 900 ft in two bores for each 2+ ton unit.
The geodesigner app had suggested (Climatemaster TT-064) 700 ft bore -- making it 1400 ft for 5-ton and for each two ton (Climatemaster tt-026) 305 ft bore -- making those 610 ft pipe....
So I added 200 bore ft to the 5-ton one and about 150 bore ft for each of the 2 ton ones as a buffer. There will be around 50-100 extra linear feet on each total loop in the manifold to basement. We are drilling relatively close to the house.
Soil composition here in Bucks county is fairly rocky... mostly red shale.
I would strongly consider combining your units, and parallel the loops and have one system for all of your units. You are talking 9 tons total. Is that right ?
The advantage is that you will have excess capacity when all 3 units are not running, and only one loop system to contend with.
The holes should be grouted with bentonite for heat transfer. If the holes are wet, my driller filles them with screenings, rather than grouting.