Geothermal HVAC loops. Decisions to be made.

[fastline12]

We are an engineering firm but NOT a geothermal firm. We are wanting to install a geo heat pump system in a new shop space. I think the open loop water system has been nearly ruled out. Concerns of any water supply issues as well as the need for a water to water exchanger to run closed loop fluid to the HVAC unit. We do not want to chance running ground water directly through the unit and as well, if water became an issue, we would have no water as well as no HVAC.

This brings us to vertical or horizontal loops. Vertical seems to make more efficiency sense but would take many years to recover the drilling costs. a horizontal excavation would be free for us.

The closer we can stay to 55-60*F inlet temp, the more efficient our system will be year round. It seems the common practice is to use PE pipe but in looking at the thermal conductivity, the question was raised if we can get anywhere with metal piping? It would then make the most sense to be in a water saturated environment (vertical loop). In a horizontal loop, it looks like the earth conductivity would be the bottle neck thus requiring hundreds of feet of piping.

Does anyone have direct experience with geo systems in the midwest to know which has been working better in practice? The thought also came to mind to have a single well dug where we could install a custom metal heat exchanger but not knowing how the layers of rock and water would interact and perform, it would be a guess for us. Dipping loops of PE pipe in a water well just does not seem an efficient use of direct water exchange but maybe that is all the ground water will take.

Looking at the thermal conductivity properties of stainless and Al compared to PE sure makes one want to reach for metal but if the earth cannot utilize it, I guess there is no use.

Thoughts?

 

[DRWeig]

Hi fastline,

Can you tell us location information, or perhaps even soil and groundwater temperature profiles by depth? For example, in the Memphis area, the groundwater is a rock-solid 60°F all year around. But the standing water table is almost 100 feet down. Soil temps vary, but I'm not sure how much. At some depth down, though, they'll be stable enough for good annual efficiency.

Getting into the water table is best, but the marginal investment may not have a very good return. You'll have to compare the two, even if a horizontal excavation might be "free." I'd advise putting the in-house cost on that one, too, unless your accounting folks say otherwise.

Good luck! There are some awfully sharp thermo people in this forum, hopefully with you providing a little more info, you'll get good help.

Good on ya,

Goober Dave

 

[Ziggypump]

I've designed several of these geothermal systems, especially here in the midwest, and if you have enough room for a horizontal, that would be my suggestion. It's less expensive than a vertical system and done in the helix (slinky?) pattern, you can get a lot of heat exchange in a smaller area.

HDPE piping is used for the geo loops. You are correct that the heat transfer is not that good. But this piping is literally fused together, so there are no joints in the field. This pipe is also flexible. Also, with metal piping, the PH of the soil can help erode the metal. HDPE piping will not work well in direct contact with water, you are correct there.

Of course you want the water temperature to be around 50-60 degrees. But this is not going to happen. You will be looking at having around 90 degree supply temp in the summer (July and/or August) and the ground temp can get down to 30 degrees (January and/or February). Think about it; in the winter you're taking heat out of the ground and putting it in the building, and in the summer you're putting heat into the ground. Geo systems do have an impact on the overall ground temp in the long run. If designed correctly, the temp will only increase 1-2 degrees (a balance will be reached). Yes 30 degrees is below freezing, but you add glycol or methenol or ethenol (propylene glycol is usually used for it low environmental impact) to the water (chemical feed tank) and make sure that the water will not freeze until it reaches 0 or so. There are extended range heat pumps that can handle these temperatures. The ground will act

The horizontal field would need to be buried about 8 ft below grade. Since you have not designed a geo system, contact an installer and have them design the field for you. You will need a VERY accurate load calculation from your building to correctly size the field. The field is sized based on peak and annual load of the building. They will calculate size of field, depths, everything needed. You might want them to run the two different sized fields to determine which is best for your situation.

This is just a little information on what I've run across. There are a lot of things to take into account. As stated above, I'm partial to a horizontal loop if it's possible, if just for the installation cost. Also keep in mind that HDPE uses iron pipe sizing, but the wall thickness of the pipe is thicker to handle the pressures. And this can have a major impact on your pumping head loss.

 

[fastline12]

Thanks guys. The installation would be in Kansas. Soil condition should be a sandy clay composition. I tested water temps today from another area site at 59* drawing from a depth of 40ft. IIRC, that temp in KS stays rather constant year round.

The issue I am facing right now is not enough data for soil temps at depth throughout the year. I am not sure how deep I need to go to get ground water like temps. Using a water to water exchanger and drawing up ground water is certainly an option if the numbers work out.

I did not mention but we will also provide cooling eventually to the entire space which will require "another" 10T of cooling. at that, I would need 30gpm of water flow. With a water to water exchanger, I would have to draw the water through an exchanger, then another pump to circulate the closed loop fluids. Certainly more complex but again, willing to jump in if the numbers work.

I would MUCH rather just bury tubing at 8ft deep and be done but in studying my efficiency tables, it might pay off both for available BTUs and efficiency. I have heard of many people going only 4ft deep for horizontal systems but the ground temp is largely affected at that depth.

 

[fastline12]

I am working with our local agricultural university to obtain any test data on any seasonal soil temp data.

I did want to run a couple thoughts by

1. Thermal conductivity through soil seems rather poor. The immediate thought was to reach for Stainless or maybe even Copper pipe to put in the earth for the closer loop. The issue really is the earth may not accept the thermal conductance well at all. It does have mass on it's side but conductance is what it is. From where I am on this, it looks like matching the tube conductance properties with the soil conductance properties is really about all one can do for an efficient installation?

2. Another thought that came to mind was to design a large surface area exchanger to be installed in a water well to more efficiently exchange. A metallic exchanger would be preferred for this. The only issue is static water does not conduct all that well and it is my understanding that water migration in the earth is next to nothing so what we may end up with here rapid heating of a column of water and dissipation into the earth and surrounding water only at the rate of conductivity of the water and earth?

 

[Ziggypump]

Be careful with metal piping in the ground. I know that copper piping will deteriorate in the soil due to electrical charge. I'm not too sure on the scientific explanation, but it's the same reason why you put a sacrificial rod in your water heater to preserve the copper tubes. This is probably the same for stainless steel. I know that HDPE isn't the best pipe to use for heat transfer, but there is a reason why it is used.

There are several companies that have a sort of heat exchanger that can be purchased. They are designed to be installed for geothermal heat exchange, and are intended to be sunk to the bottom of a pond or something. You will need about 10' of cover to make sure that it is sufficiently covered.