My name is Gregor Vialette, I am an Applications Engineer at EarthLinked Technologies. We are a manufacturer of Direct Geoexchange systems (also known as DX geothermal or refrigerant-based). We are currently working towards integrating our technology in the IGSHPA standard, so I thought I would try and cover here some of the most common misconceptions and myths that I regularly hear about DX.
1) “Copper loops corrode and fail”
First off, I’d like to mention that the reason copper tubing is used in DX is that it presents many advantages: it is strong, ductile, naturally resistant to corrosion, has a very high thermal conductivity and has a long history of use in air conditioning and refrigeration.
Copper tubing is naturally compatible with nearly all soil types in the US. Copper is extracted from the ground itself and is a noble metal - and is therefore almost completely resistant to corrosion from soils found worldwide. Additionally, EarthLinked developed an Earth Loop Protection system (EPS) based on the Impressed Current Cathodic Protection (ICCP) method, a method used for a wide range of metallic structures in various environments and applications, such as subsurface footings where steel is used in combination with concrete. Earth loop protection systems are installed with every loop field out of abundance of caution and as an added measure of insurance. As a result, EarthLinked has had copper loops in the ground and running successfully for over 30 years.
2) “Leakages could contaminate the ground”
Of course, this type of concern applies to all geothermal systems. As far as EarthLinked refrigerant-based systems are concerned there is very limited risk of contaminating the ground or the ground water. In case of a refrigerant leakage, the refrigerant used in our systems – R-410A – would immediately vaporize and seek the atmosphere. This is due to the low boiling point of R-410A: -60.5 °F. R-410A refrigerant and presents no threat to aquifers or to the ground itself.
In case of an oil leakage, the oil we use in our systems – POE32 – is insoluble in water and would therefore not contaminate aquifers.
To finish, leakages are very rare. Once installed, our systems undergo a 400 psig seal test of the underground system for a minimum of 8 hours.
3) “Bad oil return could cause a compressor failure”
Oil return was certainly an issue for some of the early DX systems. It was an especially critical factor for systems using R22 as a refrigerant and mineral oils.
To ensure good oil return, one of the most important properties of the oil is solubility with the refrigerant. Before CFC refrigerants were banned, mineral oils were widely used in refrigeration systems along with R22 refrigerant. However, R22 and mineral oil separate into two phases in the operating range, which can cause issues with return. These problems can be avoided by properly designing the system and by using oil separators.
However nowadays systems use polyolester oil (POE) and R-410A refrigerant. POE oil and R-410A mix very well together, which translates into superior oil return characteristics. As a company, EarthLinked also has a patented oil return mechanism to ensure effective oil return in our systems.
Another important factor is refrigerant velocity. We account for that when sizing our loops in order to maintain the velocity necessary for the refrigerant vapor to carry oil through the tubes.
4) “Copper is too conductive for the ground”
à Alternatives: “The ground cannot keep up” or “The loops are too short”
This type of discussion can get quite lengthy but the main misconception I am trying to address here is that DX loops are too short to perform correctly. In my experience, people’s main basis for this idea is the simple observation that DX loops are shorter than plastic loops.
To understand why loops in refrigerant-based geothermal systems can be more compact than water-based systems, I recommend reading the short article I wrote about this in the GeoOutlook magazine: http://www.geooutlook.org/epub/GO2015No4/#4
To summarize: DX systems achieve the same end than water-based systems with a different mean.
The GeoOutlook article provides a theoretical answer. A more practical approach that is also valid is to refer to direct measurements of a system’s capacity and compression ratio. Our refrigerant-based geothermal systems have similar run times and compression ratio than water-based systems.
No matter how you slice it, it would be impossible to get away with undersizing loops (no matter how it is done) without sacrificing capacity and/or efficiency.