GeoExchange and Thermally-Enhanced Grouts
We are all aware that recently we have been enjoying a fairly strong economy. For some areas of the country the recent drought has meant a robust business for water well installations. These two events might have proven to be a good economic turn of events for your company. And, because of it, you may not be overly interested in exploring new opportunities at this time. However, there is an opportunity that promises to grow by leaps and bounds over the next few decades. That opportunity is "GeoExchange."
What is GeoExchange? You may have heard of geothermal heat pumps, ground source heat pumps or GS systems. GeoExchange is just another name for the same thing. For several years this industry has tried to settle on a single name to eliminate confusion and to try to convey industry unity. "GeoExchange" seems to be the direction the industry wants to lean.
Specifically, GeoExchange is a mechanical heat pump system which is tied to a closed-loop ground heat exchanger system (which can be sub-divided into a horizontal loop or a vertical loop), an open loop well heat exchanger system (typically consisting of a supply well and an injection well), or a standing column heat exchanger system. This article will focus on the closed-loop vertical ground heat exchanger application.
A closed-loop, vertical, ground heat exchanger consists of a network of high-density polyethylene pipe installed in a predetermined number of vertical bores. The two polyethylene pipes are connected with a u-bend fitting assembly on one end and inserted into the vertical bore. The annular space in the vertical bore is then completely filled (typically with a bentonite-based grouting material) to protect the environment and to facilitate heat transfer. Nationally, bore depths will range from 50" to 600', and are usually based on local drilling conditions and local contractor capability. Generally, all vertical bores on the same project are designed to be the same depth. Once all the vertical bores are completed, individual bores are tied together with a header network and connected to the building. The system is then filled with water, or a water/antifreeze solution, which is circulated when the heat pumps are running.
A GeoExchange system can be installed on buildings that range from a small residence to very large multi-story commercial building (see "Grouting Geothermal Heat Loops At US Naval Observatory," November 2000 issue of the National Driller). On commercial applications the number of vertical bores can range from as few as 20 to several hundred or even thousands on a given project. GeoExchange offers the contractor the opportunity to perform a considerable amount of work with very little mobilization cost.
There are basically three key components that impact the amount of total vertical bore being installed on a project: 1) the heating and cooling requirements, 2) thermal properties of the local geological formation, and 3) thermal conductivity of the backfill material being used. The only design variable that is not pre-determined by site conditions is the backfill material. If the design engineer has not taken the thermal performance of the backfill material into full account, the drilling contractor has the opportunity to influence the project in a positive and profitable way.
As you know, different states and/or jurisdictions have different backfill requirements. Some states specifically require the annulus be filled with a grouting material while others may only require bore cuttings with a cap or plug at the top of the bore. Most state regulations were designed to protect the local environment. However, it is important to remember this is a mechanical system that depends on the movement of heat between the earth and the building. It is very important to stay focused on protecting the local environment, but it is also very important to understand the heat transfer aspects of this system.
Most conventional grouting materials (bentonite) offer excellent sealing characteristics but severely inhibit heat transfer. On the other hand, most materials that promote good heat transfer do little or nothing to protect the environment. In recent years, a solution to this dilemma has emerged in the form of thermally-enhanced bentonite-based grouting materials. These new products offer environmental protection, with permeabilities less than 1 x 10-7 (cm/s), and much improved heat transfer characteristics.
These new bentonite-based materials consist of a blend of sodium bentonite and sand additives that produce total solids contents that can range from as low as 45.2% to as high as 74.8% (ratio of bentonite to water is 20% at the highest value). Because a significant portion of the grout material is a component (sand) that will not absorb water and hydrate, these materials remain very fluid and extremely pumpable with a piston style grout pump. As the solids content increases, the weight per gallon increases and the pressure required to pump these materials will increase. Therefore, piston-style pumps are strongly recommended and are in widespread use by the industry.
One typical obstacle that has been encountered by drilling contractors interested in GeoExchange opportunities has been the unwillingness to invest in a high-quality grout handling system. The purchase of a specialized piece of equipment can be a substantial investment, but when dealing with thermally-enhanced bentonite-based grouts a high-quality grouting system can more than pay for itself in labor and time savings. It is common for mechanical engineers to specify the backfill material based on their understanding of its thermal characteristics. You, as the drilling contractor, need to be in a position to handle the specified material with the proper equipment that allows you to make a profit.
A second typical obstacle that has been encountered by the drilling contractor is the understanding that these new thermally-enhanced materials prove to be a win-win proposition for the contractor and the project owner. Most contractors commonly assume that, if increasing the thermal conductivity of the bore annulus reduces the total vertical bore requirement, they have less opportunity for potential profit on this project. This idea is reinforced by the fact that these materials are generally higher in price than conventional bentonite grouting materials. The reality is that, by lowering the total bore requirement and using these higher priced grouting materials, the installed cost of the vertical bores will be lower. Reducing the required total bore length results in cost savings by reducing the drilling cost per bore, the labor cost per bore, the pipe cost per bore, and the total grout volume per bore. Any additional cost in grouting material and handling labor has been shown to be more than offset by the cost savings. The result is total cost reduction and a better value to the owner.
As an example, let us assume that a contractor installs 70 vertical bores, which are 276" deep, using a 20% solids grouting material and charged a total price of $100,000 with a profit of $20,000 (20% profit margin). Now let's assume that the same system was redesigned and installed using a thermally-enhanced grout that allowed each bore to be reduced in length from 276' to 213' (22.8% reduction). Based on the cost variable listed above and common thermally-enhanced grout costs, it would be possible for that contractor to charge $84,300 for the same project, which would net $16,860 profit to the contractor at a 20% margin. However, if one-half of the savings ($100,000 - $84,300 = $15,700 / 2 = $7,850) were given to the project, with the balance being kept by the contractor, that project would have been installed for $92,150 (7.85% reduction). In addition, the contractor would now make $24,710 (26.8% profit margin), or $4,710 over the original $20,000 profit using the first assumption. The result is more profit for less effort and less wear and tear of equipment while doing the job in less time.
Lowering installation cost encourages additional growth of this technology and means additional future opportunities for the drilling contractor. Using thermally-enhanced grouts has proven time and time again to save money on the project. It is important to understand that it is not necessary for the drilling contractor to forfeit ALL his saving in order to show a final installationcost reduction.