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An article on Ground Heat Exchangers and Borehole Resistance
It is said repeatedly that lower borehole resistance shortens the loop lengths required in a ground heat exchanger (ghex) for a ground source system. The fact is: any borehole resistance adds resistance to the system and increases the loop lengths. Resistance DOES NOT affect the earth's local thermal conductivity; however, it does affect the system efficiency.
The confusion between efficiency and conductivity is alive and well. Some are selling a lot of services based on this confusion, to the muddled owner/public and even our industry, especially since the drilling industry is still water well dominated (that is, they drill larger boreholes with larger equipment because that is the size needed in water wells to install a pump). This article is written to address the vastly predominate condition where the grout thermal conductivity is less than the local earth conductivity. Most often the grout will be placed with conductivity values of 0.4 to 0.9 which will less then the local earth conductivity. Earth conductivity of less then 0.9 is not a good candidate as a vertical heat exchanger unless it is inexpensive and easy to drill, and then install a ground heat exchanger.
Let us take a look at the facts:
Thermal conductivity is just that. The quanity of heat moved is thermal conductivity, and the units are Btu / (hr-ft-°F). As an example, the number of cars in a train defines how big the train is; one train car, two train cars, or a number of train cars - similar to increasing thermal conductivity values of different types of earth. Thermal diffusivity, and the units are ft2 / day, is similar to how fast a particular train moves with all its cars, or how big (or many) was the locomotive(s) that pulled the train. The two together make an operable freight train to haul the goods (move heat). Now consider the train as it travels. All goes well until it starts uphill. The locomotives start to work harder and slows down because of the upward slope. Borehole resistance has the same affect on the ground source system. Ewbank pioneered in-situ thermal conductivity testing and worked very hard to encourage others to develop software that yielded the heat rejection and absorption loads of the facility that could be imported for design software of the ground heat exchanger (ghex) sizing. Gaining from that background please consider the following.
Sizing a vertical ghex is about:
1) The long term operation of the system, and
2) How the earth ultimately radiates the heat to deep space (cooling dominated loads) or conducts heat from the core to the crust (heating dominated loads).
Of course the sun, winds, rains, and oceans modify conditions at the air/crust boundary, but how the earth ultimately radiates or conducts is the real scenario. Horizontal ghexs never get to this as they are not significant to the deep earth and are subject to instantaneous conditions like the sun, rain, and seasons. Horizontal systems, however, can be reliable and are renewed annually.
Remember: hot flows to cold, and to enhance this (change the rate of flow of heat) heat pumps are employed. Lord Kelvin was right as he followed Nicholas Carnot's lead.
To heat and cool a facility efficiently with a ground source system, the following questions are asked: How big a block of earth is needed? What are the long term relationships that dominate? Is it largely borehole resistance? What are the loop lengths? Does borehole resistance have any affect on the peak block loads of the facility or the earth's ability to conduct and diffuse? NO! Yet those loads must pass through the borehole resistance to reach the earth or heat pump. Borehole resistance affects the efficiency of the ghex and its coupling to the facility loads. The more resistance--then a greater differential temperature needed to flow hot to cold; this lowers the capacity of the heat pumps to move the facility loads. Therefore the heat pumps must run longer.
For example, consider a heat pump cooling a facility and rejecting heat to the earth with a EWT range of 40 degrees Fahrenheit. With no borehole resistance one looks directly at long term conductivity and diffusivity for solution of how much earth is needed. With large borehole resistance then one has to start the "what if" game and sizing for the actual rates of the system and its components. If one builds in 15 degrees Fahrenheit of borehole resistance (uphill slope) then the heat pump only has 25 (40-15=25) degrees Fahrenheit of EWT to work within--and this is why the block of earth (ghex) grows in size and the loop lengths increase. There is a lower temperature differential at the earth to move heat, so we need a larger block. Also, there is a 15-degree cooling and 15-degree heating (or 30 degree Fahrenheit EWT) spread of inefficiency (uphill slope for the train) built into the system before we move any heat to the earth or the facility at acceptable rates because of the borehole resistance. This inefficiency is built into the system for the entire life of the ghex. The train never gets to go on level ground.
So how does one reduce the resistance?
1. Drill a smaller borehole, the best solution.
2. Increase the ratio of the equivalent diameter of the loop to the borehole; i.e., install a larger loop or use a standing column design.
3. Add materials to the grout that enhance the conductivity of the grout and if below static water level and permissible by appropriate jurisdictions, add nothing leaving the groundwater in contact with the loop.
4. Get the loop to contact the earth by using clips that push the loop out or by using a standing column design.
A standing column design is a tube within a tube. The center tube takes the water from the facility and places it at the bottom inside of the outer vertical tube. The flow may also be reversed. This is done with large systems and the introduction of ground water vent pumping. This system is a combination "standing column and pump-and-dump", and can be very efficient.
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