5 Tips on Designing Vertical or Slinky Geothermal Loop Fields

We’ve found it useful to focus on both articles that will help companies with their sales and marketing AND design and installation. A few weeks ago, I shared a piece – thanks to Ryan Carda – on geothermal flow path analysis for ground loop design that came from a discussion forum from our advanced geothermal design course.  My plan is to share more technical discussions that are happening within the course. If you are installing or designing geothermal projects, these articles will be useful to you if you never take the training. This is my goal.

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Here are a few tips on on vertical and slinky bore design.

Vertical Bore Design

1)      The target (optimum) flow rate versus pipe size is:

2.8 – 3.2 gpm per loop for ¾” loops
4 – 6 gpm per loop for 1” loops
5 – 9 gpm per loop for 1.25” loops

Staying within those flow ranges per loop will keep you well below the maximum recommended flow rate for head loss (4 ft per 100’ of pipe length, Figure 5.4) and above the minimum flow rate required for turbulent flow.  For the vertically-bored design, I recommend using two loops for 6 gpm per loop with 1.25” pipe.


The 4 Differences Between Geothermal Standing Column Well and Closed Loop Systems

There are four major differences between standing column well (SCW) and closed loop geothermal systems.

The use of exchange fluid
Loop materials and characteristics
Use of heat exchangers
Applications of the technology

To learn more about standing column well design, download the 13 steps to basic standing column well design by Dr. Albert Koenig here. 


1. The Use of Exchange Fluid.

The fundamental difference between the two systems is the exchange fluid used to affect the heat transfer.  In the case of grouted loops, the working fluid is water with an additive (typ. methanol) contained in the closed HDPE pipe loop that runs from the well to the building.  In the case of SCW, the working fluid is well water which fills the borehole from water bearing zones (WBZ) intersected by the bore.  There are advantages to each of these design approaches.

2. Loop Materials and Characteristics

The grouted loop provides a continuous leak-tight HDPE wall that is guaranteed for at least fifty years free from defect that requires little to no maintenance.  But, this comes with a heat transfer penalty, in that, the plastic wall together with the surrounding grout filler, impose a thermal impedance on the transfer of heat to the bore wall.  This is further limited by the HDPE pipe size that can be comfortably be manipulated down the bore, representing approximately 40% of the bore wall surface area.  Moreover, the installed loop upcomer and downcomer are not thermally isolated, allowing heat to be shunted, rather than transferred to the bore rock for storage.  The net effect of these limitations to heat transfer is to require twice the drilled footage for a given project than SCW design.  This is predicated on achieving the same working fluid temperature.