Foreword — Is the pump undoing the ground heat exchanger?

Many geothermal heat pump systems never achieve their promised energy efficiency, and the problem often lies not in the ground, but in the pumping system. An oversized circulator, or a poorly designed network that requires a large circulator to deliver the nominal flow rate, converts electricity into heat, wasting energy through hydraulic losses year after year. This parasitic energy consumption can significantly reduce overall system efficiency, even when the geothermal heat exchanger itself has been correctly designed.

For example, on a fifty-six-borehole installation, thirty months of data yielded a seasonal performance of 3.54 for the compressor alone, 2.97 when the ground-loop circulator was included, and 2.49 when every pump was taken into account. Moving the fluid cost nearly 30 % of the compressor's figure (3.54 to 2.49), while the heat pumps themselves performed only slightly below the manufacturer's data. For the excess pump energy the authors name high pipe velocities, short and frequent cycling, and faults in the control system — questions of design and commissioning rather than of the heat pumps (Naicker and Rees, 2018). Only the first is a hydraulic question, and it is the one this module addresses. Kavanaugh and Rafferty (2014) found a central pump oversized by half can eat past four-fifths of the heat pump's operating cost, and an oversized variable-speed pump does not fix itself: its minimum speed stays too high for anything but peak load.

A second common failure undermines the thermal efficiency of the GHE: a field of boreholes piped in such a way that flow is never distributed among the boreholes in the proportions assumed by the thermal model. Some boreholes receive too much flow, others too little, and the heat pump plant is blamed for a problem that originates in the piping. A well-designed ground loop is simple to define. Every borehole should receive nearly the same flow, with sufficient flow at peak heating demand to maintain turbulent conditions. A borehole operating under laminar flow transfers heat poorly precisely when the greatest amount of heat must cross the borehole wall. Away from peak demand, however, the flow should decrease, since pumping power rises—and falls—steeply with flow rate. Boreholes can be drilled and grouted perfectly, and still be connected to a hydraulic network that fails every one of these fundamental requirements. The only way to know is to calculate it.

In fact, a ground loop is judged on four questions: does every borehole get its share of the flow, does that flow stay turbulent at the peak, is each pipe the right size, and what head must the pump supply? I built GLN Analysis to make these questions easier to answer. You draw the circuit as a piping and instrumentation diagram, a node for every component and an edge for every pipe, set the properties, and instantly obtain a robust solution. What comes back is a full nodal pressure field: the spread of flow between loops, the Reynolds number in each borehole, the velocity and friction gradient in every pipe, and the head each pump must supply.

In my experience, three of those numbers are easier to get right before a system is built than after: whether the pump is the size it needs to be, not just one that will certainly be big enough; whether the antifreeze chosen for thermal protection has moved the operating point somewhere the circulator cannot follow, a question a purely thermal analysis never raises; and whether the network can be purged of air and debris with the equipment that will actually be on site, not a temporary pump nobody budgeted for.

I developed GLN Analysis, distributed by P³ Geothermal, a Polytechnique Montréal spin-off, as the third module of the Analysis Suite. I built it because users kept asking for it: thermal response and borehole sizing were covered, and the network connecting them to the plant room was not. Hydraulic calculation software is common; what I added with GLN Analysis is a place inside the same suite, at a price low enough that using a precise tool costs less than guessing. My hope is that this module will make it easier to design a good geothermal system than a mediocre one.

Professor Philippe Pasquier, eng., Ph.D.
Lead Programmer, P³ Geothermal