By: Nicole Imeson
As electrification and decarbonization gain prominence, building owners are looking to the ground to extract and reject thermal energy, establishing their own utility. In winter, fluid is pumped through ground loops to extract heat and a heat pump rejects it to the building systems. Conversely, during summer, a heat pump extracts heat from the building systems and rejects it through the ground loops. The ground serves as a heat exchanger, replacing traditional boiler and chiller plants.
Figure 1 – Winter and Summer Operation of Geothermal Loops
Source: Strategic Energy Solutions Inc. – BCxA 2023 Annual Conference
Geothermal systems operate independently as a utility, foregoing dependence on natural gas for heating. Designers, installers, and commissioning providers should optimize geothermal systems for efficiency and simplicity, ensuring long-lasting operational effectiveness. “Historically, these systems can be rather expensive. Owner’s want to protect their investment, and the best way to protect that investment is to make sure that it was designed and installed correctly, and commissioning of the system will protect that investment at the end of the day,” explains Steven Gunther, a professional mechanical engineer with Strategic Energy Solutions Inc. in Berkley, MI.
Types of Geothermal Systems
There are three common types of geothermal systems, all of which operate as a closed-loop system. The most common is the vertical ground loop, involving several 6” diameter boreholes approximately 20ft apart. The depth varies based on the building’s load profile, typically ranging from 400-600ft, with depths of 800ft possible. Another type is the horizontal ground loop, where loops extend horizontally or as a slinky, requiring a larger area for installation, but less vertical digging, typically 5-10ft deep. Lastly, a pond loop transfers heat to the pond using aspecial plate and frame heat exchangers or slinky-style loops. The pond’s size and depth are critical to ensuring adequate energy transfer for building loads.
Figure 2 – Types of Geothermal Loops
Source: Strategic Energy Solutions Inc. – BCxA 2023 Annual Conference
Design Review
Designing, installing, and commissioning a geothermal system field involves three key steps. The first and most crucial step is the thermal conductivity test (FTC), which determines ground conditions, temperatures, thermal conductivity (energy transfer capacity), and thermal diffusivity (energy transfer speed). This information guides the geothermal designer in determining bore hole quantity, depth, glycol concentration (if needed), and potential energy transfer from the ground throughout the year. A drill log informs contractors of varying soil conditions at different depths within the geothermal field.
During design reviews, the commissioning provider ensures the design engineer includes a geothermal field schedule that will provide information for installation, witnessing, testing, and informing future engineers and contractors of original design parameters in case there are any issues. The schedule covers bore hole number and depth, pipe size, soil conductivity, soil diffusivity, manifold pressure drops, fluid type, flow, temperature, and grout details. Geothermal grout, a specially blended material, maximizes energy transfer by filling the space between the pipe and bore hole. “The performance of the grout will dictate the performance and the amount of heat transfer to the surrounding earth from the geothermal piping” explained Gunther.
Drill, Loop, Grout
The second step involves drilling and grouting the bores. Once the rig drills 6” bore holes to the required depth for the geothermal field, the contractor inserts a continuous pipe length, with a U-fitting on the end, to the bottom of the borehole. Each loop arrives on-site pre-pressurized by the manufacturer. Prior to installing the pipes in the boreholes, if pressure isn’t released when removing the end caps, indicated by a ‘psst’ sound, the pipe is defective and requires replacement. Following pipe installation, the contractor fills the boreholes with grout, ensuring contact between the pipes and the ground. Witnessing the piping installation verifies material adherence to specifications, field-maintained pressure during installation, continuous grouting, and proper record keeping.
Trench, Header/Manifold
After installing the geothermal bores, the contractor digs trenches adjacent to the bore holes. These trenches are used to tie the bores together with horizontal piping. Tying the loops together will create geothermal circuits. The piping inside includes a supply and return header, branches to each geothermal circuit in the ground, isolation valves for each circuit, and pressure gauges and thermometers. In the image below, the division between the geothermal and mechanical contractor’s scopes usually occurs at the isolation valves for the manifold.
Figure 3 – Geothermal Manifold
Source: Strategic Energy Solutions Inc. – BCxA 2023 Annual Conference
The fluid is pumped from the supply and return header to heat pumps throughout the building. The hydronic loop offers an opportunity for localized temperature control using individual heat pumps for each zone. Alternatively, heat pumps integral to rooftop units or water-to-water heat pumps serving in-floor loops can be used for heating and cooling various building zones.
Flush, Purge, Test
Once the piping is installed, the geothermal loops must be flushed with water and purged to remove any air trapped in the piping. Strategic Energy Solutions Inc. recommends a flush cart for this process to ensure consistent flow and pressure during the flush. The water should flow at a rate of 2 ft/sec and “circulate through the entire circuit ten times in one direction and ten times in the other direction to make sure everything is out of the pipe, and it's just left with water” explained Gunther. Once flushed, the system is pressure tested as per local code and engineering requirements; this is typically 100 PSI or 1-1/2 times working pressure for one hour minimum.
Geothermal ground loop systems provide owners with an alternative to traditional heating and cooling methods. To ensure long term operational effectiveness, it is imperative to design, install, and commission geothermal systems correctly. The design phase involves a crucial thermal conductivity test, and subsequent steps encompass drilling and grouting. Design reviews, witnessing installation, and proper documentation are essential for maintaining system integrity. Strategic Energy Solutions Inc. emphasized the importance of meticulous flushing, purging, and pressure testing for optimal geothermal system performance.
At a recent BCxA conference in Orlando Steven Gunther from Strategic Energy Solutions Inc. in Berkley, MI hosted the session “Geothermal System Commissioning: Why It’s Important and How to Do it Effectively”. This session provided content and media for this article. The full recording of the session is available in our online learning platform.