Geothermal-based Thermal Mitigation of Stormwater Retention Pond Outflows

In 2019, the TRCA released Data Synthesis and Design Considerations for Stormwater Thermal Mitigation Measures. This report focuses on analyzing thermal mitigation measures that have been monitored for performance in a southern Ontario climate. The monitoring data set of practices reviewed and analyzed in this project included 18 subsurface draw outlets, 18 cooling trenches, 14 Low Impact Development practices (bioretention/soil cells, infiltration trench, green roof, permeable pavement), one night time release outlet, one subsurface pond, one vegetated channel and one floating island. These strategies vary in their feasibility, effectiveness, and maintenance costs, and there is currently no consensus on the best options for new construction and existing stormwater ponds.

The benefits of geothermal-based thermal mitigation are that it can be more space and cost-efficient than other approaches. The only mechanical component in place is a circulator pump that typically requires minimal maintenance or intervention. For these reasons, STEP and the City of Brampton opted to pilot and evaluate the geothermal-based stormwater retention pond cooling system. The system is a hydronic circuit where piping connects a stormwater heat exchanger to a ground heat exchanger. A pump circulates a heat transfer fluid through the hydronic circuit. At the SHX, the heat transfer fluid is cooler than the warm stormwater outflows. This temperature difference drives heat from the stormwater and into the heat transfer fluid, cooling the stormwater in the process. At the GHX, the heat transfer fluid is warmer than the deep ground. This temperature difference drives heat from the heat transfer fluid into the deep ground, cooling the fluid to its original temperature. This cycle can be used to continuously cool the warm stormwater.

The system was implemented at a small scale to study the feasibility of this approach and develop valuable insights for a full-scale system. It was commissioned in Spring 2019 and monitored into Winter 2020.

Ground Heat Exchanger

The size of the geothermal cooling system was constrained by the project budget, which allowed for a maximum borehole length of 600 ft (183 m). Borehole length describes the cumulative depth of all boreholes in a geothermal installation (i.e., the number of boreholes multiplied by the depth of the boreholes). All other system components were sized around this constraint. Note that this length was much less than was required to meet the full thermal load of the pond, but it was sufficient for a pilot that would provide insights for full-scale systems.

A single deep borehole was selected because it was expected to supply cooler temperatures than multiple shallow ones. Cooler heat transfer fluid temperatures from a deeper borehole would result in a greater temperature differential and drive a greater cooling capacity. A 1-1/4” SDR11 high-density polyethylene (HDPE) prefabricated geothermal U-bend of pipe was installed in the borehole and grouted with thermally enhanced bentonite grout forming the GHX. Drilling and GHX installation took place during Winter 2018/2019.

Storm Water Heat Exchanger

There are different ways to implement the SHX. In this pilot, it was placed in the existing vault and constructed using bundles of 3/4″ SDR11 HDPE pipe. This is the conventional approach for surface water geothermal systems, which are more common in parts of the U.S.. It was inexpensive, robust, and a familiar form factor. A major drawback of using HDPE pipe for this particular application is that the overall heat exchanger will take up a significant amount of volume for the amount of heat transfer it can provide. However, the sizing calculations indicated that an SHX made of HDPE with heat transfer capabilities to match the GHX could fit in the vault. Full-scale systems would likely need to take a different approach.

Since there was no power directly at the site, the geothermal system was powered by a small off-grid photovoltaic system. Initially, water was used as a heat transfer fluid and was replaced by 50% propylene glycol in the Fall to freeze protect the system from winter temperatures and was used for the remainder of the pilot.

Operation and monitoring took place from Spring 2019 through to Fall 2020. Monitoring data were used to calculate the system’s cooling capacity and validate the system model. The model was used to estimate the size of a full-scale system for the pond.

Monitoring points and sensors included:

• Heat to the GHX
• Heat from stormwater
• Ground temperature
• Environmental conditions
• Pond data
• Pump speed

The pilot showed that a relatively small system composed of six deep boreholes would keep outflow temperatures below 24 °C for 96% of the time during summer 2019. The cost of the GHX installation at this location was $17 000 for the single borehole. The cost for several boreholes is estimated to be lower, between $10 000 and $15 000 per borehole. It is also estimated that most of the full-scale geothermal cooling installation costs are for boreholes. It follows that the full-scale system cost for this pilot pond is estimated to be less than $200 000. Compared to other measures, particularly those that require moving large amounts of the each (like cooling trenches), the geothermal cooling approach is cost-competitive.

The pilot also demonstrates the geothermal cooling approach is highly scalable. It is possible to create a system with a single borehole or one with a dozen, or more. It is also possible to retrofit geothermal cooling into existing ponds or as an addition to existing thermal mitigation systems that are underperforming, such as cooling trenches.

The main drawback of geothermal cooling is that it requires the circulator pump’s active mechanical components. While it is not the only thermal mitigation approach to utilize active components (controlled nighttime release would be another example), the active nature of the system introduces a potential point of failure.

Another drawback identified was the cooling capacity of heat transfer liquids. Water provided higher cooling benefits than the glycol mix on a scale of 20-40%; however, due to the exposure of specific components to above-ground winter temperatures, the glycol mix was required to maintain function throughout the year.

This is the first implementation of geothermal-based thermal mitigation, and there are many remaining items that ought to be studied in future work, including:

• The implementation and evaluation of a full-scale system
• The implementation of metallic heat exchangers and other SHX options
• The implementation and optimization of other measures used in combination with geothermal
• Confirmation of findings of pre-cooling with an expanded modelling study
• Further work evaluating ground temperatures under different environmental conditions – specifically in the worst years historically speaking or moving forward in the context of climate change
• Incorporation of metallic heat exchangers into the sizing tool
• Further refinement of the sizing tool
• As well as other topics

Link to Full Case Study

Additional Resources:

• Geothermal-based Thermal Mitigation of Stormwater Retention Pond Outflows Report Addendum
• Data Synthesis and Design Considerations for Stormwater Thermal Mitigation Measures