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What is geoexchange?

When we¡¯re talking about geoexchange systems, we¡¯re referring to a heating and cooling system that uses the Earth as energy storage. The Earth absorbs significant energy from the sun and retains it through the winter. We can use that for heating and cooling. Side note: the catch-all term ¡°geothermal¡± refers to systems that collect high temperature heat from deeper in the earth and can then be used to produce electricity. But we¡¯re talking about systems specific to heating and cooling.

Geoexchange systems consist of a ground heat exchanger or loop, a heat pump, and a distribution system. Air source heat pumps work on a similar principle¡ªusing the outside air as a heat source in the winter and a heat sink in the summer. But air source heat pumps struggle in climates that are colder than -15¡ãC; although they continue to make progress with some models working down to -25/-30¡ãC. Our clients like air source heat pumps, but geoexchange ground source heat pumps perform better in a cold climate. Geoexchange systems extract stored heat from the Earth with an efficiency around 400 percent. Hybrid systems use both, for example, a ground source system for heating during the winter and an air source heat pump during the warmer months.

Interest in geothermal heating and cooling systems is peaking. This is especially true in colder areas. So, we¡¯re sharing our insights on the different ways to use these highly efficient systems in decarbonizing the built environment. And it¡¯s done with an emphasis on safety and cost control.

Why are organizations looking at geoexchange?

Many factors drive decisions about decarbonization. These include regulations, emissions targets, and ESG (environmental, social, governance) goals. One key option for decarbonization is through district energy systems. Groups are exploring new systems that leverage geothermal energy for heating and cooling.

As an alternative to conventional heating, ventilation, and air conditioning systems, a geoexchange system is more reliable and efficient, with fewer associated emissions. Compared to electric resistance heating with standard air-conditioning, geoexchange systems reduce emissions by more than 70 percent. Clients can recover upfront costs for these systems in energy cost savings over time. The systems and ground loops last for decades.

We can apply the geoexchange approach at a wide range of scales¡ªfrom a single house to a full city. We communicate the size of geothermal projects by the number of wells in the network. Today¡¯s projects can contain 200 geothermal wells or more. We¡¯ve worked on community scale and campus scale systems. We¡¯ve also used geothermal for post-secondary and secondary schools. And we¡¯re currently engaged in a city scale geothermal project.

Here are six successful steps to implement geothermal systems.

1. Know what¡¯s underground

Building geoexchange systems means drilling a network of boreholes in the ground¡ªwhether that be on a greenfield site or under a parking structure. We often see boreholes for geoexchange drilled to 400 or 800 feet below the surface. Boreholes can be as deep as 1,500 feet or more.

We need to perform due diligence and assess the environmental impact on these projects. Principally, we¡¯re concerned with preventing contaminants from getting into the building or impacting groundwater that could be used as a source of water supply.

In the environmental business, we typically do a Phase I Environmental Site Assessment using information on historical use of a property. We¡¯re looking for potential contaminants that might be on site, for example old underground storage tanks on a former gas station site. If there are potential sources of contamination, we do a Phase II ESA, which often means drilling holes, collecting soil samples, and sending them to the lab for analysis.

A Phase II ESA may include installing monitoring wells, which are designed to provide a high-quality sample of groundwater to assess the existing impacts. If needed, we can install shallow probes that sample soil gas that sits above the water table to assess if contaminants might enter a building.

There are plenty of underground challenges that we may discover. These include old mineshafts, pressurized gas pockets of methane and other gases, and contaminants from industry.

The more we know about the conditions in the borehole field before we drill, the better.

2. Prevent contamination

Whether we are drilling boreholes under buildings, parking structures, or on greenfield or brownfield sites, we are likely to hit groundwater. Once we know that contaminants are on site, we need to protect the groundwater. If we¡¯re drilling through potable water aquifers, we want to avoid spreading existing groundwater contamination or causing new impacts to an aquifer.

We can do this in the field by using cementitious bentonite materials to seal the boreholes and segregate areas above and below the aquifer. This reduces the chance for contamination of groundwater.

3. Drill safely

Many rigs use a single mud rotary drilling technique to create geothermal field boreholes. This technique relies on pumping water into the borehole to bring the drilling contents and mud to the surface which can cause contaminants to be spread throughout the borehole. So, imagine that you¡¯ve drilled that through a contaminated region above or below an aquifer. As you go through the aquifer, you¡¯ve potentially spread contaminants into the aquifer. That's the risk with this drilling technique. There¡¯s a better way.

With a dual mud rotary system or dual error rotary system, the driller installs the borehole casing all the way down as it goes. And as it brings the drilled material to the surface, it stays inside the casing. It never touches the aquifer below or above it, or even the soil above it. That¡¯s the safer way to prevent spreading contamination between zones.

To contend with the issues we discover in step one, such as gas pockets or old mineshafts, we should implement safe drilling protocols.

4. Choose the right location

There¡¯s an advantage in putting boreholes underneath the heated building. The building basically acts as a cap and warms the space above the bore field. This improves the system¡¯s heating performance by 10 percent, according to our studies. It also reduces the cooling performance by 10 percent, but that¡¯s less of an issue.

There are also complications to drilling beneath a building. When operating the geoexchange system in heating mode, it will typically run below freezing temperatures. If the system freezes the soil formation underground within 30 feet of the foundation this can cause frost heave¡ªand upward swelling of the soil. To mitigate this, we can engineer insulation for the upper portion of the borehole.

5. Make room in the schedule for geothermal field installation

On any project, whenever we want to coordinate additional activities before the foundation is laid, we¡¯re adding to the construction schedule. Drilling the boreholes for geoexchange systems takes time, so it tends to increase the upfront capital costs. To control that we need to schedule it efficiently. If the scale of the project calls for it and we¡¯re drilling hundreds of boreholes, we should add multiple rigs to reduce drilling time.

There is another way, however. Geoexchange contractors are switching the order of operations. They can come in after the foundation is laid and the building is going up. They simply drill through the foundation. They can even drill beneath existing parking structures or inside existing buildings.

6. Look at innovative geothermal strategies to maximize value

In the examples above, we¡¯re thinking about the basics of geothermal heating and cooling system installation, with an emphasis on safety and schedule. But when it comes to geoexchange systems, there is a lot of room for us to innovate, find efficiencies, and customize an approach that suits the project at hand.

We can hybridize the energy supply by combining geoexchange with other heating and cooling systems or with heat recovery to increase resiliency. Advanced technology means that boreholes need not be 90 degrees. We can drill sideways. We¡¯ll talk about some of these advanced solutions another time.?

Getting the most from going low carbon

Decarbonization is a great opportunity for our clients but also quite a challenge. Now is a great opportunity for us to offer clients guidance on implementing geothermal systems. With the approaches above in mind, we can help them implement safe, lasting, low carbon solutions to meet their buildings¡¯ thermal comfort needs.