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  • Writer's pictureMitchell Wilkie

Confusion on the difference between geothermal and GeoExchange?

Geothermal Source
Geothermal Source

Regarding retrieving energy from the earth, the terms “geothermal” and “GeoExchange” are not the same thing, but there are similarities which causes confusion to many outside (and still inside) our HVAC/R industry. Let me explain the difference.

Geothermal energy is thermal energy generated and stored in the Earth. It originates from the formation of planet Earth and the radioactive decay of materials. This thermal energy in the form of high-temperature heat conducts non-stop from the Earth’s core to the surface crust. At the core-mantle boundary (over 2,800 km deep beneath the Earth's surface), temperatures may reach over 4,000°C. In various places within the earth’s crust (8 km thick), rock and water can be heated up to 370 °C.

Geothermal energy involves extracting that high-temperature heat energy in the form of hot water or steam and can be most commonly used to generate electricity. The hot water and steam can also provide district space heating or for industrial purposes, agricultural greenhouses and spas. However, it is mostly limited to geographic areas near tectonic plate boundaries where the Earth’s crust is comparatively thin (think Iceland or around the Ring of Fire). Geothermal power is cost-effective when sources are nearby. It is also reliable, sustainable, and environmentally friendly. The Earth's geothermal resources could theoretically supply humanity's entire energy needs, but only a very small fraction may be profitably exploited. Drilling (at half an overall project cost) and exploration for deep resources is very expensive. Geothermal wells range from 3 km – 10 km deep. Spending $10 million to start on one borehole is typical and the failure rate is 20%.

Conversely, GeoExchange is the accurate description for a technology that utilises the ground (Geo) to a much lessor depth than geothermal as a heat source in winter and heat sink in summer (Exchange). GeoExchange systems are also known by other names, including earth-coupled and earth energy systems, among others which adds more confusion. GeoExchange system designs use moderate ground temperatures to boost efficiency and reduce the operational costs of heating and cooling systems. They can also be combined with geosolar heating (fluid solar panels) to recharge the ground for even greater efficiency.

The heat in the Earth’s crust primarily comes from solar radiation from the sun rather than unique geothermal anomalies such as volcanic features in the mantle. In most populated places on Earth, if you go down below 2 to 6 metres, the ground temperature is a constant 10-16°C year-round, which is roughly equal to the mean annual air temperature at that latitude at the surface. Seasonal variations drop off with depth and disappear below 7 metres.

GeoExchange involves transferring or ‘exchanging’ heat from the ground at shallow depths and often doesn’t exceed drilling boreholes more than 140 metres deep. A series of these boreholes (properly spaced apart and sized to a building or industrial process need) is known as a Ground Heat Exchange Field. It is constructed of a system of polyethylene pipe pairs in each borehole that are joined with a U-shaped cross connector at the bottom of the hole. The borehole is commonly filled with a bentonite grout surrounding the pipe to provide a thermal connection to the surrounding soil or rock to improve the heat transfer. Grout also protects the ground water from contamination and prevents artesian wells from flooding the property. The pipe contains a water and food-grade glycol solution which is circulated to the Heat Pump(s). The durable pipe lasts for over 100 years.

GeoExchange Borehole Loops
GeoExchange Borehole Loops

GeoExchange systems use a hydronic ground-source heat pump to force the transfer of heat from the Ground Heat Exchange Field. Heat pumps can transfer heat from a cool space to a warm space, against the natural direction of flow, or they can increase the natural flow of heat from a warm area to a cool one. A heat pump works much like a house-hold refrigerator or air conditioner, but it can move hot to cold and cold to hot. The core of the heat pump is a loop of freon refrigerant pumped through a vapor-compression refrigeration cycle that moves heat. Since a Heat Pump exchanges heat with the ground, it is very energy-efficient because underground temperatures are stable compared to above-ground air temperatures through the year. For efficiency differences, think of an air conditioner trying to take heat out of a building and struggling to give it up to outside air temperature of 32°C compared to a heat pump transferring the building heat to a 10°C ground temperature where it is quickly absorbed. GeoExchange hydronic Heat Pump systems reach a high coefficient of performance (CoP) of 3 to 6.

TRAK Heat Pump
TRAK Heat Pump

Ground Source Heat Pumps are one of the most energy-efficient technologies for providing HVAC, refrigeration, and water heating and chilling. TRAK International Green Energy Resources has been designing and building Smart Energy Systems that incorporated GeoExchange since 1991 and is a leader in this technology for industrial, commercial, institutional and recreational applications. TRAK manufactures modular hydronic Heat Pumps in sizes of 30, 60 and 120 Refrigeration Tons. TRAK can typically save intensive-energy users 40-60% of their energy operating costs compared to traditional HVAC/R systems.

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