Energy From the Earth: the Basics on Geothermal Heating and Cooling

Grand Prismatic Spring, Yellowstone National Park, Wyoming. © flickr

Starting at depths of  6 to 10 metres, the temperature of the earth is no longer influenced by variations in surface temperature, and stays relatively constant at around 8 to 10 C. So the underlying principle of geothermal heating and cooling is to use that consistent interior earth temperature to balance our wildly varying Canadian surface temperatures.

With the use of heat pumps, geothermal heating and cooling systems extract heat energy and transfer it into your home, saving you approximately 50 to 60%  on heating and cooling costs.

In summer months, geothermal cooling functions in a similar way to standard air conditioning, only heat is not simply ejected from your home, but rather deposited deep in the ground for future use. What you have ends up being guilt-free air conditioning because you are actually using the heat from your home to warm the earth, heat which will increase the efficiency of your system in winter months.

The three most common systems for residential use:
 

Geothermal vertical loop system © Alexandre Gilbert

Vertical closed-loop systems have a sealed U-shaped pipe of high density polyethylene that carries a heat transfer fluid (usually a water / methanol mix) in a continuously circulating loop allowing an exchange of heat by conduction. As the liquid returns to the surface, either heated or cooled depending on the season, the new temperature is used to condition your home. Required depth for this system is generally 300 feet or more, and you pay for it by the foot...through the nose, but by the foot.

Geothermal horizontal loop system © Alexandre Gilbert

Horizontal closed-loop systems function in the same manner as vertical systems, except that pipes are run back and forth 6 to 10 feet underground. Installation involves excavating trenches (at least 300 feet of them), rather than digging a well.

Horizontal systems can be cheaper to install but require a significant amount of space, and it does some pretty intense damage to any ecosystems that lay in its intended path. For a given length of pipe, horizontal loop systems are a bit less efficient than vertical loop systems, as they can be more easily affected by surface temperatures.

Geothermal open loop system © Alexandre Gilbert

Open-loop systems use ground water pumped directly from a supply well (75 to 100 feet deep) in order to draw and inject heat. Water is pumped out of the first well, and after the heat exchange is carried out, it gets injected into the second well.

Open-loop systems have a very high thermal efficiency and installation can be up to 50% less expensive than vertical closed loop systems. However, conditions necessary for the proper function of these systems are rarely found in urban areas, as they require an abundant source of ground water.

Cost, savings and return on investment: 

No geothermal system is cheap to install, and because it offers only a reduction in consumption, the return on investment is quicker for larger buildings. For this reason geothermal is more commonly found in commercial or multi-unit residential projects.

The cost breakdown is about $10,000 per 1000 square feet, so decent payback periods usually involve homes above 3000 square feet. It can be hard to justify installing geothermal in a reasonably sized high performance home, but for a bigger residence this is a great system.

Keep in mind that geothermal systems also require supplementary electrical energy. So ecologically speaking it might not be the best way to reduce your carbon footprint if you live in an area where the electrical grid is fed by heavy polluters such as coal plants.