Geothermal heating

Geothermal heating works by harnessing the heat below the Earth's surface, which it then converts into warm or cool air through geothermal heat pumps. These systems are driven by a small amount of electricity and are more efficient ways to heat or cool a building. This works as below the Earth's surface, the temperature remains relatively consistent all year. Despite the variations in air temperatures, from day and night or from summer to winter, a few feet below the surface, the temperature stays between an average of 55 to 70 degrees Fahrenheit. This is maintained because the Earth's surface absorbs 47 percent of the sun's heat, providing a reliable source of energy for geothermal heating.

Example of a geothermal heating system using a horizontal loop.

This absorption and pool of heat underground create a sustainable energy source, allowing geothermal heating to be considered a renewable resource. In some regions where there is tectonic and volcanic activity, there are higher temperatures, which can increase the efficiency of geothermal heating systems. These geologically active areas also allow for geothermal electricity generation, working on similar principles to geothermal heating. With the pool of heat or energy in the ground, the geothermal heating system works by using the difference between the temperature in a building and the temperature in the ground, using circulating fluid to transfer and transport the heat.

Geothermal heat systems are considered renewable, in part because they often use closed loops in which no material is expelled or burned, and also because the power draw of the system is much lower than other HVAC systems, which reduces the overall energy consumption of a home. And when paired with renewable energy sources, it further reduces the energy demands of a building or home.

There are four types of ground loop systems. Some of them, such as horizontal, vertical, and pond or lake, are considered closed-loop systems. There are also open-loop options and hybrid systems. The kind of system used will depend on the climate, soil conditions, available land, and local installation costs. But all approaches can be used for residential and commercial building applications.

Most closed-loop geothermal loops circulate an antifreeze solution through the closed loop. These loops are usually made of a high-density plastic-type tubing buried in the ground or submerged in water. A heat exchanger transfers heat between the refrigerant and the heat pump, although one closed-loop system, called direct exchange, does not use a heat exchanger and instead pumps refrigerant through copper tubing buried in the ground. Direct exchange systems require a larger compressor and tend to work best in moist soils and should be avoided in soils corrosive to copper tubing. And depending on the refrigerant used in the system, some locations may prohibit their use depending on local environmental regulations.

Example of a horizontal closed-loop geothermal heating system.

A horizontal closed-loop installation is generally the most cost-effective for residential installations, particularly for new constructions and where sufficient land is available. This system requires trenches at least four feet deep. The most common layouts will use either two pipes, one buried at six feet and the other at four feet, or two pipes placed side-by-side at five feet in the ground in a two-foot wide trench. And the pipe will loop to allow more pipes to be used in shorter trenches, which can reduce installation costs and make horizontal installation possible in areas that would otherwise not have enough space for a conventional horizontal application.

A vertical closed-loop system.

Vertical closed-loop systems, as the name suggests, drill holes often around 20 feet apart and 100 to 400 feet deep. These systems can be used where the soil is too shallow to trench, will minimize the disturbance to the existing landscape, and are often used for heating and cooling large commercial buildings. In the dug hole, two pipes will connect at the bottom to form a U-bend and the loop.

A pond/lake geothermal system.

If a site has an adequate body of water, it can be used to create a geothermal heating system and may be the lowest cost option for an installation. In this system, a supply line is run underground from the building to the water and coiled into circles at least eight feet under the surface to prevent freezing. This necessary depth requires that coils only be placed in bodies of water capable of meeting the minimum volume, depth, and quality requirements. When properly installed, the pond or lake loop will not adversely impact the surrounding environment as it remains a closed system in which nothing comes in or goes out.

Example of an open-loop system.

An open-loop system uses a well or surface body water as a heat exchange fluid that circulates through the geothermal heating system. Once it has circulated through the system, the water returns to the ground through the well, a recharge well, or a surface discharge. This option is only available or practical when there is an adequate supply of relatively clean water, especially as particles can create buildup in the pumps and exchangers. Further, this type of system is subject to environmental regulations regarding groundwater discharge.

Hybrid systems use several different geothermal resources, or a combination of a geothermal resource with outdoor air, offering another loop option. Hybrid approaches tend to be used when the cooling needs are significantly higher than the heating needs. Where the geology permits, a standing column well is another hybrid option, where, similar to an open-loop system, one or more deep vertical wells are drilled, and water is drawn from the bottom of a standing column and returned to the top. During periods of peak heating or cooling, the system can bleed a portion of water rather than reinjecting it all, causing water inflow to the column from the surrounding aquifer. The bleed cycle cools the column during heat rejection, heats it during heat extraction, and reduces the required bore depth.

A heat pump is an electrically driven device that extracts heat from a low-temperature place, also known as the source, and delivers it to a higher temperature place, also known as a sink. What source is used in this system often defines the system; for example, a geothermal heating system uses a heat pump with a ground-based source, whereas a more traditional heat pump system uses an air-based source. In a ground-source heat pump, or a geothermal heating system, the earth, groundwater, or both are used as a source of heat in the winter and as a reservoir to reject heat removed from the home in the summer.

Example of the flow of heat in a geothermal heat pump.

Geothermal heat pumps are considered better for heating and cooling a building as they are more efficient than air-source heat pumps, do not require natural gas or propane to run, work with thermostats and fans like other HVAC systems, and are considered to be quiet and reliable. This is because when a geothermal system absorbs heat from a home, it can transfer it to the cooler earth. When cooling a home, the outside air is at its hottest, and a traditional air-source heat pump must work harder to force the heat from a home into the heat-saturated air.

Also, during the winter, the ground source is warmer than the ambient air, which is at its coldest. As a result, traditional air-source heat pumps work hard to extract the amount of heat from the cold air needed to properly heat a home. The efficiency in both scenarios reduces the necessary energy to run a ground-source heat pump, using the Earth's abundant supply to provide heat for a home. Gas furnaces require natural gas or expensive propane to burn and are only 98 percent efficient, while geothermal systems use less energy achieving 400 to 600 percent efficiency.

Because of this efficiency and the relative stability of the pool of heat in the ground, it only takes one kilowatt-hour of electricity for a geothermal heat pump to produce nearly 12,000 Btu of cooling or heating. To produce the same amount of Btus of cooling on a 95-degree day consumes 2.2 kilowatt-hours of electricity.

These systems then use two types of sinks or distribution systems to transfer hot or cold air through a building. These are defined by the heat sink used:

Example of a distribution system for a geothermal heating system.

Water-to-air geothermal systems, also known as forced air systems, extract heat from the transfer heat transfer fluid, either water or a mixture of water and other heat-transferring fluids (such as antifreeze), which is converted into hot or cool air and used to heat or cool the house. This type of system included a compressor, blower-motor, and a circulation pump and allows users to filter the air from the home or add accessories such as ultraviolet lamps, electrostatic filters, electronic filters, and humidifiers to improve air quality.

Water-to-water geothermal systems extract heat from the heat transfer fluid to produce hot or cold water. The water is then used to heat or cool radiant floor heating systems, fan coils, or radiators. They can also be used to preheat water for domestic consumption or combined with an existing hot water system.

Installing a geothermal heating system can be cost-prohibitive. The average cost of a residential installation ranges from USD $10,000 to $45,000. The price range depends on the soil conditions, plot size, system configuration, site accessibility, and the amount of digging and drilling required. Further, when retrofitting a home, the installation may require ductwork modifications with extensive excavation; whereas in a new home or new construction the installation costs would be lower. Further, considerations such as brand selection, the capacity of the system, the type of loop used, and what features are included in the system will further influence the cost.

Depending on the geography or local climate, a geothermal heat system may require supplemental heating systems or a backup heat source, such as a furnace or electric heating elements, which would only be used on days cold enough to stop the geothermal system from working efficiently or capably.

With a geothermal system costing about 40 percent more than a traditional HVAC system, recouping costs from the installation is often an important consideration. Depending on the system and local energy costs, energy savings could return on the investment in as little as four years or as long as twenty years. Part of the recouping of the cost of installation can come through local tax credits or rebates as part of an energy-saving initiative, with some receiving as high as a 30 percent tax credit based on the energy savings and the reduction in stress on a local electrical grid.

A commercial geothermal heating system works on the same above-mentioned principles as a residential or small unit geothermal heating system, except on a larger scale. Usually a commercial unit removes heat from an energy supply source and concentrates the low-grade heat to raise its temperature and transfer it to the building's distribution system through a heat exchanger. Or vice-versa to cool a building. Using a geothermal heating system in a commercial building can provide the building with a LEED certification and increase the building's overall value, which has led to commercial installations increasing in popularity and standardization.

A commercial geothermal heating system requires a collector system, which collects the source, usually a vertical borehole loop field located under a parking lot or a recreational field. This closed-loop system works the same as a residential system, delivering the heating or cooling through a distribution system, such as radiant in-floor heating and hydronic fan coils.

These systems can cool large buildings, such as a 46,000 square foot hospital, and they can be combined into larger systems, with multiple pumps and multiple loops, held separate from each other, to create a larger heating and cooling distributions system, or they could be localized to specific parts of a building to localize the heating and cooling to certain portions of a building, which can increase energy savings for unused portions or little-used portions of a building.

Geothermal heating systems can also be used for a district heating scheme. These uses a larger implementation of a geothermal heating system to heat individual, commercial, or industrial buildings, through a distribution network. Some of the first to install those systems were areas with hydrothermal potential, but this has been since used in other districts where shallow geothermal resources can be used. These schemes are being used in Paris, Munich, and Hungary.

Example of a geothermal district heating scheme.

Despite the up-front costs of an installation of a geothermal heating system, there are various advantages to using geothermal heating: