Introduction


The coils of a ground-coupled system.
The coils of a ground-coupled system.

Ground coupling is the use of the natural, stable heat of the ground as a heat source or sink for heating or air conditioning purposes. [1] Ground temperature is nearly constant year-round at three meters below the surface.[2] Ground coupling harnesses this favorable temperature.

In its simplest form, ground coupling directly cools or warms air for use in buildings by air ducts called earth tubes or earth-air heat exchangers. The most widely used and successful application for ground coupling is the ground-coupled heat pump (GCHP) or geothermal heat pump. [3]

Sustainability




The main purpose of ground coupled systems is long term cost savings and greater freedom from traditional heating and cooling systems which run primarily on fossil fuels. The GCHP is among the most energy efficient methods for heating and cooling, both in residential and commercial applications. [4] The GCHP is far more efficient than conventional air conditioning options since the ground provides a temperature for cooling and heating that is more moderate than ambient air temperatures.[5] Because of this, GCHPs can attain desirable efficiencies of 300% to 600% in the coldest conditions. [6]

Historical Background


GCHPs have been in widespread use since the 1940s.[7] Lord Kelvin first developed the concept of the heat pump in 1852. A Swiss patent from 1912 first describes using a GCHP. [8] By the 1940’s, Robert C. Webber combined the idea of the heat pump with ground coupling. He installed copper tubing in the ground and circulated freon gas through it to exchange heat with the earth. The gas released its heat within his cellar as it circulated.[9] Interest in the technology grew between World War Two and the 1950s as fossil fuels were increasingly used for heating.[10]

Ground-water heat pump systems became widespread in the 1970’s[11] bolstered by the first oil crisis, and researchers experimented with vertical-borehole systems.[12] Even though GCHPs have been used and accepted for over five decades, the overall market for GCHPs is still quite young with room to grow as fossil fuel systems control heating and air conditioning markets. Europe has already accepted and implemented many GCHP systems, and the United States is beginning to adopt them as well.[13] Today, it is estimated that GCHPs are emerging with 10% to 30% annual global growth based on recent data. [14]

Ground Coupling Variations

Earth-Air Heat Exchangers / Earth Tubes


Earth-air heat exchangers, or earth tubes, are the simplest form of ground-coupled systems, but can be quite complex in practice. Open systems draw in ambient air, exchange air heat with ground heat, and then emit the treated air into the building. Closed systems are continuous loops that draw and expel air within the building.[15] Open systems will often require additional ventilation or treatment, since earth tubes do not control the quality of ambient air, which could contain moisture, mold, pollen, bacteria, or smoke. In areas where radon gas is a concern, closed systems must be thoroughly sealed.
An open earth-air system [15]
An open earth-air system [15]


The advantage of earth tubes over more complex systems is that there are no energy or technology needs associated with a heat pump or other mechanical system. The air can be circulated through the passive system by a simple fan or even a solar chimney, which allow the system to run naturally without any power needs during the day. Individual design of earth tube systems is crucial. For instance, it is important to keep heating-intensive systems in the sun and cooling intensive systems in the shade. Heat-conductive soils should be used whenever possible.
The tubes themselves may be made of concrete, metal, or plastics. It can be important to use antimicrobial agents on the tubes to prevent mold and bacterial growth.[16] In humid areas, additional dehumidification is required to remove water vapor. Since water build-up can lead to mold and microbial growth, sealed construction and good drainage are paramount.[17]

Earth tubes may be most efficiently located below the water table to produce the best thermal properties, but such systems must also be thoroughly sealed to prevent leakage. The length of buried pipe depends on air circulation speed (typically 2 m/s) and pipe diameter. The major cost of earth tube systems is in excavation and installation. Maintenance concerns are generally limited to inspection and duct cleaning.[18]

Construction Examples

See here for details.

The University of Ioannina in Greece has installed an earth-tube system that delivers a temperature drop of 10 ºC.

The Aler Pavia project in Italy is a housing development with an open tube system that successfully augments air temperature.[19]

Research

Researchers are actively examining ways of modelling earth-air heat exchanger systems.[20]

Ground-Coupled Heat Pumps (GCHPs)


Ground-coupled heat pumps (GCHPs) are known by many different names including geothermal heat pumps, ground source heat pumps, Earth Source Heat Pumps, Geo-exchange systems, and Earth-Energy systems, among others.[21]
GCHPs circulate a fluid (rather than air, like earth tubes), such as water or a water–antifreeze mixture, to transfer heat from the earth to the evaporator of a heat pump.


[22] Pipe material is most often HDPE.[23]
Though traditional air systems also use heat pumps, the ground’s temperature is most always closer to inside temperature than ambient air temperature, meaning that GCHPs are fundamentally more economical to run than traditional air systems,[24] with 30–50% less power consumption.[25] The simplicity of the GCHP system allows it to be built and run in most all situations with low maintenance.[26] Once the GCHP system has been installed, the only running cost is power for the heat pump. In terms of efficiency, the production of heating or cooling is often four to five times greater than the electricity for the pump. As with earth-tube systems, the greatest obstacle is initial cost.[27] It may take from five to eight years to recover initial costs of installation through operational savings.[28] Designing GCHP systems must take into account the temperature of the fluid used and the sustainability of the long-term temperature of the ground itself.[29]


Types of GCHPs


Open/Closed


The vast majority of systems are closed-
loop systems that continuously circulate a water or antifreeze solution. However, some systems are open-loop, drawing water from a well or surface source (such as a lake) to exchange heat and then expelling the water back into the environment. Open systems are highly dependent on government codes and regulations for water use.

Horizontal Systems

Horizontal construction is typically the best, cheapest option residential buildings, especially when there is available land space. The pipe work is placed in trenches over four feet deep. Systems are generally laid out as two pipes buried at six and four feet or side-by-side pipes in a two-foot wide ditch at a depth of five feet. [30]

Advantages
Shallow ditching allows trenching machines, backhoes, or excavators to prepare the site for piping. This simplicity lowers initial costs compared to vertical systems.

Disadvantages
Horizontal systems require significant land area, often do not reach groundwater (which offers greater thermal conductivity), and are more vulnerable than vertical systems to surface temperature changes.[31]

Vertical Systems

Vertical construction is implemented where large, horizontal land
A borehole vertical system [13]
A borehole vertical system [13]

areas are not available. Commercial and academic buildings often use vertical GCHEs. Typically, systems are installed by drilling downward at a diameter of around four inches, spaced at 20 feet between holes, which run 100 to 400 feet deep. Each duct is connected beneath the ground to its adjacent pipe by a U-shaped section and from above by a long, straight pipe section.[32]

Advantages
Vertical systems do not disturb existing landscaping, require small land area, and provide the most steady soil temperatures since the pipes are located far from changing surface climates.

Disadvantages
The drilling process involved with vertical systems can significantly increase costs, and system design can be difficult and expensive if geological conditions are not already known.

Slinky Systems

Slinky systems act as a modified horizontal system, where the initial ditch is dug wider and straight piping is replaced with coils of piping that cover a larger area.

Advantages
Land area is saved by using the more dense slinky method.

Disadvantages
Soil compaction can be tricky over slinky systems and higher density of piping can lead to less thermal transfer for a smaller amount of soil.[33]

Construction Examples


Joint Base Langley-Eustis in Virginia started construction in 2012 on a new Soldier in Transition Campus that uses closed-loop ground coupled heat pump systems.[34]
Workers install piping at the Juneau International Airport project [36]
Workers install piping at the Juneau International Airport project [36]


In Juneau, Alaska, multiple library projects provide sustainable and cost-effective heating and cooling with vertical ground-coupled systems. Both the Dimond Park Aquatic Center and Juneau International Airport have used GCHPs since 2010. About half of the airport’s new terminal is heated by the systems, which produced $120,000 in savings in one year.

Auke Bay School also has a ground-source system. Installation costs were projected at $2.8 million for a traditional oil-heat system and $4 million for the GCHP. Officials expect the cost difference to be made up in time by decreased operating costs, which are approximately one third of the oil system. Officials also predict that large construction projects and public buildings will increasingly use GCHP systems since their long lifespans mean substantial energy savings.[35]

Research


Topic
Link
Equations for simulation of heat exchanger effectiveness and design.[36]
http://www.sciencedirect.com.research-db.letu.edu/science/article/pii/S0306261909001743
Using thermosyphons to transmit heat in the air to the ground via a heat pump, allowing soil temperatures to recover during seasonal warming.[37]
http://www.sciencedirect.com.research-db.letu.edu/science/article/pii/S1359431113008879
Thermal response testing rigs and methods, which find thermal properties of the ground in preparation for the design of ground-coupled systems, and mathematical system modelling.[38]
http://www.sciencedirect.com.research-db.letu.edu/science/article/pii/S1364032114006947
Cost-cutting measures for vertically drilled ground-coupled systems.[39]
http://www.sciencedirect.com.research-db.letu.edu/science/article/pii/S1359431113005589
Study of size and shape of ground-coupled units, including borehole depth and its effect on heat dissipation.[40]
http://web.a.ebscohost.com.research-db.letu.edu/ehost/pdfviewer/pdfviewer?sid=9b0d8dfc-e2ee-4d72-9301-7a60c7daaf5d%40sessionmgr4005&vid=0&hid=4212
Modelling of heat transfer throughout a system’s lifespan.[41]
http://web.a.ebscohost.com.research-db.letu.edu/ehost/pdfviewer/pdfviewer?sid=2aaa1d33-a4b3-4076-9c86-dc0ff4d07eb9%40sessionmgr4005&vid=0&hid=4212
Combining ground-coupled systems with thermal energy storage (TES) systems, which allow heat energy to be collected and stored for later use.[42]
http://www.sciencedirect.com.research-db.letu.edu/science/article/pii/S1359431114005201

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