Ventilation & Air Quality

 
Underground Air Supply
 
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Definition
Heat exchange pipes buried in the ground that heat or cool ventilation air

 
Building Use
bullet institutional
bullet residential
 
 
Building Type
bullet new


Development
Status

bullet new technology

Description
The conventional approach to heating ventilation air is with energy-recovery ventilators. There are two cases where HRVs may not be suitable: where the supply and exhaust ducts are far apart or where the exhaust air is contaminated (e.g., from kitchen grease). Ground-coupled heat exchangers are an alternative means of heating (and cooling) air that overcomes these limitations. Outdoor air is brought in through an intake pipe and passed through a system of large-diameter pipes buried in the ground. The earth, at a depth of a few meters, remains at a fairly constant temperature year-round (typically 10C in southern Canada). Passing the incoming air through the buried pipes, heats or cools the air to close to the ground temperature. The ventilation air is distributed throughout the building using the conventional ducting system. 

Ground-coupled heat exchangers can also preheat inlet air to an energy recovery ventilator (ERV). At temperatures below -10C, condensate in the exhaust air may freeze and plug the ERV. Ground preheating of the inlet air can warm the air sufficient to avoid this situation. 

The size and layout of the ground piping system is quite important. The pipe diameter must be large enough that the energy gains from heating the air significantly outweigh the fan energy required to pull the air through the system. Ten to twenty centimeter (4 to 8 inch) diameter PVC piping have been used in European projects. A series of parallel pipes (instead of one long pipe) can help to minimize pressure drop through the system. 

Ground-coupled heat exchangers can also provide natural cooling when the outdoor air temperature is greater than that of the ground. The amount of cooling is limited by the surface area of the piping. In hot humid climates, a means for eliminating condensate must be provided. Piping should be sloped to a common low point for condensate removal.

   
Benefits
bullet Pre-heats ventilation air to eliminate frosting in energy recovery ventilators
bullet Eliminates use of CFCs
 
Limitations
bullet Efficiency limited by the length of piping and size of fan required
bullet Limited to low-rise buildings 
 

Application
Ground-coupled heat exchangers are suitable for low-rise residential buildings and institutional buildings. The building lot must have an area where a suitable amount of piping can be laid beneath the ground to allow for adequate heat transfer. There must be sufficient ground cover to bury the pipes at least one meter below grade. This technology is best suited to buildings where the ground is already being excavated to provide a basement.

Experience
A ground-coupled heat exchanger has been implemented in the IEA Task 13, Ultra House in Rottweil, Germany. This installation has a central balanced ventilation system that provides an air change rate of 0.2l ACH. The ground-heat exchanger consists of thirty-four meters of 0.2 m diameter, plastic piping buried 1 m below the surface of the earth, underneath the garage and carport. The piping enters the house through the basement and runs through the central part of the house to the energy recovery ventilator on the roof. The piping between the basement and the roof is well insulated to prevent heat from being drawn out of the house before reaching the heat recovery exchanger. 

When the air temperature is above 8C, the system bypasses the heat recovery ventilator. If the air is above 15C, but less than 25C, fresh air is pumped directly from outside to the house bypassing the entire ground-coupled heat exchange system. For temperatures above 25C, the air is pulled from the outside, through the buried pipes, to dissipate its heat, and then pumped into the house.

This technology is also being retrofit to a school in Egebjerg School in Ballerup, Denmark.

Cost
For sites where the excavation is being done to provide a foundation, the major cost is to lay the underground piping. The installed cost of this piping is on par with energy recovery ventilators.

Information Sources
Solar Energy Houses, J. Wiley and Sons, London, 1998
Chapter on Ground-Coupled Heat Exchangers, 
Michael Beckert and Hans Erhorn
Fraunhofer-Intitut fur Bauphysik, Stuttgart, Germany
e-mail: Erh@ibp.fhg.de

EU-Target Thermie Project
MEDUCA-Model Educational Buildings for Integrated Energy Efficient Design
Ove Morck , M.Sc., Ph.D.
Cenergia Energy Consultants
Fax: 45 44 66 01 36
e-mail: ocm@cenergia.dk

 
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