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Original article, HPAC Canada


Additional resources:

Earth Tube Ventilation Systems
Thevenard, D., Earth Tube Ventilation Systems -Applicability in the Canadian Climate, (for) Canada Mortgage and Housing Corporation, Sustainable Housing, Policy and Research, File # 6766-12, March 2011

  

Down to Earth - An ‘Exhumination’ of Earth Tube Heat Exchangers
Copyright (c) 2010, Robert Bean, All rights reserved, originally published in HPAC Canada
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F
or the uninitiated, ETHE's are air to earth heat exchangers employed to shave the peaks off heating and cooling loads, thereby lowering combustion and compression needs. Outside air (of seasonal temperature) is drawn down into relatively constant earth temperatures at 1.5m to 3m below grade into a buried air duct installed in similar fashion to PEX pipe in a radiant slab…think 150mm to 300mm buried conduit placed in a serpentine or reverse return system. In the hot summer, the heat in the outdoor air is conducted along the buried exchanger to the earth resulting in cooler entering air temperatures into the building’s HVAC system and in the winter, heat from the ground is conducted into the cooler incoming air resulting in warmer air temperatures into the building’s HVAC system. That is as complicated as it gets for members of the society of experimenters but wait, there is more.

ETHEs are not air quality control systems. Anyone making, designing or installing such a system would be ill advised to guarantee the quality of air being delivered to the indoor space. In the worst case scenario, the outside air might contain moisture and particulates, including mold spores, pollen, virus and bacteria, and smoke and debris from forest and field fires and the occasional volcano eruption. Add in some ozone ala shortwave radiation from the sun and products of combustion via the di and mon oxides of sulfur, nitrogen, hydrogen and carbon. Stir in some farm fresh herbicides or pesticides with odors from the compost pile, local dump or meat packing plant. Then pay for the electricity to run the fan that draws the smorgasbord under negative pressure into the buried duct work embedded in soils emitting radon and methane, condense the mix into condensate and let it ferment into a toxic soufflé…and the only thing left to do to yourself or client is to blow the entire recipe into the home. What you have are the results of good intentions gone bad ( maybe I could make millions selling this DVD on late night TV against Girls Gone Wild). Perhaps all of this is a little melodramatic but I did write “worst case” scenario.

So what is the upside to the downside? There is none (and here is the caveat) unless you explicitly understand that the ETHE is a H-E-A-T exchanger as I have emphasized already. As history has revealed, most weekend engineers have a propensity to confuse heat with indoor environmental quality (IEQ). Now that we have the grit and grime out of the way, the upside to these low down systems is that they can be an altruistic and maybe even a practical way of shaving the peaks off heating and cooling loads on some projects in some climates. They can and do contribute to the global quest of reducing energy use by eliminating or at least reducing the need for compression and combustion to condition occupants and spaces. Of particular interest to those way out on the limb of renewable energy, is the use of solar chimneys to induce draft through the ETHE eliminating the need for power during daylight hours. These chimneys also offer the opportunity to generate electrical power with turbines.

A WORKABLE SOLUTION

In order to specify a workable solution designers of ETHE systems should evaluate:

Location: If the system is predominantly cooling you will want the collector in a permanently shaded area near a lake or river. If the system is predominately heating it should be located in a sunny area without some aquifer competing to steal away the heat. Intakes have to be located away from sources of pollutants. These include vehicle emissions, products of combustion from fire pits, lanterns, barbeques, boilers and furnaces, exhaust fumes from bath fans and dryer vents, or sources of odors, such as garbage bins, compost piles and sewer vents.

Depth of pipe (temperature): Available shortwave radiation on the collector surface is directly related to the mode the system favors' most with consideration for the depth of pipe and can typically be between 1.5m to 3m. A system that is designed predominately for cooling in an area without shading will need the pipes buried deeper than a system designed for heating in the same locale.

Soil conductivity (heat transfer): I am disappointed at the papers and literature that promote the use of sand as the bedding material for ducts. Dry sand is the worst thing one would want in the process of conducting heat to and from buried ducts. Dense, wet and conductive (can you say moist compacted clay) is the trick.

Duct material and tube connections: Options include concrete, metal, plastics – with or without conductive fins or antimicrobial agents. It is a case of equifinality (another way of saying there are many ways to skin a cat - something you should never say to a cat lover!). The ducts cannot sag under loading. The connections have to be robust and of the highest quality as they and the ducts have to deal with ground moisture and soil gases. They must be of the most conductive material for the lowest cost with the least air flow resistance but offer the best characteristics against corrosion. For the cost evaluators I ask: What else is new?

Temperatures, flow, velocity, diameter, friction, length, layout and drainage (the thermal to hydraulic part of the calculation): According to application engineers, velocities between 6m/s to 10m/s are typical. This means the diameter has to be picked based on flows and friction losses. To optimize designs based on the thermal and pressure requirements, using several shorter lengths in parallel in a reverse return arrangement can in some cases be better than a single longer serpentine loop or it may be more suitable to use a loop that follows the perimeter of the buildings foundation.

Air entering the ground duct and the HVAC system will need to be conditioned: This means it has to be suitable for inhalation by the occupants through the decontamination of particulates, moisture, odours, gases and biological concerns. Decontamination is a continuous process with both
outdoor intake filters and indoor components and systems needing monitoring and maintenance.

Energy analysis: The designer will need to evaluate the capital and operating costs of the system, including the electricity to run the fan and decontamination equipment to assure the energy used is lower than the cooling or heating power offered by the system. Researchers are reporting the ability to shave off 10
°C to 20°C from heating and cooling loads with one source reporting COP's of 3.3 in cooling and 3.8 in heating.

Building science issues: Depending on the type of system there could be issues with infiltration and short circuiting of the ground exchanger. It is necessary that both building and ducts are sealed tightly to prevent differential pressures
across and within the home from interfering with the required differential pressures in the ETHE.

SHORT STROKES

ETHEs are not new and have always been a good idea but given all the  construction effort and IAQ concerns with them, one has to consider whether it might be better to place a sufficiently-sized glycol coil in the air handler and feed it from a closed geothermal earth loop. This takes care of the heat and IAQ issues. Until I can get my hands on some good comparisons between these two options, the jury will be out on the down in the ground ground to air exchangers.            


Bibliography

  1. Leopold Legacy Center, Construction Journal, Report 15.1 – Mechanical Systems, July 28, 2006
    Wang, G., et al, Analysis, design, and preliminary testing of solar chimney for residential air-conditioning applications, Solar 2004, Solar Harvest: Growing Opportunities, Portland, Oregon, July 2004

  2. Sharan, G., Jadhav, R., Performance of single pass earth-tube heat exchanger: An experimental study, Gujarat Energy Development Agency (GEDA), Vadodara / Ministry of Agriculture, New Delhi, July 2003

  3. U.S. Department of Energy - Energy Efficiency and Renewable Energy, Energy savers earth cooling tubes <http://www.energysavers.gov/your_home/
    space_heating_cooling/index.cfm/mytopic=12460...>
    accessed 2/7/2010

  4. Santamouris, M., Use of earth to air heat exchangers for cooling, Paper no. 11, Air infi ltration and ventilation centre, International Energy Agency Energy, Conservation in Buildings and Community Systems Programme, January 2006


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