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Original article, HPAC Canada
Additional resources:
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 |
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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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For 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
-
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
-
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
-
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
-
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
-
Earth Tube Ventilation
Systems—Applicability in the Canadian Climate, CMHC
Technical Series 11-103
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