StateoftheArt Review EarthtoAir Heat Exchanger

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State-of-the-Art Review Earth-to-Air Heat
Exchanger

• Presented by Jian Zhang
• PhD Candidate
• Supervisor Dr. F. Haghighat
• Department of Building, Civil and Environmental
  Engineering,
• Concordia University

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Component description

• Earth-to-Air Heat Exchanger (ETAHE)
• Also called Earth Tubes, Earth Coupling,
  Embedded Duct, Ground Coupled Air System, etc.

• Conventional ETAHE systems
• In mechanically ventilated buildings
• Pipes hydraulic diameter is two orders of
  magnitude smaller than their length
• Airflow is hydrodynamically and thermally fully
  developed.
• Large cross-sectional ETAHE systems
• Follow the strategy of hybrid ventilation system.
• The difference between ETAHEs hydraulic
  diameters and its length is just one order of
  magnitude.
• Airflow and heat transfer is complicated.

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Schwerzenbacherhof building, Zurich,
SwitzerlandCase study in Annex 28

• 8050 m2 of heated surface
• 43 parallel pipes
• 75 cm below the unheated basement
• Length of 23 m and diameter of 23 cm
• Axial distance between two pipes is 116 cm

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Schwerzenbacherhof building, Zurich,
SwitzerlandCase study in Annex 28

• The measured heating demand is 150 kW at -8C.
• Without ETAHE, the estimated load would be 240
  kW.
• The ETAHE itself can meet a peak demand of 60 kW.
  
• The measured heating energy consumption was 144
  MJ/m2 per year (well below the Swiss Standard of
  240 MJ/m2 per year).
• The measured electrical current to operate the
  ventilation system was 23 MJ/m2 per year which,
  again, was well below a conventional requirement
  of 90 MJ/m2 per year.
• Comfort cooling was achieved at all times.

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Jaer School, Nesodden, Norway Case study in
Annex 35

• 850 m2 floor area
• Displacement ventilation

Diameter 1.6 m 2 m width 3 m height
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Jaer School, Nesodden, Norway Case study of
Annex 35
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Mediå School, Grong, NorwayCase study in Annex 35

• 1001 m2 floor area
• Cross section 1.5 m 2 m
• Length 15 m
• Depth 1.5 m
• Displacement ventilation

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Mediå School, Grong, NorwayCase study in Annex 35
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CFD Model description
Governing equations

• Conservation of mass
• Conservation of momentum
• Conservation of energy

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Two Layer Turbulent Model
CFD Model description

• Turbulent Reynolds number
• For near-wall region i.e.
  one-equation kl model (Wolfshtein 1969)
• For outer region, i.e. standard
  k-e model (Launder and Spalding 1974)

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Model verification

• Isothermal wall Tw 30C
• Air change rate 6, 12, 50, and 100 ACH
• Tin 16C
• Experiments conducted by Spitler (1992) and
  Fisher (1995)

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CFD simulation for large cross-sectional duct
Uniform velocity at Tin -10C
Adiabatic wall at the inlet tower
ETAHE duct with constant surface temperature
Twall 10C
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Height 1 m, Q 0.45 m3/s Twall 10C, Tin
-10C
Back flow
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Effects of airflow rate and duct height on heat
convection

• Ongoing research
• To develop convective heat transfer coefficient
  correlations.
• To develop an energy simulation model for large
  cross-sectional ETAHE

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Design criteria

• Airflow rates satisfy the airflow requirement
• Maximize the heat transfer rate while minimize
  the airflow resistance.
• Long term operation of an ETAHE with a high
  heating or cooling load may exhaust its capacity.
  System recharge methods need to be decided in
  control system design.
• Condensation and moisture infiltration should be
  avoided.
• Ducts should be anticorrosive and structurally
  stable.
• Safety, insect entrance, and noise transmission
  should be taken into account.
• Ducts should be accessible for inspection and
  cleaning.

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Available design tools

• Early design guidance for different weather
  conditions and locations was developed by
  Zimmermann and Remund (2001) with a few design
  charts and tables.
• WKM is a computer program developed to size
  ETAHEs (available at http//www.igjzh.com/huber/wk
  m/wkm.htm)
• GAEA (Graphical Design of Earth Heat Exchangers )
  The Division of Building Physics and Solar
  Energy, University of Siegen, Germany
  http//nesa1.uni-siegen.de./
• An ETAHE model compatible with the TRNSYS
  (Hollmuller and Lachal 1998)

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Application field

• Successful applications in various building types
• Greenhouses and livestock houses
• Residential buildings,
• commercial buildings
• Government buildings, such as office and school.
• Applicable to wide range of climates with large
  temperature differences between summer and winter
  and between day and night.
• Primarily used for cooling purpose but also used
  for winter heating when the temperature of
  outdoor air is lower than that of the soil.

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Benefits

• Properly sized ETAHE systems may replace other
  mechanical cooling systems.
• Sometime cheaper and easier to construct than
  active cooling systems.
• Low maintenance and operation costs.
• Long lifespan.
• Compatible with other ventilation system
  components.

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Limitations

• Land availability
• Rocky ground
• Air quality may restrict the location of ETAHEs
  inlet.
• Risk to poor air quality the potential of
  microbial growth in the airway.

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Barriers to application

• ETAHE provides a path for outdoor noise
  transmitting to indoor. This may conflict with
  noise regulations.
• There is a general lack of easy-to-use design
  methods. Existing modeling methods are not
  accessible for designers. No available method for
  large cross-sectional ETAHE.
• Design of ETAHEs control strategies vary
  significantly between regions because of climate
  differences. Such variations impact technology
  transfer and adoption of best practice.
• The system costs are very dependent on the actual
  project.

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Future perspectives
Potential users

• Moderate cooling loads
• Low ground temperature
• Large daily outdoor air temperature swings
• Relatively low requirements for indoor
  environment
• Displacement ventilation system

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Thank you

• QA