Earth Tubes

HVAC tab in model data

 

An earth tube is a long, underground metal or plastic pipe through which air is drawn. During the cooling season, as air travels through the pipe, it gives up some of its heat to the surrounding soil and enters the room as cooler air. Similarly, during heating season, as air travels through the pipe, it receives some of its heat from the soil and enters the room as warmer air. Simple earth tubes in EnergyPlus can be controlled by a schedule and through the specification of minimum, maximum, and delta temperatures as described below. As with infiltration and ventilation, the actual flow rate of air through the earth tube can be modified by the temperature difference between the inside and outside environment and the wind speed. See equation below.

 

Note: Earth tubes are only available when using the Scheduled natural ventilation model option.

 

ClosedCalcSoilSurfTemp – Auxiliary Programs Document

The CalcSoilSurfTemp program is simple and requires only two input fields : soil condition and soil surface condition in addition to a valid weather file. For soil condition, the user should select the number corresponding to the actual condition of the soil surrounding the earth tube from the four following options: 1. HEAVY AND SATURATED, 2. HEAVY AND DAMP, 3. HEAVY AND DRY and 4. LIGHT AND DRY. This determines the thermal diffusivity and thermal conductivity of the surrounding soil. For soil surface conditions, the user should select the number corresponding to the actual condition of the ground surface above the earth tube from the eight following options: 1. BARE AND WET, 2. BARE AND MOIST, 3. BARE AND ARID, 4. BARE AND DRY, 5. COVERED AND WET, 6. COVERED AND MOIST, 7. COVERED AND ARID and 8. COVERED AND DRY. This determines the absorption coefficient and the fraction of evaporation rate of the ground surface.

 

From this information and an analysis of the weather for the location selected, the CalcSoilSurfTemp program (ref. Auxiliary Programs document) calculates the three parameters listed above. The user must then add these parameters as input into EnergyPlus. The full input description of an earth tube is given below.

Include earth tubes

Check this option to include zone air delivery through earth tubes.

Outside air definition method

You can set the amount, timing and control of outside air delivery through the earth tubes in the same way as for Mechanical ventilation. Refer to the Outside air section for details of how this works.

 

The outside air definition data is used to calculate the the maximum amount of air mass flow rate of the earth tube expected at design conditions (Edesign in the above equation in units of m3/s. The design value is modified by the schedule fraction (see next field) and user specified coefficients (last four fields).

Operation

Schedule

Select the schedule that modifies the maximum design volume flow rate (see previous field). The fraction in the schedules should vary between 0.0 and 1.0 and is referred to as Fschedule in the above equation.

Control

Minimum zone temperature when cooling

This is the indoor temperature (in °C or °F) below which the earth tube is shut off. This lower temperature limit is intended to avoid overcooling a space and thus result in a heating load. For example, if the user specifies a minimum temperature of 20°C, earth tube is assumed to be available if the zone air temperature is above 20°C. If the zone air temperature drops below 20°C, then earth tube is automatically turned off.

Maximum zone temperature when heating

This is the indoor temperature (in °C or °F) above which the earth tube is shut off. This higher temperature limit is intended to avoid overheating a space and thus result in a cooling load. For example, if the user specifies a maximum temperature of 20°C, earth tube is assumed to be available if the zone air temperature is below 20°C. If the zone air temperature rises above 20°C, then earth tube is automatically turned off.

Delta temperature

This is the temperature difference (in °C or °F) between the indoor and outdoor air dry-bulb temperatures below which the earth tube is shut off. This is to allow the earth tube to be stopped either if the temperature outside is too warm and could potentially heat the space or if the temperature outside is too cold and could potentially cool the space. For example, if the user specifies a delta temperature of 2°C, earth tube is assumed to be available if the temperature difference between indoor and outdoor temperature is at least 2°C. If the outside air dry-bulb temperature is less than 2°C cooler or warmer than the indoor dry-bulb temperature, then the earth tube is automatically turned off.

Fan Electric Consumption Configuration

Earth tube type

This alpha character string defines the type of earth tube as one of the following options:

 

Fan pressure rise

This is the pressure rise experienced across the fan (in Pascals (N/m2) or psi). This is a function of the fan and plays a role in determining the amount of energy consumed by the fan. This value is only entered for fan-driven earth tubes.

Fan total efficiency

This is the total fan efficiency (a decimal number between 0.0 and 1.0). This is a function of the fan and plays a role in determining the amount of energy consumed by the fan. This value is only entered for fan-driven earth tubes.

Pipe Specifications

Pipe radius

This is the radius of the earth tube/pipe (in m or ft). This plays a role in determining the amount of heat transferred from the surrounding soil to the air passing along the pipe. If the pipe has non-circular cross section, user can use the concept of hydraulic diameter as follows.

D = 4 . A / Perimeter

However, since this field requires the pipe radius, hydraulic diameter should be divided by two.

Pipe thickness

This is the thickness of the pipe wall (in m or ft). This plays a role in determining the amount of heat transferred from the surrounding soil to the air passing along the pipe.

Pipe length

This is the total length of the pipe (in m or ft). This plays a role in determining the amount of heat transferred from the surrounding soil to the air passing along the pipe. As the length of the pipe becomes longer, the amount of the heat transfer becomes larger.

Pipe thermal conductivity

This is the thermal conductivity of the pipe (in W/mC or Btu-in/h-ft2-F). This plays a role in determining the amount of heat transferred from the surrounding soil to the air passing along the pipe.

Pipe depth under ground surface

This is the depth of the pipe under the ground surface (in m or ft). This plays a role in determining the temperature of the soil surrounding the pipe.

Soil Heat Transfer Calculations

Soil condition

This alpha character string defines the actual condition of the soil surrounding the earth tube and can be one of any of the following options:

 

 

This determines the thermal diffusivity and thermal conductivity of the surrounding soil, which play a role in determining the amount of heat transferred from the surrounding soil to the air passing along the pipe.

Average soil surface temperature

This is the annual average soil surface temperature straight above the earth tube, which plays a role in determining the temperature of the soil surrounding the pipe. This field should be calculated in advance using the separate CalcSoilSurfTemp program.

Amplitude of soil surface temperature

This is the amplitude of soil surface temperature above the earth tube, which plays a role in determining the temperature of the soil surrounding the pipe. This is the difference between the maximum and minimum soil surface temperature for the whole year divided by two. This field should be calculated in advance using the separate CalcSoilSurfTemp program. It is a Delta temperature value.

Phase constant of soil surface temperature

This is the phase constant of the soil surface temperature straight above the earth tube, which play a role in determining the temperature of the soil surrounding the pipe at particular time. This is the time elapsed from the beginning of the year until the soil surface temperature reaches the minimum value of the year. This field should be calculated in advance using the separate CalcSoilSurfTemp program.

Flow Term Coefficients

The basic equation used to calculate air flow rate of earth tube in EnergyPlus is:

EarthTubeFlowRate = Edesign Fschedule(A + B . abs(Tzoneair - Todb) + C . WindSpeed + D . WindSpeed2)

Constant term flow coefficient

This number is the “A” parameter in the above earth tube equation. It is part of the user specified modifying parameters that are a function of environmental factors. This parameter, however, is a constant under all conditions and is not modified by any environmental effect. As a result, it is dimensionless.

Temperature term flow coefficient

This number is the “B” parameter in the above earth tube equation. It is part of the user specified modifying parameters that are a function of environmental factors. This parameter is modified by the temperature difference between the outdoor and indoor air dry-bulb temperatures. The units for this parameter are inverse Celsius.

Velocity term flow coefficient

This number is the “C” parameter in the above earth tube equation. It is part of the user specified modifying parameters that are a function of environmental factors. This parameter is modified by the speed of wind being experienced outside the building. The units for this parameter are s/m.

Velocity squared term flow coefficient

This number is the “D” parameter in the above earth tube equation. It is part of the user specified modifying parameters that are a function of environmental factors. This parameter is modified by square of the speed of wind being experienced outside the building. The units for this parameter are s2/m2.

Outputs

 

Note: Airflow introduced into zones by Earth tubes is included in the Mech Vent + Nat Vent + Infiltration ac/h output. Also the heating/cooling effect of Earth tubes on the zone is included in the Zone Vent heat balance output. If you would like to see further details on the impact of the Earth tubes in your results then you should include one or more of the output variables below to an external idf file, "include" it in the main IDF file generated by DesignBuilder and view results in the Results Viewer.

 

Current Earth Tube output variables:

 

Earth Tube Zone Sensible Cooling Energy [J]

Earth Tube Zone Sensible Cooling Rate [W]

These are the energy and rate associated with the zone cooling provided by the air from the earth tube. This occurs when the earth tube outlet air temperature is less than zone air temperature.

Earth Tube Zone Sensible Heating Energy [J]

Earth Tube Zone Sensible Heating Rate [W]

These are the energy and rate associated with the zone heating provided by the air from the earth tube. This occurs when the earth tube outlet air temperature is greater than the zone air temperature.

Earth Tube Air Flow Volume [m3]

The volume flow of air through the earth tube.

Earth Tube Air Volume Flow Rate [m3/s]

The volume flow rate of air through the earth tube.

Earth Tube Air Flow Mass [kg]

The mass flow of air through the earth tube.

Earth Tube Air Mass Flow Rate [kg/s]

The mass flow rate of air through the earth tube.

Earth Tube Fan Electric Energy [J]

Earth Tube Fan Electric Power [W]

These are the fan electricity consumption and power for intake or exhaust earth tube types.

Earth Tube Zone Inlet Air Temperature [C]

This is the temperature of the air entering the zone after passing through the earth tube [C]. This temperature includes the cooling or heating of outdoor air as it passes along the pipe. When intake fan assist is used, then the additional heat due to the fan is included in the inlet air temperature.

Earth Tube Ground Interface Temperature [C]

This is the average temperature of the ground along the outer surface of the earth tube [C].

Earth Tube Outdoor Air Heat Transfer Rate [W]

This is the rate of heat transfer from the earth tube to the outdoor air [W]. Positive values indicate the rate at which outdoor air is preheated; negative values indicate the rate of precooling.