This measure can be claimed if the building has an Earth Air Tunnel (EAT) system to help ventilate or pre-cool or pre-heat the intake air for the air conditioning system.
Earth Air tunnel systems help buildings reduce fossil fuel consumption and lower operating costs by pre-heating or pre-cooling the outside air entering the building. This reduces the load for space heating or for space cooling. Buildings that use energy for heating or cooling with fresh air supply have the potential to benefit from an Earth Air Tunnel system.
In a building with significant space heating or space cooling load, installing an EAT system will reduce energy consumption. In an EAT, fresh outside air is passed through a tunnel or pipe several meters under the ground. This pre-heats / pre-cools the air through conductive heat exchange with the relatively stable temperatures a few meters under the surface of the ground. This tempered air is supplied to the building for heating, cooling or ventilation.
In order to qualify, the design team must demonstrate that an EAT system is installed on the building site. EDGE calculates the savings of the system based on the local climate conditions as well as the ground temperature at which the system will be located. If the ground temperature is known, it can be input in the EDGE software.
The Earth Air Tunnel system, also called an Earth-Air heat exchanger, Air-to-soil heat exchanger, or Earth Canal, is a system that relies on the relatively constant annual temperature a few meters below the ground. The system uses the thermal inertia of the soil, and depends on its thermal conductivity (temperature difference between the soil and the air). The thermal conductivity of the soil can be affected by different factors which are detailed in the table below.
Table 35: Factors affecting thermal conductivity of the soil
|Moisture Content||Most notable impact is on thermal conductivity as it increases with moisture up to a certain point (critical moisture content)|
|Dry Density of soil||Thermal conductivity increases when soil is dry and dense|
|Mineral Composition||If the mineral content is high then the conductivity is high, while if the organic content is high then conductivity is low.|
|Soil Texture||Coarse textured, angular grained soil has higher thermal conductivity|
|Vegetation||Vegetation acts as an insulating agent moderating the effect of temperature|
The system has a wind catcher for the input air, which is driven down into an underground tunnel that runs under the building, where the air is cooled or heated based on the weather conditions (see Figure 12). The system is more commonly used in hot and dry climates for cooling, where air pre-cooled in the EAT system is input to the mechanical ventilation system or circulated directly into the building.
Figure 12. Interaction of soil with Earth Air tunnel system
If the tunnel is used for cooling, the ground can be shaded using vegetation, and can be wetted by sprinkling water to reduce the underground temperature further. The following are some of the design parameters to consider when designing an Earth Air tunnel system, as they impact its performance.
Table 36: Design parameters to be considered for Earth Air tunnel system
|Tube Depth||The deeper the better to get the lowest fluctuation in temperature. Generally, a balance between the depth and the temperature can be obtained at four meters below the ground. However, that can vary upon the external climate, the water content, the soil composition and its thermal properties.|
|Surface Area (Tube length & diameter)||
Higher surface area of the pipe (diameter and length), higher heat transfer, therefore higher efficiency. However, the length should be optimized as after certain length no significant no significant heat transfer occurs and more fan energy is required.
Also, increased diameter results in reduction of air speed and heat transfer.
The surface area of the pipe will be determined by the balance of the best performances vs. costs.
|Air Flow Rate||Increase of air flow increases the heat transfer and the outlet temperature.|
Open-loop system: Outdoor air is driven to AHU or to the building. Apart from providing ventilation, it also provides heating or cooling.
Closed-loop system: Air from the interior of the building is circulated through the Earth-Air tunnel to reduce condensation problems as well as it increases efficiency.
One-tube system: This arrangement is not efficient for cooling due to the length of the tube, but it is cost-efficient for ventilation purposes
Parallel tubes system: This arrangement increases thermal performance as it reduces the air pressure.
|Tube Material||The selection of material is based on the cost, the strength, resistance to corrosion and durability, but not based on the performance, as it has little influence on it.|
|Efficiency||It is measured using the Coefficient of Performance (COP), which is based on the amount of heating/cooling generated by the system and the amount of energy needed to move the air though the system.|
Relationship to Other Measures
The air intake is pre-cooled in hot weather; this decreases the cooling load and consumption due to ‘Cooling Energy.’ The same principle applies to the heating load if the building uses predominately space heating, in which case the reduction is in ‘Heating Energy.’ In addition, both 'Fan Energy' and 'Pump Energy' are reduced because the load for the cooling / heating system is lower, and the HVAC system does not need to work at full capacity.
The HVAC systems included in the base case do not include an Earth Air tunnel system. The improved case is assumed to have an Earth Air tunnel system that uses the local ground temperature (based on the weather file) at 4 meters below the ground.
At the design stage, the following must be used to demonstrate compliance:
At the post-construction stage, the following must be used to demonstrate compliance:
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