What is an earth air tunnel and how it works?
An earth air tunnel, also known as an earth air heat exchanger, is an essential technique that is used for the pre-cooling or pre-heating of a system that consists of a pipe, duct, or tunnel network buried underground at a reasonable depth.   It utilizes the heat-storing capacity of the earth’s surface and uses it to provide cool or hot air inside any building.  This whole cooling process works on the fact that the temperature below ground level is constant throughout the year, which is called the un-disturbed temperature of the earth.  During the summer, it accepts cool air from the ground below, and in the winter, it accepts hot air from the ground below. The rise and fall in temperature can be absorbed inside the building.
Principle of earth air tunnel
It is a technique that is used to generate cool air in the summer and hot air in the winter. The process involves moving the outside air through a duct system that is installed deep inside the earth’s surface. (Because of the earth’s constant temperature throughout the year, it exchanges heat while passing through the ductwork.
During summer, when the outside temperature reaches 35 degrees during the summer, the air that is passed through the earth’s surface (which is 10 degrees cooler than the outside temperature) absorbs the heat and transfers the cool air inside the building.
During winter, when the outside air reaches 6 degrees, the process continues and converts the outside air from 6 degrees to 16 degrees. The temperature change provides comfort to the user, and this temperature can be further brought down by other engineering controls. 
Installation Process of Earth Air Tunnel
The size of earth air tunnels varies depending on the site’s needs. In general, a 100–120 m earth air tunnel requires a depth of 6–8 m. The pipe material can be HDPE or concrete, and the diameter of the pipe should be 450 to 500 mm.
The pipework should be airtight so that it doesn’t have air leakage and other issues. One end of the pipe is exposed to the wind catchers, and the other end is fixed to the AHU, which humidifies and dehumidifies the air as needed. From here, the air is disturbed in the room. A solar chimney is also installed inside the building, which evacuates the hot air from the building. (This is the best process for Indian conditions.)
Working of the earth’s air tunnel
- A wind tower/windcatcher is installed outside the main building in the direction of the flow of air around the area.
- It is then connected to an underground duct or tunnel to force it to move in the direction of the main building.
- The depth of the tunnel should be more than 2 m or between 3 m and 4 m. 
- The optimal ground temperature required for conditioning the air is 3 m (10 feet) deep. At any other depth, the temperature is not of the optimum value. 
- Through the duct, it reaches an area where it is treated for temperature, humidity, and to remove dust particles. 
- From here, it reaches inside the building, and the air is circulated throughout the building.
Earth air tunnel design parameters (Design Guidelines)
- Ducting can be decided according to different factors like the thermal properties of the soil, the type of soil, the external climate, and the water content below the surface.
- In general, we use a depth of 4 meters. 
- An increased length of the pipe helps reduce the air pressure and also increases the fan’s energy.
- The greater the duct depth, the better the heat transfer and efficiency. But it should be economical, so various types of arrangements are available for ducting. 
Diameter of the pipe
- Because of the friction in the pipe, a smaller diameter pipe requires more energy to move the air.
- provide improved thermal performance.
- The diameter of the pipe varies from 75 mm (3 inches) to 750 mm (30 inches).  
- An increase in air velocity reduces the temperature while exiting the pipe.
Material of the pipe
- Economic factors like corrosion, strength, and durability all influence material selection.
- PVC ducts are usually preferred over galvanized iron as there is not much difference in the thermal conductivity of the two materials. 
- The metallic duct placed underground faces a certain amount of weight from the soil on top. In order to prevent the duct from collapsing due to the soil weight, its thickness is designed using a flexural formula.
- The weight of the soil is considered while designing the layout of the duct. It is necessary so that the duct doesn’t collapse during backfill and thereafter. 
Arrangement of the Pipe Network
- Closed loop System:- Inside air circulates through the earth air tunnel, and fresh air can be added as needed.
- Open loop System:- Outside air is passed through the ductwork and transferred to the buildings.
Classification of an Earth Air Tunnel Heat Exchanger
Based on the layout of the pipe below ground
- Horizontal/Straight Loop
- Vertical Loop
- Slinky/Spiral Loop
- Pond/Helical Loop 
Applications of Earth Air Tunnel
- In Commercial buildings like: Offices, Showrooms, cinema halls etc.
- Residential Buildings
- Institutional Buildings
- Hospitals 
Advantages of Earth air tunnel
- It reduces the heat gain inside the building.
- Reduces electricity consumption by 8 to 10%
- Reduces the demand for air conditioning by 5 to 10%.
- Has a long life, initial investment is a little more than the air conditioners, but their maintenance is much less.
- Low operating costs.
- Completely eco-friendly.
- This system can replace coolers in summer and heater in winter.
- It is noise-free and doesn’t require water topping as compared to a cooler.
- This system can last for 40-50 years.
- Totally a sustainable way to convert air as per weather.
Disadvantages of Earth air tunnel
- It cannot be used for changing humidity directly.
- This system requires a larger digging ground area to set up all of the ducting systems, which can be a problem at some places. 
- Give a limited amount of cooling or heating effect.
- The installation cost is higher.
- When the outside air becomes too hot or too cool, this system is not as effective.
Considerations while installing it on any site
- Various factors and parameters are considered before installing the earthen air tunnel on any site. 
- You can’t use it in all regions because of different earth properties.
- There should not be any bedrock near the site.
- Should have an ample amount of space on the site. 
- The water table should be kept below 6 to 7 meters from the ground surface.
- Opting for mechanical fans rather than going with natural airflow. 
Installation cost of Earth Air Tunnel
The cost of installing an earth air tunnel varies depending on location and several other factors. Heating and cooling can cost up to Rs. 1.5 lakhs for a room measuring 3600 x 3600 mm (12′ x 12′). According to this figure, the total cost of a boys’ hotel with 30 rooms could be around Rs. 45 lahks. (The estimated cost mentioned here is for Indian conditions only and can vary from location to location.) 
Earth air tunnel examples
To read an assignment on this topic click on the link below:-
Earth air tunnel case study will be added in the coming days.