Hi Energy! Wind Power in Canada series, focusing on the O&M of wind turbine blades, gearboxes and generators!.

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Hi Earth tunnel Air conditioning!.

Hi Earth tunnel Air conditioning!.

Earth tunnel Air conditioning system also known as passive air conditioning, is a wonder utilization of nature. It is also called as Earth tubes in Europe, Earth-air heat exchangers in North America, Earth tunnelling in India. These systems are known by several other names, including air-to-soil heat exchanger, earth channels, earth canals, earth-air tunnel systems, ground tube heat exchanger, subsoil heat exchangers, underground air pipes, and others.

Material;

Most systems are usually a network of pipes made of concrete, PVC, steel, rigid or semi-rigid plastic, plastic-coated metal pipes or plastic pipes coated with inner antimicrobial layers. These pipes are 100 – 600 mm (4 - 24 inches) in diameter and have a smooth inner surface. The diameter, layout and cleanliness of the piping is critical for the overall performance of the system.

Smaller diameter tubes require more energy to move the air and have less earth contact surface area. Larger tubes permit a slower airflow, which also yields more efficient energy transfer and permits much higher volumes to be transferred, permitting more air exchanges in a shorter time period.

Laying;

Soil temperature, at a depth of about 4.0 to 5.0m (12 - 15 feet), stays fairly constant throughout the year, and is approximately equal to the average annual ambient air temperature. Ground temperature becomes more stable with depth. The ground can, therefore, be used as a heat sink for cooling in the summer and as a heat source for heating in the winter.

The underground ambient earth temperature is typically 10 to 23 °C (50 -73 °F ) all year round in the temperate latitudes where most humans live. Daily and annual temperature fluctuations decreases with the increase in depth below the ground surface.

Daily temperature variations hardly affect the earth's temperature at a depth of more than one meter, while the seasonal variations of the ambient temperature are strongly dampened by the earth. The earth's temperature up to a depth of 6.0 to 8.0 m (18 – 24 feet) is influenced by the annual ambient temperature variations with a time delay of several months.

Avoid;

The piping system should avoid sharp 90-degree angles in the construction. Smooth wall tubes are more efficient in moving the air, they are less efficient in transferring energy. Two 45-degree bends produce less-turbulent, but more efficient air flow.

Types;

1. Closed loop system : In this system, same air is recirculated in the building. The air from inside the building is blown through a U-shaped loop of pipes which are 30.0 to 150.0 m (100 to 500 feet) in length. The air is moderated to near earth temperature before returning to be distributed via ductwork throughout the building. The closed loop system can be more effective than an open system, since it cools and re cools the same air.

2. Open system : In this system the outside air is drawn from a filtered air intake. The cooling tubes are typically 30.0 m (100 feet) long straight tubes drawn into the building. It is a process of exchanging the energy contained from outdoor ventilation. During the warmer seasons, the system pre-cools and dehumidifies, while humidifying and pre-heating in the cooler seasons. This system helps to improve the indoor air quality while reducing total HVAC equipment capacity.
   An open system combined with energy recovery ventilation can be nearly as efficient (80-95%) as a closed loop, and ensures that entering fresh air is filtered and tempered.
   
   
3. Combination system: This is a combination of both the above systems. This can be constructed with dampers that allow either closed or open operation, depending on fresh air ventilation requirements. Such a design, even in closed loop mode, could draw a quantity of fresh air when an air pressure drop is created. It is better to draw in filtered passive cooling tube air than unconditioned outside air.

Working;

A tunnel is dug 4m below ground level and network of pipes are laid. The length of the pipe is proportional to the area of the building that has to be air conditioned. One of the above system is used to circulate or recirculate the air. Fresh air which is drawn from inside or outside the building, is forced into these pipes which passes through the cooler or heater where the air is cooled or heated and it then pumped into the building. This method helps air to pass through an underground air tunnel. The air thus cooled or heated can be used directly for the conditioned space or indirectly with air conditioners or heat pumps.

The ambient air ventilated through this tunnel will get cooled in summer and warmed in winter. This method can be used for cooling in summer and heating in winter. This method can be used for either partial or full cooling and/or heating of air.

Removing hot air from building

The simple method called as Stack effect system connects the vertical ducts that project above the roof line of the building and works using the convection system of air by sucking out the stale air. The roof is fitted with a fan which takes away the hot air from the vertical pipes.

Maintaining moderate earth temperature

Sensible cooling can be aided by evaporative cooling. To reduce the underground temperature, the ground can be shaded using vegetation and can be wetted by sprinkling water. This water seeps through and dampens the tunnel walls. A slow drip watering system may improve thermal performance. Damp soil in contact with the cooling tube conducts heat more efficiently than dry soil.

Merits;

• better indoor air quality.
• 100% fresh air circulation in the premises.
• cost effective in both up-front and capital costs.
• reduces long-term operation and maintenance costs
• consumes 1/3 Rd less energy than conventional AC system.
• minimum temperature can be achieved during peak summer.

Demerits;

• performance of the system depends on location's latitude, altitude, ambient Earth temperature, climatic temperature, relative humidity extremes, solar radiation, water table, soil type, soil moisture content and the efficiency of the building's exterior envelope design.

• dry and low density soil with little or no ground shade will yield the least benefit.

• less effective in hot humid climates where the ambient temperature of the earth approaches human comfort temperature. The higher the ambient temperature of the earth, the less effective they are for cooling and dehumidification.

Environmental impact;

With reference to today's diminishing fossil fuel reserves, increasing electrical costs, air pollution and global warming, properly designed earth cooling tubes offer a sustainable alternative to reduce or eliminate the need for conventional compressor-based air conditioning systems. They also provide the added benefit of controlled, filtered, temperate fresh air intake, which is especially valuable in tight, well-weathered, efficient building envelopes.