Submitted by

Amandeep Singh

Vikas Mahala

Ashok Dhayal

Introduction

Passive cooling

Earth Air Tunnel

Principle

Factors affecting thermal conductivity

Applications of EAT

Design guidelines

Classification

Advantage and limitations

Potential issues

Conclusion

References

Energy Saving: One of the most important global challenges

Energy Efficiency:

Supply Side: Higher

Efficiency power plants,

renewable sources of energy, Smart Grids, etc.

Demand Side: Energy efficient,

Building Envelopes (direct systems),

Earth Air Tunnels(indirect systems), etc.

• Passive cooling systems are least expensive means of cooling a

home which maximizes the efficiency of the building envelope

without any use of mechanical devices.

• It rely on natural heat-sinks to remove heat from the building. They

derive cooling directly from evaporation, convection, and radiation

without using any intermediate electrical devices.

• All passive cooling strategies rely on daily changes in temperature

and relative humidity.

• The applicability of each system depends on the climatic conditions.

• These design strategies reduce heat gains to internal spaces.

- Natural Ventilation

- Shading

- Wind Towers

- Courtyard Effect

- Earth Air Tunnels

- Evaporative Cooling

- Passive Down Draught Cooling

- Roof Sprays[1]

• The Earth Air Tunnel (EAT) systems utilizes the heat-storing capacity of earth.

• The fact that the year round temperature four meter below the surface remains almost constant

throughout the year. That makes it potentially useful in providing buildings with air-

conditioning.

• It depends on the ambient temperature of the location, the EAT system can be used to provide

both cooling during the summer and heating during winter.

• The tunnels would be especially useful for large buildings with ample surrounding ground.

• The EAT system can not be cost effective for small individual residential buildings.

• The ground temperature remains constant and air if pumped in appropriate amount that allows

sufficient contact time for the heat transfer to the medium attains the same temperature as the

ground temperature.

Underground heat exchanger

Also called:

Earth-Air Heat Exchangers

Air-to-soil Heat Exchangers

Earth Canals

Earth acts a source or sink

High thermal Inertia of

soil results in air

temperature fluctuations

being dampened deeper

in the ground

Utilizes Solar Energy

accumulated in the soil

Cooling/Heating takes

place due to a temperature

difference between

the soil and the air

SOIL:

Moisture content

Most not able impact on thermal conductivity

Thermal conductivity increases with moisture to a certain point (critical moisture content)

Dry density of soil

As dry density increase thermal conductivity increase

Mineral Composition

Soils with higher mineral content have higher conductivity

Soils with higher organic content have lower conductivity

Soil Texture

Coarse textured, angular grained soil has higher thermal conductivity

Vegetation

Vegetation acts as an insulating agent moderating the affect of temperature

[2]

EAT’s can be used in a vast variety of buildings:

Commercial Buildings: Offices, showrooms, cinema halls etc.

Residential buildings

University Campuses

Hospitals

Greenhouses

Livestock houses

The design parameters that impact the performance of the

EAT are:

• Tube Depth

• Tube Length

• Tube Diameter

• Air velocity

• Air Flow rate

• Tube Material

• Tube arrangement

Open-loop system

Closed-loop system

• Efficiency

• Coefficient of Performance (COP)

[3]

Ground temperature defined by:

External Climate

Soil Composition

Thermal Properties of soil

Water Content

Ground temperature

fluctuates in time,

but amplitude of

fluctuation diminishes with depth.

Burying pipes/tubes as

deep as possible would be ideal.

A balance between going

deeper and reduction in

temperature needs to be drawn.

Generally ~4m below

the earth’s surface dampens

the oscillations significantly.

Heat Transfer depends on surface area.

Surface area of a pipe:

Diameter

Length

So increased length would

mean increased heat

transfer and hence

higher efficiency.

After a certain length,

no significant heat transfer

occurs, hence optimize length.

Increased length also results

in increased pressure drop and

hence increases fan energy.

So economic and design

factors need to be balanced to

find best performance at lowest cost.

Heat Transfer depends on surface area.

Surface area of a pipe:

Diameter

Length

Smaller diameter gives better thermal performance.

Smaller diameter results in larger pressure drop increasing fan

energy requirement.

Increased diameter results in reduction in air speed and heat

transfer.

So economic and design factors need to be balanced to find best

performance at lowest cost.

Optimum determined by actual cost of tube and excavation cost.

[4]

As the velocity of air increases the exit temp

decreases

[6]

For a given tube diameter, increase in airflow rate results in:

Increase in total heat transfer

Increase in outlet temperature

High flow rates desirable for closed systems

For open systems airflow rate must be selected by considering:

Outlet temperature

Total cooling or heating capacity

The main considerations in selecting tube material are:

Cost

Strength

Corrosion

Resistance

Durability

Tube material has little influence on performance.

Selection would be determined by other factors like ease of installation, corrosion resistance etc.

Spacing between tubes should enough so that tubes are thermally independent to maximize benefits.

EAT can be used in either:

Closed loop system

Open loop system

Open Loop system:

Outdoor air is drawn into tubes

and delivered to AHUs or

directly to the inside of the building

Provides ventilation while

hopefully cooling or heating

the building interior

Improves IAQ

Closed Loop system:

Interior air circulates through EATs

Increases efficiency

Reduces problem with humidity

condensing inside tubes.

Hybrid System:

EATHE system is coupled to another heating/cooling system, which may be an air conditioner , evaporative cooling system or solar air heater

EAT can be used in either:

One-tube system

Parallel tubes system

One tube system may

not be appropriate to meet

air conditioning requirements

of a building, resulting

in the tube being too large

Parallel tubes system

More pragmatic design option

Reduce pressure drop

Raise thermal performance

Classification of EATHE system

According to layout of pipe in ground

According to mode of arrangement

There are four different types according to layout of pipe in the

ground

Horizontal/ straight Loop

Vertical Looped

Slinky/ spiral Looped

Pond/Helical Looped

Calculating benefits from EAT is difficult due to:

Soil Temperatures

Conductivity

Performance of EAT can be calculated as:

where;

To = Inlet Air Temperature

To (L) = Outlet Air Temperature

Ts = Undisturbed ground temperature

COP based on:

Amount of heating or cooling done by EAT (Heat Flux)

Amount of power required to move the air through the EAT

Q= Heat Flux

W= Power

COP decreases as system is operated

COP can be integrated into system control strategies

When COP down to a certain point, EAT should be shut down and

conventional system should take over

[8]

ETHE based systems cause no toxic emission and therefore, are

not detrimental to environment.

Ground Source Heat Pumps (GSHPs) do use some refrigerant but

much less than the conventional systems.

ETHE based systems for cooling do not need water - a feature

valuable in arid areas like Kutch. It is this feature that motivated

our work on ETHE development.

ETHEs have long life and require only low maintenance

Low operating cost.

Require large space to make setup.

Give a limited cooling effect.

Initial cost high.

ISSUE

• Condensation inside the tubes has been observed

• Condensation occurs if temp. in the tube is lower that dew point temp.

• Condensation occurs in systems with low airflow and high ambient dew point temperature

• Removal of moisture from the cooled air is always an issue and system may be used with a regular air conditioner or a desiccant

• Water in tubes also results in growth of mould or mildew leading to IAQ issues

SOLUTIONS

• Good construction and drainage

• Tubes are tilted to prevent water from standing in the tubes

• In the service pit at the lowest point water can be captured and pumped

• Water tight tubes can be used to prevent ground water from entering into the system

EATs are based on the following principles

Using earth as a source or sink

Uses Soil Thermal inertia

Depends on the Thermal Conductivity of Soil

Various Factors affect the performance of EAT which need to be

optimized to maximize performance.

Integrate the EAT into the building systems to maximize

performance and maximize energy savings.

1. A passive solar system for thermal comfort conditioning of buildings in composite climates†,1 p. RAMAN, SANJAY MANDE and V. V. N. KISHORE received 19 august 1998; revised version accepted 13 october 2000

2. Earth air heat exchanger in parallel connection manojkumardubey1, dr. J.L.Bhagoria2, dr. Atullanjewar M.Tech student1 MANIT bhopal professor mech deptt. , MANIT bhopal asst. Professor mech deptt, MANIT bhopal(figures)

3. Jalaluddin, Miyara A, Thermal performance investigation of several types of vertical ground heat exchangers with different operation mode, Applied Thermal Engineering 33-34 (2012) 167–74.

4. Performance analysis of earth–pipe–air heat exchanger for winter heating vikas bansal *, rohitmisra, ghanshyam das agrawal, jyotirmay mathur

5. Performance analysis of earth–pipe–air heat exchanger for summer cooling vikas bansal *, rohit misra, ghanshyam das agrawal, jyotirmay mathur

6. Performance evaluation and economic analysis of integrated earth–air–tunnel heat exchanger–evaporative cooling system vikas bansal∗, rohit misra, ghanshyam das agrawal, jyotirmaymathur

7. Thermal performance investigation of hybrid earth air tunnel heat exchanger rohit misraa, vikas bansala, ghanshyam das agarwala, jyotirmay mathura,∗, tarun aserib

8. ANALYTICAL MODEL FOR HEAT TRANSFER IN ANUNDERGROUND AIR TUNNEL MONCEF KRARTI and JAN F. KREIDER (received 27 october 1994; received for publication 11 july 1995)