EARTH AIR TUNNELS

Prepared by: Angad Deep Singh

INTRODUCTION

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

EARTH-AIR TUNNEL: PRINCIPLE

Underground heat exchanger

Also called:

Earth-Air Heat Exchangers

Air-to-soil Heat Exchangers

Earth Canals etc.

EARTH-AIR TUNNEL: PRINCIPLE

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

EARTH-AIR TUNNEL: PRINCIPLE

Summer and day Winter and night

Cooling of air by charging of soil Heating of air by discharging of

soil

Performance of EAT also impacted by the thermal conductivity

of soil.

SOIL: FACTORS AFFECTING THERMAL

CONDUCTIVITY

Moisture content Most notable impact on thermal conductivity

Thermal conductivity increases with moisture to a certainpoint (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

APPLICATIONS OF EAT’S

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

NIIT University: An example of earth air tunnel being used in an

university

DESIGN GUIDELINES

IMPORTANT DESIGN PARAMETERS:

The design parameters that impact the

performance of the EAT are:

Tube Depth

Tube Length

Tube Diameter

Air Flow rate

Tube Material

Tube arrangement

Open-loop system vs closed-loop system

One-tube system vs parallel tubes system

Efficiency

Coefficient of Performance (COP)

TUBE DEPTH

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

TUBE LENGTH

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

TUBE DIAMETER

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

AIR FLOW RATE

For a given tube diameter, increase in airflow

rate results in:

Increase in film coefficient

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

TUBE MATERIAL

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

TUBE ARRANGEMENT

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.

TUBE ARRANGEMENT

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

EAT EFFICIENCY

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

𝜂 =𝑇𝑜 − 𝑇𝑜(𝐿)

𝑇𝑜 − 𝑇𝑠

CO-EFFICIENT OF PERFORMANCE(COP)

COP based on:

Amount of heating or cooling done by EAT (Heat

Flux)

Amount of power required to move the air through

the EAT

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

𝐶𝑂𝑃 =Σ𝑄

𝑊

Q= Heat Flux

W= Power

POTENTIAL ISSUES

MOISTURE ACCUMULATION AND IAQ

PROBLEM

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

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

ISSUE SOLUTIONS

INSECTS AND RODENTS

Insects and rodents

may enter into the

tubes of an open-loop

system

A sturdy grille and

insect screen should

be installed at the

tube inlet to deter

potential intruders

ISSUE SOLUTIONS

CONCLUSION

CONCLUSIONS

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

THANK YOU