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EARTH AIR TUNNEL HEAT EXCHANGER (EATHE)
Submitted by
Amandeep Singh
Vikas Mahala
Ashok Dhayal
CONTENTS Introduction Passive cooling Earth Air Tunnel Principle Factors affecting thermal conductivity Applications of EAT Design guidelines Classification Advantage and limitations Potential issues Conclusion References
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.
PASSIVE COOLING
• 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]
EARTH AIR TUNNEL
• 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.
EARTH-AIR TUNNEL: PRINCIPLE
Underground heat exchanger
Also called:
Earth-Air Heat Exchangers
Air-to-soil Heat Exchangers
Earth Canals
PRINCIPLE DIAGRAM
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
FACTORS AFFECTING THERMAL CONDUCTIVITYSOIL: 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]
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
DESIGN GUIDELINES
IMPORTANT DESIGN PARAMETERS:
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]
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.
[4]
AIR VELOCITY As the velocity of air increases the exit
temp decreases
[6]
AIR FLOW RATE
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
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.
Hybrid System: EATHE system is coupled to another heating/cooling system, which
may be an air conditioner , evaporative cooling system or solar air heater
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
CLASSIFICATIONClassification 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
CONTD….
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
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
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
ADVANTAGE AND LIMITATIONS
[8]
ADVANTAGE 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.
LIMITATIONS
Require large space to make setup.
Give a limited cooling effect.
Initial cost high.
POTENTIAL ISSUES
MOISTURE ACCUMULATION AND IAQ PROBLEM
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
CONCLUSIONS
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.
REFRENCES1. 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 *, rohit misra, 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, jyotirmay ∗mathur
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)
THANK YOU