NetZeroCities

The use of passive cooling technologies in the built environment has reached a stage of high maturity. Passive cooling techniques are based on the use of techniques of solar and heat control, heat modulation and heat dissipation. While heat modulation deals mainly with the thermal storage capacity of the building structure, heat dissipation techniques are mainly associated with the disposal of excess building heat to an environmental sink of lower temperature, like the ground, water, and ambient air or sky.

Ground dissipation techniques have gained an increasing acceptance and level of implementation. Ground techniques dissipate heat to the soil that remains at a constant and low temperature around the year at depths close to 2.5 to 3 m [1-2]. Such a cooling technique is known from the ancient time [3].

The most common technique to dissipate the excess heat of buildings to the ground is through the implementation of underground air pipes or tunnels, known as earth to air heat exchangers (EAHE). The system involves the use of buried plastic, metallic or ceramic pipes where fresh or indoor air is circulated by mechanical systems and eventually mixes it with the indoor air of the building. Due to the temperature difference between the soil and the circulated air, the temperature of the air decreases. The cooling performance of the earth to heat exchangers depends on numerous parameters like the air flow rate, the length and the diameter of the pipe, the thermal characteristics of the soil, the depth where the system is buried, the material of the pipes, etc. [4-6]. Earth to air heat exchangers are buried at depths varying between 50 and 200 cm and a spacing between them of approx. 40 cm. The exchangers are placed with a tilt of a few centimeters to drain any condensation away through a small opening at the end of the tube.

The system can be used both during the summer and winter periods to provide cooling or heating respectively. In the heating mode, cold fresh or recirculated air from inside the building is circulated through the pipes. Heat is transferred from the soil to the air stream and then either it is transmitted directly into the building or to the heating system to get the proper temperature. In cooling mode warm, fresh, or indoor recirculated air is driven through the underground pipes and is precooled. The air is driven directly into the building or to the cooling system.

Earth to air heat exchangers present important advantages like high cooling potential, simplicity, low capital, operational and maintenance cost, and environmental protection.

Experimental data from numerous real case applications, [7], has shown that for moderate climates the seasonal energy performance of the EAHE systems varies between 8–10 kWh/m2 of ground coupling area. In parallel, the peak cooling capacity at air temperature close to 32 ◦C is close to 45 W/m2 of ground coupling area, (8).

Main problems associated with the use of earth to air heat exchangers deal with the accumulation of water inside the pipes, problems of indoor air quality if the circulated air contains any biological agents, lack of efficient and dynamic control during the operation, etc. Recent projects presented below in the examples, have overcome efficiently those problems and it is well accepted that EAHE is a very mature and quite efficient technology. Earth to air heat exchangers can be fully recycled and used for other purposes.

Earth to air heat exchangers when well maintained, may operate for more than 20 years and used without significant problems.

Earth to Air Heat Exchangers

Figure 1: Operational Principle of the Earth to air Heat Exchangers

Figure 2: Installation of Earth to Air Heat Exchangers in a building.

Figure 3: Installation of Earth to Air Heat Exchangers. Photo from : Earth Tube Links (External) - Home in the Earth

MATURITY:

The technology of earth to air heat exchangers is very mature and can be used to provide precooling and preheating in buildings.

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G. Mihalakakou, On the application of the energy balance equation to predict ground temperature profiles, Solar Energy 60 (3-4) (1997) 181–190.

M. Santamouris, D. Assimakopoulos (Eds.), Passive Cooling of Buildings, James & James (Science Publishers) Ltd., London, UK, 1997

P. Jacovides, M. Santamouris, G. Mihalakakou, On the ground temperature applications profile for passive cooling in buildings, Solar Energy 57 (3) (1996), 167–175.

G. Mihalakakou, M. Santamouris, D. Asimakopoulos, On the cooling potential of earth to air heat exchangers, Energy Conversion and Management 35 (5), (1994) 395–402.

G. Mihalakakou, M. Santamouris, D. Asimakopoulos, Use of the ground for heat dissipation, Energy 19 (1) (1994) 17–25.

International Energy Agency, Energy Conservation in Buildings and Community Systems Programme, Annex 28 Low Energy Cooling, Subtask 2 Report 1. IEA Annex 28, London, UK, 1997.

Mattheos Santamouris, Dionysia Kolokotsa : Passive cooling dissipation techniques for buildings and other structures: The state of the art. Review Article, Energy and Buildings, Volume 57, February 2013, Pages 74-94,

H. Breesch, A. Bossaer, A. Janssens, Passive cooling in a low-energy office building, Solar Energy 79 (6) (2005) 682–696.

J. Pfafferott, Evaluation of earth-to-air heat exchangers with a standardized method to calculate energy efficiency, Energy and Buildings 35 (10) (2003) 971–983

K. Voss, S. Herkel, J. Pfafferott, G. Lohnert, A. Wagner, Energy efficient office buildings with passive cooling – results and experiences from a research and demonstration programme, Solar Energy 81 (3) (2007) 424–434.

A. Spieler, R. Wagner, S. Beisel, K. Vajen, Passive solar office building: first experiences and measurements, in: 4th ISES Europe Solar Congress, 2000.

R. Wagner, S. Beisel, A. Spieler, A. Gerber, K. Vajen, Measurement, modeling and simulation of an earth-to-air heat exchanger in Marburg (Germany), in: 4th ISES Europe Solar Congress, 2000.

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U. Eicker, Cooling strategies, summer comfort and energy performance of a rehabilitated passive standard office building, Applied Energy 87 (6) (2010) 2031–2039.

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P. Hollmuller, Utilisation Des changeurs Air/Sol Pour Le Chauffage Et Le Rafraichissement Des Batiments Mesures in Situ, Modelisation Analytique,Simulation Numerique Et Analyse Systemique, PhD Thesis, Universite de Geneve, Geneve, 2002.

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P.O. Tjelflaat, The Grong School in Norway the Hybrid Ventilation System. IEA ECBCS Annex-35 (HybVent) Technical Report, 2000.

P.O. Tjelflaat, Pilot Study Report – Media School. IEA ECBCS Annex-35 (HybVent), Technical Report, 2000.

B.J. Wachenfeldt, Natural Ventilation in Buildings Detailed Prediction of Energy Performance, PhD Thesis, Norwegian University of Science and Technology, 2003.

Y. Jeong, F. Haghighat, Modeling of a Hybrid-Ventilated Building – Using ESP-r, International Journal of Ventilation 1 (2003) 127–140.

P.G. Schild, “Jaer School. IEA ECBCS Annex-35 (HybVent), Nesodden Municipality, Norway, 2002.

M. Grosso, L. Raimondo, Horizontal air to-earth heat exchangers in Northern Italy – Testing, design and monitoring, International Journal of Ventilation 7 (1) (2008) 1–10.

M. Stournas-Triantis, E. Santamouris, T. Metsis, Passive Solar Retrofitting of the New School of Philosophy Building in the University of Ioannina, Greece, in: 2nd I.H.T. Conference, 1985.

M. Stournas-Triantis, M. Santamouris, Passive Solar Retrofitting in Mediterranean Climates. The case of the atrium building, International PLEA Conference (1988).

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Energy Study of the Mazerakion Athletic Center Athens, Greece. Group Building Environmental Studies, University Athens, 2007.

A. Chel, Performance evaluation and life cycle cost analysis of earth to air heat exchanger integrated with adobe building for New Delhi composite climate, Energy and Buildings 41 (1) (2009) 56–66.

R.L. Sawhney, D. Buddhi, N.M. Thanu, An experimental study of summer performance of a recirculation type underground airpipe air conditioning system, Building and Environment 34 (2) (1998) 189–196.

S. Thiers, B. Peuportier, Thermal and environmental assessment of a passive building equipped with an earth-to-air heat exchanger in France, Solar Energy 82 (9) (2008) 820–831

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Numerous projects involve the use of earth to air heat exchangers for cooling or heating purposes. Information on most of the large-scale projects is given in (8). Based on the provided information, the characteristics of several offices, residential, educational, production and commercial, multifunctional, athletic centres, hospital, and care home projects are described below.

In most of the projects the performance of EAHE has been monitored; however, the provided information varies considerably between the different projects. As expected, the performance of the EAHE systems varies as a function of the design characteristics of the project like, the number of the used pipes, their length and depth and the global management and use of the energy system.

Application of earth to air heat exchangers in an office building is Belgium. The surface of the building was close to 2000 m2. Two concrete pipes of 80 cm internal diameter and 40 m length, buried at depths 3 and 5 m, were used to provide heating and cooling. The pipes were connected to the building ventilation system and used for preheating and precooling purposes. Monitoring has shown that the maximum supply air from the buried pipes never exceeded 22 ◦C, while the earth to air heat exchangers have decreased the discomfort hours in the office by 20–30% during the whole summer period, (9).

Application of earth to air heat exchangers in three office buildings in Germany. In the first office, Netz AG (Hamm), 6000 m2, 26 buried exchangers were used providing an air flow of 12,000 m3/h. The length of the pipes varied between 67 to 107 m. The exit air temperature from the pipes was always between 20 and 5 ◦C. During the summer period when the ambient temperature was above 30 ◦C, the exit temperature in the exchanger was close to 18 ◦C, providing energy gains close to 13.5 kWh/m2/year. In the second office, Fraunhofer ISE, 13,150 m2, seven exchangers of 95 m each, providing a total flow of 9000 m3/h, were buried at 2 m depth. When the summer ambient air was close to 36 ◦C, the exit temperature from the exchangers was around 24 ◦C, providing energy gains close to 23.8 kWh/m2/year. In the third office, Lamparter, 1000 m2, two pipes of 90 m each, were buried at 2.3 m depth providing a total flow of 1100 m3/h. When the summer ambient temperature was above to 30 ◦C, the exit temperature from the exchangers was around 20 ◦C, providing energy gains close to 12.1 kWh/m2/year, (10).

Application of Earth to air heat exchangers in an office building in Marburg Germany. Four earth to air heat exchangers of 32 m length buried at 1.5 m depth were used providing a total flow close to 3100 m3/h. The cooling energy contribution of the exchangers was close to 1.5 Wh/m2/year, (12-14).

Application of earth to air heat exchangers in an office building of the University of Siegen, GIT, Germany. The total surface of the building was 3243 m2. The final energy use of the building was close to 70 kWh/m2/y, (12, 15)

Application of Earth to air heat exchangers in the UBA building in Germany. The surface of the building was 32,384 m2, and the earth to air heat exchanger used to precondition the outside air through over 5 km of tubes. The final primary energy of the building was 73.1 kWh/m2/y, (16).

Office Building in Germany. Five horizontal earth–brine heat exchangers of 100 m length having a length were installed at 1.2 m depth and were used to heat and cool an office building of 833 m2 in Germany. The maximum heat dissipation per meter of tube was 8 W, while the COP of the EAHE system for cooling was 18, (17).

Office Building In Germany. Two polyethylene pipes of 90 m length were buried at 2.80 m depth, to supply cooling to an office building of 1488 m2 in Germany. A three years monitoring shown that the temperature drop in the pipes was close to 10 ◦C while the COP of the system was varied between 35 and 50, (18).

Application of EAHE in a commercial building, Schwerzenbacherhof, Zurich, Switzerland. The building has a surface of 8050 m2, while 43 parallel high-density pipes of 23 m length were buried at 6 m depth. The ground system was in operation during the summer when the outdoor air temperature exceeded 22 ◦C. The EAHE system provided about 1/3 of total cooling demand, (18-22).

Educational building of FH BRS University Dortmund in Germany. Three ground heat exchangers have been installed in a 16000 m2 building. The length of each pipe was 75 m and were buried at 3.8 m depth. The air circulated through the pipes decreased its temperature up to 7 C, while the contribution of the earth to air heat exchangers varied between 10 and 16 kWh/m2/year, (23).

An Educational Building In Norway. An horizontal 1.5 m x 2 m concrete intake duct buried approximately 1.5 m below the ground surface was used to heat and cool the 1001 m2 building. The EAHE system is found to cover all cooling needs of the building, (24-28).

The Jaer Educational building in Norway. The EAHE system consisted of two parts: a prefabricated concrete pipe (20 m length and 1.6 m diameter) and a rectangular cast-in place concrete duct (35 m length, 2 m width, and 3 m height) with a total surface area of about 450 m2. The buried pipes covered the cooling needs and provided acceptable indoor air quality, (24-28).

School In Italy. Almost 28 buried pipes of 70 m length, were installed at 2.6 m depth in the school. Simulation predicted that the average cooling contribution of each pipe was close to 760 kWh/m2/y, (29)

Philosophical School of University of Ioannina, Greece. Four ground pipes buried at 1.5 m depth were used to provide cooling to a 4100 m2 building. The system provided 33 kWh/m2/year for cooling purposes, (30-31).

Two production buildings in Germany. The Huebner building of the University Hannover, 2122 m2, and the Surtec University Darmstadt, Passive House Institute 4423 m2 were equipped with EAHE systems. The final energy consumption of the buildings was close to 100 kWh/m2/y and 73 kWh/m2/y, respectively, (32).

Hospital In India. Earth to air heat exchangers were installed. When the maximum ambient temperature was close to 42 ◦C, the maximum exit air temperature at of the tubes was close to 25.5 ◦C, (32)

Care Home in France. Eleven pipes buried at 2 m depth have been used to provide heating and cooling to an 80 beds care home, 3950 m2, in Frejus. The EAHE system was used to cool the dining room of 380 m2. The system delivered an initial cooling power of 14 kW at 2 m3/s air flow-rate and of 9.5 kW at 1 m3/s air flow-rate. In summer the cooling power was close to 5 kW, (33).

The CircoLab Building. A ground cooling system composed by buried pipes of 40 m length, buried at 4 m depth, was installed in a multi-functional facility, 382 m2, with corporate conference, recreational, cultural and community functionalities. The exit temperature from the buried pipes was almost 5 ◦C lower than the ambient temperature, (33).

Athletic Center in Athens, Greece. Six pipes buried at 3 m depth were used to cool an athletic centre. The length of the pipes varied between 40–57 m. Thermal simulations shown that the EATHE system covered almost the 60% of the cooling needs, (34).

Multistory Building in Romania. Two parallel drums of external diameter 400 mm were buried at 3.5 m depth. The drums were connected by eight pipes and their length is 30.8 m and 5 m, respectively. When the ambient temperature was 32 ◦C the temperature at the exit of the pipes was close to 26 ◦C, (32).

Experimental Building in India. A ground heat exchanger buried at 1.5 m depth composed of 6 parallel lines of 11 m each was connected to an experimental building. The air was circulated to the pipes and then used for ventilation purposes. The annual energy conservation potential of the system was close to 10,320 kWh, while the average seasonal energy efficiency for cooling was 2.9, (35).

House In India. Two earth to air heat exchangers having a length of 74 m and buried at 2.5 m were connected to a house. When the ambient temperature was 42 ◦C the exit temperature of the exchangers was close to 27 ◦C, (36).

Passive House in France. In this application the pipes of 30 m length were buried at 1.6 m and connected to a building of 132 m2. Simulation shown that the use of the system reduced the cooling degree days from 56 to 22, (37).

Residential Building in France. A 40 m long exchanger was buried at 2.5 m depth and connected to the house to provide ventilation. When the ambient temperature was 37 ◦C, the exit temperature from the exchangers was 24 ◦C. The building connected to the EAHE was almost 3 ◦C of lower temperature than a similar conventional building, (38).

Residential Building in France. A ground exchanger of 39 m length buried at 2 m depth and connected to a residential building of 33 m2. The used exchanger is found to keep the indoor temperature of the house below 27 ◦C without the use of air conditioning systems, (39).

Six Residential Buildings in Portugal. Two ground pipes were used to provide heating and cooling in each of 6 residential buildings in Portugal. The pipes were 25 m long. During the summer period the temperature of the air circulated inside the pipes was reduced by 8 ◦C, (33).

ADDITIONAL INFORMATION FROM CASE STUDIES

Requires a precise humidity control
Requires an optimal connection to the HVAC system

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