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The potential of earth-air heat exchangers for lowenergy cooling
of buildings
Abdullahi Ahmed1, Andrew Miller2 and Kenneth Ip3
1, 2, 3School of Environment and Technology, University of
Brighton, Brighton, United Kingdom.
ABSTRACT: An Earth-Air Heat Exchanger (EAHX) is a simple
subterranean cooling/heating systemthat utilises the stable soil
temperature that is cooler and warmer than ambient temperature
insummer and winter respectively. The rise in ambient summer
temperature is bringing about buildingoverheating in the UK. There
is increased standard demanded by the building regulation to
increasebuilding energy efficiency and the use of low carbon
technologies. The paper studied the potential ofthe earth-air heat
exchanger in reducing the need for air-conditioning under UK
climatic and soilcondition. The system has been evaluated using
thermal modelling in TRNSYS simulationenvironment. Results show
opportunity for reducing indoor temperature using EAHX.
Keywords: earth-air heat exchanger, passive/low energy cooling,
ground heat sinks, thermal comfort.
1. INTRODUCTION
The Earth-Air Heat Exchanger (EAHX) also knownas earth cooling
tube is a subterranean coolingsystem that consists of a length of
pipe or network ofpipes buried at reasonable depth below the
groundsurface, Figure 1. Ventilation air supply is passedthrough
the pipes and the difference in temperaturebetween the pipe surface
and the air drives the pre-cooling/pre-heating of the ventilation
air. Themagnitude of the heat exchange between air andpipe is
dependent on factors such as, soiltemperature, air temperature,
pipe dimensions, airflow rate, pipe burial depth and soil and pipe
thermalproperties (density, heat capacity and thermalconductivity)
[1,2].
There is significant evidence of summertemperature rise in the
UK [3], and the rise in the useof air-conditioning [4]. The concept
of cooling usingEAHX is well established, but the behaviour of
suchsystems depends on climatic and soil conditions [5].The dynamic
thermal behaviour of an EAHX istherefore not universal and needs to
be studiedwithin the context of climatic, soil and building
loadconditions. The main aim of the research project isto study the
dynamic behaviour of EAHX under UKclimatic and soil conditions and
also study thepotential of the system for improving comfort
conditions in buildings and energy savings.
2. METHODOLOGY AND SIMULATION
The research was carried out using thermalmodelling of the EAHX
evaluated within theTransient System Simulation Software
Program(TRNSYS) environment. TRNSYS is a complete andmodular
simulation environment for the study ofdynamic systems [6]. The
open modular structurewithin TRNSYS allows for the use of
inter-connectionof existing components and user written
componentsto develop a simulation project.
Figure 2:Earth-air heat exchanger
After rigorous review of existing models/tools, theair-soil heat
exchanger model (Type 460) developedby Hollmuller and Lachal [7]
has been selected andadopted for this study. The earth-air heat
exchangermodel has been coupled with the building interfaceTRNBLD
to study the impact of the earth tube onindoor temperature.
3.0 HYDRO-THERMAL DYNAMICS
The thermal performance of an earth-air heatexchanger is
affected by the pipe configuration andair velocity, burial depth
and inlet air condition. In thisstudy the effect of pipe
configuration (length,diameter, burial depth) and air velocity have
beenevaluated. Ambient conditions for London have beenused as input
to the simulation programme. Thesimulation has been carried out for
the month of Julyand for a 24 hour period that corresponds to
themaximum ambient temperature. Figure 4 (a-d)shows the statistical
variation of the pipe outlet airtemperature for different
scenarios.
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2 4 5 6 816
18
20
22
24
26
28
30
(a) Air velocity (2-8m/s) 0.4m diameter, 2m deep,
30mlength, inlet temperature (21.3-28.3oC)
Air velocity (m/s)
Airtemp
erature(oC)
1 2 3 4
16
18
20
22
24
26
28
30
(b) Pipe depth (1-4m), 30m length, 2m depth,
0.4m diameter, inlet temp. (21.3-28.3oC)
Pipe depth (m)
Airtemp
erature(oC)
0.3 0.5 0.6 0.814
16
18
20
22
24
26
(c) Pipe diameter (0.3-0.8 m), 30m length, 4m/s air
velocity,
2m deep, inlet temperature (21.3-28.3 oC)
Pipe diameter (m)
Airtemperature(oC)
30 40 50 60 70 80
14
16
18
20
22
24
26
(d) Pipe length (30-80m) 0.4m diameter, 2m deep,
air velocity 4m/s, inlet temperature (21.3-28.3 oC)
Pipe Length (m)
Airtemperature(oC)
Max
Min
Mean
Max
Min
Mean
Max
Min
Mean
Max
Min
Mean
Figure 2:Outlet air temperature for different pipe
configurations: a) Air velocity, b) pipe depth c) Pipe diameter
d)Tube depth.
The figure reveals some of the inter-relationships oflength, air
velocity, diameter and depth. In coolingmode outlet air temperature
decreases with length ofpipe and pipe depth. The pipe outlet
temperatureincreases with increased air velocity pipe diameter
and higher air velocity.
5. INDOOR TEMPERATURE
The potential of the earth-air heat exchanger forreducing indoor
temperature has been studied. Abuilding model has been developed
for a singlestorey office building in London and the
indoortemperature simulated for a free running buildingand a
building with EAHX. as a source of ventilationair. The results show
significant improvement inindoor temperature using EAHX. The
building hasdimensions (5m X 10m), values of internal heat gainand
occupancy levels have been taken from BSRIARules of Thumb [8]. The
indoor temperature of thefree running building rises above 27C for
over 6hours, while temperature of the building with earth-air heat
exchanger remained below 27C for thesimulation period.
CONCLUSION
The paper presented results of a study of thepotential of the
earth-air heat exchanger for reducingindoor temperature in
buildings. It has demonstratedthe impact of the various parameters
on the thermalbehaviour of the earth-air heat exchanger.
The EAHX has been evaluated within TRNSYSenvironment to study
the performance of the earth-air system and its impact on the
indoor environment.
REFERENCES
1. Kumar, R., S. Rajesh, and S.C. Kaushik,Performance evaluation
and energy conservationpotential of earth-air-tunnel system coupled
withnon-air-conditioned building. Building andEnvironment, 2003.
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2. Roaf, S., Fuentes, M., and Thomas, S., Casestudy 5 Monama:
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2003,Architectural press: London. p. 304-309.
3. CIBSE, Climate Change and the IndoorEnvironment: Impact and
Adaptation, in CIBSEKnowledge series, K. Butcher, Editor.
2005,CIBSE: Norwich.
4. Market Transformation Programme, Policy Brief:UK energy
consumption of Air-Conditioningsystems. 2006. p. 1-6.
5. Piechowski, M., Heat and Mass Transfer Modelof a Ground Heat
Exchanger: TheoreticalDevelopment. International Journal of
EnergyResearch, 1999. 23: p. 571-588.
6. TRNSYS16.0, A Transient System SimulationProgramme. 2005,
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7. Hollmuller, P. and B. Lachal, Cooling andpreheating with
buried pipe systems: monitoring,simulation and economic aspects.
Energy andBuildings, 2001. 33(5): p. 509.
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PLEA2007 The 24th Conference on Passive and Low Energy
Architecture 777
