3358
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
4 PROFITABILITY ANALYSIS
To achieve a pressure difference between the well and the
surrounding groundwater it is necessary to inject air into the
well with an air compressor, which uses electricity. An air-
injection borehole heat exchanger is only profitable when the
increase of the heat abstraction capacity is higher than the
energy used by the air compressor.
To calculate the energy necessary for the air compressor to
work, two parameters are needed: operating pressure and air
flow rate.
To calculate the injected air a perfomance record is chosen,
which considers not only effective power for water production
and air expansion but also includes a performance loss ratio
(Rautenberg 1972):
N
L
± N
W
+ N
R
+ N
S
+ N
B
+ N
E,U
With
N
L
air expansion
N
W
effective power for water production
N
R
dissipation loss due to friction of the two-phase flow
N
S
dissipation loss due to slip between air bubbles
N
B
dissipation loss to accelerate the water
N
E,U
entry and exit dissipation loss
The dissipation loss N
E,U
is very small compared to the other
factors and can be neglected.
By iteratively solving equation 5 the necessary air flow rate
for inducing a groundwater circulation can be calculated. For a
density of Δρ = 10 kg/m³ the through air injection induced water
flow rate is so low that effective power for water production can
be disregarded. The amount of air necessary for achieving a
pressure gradient in the well, which is the minimally necessary
air flow rate (Luber 1999) and does not depend on soil
permeability, is the decisive factor for calculating the total
amount of air. This leads to a small-scale dependency of the
total amount of air from the soil permeability.
Up until a permeability of 4 · 10
-5
m/s the coefficient of air
injection (COA) is smaller than 1, which means that the amount
energy used for air injection is higher than the increase in the
heat abstraction rate and the use of the air injection technique is
not favorable. With increasing k the COA also increases. In a
soil with a permeability of k = 10
-3
m/s, the COA is expected to
be about 100. In this case the 100 times of the energy used for
the air compressor is converted into usable energy for air
conditioning.
The coefficient of performance (COP) for ground coupled
heat pumps can reach a maximum of 5 (Pahud and Hubbuch
2007, Wood, Liu and Riffat 2009). This value can already be
exceeded by the COA-value of the air-injection borehole heat
exchanger with a value for k = 6 · 10
-5
m/s. In a permeable soil
the COA shows the profitability of the air-injection borehole
heat exchanger.
5 CONCLUSIONS
Combining an air-sparging well with a borehole heat exchanger
offers the opportunity of increasing the heat-abstraction
capacity of closed geothermal systems without pumping
groundwater. The induced groundwater circulation accelerates
the heat transfer through convection.
For a permeability of k < 10
-5
m/s, the induced circulation is
too slow to have an effect on the heat transfer. But with
increasing permeability the positive effect of the air injection
increases as well. In soils with k > 10
-4
m/s convection is the
dominant method of heat transfer. For soils with k = 10
-3
m/s
and λ = 2,5 W/(m · K) the heat abstraction capacity can increase
about ten times through use of air injection when Δρ = 10
kg/m³.
Simulations so far have only been done for one air injection
borehole heat exchanger and one operating period. Long term
simulations as well as an in-situ test in Hamburg are planned
(Ma und Grabe 2010).
With certain groundwater chemistries the use of this
technique can lead to the sedimentation of iron ochre over time.
This may lead to the necessity of cleaning the well with suitable
methods (Herth and Arndts 1995). An alternative for this would
be the usage of different gases like N
2
or CO
2
. In those cases the
air escaping the well should be collected and reused.
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Herth, W., and Arndts, E. 1995. Theorie und Praxis der
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M.
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Luftinjektionsbrunnen
zur
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situ
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