3355
Combination of borehole heat exchangers and air sparging to increase geothermal
efficiency
Combinaison de sondes géothermiques et barbotage d’air pour augmenter l’efficacité
géothermique
Grabe J., Menzel F., Ma X.
Technische Universität Hamburg-Harburg
ABSTRACT: Closed and open systems are available for the usage of shallow geothermal energy. In closed systems heat can only be
transferred conductively for the case of no groundwater flow. Unfortunately heat conduction is a relatively slow heat transfer
mechanism, which causes limited heat-abstraction capacities in geothermal systems. A patented method is presented, in which a
closed system is combined with groundwater-circulation technology. In this way a groundwater circulation will be created artificially,
which increases convective heat transfer in the soil and therefore the heat capacity of the geothermal system. In this paper a borehole
heat exchanger combined with an air sparging well is numerically simulated. The induced groundwater circulation and the heat
propagation are calculated sequentially. The heat capacity of this system is compared to a normal borehole heat exchanger.
Furthermore, variation calculations are performed to investigate the influence of density of the water-air-mixture in the well,
permeability and hydraulic conductivity of the soil. A profitability analysis is carried out based on the numerical results.
RÉSUMÉ : Des systèmes ouverts et fermés sont disponibles pour l’utilisation de l’énergie géothermique peu profonde. La chaleur,
présente dans les systèmes fermés ne peut être transférée par conduction s’il n’y a pas un écoulement d’eaux souterraines. La
conduction thermique est malheureusement un mécanisme de transfert thermique assez lent. Cela limite donc la capacité thermique
dans ces systèmes géothermiques. On présente un procédé breveté où un système fermé est combiné avec une technologie de
circulation des eaux souterraines. La circulation des eaux souterraines est artificiellement créée, ce qui permet d’augmenter le transfert
thermique par convection dans le sol et, par conséquent, la capacité calorifique du système géothermique. Dans cet article, une sonde
géothermique combinée à une injection d’air sont simulées numériquement. La circulation des eaux souterraines induite, ainsi que la
propagation de chaleur, sont calculées de manière séquentielle. La capacité calorifique du système est comparable à celle d’une sonde
géothermique normale. En outre, des calculs de variations sont effectués afin d’étudier l’influence de la densité de l’eau/air mélangé
dans le puits de conductivité, de la perméabilité et l’état hydraulique du sol. Une analyse de rentabilité est ensuite effectuée à partir de
ces résultats numériques.
KEYWORDS: shallow geothermal energy, air sparging, induced groundwater flow, numerical simulations
1 INTRODUCTION
Shallow geothermal systems use the energy that is available
within the top 400 m of the Earth’s crust. The relatively
constant temperatures of the soil can be used to heat or cool
buildings.
In closed systems without groundwater flow, heat is
transported by conduction only. This is a very slow process and
it limits the heat-abstraction capacity of the system. Systems
that induce groundwater flow have the advantage of being able
to use convection as a much faster heat transfer mechanism (Ma
and Grabe 2009, Wang et al. 2009). These are open systems
that, in spite of the higher efficiency, are rarely used because
permissions for these systems are difficult to obtain. Also the
hydrological boundary conditions for these systems can be hard
to fulfill and limit the usage of open systems. Alternative
methods are possible for closed systems.
Ma and Grabe (2009) suggest the combination of a
groundwater-circulation system with a borehole heat exchanger.
Several methods exist to induce groundwater circulation. For
this example the air-injection well (Wehrle 1990) was chosen.
The objective of this entry is to numerically show to what extent
the efficiency of borehole heat exchangers can be increased
through the use of air-injection at sites with low to zero
groundwater flow (Ma and Grabe 2011).
2 NUMERICAL SIMULATION OF AIR-INJECTION
BOREHOLE HEAT EXCHANGER
2.1 System description
Figure 1 shows the concept of the combined air-injection
borehole heat exchanger. In this example the system is coupled
with an A/C. Unlike with regular borehole heat exchangers the
borehole in this case is not filled with grouting. Instead a
drainage layer is installed below the water table and the heat
exchanger pipes as well as the air-injection pipe are fitted into
the borehole. Above the water table the borehole is sealed, save
for a small opening to let the air escape.
For the following calculations summer usage is assumed as
well as temperature independent thermal and mechanical
characteristics. The results can easily be converted for a winter
scenario as long as the fluid in the pipes does not reach
temperatures below 0°C.
When the system is in use, air is injected to the lowest point
of the well to create an air-water mixture with a density lower
than that of the surrounding water. The water level in the well
rises higher than the groundwater table. That way water can
flow from the well into the aquifer in the top part of the well,
while in the bottom part water is flowing from the aquifer into
the well.
