Sustainability and renewability aspects of a group of
borehole heat exchangers (BHE) are treated by
numerical model simulations. The software
FRACTure (Kohl and Hopkirk, 1995) is used as the
modeling tool which implies transient, coupled heat
and mass transfer, rock mechanics and rock/water
interaction in 3D. The tool is equipped with a
semiautomatic mesh generator. The long-term
thermal behavior (heat extraction / recovery) of an
array of six 100 m deep BHEs has been simulated
over 100 years. The distance between the BHEs is 7.5
m. The model simulations are based on load profiles
with monthly differing heating demands, with a total
of 1800 h/a heat pump runtime.

The results (ground temperatures and BHE delivery
temperatures) have been compared to a single BHE
of the same length. Single and multiple BHEs show
the same cooling and recovery characteristics: the
cooling is strong at the beginning and slows later
down asymptotically; also the recovery is strong in
the beginning and with time it levels off. The BHE
array spacing is a critical parameter; the minimum
distance shall not fall short of 8 m to provide
sustainable production. In a BHE array the recovery
time is longer than for a single BHE; the lower
temperatures of the produced fluid can be
compensated for by additional drilling meters.
Numerical values are given for the latter. Sustainable
production from a BHE field can be achieved by
proper design.

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