Heat supply by shallow geothermal energy in Karlsruhe

By employing shallow geothermal systems, heat is extracted from the subsurface and utilized for
space heating and domestic hot water (DHW). In built-up areas the available thermal energy is even
larger, if the subsurface urban heat island (UHI) effect is also considered. Increased surface
temperatures combined with underground anthropogenic heat sources, such as basements and
sewage systems, can raise urban groundwater temperatures by 3 K to 7 K above those in rural areas.
Previous studies calculated the annual average anthropogenic heat flux into the ground by means of
a spatially resolved heat transport model (Benz et al., 2015).
In this study, the geothermal potential is compared to the energy demand for space heating as well
as DHW for the urban quarter “Rintheimer Feld” in Karlsruhe, Germany. In this quarter the housing
association (Volkswohnung GmbH) is proprietor of 30 multifamily-houses with about 70,000 m² of
living space. These houses were built in the 1950/60s and meanwhile have been refurbished (Rink
and Kuklinski, 2015). The calculation is based on the consumption data of space heating and DHW of
all buildings before and after the refurbishment. ... mehrBy merging the anthropogenic heat flux and the
energy stored underground we obtain the geothermal potential. Based on these results and
considering space availability as well as (hydro)geological boundary conditions, we determine the
required number of open and closed geothermal systems. Furthermore, we determine how much of
the energy demand – before and after refurbishment, respectively – can be covered by one of the
three following geothermal systems: (1) horizontal ground heat exchangers (HBHE), (2) ground
source heat pump (GSHP) systems with vertical borehole heat exchangers (BHE) and (3) ground
water heat pump (GWHP) systems. Our results show that energy supplied by the HBHE is not
sufficient, since the area required for the system installation is too small. Totally 90% of the heating
energy demand can be covered. Assuming a BHE length of 100 m and a spacing of 5 m, the energy
demand before and after refurbishment can be fully covered by GSHP systems. GWHP systems can
only partly cover the demand, due to the higher space demand, which is required to avoid thermal
interaction between the wells. In case of a conservative assumption (1 K plume isotherm), 6% of the
energy demand can be obtained before and 13% covered after refurbishment. Assuming a 3 K plume
isotherm, an entire coverage of the demand is possible after and at 76% before refurbishment.
To conclude, GSHP systems can cover the energy demand before and after refurbishment in the
“Rintheimer Feld” and are therefore also the preferred geothermal system variant