Increased Temperature in Urban Ground as Source of Sustainable Energy

Increased temperatures beneath cities have been reported in several studies in northern countries. Local surveys in North America (e.g., Ferguson and Woodbury [1], Taylor and Stefan [2], Eggleston and McCoy [3]), Europe (e.g., Balke [4], Menberg et al. [5], Müller et al. [6], García-Gil et al. [7], Epting and Huggenberger [8]), and Asia (e.g., Taniguchi et al. [9], Yamano [10], Liu et al. [11], Zhan et al. [12]) have shown regional thermal anomalies of around 2–5 K in comparison to the undisturbed surrounding. In these sub- surface urban heat islands (SUHIs), heat from anthropogenic sources propagates into the urban subsurface increasing the temperature and changing the thermal conditions in shal- low ground and groundwater (Fig. 1). The regionally increased temperature is also called SUHI intensity.

In Europe, one of the first comprehensive studies was presented by Balke [4], who investi- gated the temperature regime in the shallow aquifer of Cologne, Germany. This was done by temperature measurements in multiple groundwater wells in the city. By constructing contour plots at 20 m depth, a SUHI reaching an intensity of 5 K was resolved beneath the city centre. In similar detail, Hötzl and Makurat [13] investigated the shallow thermal conditions beneath the city of Karlsruhe in 1981. It was shown that the SUHI intensity beneath the city centre reached at least 3 K, and that horizontal groundwater flow promotes the downstream transport of heat.

While more recent studies focused on the environmental impact of SUHIs (e.g Müller et al. [6], Bonte et al. [14], Brielmann et al. [15]), others have studied their associated geothermal potential (e.g., Allen et al. [16], Arola and Korkka-Niemi [17], Sinnathamby et al. [18]). Zhu et al. [19], Zhang et al. [20] for instance evaluated this potential in several cities including London, Prague, Cologne, Winnipeg and Beijing. The results indicate that by using geothermal systems for decreasing the SUHI intensity, the amount of energy extracted is several times higher than the annual heating demand in these cities. Addition- ally, the elevated temperatures are crucial for adjusting and enforcing environmental regulations (Hähnlein et al. [21], Vienken et al. [22]), and they are relevant for technolog- ical issues related to conventional shallow geothermal systems (e.g., Banks [23], Rybach and Eugster [24]).

Extracting this shallow geothermal potential requires a comprehensive underground man- agement of the energy content since an unbalanced exploitation scheme progressively degrades the low-enthalpy energy resources (e.g., Rivera et al. [25]). Possible ways around are implementation of centralized district heating and/or cooling networks for distributing locally unbalanced energy loads (e.g. Arola and Korkka-Niemi [17], Zhang et al. [20]) or direct heat sources. In such “smart energy” management, the urban underground can be con- ceived as a reservoir that buffers the spatial and seasonal variability of the energy demand.

This work presents a description of the spatial distribution of SUHIs in several German cities including Cologne, Berlin, Munich and Karlsruhe. Finally, the

associated energy content is estimated and compared with case-specific space-heat- ing demands.

The spatial distribution of shallow groundwater temperatures (GWT) was estimated in four German cities. The study cases (Table 1) cover a wide range of population densities with specific patterns in the subsurface thermal alteration. The measured surface air temperatures (SAT) reflect a moderate climate, and the dominant sedimentary deposits are likely sensitive to intensified downward heat fluxes.

The spatial distribution of GWT is obtained from a fairly large number of observation wells that the respective municipalities use for monitoring groundwater quality among other parameters. The wells are not only located within the urban areas but also in the suburban surroundings, which is fundamental in estimating the relative intensity of the subsurface urban heat islands.

In Berlin, these measurements were carried out by the Senate Department for Urban Devel- opment and the Environment in 2010 at 0 m above sea level (asl), corresponding to a depth of about 30–60 m below ground level [26]. In Munich, a measurement campaign was conducted in 1983 on a dense network of observation wells along the subway system [27]. In Karlsruhe in turn, the shallow GWT is daily recorded by the Public Works Service through data loggers installed at depths of about 9–12 m. Due to the typical seasonal variability of the temperature data at these depths, the arithmetic annual mean for the year 2012 is used for the present analy- sis. Finally in Cologne, this study uses the data from Zhu et al. [19] that describes the spatial distribution of groundwater temperature in 2009. In all cases, the spatially distributed tempera- tures are visualized and interpolated via kriging in GIS (ESRI® ArcInfo™ 10.0).

The volumetric heat capacity of the porous medium Cm (kJ m−3 K−1) can be used for estimating the energy yield due to a change of temperature ΔT (K) in a given reference volume V (m3). This approach was followed by Zhu et al. [19] to quantify the theoretical geothermal potential in urban aquifers with elevated GWT. Provided that the thermo-physical and hydrogeological parameters of the aquifer are known, the heat content Q (kJ) can be calculated by:

where $C_m$ is obtained assuming a fully saturated medium in which volumetric energy contributions from water $C_w$ and from solids $C_s$, are weighted according to the medium porosity $n$. This parameter and the aquifer thickness (required to calculate the volume $V$ ) are listed in Table 2 for the studied cities.

The parameters included in eqn. (1) considerably vary in space. In order to cope with this geological heterogeneity, parameter ranges and a respective mean value were defined based on hydrogeological data from literature (Table 2). For the heat capacity of water a fixed value of $4150 \mathrm{~kJ} \mathrm{~m}^{-3} \mathrm{~K}^{-1}$ was taken [32].

Following Zhu et al. [19], the temperature reduction is set to $\Delta T=2-6 \mathrm{~K}$, with a mean value of $\Delta T=4 \mathrm{~K}$ (oriented at SUHI intensity). The calculated energy content $Q$ is contrasted with the space-heating demand of the individual cities. The latter is estimated by the average living space and the average annual unit heating demand of $50 \mathrm{kWh} \mathrm{m}^{-2}$ [19]. The average living space areas are taken from Timm [37] for different regions in Germany as follows: Berlin $40 \mathrm{~m}^2$, Munich $44 \mathrm{~m}^2$, Cologne $42 \mathrm{~m}^2$ and Karlsruhe $43 \mathrm{~m}^2$.

The interpolated GWT fields for each city are shown Fig. 2. They exhibit a common pattern in SUHIs with closed isotherms and generally maximum temperatures around the city cen- tres. The observed SUHI intensities decrease towards suburban or rural areas, where anthropogenic affectation is less intense. In these areas, the shallow GWT ranges between 8°C and 11°C in the studied cities mirroring the annual mean SAT indicated in Table 1. The relative large-scale thermal anomalies observed in the city downtowns are likely associated with long-term and extensive changes in land use. For the largest considered cities, the tem- peratures vary between 13°C–18°C. In Karlsruhe, however, the highest GWT is observed in an industrial area (western part of the city) where important reinjections of thermal waste water are operated [38].

Another common feature in SUHIs is their spatial heterogeneity which is partially caused by the spatial density and depth of the temperature measurements. This is more evident in Munich where the dense network of observation wells allows to identify hot spots with tem- perature of up to 20°C nearby building cellars and at the same time, low temperatures close to the background values in green spaces in the inner-city [27]. This heterogeneity is also enhanced by the variety of heat sources within a typical urban area including subway systems, district heating networks, shallow geothermal systems, heat losses from basements in build- ings and increased surface and air temperatures [1–2] and [38].

With the parameters ranges listed in Table 2, the theoretical heat content Q can be esti- mated for each city. The corresponding minimum, maximum and mean values are shown in Table 3. Given the similarity in hydro-physical parameters and groundwater warming between the different cities, the estimated potential only varies within one order of mag- nitude. For comparison, Table 3 also includes the area-averaged space-heating demand. It is shown that the underground stored energy could cover this demand in the individual cities for at least 1.3–5.6 years while more optimistic estimations yield capacities of up to 17.1–31.4 years. These assessments can be judged as conservative since only the actual heat content in the urban aquifer is considered excluding the continuous power input from the associated anthropogenic sources. In fact, provided an appropriate exploitation technology, this continuous replenishment of thermal energy can supply part of the heating demand in a sustainable way. For the city of Karlsruhe for instance, by tapping only the annual anthropogenic heat loss, about 20%–30% of the annual space-heating demand could be covered sustainably [39]. However, these estimations of the geothermal potential are based on a lumped parameterization, which neglects the high variability of heat sources and subsurface hydrogeological conditions. These factors together with techno- logical issues, will ultimately determine the site-specific heat gain from the urban aquifers.

The presented study cases exhibit the increased temperature in the shallow urban subsur- face of several German cities. The thermal anomalies have a strong spatial heterogeneity with observed persistent values of up to 16°C. Local temperatures however can reach up to 20°C near dominant heat sources yielding groundwater warming by more than 6 K in comparison to background conditions. Several anthropogenic sources such as heat losses from buildings and other infrastructure, direct geothermal use of aquifers and changes in land cover drive the long-term and multi-scale thermal alteration in the studied urban aquifers.

The accumulated heat content is substantial and urban aquifers can act as attractive and easily accessible reservoirs and also storage for thermal energy and waste heat. The presented estima- tion of the individual space-heating capacities reveals that the theoretical geothermal potential could fulfil the residential heating demand in all studied cities for at least several years, and assuming more optimistic values even for decades. Considering the permanent anthropogenic heat input from the urban structures into the shallow aquifer, the heating demand of the cities could also be partly satisfied for a longer period. However, to meet the requirements for such a sustainable underground management, a more detailed examination of the transient behaviour of these subsurface urban heat islands with geothermal usage is still needed.