Introduction

The thermal use of shallow subsurface systems is increasingly discussed, promoted, and implemented as one of many promising measures to reduce fossil fuel use. The energy extracted from such systems is referred to as shallow geothermal energy or low-enthalpy energy. These systems may consist of groundwater abstraction by pumping wells, energy generation, or abstraction with the support of heat pumps such as groundwater heat pump (GWHP) systems, and the reinjection of the cooled or warmed water into the aquifer (open system, Figure 1.1b). On the other hand, thermal use may also include pumping of groundwater for cooling purposes and reinjection of warm water. Both systems may even be seasonally combined. A related topic is heat storage (Dinçer and Rosen 2011). Since temperature in undisturbed shallow aquifers is around annual mean air temperature at a given location outside the range of influence of infiltrating rivers or lakes, energy production by ground-source heat pumps and GWHPs is attractive also at low air temperatures in winter. Thermal use of shallow underground (saturated or unsaturated zone) can also be accomplished by low-enthalpy geothermal heat exchanger systems combined again with heat pumps (closed systems or ground source heat pump systems [GSHP], Figure 1.1a). With respect to the thermal management of underground and groundwater systems, a series of questions arise in this context. What is the tolerable temperature increase or reduction of groundwater? What is the long-term usable energy potential of an aquifer? What is the long-term sustainability of the thermal use of groundwater? What are management problems with respect to thermal use? What are the geotechnical risks related with the thermal use of the underground? What harm comes from the thermal use of groundwater? Is groundwater quality and/or groundwater ecology affected? Is there a competition between drinking water production and thermal use? How much does a city heat up shallow groundwater? How can the temperature development be assessed? How is the heat balance affected? In order to answer these questions and to design thermal systems, it is necessary to provide methods to compute their effects on the development of temperature in the

underground and, in particular, the groundwater. Accordingly, the theoretical fundamentals of heat transport in groundwater systems are recalled, and the essential thermal properties and parameters are reviewed and discussed. Hydrogeological-thermal investigations have to be combined with modeling. Therefore, a series of mathematical tools and simulation models based on analytical and numerical solutions are presented and discussed. Case studies are shown for locations in Austria, Germany, and Switzerland. They concern the urban thermal energy use as well as heat storage and cooling.