Definition of harmonized workflows
There are different workflows for urban and non-urban areas as some constraints are posed or influenced by the respective environment. For example, due to human interventions and built-up surfaces, groundwater temperatures in urban situations can be significantly higher than those in rural locations. Hazards and conflicts may differ: Subway tunnels exist only in urban areas, whereas active mining is much more likely in non-urban areas. Finally, there are different requirements regarding the output: Urban areas require a higher data resolution.
Nevertheless, the general procedure and the sequence of steps are similar.
First, numerous sources of input data are incorporated to generate a 3D-model of the subsurface: A digital elevation model is fitted with geological data ranging from lithology to rock properties and tectonic faults.
A conflict map is then generated for each of the risk factors identified. Examples for potential conflicts that could evoke additional stipulations or prevent geothermal use altogether are active or historic mining, existing or planned infrastructure, or geological hazards such as faults or swelling evaporites. Each region will create its own specific set of conflict maps to present local constraints and requirements.
For each potential conflict, a traffic light map indicating the status of the conflict is created. The red color denotes that geothermal energy use is not possible or not allowed, green indicates no foreseen problems, and yellow refers to areas of either missing or inconclusive data, e.g. areas with additional stipulations or permitting procedures based on case-by-case assessments.
All traffic light maps of the different conflicts are then combined to obtain a single traffic light map. For each surface location, this map provides a first »yes/no/maybe« indication of a project being permissible or not; of course, the traffic light map does not replace professional surveys and permits.
For closed-loop systems, additional maps show physical parameters quantifying the geothermal potential. In order to generate these geothermal potential maps, the 3D-model is fitted with further data derived from temperature measurements in boreholes, groundwater levels, and thermal conductivities obtained from rock samples or TRT-measurements. From this data, maps for thermal gradient, thermal conductivity and heat extraction capacity can be calculated at various depth levels. These parameters help to estimate the output of a borehole heat exchanger, i.e. the required drilling depth.
Generating geothermal potential maps for open-loop systems is somewhat more complicated. Each aquifer has to be delineated in the 3D-model and split into smaller parts to enable processing of large data volumes. Groundwater temperature data is subjected to statistical analysis, and temperature shift maps for heating, cooling and balanced use are created. These maps are combined and evaluated with respect to legal regulations; the output is a thermal productivity map indicating whether thermal constraints are complied with. Like the traffic light map, the thermal productivity map serves as a first indication whether open-loop systems are possible, and if so, for which use (heating/cooling/both). Adding further data to the 3D-model, such as hydraulic conductivity, net aquifer thickness, and volumetric heat capacity of the aquifer, maps for energy content, hydraulic productivity and thermal capacity can be calculated. These parameters aid in sizing the planned installation.