## 1 Introduction
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Project Name** & **Reference Number** & **Timespan** & **Short Summary** \\ \hline BestHeatNet & 03 ET1626A & 12.2018–10.2023 & \begin{tabular}{c} Practical development and testing of a \\ low-temperature DHC network with \\ variable renewable energy sources \\ \end{tabular} \\ \hline Envisage Plus & 03 ET1465A & 01.2017–03.2021 & \begin{tabular}{c} Low-temperature DHC network with \\ agrothermal collector \\ \end{tabular} \\ \hline ErdEis & 03 ET1382A & 09.2016–03.2019 & \begin{tabular}{c} Utilization of latent heat and geothermal \\ heat storage for the districts \\ \end{tabular} \\ \hline ErdEisII & 03 ET1634A & 03.2019–02.2022 & \begin{tabular}{c} Low-temperature DHC network with a \\ frozen soil storage system \\ \end{tabular} \\ \hline Grubenwasser in & 0327400Z & 10.2011–19.2016 & \begin{tabular}{c} Use of heat from mine water for a \\ low-temperature DHC network \\ \end{tabular} \\ \hline Kassel Zum & 03 ET1336A & 11.2015–08.2019 & \begin{tabular}{c} New build district in Kassel (Germany) \\ with solar thermal and vertical geothermal \\ systems as the heat supply \\ \end{tabular} \\ \hline Living Lab Energy & 03 ET1551A & 01.2018–12.2020 & \begin{tabular}{c} Field site to test a self-sufficient area \\ network with renewable energy sources \\ \end{tabular} \\ \hline LowExTra & 03 ET1237A & 07.2014–12.2017 & \begin{tabular}{c} Low-temperature DHC network for the \\ integration of multiple renewable energy \\ sources (solar, thermal, geothermal) \\ \end{tabular} \\ \hline NATAR & 03 ET1425A & 01.2017–09.2020 & \
\begin{tabular}{c} Low-temperature DHC network to provide \\ regulatory power \\ \end{tabular} \\ \hline Neckarpark Stuttgart & 03 ET1156A & 05.2013–04.2020 &
\begin{tabular}{c} Low-temperature DHC network fed by \\ sewage out of the sewer \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of current research projects in Germany developing low-temperature district heating and cooling (DHC) networks with geothermal energy sources. A wider range of different technologies and national differences is addressed by the European funded research project called \"REWARDHeat--Renewable and Waste Heat Recovery for Competitive District Heating and Cooling\", which focuses on demonstrating low-temperature DHC networks and their business models. Twenty-eight partners around Europe use their knowledge of low-temperature DHC to make it even more competitive. The project is coordinated by EURAC (the European academy of Bozen / Bolzano). The knowledge gained from the Plus-Energy settlement \"Wustenrot\" will be also considered in the project. Decentralized brine/water heat pumps complement low-temperature DHC networks ideally. Low-temperature DHC networks and heat pump systems can be used for sector coupling--a very promising approach [9]. This is relevant, as the current situation in Germany points toward that in 2018, the share of RES (renewable energy sources) within the entire consumed electricity was at approximately 40% [9], but the share of RES within the consumed heat was approximately 14% [10]. The transfer of excess electricity can serve as heating (power-to-heat), but it also allows direct grid-stabilizing measures. For instance, it can counteract the increasing challenge that the share of RES in the (German) electricity grid poses due to its high volatility. Power-to-heat applications based on heat pumps show a promising and largely untapped potential as they could provide a DR (demand response) potential of up to 20 GW\({}_{\text{el}}\) in the year 2030 [11]. Together with DR, DSM (demand-side management) can play an important role to ensure future power grid stability [12]. In addition, technologies such as battery storage, regulatory measures such as strategic stability reserve plants, or innovative incentives in the private sector such as flexible end-user electricity tariffs can provide solutions for this demanding task [13; 14].