# IoT Sensor Challenges for Geothermal Energy Installations Monitoring: A Survey
## 4 Research Challenges
### New Potential Applications  
Underground geothermal structures are designed for long-term use; therefore, the proper management of the installation's operational life is required. As mentioned in Section 1, eliminating undesirable states (e.g., thermal imbalance, the absence of turbulent fluid flow and the unforced operation of heat pumps during cooling) during the operation of an underground geothermal structure is beneficial. Long-term monitoring with IoT sensors can facilitate the optimal operation of underground geothermal structures, prolong system endurance and optimize standard operations. Table 10 lists several undesirable states and potential solutions. Thermal imbalance during the operation of an underground geothermal structure is caused by the heterogeneity of the rock environment. To eliminate thermal imbalances, the monitoring of individual boreholes is a potential solution. The absence of turbulent fluid flow can be avoided with higher flow rates. Low flow rates in the geothermal structure can be specifically addressed with turbocollectors (pipes with spiral ribbing), which guarantee that the Reynolds number of the flow is exceeded. The last undesirable state is the operation of the heat pump during cooling. The thermal stress of the pile must be monitored for mechanical property stability (strength and deformation parameters). A change in concrete temperature due to heat transfer (charging and discharging) produces a change in the stresses experienced by the concrete in the pile. This change in stress consequently produces volumetric changes in the concrete. In critical cases, the brittle failure of the concrete occurs, and the bearing capacity of the pile as a whole is reduced. Another benefit of the long-term monitoring of currently deployed geothermal installations is the continuous data collection of operational parameters. Large datasets describing geothermal installation behavior during regular operation and also non-standard conditions can be created. IoT technologies allow the extensive monitoring of existing structures, and therefore, the knowledge base will contain new and important information that is useful for design work for future underground geothermal structures. Table 11 lists parameters whose monitoring could aid in producing improved designs for future applications. Utilizing measurement and monitoring techniques for borehole properties, particularly temperature and flow rate, presents an opportunity to enhance future applications. Flow rate monitoring could aid in techno-economic optimization through the selection of suitable pipe diameters and optimal energy consumption models. Temperature monitoring could be applied to optimize designs for the well depth (the length of the borehole). \begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt}} \hline \hline
**Undesirable State** & **Solution** \\ \hline Thermal imbalance & Monitoring of individual boreholes \\ Absence of turbulent fluid flow & Higher flow rate or pipes with spiral ribbing (to address low flow rates) \\ Unforced operation of the heat pump during cooling & Heat pump not operational during cooling \\ Critical stress change & Temperature monitoring of critical limits with adequate reduction of flow rate \\ Volumetric change in concrete in pile & Application of pressure cell monitoring at the head of the pile between the pile and concrete slab \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Undesirable states and potential solutions. Many underground geothermal structure walls do not permit the use of sensor technologies due to wired sensor limitations or the absence of power grid connections. Such installations may include sensors that are installed during the construction process and are subsequently inaccessible or insular systems where a power supply connection is difficult to build. Energy harvesting technologies have excellent potential as solutions for such cases through their ability to generate electricity for sensor operation directly from the ambient environment.