# IoT Sensor Challenges for Geothermal Energy Installations Monitoring: A Survey
###### Abstract  
Geothermal energy installations are becoming increasingly common in new city developments and renovations. With a broad range of technological applications and improvements in this field, the demand for suitable monitoring technologies and control processes for geothermal energy installations is also growing. This article identifies opportunities for the future development and deployment of IoT sensors applied to geothermal energy installations. The first part of the survey describes the technologies and applications of various sensor types. Sensors that monitor temperature, flow rate and other mechanical parameters are presented with a technological background and their potential applications. The second part of the article surveys Internet-of-Things (IoT), communication technology and cloud solutions applicable to geothermal energy monitoring, with a focus on IoT node designs, data transmission technologies and cloud services. Energy harvesting technologies and edge computing methods are also reviewed. The survey concludes with a discussion of research challenges and an outline of new areas of application for monitoring geothermal installations and innovating technologies to produce IoT sensor solutions. geothermal energy; geothermal sensors; energy harvesting; edge computing; IoT 2023235772023357235  
are the fluid temperature, ambient rock environment temperature, volumetric flow rate, and physical characteristics of underground structures; minor monitoring parameters include scale formation [3] and slope movement [4]. Monitoring the specific characteristics and behaviors of underground structures assists in detecting problems and failures that prevent optimal system operation and energy management. The monitoring focus for an underground geothermal structure is dependent on the geothermal application area, examples of which include heat pumps [5], mines or wells [6], pipelines [7], reservoirs [8] and borehole exchangers [9]. Monitoring aims to determine or continuously update the status of key parameters necessary for the proper operation of an underground geothermal structure. To monitor these parameters, devices such as those connected to the Internet of Things (IoT) can be used. These devices must be able to monitor several variables and wirelessly transmit measured values to the cloud [10]. IoT technology is widely used and can also be applied to many areas of monitoring underground geothermal structures. This type of technology is especially useful in locations where wireless communications are required. These areas are generally difficult or impossible to connect to a power grid, and therefore, alternative power sources such as batteries or energy harvesting methods are necessary [11]. In this case, monitoring devices must consume low amounts of energy, and suitable technologies with powering constraints, for example, low-power wide-area network (LPWAN) technology, are therefore required. LPWAN technology was designed for reliable wireless data transmission with minimal power consumption. The commonly used LPWAN technologies include LoRaWAN, Sigfox and NB-IoT [12]. Table 1 lists recently published reviews of monitoring performed in specific underground geothermal structures. The listed studies examined a variety of systems that monitor different parameters, are deployed in specific types of geothermal structures and employ IoT technology. In Refs. [13; 14; 15], the authors discussed temperature monitoring and the temperature measurement methods, sensors and tools suitable for geothermal applications, such as vertical borehole heat exchangers, downholes and power plant devices. In Ref. [1; 2], the authors discussed flow rate sensing techniques, providing comprehensive descriptions and comparisons of the two-phase flow monitoring methods applied in geothermal pipelines and geothermal wells. In addition to monitoring temperature and flow parameters, geothermal applications also use sensors to monitor mechanical parameters, such as scale formation [3] and slope movement [4]. In Ref.