## 1 Introduction  
Combined heat and power (CHP) generation presents several advantages in terms of lower costs, emissions, fuel consumption, and higher decentralised generation share as compared to conventional separate electric and thermal production [1; 2; 3]. During the last few years, the research community and the industry have been focusing increased attention on renewable sources, sustainability and global efficiency [4; 5; 6]. In this framework, geothermal energy represents an interesting resource and, due to its not intermittent availability, an attractive solution for lowering fossil-fuel dependence and their environmental impact [7; 8; 9]. Furthermore, geothermal sources are slightly dependent on the season, climate and local conditions, presenting the advantages of different viable technologies [10] with direct (e.g., heat production for industrial and domestic users) and indirect applications (i.e., electricity generation at large, medium and small scales). In geothermal applications, a geo-fluid transports the heat stored underground to the users through wells drilled into the geothermal reservoirs (up to 200-300 \({}^{\circ}\)C) [10]. The installed capacity of geothermal power has been increasing rapidly in the last few years [11] (from 6.8 GW in 1995 to an estimate of 21.4 GW for 2020 [12]). On the other hand, the direct use of geothermal sources is one of the first, most flexible and common methods to exploit geothermal potential [13]. A worldwide review of published data shows that the average electric conversion efficiency of geothermal systems is about 12%, while the highest values reach 21% [14]. For this purpose, it is worthy noting that several factors influence the system efficiency, including the conversion technology (i.e., dry steam, single, double and triple flash, binary and hybrid systems), the desired output (e.g., electricity production, combined heat and power generation, polygeneration), the size (large, medium, small, micro-scale), the ambient conditions, the temperature, the flow rate and the physic-chemical properties of the geothermal source [14]. In particular, several technologies can be adopted for geothermal power extraction, which depend on the thermodynamic state of the resource. At high temperatures, dry steam systems are usually adopted [12]. They represent a well-established technology and account for 23% of the global geothermal capacity. Furthermore, single and double flash steam power systems are the most diffused technology (42% for single flash and 19% for double flash) [12]. Finally, binary cycles represent the most recent solution that accounts for the remaining 16% of the total installed capacity in operation [15]. In this context, binary organic Ranking cycles (ORCs) play a crucial role [16; 17; 18; 10]. ORCs represent an attractive and flexible technology for small applications owing to their capability to convert low enthalpy heat sources (i.e., waste heat [19; 20; 21; 22; 23] and renewable sources [24; 25; 26]) to electricity. Specifically, lower maintenance costs, better partial load performance, faster start-up and stop procedures, and higher safety and lifetime (up to 30 years) with respect to conventional installations are guaranteed [7; 27]. Furthermore, a geothermal ORC does not need an intermediate thermal oil circuit, as usually present in other ORC applications [28; 29] with a significant advantage in terms of performance, cost and architecture complexity [30]. The electric efficiency of the system can reach values up to 20% for the sole power generation [31]. Specifically, several factors affect the performance of ORC units: the thermal level of the external source, the working fluid, as well as the type of expander and operating conditions; therefore, the system architecture can vary significantly with the specific application and a standard setup is not completely established [11; 32].