## 1 Introduction
However, to the best of the author's knowledge, there are no research studies which focus on the economic benefit derived from considering a BTS as an MG in terms of optimal energy flow control. Needless to say, achieving energy efficiency needs to go hand in hand with providing uninterruptible and reliable power supply to the critical loads for operation. In this regard, BTSs powered by only solar energy pose a challenge of dimensioning the PV system and the Battery Energy Storage System (BESS) [22]. On the other hand, in grid-connected BTSs, when a power outage occurs, the communication networks should remain operative. Moreover, a BTS based on renewable energy without an operational strategy may increase the OpEx, thereby delaying the return on the initial investment and making this alternative less attractive for network operators. Design of an operational strategy must consider the BTS energy profile in terms of the energy consumed by each component, and thus, the energy allocation per function within the BTS. This topic has been properly researched in [23; 24], showing that the radio equipment and amplifiers account for more than 60% of the power consumed; 11% is due to the DC power system, while the cooling equipment is responsible for 25% of the power consumed. Therefore, an optimal design of the radio equipment and the cooling system can reduce the power drawn by the BTS. Likewise, it is very important to develop strategies directed towards BTS Energy Savings, which can be applied to both the radio equipment, e.g., radio sleep mode [25], and to the cooling, e.g., passive cooling, advanced climate control [26] and power electronics. Additionally, the forecasting of the available energy from renewables should be considered due to their stochastic nature, and the electricity price, battery health and lifespan, charging and discharging cycles for the BESS etc. are also required. Hence, an intelligent energy management of the BTS components has to be adopted. To develop the above-mentioned operational strategy, a microgrid (MG) paradigm can be used to model a BTS, consisting of a low-voltage network (DC and/or AC) that integrates renewable energy sources (RES), a BESS and controllable loads connected to the main grid. To provide the MG-based BTS with intelligent energy management, a local controller and an Energy Management System (EMS) should be implemented. By supervising and coordinating the operation of the sources of renewable energy, the BESS, the controllable loads (critical and non-critical) and the network devices, the BTS performance can be improved. The operation of an MG-based BTS should be based on a sensing system for determining the value of electrical and environmental parameters. In [27], a comprehensive review of several PV sensing systems is presented. Overall, these systems have either a high cost [28] or reveal limitations regarding the number and type of the parameters to be measured [29]. The practical application of monitoring systems poses many challenges, such as the harsh environmental conditions the different components must withstand and the degradation of the electrical components e.g., connector corrosion, resource restrictions with respect to the energy efficiency, real-time constraints and low-cost implementations, to name but a few [27]. To successfully address these challenges, the monitoring system proposed in this paper is based on up-to-date technologies which guarantee real-time operating conditions, low energy consumption, protection against harsh environmental conditions, firmware OTA updating, local and cloud-based data storage, high computational capacity, accurate measurements and support for several communication protocols, e.g., I2C, SPI, Modbus and MQTT, implementing a low-cost approach. Another strength of this approach is that the system not only monitors the BTS variables but also is able to control the energy flow among the different elements comprising the BTS. This is carried out by an EMS algorithm developed for this application.