## 1 Introduction  
According to the FAO (Food and Agriculture Organization of the United Nations), increasing agricultural production with the aim of meeting future food demand is one of the most important challenges for the 21 st century [1]. Global human population is projected to reach--with a certainty of 95%--between 9.4 and 10.1 billion people in 2050, and between 9.4 and 12.7 billion people in 2100 [2]. This significant increase of global human population will cause a rise in food demand [3]. As a consequence, agricultural production should increase by about 70% over 2005-2007 levels [1], maintaining high quality products but using the minimum amount of resources [3]. This increase will also regard livestock products. Before 2050, the demands of meat and dairy products are expected to increase by about 70 and 60%, respectively, in comparison with the levels at the beginning of this century [4]. Currently, food from animal sources represents 18% of the kilocalories consumed worldwide and it provides various micro-nutrients, such as vitamin A and iron, that are difficult to obtain in adequate quantities exclusively from plant-source food [5]. In addition, animal proteins are considered of high quality since their amino acid pattern is closer to the human body requirements [6]. Animal proteins are also more easily digestible and available for humans [7]. For these reasons, 25% of the total amount of proteins consumed worldwide are derived from animals [5] that have been farmed by humanity for this specific purpose for millennia. As agriculture developed, it became evident that controlling the thermal environment in which animals are farmed contributes materially to their production [8]. Thus, animals started being farmed in confined systems, i.e., livestock houses, to providing them an environment of controlled temperature and humidity. In this way, the environmental extremes--such as very low/high air temperatures--that are typical of outdoor weather conditions are considerably reduced, increasing productivity [9]. In addition, in the last decades, livestock houses have been subjected to a continuous engineering process that has strongly transformed them, by introducing new technologies. Currently, many livestock houses are fully-mechanically controlled and equipment for supplemental heating and cooling are usually adopted. Supplemental heating systems can be classified into localized heating and space heating systems. Localized heating systems create temperature variations in the zones where animals are reared, while space heating systems create a more uniform thermal environment. The most adopted technologies for localized heating are infrared lamps or infrared gas catalytic radiant heaters, which emit 70% of their heat by radiation. In contrast, the most adopted solutions for space heating are technologies based on convection systems, such as air heaters [10]. Inside livestock houses, free cooling systems are often present. The needed ventilation air flow rate is provided by fans, whose design and energy efficiency have considerably changed during the last decades. In modern livestock houses, fixed angular speed fans equipped with brushless motors are adopted. These motors are characterized by less maintenance, reduced overheating, and increased safety regarding fire risk, compared to brushed motors. A new technology regarding ventilation that is gaining ground in livestock houses is variable angular speed fans [11] that enhance a refined control of the ventilation flow rate with reduced energy consumption. The engineering process that is transforming livestock houses is also affecting control and monitoring systems. In fact, the continuous innovation in information and communication technologies is spreading the adoption of precision livestock farming (PLF) technologies, based, for example, on wearable Internet of Things [12] and new artificial intelligence algorithms [13].