## 1 Introduction  
Energy conversion processes are associated with the release of waste energy, usually in the form of heat, which is commonly referred to as waste heat and is often dumped to the environment. The amount of waste heat produced at EU level was estimated to ca. 305 TWh/y, with a large portion of it in the temperature range 100-200 \({}^{\circ}\)C [1]. The proper exploitation of these waste energy streams represents a critical task to further increase the energy efficiency of industrial processes, which can have a huge impact from both economic and environmental points of view. Indeed, when the temperature level is sufficiently high (i.e., above 150 \({}^{\circ}\)C), there are several established technologies that can be employed to produce power, heating, and cooling, namely sorption heating and cooling technologies, organic Rankine cycles (ORC), steam, and Kalina cycles [2]. When the waste heat temperature is too low for being efficiently exploited, i.e., below 100 \({}^{\circ}\)C, the only way to make use of it is to upgrade the temperature level, thus re-using the energy in the same industrial process or providing external services, like feeding district heating networks (DHN). Recently, the development of high-temperature industrial heat pumpshas been deeply investigated for this application, showing a high potential [3] and also the possibility of employing low Global Warming Potential (GWP) refrigerants [4], thus also representing an environmentally compatible technology. Nevertheless, the application of high-temperature heat pumps requires the consumption of electricity and still suffers from technical limitations i.e., linked to the compressor efficiency and the lubricant [5], making them not always attractive. A different technology, recently investigated for upgrading low-temperature waste heat, is the one based on a thermally activated processes, exploiting thermochemical [6], liquid absorption [7], or physical adsorption processes [8]. These three technologies are characterized by the same operating principle, in which a working fluid reacts with another component and, exploiting the temperature difference between waste heat source and ambient, performs a working cycle whose final output is the temperature upgrade of the waste heat. The thermochemical approach exploits the chemical reaction occurring between a working fluid (e.g., ammonia, water) as a refrigerant and a solid phase (e.g., salts). This technology is currently under investigation, with activities dedicated to the definition of selection criteria for working pairs [9] as well as design and testing of lab-scale prototypes, showing relevant performance in terms of coefficient of performance (COP) defined as the ratio between the upgraded heat and the heat spent to drive the process, and achievable temperature lift [6]. Nevertheless, the thermochemical approach suffers from some intrinsic limitations, such as working pairs' stability and reduced process kinetics, due to the associated chemical reaction as well as mass transfer resistance inside the reactor [10]. In the case of liquid absorption, a refrigerant is being absorbed/desorbed in or out of a liquid solution (e.g., LiBr/water, water/ammonia). Further, in this case, the development activities focus on innovative working pairs, especially investigating ionic liquid as alternative to standard salt solutions, showing promising thermodynamic performance [11]. Furthermore, large-scale prototypes were recently developed, based on standard LiBr/water working pairs [12], demonstrating the possibility of being properly operated as a double effect heat upgrade, in a waste heat temperature range of 130-160 \({}^{\circ}\)C, with COP up to 0.61, when the ambient temperature was set to 30 \({}^{\circ}\)C and achieving a temperature upgrade of 5-20 K. Other relevant development activities focused on the optimization of the operation strategy of absorption heat transformers, mainly based on numerical models [13].