## 1 Introduction
The results demonstrated that some parameters must be taken into account to properly optimize the performance, such as the amount of solution inside the system, which must be carefully managed depending on the user's needs. Despite the interesting features above reported, absorption heat transformers suffer limitations related to the cost and harmfulness of the novel working pairs under development [11], as well as difficulties in managing standard working pairs, like LiBr/water, under typical low-temperature waste heat operating conditions, due to corrosion and crystallization issues [14]. A different option is represented by the adsorption heat transformer (AdHT) technology, in which the refrigerant (e.g., water, alcohol, ammonia) reacts with a solid sorbent (e.g., silica gel, zeolite, composite sorbent). The main difference compared to the thermochemical approach is that the involved reaction is commonly a physical one, characterized by weak interactions (e.g., van der Waals) between the working fluid and the solid. This guarantees much more stable processes as well as the possibility of effectively operate the system even at very low waste heat temperature (i.e., below 60 \({}^{\circ}\)C). For industrial waste heat applications, the AdHT approach was firstly proposed using a zeolite/water vapor working pair [15], showing promising features from the thermodynamic point of view. More recently, a detailed numerical dynamic simulation was setup to investigate the achievable specific power of an AdHT using the working pair AQSOA Z02/water for industrial waste heat ranging between 90 and 100 \({}^{\circ}\)C [8]. The model assumed an ideal condenser and evaporator and neglected any thermal losses to the environment during the operation, but the results showed the possibility of achieving a promising specific heating power in the range of 400-600 W/kg with an exergetic COP between 0.55 and 0.62. It is then needed to validate these results by means of lab-scale prototypes. To the best of the authors' knowledge, so far, no experimental results were reported in the literature about AdHT technology for industrial waste heat applications. Recently, the AdHT cycle was proposed to exploit extremely low ambient temperatures under specific climatic conditions, for instance in Siberia (Russia), to upgrade waste heat in residential applications for space heating purposes [16]. The authors performed a deep development activity, starting from thermodynamic analysis [16; 17] up to the experimental characterization of small-scale dynamics of the process [18]. They were then able to design, realize, and test a lab-scale prototype, based on the working pair silica gel-LiCl/methanol, which was tested under controlled operating conditions [19]. The obtained results demonstrated the ability of the technology to achieve up to 25 K of temperature lift, using 20 to 30 \({}^{\circ}\)C of waste heat and \(-\)20 \({}^{\circ}\)C of ambient temperature. An even more attractive result is the achievable specific energy, which reached up to 620 kJ/kg at a specific power ranging between 6.0 and 10.8 kW/kg. Based on the above reported literature, it is evident how the AdHT technology could be considered as an interesting option for waste heat upgrade, able to overcome most of the limitations of other concurrent technologies. Nevertheless, the literature still lacks a detailed thermodynamic investigation of possible working pairs suitable for the expected working operating conditions. Accordingly, this paper focuses on the description of AdHT thermodynamics of two different AdHT architectures. Subsequently, a comparative analysis of the achievable performance, in terms of COP and specific energy, of several working pairs available in the literature is conducted. It can be considered as a screening tool for defining the most suitable working pairs to be used for the design and operation of AdHT for different industrial applications.