## 2 Description of the Heat Transport and Distribution Networks Based on Absorption Systems
### Absorption Heat Transformers (AHT) in Step-Up Thermal Stations (SUS)  
The step-up thermal station (SUS) consists of an absorption heat transformer (AHT) coupled with a water-to-water heat exchanger as shown in Figure 1b. The role of the step-up thermal station is to upgrade the temperature of the waste heat. The temperature output of the AHT system is at a higher temperature than that of the waste heat source. In contrast to the conventional waste heat recovery system configuration (Figure 4a), where the temperature of the primary circuit water is lower (65 \({}^{\circ}\)C) than that of the waste heat (70 \({}^{\circ}\)C). The main performance parameter is the COP defined as the ratio between the heat delivered at the absorber and that supplied at the evaporator and generator. Another fundamental operation parameter for AHTs is the gross temperature lift (GTL) defined as the temperature difference between the upgraded and the waste heat input temperature. There are many papers reviewing the absorption heat transformer technology [21,22] and studying their design and application in many different cases: Production of steam ([PERSON] et al. [23]), integration in a CO\({}_{2}\) capture system ([PERSON] et al. [24]), integration with a seawater desalination system ([PERSON] et al. [25]), etc. This study does not intend to present an exhaustive review on heat transformers but to introduce the most relevant aspects in relation with its application working as an \"absorption heat exchanger\" in a step-up station. Two of the most studied AHT configurations are considered here: A single-effect AHT (SEAHT) and a double-absorption heat transformer (DAHT) both working with the H\({}_{2}\)O/LiBr pair, although other pairs have been analyzed in the literature [26] and other configurations, such as the double-lift AHT, are under development [27]. The reason for this choice is that this working pair and the mentioned two AHT configurations are those that offer a contrasted performance for the upgrade of low temperature waste heat at a reasonable performance. Figure 3a,b show the Duhring scheme of the considered SEAHT and DAHT, respectively. The SEAHT is a tri-thermal absorption system (Figure 3a): High temperature in the absorber, a medium temperature in the evaporator and generator or desorber, and a low temperature at the condenser and working at two pressure levels: High pressure in the absorber and evaporator and low pressure in the condenser/generator. At low pressure, water vapor is driven out of the solution in the desorber (Figure 3a, stream 7) by heating this component, usually with waste heat. The desorbed vapor flows to the condenser, where it changes its phase and heat rejection occurs. The liquid refrigerant is then pumped up to the evaporator at high pressure (stream 8). Waste heat is applied at the evaporator, as was done at the desorber, in this case to produce a liquid-to-vapor phase change (stream 10). The vapor then flows to the absorber, where it recombines with the salt solution coming from the desorber (stream 4) to complete the cycle. The absorption process is accompanied by a thermal energy release that is the output product of this cycle. By transferring energy out at high temperatures (stream 12), the AHT cycle upgrades the temperature of a waste heat stream (streams 13 and 15) to a temperature level at which it is more useful. The pump work requirements are larger than in other  
Figure 4: Scheme in a Duhring chart of (**a**) the single-effect (SEAHP) absorption heat pump, (**b**) the double-lift absorption heat pump (DLAHP), and (**c**) the double-effect absorption heat pump (DEAHP). absorption heat pumps but usually still low in comparison with the thermal energy input requirements. For favorable applications, single-effect AHT (SEAHT) using water/LiBr offers a COP close to 0.5 and are limited to obtaining a gross temperature lift (GTL) that is usually not higher than 50 \({}^{\circ}\)C ([PERSON] and [PERSON] [28]).