## 3 System Configurations and Modelling  
### SUS and SDS Configurations  
The proposed system configurations are intended to recover waste heat at 60 to 70 \({}^{\circ}\)C or slightly lower. This temperature level is known to be abundant and in excess in industrial facilities and would not be so easily recovered inside the same industrial process due to its low temperature. In the SUS,a water-to-water heat exchanger is used in combination with the AHT to build an \"absorption heat exchanger\" in such a way that the inlet return water from the DH network is preheated before it enters the absorber of the AHT to upgrade the water temperature to its final target, as shown in Figure 1b. If the return temperature of the transport circuit is at a temperature that is low enough (Figure 2b,d), it is used as cooling media at the condenser of the AHT. If not, a cooling water tower has to be included as is the case shown in Figure 2c. Six system configurations (Figure 5) are studied to recover low-temperature waste heat, upgrade it, transport this heat at a higher temperature, and deliver it efficiently at a lower temperature to the final users. The used nomenclature to name each configuration is: \"Absorption unit for SUS\" + \"absorption unit for SDS\"/\"H or H&C\" depending on only heating or heating and cooling is delivered to the final user. In this figure are shown some typically expected temperatures for each input and output stream. Configurations differ from each other by the type of absorption technology used and by: (1) The temperature of the hot water in the heat transport network, (2) the capability (or not) to produce chilled water to supply cooling using the distribution network, and (3) the need (or not) of a cooling tower in the step-up thermal station and the step-down thermal station. Figure 5: Analyzed system configurations based on absorption technology to recover low-grade waste heat. (Color lines: Orange—Waste heat, Red—Heat supply, Dark blue—Chilled water, Light blue—Cooling water). Table 1 summarizes the main features of each configuration shown in Figure 5. Configuration (a) (SE + SE/H) can deliver only heating while all the others can supply heating and cooling. Configuration (a) and (d) do not require the use of a cooling tower. System configurations that produce hot water at 40 to 50 \({}^{\circ}\)C are suitable for terminal units, such as fan-coils or a combination of radiant floor and fan-coils [35]. However, the configuration DA + DL/H&C (configuration (f)) produces hot water at 60 \({}^{\circ}\)C, so this temperature might be suitable for heat radiators. For those applications requiring temperatures higher than 40 \({}^{\circ}\)C, the double lift SDS is required. Except configuration (a), all the others are designed to produce both heating and cooling. Next, each configuration is described in more detail. * _SE + SE/H configuration_ (Figure 5a): This configuration was proposed by [PERSON] and [PERSON] [6]. It consists of an SEAHT in the step-up thermal station (SUS) and a single-effect absorption machine in the step-down thermal substation (SDS). This configuration is designed to be coupled with a distribution district heating network at 40 \({}^{\circ}\)C using the heat released at the absorber and condenser of the SDS. The evaporator of the SDS unit is used to increase the temperature difference of the heat transport circuit and consequently reduce its required flowrate. * _SE + SE/H&C configuration_ (Figure 5b): It is designed to provide not only heating as in the previous configuration but also cooling. The AHT of the step-up thermal station is a single-effect AHT. Both the absorber and the condenser of the AHP and an additional heat exchanger are used to produce hot water at 40 \({}^{\circ}\)C. A single-effect AHP is used in the step-down thermal substation using its evaporator to produce chilled water at 6 \({}^{\circ}\)C. Thus, a cooling tower is necessary in the step-up thermal station, because the return temperature could be too high to be used in the condenser of the AHT.