## 1 Introduction and Objectives
This configuration is very useful to use heat at high temperatures from the heat transport network and provide heat to low temperature distribution networks. This absorption system provides a temperature drop much higher than that obtained with a conventional heat exchange [4]. Therefore, heat is used more effectively. An AHE consists of an absorption heat pump and a heat exchanger connected as shown in Figure 1a. This type of system has been referred to in the literature as \"first type AHE\". The external thermal energy at high temperatures (>80 \({}^{\circ}\)C) drives the absorption system and later is used to provide useful heat at a lower temperature (40 \({}^{\circ}\)C). After leaving the heat exchanger, its temperature is further reduced at the evaporator before leaving the system. In this way, a much lower return temperature is achieved compared with a simple heat exchanger. The heat rejected at the condenser and absorber is used to provide additional useful heating (Figure 1). Some applications for this AHE have already been analyzed in the literature. [PERSON] et al. [5] reviewed the potential use of industrial waste heat in DHC networks and developed a DH system with a combined heat and power (CHP) with waste heat recovery. Absorption systems are considered to increase the heating capacity of the CHF system by recovering waste heat and using it locally in a DHC network. [PERSON] and [PERSON] [6; 7; 8] developed a system for long distance heat transportation based on AHEs. The authors defined two types of AHE: First type and second type, as mentioned above. The first type is an AHE identical as described in the references above (Figure 1a) and it is located near the end users. These step-down thermal substations receive high temperature water from a so-called \"second type AHE\" that consists of an \"absorption heat transformer\" (AHT) combined with a conventional heat exchanger (Figure 1b) and is located at the industry plant side (waste heat source). The aim of this second type AHE is to upgrade the temperature of the return hot water of the district heating network (20 \({}^{\circ}\)C) up to a temperature (85 \({}^{\circ}\)C) higher than that of the input waste heat (70 \({}^{\circ}\)C). This configuration increases the hot water temperature difference between the supply and return with respect to conventional DH networks. To achieve the desired performance, large temperature differences between the input and output streams of the cycle components are desirable. Thus, conventional heat pumps cannot be used as AHE and need to be redesigned. This problem could be solved with multi-section components and a vertical configuration to obtain compactness. One of these small capacity AHE was tested satisfactorily in an office building in the city of Chifeng (China) according to [PERSON] et al. [9]. The authors claim that this developed system is able to replace plate heat exchangers in building substations. In all the mentioned projects, simple absorption systems are considered only to produce heating. [PERSON] et al. [10] presented a model and an experimental study of an AHE with a heat exchange capacity of 6 MW. Also, they highlighted the advantages of AHE over a typical plate heat exchanger (PHE): The heating capacity of the heating system is enlarged without changing the flow rate due to a lower return water temperature of the primary side of the DH system with respect to the case using a PHE. The concept of AHE has been used not only to efficiently recover waste heat but proposed also to be used in DH networks in combination with natural gas boilers by [PERSON] et al. [11; 12]. The concept of cogeneration with absorption heat exchange (Co-ah cycle) is an interesting configuration proposed in the literature, such as in [PERSON] et al. [4] and [PERSON] et al. [13]. This concept is similar to that of heat transport proposed by [PERSON] and [PERSON] [6] but replacing the second type AHE by a steam Rankine cycle and an absorption heat pump.