## 1 Introduction and Objectives
In this Co-ah configuration, the steam extracted from the steam turbine drives the absorption heat pump. The steam at the lowest pressure (condensing stage of the steam turbine) is used as a heat source at the evaporator of the heat pump. Thus, the heat for the DH network is provided both by the condensing stage of the steam plant and the heat rejected at the absorber and condenser of the heat pump. With respect to the operation of AHEs, the required input and output temperatures of an AHE can change during a given heating period. For this situation, [PERSON] et al. [14] studied the operating conditions to ensure an optimal performance. [PERSON] and [PERSON] [6] used different indicators defined to analyze the benefits of this kind of absorption system for long distance heat transport. Also, AHEs have been optimized using the so called \"entransy\" concept to properly measure the irreversibility of the heat transfer processes [15] as an alternative to the exergy method, which is more focused on the measurement of the heat-work conversion. In summary, the strategy mentioned in the literature above consists on the upgrade of low temperature waste heat (e.g., 60 \({}^{\circ}\)C) to a higher temperature (e.g., 90 \({}^{\circ}\)C) in order to be transported through a network to a final user or multiple users using a distribution network. An example is given in [PERSON] and [PERSON] [6], where a real long-distance transport system of 37.8 km is mentioned. At the user side, an AHE will efficiently provide low temperature heat (e.g., 45 \({}^{\circ}\)C) and at the same time, producing a high temperature difference between supply and return temperatures in the DH network  
Figure 1: Example diagram of an “absorption heat exchanger” (AHE) known as (**a**) first type AHE and as (**b**) second type AHE, both introduced by [PERSON] and [PERSON] [6]. as additional effect. This configuration is an alternative or complement to the shorter distance low temperature DH networks for smart energy grids ([PERSON] et al. [16]) mainly designed for the distribution of heat instead of its transportation at long distances. The current trend in DHC networks is to reduce their temperature, but in our case, we aimed to introduce absorption systems to effectively recover industrial waste heat at low temperatures. Therefore, the temperature for the transport of heat is set by the driven temperature of the absorption heat pump. Anyway, this temperature is not higher than the current state-of-the-art DHC networks (80-90 \({}^{\circ}\)C) and additionally, the studied configurations allows for a reduced mass flow rate circulation with respect to the mentioned conventional DHC networks due to its much lower return temperatures. Thus, the studied system will have a better performance than current state-of-the-art DHC networks, benefiting from the recovery of low temperature industrial waste heat. The purpose of the presented short review is not to provide a full state-of-the-art on district heating and cooling networks, for which there are many reviews in the literature [17; 18], but to concentrate on the integration of absorption systems to boost the efficiency and profitability of these networks. A common point of the existing literature is that all efforts are concentrated in the field of heating applications. Only a few references explore the possibility of using a small capacity absorption heat pump to provide heating and cooling depending on the corresponding season ([PERSON] et al. [19]). That paper also addressed the potential of absorption systems to reduce unbalanced heat demand between the winter and summer seasons for district heating networks. However, many buildings suffer simultaneous thermal inversion; therefore, space heating and cooling are both required simultaneously. Another important weak point of previous studies on AHE systems is that usually no details of the absorption system operation are provided, so there is no thermodynamic evidence that the absorption heat pumps can actually work in the required conditions.