## 1 Introduction and Objectives  
Social concerns about climate change are growing. Since the Copenhagen Climate Conference (1997), several countries began implementing energy policies to reduce global warming gases and air pollution. To help this, efficient use of energy, renewable energy sources, and the recovery of waste energy are key actions to mitigate the effects of the global warming and contribute to the sustainable development of our society. Due to industrial activities, such as washing, drying, cooling processes, emission of furnace exhaust gases, etc., huge amounts of waste heat are released to the ambient atmosphere [1] in many industrial sectors: Petrochemical plants, oil refineries, municipal waste incinerator plants, cement plants, etc. This waste heat can be classified into three groups depending on its temperature, although there is no consensus on the limits for the temperature ranges: High temperature (>400 \({}^{\circ}\)C), medium temperature (100-400 \({}^{\circ}\)C), and low temperature (<100 \({}^{\circ}\)C) waste heat. The technologies for waste heat recovery vary depending on the temperature level of the waste heat and on its final application. In many cases, industrial plants generate large amounts of waste heat, for example, at temperatures of 60 to 70 \({}^{\circ}\)C or lower, that has to be simply rejected to the ambient atmosphere using cooling towers or air coolers. Because this temperature is usually too low to be reused in the same industrial processes or in other cases, the demand for this low temperature heat is too small and not economically feasible to be recovered. [PERSON] et al. [2] is presented an interesting estimation of the industrial waste heat available in different industrial sectors per temperature level and country. The use of this industrial excess heat in urban district heating networks would be very attractive. Modern smart cities are expected to use the heating and cooling of district networks generated with renewable energies or recovered waste heat, thus avoiding as much as possible the use of gas networks inside cities. With respect to the recovery of waste heat, industrial waste heat sources are usually located far away from urban energy demand users. One of the options to transport heat long distances is to upgrade the heat at a higher temperature and transport it at larger distances in a similar way as what is done with electricity, increasing, in that case, its voltage to transport it. However, the typical urban end user does not require heat at a high temperature. Thus, at the user connection substation, the temperature could be reduced accordingly. Another desirable characteristic of the heat transport network is a high temperature difference between the supply and return temperatures of the heat transport network in order to reduce as much as possible the required mass flow rate circulation. Absorption technologies are heat-driven systems and excellent candidates to perform the two mentioned tasks as follows:  
1. Upgrade low or medium temperature industrial waste heat to a higher temperature in order to be later transported through a network. 2. Use the transported heat at high temperatures to deliver heat efficiently to a low temperature district heating network while at the same time increase the temperature difference between the supply and return temperatures of the heat transport circuit. Next, both the characteristics of absorption systems and thermal energy networks are briefly reviewed. The concept and working principles of absorption systems and technologies, including chillers, heat pumps, and heat transformers, are assumed to be known by the interested reader and are already very well covered in the literature [3]. Only the most relevant absorption systems for the heat transport and distribution problem will be covered in detail here. [PERSON] et al. [4] introduced the concept of \"absorption heat exchanger\" (AHE) that consists of an absorption heat pump (AHP) combined with a water-to-water heat exchanger.