## 1 Introduction  
Approximately 50% of the energy consumed in the European Union (EU) is in the heating and cooling sector [1]. The carbon footprint of heat supplied through district heating depends heavily on its source. Over time, a shift from fossil fuel heat sources to renewable sources has been observed. This shift is most visible in Nordic countries where biofuels dominate the fuel mix [2]. One important feature of district heating networks (DHNs) is that waste heat from other processes can be recovered and distributed through the network. According to a survey aimed at quantifying the potential use of waste heat in the EU, the technical potential could be up to 2.7 EJ/year [3]. The use of waste heat is advantageous in reducing carbon emissions, and therefore in meeting climate targets. Industrial waste heat is often of a high temperature, which facilitates effective recovery into district heating systems. Waste heat at an urban level provides an alternative source. Urban waste heat stems from commercial processes or from structured operations that are undertaken daily. For example, heat can be recovered from data centres and sewage water [4; 5; 6; 7; 8; 9; 10]. The main challenge of urban waste heat recovery is that the heat is normally low-enthalpy (around 20-40 \({}^{\circ}\)C). This temperature is lower than the heat distributed in conventional DHNs (around 90 \({}^{\circ}\)C) [11]. According to the definition of 4 thGeneration DHNs provided by [PERSON] et al. [12], future DHNs may need to supply low-temperature heat for space heating and domestic hot water (DHW) to energy-renovated existing buildings and new low-energy buildings. Heat from urban waste heat sources is able to be upgraded to fulfil end-user requirements and is thus expected to play a fundamental role in meeting global thermal energy demand [13]. However, urban waste heat recovery implementation is at an early stage and its feasibility needs to be demonstrated for the purpose of scaling up. New urban waste heat recovery system solutions are currently being designed across Europe. In an ongoing International Energy Agency project (the TS2 project on 4 th generation district heating technology implementation), over 130 low temperature district heating installations have been identified (www.iea-dhc.org), confirming that urban waste heat recovery is an upward international trend. The ReUseHeat project, on which the authors of this paper are partners, is a Horizon 2020 EU funded project (see acknowledgements) addressing the input of urban waste heat into existing district heating networks through the implementation of four demonstrator sites. The ReUseHeat project aims to provide answers to open issues regarding urban waste heat recovery investments. It demonstrates the techno-economic viability of four large scale systems enabling the recovery of heat from urban heat sources (data centres, sewage water networks, metro stations and hospitals). At this stage, due to technical difficulties, the metro station demonstrator site is in a less developed state than originally envisaged, and thus results from this site are not currently available. One key finding of the ReUseHeat project is that heat recovery potential from unconventional waste heat sources in the EU is significant. There is potential to recover 1.2 EJ/year from data centres, metro stations, service sector buildings, and waste water treatment plants. This corresponds to more than 10 percent of the EU's total energy demand for heat and hot water, which is approximately 10.7 EJ [14]. There are a number of known barriers to investment in industrial waste heat recovery. One obvious barrier is the absence of a DHN in the proximity of the heat source. Other barriers are the existence of cost competitive heat supply alternatives and policy incentives for other forms of heat supply such as biomass or waste fueled CHP plants.