# Advanced Control and Fault Detection Strategies for District Heating and Cooling Systems--A Review
## 1 Introduction  
The world's level of urbanization is expected to increase from about 55% in 2018 to 68% in 2050, and 90% of this increment is expected to take place in Asia and Africa, which were home to about 90% of the world's rural population in 2018 [1]. In Europe, 74% of the population lives already in urban areas today, and this percentage is expected to grow up to 84% by 2050 [1]. Furthermore, in urban areas where the heating and cooling demand exhibits the highest density and the largest load simultaneity, a huge amount of low-grade excess heat is wasted. Moreover, for historical reasons, cities and towns have been born along rivers, lakes, and seashores that are all ambient heat sources in which utilization is highly replicable, because it is accessible right where it is needed. Even, in some cases, such as in London [2], the total heat wasted from secondary sources has been estimated larger than the city's total heat demand. Six out of ten of the top European heatwaves between 1950 and 2014 have appeared in the latest 20 years [3]. Extreme weather events, such as wind storms and flooding, have increased in number and intensity worldwide [4]. Some scientists claim that climate change's evolution and consequences have several similarities with the current pandemic crisis of COVID-19, but in slow motion [5, 6]. Nevertheless, in the latest years, several public entities recognized the climate crisis and are going to implement serious actions to mitigate global warming effects. As a result, 2019 has been defined the year of the \"climateemergency\" declaration. In this context, the European Union, which already demonstrated between 1990-2018 that it is possible to decouple gross domestic product (GDP) growth from greenhouse gas emissions [7], set a very ambitious target to achieve carbon neutrality by 2050. Moreover, the EU admitted that this could not be accomplished with the current commitments under the Paris Agreement that foresee a total greenhouse gas emissions reduction by only 40% in 2030 with respect to 1990 levels. To boost the global warming fighting process, the new European Green Deal [8] set a more ambitious target that corresponds to reach a greenhouse gas emissions reduction by at least 50% by 2030 compared with 1990 levels with dedicated strategies for the different sectors. In particular, to decarbonize the heating and cooling sector and to improve the air quality in urban areas, the European Green Deal Investment Plan will support the implementation of district heating and cooling (DHC) networks [9]. This technology is the most promising to implement circular economy principles in the heat sector by harvesting and distributing local excess heat that otherwise will be wasted. Moreover, it allows boosting the share of modern renewables to cover the heat demand of buildings that was only equal to about 13.6% in 2017 at a worldwide level [10]. The digital roadmap for district heating and cooling (2019) [11], which has been developed as part of the H2020 project STORM [12], carefully presents how digitalization and the concept of Industry4.0 can push forward the efficiency and operation of DHC networks, empowering their role in an integrated smart energy system. The barriers identified that could hinder the rollout of digital technologies in DHC are not technical but are mainly related to limitations in acting on the building substation, absence of business models or dynamic tariffs to stimulate the flexibility that can be provided by buildings and regulations on private data protection. This study, performed in the context of the H2020 REWARDHeat project [13], aims at reviewing the state-of-the-art in scientific publications and to create a common knowledge from different European research projects of two parts that are fundamental to achieve innovative and robust DHC systems in the near future: advanced control strategies and fault detection and diagnosis solutions.