# Advanced Control and Fault Detection Strategies for District Heating and Cooling Systems--A Review
## 2 State of the Art Control Strategies for District Heating (DH) and District Cooling (DC) Systems  
### DHC as an Enabling Infrastructure for Greater Renewable Sources Penetration  
DH/DC networks include a variety of subsystems from heat/cold generation units, distribution pipelines and user substations. In the last decades, while renewable energy sources for electricity generation, such as solar and wind, have made considerable progress, it is not the same for renewable thermal energy generation. Even if the technology is mature, only a few countries such as Denmark, Sweden and Lithuania [16] already achieved a good share of renewable thermal energy production in DH/DC systems thanks to their ambitious decarbonization policies. However, it is worth mentioning that DHC is the key solution to scale up renewable thermal energy in the built environment. On one hand, thanks to economies of scale and, on the other hand, by solving issues that are relevant at the building level such as the lack of space or architectural heritage preservation. In Europe, DH cover about 12% of the heating market share [17] and DC only 2% of the cooling market share [18]. Worldwide, district energy systems supply only 6% of the thermal energy demand and it is strongly based on fossil fuels. In fact, in 2018 only 8% of the energy sources employed in DHC were based on renewables with 95% of this consisting of biomass according to [16]. This high share confirms that biomass is the most competitive source for renewable heat supply in DHC systems of several countries. Moreover, it is easy to implement, since the management of the heat generation plant remains very similar to a traditional dispatchable fossil-based one. Biomass boilers can be derived from the upgrade of existent coal-fueledplants and, in some cases, are coupled with steam-based or Organic Rankine Cycles (ORC) combined heat and power (CHP) systems. The years between 2020 and 2030 have been defined as the \"Geothermal Decade\" [19], since a large growth is foreseen for the exploitation of geothermal energy in Europe for both electricity production and direct/indirect use. Even if the location of high enthalpy geothermal reservoirs is geographically constrained, medium enthalpy resources are more diffused. Deep geothermal for direct use in DH can meet the heating demand of about 25% of the European population according to the results of the research project GeoDH [20]. With this regard, Iceland is the leader country in the installed capacity of geothermal DH. The projections for coming years foresee important market growth of geothermal DH, especially in France, Germany, Denmark, the Netherlands and Hungary [19]. Deep geothermal for direct use in DH provide dispatchable baseload capacity, achieving large equivalent full-load hours. Shallow geothermal can be exploited almost everywhere but requires centralized or decentralized heat pumps to lift the supply temperature. Solar thermal has great potential for DH application, but conversely from geothermal and biomass might introduce more effort for the control of the system that can be supported by forecast tools for both the heat demand and the not-programmable solar heat generation. Denmark is the pioneer country in solar district heating (SDH) and the worldwide leader in large-scale solar thermal fields achieving 1.1 GW\({}_{\text{th}}\) of installed capacity in the first half of 2019 (almost equal to the PV capacity installed in the country) [21]. This was possible thanks to the competitive heat price from solar thermal with respect to gas (affected by high taxes), high penetration and sound experience with DH networks, cheap land prices and willingness of the community-owner DH companies that run a business that is non-profit by law. From the Danish experience [22], the average efficiency of the solar collector fields is about 40%, and the heat production cost ranges from 20 to 40 EUR/MWh.