# 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
### Basic Control Strategies in 4 GDH and 5 GDHC Systems  
The basic control strategies shown in the previous section are usually applied also in 4 GDH systems. Generally, these have a supply temperature of 50-60 \({}^{\circ}\)C, while if it is lower such as in ultra-low temperature district heating (ULTDH), small booster heat pumps (HPs) are required at the customer-sited substation to satisfy minimum DHW comfort temperature and limiting legionella proliferation. The challenge that the current 3 GDH systems are facing in the transition towards 4 GDH is the supply temperature reduction. This is achievable mainly in those areas with new or refurbished buildings by zoning the DH network, maintaining a high-temperature backbone and low-temperature subnetworks. In this regard, a prototype of a mixing shunt system in a containerized solution has been installed in the DH network of Brescia (Italy) in the framework of the H2020 TEMPO project [26]. In contrast, a commercial solution has been recently installed in the DH network of Albertslund (Denmark) in the framework of the H2020 REWARDHeat project [27], as shown in Figure 2. These shunt systems usually include the minimum supply temperature and the minimum differential pressure controls for the subnetwork and unlock the possibility to absorb local peak loads by operating the subnetwork according to a dynamic heat storage strategy. Other benefits of the network supply temperature reduction are:  
* The diminution of the distribution heat losses since also the return temperature decreases. * The increment of the DH network capacity thanks to the larger temperature difference between supply and return at the backbone. This means that more customers could be connected to the existing infrastructure and that a lower flow rate is needed to supply the same thermal power. * The increment of the life of some DH components thanks to the lower operating temperature. * The increment of the efficiency in condensing boilers, CHP plants, HPs and solar thermal fields thanks to the lower return temperature. Figure 2: (**a**) Piping & instrumentation diagram of the mixing station installed at the DH system of Brescia [26]; (**b**) shunt system installed in the district heating and cooling (DHC) system of Albertslund [27]. Nevertheless, to maximize these benefits, it is essential to reduce the return temperature of the subnetwork by acting on the customer's substation and heating system. Apart from the substitution of some critical radiators with high panels or the extension of the heat emission surface, it might be useful to mitigate the effect of high flow rate occurring for instance due to night set back, with control using thermostatic radiator valves (TRV) and return temperature sensor [28]. As far as 5 GDHC networks are conceived, basic control strategies can differ from traditional DH/DC systems and 4 GDH. 5 GDHC systems, which usually operate with a network temperature in the range of 0-30 \({}^{\circ}\)C, include heat pumps (HPs) at the customer-sited energy transfer station that are in charge of meeting the required heating/cooling supply temperature. For this reason, supply temperature control is usually not implemented at centralized level, and several systems operate with free-floating temperature that is a result of the balance between the heating and cooling loads as well as thermal energy source temperature throughout the year [24]. Minimum supply temperature control is in some cases implemented to limit the risk of very low temperature and possible freezing in the pipes using back-up systems [29; 30; 31]. Differential pressure control is usually present only when centralized pumping stations are used and, in some cases, together with redundant heat exchangers that separate the HPs from the thermal grid. In other cases, such as in small loops where decentralized hydraulic pumps are installed at the substation level, differential pressure control is absent. Anyhow, the minimum pressure in the network is met by means of pressurization stations.