# 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
### Demand-Side Management (DSM) in DHC System  
Electrical/thermal demand is mainly uncontrollable, and its quantity varies during the day and along the season. Accordingly, to flatten the load curve, system operators usually implement demand-side management (DSM) strategies that consist of a portfolio of practices aiming at modifying the demand side of an energy system by promoting information programs, energy efficiency, energy saving but also with the implementation of demand response (DR) programs. Peak-shaving and load management are two main applications of DR that add flexibility to an energy system by making the demand more elastic. In general, DR strategies are classified into price-based and incentive-based programs [78]. To the first group belongs all the solution that aims at influencing end-user choices with time-dependent tariffs such as real-time pricing (RTP) or common time-of-use (TOU) tariffs. Differently, incentive-based programs are based on contractual agreements with grid operators or utility companies and foresee a reward to the customer for the reduction in their load upon request or with the direct remote control of some equipment. So far, DR has widely studied to shave the peaks in power grids and on pooling of individual air-source heat-pump-based heating ventilation and air conditioning (HVAC) systems as in [79], [80] and [81]. Recently, aggregation of the HP-based substations in the 5 GDHC network of Wustenrot has been analyzed for the implementation of power-to-heat strategies in the context of the Sim4 Blocks H2020 project [82]. In particular, heuristic optimization of HP operation for the maximization of PV electricity self-consumption and exploitation of flexible electricity tariffs have been mainly investigated in [PERSON] et al. (2019) [83], whereas cluster aggregation for power market participation such as frequency restoration reserve (FRR) has been analyzed in [PERSON] et al. (2019) [84]. The latter showed how the requirement of a short activation period in the FRR market can be in contradiction with the minimum running time of the HPs for both technical and economic aspects. As explained in Section 2.3, the critical issue in DHC networks is the presence of peak requests, particularly in the morning. Here, DR can play a vital role on the demand side to shave these peaks and avoid large investments in centralized TES systems. Thermal peak shaving using DR in a real test case of a distribution network of the Turin DH system is investigated in [PERSON] et al. (2019) [85]. A genetic optimization algorithm has been adopted to select the binary values of on/off switching every 10 min with strong restrictions such as maximum anticipation of 20 min and acting only on 32 out of the 104 buildings connected to the distribution network. The field results revealed a reduction of 5% of the global load. However, the authors demonstrated with a simulation activity that if the controller acts on all connected buildings and with maximum anticipation of 90 min, a peak reduction up to 37% can be obtained. In a previous work [86], the same authors assessed the fact that modeling both supply and return pipelines provide a more precise assessment taking into consideration how the on/off switching of some substations could affect the delay in the temperature distribution to the farthest consumers. Moreover, [PERSON] and [PERSON] (2019) [87] proposed a simplified approach of DSM to shave the peak of DH substations by means of a modification in the proportional-integral control logic that regulate the flow rate on the primary side. A differential of return temperatures (DRT) control strategy is introduced, and it aims at maintaining the temperature difference between the inlet of the secondary side and the outlet of the primary side below a threshold so that during the start-up the valve opens slower, the flow rate on the primary side is reduced and the efficiency of the heat exchanger is increased.