# 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
### Advanced Control Strategies in DHC Systems
In the case study, the cogeneration unit directly reaches 70% of both the thermal and electric yearly demand, while 16% of the subsequent is met by storage. Likewise, a hierarchical optimization strategy is used in [PERSON] et al. (2018) [69]. The higher layer for binary decision is dedicated to the genetic algorithm (GA), while the MILP algorithm is used in the lower to choose the system's optimal operation. Results show a remarkable reduction in computing time. The multi-agent system (MAS) approach benefits several critical computer applications such as communication network configuration, process control, planning or concurrent systems. MASs are formed by several agents that have a considerable number of communications with each other. In general, agents act to support the users and have different goals and motivations. An agent is defined as an autonomous entity that can be viewed as perceiving and acting upon its environment. An agent must be able to communicate with other agents [70]. To interact successfully, agents must have the ability to cooperate, coordinate and negotiate with each other, almost as people do. To improve the monitoring and control of a DH system through the use of agent technology, [PERSON] and [PERSON] (2002) [71] developed a MAS approach. In order to share the peaks and prevent entering the boilers in the circuit, the authors of [72] have applied the MAS approach. Yielded results show that MAS was capable of decreasing the peak loads by up to 20%. In the same way, [PERSON] et al. (2012) [73] used the MAS approach to control a compressed unit providing heating, ventilation and domestic hot water production in a low-energy building. However, findings show that the suggested methodology managed to increase costs by only 2.5%, but they improve thermal comfort by 35%. MAS and PSO's combination is the main idea in [PERSON] et al. (2012) [74]. The aim is to maximize the comfort index using minimum power consumption, and the findings indicate the effectiveness of the proposed method. The MAS approach has been recently applied to 5 GDHC systems that enable the concept of decentralized prosumers for all the connected substations. In particular, in [PERSON] et al. (2018) [75] the performance of two 5 GDHC systems located in San Francisco and Cologne exploiting an agent-based control have been assessed in comparison with a traditional gas-fired DH and individual chillers solution for space cooling. By exploiting the coordinated balancing effects of several decentralized prosumers with heat pumps and chillers, this control strategy was able to maintain the average temperature of the system within 2 K around the setpoint all year around showing that this approach is beneficial with respect to free-floating temperature control. Future technology could match blockchainwith the MAS approach. Blockchain application in 5 GDHC have been investigated in the framework of the D2 Grid project [76] and is going to be demonstrated at the demo case of Paris-Saclay (France) [77]. The goal is to apply a new business model where smart contracts can be signed automatically and with the exploitation of dynamic energy tariffs to flatten the peaks in the heating and cooling demands by promoting flexibility and demand response (DR) strategies at the customer level. In this regard, an overview of DR applications in the DHC sector is presented in the next section.