The design and control of a district heating substation has an important impact on the energy exchange between the thermal grid and the building heating system. A properly controlled energy exchange will improve the economy of the entire system while minimizing the pollutant emissions and the fossil fuel consumption; the main goal of the European energy plan till 2030 . A substation typically consists of a plate heat exchanger equipped with pumps and control valves that connect the district heating primary side with the secondary side i.e, the building heating system. The temperature of the secondary side is controlled by regulating the flow rate of water at the primary side of the heat exchanger. As a result, an optimal heat transfer from the primary side of the network to the secondary side can be achieved [2, 5]. Existing methodologies are capable to provide steady state form of the secondary supply temperature but they cannot provide the desired low return temperature from the substation to the network. However, achieving the low return temperature from the substation is also an important aspect of the district heating system as it can reduce the operational cost by 10 – 15% of fuel by saving fuel at the production side . Hence, in this research work, a model-based control methodology is employed to achieve low primary return temperature while satisfying the desired comfort levels in the building. In the designed approach, first, a control-oriented mathematical model of a plate heat exchanger is developed, since the existing mathematical model of a plate heat exchanger consists of nonlinear partial differential equations (PDEs) and such model cannot be used directly in model-based control framework due to its complexity and computational inefficiency . Thus, nonlinear PDEs are approximated by using a finite difference method to obtain control-oriented nonlinear ordinary differential equations (ODEs). The ODE model is validated with the experimental data given in  and then implemented with the model predictive control (MPC). The low return temperature from the substation is considered as a constraint in the MPC framework. Furthermore, the constraint on input flow rate of water through the substation is also added to reduce the pumping cost of the network. The designed MPC will provide the desired supply temperature of the secondary side while satisfying all the above mentioned constraints and yields less operating cost. The proposed methodology is also compared with an existing proportional integral (PI) controller. The comparison between the designed MPC and PI controller shows that MPC performs better against the variations of input flow rate and temperature.
|Media of output||Portuguese Meeting on Optimal Control - EPCO 2021|
|Publisher||EPCO 2021 Optimal Control Meeting|
|Number of pages||2|
|Place of Publication||University of Porto|
|Publication status||Published - 28 Jun 2021|