"Model-based analysis of bypass control strategies in district heating: Revisiting constant and thermal bypass in a real network"

The transition to low-temperature district heating systems is important for achieving higher energy efficiency and integrating renewable energy sources. A Modelica-based dynamic simulation framework is used to analyze the thermo-hydraulic behavior of an existing third-generation district heating network in Losone during two weeks of summer operation to evaluate the impact of constant and thermal bypasses on supply temperature, energy consumption and hydraulic efficiency. The results show that both constant and thermal bypasses are successful in maintaining the required supply temperature. The thermal bypass shows greater adaptability to demand fluctuations, reducing energy consumption and hydraulic load compared to the constant bypass. However, thermal bypass operation is sensitive to sensor placement and measurement uncertainty, which can significantly reduce performance. This finding has direct practical implications for system designers as it shows that thermal bypasses require precise sensor configuration to realize their efficiency benefits, while constant bypasses offer greater robustness under uncertain operating conditions. This study emphasizes the dynamic nature of bypass operation and highlights the limitations of static sizing approaches. Advanced optimization methods, such as digital twins and model predictive control, combined with functional mock-up interfaces, are proposed to overcome these limitations by enabling real-time optimization of bypass operation based on demand forecasts and specific performance objectives such as minimizing energy consumption or operating costs. The study provides practical recommendations for bypass design and operation, advancing control strategy development for 3rd generation district heating systems and facilitating the transition to more efficient 4th and 5th generation district heating and cooling systems.