Pathway towards a non-linear integrated optimal control and sizing methodology for clean hybrid, multi-energy vector district heating and cooling systems
The heating and cooling of buildings represents a major share of global energy demand and associated carbon dioxide emissions, motivating the transition towards clean hybrid district systems integrating heat pumps, solar collectors, PV panels, PV-T panels, and energy storage. To minimize both investment and operational costs in these multi-energy vector districts, this paper presents a pathway towards a Integrated Optimal Control and Sizing (IOCS) framework that combines detailed physics-based models in Modelica with non-linear Model Predictive Control implemented via the Toolchain for Automated Control and Optimization (TACO). The methodology is demonstrated through two Belgian district use cases, representing a virtual mixed-use district (which is cooling-dominated) and a real-life heritage district (which is heating-dominated). A comparative analysis evaluates (1) conventional sizing methods, (2) an earlier developed two-step iterative IOCS method, and (3) a direct IOCS approach, which directly optimizes component sizes within a single optimal control problem. Results show that both IOCS methods substantially reduce the total cost of ownership (TCO), with the direct IOCS achieving the lowest TCO in all scenarios for both use cases (up to 69% reduction for the virtual district and 30 % for the heritage district under typical weather conditions, while stress-testing in extreme weather conditions still yields TCO reductions of respectively 55 and 19 %). Key findings include significant downsizing of ground-source heat pumps and borefields, enhanced utilization of building thermal inertia, and hybridization with air-source heat pumps and chillers. The direct IOCS approach also significantly reduces computational effort compared to the two-step method, with optimization times decreased by up to 50%, suggesting practical feasibility for larger-scale. These results underscore the value of incorporating operational flexibility into the design phase, enabling more compact, cost-efficient, and renewable-based energy systems for future districts to make the energy transition more feasible and affordable.