Thermal and hydraulic modeling and control of a district heating system

A district heating system is a centralized heating system widely used for space heating. They offer economic benefits and are acknowledged to be more energy efficient. Their energy efficiency can be further improved by optimally controlling and operating the overall system. With this as the motivation a thermal and hydraulic model of a district heating is developed in this thesis. The developed model consists of a boiler, six buildings, hot water distribution network, circulating pump, balancing and control valves and terminal heaters. Both dynamic and steady state hydraulic simulations were made to study supply, return water and zone temperature response; Pressure distribution in the piping network under different load conditions. A relationship between balance valve settings and outdoor temperature (heating load) was determined. It was shown that proper setting of balance valves as a function of heating load improves energy efficiency. PI controllers were designed for the boiler and zone temperature control. Closed loop simulations are presented to show the control performance. By using steady state optimization technique optimal set points for the boiler temperature and near-optimal balance valve positions as a function of outdoor temperature were determined. Simulation results show that the use of optimal set points and balance valve settings up to 20% energy can be saved compared to the conventional outdoor air reset control strategy. Simulation results under operating conditions are presented