Multiobjective optimization and analysis of low-temperature district heating systems coupled with distributed heat pumps

Abstract

Developing district heating can help decarbonizing energy systems. Modern concepts of district heating revolve around low operational temperature to reduce heat losses and enhance the efficiency of the supply plants with technologies such as cogeneration, condensing boilers or geothermal heat pumps. However, temperature reduction in district heating networks may be limited by technical requirements on the building side. Booster heat pumps on the end-users’ side can be a solution to bridge the network-building temperature gap. However, the current state-of-the-art on such systems does not provide ways for optimizing in real-time the operating temperature, which is needed to fully exploit its benefits, and proposed control strategies focus solely on reducing costs without considering environmental aspects. This article proposes a new multi-objective optimization framework to determine the real-time supply temperature for low-temperature district heating with booster heat pumps. The optimization approach uses rolling horizon and both operating costs and GHG emissions are minimized simultaneously. The model on which the optimization relies includes efficiency curves for equipment and water transport delays in the network. Many scenarios are simulated to understand how key factors affect the optimal supply temperature, such as the fuel type (i.e., wood chips or natural gas), price of electricity, emission factor of electricity, and relative weight of costs and emissions in the objective function. Results show that the proposed framework can indeed reduce costs and emissions, but the viability of the system depends strongly on the above-mentioned factors. In the case-study with typical electricity costs and emissions for Quebec (Canada), the average operating temperature could be reduced to 42 and 66 °C depending on the type of boilers in the system. While emissions were always smaller than with a conventional network, operating costs could be higher (with wood chip boiler) or lower (with natural gas boiler). This work can contribute to the deployment of efficient low temperature networks.