Multi-criteria analysis of district heating system operation strategy

Highlights: • A mathematical model of simplified district heating system is proposed. • Dynamical performances of district heating systems are researched. • District heating system can be deployed as a dynamical energy storage. • District heating system can provide additional flexibility to cogeneration plants. - Abstract: The contemporary energy system is faced with new challenges on the energy market. Both the rising share of renewable-based electricity (characterised by intermittent production) and increasing energy efficiency in buildings induce a reconsideration of the traditional role of conventional power plants coupled with district heating systems along with its impact on the energy system. Moreover, the price of electricity determined by the merit-order system additionally decreases the load factor of such plants, making them less competitive or even inflicting financial loss in operation. Research presented in this paper focuses on a novel approach towards conventional combined heat and power (CHP) plants coupled with district heating systems. It involves an analysis of dynamical performances of the district heating system – capability of energy accumulation and thermal inertia – in order to assess its potential to become part of ancillary services. It is concluded that the district heating system, i.e. network of pipelines can be considered as dynamical thermal energy storage in which excess energy can be stored during operation of the CHP plant. A comprehensive analysis of dynamic behaviour of the district heating system has been performed by means of a mathematical model developed as a part of this research. The model is implemented on a theoretical case consisting of a simplified district heating system with three final users and the pipeline network of 9000 m in length. The simulation has shown that the storage capacity of the network depends on the thermal load in the network and in such circumstances specific thermal capacity of the network amounts to 10.1 Wh/(km). If the thermal load is decreased by 13% at the peak the accumulation capability is decreased by 40%. Moreover, the thermal capacity of the network increases up to 13.7 Wh/(km) as the pipeline length declines to 1000 m which is characteristic of densely populated areas. The capability of energy accumulation is explored for different parameters, such as: external temperature, distance of the network, supply water temperature etc. It is shown that distance of final users from CHP plant has certain impact on the operation strategy as well. Final users closer to the CHP plant determines operation strategy in the case of short-term disturbances in heat supply, while the farther ones in the case of long-term disturbances.