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The aim of this document is to present the topic of modeling district heating systems in order to enable optimization of their operation, with special focus on thermal energy storage in the pipelines. Two mathematical models for simulation of transient behavior of district heating networks have been described, and their results have been compared in a case study. The operational optimization in a DH system, especially if this system is supplied from a combined heat and power plant, is a difficult and complicated task. Finding a global financial optimum requires considering long periods of time and including thermal energy storage possibilities into consideration. One of the most interesting options for thermal energy storage is utilization of thermal inertia of the network itself. This approach requires no additional investment, while providing significant possibilities for heat load shifting. It is not feasible to use full topological models of the networks, comprising thousands of substations and network sections, for the purpose of operational optimization with thermal energy storage, because such models require long calculation times. In order to optimize planned thermal energy storage actions, it is necessary to model the transient behavior of the network in a very simple way -allowing for fast and reliable calculations. Two approaches to building such models have been presented. Both have been tested by comparing the results of simulation of the behavior of the same network. The characteristic features, advantages and disadvantages of both kinds of models have been identified. The results can prove useful for district heating system operators in the near future.

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The main aim of this paper is to evaluate the possible transition routes from the existing centralized energy systems in Polish cities to modern low-emission distributed energy systems based on locally available energy sources, mainly solar energy. To evaluate these possibilities, this paper first presents the current structure of energy grids and heating networks in Polish cities. A basic review of energy consumption in the building sector is given, with emphasis on residential buildings. This paper deals with the evaluation of the effectiveness of operation of central district heating systems and heat distribution systems; predicts the improvement in the effectiveness of the energy production, distribution, and use; and analyzes the possible integration of the existing system with distributed energy sources. The possibility of the introduction of photovoltaic (PV) systems to reduce energy consumption by residential buildings in a big city (Warsaw) is analyzed. It is assumed that some residential buildings, selected because of their good solar insolation conditions, can be equipped with new PV installations. Electricity produced by the PV systems can be used on site and/or transferred to the grid. PV energy can be used not only for lighting and electrical appliances in homes but also to drive micro- and small-scale heat pumps. It is assumed that the PV modules are located on roofs of residential buildings and are treated as individual micro scale energy systems of installed capacity not larger than 50 kW for each of the buildings. In such a case, the micro energy system can use the grid as a virtual electricity store of 70% or 80% efficiency and can produce and transfer electricity using a net-metering scheme. The results show that the application of micro-scale PV systems would help residential buildings to be more energy efficient, reduce energy consumption based on fossil fuels significantly, and even if the grid cannot be used as a virtual electricity store then the direct self-consumption of buildings can reduce their energy consumption by 30% on average. Development of micro-scale PV systems seems to be one of the most efficient options for a quick transformation of the centralized energy system in large Polish cities to a distributed energy one based on individual renewable energy sources.