District heating: a dynamic simulation approach for energy and economic analyses

In this paper the dynamic energy and economic performance of district heating systems is investigated. As well known, such plants can be convenient if thermal energy is available at low costs or as waste heat. Generally, such condition can be obtained in case of proximity of building clusters or town neighbourhoods to thermal power systems, urban waste heat treatment plants or some industry typologies. In this case, through a suitable pipeline network such heat can be supplied to buildings for balancing their heating and cooling demands. In general, the economic profitability of district heating systems depends on different variables, such as for example the unitary cost of heat to be offered to users and the related yearly amount of the sold thermal energy. The system performance is also function of several dynamic parameters such as the weather conditions (influencing the buildings heating/cooling demands). In case of thermal power plants, the related activation typically depends on the hourly national single price of the sold electricity to the grid. In order to take into account all such time dependant variables, a suitable dynamic simulation model for the system energy and economic performance assessments was developed. Such tool is conceived for feasibility analyses of new district heating plants as well as for checking and controlling the existing ones. The model was implemented in a MatLab code where the heating/cooling demands of buildings are dynamically assessed by taking into account suitable hourly weather data files (TMY, IWEC, etc.). A specific subroutine is implemented in the code for the economic optimization of the diameter of each network section. In addition, by the presented computer code the effects on the system performance of the variation of different design and operating parameters can be carried out by varying suitable objective functions. Finally, in order to show the potentiality of the developed simulation tool, a suitable case study is presented. It refers to a district heating hypothesis in the S. Giovanni neighbourhood of Naples (South-Italy). Here, the system is supplied by the heat produced through the thermal power plant of Vigliena in which a novel combined cycle system is currently operating. For such district heating system an energy assessment and an economic feasibility investigation are performed. Furthermore, a system parametric analysis is also carried out for computing the optimal number of district heating users and the optimal selling electricity price with different objective functions, such as: minimum simple and discounted paybacks, maximum net present value and profitability index, maximum primary energy saving and CO2 emission reduction. Useful design criteria and interesting energy and economic results were obtained.