EXERGETIC AND THERMOECONOMIC APPROACH FOR OPTIMAL PLANNING OF DISTRICT ENERGY SYSTEMS

Abstract

A sustainable urban energy planning for achieving the EU 2020 and 2050 energy goals requires adopting a systemic approach based on reducing end-user energy requirements, recycling energy that otherwise would be wasted and replacing fossil fuels by renewable. District Heating and District Cooling play a key role in such a concept. From the sustainability viewpoint, district heating is an important option to supply heat to the users in urban areas. The energy convenience of such option depends on the annual energy request, the population density and the efficiency in heat production. Among the alternative technologies, geothermal heat pumps (both open loop and closed loop heat pumps) play a crucial role. In order for the DHN to remain an effective solution with respect to alternative technologies, the optimal configuration, design and operation must be investigated. This thesis aims to propose a methodology for the Multiobjective Optimizations of district heating networks, where the objective functions (the minimum specific primary energy consumption or the minimum economic cost) of a district heating network are investigated using a thermoeconomic based probabilistic procedure. A procedure, derived from Simulating Annealing optimization technique, to select which users in a urban area should be connected with a district heating network and which ones should be heated through an alternative technology is proposed. The goal of this procedure is to reach a globally optimal system from the energy and economic viewpoints. The procedure proposes district heating as the initial choice for all the users. The users are then progressively disconnected to the network, according with the primary energy required to supply them heat, and the alternative technology is considered for disconnected users. Here, ground water heat pump and condensing boilers are considered as the alternative technologies. The optimization technique developed in this PhD thesis develops the three levels of the optimization of energy systems: - Development of a Synthetic Method: The optimal synthesis is performed though a method which starts with a superstructure (where all the buildings (users) in the considered area for the expansion of DH network are supplied by district heating network) and then reduced to the optimal configuration (some of the users are disconnected from the DHN and supplied with an alternative technology such as geothermal heat pumps or condensing boilers). - Development of Optimal Design Method for the components and the properties at the nominal load selected in order to reach optimal performances: - as the users are disconnected from the district heating network, the mass flow rate flowing in the pipes is reduced resulting in different pipe diameters in comparison to the initial configuration. The optimal value velocity in the pipes is obtained as a function of the pipe diameters; - The cogeneration ratio (the ratio between the thermal power of the CHP appliances and the total thermal power installed in the power station ) has been considered as a parameter in the optimal design of the system. - Development of Optimal Operating properties: the operating properties under specific conditions has been changed, like the operating supply temperatures, but also the evolution of the network during its construction is considered. The application to an Italian town is considered as a test case. The main advantage of this procedure is that complex networks, like the DHN in Casale Monferrato characterized by 198 users, grouped in 21 macrozones, can be easily processed. The optimal configuration of the overall urban heating system is obtained. This configuration corresponds to the minimum primary energy request to supply heat to all the users (those connected to the network and those using an alternative heating system). After a brief introduction where the district heating technology is presented, the Thesis is divided in two parts: the first parts introduces the methodological approach proposed for the optimization of a District heating network, together with the description of the optimization model. The second part focuses on a specific application case, showing the preliminary operations required for the application of the model and the results obtained from the optimizations performed. The results have been interpreted trying to reach a more general conclusion which is not related only to the specific case study