Abstract
The purpose of this thesis stems from the need to make the existing building stock more energy efficient, which consist mostly of poorly insulated buildings with high energy consumption. The contribution of the building sector to the decarbonization goals set by the European community is essential since they represent a significant share of the total energy consumption. The choice of the technology that must be applied to heating systems in the residential sector in order to reach these decarbonization goals is not clear yet. Heat pumps are currently the most promising systems, but they have drawbacks. The most widespread type is the air source heat pump, which suffer from efficiency drop when producing water at high temperatures or in cold climates. This is mainly due to thermodynamic reasons. Therefore, heat pumps are usually supplied in combination with backup generators, such as electric heaters. In this work, the different technologies that can be combined with heat pumps, such as solar, biomass or gas appliances, have been analyzed. Chapter 1, describing the relative literature review, identifies a literature gap regarding hybrid systems that combine air source heat pumps and gas boilers. In fact, the technology is commercially mature and its spreading in some countries, such as Italy, has grown considerably in recent years. However, there is a lack of guidelines regarding proper application, or comparison among different control strategies, which are the central part of hybrid systems. This work is based on these assumptions and aims to provide an overview of hybrid systems (HSs) - combining heat pump and gas boiler - ranging from system modeling, to the identification of the most suitable applications and the assessment of control strategies, to the analysis of system settings parameter. Chapter 2 presents the development of a HS model based on performance maps, suitable for application to dynamic building simulations. The chapter presents the discussion and the reasons behind the choice of this type of model, and the combination of the model with a TRNSYS building simulation. In addition, the chapter presents a set of simulation results to provide insightsinto the most suitable applications for hybrid systems, along with a discussion about control strategies. Chapter 3 presents the results of experimental tests that allowed for a validation of the HS model. Based on the results obtained in Chapter 2, the application chosen for the validation was that of poorly insulated buildings provided with radiators. In addition, an analysis based on laboratory tests has been conducted on the main transients of the HS, i.e., generators startups and heat pump defrosting cycles. Correlations were derived that allow for a more detailed simulation of these phenomena in the modeling. Chapter 4 describes the analysis performed to determine the influence of some HS setup parameters on the energy consumption. The effect that the simulated generator startups and defrosting cycles have on the energy consumption calculated by the model was analyzed. Finally, chapter 5 presents the development of a second, detailed, HS model. The model is based on the subdivision of the HS into its major components, most of which are modeled according to the physical or heat transfer laws governing the process. This model serves as a basis for studying the design of a hybrid 2 system, as it gives the possibility to analyze the influence of the variation of certain components or construction parameters on the overall efficiency of the system. The results presented in the various chapters are the basis for the conclusion section, in which the technology of hybrid systems is reviewed and their role and proper application in the context of heating systems for residential applications is outlined.