Passarelli, Adelso Flaviano (2025) Large size heat pumps for district heating systems with frost formation: modelling, experimental calibration and seasonal consumption assessment. [Tesi di dottorato]

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Abstract

Nowadays, about half of global energy consumption is related to heating purposes. According to the path to reach decarbonization and improve energy efficiency, the role of heat pumps in the heating sector is crucial. To ensure their efficiency, it is essential to have not only a proper system design, but also off design working conditions analysis such as faulty or critical operating modes. Among the different types of heat pumps, air-source one represents the most widespread solution. However, in cold and wet locations, frost formation is a critical operative issue, affecting performance and involving the need for defrosting. A reliable prediction of the actual seasonal performance is then related to the accurate modelling of this critical operation. The objective of this thesis is to carry out a system analysis on large size air-source heat pumps deepening the effect of frost formation and defrosting on seasonal performance evaluation. The dynamic behaviour of the system has been analysed using a detailed modelling approach for the multi-evaporator system. These heat exchangers are characterized by two-phase flow within tubes and the wet air interaction with the coil surface. Therefore, this thesis presents a first part dealing with experimental assessment and development of predictive methods for flow boiling heat transfer and pressure drop within smooth and internal micro-fin. A very accurate mechanistic model for pressure drop has been developed while a critical analysis on the use of physics-based and machine learning predictive approaches shows that although the latter are very accurate when used for interpolation problems, physics-based methods are more robust and suitable for extrapolation. Then, to complete the evaporator modelling, the air-side heat and mass transfer is defined and followed by an experimental validation for the whole heat exchanger. In the second part, the analysis goes to the system level where the dynamic balancing of the multi-evaporator system with the rest of the heat pump during heating season is carried out. Two case studies involving a large size air-source ammonia heat pump serving a district heating network in different locations are presented. In the first one, it is found that a preliminary design analysis neglecting frost formation can suggest a domain of nearly equivalent thermo-economic solutions but, when frosting and defrosting are considered, substantial differences emerge within this seemingly equivalent domain, leading to a significant shrinking of the truly convenient domain. In the second case study, frost formation resulted less intense than the first one. However, it has been confirmed that the average results with respect to design parameters or defrosting strategies can provide an indication to reach a subdomain of thermo-economic convenience, but it is the combination of effects of both design and defrosting variables that leads to the best seasonal performance. Lastly, a demand-based defrosting analysis based on air-side pressure drops resulted representative of the evaporator cooling capacity decay. This represents a point of further development of the model, involving demand-based defrosting to find the ideal number of evaporators to be defrosted simultaneously and with what defrosting trigger thresholds.

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