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Thesis (PhDMechanicalEng)--University of South Australia, 2005. Includes bibliographical references. Space heating is one of the simplest and most appealing applications of solar energy. The Roof Integrated Solar Heating System (RISHS) initiated by the Sustainable Energy Centre (SEC) of the University of South Australia offers a practical solution for this application. The main objective of this project is to study the technical viability of this system for domestic applications in Australia. The research work carried out and reported in this thesis brings together the pioneering work on RISHS, namely: modelling and analysis of both the roof integrated heating system and the phase change material (PCM) thermal storage. The work accomplished includes: developing an improved model for the PCM thermal storage unit (TSU), the development of a comprehensive collector model, the development of control strategy of the total system, the development of TRNSYS-based simulation tool for analysing individual component as well the total system, and analysis of the thermal performance of the system. The new phase change thermal storage model allows for the inclusion of sensible heat both prior to and after the phase change is complete. It relies on the newly developed phase change processor (PCP) algorithm for accurately predicting the outlet temperature and heat transfer rate. The PCP algorithm facilitates the iterative process required to simulate the phase change process, a phenomenon which cannot be dealt with using the general conduction heat transfer equations. The work has also identified that a melting or freezing process of PCM can be divided into three stages; (a) the sensible heat exchange stage characterised by very high heat transfer rates, (b) predominant latent heat transfer stage with much reduced but relatively constant heat transfer rate, and (c) the stage of combined sensible and latent heat exchange with much a further reduced rate. Two temperature differences governing the melting and freezing processes have been introduced. It has been found that these two quantities affect melting and freezing significantly. Given the clear significance of these two newly introduced quantities, the research work questions some previous claims about the effects of natural convection in certain PCM TSU geometries. The work on the roof integrated solar collector includes the development of a comprehensive solar collector model where the collector dimensions, construction and material properties become inputs or parameters. Using this model, thermal performance of the collector can be evaluated and the collector standard thermal parameters such as collector heat removal factor and collector heat loss coefficient can be evaluated. The development of a control strategy of the total system which includes the house being heated, the collector, the PCM, the fan and the auxiliary heater, has been carried out. In the control strategy, the characteristics of each component are taken into account and the schedule of energy flow from the heat source and / or thermal storage are designed to maximise the solar contribution and minimise the auxiliary heating required throughout the heating period. To achieve this, an integrated control strategy of auxiliary and solar heating / thermal storage unit has been proposed. Using this approach, the optimum solar contribution for a specified RISIS can be established. The air mass flow rate and the room space temperature difference are the key parameters which influence the system thermal performance. The subroutines for modelling the PCM thermal storage and the roof integrated solar collector and the system control strategy have been incorporated into the TRSNSYS simulation package to analyse the total system. The analysis carried out indicates that the RISHS is a technically viable system which can provide significant heating contribution for house heating in Adelaide and Melbourne. The introduction of the PCM thermal storage in the system is a technically viable option; it is in fact the main factor which improves RISHS overall performance. The analyses on three PCMs have revealed that the main factors which dictate their thermal performance are the charge temperature and the melting point (which affect the chargeability), mass flow rate and charge and discharge temperature differences. The TRNSYS-based simulation tool developed during the research project is expected to become a reliable tool for designing any real system for any location and applications. |
