Summary of Chapter 2

Chapter 2 introduced the concept of phase change materials, with a focus on solid-liquid PCMs. The important aspect of high heat content at narrow ranges of temperature supports the use of PCM in buildings, but various other thermo-physical, kinetic and chemical properties have also been identified as essential to the proper utilisation of PCMs. The lack of certain desired properties can be overcome through appropriate system design. The classification of PCMs into organic, inorganic and eutectics has been presented, with a description of some commonly employed experimental and commercial PCMs. Classical thermal analysis techniques consisting of the DSC and T-history methods have also been described.

The choice of PCMs is primarily related to the comfort temperatures required in the conditioned space. In passive systems, PCMs are generally chosen such that the peak phase change temperature lies within the comfort temperature range; although this does not imply that the PCM is most effectively used. The overall phase change temperature range should also ensure that most of the phase change occurs within the desired temperature range, in order to improve the effectiveness of the PCM and prevent energy

‘wastage’. In semi-active or active systems, the phase change temperature and enthalpy ranges are more system dependent. The phase change enthalpies are determined based on the heat loads of the building, and are also system dependent. Thus, as the performance of PCM systems depends on the type of building, weather conditions, thermal comfort conditions, etc., the sizing of PCM systems is also case-dependent.

Solid-liquid PCMs are found to have poor heat transfer properties and hence are not used directly as a heat transfer medium. Micro-encapsulation, heat transfer enhancements and efficient system designs have been popular methods to improve the heat transfer properties. The application of PCMs to buildings has mainly been through their incorporation into the building envelope or their use in air-conditioned related systems. They are used to shift the thermal loads by providing additional thermal inertia. They have also been incorporated in gypsum wallboards, wooden structures and concrete, with very encouraging results.

The Rubitherm CSM® plates have been popular in the design of semi-active AC-related systems, where they have been used to provide free-conditioning or to reduce the size

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and energy consumption of the AHU. Cold night ambient air is commonly used for recharging the PCMs in cooling applications, while for heating purposes, the relatively warmer room air can be re-circulated to restore the heating potential of the PCM. Other less popular PCM heat exchanger systems include the use of PCM spheres in the AC ducts, or the use of conventional heat exchangers with PCM incorporated in them. In passive systems, the PCM is allowed to operate on its own, based on the physics of the building, and therefore emphasising the importance of appropriately sizing the passive PCM system. Conversely, the employment of semi-active systems offers more flexibility in the performance of the PCM, as secondary parameters such as flow rates or temperatures can be further controlled during the operation of the PCM system. With regards to costs, the capital costs of implementing PCM systems in AC buildings may be high, but compared to conventional AC systems, the running costs are very favourable, with encouraging payback periods (Zalba et al, 2004).

Other uses of PCMs include heat/cold stores and PCM glazed units. Heat/cold stores can consist of macro- or micro- encapsulated PCM, possibly combined with water to allow for stratification. Direct contact methods were found to be more efficient than indirect methods. In general, the stores can be found to have enhanced thermal storage capacity, thus enabling smaller store sizes to be used. PCM glazed units can be mainly used to improve the thermal mass of lightweight and glazed structures by absorbing solar radiation and providing a thermally stable indoor climate. However, the optical properties of PCMs may affect the aesthetics of the space, and PCM-glazed units may therefore require concealment.

The majority of PCM systems encountered in the literature have been studied for relatively small indoor spaces such as offices, and for intermittently occupied buildings.

The performance of PCM systems were found to be heavily dependent on the phase change temperatures, thermophysical properties, melting enthalpy, amount and type of PCM, orientation of the building, climatic conditions, type of systems (passive, active or semi-active), design of the system and building operation (ventilation schedules and heat loads) – i.e. a PCM system’s performance is case dependent. Therefore, this emphasises the use of numerical models in the prediction and optimization of the performance of PCM systems.

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Section 2.2 provides a general understanding of the thermal comfort criteria for airport terminal buildings – the two comfort models, i.e. Fanger’s equation and the adaptive model are described. As specified in section 2.2.1, the combined use of both models is a more accurate way of quantifying thermal comfort. The adaptive model basically specifies that occupants can adapt themselves to a specific environment, while Fanger’s equation of comfort provides a mechanistic approach to comfort. Using both methods simultaneously enables a relaxation of the thermal comfort requirements by closely tracking the external temperature patterns. This allows reduction in the AC running costs.

The following chapters describe the numerical methodologies employed in this study to evaluate the performance of PCM systems in the airport terminal space.

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