| Recently, the phase change materials have been increasingly used in building's envelopes in the hope to save more energy than usual in conditioning the indoor environment. The existing studies, experimental or numerical, are focused on local sites of buildings and special conditions. Considering the universal effect of PCMs in buildings, we conduct research on the responses of buildings provided with PCMs to nationwide change of weather in terms of energy savings.A transient model of heat transfer is established for a single room that is used as a standard model in our study. The outside surface of envelope is thermally insulated by Expandable Polystyrene (EPS), while the inside surface is covered by PCMs or PCMs-based components. In the first place, we choose Beijing as the typical weather zone to check the effects of the insulation material and PCMs. In the study, we make full use of the natural cooling in the night to observe the performance of thermal storage of PCMs in the passive mode, particularly the indoor temperature change due to PCMs; and in the air-conditioning mode, the efficiencies in energy due to both PCMs and EPS are also checked. We also pay some close attention on their effects on room temperature and energy consumption in the room, which is heated by using electric or solar hot water floor heating system with shape-stabilized PCMs plates in winter.Based on the above studies, we find that: in the passive mode, if the inner walls are completely made of PCMs, the room temperature is reduced effectively and the requirement of indoor comfort in summer could be met, but it is impossible to use one PCM of definite melting point to meet requirements both in summer and winter; in the active mode, applying PCMs to the surface of inner wall cannot reduce the air-conditioning energy consumption per unit area of construction effectively, but the effect would be enhanced if the amount of PCMs are increased; the reducing of the energy consumption for heating is significant when the outside wall is covered with EPS, however, it will block the heat transfer from indoor to outside in summer, so it needs to take the performance of EPS in one year into account when evaluating its capacity of energy saving; The floor heating system integrated with PCM can not only increase the room temperature, but also reduce the heating-power consumption.We choose five cities to represent five distinct climate zones in China, and the passive and active performances of PCMs or EPS in buildings are discussed in terms of hours falling in the range of comfort and associated energy-consumption per unit area of construction respectively. In the analysis, some economic factors are also considered, including peak-load shifting, investments and operating costs of heating and air-conditioning systems.Our results indicate that the insulation on the outside surface of external wall is most significant in terms of energy savings, and the amount of saved energy is proportional to the ratio of heating energy to the total energy consumed in buildings. For cold zones, 1000RMB around could be saved in one year, and the number is reduced to about 150 in hot-summer and cold-winter zones. However, in the southern region, the external shading and adjustable ventilation are preferable whereas the external insulation is not recommended; PCMs inner walls can make room comfortable and the air-conditioning consume less energy in summer, however, it needs a long period for payback of investment, that is 15 years at least in the Beijing, Shanghai and Guangzhou because of its high investment; For cold zones where only heating is needed, the floor heating system can give a relatively satisfactory result in energy saving when using PCM as thermal mass, and the payback period falls within 5 years. It's more cost-effective to use external insulation materials than PCMs, for the payback period of the former is only one and a half year. |
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