A new kind of phase change material (PCM) for energy-storing wallboard

Abstract

A new kind of phase change material (PCM) for energy-storing wallboard is introduced in this paper. By establishing the one-dimensional non-linear mathematical model for heat conduction of the PCM energy-storing wallboard and according to the “effective heat capacity method”, simulation and calculation were made using the software MATLAB to analyze and solve the heat transfer problem of the PCM room. Meanwhile, the property can be found that the heat storing/releasing ability of the new PCM is significantly higher than that of ordinary materials by the experiment-based method. The result indicates that applying proper PCM to the inner surface of the north wall in the ordinary room can not only enhance the indoor thermal-comfort dramatically, but also increase the utilization rate of the solar radiation. So the heating energy consuming is decreased and the goal of saving energy has been achieved. If the parameters of the PCM is given as follows: the phase change temperature is set at 23 °C, the thickness is set at 30 mm, the phase change enthalpy is set at 60 kJ/kg, and the heating temperature is set at 20 °C, the energy-saving rate of heating season η can get to 17% or higher. So the energy is effectively used and saved obviously.

Introduction

In many areas of north China, the solar energy resource is abundant in winter with the properties as long-time sunshine and intense solar radiation. Let us take Beijing as an example, the winter sunshine rate reaches as high as 76%; meanwhile, the solar elevation angle is small. So the intensity of solar radiation directly entering into the room is great. As shown in Fig. 1: the solar radiation directly radiating on the south wall or permeating the north window in winter is much higher than that in summer, and extremely higher than that on the east wall and the west wall. This property provides a great advantage for the application of reused sources in the building energy-saving aspect. However, the amount of the solar radiation is affected by many factors, such as the rotation and revolution of the earth, the weather and climate change and so on, which results in a large fluctuation of solar energy amount everyday. Thus how to effectively store the extra solar energy that enters the room in the daytime and release it at night when demanded, and achieve the goal of “time shifting” of solar energy to reduce the energy consumption in air conditioning and heating will become a crucial topic.

In order to make buildings have the ability of absorbing solar energy and achieving the goal of “time shifting”, it is not only necessary to ensure the building envelopes have a certain heat resistance, but also have a big heat capacity, by which we should enhance the heat storage capacity and the thermal inertia. Having this property, the PCM could absorb or release a large amount of phase transition latent heat under an isothermal condition. In recent years, scholars all over the world have conducted various investigations on PCMs. The researchers Feldman and co-workers [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12] have made theoretical analysis and experimental research on the manufacture method, heat storing/releasing characteristic, stability, flammability and the security of the PCM in the Centre for Building Studies of Concordia University in Canada. And they have gained very outstanding result on this subject. Neeper [13] made some researches on the thermal performances of the PCM used in passive solar house which provides references for the choosing of better PCM and make better estimation on the energy conservation characteristic for the PCM. Tyagi and Buddhi [14] in Indian made a comprehensive review on different possible modes for heating and cooling in buildings. Furthermore, they also made a detail introduction on different ways of the thermal storage for the system like PCM trombe wall, PCM wallboards, PCM shutters, PCM building blocks, air-based heating systems, floor heating, ceiling boards, etc. The research result indicated that applying the PCM in the heating and cooling systems has very great application potential. Moreover, it can decrease the energy consumption for the building dramatically.

Researches on PCMs and PCM wallboards in China are later than overseas. Zhang [15], [16], [17] in Tsinghua University has elaborated the classification and selecting, the thermal performances, the analysis method of heat transfer and the design method of heat storage equipment of the PCM since 19th century. Based on the research, a new type of shape-stable PCM that is made of paraffin as a dispersed phase change material is developed and comparisons have been made about the heat storing/releasing characteristic, the uniformity and the stability of the material between different PCMs whose supporting material is HDPE and LDPE. F. Guohui [18], [19] have developed a new kind of PCM wallboard through immersion method and carried out the PCM room experiment in the north climate of China.

In recent years, our research group [20], [21] has been engaged in the theoretical and experimental study on the PCM technology. And a new kind of phase change material (PCM) has been developed according to the meteorological characteristic of China and construction characteristic of the building in China. It can be mixed with ordinary constructive materials and directly applied to the inner surface of the ordinary wall to get a new type of PCM energy-storing wallboard. This paper makes a discussion on the phase change heat transfer of the building envelope with the new PCM energy-storing wallboard. Through establishment of phase change heat transfer model of the room [22], simulations and calculations according to the heat storing/releasing characteristics of the PCM are made by effective heat capacity method [20] and the software MATLAB. The effects of the PCM energy-storing wallboard on the heating energy consumption of the room and the energy conservation rate are studied according to phase change temperature, the phase change enthalpy and the thickness of the PCM energy-storing wallboard. Furthermore, by the experimental method, the validity of this new type material has also been approved.