Nowadays, climate change, environmental protection and energy shortage are global significant issues requiring for prompt solution. The greenhouse gases emission generated by energy utilization accounts for 69% of the global total emissions [1
]. Some 30–40% of the world energy is consumed by buildings, mainly for climatization purposes [2
]. Thereby, investigating new technologies that combine clean energy, such as the solar energy, with the building energy consumption is a hot topic for researchers around the globe.
The incorporation and integration of solar energy with phase change material storage wall technology in buildings has many advantages. For example, it can use the limited external surface area of envelope structure of buildings to enhance the overall utilization ratio of solar energy, thereby reducing the percentage of conventional energy and thus realizes building energy conservation. Meanwhile, features such as the small temperature change in phase change process and high phase change latent heat of phase change material (PCM) are used to enhance the heat storage capability of envelope structure of building. This can be helpful in reducing the indoor temperature fluctuation, and improving the indoor thermal comfort conditions [3
Different types of research studies dealing with the incorporation of PCM in buildings have been conducted. Neeper [4
] proposed a system in which PCM was incorporated with the building envelope. It was concluded that the largest storage can be obtained when the phase change temperature is close to the average air temperature in the room. Zhou [5
] established a south-facing direct-gain room with shape-stabilized phase change material (SSPCM) model and discussed the selection criteria of some parameters such as phase change temperature, and latent heat. Zhang [6
] simulated the indoor temperature variation and energy saving performance of buildings constructed with PCM under different ventilation conditions in varying ambient conditions (June to September). Different optimal ventilation schemes were suggested for different climatic conditions. Soares et al. [7
] concluded the combination of solar energy technology and PCM storage technology with envelope structure of the building can effectively reduce room temperature fluctuation and enhance thermal comfort. Sun [8
] proposed a proposed a mathematical model for the building wall incorporated with PCM, presented calculation methods of energy-saving rate and power-conserving value, created a mathematical model and discussed the usage and investment cycle in different regions. Reddy [9
] investigated the thermal performance of roof integrated with PCM. The results showed that using multi-PCM layers of appropriate thickness are more beneficial compared to the single PCM. It was observed that using multiple PCMs a constant comfortable temperature of ~28 °C can be maintained in the building throughout the day in hot and humid conditions of Chennai, India. Maha [10
] proposed a PCM wall combined with light-weight insulation plate to enhance the thermal inertia of the entire wall and the internal thermal comfort. Simulation study was performed using TRNSYS software and results were validated with the experimental study. It was observed that good experimental effects exist in winter and summer, as well as long-term durability. The above mentioned literature shows the positive effect and profound significance of combining PCM and solar energy technology in buildings on the thermal conditions of the space. Gu et al. [11
] proposed a heat recovery system using PCMs to recover the rejected heat from air-conditioning systems to produce domestic hot water for washing and bathing. The thermodynamic calculation showed that the integrative energy efficiency ratio of the system can be improved effectively when all rejected sensible and latent heat from air-conditioning systems can be recovered. Saman et al. [12
] investigated a roof integrated solar heating system using a PCM storage experimentally and numerically. The results showed that the effect of sensible heat was perceived in the initial periods of melting and freezing processes. A higher inlet air temperature and air flow rate can increase heat transfer rates and shortens the melting time, but a higher air flow rate increased outlet air temperatures. For freezing, a lower inlet air temperature and a higher air flow rate can increase heat transfer rates and shortens the freezing time, but a higher air flow rate reduced outlet air temperatures. Ismail and Henríquez [13
] proposed a concept of a window with moving PCM curtains. The window was double sheeted with a gap between the sheets and an air vent at the top corner. The experimental and simulation results showed that the proposed concept of the PCM filled window system is thermally effective, and the green colored PCM is more effective in reducing radiated energy gains. Literature above show the positive effect and profound significance of combining PCM and solar energy technology in buildings.
It is revealed that there exists a large regional discrepancy in indoor thermal environment, the worst performing region being the hot summers and cold winters zone in China [14
]. To fulfill the heating and cooling/insulation demand during winter and summer seasons that coexists in buildings especially located in regions with hot summers and cold winters, based on the principle of combining solar energy utilization technology and PCM storage technology, the effectiveness of a dual-channel and thermal-insulation-in-the-Middle type Solar PCM storage Wall (MSPCMW) system is analyzed experimentally and discussed.
The analysis is carried out through comparative tests conducted in a hot-box using the south-facing wall as the insulation wall [16
]. However, the light-weight insulation wall which has negligible storage capability is used in the comparative hot-box tests, while the actual building walls are mostly solid walls with storage capability. The standard test approach is conducted by shutting doors and windows whereas in actual situation the buildings usually use vented working conditions-keeping the doors and windows open in summer. Hence, some previous experimental work related to this novel system cannot fully reflect the real performance of the working scenarios in summer. To extend the performance characteristics and the effectiveness of the system under practical application scenarios, this work compares the performance when the experimental facility is installed on a light-weight insulation wall and ordinary wall, as well as under different ventilation situations, i.e., shutting doors and windows, and opening doors and windows, respectively.
2. Test Setup and Introduction of Test Approach
The working principle of dual-channel and thermal-insulation-in-the-middle type solar PCM storage wall (MSPCMW) system is shown in Figure 1
. It is mainly composed of PCM wall, thermal insulation wall, interior and exterior flow channels, heat absorbing aluminum plate covered by selective absorption coating, indoor upper and lower vents (5, 6), outdoor upper and lower vents (1, 2), middle layer upper and lower vents (3, 4), glass cover and frame. The system is installed on the south-facing vertical surface of the building. Structural design of dual-channel and thermal insulation layer, appropriate design of vent opening switch, as well as the use of characteristics of PCM help achieving passive heating, heat preservation, heat insulation and cooling functions of buildings. Two operation modes are adopted to realize heating, heat preservation, thermal insulation, and cooling for the demands in different seasons. The operation modes are:
winter operation mode, passive solar heating during daytime and heat preservation in nighttime;
summer operation mode, thermal insulation in daytime and passive cooling in nighttime. The working condition of summer operation mode is introduced as follows:
Heat insulation mode: when the building needs the protection of thermal insulation in daytime, e.g., in summer’s daytime, the indoor upper and lower vents (5, 6) and the middle layer ones (3, 4) shut while the outdoor ones (1, 2) open. The combination of ambient wind pressure and thermosiphon pressure forms a circular flow between the external channels and the outdoor air that brings the solar energy absorbed by the aluminum plate back to the environment; meanwhile the inner air channel acts as heat protection between the thermal insulation layer and the south wall of the room. Thus achieving the purpose of reducing building’s absorption of solar energy.
Passive cooling mode: when the building needs thermal insulation protection during nighttime, e.g., in summer nighttime, the indoor upper and lower vents (5, 6) shut while the middle layer ones (3, 4) and the outdoor ones (1, 2) open. Primarily under the action of ambient wind pressure, circular flow forms between internal channel and external channel; the relatively cool outdoor air during the nighttime cools down the PCM wall. In this way cold energy can be stored in the PCM wall reducing the indoor temperature.
The present comparison test is conducted establishing the two experimental test systems on a comparative hot-box using the light-weight insulation wall as south-facing wall and a comparative-hot-box-like hot-box using the brick wall as south facing wall, respectively. The dimensions of both hot box are 3 m × 3 m × 2.6 m (L
). The dimensions of the door and the window are 1.6 m × 0.6 m (L
) and 1 m × 1 m (L
), separately. For brevity, insulation wall test system and solid wall test system indicate the two systems, respectively. Each test system consists of two rooms. The one installed with the present experimental facility is the experimental room, and the other with structures and dimensions consistent with those of the experimental room but without the experimental facility is the control room. Figure 2
a,b show the exterior appearances of the insulation wall test system and the solid wall test system, respectively; the structures and the thermal material properties of the walls are listed in Table 1
. In each system, the working conditions of closed doors and windows, and opened doors and windows (mentioned as unvented working condition and vented working conditions hereinafter) are compared.
The measurements mainly include the temperature and solar radiation intensity. A conventional copper-constantan thermocouple (accuracy of ±0.5 °C) is used for temperature measurements. The configuration of the main temperature monitoring points in the test system are shown in Figure 1
and Figure 3
. Figure 1
shows a section view of the structure in which the positions of the points, marked by ‘×’, are indoor air temperature (1 point), exterior surface of the wall on which PCM plate is attached (3 points evenly distributed along the height) and internal surface of the wall (1 point). Figure 3
shows the locations of the thermocouples on the external surface of PCM plate, marked by ‘●’, and the thermocouples on the interior side counterpose those on the exterior side. The measuring system also includes ambient temperature measurement and solar radiation intensity on the south-facing vertical surface obtained by a TBQ-2 pyranometer (Agilent, Hefei, Anhui Province, China). Real-time temperature and radiation data was collected by data acquisition unit.
The plate was wrapped in aluminum and plated with anti-corrosive coating. Each plate measured 0.45 m × 0.3 m × 0.01 m (L
). Its interior components are crystalline hydrate and organic PCM thus it benefits from both phase change materials of hydrate and organic matter [16
]. The material properties of the PCM are listed in Table 2