Yanping Yuan

Thermal properties of polyethylene glycol/carbon microsphere composite as a novel phase change material

Polyethylene glycol (PEG) as phase change materials has been extensively studied. However, it is difficult to uniformly disperse the PEG in the unmodified matrix material owing to the impregnation problem. It is therefore challenging to overcome the shortcoming of low thermal conductivity. In order to solve this problem, carbon microspheres (CMPs) prepared by the hydrothermal method is firstly proposed as the supporting matrix for preparation of PEG composite. The PEG/CMPs composite is prepared via a mutual diffusion methodology in a high-temperature environment. The test result shows the CMPs nano-material has rich oxygen-based functional groups and can be uniformly dispersed in the PEG phase. As the carbon content increases, the CMPs become gradually connected in the PEG/CMPs composite, which can result in a network with good thermal conductivity. Compared with the thermal conductivity of pure PEG, the thermal conductivity of the composite is increased by 65.07%. Owing to the existence of hydrogen bonds in the composite, the crystallinity fraction of the PEG is in the range of 102–105%. It means that the test result is higher than the theoretical value of latent heat for the composite. Finally, its cycling performance was measured. After 500 thermal cycles, the phase transition temperature of the composite remains almost constant, and the latent heat values of the melting and freezing decrease by 1.05 and 1.45%, respectively. The PEG/CMPs composite would be a promising material for thermal energy storage applications and can be used in various engineering disciplines.

Study on thermal property of lauric-palmitic-stearic acid/vermiculite composite as form-stable phase change material for energy storage

The form-stable composite phase change material of lauric–palmitic–stearic acid ternary eutectic mixture/vermiculite was prepared by vacuum impregnation method for thermal energy storage. The maximum mass fraction of lauric–palmitic–stearic acid ternary eutectic mixture retained in vermiculite was determined as 50 wt% without melted phase change material seepage from the composite phase change material. Fourier transformation infrared spectroscope and scanning electron microscope were used to characterize the structure and morphology of the prepared lauric–palmitic–stearic acid ternary eutectic mixture/vermiculite form-stable composite phase change material, and the results indicate that lauric–palmitic–stearic acid ternary eutectic mixture was well confined into the layer porous structure of vermiculite by physical reaction. The melting and freezing temperatures and latent heats were measured by differential scanning calorimeter as 31.4°C and 30.3°C, and 75.8 and 73.2 J/g, respectively. Thermal cycling test showed that there was no significant change in the thermal properties of lauric–palmitic–stearic acid ternary eutectic mixture/vermiculite form-stable composite phase change material after 1000 thermal cycles. Moreover, 2 wt% expanded graphite was added to improve the thermal conductivity of lauric–palmitic–stearic acid ternary eutectic mixture/vermiculite form-stable composite phase change material. All results indicated that the prepared lauric–palmitic–stearic acid ternary eutectic mixture/vermiculite form-stable composite phase change material had suitable thermal properties and good thermal reliability for the application of thermal energy storage in building energy efficiency.

Inorganic composite adsorbent CaCl2/MWNT for water vapor adsorption

CaCl2 has been proposed as an advanced chemical adsorbent for cheap, non-toxic and powerful in water vapour adsorption. One of the key barriers for practical application is poor in mass and heat transfer due to large volume expansion, serious agglomeration and low thermal conductivity. A novel CaCl2/MWNT adsorbent is prepared with carbon nanotube as matrix. In the composite, a part of CaCl2 nanoparticles are adsorbed on the matrix surface, and the else hygroscopic salts are filled in the space of matrices. The nanoparticles with big specific surface areas can be fully contacted with water vapour, enhancing water uptake and adsorption rate. Furthermore, the paper has reported that hydrophilic/hydrophobic property of the MWNT is relation with the adsorption performances of the CaCl2/MWNT composite. It is found that hydrophilic matrix can enhance water-retaining property, and keep a stable porous structure of the material. Finally, a simple modified method adding PVP as a water-based adhesive into the CaCl2/MWNT composite adsorbent is also reported for the first time. The PVP powder improves water-retaining property of the CaCl2/MWNT adsorbent during adsorption process. For regeneration process, the PVP water-based adhesive makes adsorbent particles agglomerate macroscopically under unchanged their microcosmic structure. At low humidity condition, adsorption rate of CaCl2/MWNT composite is obviously enhanced by the PVP powder. Consequently, the modified CaCl2/MWNT composite with the hydrophilic matrix will promise for commercial application in water vapour extraction, thermal storage energy and other applications.

Core-shell microstructured nanocomposites for synergistic adjustment of environmental temperature and humidity

The adjustment of temperature and humidity is of great importance in a variety of fields. Composites that can perform both functions are prepared by mixing phase change materials (PCMs) with hygroscopic materials. However, the contact area between the adsorbent and humid air is inevitably decreased in such structures, which reduces the number of mass transfer channels for water vapor. An approach entailing the increase in the mass ratio of the adsorbent is presented here to improve the adsorption capacity. A core-shell CuSO4/polyethylene glycol (PEG) nanomaterial was developed to satisfy the conflicting requirements of temperature control and dehumidification. The results show that the equilibrium adsorption capacity of the PEG coating layer was enhanced by a factor of 188 compared with that of the pure PEG powder. The coating layer easily concentrates vapor, providing better adsorption properties for the composite. Furthermore, the volume modification of the CuSO4 matrix was reduced by 80% by the PEG coated layer, a factor that increases the stability of the composite. For the phase change process, the crystallization temperature of the coating layer was adjusted between 37.2 and 46.3 °C by interfacial tension. The core-shell CuSO4/PEG composite reported here provides a new general approach for the simultaneous control of temperature and humidity.

Thermal properties of phase change cement board with capric acid/expanded perlite form-stable phase change material:

Capric acid/expanded perlite form-stable phase change materials were prepared by vacuum adsorption. The phase change cement boards were prepared by adding capric acid/expanded perlite into cement mortar, and their thermal properties were tested. The results show that capric acid/expanded perlite improves the thermal storage capacity of the cement board; the latent heats of melting and freezing were 14.25 and 14.1 J g−1, respectively, at 20 wt% capric acid/expanded perlite. Capric acid/expanded perlite enhanced the specific heat capacity of the cement board, and the higher the content of the capric acid/expanded perlite in the cement board is, the higher the equivalent specific heat capacity is. Capric acid/expanded perlite also significantly reduced the thermal conductivity and the heat storage coefficient, while at the same time, increased the thermal inertia coefficient of the cement board. Compared with the ordinary cement board, the thermal conductivity of phase change cement board with mass ratios of 10, 15, and 20 wt% of capric acid/expanded perlite decreased by 39.4%, 47.83%, and 52.49% at 20°C and 37.94%, 46.84%, and 50.63% at 50°C, respectively; the heat storage coefficients decreased by 34.07%, 40.62%, and 44.87% at 20°C and 30.25%, 35.59%, and 37.65% at 50°C, respectively; the thermal inertia coefficients increased by 8.75%, 13.78%, and 15.96% at 20°C, and 8.75%, 21.53%, and 26.21% at 50°C, respectively.

Steady-state equation of water vapor sorption for CaCl2-based chemical sorbents and its application

Green CaCl2-based chemical sorbent has been widely used in sorption refrigeration, air purification and air desiccation. Methods to improve the sorption rate have been extensively investigated, but the corresponding theoretical formulations have not been reported. In this paper, a sorption system of solid-liquid coexistence is established based on the hypothesis of steady-state sorption. The combination of theoretical analysis and experimental results indicates that the system can be described by steady-state sorption process. The steady-state sorption equation, μ = (η − γT) , was obtained in consideration of humidity, temperature and the surface area. Based on engineering applications and this equation, two methods including an increase of specific surface area and adjustment of the critical relative humidity (γ) for chemical sorbents, have been proposed to increase the sorption rate. The results indicate that the CaCl2/CNTs composite with a large specific surface area can be obtained by coating CaCl2 powder on the surface of carbon nanotubes (CNTs). The composite reached sorption equilibrium within only 4 h, and the sorption capacity was improved by 75% compared with pure CaCl2 powder. Furthermore, the addition of NaCl powder to saturated CaCl2 solution could significantly lower the solution’s γ. The sorption rate was improved by 30% under the same environment.

Thermal storage/management system with phase change materials for building:

During the processes of melting or solidification, phase change materials (PCMs) can store or release a significant amount of latent heat. Therefore, PCMs can be used in building to solve the mismatch between energy supply and demand in time, space, and intensity. Moreover, it is also used to recover residual heat and waste heat of buildings to improve the building energy efficiency. The temperature of PCMs can be stably maintained during the melting/solidification; thus, PCMs can be used for thermal management, such as adjusting the temperature of the photovoltaicthermal (PVT) system to improve the power generation efficiency and employing phase change wall (PCW) to adjust the indoor air temperature. Moreover, PCW can not only reduce the capacity of the air-conditioning system but also reduce the fluctuation range of the indoor air temperature and thus improve human body comfort. Therefore, this special collection features recent PCM-related research works in the field of building energy efficiency. The selected 14 papers illustrate the broad impacts of PCM techniques in this challenging area. Three papers reviewed previous work about PCM application in buildings. ‘‘A review on phase change material application in building’’ by Y Cui et al. presents a review on PCM application situations in building, and several aspects are discussed: major applications in building, thermal-physical properties, and effects of application. ‘‘Review on application of phase change material in water tanks’’ by L Xie et al. reviews the research on the water tank integrated with PCM in terms of existing research methods and heat transfer enhancing technologies and then summarizes the applications of various PCM-based water tanks. Finally, the further research suggestions on the PCMbased water tank are proposed. ‘‘A review about phase change material cold storage system applied to solarpowered air-conditioning system’’ by L Zheng et al. is a novel investigation of the PCMs usage in cold storage system, the PCM cold storage working principles and features that are applied in the different solar-powered air-conditioning systems. Three papers of this collection concern preparation and application of new PCMs. New microencapsulated PCMs applications in construction material and with fabric clothing are, respectively, proposed by X Han et al. in the papers of ‘‘Experimental study on effect of microencapsulated phase change coating on indoor temperature response and energy consumption’’ and ‘‘Experimental studies on phase change and temperature-adjusting performance of phase change fabric clothing.’’ Capric acid/expanded perlite formstable PCMs were prepared by vacuum adsorption in the paper of ‘‘Thermal properties of phase change cement board with capric acid/expanded perlite formstable phase change material.’’ The phase change cement boards were prepared by adding capric acid/ expanded perlite into cement mortar, and their thermal properties were tested. In the topic of thermal storage device, experimental and theoretical researches are both involved. The performances of the PCM thermal energy storage tank during the heat charging processes are investigated experimentally, and a series of experiments are carried out under different heat transfer fluid flow rates and distances between PCM plates in the paper of ‘‘Heating storage performance of a water tank-combined phase change material: An experimental case study.’’ It is found that water flow rate of 1.3m/h is taken as the optimal working condition, and the 3-cm plate distance is considered as the optimal design. Another paper named ‘‘Numerical study of the influences of geometry orientation on phase change material’s melting process’’ reveals how the melting rate could be affected by changing the orientation of a rectangular PCM container with a constant temperature boundary. The transient melting processes of lauric acid in a twodimensional (2D) rectangular container with five orientations were simulated using the computational fluid dynamics software. The computational fluid dynamics model was validated against the available experimental data obtained from published literature. A thermal energy storage system using U-tube heat exchanger is proposed in the paper of ‘‘Numerical investigation of