A novel strategy for preparing hydrate salt/expanded graphite (EG) composite phase change materials (PCMs) with large latent heat capacity and high thermal conductivity is explored, which involves modifying EG with a surfactant, compressing the modified EG into a block, and immersing the block into a melted hydrate salt. TritonX-100 is employed to modify EG for improving its hydrophilicity. The block fabricated from the modified EG exhibits larger adsorptive capacity for a MgCl2·6H2O-NH4Al(SO4)2·12H2O eutectic as compared with that of the unmodified one. The obtained eutectic/modified EG composite block exhibits a melting temperature of 63.40 °C and a melting enthalpy of as high as 157.8 J/g. The thermal conductivity of the composite block is measured to 4.789 W/m·K, about 10 times that of the eutectic. The novel eutectic/modified EG composite block shows great potentials in solar utilization systems, and this work sheds light on the development of EG-based composite PCMs containing hydrate salts.

•Preparation of graphite nanoparticles-modified phase change microcapsules.•The microcapsules dispersed slurry for direct absorption solar collectors.•The slurry has great photo-thermal performance and large thermal storage capacity.A microencapsulated phase change composite, in which paraffin as the core and graphite nanoparticles embedded melamine–formaldehyde (MF) as the shell, has been prepared and characterized. The paraffin@MF/graphite composite is composed of spherical particles with diameters ranging from hundreds of nanometers to several micrometers. Raman spectroscopy analyses confirm the existence of graphite on the shell of the paraffin@MF/graphite composite. DSC results indicate that the melting temperature and latent heat of the paraffin@MF/graphite composite are 50.5 °C and 90.8 J g−1, respectively, in which the mass ratio of paraffin is calculated to be 51.1%. The paraffin@MF/graphite composite can be dispersed into the ionic liquid to form a novel latent functional thermal fluid (LFTF). It is found that the temperature of the LFTF can increase from 30 to 113 °C under irradiation, indicating its remarkable photo-thermal conversion performance. The thermal storage capacity of this new kind of heat transfer fluid (HTF) is twice larger than pure ionic liquid. The high heat storage capability and excellent photo-thermal conversion performance enable the paraffin@MF/graphite composite as a potential material for solar energy utilization.

•A novel process was explored to prepare molten salt/EG composite PCM blocks.•The novel process involves mixing, compression and heating followed by cooling.•An optimal mass fraction of the MgCl2-KCl eutectic in the block is around 85%.•Good uniformity and small volume expansion are found for the composite PCM block.•This process is superior to the method involving adsorption and then compression.Herein a novel process was explored for preparing molten salt/expanded graphite (EG) composite phase change material (PCM) blocks, which involves mixing a solid molten salt with EG thoroughly, compressing the mixture into a block with a designed shape and then heating the block to a temperature above the melting point of the molten salt followed by cooling. An MgCl2-KCl eutectic salt was used as the PCM, and its EG-based composite PCM block was prepared by this process, in which the optimal mass fraction of the eutectic was determined to be around 85%. The microstructure of the MgCl2-KCl/EG composite PCM block containing 85% the eutectic shows a uniform distribution of the molten salt. The composite PCM block has a melting point of 424.14 °C and a solidification point of 418.39 °C, and its latent heat values are 161.37 J/g for melting and 160.28 J/g for solidification. Compared with the eutectic salt, the composite PCM block exhibits a reduction in supercooling by 3.7 °C and an enhancement in thermal conductivity by 11-fold. It has been verified that the composite PCM block possesses excellent thermal reliability. Furthermore, the composite PCM block has been compared with the one prepared by the conventional method (first adsorption and then compression). It is found that the composite PCM block fabricated by the novel process exhibits better uniformity and smaller volume expansion than the one obtained from the conventional method. The MgCl2-KCl/EG composite block shows great promise in high-temperature thermal energy storage systems, and this novel process is very suitable for preparing molten salt/EG composite PCM blocks.

•Preparation of ionic liquid-modified graphene.•The MGE/[HMIM]BF4 nanofluids have better dispersion stability than the unmodified GE based nanofluid.•The MGE/[HMIM]BF4 nanofluids has good photo-thermal performance.•A transient model was used to predict the temperature of MGE nanofluid under higher solar concentration.•The MGE/[HMIM]BF4 nanofluids based DASCs was optimized.Dispersion stability has been long considered as a critical issue for applying nanofluids in various fields, especially for the applications at elevated temperatures. Herein a novel route is explored to improve the dispersion stability of graphene (GE)/ionic liquid (IL) nanofluids for use as working fluids in medium- and high-temperature direct absorption solar collectors (DASCs), which involves modifying GE according to the molecular structure of the IL. Specifically, GE was modified using the reagents and process for synthesizing [HMIM]BF4, followed by dispersing the modified GE (MGE) into [HMIM]BF4. It is verified that the molecular chains similar to [HMIM]BF4 have been grafted on the nanosheets of GE, and the MGE/[HMIM]BF4 nanofluids exhibit much better dispersion stability than the one containing the unmodified GE, even at elevated temperatures. Moreover, the temperature profiles of the nanofluids containing MGE and GE were obtained both from the experimental measurement and the theoretical prediction using a one-dimensional transient heat transfer model. It is shown that the experimental data are in good agreement with the numerical ones for the MGE nanofluids, while a large deviation between them is found for the one containing the unmodified GE. And the MGE nanofluid shows enhanced receiver efficiency as compared to the GE one due to its much improved dispersion stability. Further, the transient model was used to predict the performance of the MGE nanofluid based DASCs under high solar concentrations. And by integrating the MGE concentration and the receiver height into a parameter, namely optical thickness, the optimization of the MGE nanofluid based DASC was carried out varying solar concentration, MGE concentration, nanofluid height and exposure time. It is revealed that the photo-thermal conversion performance of nanofluids greatly depends on its dispersion stability at elevated temperatures, and the MGE/[HMIM]BF4 nanofluids possess excellent dispersion stability and show great potentials for use as the working fluids in DASCs. This work sheds light on effective routes for improving dispersion stability of nanofluids as well as numerical investigations on nanofluid based DASCs.Download high-res image (179KB)Download full-size image

•Composition of MgCl2·6H2O-Mg(NO3)2·6H2O eutectic is determined.•No liquid leaks from eutectic/SiO2 composite even after melting.•SiO2 improves thermal and cycle stability of the eutectic salt hydrate.•SiO2 enhances thermal conductivity of the eutectic salt hydrate.MgCl2·6H2O-Mg(NO3)2·6H2O eutectic has a proper melting point and high latent heat for solar water heating systems with latent heat storage. In this paper, the exact composition of the eutectic and the corresponding phase change temperature are determined by measuring the phase change characteristics of mixtures of MgCl2·6H2O and Mg(NO3)2·6H2O in various ratios. MgCl2·6H2O-Mg(NO3)2·6H2O eutectics, the melting temperature of which is around 58 °C, can be obtained with the mass fraction of MgCl2·6H2O ranging from 35% to 50%. The eutectics are composited with porous fumed silica to improve its thermo-physical properties. The eutectic/SiO2 composite retains a similar melting point of the pure eutectic, is of no liquid leakage even after melting while the SiO2 weighs more than 15 wt%, and the specific enthalpy reaches 88.13 kJ kg−1. The eutectic/SiO2 composite has a thermal conductivity 5% higher than pure eutectics and better thermal stability and cycle stability. The thermogravimetric analysis shows its weight loss is 8.03% less than pure eutectic at 300 °C. And the eutectics suffers a decrease in phase change enthalpy after 100 thermal cycles by 9.2%, much lower compared with the 37.23% for pure eutectics due to the phase separation during the thermal cycles. Thermo-physical properties and stability of MgCl2·6H2O-Mg(NO3)2·6H2O eutectic have been enhanced by fumed silica.

•GO/water nanofluids has good long-term stability.•The optical absorption property of GO/water nanofluids is significantly improved.•The thermal conductivity of GO/water is enhanced.•The highest efficiency was achieved by GO/water nanofluids containing 0.02 w% GO.Graphene oxide (GO) dispersed nanofluids were first explored to be used as working fluids for direct absorption solar collectors. The stability, optical absorption property, thermal-physical property and photo-thermal conversion performance of the GO/water nanofluids with different mass fractions of GO were symmetrically investigated. The sedimentation experiment and Zeta potential (ζ) analysis showed that the GO/water nanofluids not only possess excellent long-term stability but also keep stable at elevated temperatures. A remarkable enhancement in optical absorption property was observed for the GO/water nanofluids due to the dispersion of GO into water. The GO/water nanofluids exhibited an obvious improvement in thermal conductivity as compared with water. The photo-thermal conversion performance of the GO/water nanofluids were higher than that of water, and the highest efficiency was achieved by the one containing 0.02% of GO, which was 97.45% at 30 °C and 48.92% at 80 °C. The GO/water nanofluids with excellent dispersion stability, good optical absorption property and photo-thermal performance show great promise for use as the working fluids in low temperature direct absorption solar collectors.

•A novel form-stable CaCl2·6H2O/EP composite PCM with nonflammability was prepared.•Composite PCM has low thermal conductivity, little supercooling, low cost and good thermal reliability.•Composite PCM has great thermal characteristics and insulation properties.•Composite PCM shows great potentials to be applied in building energy conservation.A porous supporting matrix, expanded perlite (EP) is composited with a salt hydrate mixture of CaCl2·6H2O and SrCl2·6H2O (98:2 in mass ratio), to develop a nonflammable thermal storage material for building use. The salt hydrate is uniformly adsorbed on sheets of EP, and a maximum absorption capacity test shows EP can contain as high as 55 wt% the salt hydrate and keep the stable form. It is shown that the melting temperature of the composite PCM is 27.38 °C, close to that of CaCl2·6H2O, and its latent heat is 87.44 J/g, equivalent to the calculated value based on the mass fraction of CaCl2·6H2O in the composite. EP further reduces the salt hydrate thermal conductivity by over 70% to enhance its thermal insulation, and suppresses the supercooling. A 1000 heating-cooling cycles test verifies the CaCl2·6H2O/EP composite possesses good thermal reliability. The CaCl2·6H2O/EP composite is then fabricated into a board for replacing the foam board employed as the core in a commercial foam insulating brick to obtain a PCM brick. It is found that, when applied as the roof of a test room, the PCM brick has the function of decreasing the indoor peak temperature and causing the hysteresis in the indoor temperature rise, compared with the foam insulation brick. The nonflammability along with good thermal storage and insulation properties makes the CaCl2·6H2O/EP composite PCM show great promises for use in building energy conservation.

•The thermal performances of SIP structure containing CaCl2·6H2O/EG are studied numerically.•Increasing the thickness of PCM panel can improve the thermal performance of SIP structure.•When the thermal conductivity of PCM is above 0.5, it has no effect on the room thermal performance.•The PCM panel in different building orientation and construction will play a different role.•The PCM panel is more suitable for use in areas with large temperature different between day and night.Integrating phase change material (PCM) into traditional building envelopes can decrease the building energy consumption and improve the indoor thermal comfort by enhancing the heat storage capability. In this work, CaCl2·6H2O/expanded graphite (EG) was employed to fabricate the PCM panels for building envelope as a low-cost PCM. In order to give full play to the role of CaCl2·6H2O/EG, its characteristic in structural insulated panel (SIP) was investigated numerically under different conditions. The enthalpy-porosity technique was used for modeling the solidification/melting process and it was validated by experiment. Temperature time lag, temperature decrement factor and energy saving rate were used to evaluate the dynamic thermal characteristics of the building envelope. The results showed that the density and thermal conductivity of PCM had little effect on the thermal performance of building envelope. However, the change of PCM panel thickness, building orientation, climate region and building construction would affect the phase change process of the PCM panels, which would have a certain influence on the room temperature. By analyzing the effect of CaCl2·6H2O/EG on the thermal performance of the building envelope under different influencing factors, it will provide effective guidance for the selection and use of PCM.

An aqueous solution of Xanthan Gum (XG) at a weight fraction as high as 0.2% was used as the base liquid, the stable MWCNTs-dispersed non-Newtonian nanofluids at different weight factions of MWCNTs was prepared. The base fluid and all nanofluids show pseudoplastic (shear-thinning) rheological behavior. Experiments were performed to compare the shell-side forced convective heat transfer coefficient and pressure drop of non-Newtonian nanofluids to those of non-Newtonian base fluid in an integrally helical baffle heat exchanger with low-finned tubes. The experimental results showed that the enhancement of the convective heat transfer coefficient increases with an increase in the Peclet number and the nanoparticle concentration. For nanofluids with 1.0, 0.5 and 0.2 wt% of multi-walled carbon nanotubes (MWCNTs), the heat transfer coefficients respectively augmented by 24.3, 13.2 and 4.7% on average and the pressure drops become larger than those of the base fluid. The comprehensive thermal performance factor is higher than one and increases with an increasing weight fraction of MWCNTs. A remarkable heat transfer enhancement in the shell side of helical baffle heat exchanger with low-finned tubes can be obtained by adding MWCNTs into XG aqueous solution based on thermal resistance analysis. New correlations have been suggested for the shell-side friction coefficient and the Nusselt numbers of non-Newtonian nanofluids and give very good agreement with experimental data.

•A stable OP10E emulsion is prepared under the optimized emulsifying conditions.•Nano-graphite contributes little supercooling and enhanced thermal conductivity.•The graphite dispersed phase change emulsion exhibits good thermal reliability.•The viscosity of the novel emulsion meets the requirement for transportation.•The graphite dispersed OP10E emulsion is a promising cold storage medium.Graphite nanoparticles used as a nucleating agent have been dispersed into a phase change emulsion containing 30 wt% of OP10E, aiming at eliminating its supercooling and improving its thermal conductivity. Based on the optimizations using surface response methodology, the stable OP10E emulsion with small droplets can be prepared with the aid of the emulsifier with a HLB value of 8.9 at a loading of 5 wt%. It is found that the degree of supercooling for the emulsion decreases from 9.9 °C to almost 0 °C after the dispersion of the graphite nanoparticles at an optimal concentration of 2 wt%, suggesting a complete elimination in supercooling. The addition of the graphite nanoparticles has no effect on latent heat of the OP10E/water emulsion. The thermal conductivity of the emulsion containing 2 wt% graphite is 0.578 W m−1 K−1, achieving an enhancement by 88.9% as compared to the one without graphite. The viscosity of the emulsion containing 2 wt% graphite is lower than 11.5 mPa s, meeting the transportability requirements for pumping in practical applications. No distinct changes in these properties have been observed for the emulsion after being stored for 30 days or being experienced 300 heating-cooling cycles test, implying its good dispersion stability and thermal reliability. The high energy storage capacity, little supercooling, enhanced thermal conductivity, excellent thermal reliability and good fluidity make the graphite nanoparticles-dispersed OP10E/water emulsion show great potential for use as a novel cool storage fluid for high efficiency cool storage device.

•Successfully prepared graphite nanoparticles-dispersed paraffin/water emulsion.•The paraffin/water/graphite emulsion has high energy storage capacity and good fluidity.•The paraffin/water/graphite emulsion has excellent photo-thermal performance.In this paper, graphite nanoparticles have been dispersed into a paraffin/water emulsion with the purpose of improving the thermal conductivity and photothermal conversion performance of the emulsion. The melting enthalpy and apparent specific heat of 20 wt% paraffin/water emulsion is 39.2 J g−1 and 9.105 J g−1 K−1 and its melting and freezing temperatures are very close to those of paraffin. Moreover, the 300 heating–cooling cycles test indicates that the paraffin/water emulsions containing 0.1 wt% graphite exhibits good thermal reliability. The viscosities of all samples are lower than 0.0750 Pa s, meeting the transportablilty requirements in pump systems for applications. In addition, the thermal conductivity of the emulsion containing 0.1 wt% graphite increases by 20.0% as compared to the base fluid and its receiver efficiency is higher than 86.0% as the temperature ranges from room temperature to 80 °C. As a result, the paraffin/water emulsion containing graphite with high energy storage capacity, enhanced thermal conductivity and excellent photo-thermal performance, good thermal reliability and good fluidity shows great potential for use as advanced heat transfer fluid (HTF) in the low temperature applications of direct absorption solar collector (DASCs).

•Combined numerical and experimental study on graphene/ionic liquid nanofluid based high temperature receiver.•Effects of solar concentration, graphene concentration, nanofluid height on receiver efficiency were obtained.•Receiver efficiency was beyond 0.7 below 700 K.Novel heat transfer fluids with very low vapor pressure and high thermal stability are highly desirable for both high temperature direct solar collectors and concentrated solar collector. Herein a combined analytical and experimental study has been conducted on high temperature direct solar thermal collectors using graphene/ionic liquid nanofluids as the absorbers. A one-dimensional transient heat transfer model has been used to predict the receiver temperature and efficiency with varying parameters such as solar and graphene concentration and receiver height. The results show that the experimental temperature is in good agreement with numerical results under the same conditions. Based on the model, it is shown that the receiver efficiency increases with the solar concentration and receiver height, but decreases with the graphene concentration. The receiver efficiency could be maintained 0.7 under the conditions of 0.0005 wt% of graphene in 5 cm receiver under 20×1000 W m−2 at 600 K. This work provided an important perspective to the graphene/ionic liquid nanofluids for use as a kind of novel heat transfer fluid in direct solar thermal collectors under concentrated solar incident radiation.

•Successfully prepared microencapsulated paraffin@SiO2.•Successfully modified microencapsulated paraffin@SiO2 with GO.•The phase change slurry has a high heat storage capability.•The phase change slurry also has a remarkable photo-thermal performance.In this work, a new microencapsulated phase change material, paraffin@ silica (SiO2)/graphene oxide (GO), is prepared by two steps: engulfing paraffin with silica by in situ hydrolysis and poly condensation of tetraethoxysilane and the modification of the SiO2 shell with GO. The paraffin@SiO2/GO composite is composed of spherical capsules with diameters of ~20 um. Raman spectrometer analysis verifies the embedding of GO in the SiO2 shell. The melting and freezing temperatures of the composite are very close to those of paraffin. Based on the melting and freezing enthalpy of the composite, the encapsulation ratio of paraffin is calculated to be 50.8% in the paraffin@SiO2 composite and 49.6% in the paraffin@SiO2/GO composite. It is shown that the paraffin@SiO2/GO composite exhibits enhanced thermal stability and excellent thermal reliability. The phase change slurry prepared by dispersing the paraffin@SiO2/GO composite in water shows higher thermal conductivity and heat capacity along with remarkable photo-thermal conversion performance, making it potential for use as the heat transfer fluid in direct absorption solar collectors. The high heat storage capability and excellent photo-thermal conversion performance of the paraffin@SiO2/GO composite enable it to be a potential material to store solar energy in practical applications.

Experiments were performed to compare the shell-side heat transfer coefficient and pressure drop of an integrally helical baffle heat exchanger with rib-shaped fin tubes to those of that with low-fin tubes for oil cooling using water as a coolant. The experimental results showed that for the heat exchanger with rib-shaped fin tubes, the shell-side Nusselt and Euler numbers were augmented by 90−130% and 10%, respectively. The increase in heat transfer is significantly greater than that in pressure drop for rib-shaped fin tubes. Correlations have been suggested for both the shell-side Nusselt and Euler numbers for the two heat exchangers with different tube types and give very good agreement with experimental results. It is a promising route to use rib-shaped fin tubes instead of low-fin tubes for improving the performance of an integrally helical baffle heat exchanger.

Flexible composite phase change materials are highly desirable for fabricating novel masks for thermotherapy of allergic rhinitis. Herein, a novel flexible composite phase change material was prepared by adsorbing paraffin with a melting point of 44 °C into a kind of polypropylene hollow fiber via simple impregnation. The maximum mass fraction of paraffin encapsulated in the hollow fiber was determined to be as high as 82.1%. It is shown that the paraffin/fiber composite exhibits a high latent heat value of 199.9 J g−1, making this flexible composite phase change material suitable for thermal therapy. Then, the as-prepared flexible composite was woven to a cuboid block. The thermal conductivity of the cuboid block is much lower than the pure paraffin block, which has an advantage of thermal therapy mask for allergic rhinitis. Therefore, a channel-type mask was designed for the thermal therapy of allergic rhinitis. Experimental and simulative investigations on its thermal release performance (2250 s of the plateau at ~43 °C) prove this new kind of mask has great potential in thermal therapy of allergic rhinitis. With high thermal storage capability, flexibility, low thermal conductivity and excellent thermal release performance, this new kind of phase change composite shows great potential in thermal therapy of biomedical fields.

•Three kinds of nanofluids with carbon nanomaterials dispersing in ionic liquid were prepared.•Effect of morphology of carbon nanomaterials on photo-thermal characteristics was studied.•Grephene-dispersed nanofluids exhibit the highest thermal conductivity enhancement ratios.•Grephene-dispersed nanofluids show the lowest transmittance.•Grephene-dispersed nanofluids possess the highest extinction coefficients.Graphite nanoparticles (GNPs), single-wall carbon nanotubes (SWCNTs) and graphene (GE) were dispersed into an ionic liquid (IL) to prepare nanofluids at different mass fractions, respectively. The thermo-physical characteristics, radiative properties, and photo-thermal conversion performance of the IL-based nanofluids containing the carbon nanomaterials with different morphologies were investigated in details. It is shown that all the nanofluids exhibit an increase in thermal conductivity and a decrease in viscosity as composed with the base liquid, and the enhancement and reduction ratios varied with their morphologies. The GE-dispersed nanofluids exhibit the highest thermal conductivity enhancement ratios as compared to the GNPs- and SWCNTs-dispersed ones at the same mass fractions. Among the nanofluids containing different carbon nanomaterials, the GE-dispersed nanofluids show the lowest transmittance and possess the highest extinction coefficients. It is revealed that the photo-thermal conversion performance of the IL has been enhanced by the addition of the carbon nanomterials, and the GE-dispersed nanofluids exhibit the highest photo-thermal conversion efficiency among the nanofluids containing different carbon nanomaterials. The superiorities in thermal conductivity, optical property and photo-thermal conversion efficiency make the GE-dispersed nanofluids show great potential for use as high-performance HTFs in solar thermal systems such as working fluids for DASCs.