In this study, a new cold storage phase change material eutectic hydrate salt (K2HPO4·3H2O–NaH2PO4·2H2O–Na2S2O3·5H2O) was prepared, modified, and tested. The modification was performed by adding a nucleating agent and thickener. The physical properties such as viscosity, surface tension, cold storage characteristics, supercooling, and the stability during freeze-thaw cycles were studied. Results show that the use of nucleating agents, such as sodium tetraborate, sodium fluoride, and nanoparticles, are effective. The solidification temperature and latent heat of these materials which was added with 0, 3, and 5 wt% thickeners were −11.9, −10.6, and −14.8°C and 127.2, 118.6, 82.56 J/g, respectively. Adding a nucleating agent can effectively improve the nucleation rate and nucleation stability. Furthermore, increasing viscosity has a positive impact on the solidification rate, supercooling, and the stability during freeze-thaw cycles.
The development of cities is accompanied by huge energy consumption; people have gradually realized that the energy storage technology has very good ability to improve this situation, which can effectively improve the energy utilization ratio and reduce losses. Phase change material (PCM) is an effective latent heat thermal storage material. It has been widely used as a thermal functional material in thermal and cold energy storage fields, like solar energy storage [
Salt hydrates [
It can be seen above that most of the research on hydrated salt cold storage has focused on the development of energy storage materials for air conditioning. There is a lack of research on the energy storage materials with phase change temperature below 0°C. Otherwise, it is a meaningful attempt to study the improvement of supercooling and cycle stability of ternary eutectic system from the point of fluid viscosity and different nucleating agents added. In this paper, firstly, a K2HPO4·3H2O–NaH2PO4·2H2O–Na2S2O3·5H2O ternary salt system was prepared, modified, and synthesized. Then the contrast experimental method, step cooling method, and DSC technology were proceeded to investigate the thermal storage property and solidification behavior of eutectic hydrate when adding different amounts of nucleating agent and thickener.
Preparation of hydrated salt phase change material (HSPCM) is the major key step to ensure the performance of PCM in cold storage application. The proper mixing and stabilization are required in order to achieve stable HSPCM. In the present study, deionized water (DI water) as the base PCM; sodium dihydrogen phosphate dihydrate, dipotassium hydrogen phosphate trihydrate, and sodium thiosulfate pentahydrate as hydrated salts; nanoactivated carbon, sodium tetraborate, and sodium fluoride as nucleating agents; polyethylene glycol 400 as a dispersant; and sodium alginate as a thickener were used to prepare the HSPCM. The used materials are listed in Table
Experimental materials.
Experimental materials | Purity | Application |
---|---|---|
Sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O,) | AR | Hydrated salt |
Sodium alginate | CP | Thickener |
Sodium thiosulfate pentahydrate (Na2S2O3·5H2O) | AR | Hydrated salt |
Dipotassium hydrogen phosphate trihydrate phosphate (K2HPO4·3H2O) | AR | Hydrated salt |
Nanoactivated carbon (100 nm, heat treatment) | AR | Nucleating agent |
Sodium tetraborate | AR | Nucleating agent |
Polyethylene glycol 400 | AR | Dispersant |
Sodium fluoride | AR | Nucleating agent |
Nanocomposite PCM preparation flow chart.
Table
Result uncertainty analysis of experimental equipment.
Measured quantities | Deviation |
---|---|
Latent heat | ±6 kJ/kg |
Mass | ±0.002 g |
Temperature data logger | ±0.06% |
Volume (10 ml, 20 ml) | ±0.06 ml |
Volume (100 ml) | ±0.02 ml |
Thermocouple | ±1.0% |
Surface tension | ±0.01 mN/m |
Viscosity | ±2% mps |
Schematic diagram of the experimental system.
In order to assess the effect of thickener concentration on onset/end temperature and phase change heat absorption capacity during the phase change process, the thermal analysis was conducted in different cases of 0, 3, and 5 wt% sodium alginates, while the latent heat of melting process and onset/end temperature was measured with a differential scanning calorimeter (DSC), using measuring temperature range from −25 to 5°C and constant heating rates of 2 K·min−1. In the process of measurement, the temperature of the material rises constantly, and the heat flux of the sample was measured continuously. The heat flux is proportional to the instantaneous specific heat of the material. Melting phase change enthalpy was calculated through the area of the endothermic peak in the DSC picture. In addition, the specific heat, heating rate (
Figure
DSC curves of eutectic hydrate salt with three different amount of thickening agent.
From Table
DSC measurements.
Data items | Thickener (wt%) | ||
---|---|---|---|
0 | 3 | 5 | |
Area (J/g) | 82.56 | 118.6 | 127.2 |
Peak sample temperature (°C) | −8.3 | −2.4 | −5.0 |
Onset temperature (°C) | −14.8 | −10.6 | −11.9 |
End temperature (°C) | −7.5 | −2.3 | −4.6 |
Table
Melting latent heat of water and three samples.
Material | Melting latent heat (kJ/kg) |
---|---|
Ice | 335 |
Hydrated salt with 0 wt% thickener added | 127.2 |
Hydrated salt with 3 wt% thickener added | 118.6 |
Hydrated salt with 5 wt% thickener added | 82.56 |
Supercooling is a process closely related to the crystallization process. Before the temperature returns to the original freezing point, the liquid solidifies below its normal freezing point and continues to decrease until complete solidification. When the degree of supercooling increased, the degree of the deviation from the equilibrium state has risen, and the critical dimension of the ice core and the formation energy also decrease dramatically, which ultimately increases the probability of forming the nucleus. Calculating the absolute degree of supercooling following relation was used to measure the thermal storage performance of materials:
The supercooling degree was measured by temperature change curves during the cooling process by putting a thermocouple at the three sample’s center separately. During the solidification and crystallization process, the general liquid crystal is divided into the stable region (noncrystalline), substable region, and unstable region as shown below in Figure
Supersaturation and super solubility curves of the solution.
Figure
Step cooling curves of adding 1, 3, and 5 wt% thickeners in the solution without the nucleating agent.
In this experiment, the characteristics of the fluid itself were significantly changed with a constant increase in viscosity. From Table
Viscosity of water and hydrated salts.
Material | Viscosity (mpa·s) |
---|---|
Ice | 1 |
Hydrated salt with 0 wt% thickener added | 2.04 |
Hydrated salt with 3 wt% thickener added | 6.3 |
Hydrated salt with 5 wt% thickener added | 8.77 |
Figure
Step cooling curves of adding 0, 3.12, and 7.46 wt% thickeners and 2.5 wt% nucleating agent in the solution.
Figure
Solidification mass friction curve of three samples with different nucleating agents added.
The viscosity of the hydrated salt affects not only the internal thermophysical properties of the fluid but also the properties of the fluid surface. Figure
Surface tension of three samples within three different temperatures.
In the above test, it was found that the supercooling phenomenon is a serious defect of the inorganic phase change material. Through many studies of the nucleation mechanism, it is proved that the supercooling also improved effectively by adding a nucleating agent. According to the crystal nucleation theory, for homogeneous nucleation process, the formation of the crystal needs to be larger than the critical size of the particles, but for heterogeneous nucleation, the surface affinity should also be considered [
(a), (b), and (c) represent step cooling curves of adding three different kinds of nucleating agent (nanoactivated carbon, NaF, and Na2B4O7·10H2O) in ternary eutectic hydrate salts without a thickener.
When using a cold storage material, life span is an important factor to measure the stability. In this experiment, adding a thickener in inorganic hydrated salt is an effective method to enhance the working life of materials and prevent phase separation phenomenon. The salt solution which was added 3 wt% and 5 wt% thickeners still maintained good stability, and there was no obvious phase separation phenomenon. However, a thin layer of crystal at the bottom after about 10 times of freeze-thaw cycles in the salt solution without a thickener appeared. As the freeze-thaw cycles continue, crystal thickness was increased, which severely affected the cold storage characteristics of the salt solution. After stirring or heating, the sample changed back to its initial state and no rapid degradation or irreversibility was observed. Therefore, to avoid the shortcoming of the phase separation mentioned above, the mixture can be stabilized by adding a thickener.
Figure
(a) and (b) represent the change of step cooling temperature curve after 20, 30, and 50 freeze-thaw cycles.
Adding 3 wt% thickener
Adding 5 wt% thickener
This experimental study was conducted on the preparation, modification, and characterization of a new ternary eutectic of inorganic salt hydrate phase change materials with 0, 3, and 5 wt% thickeners added into salt hydrate solution, and the effect of viscosity and nucleating agents on the thermophysical properties and cold storage ability of PCM was measured by DSC and temperature acquisition system. The results of this study can be summarized as follows:
The increase of the viscosity will reduce the phase change enthalpy, which reduces the storage capacity. Meanwhile, the phase stability, supercooling, and phase separation will be significantly improved. NaF, Na2B4O7·10H2O, and nanoactivated carbon were all proved to be effective nucleating agents. With the increasing content of a nucleating agent, the degree of cooling can be obviously reduced or even eliminated. During the stability test of the inorganic material, the supercooling of inorganic materials will become smaller after 20, 30, and 50 freeze-thaw cycles.
The degree of supercooling
Heat flux
Melting phase change enthalpy
The melting point temperature
The freezing point temperature
Analytically pure reagent
Chemically pure reagent
Deionized water
Phase change materials
Hydrated salt-based PCMs
Simultaneous thermal analyzer
X-ray diffraction
Differential scanning calorimeter.
The authors declare that they have no conflicts of interest.
This work is financially supported by the Fundamental Research Funds for the Central Universities (Grant no. 2014QNB05).