Salt Hydrate Phase Change Materials Research Articles

Parametric modeling and simulation of Low temperature energy storage for cold-climate multi-family residences using a geothermal heat pump system with integrated phase change material storage tank

A novel geothermal heat pump (GHP) system with an integrated low- to moderate-temperature salt hydrate phase change material (PCM) storage tank for buildings in cold climates is proposed in this study. The purpose of the PCM storage tank is to dampen peak heating loads and to remove annual ground thermal load imbalances on the ground heat exchanger (GHX) to assist in achieving an optimally-sized GHX. As heat is extracted from the closed-loop system by heat pumps in heating mode, a significant portion of this heat is used to solidify a salt hydrate PCM. This heat of fusion is later released back into the heat transfer fluid, storing it in the PCM tank and GHX for later diurnal and seasonal use. To examine the merits of the proposed concept, electric utility meter data on 15-minute time intervals were mined from an actual apartment building and used to estimate space heating, cooling, and hot water heating loads. Those data were used in an hourly, dynamic 20-year life-cycle simulation model in TRNSYS to design an optimum combination of GHX and PCM storage, where each component was sized to balance the annual ground thermal loads. The system simulation results show significant potential for GHX size reduction with a PCM storage tank, but the system is quite sensitive to the PCM melt temperature due to significant hysteretic nature of the salt hydrate PCM heating and cooling curves. We also find that there is no unique optimum unless other factors are considered such as installation cost and physical constraints; many combinations of GHX size and PCM mass are capable of achieving the design goal with similar annual electric energy consumption. For the cases examined here, a PCM melt temperature of 27 °C yields the most favorable economic results, and a preliminary economic analysis suggests that with typical drilling cost and PCM tank cost values, the GHX size can be reduced by over 50 %.

Effect of phase separation and supercooling on the storage capacity in a commercial latent heat thermal energy storage: Experimental cycling of a salt hydrate PCM

• Experimental study of a novel latent cold storage design in a laboratory unit. • Phase separation and supercooling has been determined to cause a significant storage capacity decrease as soon as after the first cycle. • The phase separation and supercooling effects in the storage is reversible and repeatable results are presented. • A sample analysis only on material scale is not enough to predict this storage decrease. Latent heat storage technologies offer process benefits like daily peak shaving. In this work a commercial storage design for storing cold thermal energy has been studied using a laboratory prototype containing 168 kg of a commercial salt-hydrate phase change material (PCM). The storage was charged and discharged with subsequent cycles at different mass flow rates over a fixed temperature range and duration. It was found that the PCM TES design exhibits phase separation and increased supercooling with continuous cycling. Both phenomena lead to a gradual decrease of the effective storage capacity. With later cycles only the bottom part stores latent heat, while the top and middle parts of the storage remain liquid. The results were repeatable and are consistent with T-History measurements of samples from the PCM TES before and after cycling. It is likely that the PCM itself does not suffer from incongruent melting. Instead, the phase separation is likely to occur due to a segregation of different liquid phases across the height of the storage. It was found that T-History measurements alone are not able to predict this behavior. Moreover, it is shown that phase separation in the storage can be reversed by increasing the PCM temperature and mechanical mixing of the liquid phase. This phase separation has to be prevented in future work in order to achieve stable performance with the studied storage design.

Study of an autothermal-equilibrium metal hydride reactor by reaction heat recovery as hydrogen source for the application of fuel cell power system

This paper proposes a novel autothermal-equilibrium metal hydride reactor as the hydrogen source for the fuel cell power system, which employs phase change material (PCM) to recycle the hydrogen storage heat. A three-dimensional model of the metal hydride reactor coupled with a salt hydrate PCM for heat recovery is developed. Based on the model, the effects of key operating and design parameters on the reactor are investigated for performance optimization, including operating pressure, melting temperature, latent heat and thermal conductivity of PCM. Through the parametric analysis, it is found that increasing the operating pressure is beneficial to accelerate the absorption reaction. The average reaction fraction at 2400 s is increased by 24% with the pressure increasing from 6 to 10 bar. The moderate melting temperature and the thermal conductivity of the PCM that is comparable to that of metal hydride bed contribute to the improvement of hydrogen storage efficiency. Using this kind of hydrogen source reactor in a fuel cell power system, stable hydrogen storage efficiency of approximately 60% in the experiment is presented. In addition, no obvious performance deterioration of the power system occurs after ten cycles.

Metal nanoparticles enhanced thermophysical properties of phase change material for thermal energy storage

Phase change materials (PCMs) are one of the promising materials in thermal energy storage systems. In this work PCM nanocomposites were prepared using melt-blending technique by dispersing metal nanoparticles (Fe, Cu) at mass fraction of 0.5 wt% in magnesium nitrate hexahydrate (MNH), an inorganic salt hydrate PCM. The as-prepared PCM nanocomposites were analyzed by scanning electron microscopy (SEM) and X-ray diffractometer (XRD). Fourier transform infrared spectroscopy (FTIR) analysis was carried out to monitor the changes in chemical nature of PCM nanocomposites. The heat transfer characteristics were investigated by conventional heating system, which were used to carry out melting (charging) and solidification (discharging) cycle of MNH-metal nanocomposites. The experiment results clearly indicates that the rate of melting and solidification of MNH-metal nanocomposites increased at 0.5 wt% mass fraction of metal nanoparticles as compared to MNH. The thermal conductivity of MNH-metal nanocomposites at 0.5 wt% mass fraction of metal nanoparticles (Fe, Cu) in solid phase was measured using the transient hot method, which clearly indicates that thermal conductivity improved to (0.61) W m−1 K−1 for MNH-Fe nanocomposite & (0.63) W m−1 K−1 for MNH-Cu nanocomposite than that of pure MNH (0.4) W m−1 K−1. The prepared nanocomposites showed good heat transfer characteristics and better thermal conductivity. Therefore, this study demonstrates that metal nanoparticles, added to inorganic PCM (MNH) had a significant potential for enhancing the thermophysical properties and makes it promising candidate for thermal energy application.

Performance Evaluation of a Plate Encapsulated Salt Hydrate PCM Mixed with a Gel

An alternative method of cooling is required to meet cooling demands and simultaneously decrease conventional energy consumption. The current research aimed to investigate the feasibility of using a phase change material (PCM) in the form of a salt hydrate mixed with a CSIR-developed gel for PCM based cold storage that used nighttime cold to cool ambient air during the day. The experiments were conducted over a time period of three hours between 12:00 pm and 15:00 pm in the afternoon. The total temperature drops were found to be 3.8, 2.9 and 2.6 degrees for air flow rates of 0.03, 0.05 and 0.06kg/s respectively. The total energy transferred for each of the mass flow rates averaged 140.9, 144.4 and 158.9J for air flow rates of 0.03, 0.05 and 0.06kg/s respectively.

Fabrication and characterization of CaCl2·6H2O composite phase change material in the presence of CsxWO3 nanoparticles

Phase change materials (PCMs) with excellent energy conservation performance have been extensively utilized in solar energy storage system. In this study, a novel kind of thermal storage composite material based on calcium chloride hexahydrate (CaCl2·6H2O) salt hydrate as core PCM and cesium tungsten bronze (CsxWO3) nanoparticles as near-infrared (NIR) light absorbent additives is fabricated. The effect of CsxWO3 nanoparticles on crystalline morphology, thermal physical behaviors and light transmittance of CaCl2·6H2O are investigated. Additionally, thermal cycle tests are conducted for stability examination. Furthermore, thermal regulating performance of double layer glass window filled with the PCM composite is tested in small chambers by being sunlight irradiated under a solar simulator. All results manifest that with the incorporation of CsxWO3 nanoparticles, the salt hydrate PCM composite exhibits large latent heat, low supercooling degree and high NIR light shielding ability as well as good thermal stability. When employed as the glazing window of a test room, the PCM panel has functions of reducing indoor temperature fluctuations and increasing thermal inertia. Hence, the salt hydrate/nano CsxWO3 PCM design in this work shows great potentials for the development of solar energy storage products in energy conservation fields.

Thermal and mechanical properties of PCM-incorporated normal and lightweight concretes containing silica fume

Improving the thermal performance of concrete, as an important construction and pavement material, by incorporating phase change materials (PCMs) has been the topic of much research. Also, various carrier agents such as lightweight aggregate (LWA) have been introduced to incorporate PCMs into concrete. However, incorporation of PCM-impregnated LWA reduces the compressive strength of concrete. In this study, the application of silica fume to improve the compressive strength of PCM-incorporated concrete is investigated. Two types of PCMs, salt hydrate PCM and polyethylene glycol PCM, were incorporated into concrete via scoria LWA, and different physical and mechanical characteristics of the concrete were studied when silica fume was incorporated into the mix. The results show that incorporation of silica fume increases the compressive strength of PCM-incorporated concrete, and at the same time does not diminish the thermal performance of the incorporated PCM. Therefore, incorporation of silica fume was found to be a promising strategy to improve the compressive strength of PCM-incorporated concrete.

Experimental analysis for thermal storage performance of three types of plate encapsulated phase change materials in air heat exchangers for ventilation applications

Due to climate change and rising global temperatures, energy demand associated with commercial and office building cooling, is projected to increase. Passive-cooling, based on phase change materials, can assist to moderate this increase, however relatively few modelling equations to describe its operating behaviour exists. In this experimental investigation the melting performance of three commercially available encapsulated phase change materials were evaluated for passive cooling applications in air ducts systems. Two paraffin type phase change materials and one salt hydrate phase change material with melting temperatures in the range of 22 °C and 28 °C were considered. Vertical orientated plate type encapsulations with a thickness of 10 mm and a pitch of 15 mm were tested for air inlet temperatures ranging from 30 °C to 35 °C and upstream air velocities ranging from 0.4 m/s and 0.9 m/s. The average effectiveness, cooling power, energy absorption, and phase transformation durations were determined. It was found that the average thermal effectiveness decreased with increased velocities and that the cooling power, which was inversely proportional to the phase transformation duration, increased with air flow rate and inlet air temperature. Based on the data a new empirical correlation model was developed which describes the cooling capacity of the in-duct phase change material plates.

Experimental Investigation on Graphene Oxide/SrCl₂·6H₂O Modified CaCl₂·6H₂O and the Resulting Thermal Performances.

Although the inorganic salt hydrate phase change materials (PCMs) such as CaCl2·6H2O have promising potential for thermal energy storage in building application, the issue of supercooling has restricted their practical application. In this study, graphene oxide (GO) and SrCl2·6H2O as binary nucleation agents were used to modify CaCl2·6H2O and reduce its supercooling degree. Compared with pure CaCl2·6H2O, the incorporation of graphene oxide (GO)/SrCl2·6H2O reduced the supercooling degree to 0.3 °C significantly. In addition, the supercooling degree of modified CaCl2·6H2O after 200 thermal cycles was still much lower than that of non-modified CaCl2·6H2O. From the results of differential scanning calorimetry (DSC), the latent heat value and phase change temperature of the modified CaCl2·6H2O were 207.88 J/g and 27.6 °C, respectively. Aluminum capsules were used to encapsulate the modified PCM and placed inside the composite wallboard. The thermal performances of the composite wallboard with modified PCM were investigated using infrared thermography. Experimental results showed that the average temperature difference between the top and bottom surfaces of modified CaCl2·6H2O/wallboard composite after 1 h heating was kept around 15.8 °C, while it was 4.9 °C for the control wallboard. The above test results proved that the modified CaCl2·6H2O demonstrated good thermal performance and can be used in buildings to maintain thermal comfort.

Investigation of different materials for macroencapsulation of salt hydrate phase change materials for building purposes

Most of the installed phase change material in buildings is expensive microencapsulated paraffin, which is flammable. Salt hydrates are corrosive and hygroscopic but cheap. A suitable salt hydrate encapsulation material would be of great interest to the construction industry. Thus, macroencapsulation can be a possible and cheap opportunity. Therefore, different methods to macroencapsulate phase change material, consisting of 50 wt% MgCl2 × 6H2O and 50 wt% CaCl2 × 6H2O, were investigated. The investigation results show that the foils with a layer of aluminum or polyvinylidene chloride and liquid spar varnish with additives (mica, graphite, glass flakes) are suitable materials. Furthermore, liquid rubber (Noxyde®), liquid EPDM (ethylene propylene diene (M-class)) and liquid polyvinylidene chloride as well as butyl rubber as extruded tape, malleable modelling clay or injectable mass were tested. These materials are also usable, whereas liquid two-component polysulfide and other foils are not suitable. In conclusion, the supplier can decide to use foils or liquid materials based on the application, the construction material and the price.

Effect of percussion vibration on solidification of supercooled salt hydrate PCM in thermal storage unit

Abstract Experiments are performed on the effect of percussion vibration on triggering solidification of supercooled sodium acetate (CH3COONa) salt solution in the rounded - rectangular thermal storage unit. To provide quantitative analysis, the steel ball freely falling down to the surface of PCM (phase change material) unit is applied for percussion vibration with parameters of percussion number and crystallization induction time. Factors such as the steel ball diameter and falling height, as well as the percussion position on the PCM unit are examined about their effects on the crystallization induction. The results show that it is favorable to activate solidification of supercooled sodium acetate solution with larger percussion momentum (larger ball diameter and higher falling height), percussion near the cover lid and edges of unit. A term named percussion effectiveness is also introduced for evaluation of percussion effect on solidification activation. The results in this paper are useful for seasonal thermal storage of solar energy within supercooled inorganic PCMs and control of discharging the stored energy for space heating.

The melting of salt hydrate phase change material in an irregular metal foam for the application of traction transient cooling

To cope with the increasing cooling loads in power electronics especially in traction, a new heat sink based on the irregular aluminium foam/salt hydrate phase change material composite was proposed for the integration into liquid cooling or heat pipe systems to provide transient cooling during the traction stationary phase. This heat sink would also be applicable to accommodate power surges or thermal buffering-related conditions. The melting point of the salt hydrate PCM was 46 °C. The coolant water was heated up to 60 °C, similar to the inlet condition in traction liquid cooling systems. The vacuum impregnation method used to create the composite resulted in a 96% impregnation ratio. The heat sink absorbed a thermal load up to 0.092 kW h with increasing charging temperature from 50 to 60 °C (from ambient: 20 °C) within an hour of heating and the PCM was fully melted. This was a combination of latent and sensible heat storage methods. The thermal capacity value matched closely with the theoretical calculation of 0.103 kW h (11% deviation). An hour of complete melting would match the stopping period of intercity trains at the terminus and the sink would be able to cool a group of 4–5 IGBTs, rated at 20 W power dissipation for each IGBT, during this stopping period. The heat balance formulated was commendable due to the small mismatch (7% deviation) between the released and absorbed heat, from the coolant to the heat sink. Therefore, it could be used to further optimise the design with alternative PCMs, foam porosities, operational conditions, thermal masses, heat exchanger configurations and so on. The effective thermal conductivity (Keff) for the composite was calculated as 10.8 W/m K and the ratio of Keff/KPCM was 24, higher than many composites previously studied in literature. In general, the heat sink would benefit in providing a transient, compact and inexpensive solution.

Preparation and Supercooling Modification of Salt Hydrate Phase Change Materials Based on CaCl2·2H2O/CaCl2

Salt hydrates have issues of supercooling when they are utilized as phase change materials (PCMs). In this research, a new method was adopted to prepare a salt hydrate PCM (based on a mixture of calcium chloride dihydrate and calcium chloride anhydrous) as a novel PCM system to reduce the supercooling phenomenon existing in CaCl2·6H2O. Six samples with different compositions of CaCl2 were prepared. The relationship between the performance and the proportion of calcium chloride dihydrate (CaCl2·2H2O) and calcium chloride anhydrous (CaCl2) was also investigated. The supercooling degree of the final PCM reduced with the increase in volume of CaCl2·2H2O during its preparation. The PCM obtained with 66.21 wt % CaCl2·2H2O reduced the supercooling degree by about 96.8%. All six samples, whose ratio of CaCl2·2H2O to (CaCl2 plus CaCl2·2H2O) was 0%, 34.03%, 53.82%, 76.56%, 90.74%, and 100% respectively, showed relatively higher enthalpy (greater than 155.29 J/g), and have the possibility to be applied in buildings for thermal energy storage purposes. Hence, CaCl2·2H2O plays an important role in reducing supercooling and it can be helpful in adjusting the solidification enthalpy. Thereafter, the influence of adding different percentages of Nano-SiO2 (0.1 wt %, 0.3 wt %, 0.5 wt %) in reducing the supercooling degree of some PCM samples was investigated. The test results showed that the supercooling of the salt hydrate PCM in Samples 6 and 5 reduced to 0.2 °C and 0.4 °C respectively. Finally, the effect of the different cooling conditions, including frozen storage (−20 °C) and cold storage (5 °C), that were used to prepare the salt hydrate PCM was considered. It was found that both cooling conditions are effective in reducing the supercooling degree of the salt hydrate PCM. With the synergistic action of the two materials, the performance and properties of the newly developed PCM systems were better especially in terms of reducing the supercooling degree of the PCM. The novel composite PCMs are promising candidates for thermal energy storage applications.

Melting of phase change material assisted by expanded metal mesh

Accelerating the melting of phase change material (PCM) via metal foam impregnation is an established method. In this work, it is proposed to use expanded metal mesh (EMM) as an inexpensive alternative. Raised aluminium EMM sheet was cut into five layers, arranged perpendicularly and attached with wire columns, creating a 90% porosity unit. Salt hydrate PCM, which melted at 46 °C, filled the void space creating a PCM/EMM composite. The composite was heated at 55 °C, and the melt completion time was compared with the control i.e. pure PCM. It was found that EMM reduced the melting time by 14%, which could be increased if better thermal interfaces such as soldering or brazing were used. Small scale modelling compared very well with the experimental results, which verified its viability to model PCM/EMM systems. Simulation showed that an excellently contacting EMM layers arranged in parallel, resulted in an 81% melting time reduction, far superior than any metal foam/PCM performances to date. EMM is a promising concept in enhancing heat transfer in PCM, which requires further studies to optimize the performance.

Effects of PCM state on its phase change performance and the thermal performance of building walls

To figure out the effects of PCM location on the thermal performance of building walls, the effects of the state of a kind of salt hydrate phase change material (PCM) on its phase change performance were researched by the differential scanning calorimeter (DSC) tests and the cooling experiments. The three different PCM states are the not-melted state, the partially-melted state, and the fully-melted state, respectively. The experimental results showed that PCM state had great effects on the degree of supercooling and the phase change performance of PCM. When the PCM was in the partially-melted state before cooling, it was able to release the latent heat quickly, while when the PCM was in the fully-melted state before cooling, its degree of supercooling was large. Because the temperature ranges of PCM during the thermal cycles were different when the PCM was placed in different locations in the wall, the PCM location had significant effects on its state, and as a result, on the phase change performance of PCM and the thermal performance improvement of building walls.

Corrosion of metal and metal alloy containers in contact with phase change materials (PCM) for potential heating and cooling applications

Thermal energy storage (TES) using phase change materials (PCM) can be used for load shaving or peak load shifting when coupled to a heating, ventilation, and air-conditioning (HVAC) system such as heat pump. In these systems the PCM is embedded in packages or used in bulk, so the compatibility of the encapsulation materials and the PCM is a key factor to ensure long operational life of the system. Although corrosion caused by salts is known from the chemical industry, when these salts are used as PCM no corrosion data is available, since the salts are used without being in water solution. Producing new corrosion data is essential for PCM utilisation in new applications. In this study the corrosion rate of two metals and two metal alloys when they are in contact with different salt hydrate PCM is evaluated; in total eleven PCM, being four of them commercial PCM, are tested. Since they are PCM to be used for heating and cooling applications they are classified in two different groups to present the corrosion study. Results present the recommendation of using each PCM with the different metals and metal alloys according to the obtained corrosion rate and visual observation of the samples.

Solar Energy Storage in Phase Change Materials: First-Principles Thermodynamic Modeling of Magnesium Chloride Hydrates

Thermal energy storage in salt hydrate phase change materials, such as magnesium chloride hydrates, represents an attractive option for solar energy applications. In this study, the structural, electronic, and thermodynamic properties of magnesium dichloride hexahydrate, MgCl2·6H2O, and its dehydrated phases, MgCl2·nH2O (n = 4, 2, 1), were computed within the framework of density functional theory. Densities of states were predicted, and phonon analysis using density functional perturbation theory was performed at equilibrium volume to derive isochoric thermal properties (i.e., Helmholtz free energy, entropy, and isochoric heat capacity). Isobaric thermal properties (i.e., Gibbs free energy, isobaric heat capacity, and latent heat) were also calculated within the quasi-harmonic approximation. Overall good agreement is observed between the computed thermodynamic properties and the scarce experimental data available for these materials.

Thermographic validation of a novel, laminate body, analytical heat conduction model

The two-region fin model captures the heat spreading behaviour in multilayered composite bodies (i.e., laminates), heated only over a small part of their domains (finite heat source), where there is an inner layer that has a substantial capacity for heat conduction parallel to the heat exchange surface (convection cooling). This resulting heat conduction behaviour improves the overall heat transfer process when compared to heat conduction in homogeneous bodies. Long-term heat storage using supercooling salt hydrate phase change materials, stovetop cookware, and electronics cooling applications could all benefit from this kind of heat-spreading in laminates. Experiments using laminate films reclaimed from post-consumer Tetra Brik cartons were conducted with thin rectangular and circular heaters to confirm the laminate body, steady-state, heat conduction behaviour predicted by the two-region fin model. Medium to high accuracy experimental validation of the two-region fin model was achieved in Cartesian and cylindrical coordinates for forced external convection and natural convection, the latter for Cartesian only. These were conducted using constant heat flux finite heat source temperature profiles that were measured by infrared thermography. This validation is also deemed valid for constant temperature heat sources.

Thermal characteristics of oleochemical carbonate binary mixtures for potential latent heat storage

AbstractThe thermal properties for binary mixtures at various weight fraction ratios between dodecyl carbonate (1a), tetradecyl carbonate (1b), hexadecyl carbonate (1c), and octadecyl carbonate (1d) were examined by DSC at 0.5, 1.0, and 5.0°C/min. At 5°C/min sharp well defined phase transitions were observed at 80:20, 88:12, and 90:10, for 1a:(1b, 1c, and 1d), respectively, at 74:26 and 80:20 for 1b:(1c, and 1d), respectively, and at 82.5:17 for 1c:1d. The melting and solidification temperatures for these mixtures ranged between 12–37°C (melting) and 8–34°C (solidification). The latent heat of melting and solidification determined for the binary carbonate mixtures ranged between 135 and 175 J/g. At 0.5 or 1.0°C/min, mixtures of 1a:(1b, 1c, and 1d) and 1b:1d melted and solidified similarly when run at 5°C/min while the melting curves for 1b:1c and 1c:1d were not as well defined. These carbonate mixtures extend the thermal properties of the pure oleochemical carbonates and compliment other biobased phase change materials (PCMs) substances such as fatty acids, esters, and alcohols and may provide useful biobased alternatives to paraffin wax and salt hydrate PCM currently dominating the market.

Measurements of Heat Capacity and Enthalpy of Phase Change Materials by Adiabatic Scanning Calorimetry

Phase change materials (PCMs) are substances exhibiting phase transitions with large latent heats that can be used as thermal storage materials with a large energy storage capacity in a relatively narrow temperature range. In many practical applications the solid–liquid phase change is used. For applications accurate knowledge of different thermal parameters has to be available. In particular, the temperature dependence of the enthalpy around the phase transition has to be known with good accuracy. Usually, the phase transitions of PCMs are investigated with differential scanning calorimetry (DSC) at fast dynamic scanning rates resulting in the effective heat capacity from which the (total) heat of transition can be determined. Here we present adiabatic scanning calorimetry (ASC) as an alternative approach to arrive simultaneously at the equilibrium enthalpy curve and at the heat capacity. The applicability of ASC is illustrated with measurements on paraffin-based PCMs and on a salt hydrate PCM.