Latent Thermal Energy Storage Research Articles

The evaluation of the effective thermal conductivity of metal-foam loaded phase change materials

Phase Change Materials (PCMs) are suitable materials to be included in Latent Thermal Energy Storage Systems (LTESS) to enhance the storage capacity per unit of volume. In order to increase their typical low thermal conductivity, PCMs are often loaded with high-porosity open-cell metal foams. Several correlations have been proposed in the literature to evaluate the effective thermal properties of the composite medium made of PCM and metal foam. However, the values of the effective thermal conductivity (keff) predicted by the different relationships can be very different from each other, with a consequent strong impact on the results of numerical simulations. In this work, a critical overview of the accuracy of the most used literature correlations for the evaluation of the effective thermal conductivity of open-cell metal-foam loaded PCMs is made: the temperature distribution obtained through a numerical model using different correlations is compared with the experimental values measured by testing different commercial paraffins loaded with copper or aluminum foams, subjected to complete melting. Since no correlation proves to yield accurate results for all the composite PCMs tested in this work, a new method for the calculation of the effective thermal conductivity of the PCM-metal foam medium is suggested.

Performance enhancement of latent energy storage system using effective designs of tubes and shell

This study aims to numerically investigate the effects of geometric designs of tubes and shell on thermal performance enhancement of latent thermal energy storage system (LTESS). Stearic acid is used as a phase change material (PCM) while water acts as heat transfer fluid (HTF). Starting with a base case consisting of three circular HTF tubes within a circular shell, the tube and shell geometries are modified systematically. First, the effect of tube shapes and their orientations are investigated in detail. The circular exteriors of HTF tubes are modified with hexagonal, pentagonal, square and triangular shapes. The performance of triangular tubes with the vertex pointing downward exceeds all the other tube configurations. It augments the melting rate of the PCM by 27.2% and the energy storage capacity of the LTESS by 3.72%, as compared to the base case. The bottom vertex angle of the best HTF tube design is then varied yielding 45∘ as the optimum triangular tube configuration. It improves the energy storage capability of the LTESS by 7.61% and the melting rate of the PCM by 41.4%. Following the optimum HTF tube design, the triangulated shell designs with various bottom vertex angles are explored. The 75∘ bottom vertex angle of the shell offers maximum improvement. It accelerates the melting rate of the PCM by 66.9% while enhancing the energy storage capacity of the LTESS by 23.7% in comparison to the base case. Lastly, two new correlations of melting Fourier number and average Nusselt number are proposed for the optimum LTESS design configuration.

Phase Change Material numerical simulation: enthalpy-porosity model validation against liquid fraction data from a computed tomograph

Nowadays, Latent Thermal Energy Storage systems (LTESs) are becoming the key enabling technology for the achievement of the 2050 EU targets on energy savings and CO2 emission reduction. LTESs are based on Phase Change Materials (PCMs), which, during the liquid-solid phase change, can store and release a great amount of thermal energy compared to sensible systems. Several studies have already been conducted to properly design LTESs based on PCMs and many of those aimed at developing reliable numerical tools. The main way to model the phase change with a CFD approach is through the use of the enthalpyporosity method, which has already been successfully validated in several articles, especially against the average PCM temperature. However, due to the experimental complexity, the liquid fraction evolution is hard to be measured and monitored and only symmetrical systems could have been tested so far. Computed Tomography (CT) can become a strong and reliable tool to experimentally evaluate the liquid fraction profile during either the melting, and solidification process in real geometries becoming extremely useful to validate the numerical models. In the present study, a first attempt to combine the described experimental-numerical approach is proposed: an enthalpy-porosity model was developed and validated against liquid fraction data from a computed tomography system. The liquid fraction profile of an ice cylinder melting at ambient temperature was measured using the CT and then compared against the numerical results. The results show the promising capabilities of the proposed method.

Enhancement of thermal performance of latent thermal energy storage systems using periodically reciprocating flows

• Periodic reciprocating flow enhances radial heat conduction in PCM. • Lower sensible heat transfer and higher latent heat transfer rates are observed. • Increasing flow rate or fluid temperature increases effects of reciprocating flow. • An optimal reciprocation period maximizes effects of reciprocating flow. • Higher thermal conductivity of PCM increases effects of reciprocating flow. From the perspective of increasing heat transfer rates in latent thermal energy storage systems (LTESS), less attention has been paid to the thermodynamic characteristics at the physical boundary between phase change material (PCM) and heat transfer fluid (HTF). The current numerical study analyzes a simple method of enhancing the thermal performance of a shell and tube LTESS, wherein the flow direction of HTF is periodically reversed. The periodically reciprocating flow creates higher temperature gradients across the HTF-PCM boundary which leads to higher and more uniform heat transfer rates and thereby lower average temperatures, lower maximum temperatures, and lower temperature differences across the PCM than unidirectional flow. Detailed parametric studies are performed to understand the effects of flow and heat transfer parameters of HTF, thermophysical properties of PCM, and design of PCM container on the relative thermal efficiency of the LTESS with reciprocating flow to unidirectional flow. For certain conditions, the time required for melting could be reduced by over 11.5% by using periodically reciprocating HTF flow instead of unidirectional HTF flow. Increasing the heat transfer rate by increasing the temperature or velocity of the HTF, or enhancing thermal conductivity of PCM, increases the relative effects of reciprocating flow compared to unidirectional flow.

Fully volumetric tracking of melting fronts in phase change materials with computed tomography

Recent advances in laboratory X-ray computed tomography (XCT) enabled short acquisition times on the order of a few minutes, which enable the investigation of dynamic processes in-situ by time-lapse measurements. These advances open new opportunities for revealing information on the solid-liquid phase transformation occurring in phase changes materials (PCMs), which are a promising solution for designing compact latent thermal energy storage systems (LTES). In this contribution we report results obtained from the XCT System, “LuCi”, recently installed at Lucerne university of Applied Sciences and Arts, where the evolution of the melting process in an ice cube confined within a cylindrical, X-ray transparent sample holder is tracked. Our results indicate that even tiny differences of a few percent in the density of the two different phases can be differentiated in XCT measurements. The high-power X-ray source and a fast X-ray detector of LuCi enable the acquisition of a XCT measurement in a short period (< 1 – 6 minutes). By using proper image processing methods, estimates for the volumetric melting rate, the local melting front velocity, and the tree dimensional (3D) shape of the melting front were obtained from the four-dimensional (4D) imaging data. These results will help to build more efficient thermal energy storage systems in the future, which will support the energy turnaround.

Development and experimental testing of an integrated prototype based on Stirling, ORC and a latent thermal energy storage system for waste heat recovery in naval application

This paper focuses on an advanced energy system, able to increase the overall efficiency of ships by recovering the heat wasted by the propulsion system. A small-scale prototype has been realized, developed, and tested under realistic operating conditions of boat during a winter and summer cruise. The main novelty of the research that has been presented in this paper lies in a significant contribution by developing an integrated dynamic device for electrical production and thermal energy storage. It is based on a 1000 cm3 light duty compression ignition engine coupled with a properly adapted Stirling Engine (SE), an Organic Rankine Cycle group (ORC) and a latent Thermal Energy Storage system (TES). All the components have been managed by means of a specifically developed electronic control, simulating two standard cruise profiles. Scaled engine power data have been imposed to simulate port, manoeuvring and open sea navigation phases. The consumption of hot water has been obtained by considering the typical hourly use profile of a cruise ship.It has been demonstrated that the proposed integrated system allows recovering all the thermal energy needed to satisfy the hot water request during the cruise, avoiding the use of auxiliary boilers. The results indicate a favourable effect because the recovered thermal energy represents the 7.7 % of the total energy consumed by fuel. The net electrical energy generated by ORC and Stirling engine resulted to be about 1 % of the total fuel energy consumption, respectively 0.8 % and 0.2 %. The developed prototype can be a useful tool in viability analysis and can easily be reproduced for several uses. In conclusion, the integration of different systems with an optimal integration and sizing of the thermal energy storage considerably improve the thermodynamic, economic and environmental results for future clean ships.

Experimental investigations on dynamic performance of organic Rankine cycle integrated with latent thermal energy storage under transient engine conditions

To succeed on the aim of achieving a carbon-neutral energy system, improving the efficiency of waste heat recovery is an important technique route. However, most of available heat sources in energy-intensive industries and transport sectors always present fluctuating nature that prohibits the safe and efficient operation of organic Rankine cycle (ORC) based waste heat recovery systems. Hereby, this study proposes a novel ORC system integrated with latent thermal energy storage (LTES), using LTES to reduce the thermal power fluctuations of engine exhaust and improve the safety performance of downstream ORC system. A dual-model test bench including the traditional basic ORC and ORC-LTES directly driven by engine exhaust is designed and established. The dynamic performance of basic ORC and ORC-LTES is experimentally investigated and compared under different step-change and cyclic engine conditions. The experimental results indicate that the existing of LTES can significantly reduce the fluctuating range of exhaust temperature at the evaporator inlet under both step-change and cyclic engine conditions. Under step-change engine conditions, ORC-LTES can completely resist the sudden reduction of exhaust temperature and mass flowrate under small step-change ratio of engine load, and the superheat degree of ORC-LTES lasts much longer time to drop to zero than the basic ORC under larger step-change ratio of engine load, while superheat degree of the basic ORC fast drops to zero under different step-change engine conditions. Under cyclic engine condition, ORC-LTES keeps safely operating all the time while the basic ORC has to stop working in each period, in addition, the net power and thermal efficiency of ORC-LTES are 23.5% and 23.2% higher than that of the basic ORC respectively.

Simulation study on the effect of fins on the heat transfer performance of horizontal dual-inner-tube latent thermal energy storage heat exchangers

Compared with the traditional horizontal double-tube heat exchangers, the horizontal dual-inner-tube latent thermal energy storage (LTES) heat exchangers can effectively shorten the melting time of the phase change materials (PCMs). This paper analyzed the liquid phase distribution, temperature field, melting rate and heat storage about finless horizontal dual-inner-tube latent thermal energy storage heat exchangers during the melting, and researched the effects of the fin position and number on melting progress and heat storage performance. The results showed that compared with the variation of fin number, the reasonable fin arrangement is better to improve heat transfer performance, and the heat transfer performance reaches to the optimal value when the single fin is placed at the bottom of the lower tube, which can shorten the melting time by 23.91% compared with the finless condition, but the increase of fin number on the basis of the optimal single fin arrangement only shortens the melting time by 1,2%. Additionally, it also obtained that the time of the high melting rate reduces by 9.14% when the fin is installed in the middle of the exchangers body, which weakens the effect of strengthening heat transfer. These results indicate that the less fins, if the arrangement is designed properly, can be as efficient as more fins in horizontal dual-inner-tube LTES heat exchangers.

Phase Change Material Evolution in Thermal Energy Storage Systems for the Building Sector, with a Focus on Ground-Coupled Heat Pumps.

The building sector is responsible for a third of the global energy consumption and a quarter of greenhouse gas emissions. Phase change materials (PCMs) have shown high potential for latent thermal energy storage (LTES) through their integration in building materials, with the aim of enhancing the efficient use of energy. Although research on PCMs began decades ago, this technology is still far from being widespread. This work analyses the main contributions to the employment of PCMs in the building sector, to better understand the motivations behind the restricted employment of PCM-based LTES technologies. The main research and review studies are critically discussed, focusing on: strategies used to regulate indoor thermal conditions, the variation of mechanical properties in PCMs-based mortars and cements, and applications with ground-coupled heat pumps. The employment of materials obtained from wastes and natural sources was also taken in account as a possible key to developing composite materials with good performance and sustainability at the same time. As a result, the integration of PCMs in LTES is still in its early stages, but reveals high potential for employment in the building sector, thanks to the continuous design improvement and optimization driven by high-performance materials and a new way of coupling with tailored envelopes.

Latent Thermal Energy Storage Application in a Residential Building at a Mediterranean Climate

An innovative thermal energy storage system (TESSe2b) was retrofitted in a residential building in Cyprus with a typical Mediterranean climate. The system comprises flat-plate solar collectors, thermal energy storage tanks filled with organic phase change material, a geothermal installation consisting of borehole heat exchangers with and without phase change material and a ground source heat pump, an advanced self-learning control system, backup devices and several other auxiliary components. The thermal energy storage tanks cover the building’s needs at certain temperature ranges (10–17 °C for cooling, 38–45 °C for heating and 50–60 °C for domestic hot water). A performance evaluation was conducted by comparing the TESSe2b system with the existing conventional heating and cooling system. The systems were simulated using commercial software, and the performance of the systems and the building’s energy needs were calculated. Based on the energy quantities, an economic analysis followed. The equivalent annual primary energy consumption with the conventional system resulted in being 43335 kWh, while for the storage system, it was only 8398 kWh. The payback period for the storage system was calculated to be equal to 9.76 years. The operation of the installed storage system provided data for calculations of the seasonal performance factor and storage performance. The seasonal performance factor values were very high during June, July and August, since the TESSe2b system works very efficiently in cooling mode due to the very high temperatures that dominate in Cyprus. The measured stored thermal energy for cooling, heating and domestic hot water resulted in being 14.5, 21.9 and 6.2 kWh, respectively. Moreover, the total volume of the phase change material thermal energy storage tanks for heating and domestic hot water was calculated to be roughly several times smaller than the volume of a tank with water as a storage medium.

Experimental Analysis of the Thermal Performance of a Latent Heat Energy of Helical Coil for the Application of Solar Energy

The latent thermal power storage system has more characteristics than the sensitive storage system. The heating system is a process of releasing and absorbing heat energy using phase transfer material (PCM) and it provides more efficient energy than sensible heat storage. This also consist of high energy storage and high density. It provides high heat transfer in low volume and thereby enhances heat transfer. This enhances the capacity and efficiency of the EFU while extending the service life. The coil tube is designed for latent thermal energy storage to implement and enhance thermal performance during the loading and unloading process. The offloading time, however, was not affected by the flows. Higher throughput has also been shown to improve the effectiveness of recovery. The direction of flow of the HTF did not affect the total time of loading and recharging but affected the temperature changes of the PCM in the energy storage element. It is intended to predict the ability to store maximum energy as higher energy efficiency during the phase shift process. Parameters such as the mean temperature of the PCM, the growth of the melting front, the energy efficiency, and the number of generations of entropy are studied.

Latent thermal energy storage for refrigerated trucks

Latent Thermal Energy Storage (LTES) systems adopting Phase Change Materials (PCMs) have been proposed to be implemented along the cold chain over the last years. Hence, in this work, a novel insulated wall concept for refrigerated truck is proposed. A 2D transient numerical model of the truck cell is developed. Results are obtained considering the varying hourly solar radiation from 6 AM to 4 PM of a typical summer day in Vicenza (Italy). The innovative composite wall consists of a traditional polyurethane (PU) insulation layer wrapping a layer of PCM with different thickness from 0.5 cm to 2 cm. Three paraffin waxes with different reference melting temperature of 2 °C, 4 °C, and 5 °C are studied. The results confirm the validity of our hypothesis because even the minimum thickness layer of PCM considered here can ensure an adequate temperature inside the cell for almost the entire simulated period.

Estimating the state of charge in a latent thermal energy storage heat exchanger based on inlet/outlet and surface measurements

• Estimators for the total and partial stored energy are developed and validated. • The estimator for the heat transfer fluid energy is generally applicable. • A finite volume latent thermal energy storage model is developed and fitted. The performance of latent thermal energy storage (LTES) heat exchangers is related to the stored energy (i.e. state of charge) during the (dis)charging of the energy storage system. Therefore, measuring the stored energy is crucial to understand the behavior of LTES systems. However, technical considerations often oppose the measurability of the stored energy. The present article presents a case where only measurements at the in- and outlet of the heat transfer fluid and on the outer surface of the heat exchanger were possible. Based on this sparse set of measurements, estimators for the stored energy are developed. To test these estimators, a finite volume model of the LTES heat exchanger is made. The finite volume model is fitted by comparing the predicted heat transfer fluid outlet temperature for 36 experiments. The energy estimators are compared to simulation results. The estimators for the stored energy in the heat transfer fluid, metal and phase change material obtain an average deviation of respectively 0.5 %, 3.9 % and 0.6 % with the stored energy predicted by the finite volume model. The estimator for the stored energy in the heat transfer fluid should be applicable to all LTES heat exchangers while the applicability of the estimators for stored energy in the metal and phase change material depend on small temperature differences between surface and interior of the metal and negligible heat losses.

Novel method for microencapsulation of eutectic hydrated salt

Phase change materials (PCMs), as latent thermal energy storage, have received plenty of attention in recent decades due to their excellent performance in energy conservation. Inorganic hydrated salts have many advantages over organic phase change materials, such as higher thermal storage density, low cost, nontoxicity and nonflammability. This work presents a simple and environmentally friendly method for the preparation of microcapsules with inorganic hydrated salts as core materials. Microcapsule using EHS composed of SSD and DHPD as core material and EC/ABS as shell material has been prepared and investigated by various characterization techniques. The scanning electron microscope (SEM) results indicate that the microcapsules are basically spherical with a core-shell structure. In addition, Fourier-transform infrared spectroscopy (FT-IR) analysis results suggest that the microcapsules are only the physical coating of composite shell material to core material. Furthermore, the phase transition enthalpy is 168.3 J/g according to the differential scanning calorimeter (DSC) analysis. Besides, the method of microencapsulation may be extended to other liquid hydrates salts at room temperature.

A decision support model for waste heat recovery systems design in Data Center and High-Performance Computing clusters utilizing liquid cooling and Phase Change Materials

The Data Center (DC) and High-Performance Computing (HPC) sector is increasingly becoming one of the single largest energy consuming sectors in the energy system and the demand for new DCs is growing with the rising use of cloud-based services, social media and internet usage in general. DCs and HPC clusters employ energy intensive cooling in order to dissipate the generated heat from the Information Technology (IT) equipment. Conventionally, the cooling is almost exclusively done by air-based systems; however, the use of liquid based cooling has shown potential to increase computational efficiency of the systems and decrease cooling needs by operation above the free cooling limit. Furthermore, the liquid coolant can operate at higher temperatures allowing high temperature waste heat recovery. In addition, load shifting through Latent Thermal Energy Storage (LTES) will allow the non-controllable waste heat resource to be stored seasonally, thereby granting the ability to cover larger District Heating (DH) loads. In this work, a decision support model is developed that will take basic information regarding a HPC cluster or DC as inputs. The decision support model will provide a parameterized output that shows different configurations and design parameters that can be utilized for the system. The main outputs includes yearly energy savings, yearly cost savings and efficiency gains through the Power Usage Efficiency and the Energy Reuse Efficiency. The decision support model is demonstrated in a Danish case study. Electricity savings between 8.14 % and 10.8 % of the total cluster electricity consumption and a waste heat recovery potential of 85 MWh/year to 576 MWh/year are obtained. It is shown that if the DH covered is needed to be self-sufficient the configuration would require an LTES with PCM mass of 500 kg, but system configurations that operate as an addition to an existing local heating source shows increases in yearly energy savings of 332 % compared to self-sufficient configurations. The goal of the decision support model is to assist the design of future waste heat recovery applications through selection of system parameters including coolant temperatures, energy storage design parameters, DH supply temperatures and DH load coverage from the DC or HPC cluster.

Experimental investigation of high performance composite phase change materials based on sodium acetate trihydrate for solar thermal energy storage

Latent thermal energy storage is a novel technology based on phase change materials (PCMs) for storing and transporting energy. Sodium acetate trihydrate (SAT) has a large latent heat, but its application is severely restricted by supercooling and phase separation. In this study, a high-performance composite PCM (SAT/SiC/EG) containing SAT (as the PCM matrix), silicon carbide nanoparticles (SiC; as a nucleating agent), and expanded graphite (EG; as a supporting material) were synthesized by stirring and ultrasonic dispersion to ameliorate supercooling and phase separation. The maximum supercooling degree of the composite PCM with 2 wt% SiC and 2 wt% EG was 1.1 °C after 200 heat storage and release cycles. Interestingly, the introduction of EG significantly inhibited the phase separation of SAT, and the composite PCM exhibited excellent dispersion stability after 200 cycles. The addition of 2 wt% and 3 wt% EG significantly improved the thermal conductivity of the composite PCM by 1.09- and 1.77-fold, respectively, as compared to that of SAT. Additionally, the prepared composite PCMs exhibited a large latent heat capacity and a good thermal cycling performance, where the melting and freezing enthalpies of SAT/SiC/EG containing 96 wt% SAT, 2 wt% SiC, and 2 wt% EG was 252.1 J/g and 222.1 J/g, respectively. This study demonstrates that the SAT/SiC/EG composite PCMs have extensive application prospects in middle-low temperature solar thermal energy storage. • The composites PCMs were synthesized by stirring and ultrasonic dispersion method. • Silicon carbide can effectively reduce the supercooling of sodium acetate trihydrate. • The addition of expanded graphite improves phase separation and thermal conductivity. • The heat storage and release rate of the composite PCMS was increased. • The composite PCMs still has a large enthalpy of melting/freezing after 200 cycles.

Experimental investigation of clay brick with sensible, latent, and hybrid thermal energy storage in buildings

Global warming has caused uneven extremities in the climate such as heat waves, floods, and extreme rise in temperatures. Consequently, there is increase in demand for energy for indoor space cooling in the buildings to create better indoor thermal comfort. Thermal energy storage (TES) using Phase Change Material (PCM) is an effective technology for improving the thermal performance of the building envelope. Energy efficient building envelope can reduce the external heat gain and maintains indoor thermal comfort level easily. In this study, the thermal performance of clay bricks integrated with sensible heat storage material, latent heat storage material, and hybrid (sensible + latent) was investigated. A comparative study of indoor thermal performance in terms of peak temperature, thermal amplitude, time lag, decrement factor of sensible heat storage brick, latent heat storage brick, and hybrid brick in comparison with conventional brick was prepared. Latent heat storage brick has shown the highest percentage average peak temperature reduction of 3.86 % in comparison to conventional brick. Maximum reduction of 13.74 % was shown by latent heat storage brick followed by hybrid heat storage brick and then by sensible heat storage brick. Time lag of 180 min is observed in brick embedded with latent heat storage capacity. Additionally, infrared thermal images suggest that the brick integrated with latent heat storage material shows better indoor thermal performance.

A detailed energy analysis of a novel evaporator with latent thermal energy storage ability

• A detailed energy analysis of a novel evaporator-TES module was carried out. • The energy storage contribution in each material was evaluated. • More than 54% of the total energy was stored in the PCM despite its low mass. • The effective charging of the module was observed between −2 °C and 7 °C. • The charge level varies depending on energy supply and demand conditions. The direct integration of phase change materials (PCM) into refrigeration and air conditioning systems through compact modules is an identified literature gap. In response to the literature gap, this paper provides a detailed energy analysis of a novel compact thermal energy storage module, that allows its direct integration into a refrigeration system as the evaporator. The study addresses key aspects of thermal energy storage (TES) and heat transfer mechanism that complement the previous analyses of the novel concept. Here the total energy stored in the module (including in all auxiliary parts), the charging/discharging power, and the behaviour of the module when used as a TES module and as a heat exchanger (HEX) are assessed. The results demonstrate the feasibility of the module to work as a TES and as a HEX. When working as a TES, complete charging and discharging was achieved, and 54% of the total energy was stored in the PCM although the PCM only accounts for around 14% of the total mass. Moreover, the highest charging/discharging power was obtained within the temperature range where most of the phase change occurred. When the module works as a HEX, it initially charges/discharges partially until a thermal equilibrium is achieved and the level of charge responds to the variation in the energy supply and demand.

Molecular dynamics approach to study the effects of MgCl2 salt atomic ratio on the phase transition phenomenon in the phase change materials

In general, phase change materials reduce energy consumption in different structures by increasing the thermal conductivity or decreasing the phase change time. On the other hand, hydrated salts are the most important group of phase change materials with high phase change temperatures and are extensively studied in latent thermal energy storage systems. Today, these atomic structures are used in different usages such as thermal transfer compounds, commercial and industrial applications, for cooling and heating on a small scale, etc. In the current computational study, we utilized Molecular Dynamics procedure to explain the thermal and phase transition (from liquid to solid-state) behavior of water liquid in the presence of MgCl2 salt as salts hydrate compound. Computationally, physical parameters like temperature, potential/total energy, phase transition time, thermal conductivity, and velocity/density profile of H2O-MgCl2 nanofluid were reported for time=15 ns. Molecular Dynamics outputs indicated that the phenomenon of phase transition in simulated nanofluid happens in a shorter time using a 4% atomic ratio salt structure. This atomic behavior of simulated structure causes thermal behavior improvement is defined nanofluid. Numerically, by MgCl2 salt ratio optimizing, H2O-MgCl2 nanofluid thermal conductivity converged to 0.67 W/m.k. Technically, thermal conductivity calculation in current research is done by using the Green-Kubo method.

OBTAINING AND PROPERTIES OF NANOSCALE SOLID-STATE HEAT STORAGE WITH CARNAUBA WAX

Latent thermal energy storage using phase change materials has attracted interest in the use of solar and other types of energy due to their ability to provide high density lateral energy storage. Materials with a latent heat of storage have become attractive for their use in many branches of human activity. However, the materials use is often limited by problems of low thermal conductivity, the transition from a solid to a molten state causes difficulties in storing materials in a container, and special heat exchangers are needed to increase the energy cost. The solution to the above problems may be to create solid-state, form-stable heat storage elements. In this work, a number of shape-stable materials with a phase transition were obtained from melts by mixing halloysite nanotubes with carnauba wax in order to improve the heat accumulation characteristics. Halloysite nanotubes were mixed at elevated temperatures with carnauba melted wax and rapidly cooled to prevent the nanotubes sedimentation. As a result, a series of solid wax/nanotube samples were prepared with weight ratios of 70/30, 60/40 and 50/50. Pure wax showed a accumulation heat of the solid-to-liquid phase transition of 189.09 J/g. Carnauba wax has a latent heat greater by about 25 % compared to paraffin. Composite materials had significantly lower latent heat, respectively, 99.39 J/g for 70/30, 90.25 J/g for 60/40, and 81.26 J/g for 50/50 samples. Elemental mapping of the nanomaterial revealed a nanotubes uniform distribution in the wax. According to the data of X-ray analysis, as a result of the composite materials preparation, the components did not form new crystalline phases, but they were physical mixtures. When heated, the components did not chemically interact with each other, which is useful for the accumulation of thermal energy by materials. Analysis of the IR spectra of the samples confirmed the change in the absorption bands of functional hydroxyl groups at 3696 sm–1 (Al–O–H) and 3621 sm–1 (Si–O–H). In primary nanotubes, the intensities ratio of silanol to aluminol groups is greater than unity, while in the composite it is already less than this value. This manifestation can be explained by the fact that, during the wax melting, the interaction of wax molecules on the outer surface of the nanotubes occurs. Bibl. 16, Fig. 5.