Low-temperature Thermal Energy Storage Research Articles

Solid–Liquid Phase Equilibrium of the n-Nonane + n-Undecane System for Low-Temperature Thermal Energy Storage

The current article presents an exploration of the solid–liquid phase diagram for a binary system comprising n-alkanes with an odd number of carbon atoms, specifically n-nonane (n-C9) and n-undecane (n-C11). This binary system exhibits promising characteristics for application as a phase change material (PCM) in low-temperature thermal energy storage (TES), due to the fusion temperatures of the individual components, thereby motivating an in-depth investigation of the solid–liquid phase diagram of their mixtures. The n-nonane (n-C9) + n-undecane (n-C11) solid–liquid phase equilibrium study herein reported includes the construction of the phase diagram using Differential Scanning Calorimetry (DSC) data, complemented with Hot–Stage Microscopy (HSM) and low-temperature Raman Spectroscopy results. From the DSC analysis, both the temperature and the enthalpy of solid–solid and solid–liquid transitions were obtained. The binary system n-C9 + n-C11 has evidenced a congruent melting solid solution at low temperatures. In particular, the blend with a molar composition xundecane = 0.10 shows to be a congruent melting solid solution with a melting point at 215.84 K and an enthalpy of fusion of 13.6 kJ·mol–1. For this reason, this system has confirmed the initial signs to be a candidate with good potential to be applied as a PCM in low-temperature TES applications. This work aims not only to contribute to gather information on the solid–liquid phase equilibrium on the system n-C9 + n-C11, which presently are not available in the literature, but especially to obtain essential and practical information on the possibility to use this system as PCM at low temperatures. The solid–liquid phase diagram of the system n-C9 + n-C11 is being published for the first time, as far as the authors are aware.

Three-dimensional topology-optimized structures for enhanced low-temperature thermal energy storage

In this study, a three-dimensional topologically-optimized structure was developed to enhance the thermal energy storage performance of low-temperature phase change materials. The topology of the structure employed in the thermal energy storage device was developed using COMSOL Multiphysics by maximizing heat diffusion in a design domain with a constant temperature plate and adiabatic boundary conditions. The optimized thermal energy storage device was additively manufactured, and its thermal performance was experimentally characterized and compared against two conventional structures as baselines, viz., a plate fin and a pin fin structure. For the first time, this study seeks to determine the sole influence of fin topology on thermal energy storage performance by designing the fins with the same physical parameters, viz., surface area, volume, base plate size, and material. The fin structure volumes were set at approximately 5% of the simulated domain volume and were fabricated by Selective Laser Melting, a metal additive manufacturing technique. The fin structures were experimentally tested under three different constant plate temperatures (65 °C, 70 °C, and 75 °C) using two different phase change materials (RT35 and PEG1000). Their performances were evaluated by comparing the total charging time, melt fraction, and base plate temperature. Our results show that the topology of the optimized fin structure can reduce charging times by up to 9.1% when a constant plate temperature of 65 °C is applied. The topology of the optimized fins also achieved base plate temperatures that were up to 4 °C lower than conventional fins while having a more uniform distribution of heat to the phase change material within the housing. Additionally, by fixing the critical physical parameters of the fin structures, this work also shows that the fin topology plays a significant role in enhancing the melting performance of thermal storage devices.

Solid–Liquid Phase Equilibrium: Alkane Systems for Low-Temperature Energy Storage

The thermal characterization of two binary systems of n-alkanes that can be used as Phase Change Materials (PCMs) for thermal energy storage at low temperatures is reported in this work. The construction of the solid–liquid binary phase diagrams was achieved using differential scanning calorimetry (DSC) and Raman spectroscopy. The solidus and liquidus equilibrium temperatures were determined using DSC for thirty-nine different samples, three for the pure n-alkanes and the remaining for binary mixtures at selected molar compositions and used to acquire the corresponding solid–liquid phase diagrams. The two binary systems of n-octane/n-decane (C8/C10) and n-decane/n-dodecane (C10/C12) are characterized by a eutectic behavior at low temperatures. The eutectic temperature for the system C8/C10 was found at 211.95 K and the eutectic composition appeared at the molar fraction xoctane = 0.87. For the system C10/C12, the eutectic temperature was found at 237.85 K, and the eutectic composition appeared for the molar fraction xdecane = 0.78. This work aims to fulfill the lack of available data in the existing literature, considering the potential application of these systems for low-temperature thermal energy storage. Raman spectroscopy was used to complement the DSC data for the construction of the solid–liquid phase equilibrium diagrams, enabling the identification of the solid and liquid phases of the system. Additionally, the liquidus curve of the phase diagram was successfully described using a modified freezing point depression curve as fitting equation, the absolute root mean square deviation for the data correlation of the C8/C10 and C10/C12 systems being 2.56 K and 1.22 K, respectively. Ultimately, the fitting procedure also enabled a good prediction of the eutectic point for both studied systems.

Performance investigation and evaluation of a low-temperature solar thermal energy storage system under dynamic weather conditions

Designed and operational parameters matter greatly in how successful an energy storage system performs. However, a current challenge in matching suitable these parameters lies in the poor understanding of such systems’ performance under dynamic weather conditions. In this paper, we investigated a phase change material (PCM) storage unit that is particularly aimed for poor-solar areas, and connected the unit to a flat plate solar collector to establish a complete solar thermal energy storage system (STESS). To optimize the system performance, different grades of solar radiation and outdoor ambient temperatures are introduced, and the effects of heat transfer fluid (HTF) flow rates and solar collecting areas are investigated numerically through multiple performance evaluation parameters. The results indicate that the solar collecting area has a significant effect on the performance of the STESS compared to that of the water flow rate. And different collecting areas of 1 m2, 1.5 m2, and 3 m2 with an optimal water flow rate of 0.06 m3/h are matched to different average daily solar radiation of 203.4 W/m2, 155.2 W/m2, 92.7 W/m2, respectively. Further for the heat release process, different heat release efficiencies from 88.32 % to 97.12 % with different energy consumption of the fan from 0.003 kWh to 0.063 kWh are obtained when the air flow rate is increased from 30 m3/h to 90 m3/h. One of the air flow rates of 60 m3/h is superior for the STESS considering higher heat release efficiency and lower energy consumption of the fan. The findings of this paper will contribute to optimizing the performance of a STESS in practical applications.

Enhancing thermal energy storage in buildings with novel functionalised MWCNTs-enhanced phase change materials: Towards efficient and stable solutions

Phase change materials (PCMs) are a promising panacea to tackle the intermittency of renewable energy sources, but their thermal performance is limited by low thermal conductivity (TC). This pioneering work investigates the potential of organic PCM-enriched surface-modified and un-modified multi-walled carbon nanotubes (MWCNTs) for low-temperature thermal energy storage (TES) applications. The functionalised and un-functionalised MWCNTs enhanced PCM have demonstrated a TC enhancement of 158 % and 147 %, respectively, at 25 °C. However, the TC value of the unmodified MWCNTs-based PCM dropped by 52.5 % after 48 h at 25 °C, while that of the functionalised MWCNTs-based PCM remained stable. A DSC analysis of up to 200 thermal cycles confirmed that the surface-modified and un-modified MWCNTs had no major effect on the peak melting and cooling temperatures of the nano-enhanced PCMs although a minor decrease of 7.5 % and 7.7 % in the melting and crystallisation enthalpies, respectively, was noticed with the inclusion of functionalised MWCNTs. Moreover, functionalised MWCNTs incorporated PCMs have led to increases in specific heat capacity by 23 % with an optimal melting enthalpy value of 229.7 J/g. In addition, no super-cooling, no phase segregation, and a small phase change temperature were noticed with these nano-enhanced PCMs. Finally, no chemical interaction from nano-PCMs was seen in the FT-IR spectra with the incorporation of both functionalised and un-treated MWCNTs. It is evident that the functionalised MWCNT-based PCM has better thermal stability and it offers a promising alternative for improving thermal storage and management capabilities in buildings, contributing to a sustainable and energy-efficient building design.

Experimental analysis of one and two-stage thermoelectric heat pumps to enhance the performance of a thermal energy storage

This experimental study demonstrates the possibility to enhance the performance of a low-temperature thermal energy storage system (∼160 °C) based on airflow heating using electrical heaters by including thermoelectric technology. An improvement of the 17 % on COP is reached by using an optimized thermoelectric heat pump system to preheat the airflow, consisting of three one-stage and three pyramidal two-stage thermoelectric heat pumps sequentially installed along the airflow that is heating.This research experimentally analyses and compares the COP of three different configurations of thermoelectric heat pumps: one-stage, square two-stage, and pyramidal two-stage thermoelectric heat pumps. The experimental study aims to characterize the operation of each configuration for heating an airflow of 16.5 m³/h at 25 °C as ambient temperature. To that purpose, the airflow inlet temperature, voltage supply, and voltage ratio between stages have been modified.The experimental results show that for 25 °C as inlet temperature the one-stage thermoelectric heat pump has the best performance with a maximum generated heat of 78 W. Whereas, a two-stage thermoelectric heat pump is required when the inlet temperature increases. At 40 °C as inlet temperature, the square two-stage configuration provides the best performance with a voltage ratio of 2, which reaches a COP of 3.29 generating only 20 W of heat. However, the pyramidal two-stage configuration is able to achieve the maximum heat outputs with a voltage ratio of 1, generating 172; 161; 149 and 138 W, with corresponding COP values of 1.17; 1.16; 1.14 and 1.11 for inlet temperatures of 25; 40; 55 and 70 °C. This configuration is the one that achieves the greatest COP values with high inlet temperatures.

Preparation and characterization of quinary nitrate salt based composite phase change material with low melting point for low and medium temperature thermal energy storage

This work concerns the development of a shape-stable molten salt based composite phase change material (PCM) for low and medium temperature thermal energy storage. The composite is fabricated by using a cold compression and hot sintering method with the employment of a eutectic quinary nitrate salt of NaNO3-NaNO2-KNO3-KNO2-LiNO3 as PCM, halloysite nanotube (HNT) as skeleton supporting material and natural graphite as thermal conductivity enhancement additive. A sequence of characterizations is performed to investigate the composite microstructure, chemical and physical compatibility, thermal stability, phase change behaviour, and thermal conductivity as well as cycling performance. The results indicate that an excellent chemical compatibility has been achieved among the ingredients of quinary salt, HNT and graphite within the composite. A mass concentration of 50 wt% HNT endows the composite with the optimal formulation in which 10 wt% graphite can be successfully accommodated and a thermal conductivity around 1.31 W/m·K can be acquired. Moreover, in such a formulation, the composite presents a considerably low melting temperature of 72.4 °C and a high thermal decomposition temperature of 530 °C, which achieves the composite a relatively high energy storage density nearly 500 kJ/kg at a temperature range of 25–510 °C. The results presented in this work demonstrate that the quinary salt-HNT-graphite composite with fairly low phase transition temperature and a splendid combination of thermal properties and cycling performance could be a promising candidate to replace the conventional organic based PCMs utilized in low temperature thermal energy storage fields.

Fabrication and investigation of paraffin based shape-stable composite phase change materials with styrene ethylene butylene styrene as supporting skeleton for low temperature thermal energy storage and management

Paraffin based composite phase change materials (PCMs) have gained intensive attentions in low temperature thermal energy storage (TES) and management application domains because of its suitable melting temperature range and advanced thermoproperties. Seeking decent structural supporting materials (SSM) and the associated manufacturing approaches that capable of fabricating paraffin composites at appropriate temperatures to avert the thermal decomposition of the paraffin particularly for low-molecule ones are in high demand. Herein, we developed a low-molecule paraffin (RT18HC) composite through a simple blending approach by using styrene-ethylene/butylene-styrene (SEBS) as SSM, and showed that such SEBS-paraffin composites could be able to fabricate at a low operation temperature of 80 °C, significantly lower than that required for the preparation of conventional high density polyethylene (HDPE)-paraffin composites. To enhance the composite effective thermal conductivity, a micro-sized graphite power as enhancer was also involved over the fabrication process. Owing to the desirable solubility of paraffin in SEBS, a crosslinking structure inlaying paraffin and graphite was formed, endowing the composite with ability to maintain the structure stabilization and eliminate the graphite sedimentation over the repeated thermal cycles. 10 wt% of SEBS gives the composite optimum formulation at which 10 wt% graphite can be involved, and also an excellent shape stability and thermal adjustment ability can be achieved.

Bubble-injection and seeding enabled crystallization of erythritol/xylitol eutectic phase change material

The utilization of Erythritol/xylitol eutectic material for low-temperature thermal energy storage and thermal management holds great promise. However, its inability to undergo spontaneous nucleation and crystallization poses a significant obstacle. This research paper introduces a novel nucleation technique that combines gas injection agitation and seeding to address this issue. The study aims to investigate the impact of gas injection tube diameter, thermal environment, and gas injection duration on the nucleation and crystallization characteristics of the material. Experimental analysis reveals that, under optimal conditions, the solidification enthalpy reaches 203.1 J/g, equivalent to 85% of the melting enthalpy. Additionally, the material exhibits excellent thermal cycle stability, with no observed phase separation or deterioration after 100 heating and cooling cycles. Furthermore, the influence of Multi-Walled Carbon Nanotube (MWCNT) nanoparticle additives on nucleation is explored. Different MWCNT additive mass fractions are compared. The obtained results underscore the significance of understanding the effects of gas injection parameters and nanoparticle additives on nucleation and crystallization processes. The enhanced solidification enthalpy, remarkable thermal cycle stability, and insights into MWCNT effects pave the way for broader applications of this promising phase change material in thermal energy storage and thermal management.

Design optimization of low-temperature latent thermal energy storage for urban cooling applications

This study investigates a novel design optimization method for a low-temperature latent thermal energy storage (LTES) in a shell-and-tube heat exchanger configuration for a district/urban cooling network. An ordered hexagonal tube-packing method is first developed for the inner configuration of the LTES followed by the implementation of a differential evolutionary multi-objective optimization scheme for key design variables. Using a numerical model, transient characterisation of the optimized designs from a prior work confirmed that the method was capable of producing improved outcomes of up to 54.7% increased storage capacity. Key observations from the optimization revealed that heat transfer maximization scales costs by up to 10 times while minimizing costs translates to a mere three-fold decrease in heat transfer characteristics. Further characterisation of the optimized LTES designs under district cooling application showed that optimizing for discharge exergy efficiency in addition to cost and heat transfer efficiency was crucial in maximising discharge efficiency by up to 30%, as well as producing 33% and 20% greater capital cost savings compared to the single- and double-objective optimization cases respectively.

Carnot Battery Based on Brayton Supercritical CO2 Thermal Machines Using Concentrated Solar Thermal Energy as a Low-Temperature Source

Carnot batteries store surplus power as heat. They consist of a heat pump, which upgrades a low-temperature thermal energy storage, a high-temperature storage system for the upgraded thermal energy, and a heat engine that converts the stored high-temperature thermal energy into power. A Carnot battery is proposed based on supercritical CO2 Brayton thermodynamic cycles. The low-temperature storage is a two-tank molten salt system at 380 °C/290 °C fed by a field of parabolic trough collectors. The high-temperature storage consists of another two-tank molten salt system at 589 °C/405 °C. Printed circuit heat exchangers would be required to withstand the high pressure of the cycles, but shell and tube heat exchangers are proposed instead to avoid clogging issues with molten salts. The conventional allocation of high-temperature molten salt heat exchangers is then modified. Using solar energy to enhance the low-temperature thermal source allowed a round-trip efficiency of 1.15 (COP of 2.46 and heat engine efficiency of 46.5%), thus increasing the stored power. The basic configuration has a levelised cost of storage of USD 376/MWh while replacing the shell and tube heat exchangers with hybrid printed circuit heat exchangers is expected to lower the cost to USD 188/MWh.

Predictive control of low-temperature heating system with passive thermal mass energy storage and photovoltaic system: Impact of occupancy patterns and climate change

With the increasing energy prices and growing concerns over energy security, an accelerated transition to net zero carbon built environment has never been more important. Many studies have shown the capabilities of advanced control strategies such as model predictive control (MPC) to achieve energy efficiency, balance with thermal comfort and air quality. It has also shown its capability to provide demand flexibility, minimising peak load demands and maximising the production of renewable energy sources in buildings. This work investigates the potential of integrating price-responsive MPC with a low-temperature heating system and passive structural thermal energy storage (STES). Integration with a photovoltaic (PV) system is also explored. The system performance under future climate conditions is evaluated considering different design and operating conditions, including different thermal mass, occupancy patterns and internal heat gains, setpoint strategies and operation temperatures of the low-temperature heating system. The coupled model developed has been verified and validated with numerical and experimental data, and good agreement is observed. The results showed that mediumweight thermal mass and a medium-temperature (45 °C) under-floor heating inlet temperature provided a higher load shifting ability, based on a realistic occupancy profile for a residential building and a high tolerance setpoint strategy during unoccupied periods. The result also showed that higher low-price energy usage and lower heating energy usage could be achieved in future climate conditions. Finally, an increase in the load shifting ability was observed after the integration of rooftop solar PV system.

Thermophysical characterization of paraffin-based PCMs for low temperature thermal energy storage applications for buildings

Trustworthy information about the thermophysical properties of phase-change-materials (PCMs) is vital for designing low temperature thermal energy storage systems for buildings and for modelling their thermal behaviour. However, data provided by manufacturers are recurrently optimistic, insufficient and/or uncertain for developing reliable numerical approaches, which requires further investigation. This paper aims to assess the key thermophysical properties of ten paraffin-based PCMs: microencapsulated and bulk paraffins.The melting/solidification peak temperatures, the enthalpy of fusion/solidification and the specific heat capacity were measured by Modulated-Differential-Scanning-Calorimetry. The Transient-Plane-Source-method was used to estimate the variations of the thermal conductivity/diffusivity with temperature. The thermal stability was assessed by High-Resolution-Thermogravimetric-Analysis. The microstructures of the microencapsulated PCMs were examined via Scanning-Electron-Microscopy.It was concluded that some information provided in the datasheets is roughly corroborated by the experimental results. However, in some cases, it was too optimistic (particularly the enthalpy of fusion/solidification), incoherent (mainly the melting/solidification peak temperatures) or scarce (e.g., the values of the thermal conductivity and the specific heat of the solid/liquid phases). The major contribution of this work is a reliable dataset for upcoming numerical modelling, saving time and resources in future studies. The results can also be used as a reference for future inter-laboratorial comparison studies.

Energy and exergy performance evaluation of a novel low-temperature physical energy storage system consisting of compressed CO2 energy storage and Kalina cycle

To improve the overall performance of the Compressed CO2 Energy Storage (CCES) system under low-temperature thermal energy storage conditions, this paper proposed a novel low-temperature physical energy storage system consisting of CCES and Kalina cycle. The thermal energy storage temperature was controlled below 200 °C, and the Kalina cycle was used to optimize the reuse of the stored thermal energy. A thermodynamic model of the integrated system was constructed, and the system performance was analyzed from the energy and exergy perspectives. Several evaluation indicators, such as system exergy efficiency (SEE), round trip efficiency (RTE), system energy density (SED), Kalina exergy efficiency (KEE), and Kalina energy efficiency (KEnE), were defined to evaluate system performance. In addition, the influences of several core parameters (e.g., split ratio (r), storage pressure (ps), turbine efficiency (te), pinch point temperature difference (pt) of heat exchangers, and ambient temperature (T0)) on the system performance were analyzed. The results of the base condition showed that SEE, RTE, SED, KEE, and KEnE were 58.17 %, 59.38 %, 6.32 kWh/m3, 38.62 %, and 8.64 %, respectively. The exergy destruction in heat exchanger 3 was the largest, accounting for 71.7 % of total exergy destruction. The sensitivity analysis results showed that increasing r could increase the SEE of the system, increasing ps could lead to a decrease in the RTE of the system, and increasing T0 could cause the RTE and SED to increase. The increase in te could improve all evaluation indices, while the increase in pt could reduce the system's overall performance. This study provides a new solution for the integrated application and optimal design of CCES systems under low-temperature thermal energy storage conditions.

Thermal performance improvement of a low-temperature thermal energy storage with configuration optimization

To improve the performance of solar heat storage and reduce the energy consumption of the fresh air system in areas with poor solar conditions, this study investigates the thermal storage and release performance of a low-temperature rectangular thermal energy storage unit (RTESU). A validated Computational Fluid Dynamics (CFD) model is used to address the impacts of configuration parameters on the performance of the thermal storage process in the Phase change material (PCM) module, including the number and eccentric arrangement of tubes. Subsequently, the effect of the air channel width on the air outlet temperature and the heat transfer rate is analyzed for the heat-releasing process. The numerical results show that the number and the eccentricity of the heat exchanger tube have significant impacts on the heat storage performance, and a minimum duration of 117 min can be achieved with a tube pitch of 30 mm (corresponding to 5 water tubes) and an eccentric of 5 mm, respectively. The air channel width of 20 mm is found as the optimal width due to its higher heat release efficiency (83.24 %) and lower pressure drop (4.3 Pa). The identification of key structural impacts helped to optimize the RTESU configuration, which performs at a low heat storage time of 117 min and achieves a heat release efficiency as high as 83.24 % to satisfy the need for poor solar conditions.

A novel one-step ultraviolet curing fabrication of myristic acid-resin shape-stabilized composite phase change material for low temperature thermal energy storage

In this work we present for the first time a simply, fast and efficient process for the fabrication of form-stable composite phase change material (PCM) by ultraviolet (UV)-curing technique. Simply by UV-curing of the PCM with a desired UV curable carrier for several minutes, a neoteric UV-cured composite that suitable for low temperature thermal energy storage (TES) can be fabricated. The feasibility of such a novel technique is demonstrated by investigating a case of myristic acid (MA) dispersed into a UV curable resin containing polyurethane acrylate (PUA). The results show that a crosslinked structure is formed in the composite after UV-curing due to the occurrence of photochemical reaction, which offer sufficient paths to accommodate the PCM, endowing the composite with the great ability to maintain the structure stabilization and prevent the liquid leakage. A good physical and chemical compatibility has been achieved in the composite and 40 wt% of UV-curing resin grants the composite optimum formula at which a melting temperature of 52 °C and an energy storage density over 210.8 kJ/kg at temperature ranged 25–100 °C are obtained. The results also reveal that the material mechanical strength can be linked to the UV-curing duration. For a given V-curing time of 5–30 mins, the composite mechanical strength is measured over 25–34 MPa, almost over twice as high as that of high-density polyethylene (HDPE) and styrene ethylene butylene styrene (SEBS) based composites. Owing to the high shape strength and excellent thermal cycling performance, such UV-curable composites can be utilized for modular design of TES devices, and also be used to precisely and directionally produce TES devices through additive manufacturing. This work exploits a new perspective for the energy-saving and time-effective fabrication of shape stabilized composite PCMs for low temperature thermal energy storage.

Valorization of Spent coffee Grounds: A sustainable resource for Bio-based phase change materials for thermal energy storage

Spent coffee grounds (SCGs) are waste residues arising from the process of coffee brewing and are usually sent to landfills, causing environmental concerns. SCGs contain a considerable amount of fatty acids and is therefore a promising green alternative bio-based phase change material (PCMs) compared to conventional organic and inorganic PCMs. In this study, the extraction of coffee oil from SCGs was conducted using three different organic solvents-ethanol, acetone, and hexane. The chemical composition, chemical, and thermophysical properties of these coffee oil extracts were studied to evaluate their feasibility as a bio-based PCM. Gas chromatography-mass spectroscopy (GC–MS) analysis indicated that coffee oil contains about 60–80 % of fatty acids while the phase transition temperature of the coffee oil extracts is approximately 4.5 ± 0.72 °C, with latent heat values of 51.15 ± 1.46 J/g as determined by differential scanning calorimetry (DSC). Fourier Transform Infrared Spectroscopy (FTIR) and DSC results of coffee oil extracts after thermal cycling revealed good thermal and chemical stability. An application study to evaluate coffee oil extract as a potential cold therapy modality showed that it can maintain temperatures below normal body temperature for up to 46 min. In conclusion, this work exemplifies the potential of SCGs as a promising green and sustainable resource for bio-based PCMs for low-temperature thermal energy storage applications such as cold-chain transportation and cold therapy.

A VIKOR based selection of phase change material for thermal energy storage in solar dryer system

Phase Change Material (PCM) has vast applications in Thermal Energy Storage (TES). An efficient thermal energy storage system design depends on properly selecting PCM. This study found a detailed and organized process for evaluating PCMs for the design of latent heat based thermal energy storage that includes pre-screening, ranking, and objectively evaluating performance utilizing Multi-Criteria Decision-Making (MCDM) methods. Among some PCMs suitable for low-temperature thermal energy storage applications, organic PCMs are a desirable option over inorganic PCMs. The following properties, such as melting point temperature, latent heat, thermal conductivity, specific heat capacity, density, and cost, are considered in PCM selection. This work aims to select an eco-friendly, economical, and thermal-efficient PCM for low temperature (40 °C – 70 °C) solar dryer application from the thirteen identified PCM’s. VIKOR technique is used to select the suitable PCM from the identified PCMs like Natural Soya Wax, Bee Wax (C15 H31 COO C30 H61), Paraffin Wax, Palm Wax, Stearic Acid (CH3(CH2)16 CO2), Lauric Acid (C12 H24 O2), Palmitic Acid (C16 H32 O2), Medicine Paraffin Wax, RT55, RT60, P116, Shellac Wax (C30 H50 O11), and CR + 55A. This technique ranks the identified PCM using the appropriate properties. Paraffin wax ranks at the top, satisfying all the required properties. The results showed that this technique is a feasible tool for choosing the appropriate PCM.

Parametric study of low-temperature thermal energy storage using carbon dioxide as the phase change material in pillow plate heat exchangers

Industrial low-temperature freezing applications are often batch processes, requiring a lot of energy, exerting stress on the electrical grid. To relieve this stress, thermal energy storage can be used. However, there is a lack of suitable storage material for low temperature applications (around −50 °C). Under high pressures, carbon dioxide can be used as the phase change material for storage temperatures around −55 °C. In this study, a parametric study was conducted on the design and operational parameters of an industrial-scale pillow plate heat exchanger with carbon dioxide. Two responses were selected for the analysis where R1 considered the storage size over the phase change time (kWh/h), while R2 indicated the cost over the storage size (USD/kWh). Using design of experiments, a total of 52 simulations were carried out to investigate the parameters under constant heat transfer surface area. Analysis of variance was then carried out followed by correlation development and optimization. It was found that regardless of the process (charging or discharging), for R1 and R2, the difference between refrigerant and carbon dioxide phase change temperatures followed by plate material had the highest significance. In contrast, the refrigerant flow rate had the lowest significance in almost all cases. Moreover, considering an equal weight for the responses, overall optimal conditions were determined for the processes. The recommended values for plate pitch, plate material, difference between refrigerant and carbon dioxide phase change temperatures and refrigerant flow rate were 25 mm, aluminum, 15 °C and 4 kg/s, respectively.

Development of phase change materials for low-temperature thermal energy storage application

Background: Energy storage is very critical for reducing the mismatch between demand and supply thus offering better management capabilities. It reduces the peak energy demand and increases efficiency, security, and reliability. There is unavailability of low-cost phase change materials (PCMs) in the lower temperature range. Methods: This study discusses the creation of eutectic from capric acid and paraffin wax. A series of blending of capric acid/paraffin wax (CA/PW) were prepared, having variable weight-composition. The thermophysical properties were obtained using differential scanning calorimetry, Further, thermal cycle testing was done to understand the thermal stability and reliability of the prepared eutectics. Results: The area underneath the peak is used to calculate the latent heat of fusion, and the tangent of the steepest slope at the peak of the crest is used to calculate the melting temperature (Tm). Differential scanning calorimetry results showed the developed eutectic had an appropriate melting temperature and adequate latent heat of fusion of 29.86 °C - 30.60 °C and 154.15–198.62 kJ/kg respectively, and can be used for various thermal energy storage applications in buildings, solar absorption chillers, surgical dress/clinical bed, and photovoltaic systems. Conclusions: The accelerated thermal cycle of the same confirmed its thermal stability up to 500 heating and cooling cycles. It was discovered that variable heating and cooling speeds had no significant influence on the melting temperature and latent heat of fusion of PW/CA eutectics. Further, the economic study revealed that the created PCM is inexpensive and readily available in the Indian market.