Phase change materials (PCMs) can reduce building peak loads and enable demand-responsive thermal energy storage (TES), but their deployment depends on reliable measurement and interpretation of thermal properties across laboratory, intermediate, and application scales. This review systematically examines characterization methods, testing protocols, and recent advances for neat PCMs and PCM composites, emphasizing thermal conductivity, enthalpy-related properties (phase change temperature, latent heat, specific heat), and cycling stability. For thermal conductivity, we compare steady-state and transient techniques and note limitations when phase transition and contact resistance affect measurements. For enthalpy–temperature characterization, we discuss differential scanning calorimetry together with intermediate- and bulk-scale methods, including T-history, heat flow meter testing, and three-layer calorimetry (3LC), to generate application-relevant enthalpy–temperature profiles. Cycling stability is organized into four experimental families: thermoelectric–air, fully thermoelectric, water-bath, and in situ chamber approaches, with attention to separating reversible supercooling from true degradation such as phase segregation. We highlight emerging noncontact diagnostics, including infrared thermography and embedded sensing, for spatially resolved validation and multiscale interpretation. Finally, we review the growing use of AI and machine learning for property prediction, inverse characterization from experimental signals, and real-time state estimation in building-integrated TES. Key needs include harmonized protocols, interlaboratory benchmarking, uncertainty reporting, and metadata-rich datasets to accelerate reproducible PCM qualification for grid-flexible buildings.

Thermal energy storage-integrated heat pump (TES-HP) systems offer a promising approach to reducing peak electricity demand while maintaining thermal comfort; however, field-scale validation under real operating conditions remains limited. This paper presents a field demonstration of a TES-HP system installed in an occupied residential building and operated during the cooling season. A phase change material (PCM)-based TES unit is integrated on the condenser side of a conventional split HP and operated using a supervisory control strategy to enable on-peak load shifting. Both component-level TES charging/discharging tests and integrated building-level TES-HP tests were conducted. Field measurements demonstrate stable TES operation, with outlet temperatures maintained within or near the PCM phase-change range. At the building level, TES discharge maintains conventional air-duct performance and indoor temperature regulation while delivering approximately 8 kW of cooling, comparable to the baseline system. During TES-assisted operation, condenser heat rejection is shifted from ambient air to the TES, reducing the condenser saturation temperature from approximately 40 degrees C to 25 degrees C and lowering on-peak outdoor-unit power consumption from 2.21 kW to 1.35 kW. Compared with an HP-only baseline, energy-based evaluation shows an on-peak electricity reduction of 38.4% and a daily electricity cost reduction of 27.7%, corresponding to a calibrated cost saving of 14.2% when normalized to equal cooling delivery. Results also indicate effective TES operation under part-load conditions. Overall, this study demonstrates the practical feasibility and load-shifting benefits of TES integration for residential HPs under field conditions.

Household dishwashers must address several performance goals: maximize washing and drying performance, while minimizing cycle duration, energy consumption, and water consumption. This study develops and examines a novel thermoelectric heating and cooling (TEHC) system with thermal energy storage applied to a household dishwasher (DW), aiming to improve the energy and drying performance of a commercially-available dishwasher while maintaining its washing performance, water consumption, and cycle duration. Experimental testing conducted on the novel TEHC-DW system demonstrates an 8.7% reduction in total energy consumption, lowering the per-cycle usage to 0.952 kWh, and a 40% reduction in energy consumption for internal water heating. The novel TEHC-DW system also demonstrates better drying performance, shortening the drying time by 42% to reach the same remaining moisture content as a commercially-available system. Additionally, a resistance-capacitance network model is developed that predicts total drying time, total energy consumption, and the highest temperature reached in the tub (53.4 degrees C). The model accuracy is validated with experimental data (within +/- 1 K), and the model functions as a design tool via a parametric study to evaluate the nextgeneration design and the effect of the number of thermoelectric modules and thermoelectric driving force (i. e., current). Overall, this study demonstrates the potential for TEHC technology to improve energy efficiency and drying performance in household dishwashers.

In recent years, thermal energy storage (TES) has gained attention for its role in enhancing renewable energy solutions and sustainable energy consumption. The usage of strontium chloride hexahydrate (SCH), graphene nanoplatelet (GNP), and cellulose nanofibril (CNF) additives were investigated to enhance the performance of calcium chloride hexahydrate (CCH) based on the melting/solidification behavior for TES applications. In this work, we develop a promising phase-change-material (PCM) formulation by introducing these additives that reduce supercooling, improve the thermal conductivity and stabilizing the energy storage capacity of CCH. Rheological characterizations demonstrated that the addition of 1 wt% of CNF into CCH produced the required improvement in viscosity and boosted solid-like rheological behavior. Structural characterizations show a physical mixing of the materials within the PCM composites. Our observations show that the amphiphilicity of CNF enables the surface attachment to GNP via hydrophobic interactions providing effective dispersion of GNP throughout the PCM composite. The addition of a nucleating agent, SCH decreased the degree of supercooling of similar to 20 g of CCH from >20 degrees C to 3 degrees C at a cooling rate of 5 degrees C/min. Thermal characterization showed the resulting PCM composite has a latent heat of melting of 186 Jg(-1), phase change temperature of 32 degrees C, and stable thermal properties after being subjected to 70 melt-freeze cycles. Adding CNF and GNP to pure CCH increased its thermal conductivity by 76 %. The high thermal conductivity of GNP and its effective dispersion by CNF is responsible for this enhancement. The study highlights the use of biodegradable nanocellulose for the preparation of sustainable PCM composites with improved performance. These PCM composites are scalable, they have potential to increase energy efficiency and revolutionalize the heating/cooling applications in buildings and other TES systems.

The worldwide increasing energy demand and 2050 net zero carbon target urge the globe to solve the energy challenge. Thermal Energy Storage (TES) has received significant attention in recent years as TES can be integrated into heating, ventilation, and air-conditioning systems where the energy would be stored during low-demand times and dispatched during high-demand times, resulting in controlling the peak load and improving energy savings. Material development is an integral part of TES. Salt hydrates are appealing due to cost-effectiveness, low- to no toxicity, and their high melting enthalpy, where energy is stored as latent heat. However, most salt hydrates are prone to incongruent melting (i.e., phase separation upon melting), which results in poor stability and large supercooling. In this study, we produced a highly stable novel energy storage material at a composition of 32 wt% sodium sulfate decahydrate, 52 wt% sodium phosphate dibasic dodecahydrate, 12 wt% milled expanded graphite, and 4 wt% borax. The material has a melting temperature of 28°C and an energy storage capacity of 167 kJ/kg with a supercooling of less than 3°C. The system showed no loss in energy storage performance after 150 cycles. The findings suggest that the novel energy storage material developed in this might be utilized in building heating and cooling applications.

Thermoelectric (TE) heat pumps (TEHPs) are advantageous for heating and cooling in various applications because of their modularity and simple design. A TEHP system includes the TE modules with p- and n-type materials bonded to substrates, plus heat exchangers, thermal interfaces to the heat exchangers, and heat transfer fluids. Although modeling an individual TE module has been extensively studied, limited studies have reported performance at the larger system-level. Furthermore, no prior study has addressed the impact of temperaturedependent TE material properties (e.g., electric resistivity, thermal conductivity, and Seebeck coefficient) on overall heat-pump-system-level performance. This work presents a mathematical model for TEHP system performance based on Goldsmid's approach for TE material performance, "effective" TE material properties, Gnielinski's correlation for convective heat transfer, and thermal balance theory for a heat exchange network. This combined approach provides an accurate model of the liquid-to-liquid TEHP system. Three different approaches-one empirical, one based on the manufacturer's specifications, and one drawn from the literature-were then used to determine values for TE material properties. The first two methods treated properties as constants, while the last approach treated properties as surface-temperature-based functions. Finally, experimental TEHP data was used to validate the models, all with relative absolute deviations of approximately 10% when predicting heating capacity and 10%-25% when forecasting cooling capacity up to a 30 K surface temperature lift. The results demonstrated that, at the TEHP system level, the TE material properties could be treated as constants, avoiding solver iterations and reducing the performance uncertainty by up to 95%.

Phase change material (PCM)-based thermal energy storage (TES) can provide energy and cost savings and peak demand reduction benefits for grid-interactive residential buildings. Researchers established that these benefits vary greatly depending on the PCM phase change temperature (PCT), total TES storage capacity, system configuration and location and climate of the building. In this study, preliminary techno-economic performance is reported for a novel heat pump (HP)-integrated TES system using an idealized approach. A simplified HP-TES was modeled for 1 year of space heating and cooling loads for a residential building in three different climates in the United States. The vapor compression system of the HP was modified to integrate with TES, and all heat transfer to and from the TES was mediated by the HP. A single PCM was used for heating and cooling, and the PCT and TES capacity were varied to observe their effects on the building’s energy consumption, peak load shifting and cost savings. The maximum reduction in electric consumption, utility cost and peak electric demand were achieved at a PCT of 30 °C for New York City and 20 °C for Houston and Birmingham. Peak energy consumption in Houston, New York City, and Birmingham was reduced by 47%, 53%, and 70%, respectively, by shifting peak load using a time-of-use utility schedule. TES with 170 MJ storage capacity allowed for maximum demand shift from on-peak to off-peak hours, with diminishing returns once the TES capacity equaled the daily building thermal loads experienced during the most extreme ambient conditions.

Sodium sulfate decahydrate (Na2SO4.10H2O, SSD), a low-cost phase change material (PCM), can store thermal energy. However, phase separation and unstable energy storage capacity (ESC) limit its use. To address these concerns, eight polymer additives-sodium polyacrylate (SPA), carboxymethyl cellulose (CMC), Fumed silica (SiO2), potassium polyacrylate (PPA), cellulose nanofiber (CNF), hydroxyethyl cellulose (HEC), dextran sulfate sodium (DSS), and poly(sodium 4-styrenesulfonate) (PSS)-were used to explore several stabilization mechanisms. The ESC of PCMs deteriorated when thickeners, SPA, PPA, and CNF, were added. DSS-modified PCMs exhibited greater stability up to 150 cycles. Rheology measurements indicated that DSS did not impact SSD viscosity significantly during stabilization. Dynamic light scattering showed that DSS reduces SSD particle size and electrostatically suspends salt particles in a stable homogeneous solution, avoiding phase separation. This study proposes a promising method to improve the thermal stability of salt hydrate PCMs by utilizing polyelectrolyte-salt hydrate mixture for thermal energy storage applications.

Experimental results for phase transition during extended thermal cycling of the salt hydrate LiNO3 center dot 3H2O in both heterogeneous nucleation and self-seeding as well as material compatibility are presented. Controlled samples of this phase -change material (PCM) are heated and cooled between 20 and 40 degrees C. This was carried out for 1000 thermal cycles to ascertain the long-term suppression of subcooling without degradation in latent heat (hsf). In heterogeneous nucleation, two additives were considered, Zn3OH4(NO3)2, and Zn(NO3)2 center dot 6H2O, whereas the self-seeding was induced by less than complete (90%) melting. The two nucleating agents were selected by the minimal lattice-mismatch method for salt crystals. Self-seeding was found to completely suppress subcooling without any loss in the latent heat of fusion, whereas heterogeneous nucleation with additives resulted in a subcooling of Delta Ts > 3 degrees C and an up to 8-52% loss in latent heat after 1000 thermal cycles. Additionally, the material compatibility test was conducted with Al 3003, Al 1100, and SS 304 for 4000 heating-cooling cycles. The corrosion rates observed without nucleating agents were insignificant and less than typically observed in similar metals exposed to marine conditions.

Heating and cooling systems in building infrastructure utilize conventional materials that account for a considerable amount of energy usage and waste. Phase change material (PCM) is considered a promising candidate for thermal energy storage that can improve energy efficiency in building systems. Here, a novel salt hydrate-based PCM composite with high energy storage capacity, relatively higher thermal conductivity, and excellent thermal cycling stability was designed and developed. The thermal cycling stability of the PCM composite was enhanced by using dextran sulfate sodium (DSS) salt as a polyelectrolyte additive, which significantly reduced the phase segregation of salt hydrate. The energy storage capacity and the thermal conductivity of the composite were enhanced by the addition of various graphitic materials along with Borax nucleator. A significant increase in thermal cycling stability was observed for the DSS-modified composite, with over 100 thermal cycles without degradation. The final PCM composite exhibited as much as 290% increase in energy storage capacity relative to the pure salt hydrate, and approximately 20% increase in thermal conductivity. In addition, the PCM composite developed can be produced at larger scale, and can potentially change the future of heating/cooling system in building infrastructure.

Inorganic salt hydrate phase change materials (PCMs) are of interest for near-room temperature thermal energy storage (TES) systems, but their low thermal conductivity, similar to 0.5 W/m-K, limits their performance. In this work, we report the thermal conductivity and bulk density of composites containing sodium sulfate decahydrate (SSD) Na2SO4 center dot 10H(2)O with three types of graphite: expanded graphite (EG), milled EG (MG), and graphite nanoplatelets (GnP). The effect of these thermophysical properties on TES performance is presented. The composites were made using a readily scalable one-pot synthesis procedure with graphite received as-is. A 583% increase in thermal conductivity (4.1 W/m-K) was achieved with 25 wt% EG. However, as EG fraction increases, bulk density decreases and thermal conductivity plateaus. This ultimately resulted in lower thermal performance at higher EG fractions despite higher thermal conductivity. This highlights the tradeoff between PCM composite properties and performance, and why thermal conductivity is insufficient to describe PCM thermal performance. GnP is added to EG-SSD to increase bulk density and energy storage density, but these density improvements do not offset lower thermal conductivity and thus thermal performance declined. Similarly, MG-SSD composites had a higher bulk density and energy storage density, but lower thermal conductivity and thermal performance than EG-SSD composites at similar compositions. Atomistic molecular dynamics simulations were performed to understand the structure-property relationship of graphite-SSD interfaces. The simulations support the hypothesis that atomic level contact resistance between graphite and SSD increases thermal resistance at the interfaces resulting in effectively lower bulk thermal conductivity in MG-SSD compared to EG-SSD.

Energy storage needs to support commercial and residential buildings in the U.S. in 2050 for various 100% renewable energy scenarios.

This study reports on the use of sodium alginate to effectively stabilize sodium sulfate decahydrate (Na(2)SO(4)middot10H(2)O, SSD) based phase change material (PCM) for application as a thermal energy storage material. Alginate/SSD composite PCMs were prepared by blending SSD with different concentrations of alginate polymer. The resulting composite PCMs demonstrate high phase change enthalpy similar to 160 J/g and extended cycling stability compared to existing PCM composites. The analysis carried out by optical microscopy, X-ray scattering, and periodic density functional theory (DFT) calculations demonstrated that the stabilization effect was caused by the interplay between ionic and hydrogen bond interactions between the alginate and SSD. Additionally, the variation in mechanical properties of PCM composites with polymer concentrations made it possible to formulate a composite that maintains stable performance after 3D printing. The advanced properties make this composite a promising candidate for application as a thermal energy storage material.

Air dehumidification is essential since excess moisture in the buildings causes discomfort to the occupants, encourages the production of air pathogens such as mold or mildew, and causes corrosion and rotting that degrade building materials. Existing moisture removal processes are mainly focused on condensation and desiccant (liquid or solid) techniques with direct contact between air and desiccant. However, these methods are energy-intensive, or desiccant might be lost or cause corrosion in the process. The main objective of this study is to investigate an ionic liquid-based liquid desiccant absorber based on a membrane fiber bundle. A novel membrane contactor system was fabricated with a bundle of 10,000 polypropylene fibers. Each fiber has 0.3 micron outer diameter, with ionic liquid flowing inside, and air flowing outside. The fibers provide a high contact area among phases: 1.4 m contact surface area in a 0.00015 m volume (9,333 m/m ratio of surface area to volume). The ionic liquid as a sorbent has selectivity for water vapor (i.e., the ionic liquid has higher affinity for water vapor) prevents the loss of solvent in the operation due to negligible volatility, provides fast diffusion due to low viscosity compared to common ionic liquids, and has high affinity and solubility in water. The dehumidification capacity of the prototype membrane system was experimentally investigated using six modules with 10,000 fibers each. The experimental results show that the ionic-liquid based membrane system can effectively remove excess moisture from the air. The novel fiber bundle dehumidification system has a total system volume of 0.00798 m (7.98 L) and active heat and mass transfer surface area of 8.4 m. It achieved an average dehumidification of 320 ± 25 W with a volumetric air flowrate of 3.1 m/min (108 ft/min). Notice: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the US Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-publicaccess-plan). 18 International Refrigeration and Air Conditioning Conference at Purdue, May 24-28, 2021

Salt hydrate-based phase-change materials are considered promising for future heat storage applications in residential heating/cooling systems. Smooth phase transition from the liquid to solid phase and vice versa is essential for effective heat exchanger; however, supercooling in salt hydrates delays the onset of liquid–solid phase transition. We investigate the molecular level mechanism of supercooling in sodium sulfate decahydrate (SSD). SSD is a complex salt hydrate whose properties are governed by electrostatic forces that include pure Coulombic interactions as well as hydrogen bonds. Experimentally, we examine the importance of a nucleator in reducing supercooling temperatures. We investigated the effect of various mass concentrations of a borax nucleator on a decrease of supercooling temperatures. Molecular dynamics simulation techniques are used to obtain a basic understanding of supercooling in SSD. We observe that by introducing borax as a nucleator, there is a decrease in the supercooling temperature before nucleation. Our molecular dynamics simulations show that long-range electrostatics between sodium and sulfate ion pairs and that with polar water molecules is responsible for delayed nucleation in SSD that results in supercooling, and also, dynamics of charged molecules slows down. The lack of crystallization leads to amorphous structures in supercooled SSD.