Passive Thermal Energy Storage Research Articles

Thermal-Responsive Smart Windows with Passive Dimming and Thermal Energy Storage.

Chromogenic smart windows are one of the key components in improving the building energy efficiency. By simulation of the three-dimensional network of polymer hydrogels, thermal-responsive phase change materials (TRPCMs) are manufactured for energy-saving windows. For simulated polymer hydrogels, tetradecanol (TD) and a color changing dye (CCD) are filled in situ in poly(n-butyl isobutyrate) (PBB) networks. TRPCMs convert solar energy into thermal energy through a dark blue CCD. The TD phase change material (PCM) absorbs heat energy to become a transparent liquid. Simultaneously, the CCD changes from blue to colorless and transparent at around 45 °C. As a result, as-prepared TRPCMs transform from an opaque state at room temperature to a high-transparency state after melting (74.5%). TRPCMs also show a good thermal storage capacity, with a phase transition enthalpy exceeding 161.9 J g-1. As-prepared smart materials can simultaneously achieve photothermal conversion, thermal energy diffusion, latent heat storage, and resistance to liquid leakage at the phase interface between opaque and transparent states, providing more options for the design of energy-saving buildings.

Effect of steam flow parameters on the partial load behaviour of a PCM storage

To estimate the achievable thermal power in passive latent heat thermal energy storage (LH-TES) systems and the corresponding charging/discharging times for customised storage design and operation, relevant influencing parameters and their impact must be sufficiently known and understood. These parameters include not only the well-researched parameters on the phase change material (PCM) side, but also the parameters on the heat transfer fluid (HTF) side, such as the inlet temperature, the mass flow rate and the resulting heat transfer coefficient (HTC) between the HTF flow and the tube wall. For heat exchangers, these influences have been studied well, however for highly transient LH-TES studies are scarce. This article presents the influence of relevant parameters focusing on the ones related to the HTF side, using analytical and numerical parameter studies in Matlab®. The effect of different HTCs for varying flow conditions of the HTF in different zones of the tube is utilised to introduce the dominant heat transfer surface between the storage and the HTF (DHTS) as an indirect operating parameter. It corresponds to the area where the HTF phase change occurs and where most of the total energy is transferred. The numerical study indicates that the size of the dominant heat transfer area and thus the DHTS can be adapted by the HTF mass flow rate and the temperature difference between the HTF inlet temperature and the PCM phase change temperature. Based on this knowledge, new conjugate design and operating strategies for a test rig with a nominal thermal power of 1 kW using the commercial PCM PLUSICE A133 as storage material are proposed. They lead to a peak shaving behaviour of the storage with defined charging and discharging times.

Fundamental limits of dynamic phase change materials

To accommodate societal electrification and decarbonization, renewable energy resources continue to expand their share of the global energy market. The intermittency of renewable energy technologies as well as the high power density of modern electrified platforms necessitates the need for both efficient thermal management and high-density thermal storage. Phase change materials are a promising passive thermal energy storage solution. However, difficulties with efficient system implementation stemming from the inherent melt pool formation hinder their potential. We develop an innovative strategy, termed dynamic phase change material “dynPCM,” to address this thermal transport issue by ensuring a constant, thin, melt layer. We analyze the fundamental limits of dynPCMs, characterize the peak achievable heat flux and energy/power densities, estimate the power consumption of dynPCM systems, and investigate the fundamental physics which govern dynPCM behavior. We show that dynPCM can eliminate the classical trade-off seen between energy density and power density and achieve ultrahigh heat fluxes, ∼105 W/cm2, with heat flux-to-required power ratios as high as ∼107. We also demonstrate achievable power densities as high as ∼100 W/cm3 at energy densities as high as ∼10 kJ/cm3. Throughout this work, we develop a methodology to evaluate the operating limits, enabling adaptation of the dynPCM system concept to a variety of applications and industries.

N-octanoic acid-based phase change composites synthesized by absorption polymerization for efficient thermal energy storage

Due to their high latent heat capacity near the phase change temperature, phase change materials (PCMs) have been extensively employed in passive thermal management and energy storage. Low thermal conductivity and leakage are, nevertheless, two ongoing limitations in PCMs thermal applications. In this work, a novel absorption polymerization process was carried out to prepare poly(methyl methacrylate)/expanded graphite/n-octanoic acid (PMMA/EG/OA) phase change composites (PCCs) having high thermal conductivity, enthalpy, encapsulation efficiency, as well as leakproofness. The PMMA/EG/OA PCCs are prepared in a two-step process: absorption of OA and methyl methacrylate (MMA) into the EG networks; and subsequent polymerization of MMA to cover the EG absorbed OA. The PCCs exhibit a latent heat capacity greater than 110 J/g, an encapsulation efficiency of approximately 100 %, and a thermal conductivity of 0.75–3.58 W/(m·K) at EG loadings of 6–15 wt%. Moreover, the PCCs display outstanding dynamical responsiveness and prolonged cycling reliability. Absorption polymerization is an efficient and profitable approach for producing high-performing PCCs that can be used for a variety of thermal-related areas, including thermal harvesting for renewable energy, building energy conservation, as well as thermal management of electrical devices.

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.

N-Eicosane-Impregnated nonwoven phase change mats of electrospun Poly(ethylene oxide)/Poly(methyl methacrylate) blended fibers

The ever-increasing energy demand led scientists turn to more efficient, environmentally friendly, and renewable technologies. Phase change materials (PCMs) are gaining popularity in the field of passive thermal energy storage. In the current work, polyethylene oxide/poly (methyl methacrylate) (PEO/PMMA) electrospun fibers were employed as encapsulation matrices for n-eicosane and their physicochemical properties were tested, aiming to develop stable phase change fibers (PCFs). The effect of the n-eicosane content on the morphology and thermal properties of the form-stable PEO/PMMA/n-eicosane PCFs was extensively investigated by using X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Differential Scanning Calorimetry (DSC) and Thermogravimetric analysis (TGA). The melting and crystallization enthalpy of PEO/PMMA/n-eicosane remained unchanged during the first and second cycle. After 30 heating-cooling cycles, the samples with 2 and 5 wt% n-eicosane exhibited inferior thermal reliability, while no temperature and enthalpy change were observed after the thermal cycling of PCF with 7.5 and 10 wt% n-eicosane content, thus rendering these systems suitable materials for thermal energy storage applications. The most promising system was the one containing a 10 wt% n-eicosane, due to the higher crystallization enthalpy (184 J/g) and the stabilization of the melting and crystallization peaks for all cycles. The degradation kinetics were investigated for n-eicosane, the pristine PEO/PMMA fibers and the PEO/PMMA/n-eicosane fibers with a 10 wt% n-eicosane content. Moreover, the Activation Energy (Ea) and the pre-exponential factor (A) were calculated using the Friedman and Vyazovkin methods whereas the reaction mechanisms were studied using the model fitting method.

Paraffin@SiO2 microcapsules-based phase change composites with enhanced thermal conductivity for passive battery cooling

Phase change materials (PCMs) are potential candidates in passive thermal regulation and energy storage fields due to their high latent heat capacity around phase transition temperature. However, the leakage problem and low thermal conductivity are two obstructive factors for the extended application of PCMs. Herein, a series of paraffin@silicon dioxide microcapsules (Pa@SiO2)/graphene sheets (GS)/silicone rubber (SR) phase change composites (PCCs) were prepared. It is found that the inorganic SiO2 shell is conducive to enhancing the thermal conductivity of PCCs and the double encapsulation by the SiO2 shell and SR skeleton can restrict the leakage of liquid Pa during phase transition. With a Pa@SiO2 content of 70 wt%, the PCCs have a high latent heat of 126.1 J/g and enhanced thermal conductivity of 0.37 W m−1 K−1, which is 131.25% higher compared to that of pure SR. In addition, the introduction of graphene sheets further boosts the thermal conductivity of PCCs to 2.69 W m−1 K−1. The obtained PCCs lead to a surprising temperature decline of nearly 35 °C of a commercial lithium-ion battery during a high discharge rate (7.4 C). This work provides an efficient route to fabricate microcapsules-based PCCs for passive thermal regulation.

Evaluation of global energy performance of building walls integrating PCM: Numerical study in semi-arid climate in Morocco

Passive latent heat thermal energy storage technologies with phase change materials (PCM) provide a potential solution to reduce energy demand and regulate the thermal comfort in occupied buildings. In this study, a numerical investigation is carried out on PCM-enhanced integrated building walls under the semi-arid climate to set the key parameters for the effective utilisation of PCM. A parametric study of PCM-composites walls is performed, including phase-change temperatures range, thicknesses, location, configurations, and mechanical ventilation effect on the performance of PCM integrated building walls. The results showed that the PCM-integrated walls improve indoor comfort and reduce cooling and heating loads and temperature fluctuations. The optimal PCM was RT-28 HC for the semi-arid climate with the highest annual average ATFR of 1.91 °C. In addition, the use of the double layers and triple layers PCM is more efficient compared to the single-layer PCM with energy consumption reduction from 7,30% to 15,21%. Also, the triple-layer system with mechanical ventilation showed the best energy efficiency and load shifting performance in the summer season. Finally, this research reveals that a triple configuration PCM wall comprising the paraffin RT-21+RT-25+RT-28 HC as PCM provides an optimal energy performance for heating in winter and free cooling in summer, as well as an annual energy saving of 102 kWh and 324 kWh in the heating and cooling season respectively, which considerably reduces the annual energy consumption.

Study on thermal energy storage properties of bio-based n-dodecanoic acid/fly ash as a novel shape-stabilized phase change material

In the present study, a novel BSPCM was prepared with the n-dodecanoic acid (or) LA phase change material, which was vacuum impregnated into the porous FA pebbles. The surface morphology of the BSPCM has confirmed the presence of the lauric acid in the porous supporting matrix. The XRD analysis of the BSPCM revealed that the crystalline nature of the PCM was unaltered after impregnation. The surface structure study verified the chemical compatibility between the PCM and supporting material. The DSC results showed that, BSPCM has exhibited a good latent heat of fusion of 68.93 kJ/kg with high thermal energy storage capability of 97.62%. The BSPCM was thermally stable up to 150 °C and showed good leakage stability up to 85 °C. Thermal reliability of the BSPCM performed for 1000 thermal cycles revealed excellent thermal reliability index of 96.76%. Further, thermal conductivity of BSPCM was found to be 0.1702 W/mK, which was attributed to the effective impregnation of the PCM into the fly ash pebbles. In total, the developed BSPCM can be a viable candidate for achieving passive thermal energy storage in buildings.

Review on the Integration of Phase Change Materials in Building Envelopes for Passive Latent Heat Storage

Latent heat thermal energy storage systems incorporate phase change materials (PCMs) as storage materials. The high energy density of PCMs, their ability to store at nearly constant temperature, and the diversity of available materials make latent heat storage systems particularly competitive technologies for reducing energy consumption in buildings. This work reviews recent experimental and numerical studies on the integration of PCMs in building envelopes for passive energy storage. The results of the different studies show that the use of PCMs can reduce the peak temperature and smooth the thermal load. The integration of PCMs can be done on the entire building envelope (walls, roofs, windows). Despite many advances, some aspects remain to be studied, notably the long-term stability of buildings incorporating PCMs, the issues of moisture and mass transfer, and the consideration of the actual use of the building. Based on this review, we have identified possible contributions to improve the efficiency of passive systems incorporating PCMs. Thus, fatty acids and their eutectic mixtures, combined with natural insulators, such as vegetable fibers, were chosen to make shape-stabilized PCMs composites. These composites can be integrated in buildings as a passive thermal energy storage material.

Porous biochar/heptadecane composite phase change material with leak-proof, high thermal energy storage capacity and enhanced thermal conductivity

Solid-liquid phase change materials (PCMs) have been preferred for solar passive thermal energy storage (TES) applications. However, low thermal conductivity and leakage issue of molten PCMs considerably restrain their TES potential. In this framework, n-Heptadecane (HD) as a solid-liquid PCM was incorporated with carbonized lemon peel (CLP) for development of a novel leak-proof composite PCM. Chemical compatibility between the constituents of the leak-proof composite PCM was examined by using FTIR spectroscopy and XRD diffraction analyses. The DSC results revealed that the developed leak-proof CLP/HD composite PCM had a melting temperature of 19.79 °C and LHS capacity of 141.8 J/g. The composite PCM exposed venerable thermal degradation stability after a 1000-cycling heating-cooling process. Thermal conductivity of the CLP/HD composite PCM (0.46 W/m.K at 10 °C) was measured as approximately 77% higher than that of pristine HD (0.26 W/m.K at 10 °C).

Empirical validation and comparison of methodologies to simulate micro and macro-encapsulated PCMs in the building envelope

Thermal Energy Storage (TES) has the potential to shift peak electricity demand. Passive TES is usually implemented in building envelope as micro and macro encapsulated phase change materials (PCM) to shift electric energy demand and therefore requires careful heat transfer analysis. Whole building energy modelling with simplified heat transfer analysis has become extremely important for designers, architects, engineers, and researchers to predict energy performance of buildings. It is important to validate PCM modelling algorithms used in building energy programs to quantify their error and prove their capacity to model different PCM encapsulation types. This study uses data from a microencapsulated PCM and two macroencapsulated PCMs (Bio based PCM and hydrate salts) tested in full-scale using the Advanced Multiscale Building Energy Research (AMBER) Lab located at the Colorado School of Mines and is used to validate a numerical algorithm written in MATLAB language. To approximate the heat transfer through a wall assembly with macroencapsulated PCM pouches, several modelling techniques that can reduce 3D heat transfer characteristics to 1D are explored in this research. A parallel path heat transfer modelling approach is found to give the closest agreement with the experimental data for the pouched PCMs in building envelope applications.

Experimental apparatus and methodology to test and quantify thermal performance of micro and macro-encapsulated phase change materials in building envelope applications

Abstract Thermal energy storage (TES) is used as a viable technology to shift peak electricity demand caused by the space cooling requirements in buildings. Passive TES is implemented in building envelope via micro and macroencapsulation methods. This study describes the use of a state-of-the-art laboratory to test different PCM inclusions in simplified building walls. A microencapsulated PCM and two macroencapsulated PCMs are tested in a controlled environment to gather data for validation purposes of PCM modelling algorithms in building energy modelling programs. Data indicates that the chamber environment and the heating and cooling system can conduct full-cycle tests of wall panels with PCM inclusions. This study also generates data from parallel tests on 4 wall panels which can be used in building energy modelling programs to validate the PCM modelling algorithms. The cyclic tests also capture the thermal effects of PCMs and complex PCM behaviors like sub-cooling in PCM hydrate-salts.

Thermo-optically responsive phase change materials for passive temperature regulation

Many phase change materials (PCMs) experience a change in transparency when undergoing a phase transition. These thermo-optically responsive materials can be used to generate passive temperature control systems for building enclosures. The integration of optical and thermal switches into smart temperature-controlling elements requires rationally designed PCMs featuring tunable optical and thermal properties. Two polymers, poly (Octadecyl methacrylate) (PSMA) and poly(2-(2-(octadecyloxy) ethoxy) ethyl methacrylate) (PE2SMA) were synthesized and evaluated for their potential use in passive thermal energy storage systems. UV–Visible Spectroscopy, Near Infra-Red Spectroscopy, and Differential Scanning Calorimetry were used to evaluate the effect that changes in the polymer chemical structure had on the optical and thermal properties of the resulting materials. Insertion of a 6-atom flexible spacer (diethylene glycol) between the pendant crystalline motif and the polymer backbone of PSMA resulted in increases of latent heat storage capacity from 62 J/g to 94 J/g and thermal conductivity from 0.218 W/mK to 0.318 W/mK. Notably, insertion of a flexible spacer also resulted in a melting transition temperature increase from 37.7 °C for PSMA to 48 °C for PE2SMA. The visible transmittance of the polymers increased from 0% to 90% upon transition from crystalline to amorphous state. This study presents a synthetic strategy to control thermal and optical properties of polymeric PCMs materials. The material properties and structure-property relationships derived from this study will enable the refinement of the models used to predict the performance of passive temperature-regulating systems. More accurate models will guide the development of the thermo-responsive polymeric materials required for better perfoming temperature-regulating building enclosures.

Using Real-Time Electricity Prices to Leverage Electrical Energy Storage and Flexible Loads in a Smart Grid Environment Utilizing Machine Learning Techniques

With exposure to real-time market pricing structures, consumers would be incentivized to invest in electrical energy storage systems and smart predictive automation of their home energy systems. Smart home automation through optimizing HVAC (heating, ventilation, and air conditioning) temperature set points, along with distributed energy storage, could be utilized in the process of optimizing the operation of the electric grid. Using electricity prices as decision variables to leverage electrical energy storage and flexible loads can be a valuable tool to optimize the performance of the power grid and reduce electricity costs both on the supply and demand sides. Energy demand prediction is important for proper allocation and utilization of the available resources. Manipulating energy prices to leverage storage and flexible loads through these demand prediction models is a novel idea that needs to be studied. In this paper, different models for proactive prediction of the energy demand for an entire city using different machine learning techniques are presented and compared. The results of the machine learning techniques show that the proposed nonlinear autoregressive with exogenous inputs neural network model resulted in the most accurate predictions. These prediction models pave the way for the demand side to become an important asset for grid regulation by responding to variable price signals through battery energy storage and passive thermal energy storage using HVAC temperature set points.

A numerical investigation of the effects of metal foam characteristics and heating/cooling conditions on the phase change kinetic of phase change materials embedded in metal foam

Abstract In the last decade, the use of Phase change materials (PCM) as passive thermal energy storage has been widely studied both numerically and experimentally. Despite their advantages, their thermal conductivity is very low, with a large change in volume during the melting and solidification process. One way to increase their poor thermal conductivity is to embed them into open cells metallic foams. In this paper, a numerical study is conducted on the effects of metal foam properties and heating/cooling conditions (cyclic heating and cooling) on the phase change kinetic of PCM embedded in metal foam. Two types of heating condition were studied: constant and sinusoidal heat flux. The mathematical model is based on the volume-averaging technique. The Brinkman–Forchheimer-extended Darcy equation and the local thermal non-equilibrium model that applies a two-energy equation are used. The phase change of PCM is modeled by an enthalpy-porosity method. The numerical results are validated by comparing with experimental data. It is observed from the numerical results that the foam morphology and materials have a significant influence on the melting and solidification of phase change materials. The results also show that the melting time is reduced with sinusoidal heating.

Numerical Study of Application of PCM for a Passive Thermal Energy Storage System for Space Cooling in Residential Buildings

In this paper, effectiveness of phase change materials (PCMs) for application in passive thermal energy storage (TES) system for space cooling in residential buildings is investigated numerically. PCM is encapsulated in steel containers and integrated into the building interior under a ceiling slab. Thermal response of two phase change materials with different melting/solidification temperatures under the typical summer conditions of the Baltic States is analysed by using computational fluid dynamics (CFD) software Ansys Fluent. The results showed that integration of the passive TES system can reduce the overheating problem and improve thermal comfort; however full advantage of the PCM storage capacity cannot be taken without active regeneration of the PCM.

Heat transfer reduction in buildings by embedding phase change material in multi-layer walls: Effects of repositioning, thermophysical properties and thickness of PCM

Passive latent heat thermal energy storage approach incorporating phase change materials (PCM) is a brilliant technique to tackle high energy consumption issue in buildings. This paper investigated the thermal performance of the conventional walls of buildings in Isfahan, Iran with the inclusion of thirteen different phase change materials. The studied base wall was composed of plaster (2 cm), clay brick (15 cm), and cement (3 cm). The effect of PCM position inside the wall on the heat transfer was assessed in two scenarios, namely: close to the interior and close to exterior. The nonlinear governing equations were solved using the finite volume method. The results show that the performance of PCM-based wall is strongly influenced by the thermal conductivity, phase-change enthalpy and melting temperature of PCM. A PCM can more efficiently reduce the heat transfer to the interior space in case it has a lower thermal conductivity, has a higher latent heat of phase-change, and its phase-change temperature is closer to the room temperature. Moreover, the thermal conductivity has priority over other PCM thermophysical properties. The lower PCM thermal conductivity leads to transfer the lower amount of heat to the interior space. A two-fold increase in the thickness of the PCM leads to less than a twofold reduction in the heat transfer. Among the studied PCMs, the heat transfer reduction by Enerciel 22 was within the range 15.6–47.6%, while this range was 2–7.8% for CaCl2.6H2O.

Investigation of thermal properties and enhanced energy storage/release performance of silica fume/myristic acid composite doped with carbon nanotubes

Myristic acid (MA) with a melting temperature around 54 °C has huge prospects for solar passive thermal energy storage applications. However, like all other fatty acids (FA), it is also suffers from issues like leakage and low thermal conductivity (TC), when considered for latent heat thermal energy storage (LHTES) applications. Here, to overcome the seepage problem, MA was incorporated within the silica fume (SF) using simple direct impregnation method. To deal with the TC issue, prepared SF/MA pre-composite was doped with carbon nanotubes (CNTs) at three different weight percentage (0.3, 0.5 and 1.0 wt%) and eventually enhanced its TC by about 9.4%, 21.9% and 43.8%, respectively. The chemical and morphological structures of the SF/MA/CNTs developed as novel shape-stabilized composite PCMs (SS-CPCMs) were evaluated by FTIR, XRD, SEM and DSC analysis. The heat storage capacities of the doped and undoped SS-CPCMs were found in the range of about 87–91 J/g with good thermal stability up to 190 °C. Long standing-thermal cycling study was performed with the selected SS-CPCM doped with CNTs (1.0 wt%). No visible degradation or chemical changes were observed even after 1000 thermal cycles. CNTs doping significantly improved the heating and cooling performance of SF/MA composite.

Experimental investigation of latent heat thermal energy storage using PCMs with different melting temperatures for building retrofit

Recent architectural trends in high-rise residential buildings are favoring more transparent facades with fully glazed exteriors. While aesthetically pleasing, high window to wall ratios (WWR) entail significant discomfort due to higher solar heat gains and heat losses. In this architectural context, passive latent heat thermal energy storage (LHTES) is a possible solution to regulate indoor thermal comfort by limiting the room temperature swings. This study investigates the effectiveness of adopting Phase Change Materials (PCMs) in retrofitting high-rise apartments with 80% WWR. A composite PCM system, comprised of two PCM products with melting temperatures of 21.7 °C and 25 °C is introduced to provide thermal energy storage across the year. The phase change cycle of the PCMs and their impact on indoor air and interior building surface temperatures are assessed. For this, two test cells were constructed, one acting as a baseline and another enhanced with the composite PCM system positioned on the walls and the ceiling. The results indicate improved performance of the test cell containing the composite PCM system in lowering peak indoor and surface temperatures up to 6 °C. The melting and solidification patterns of the PCMs showed a close correlation with peak solar heat gain and heat loss periods respectively. The benefits of the PCMs were more pronounced in stabilizing indoor cell temperatures in extreme conditions with sharp temperature variations. Aspects such as the effectiveness of night natural ventilation and shading design optimization were also assessed.