Active Thermal Energy Storage Research Articles

Flow stability and distribution in horizontal serpentine tube bundles with vertical headers

Particle-based systems have immense potential for combining thermal energy storage (TES) with renewable energy sources. The so-called sandTES system, which is an active TES system, utilizes sand or other small particles as a storage material and consists of a hot tank, a cold tank, and a reversible fluidized bed heat exchanger. In the preferred design, the tubes are arranged in horizontal serpentine tube bundles; thus, the headers are positioned vertically, for one phase subcritical, two-phase and supercritical water/steam conditions. So far, no design principles have been published for macro-sized vertical headers and horizontal tube bundles. This paper investigates the unbalance in flow distribution and other flow instabilities of such an unusual heat exchanger and its scale-up using APROS simulations.The steady-state simulations of the tallest investigated heat exchanger show a strong unbalance in the mass flows between individual heat exchanger tubes: the smallest mass flow was only 55% of the largest one when generating steam. Smaller heat exchangers experience less pronounced unbalance factors in the order of 88%. When steam is condensed, the unbalance is insignificant. Dynamic simulations of start-up and shutdown revealed instabilities and that the flow maldistributions even lead to flow reversals in most investigated cases although neither last for stationary operation. Part-load cases show higher unbalances than their respective full-load case by 9 to 17 percentage points. Header design modifications – secondary headers with submodules and orifices – improve the unbalance factor from 55% to 88%.

Fabrication and evaluation of polyurethane supported form‐stable PCMs

AbstractThe current research is oriented towards the development and assessment of the properties of the form‐stable phase change materials (PCMs), comprising of the thermoset polymer matrix (polyurethane, PU) enclosing capric acid (CA) as an active thermal energy storage (TES) component. The Fourier Transform Infrared Spectroscopy (FTIR) and Field‐Emission Scanning Electron Microscopy (FE‐SEM) have been used to characterize the chemical nature and morphological details of the developed PU‐based PCMs (PUPCMs). TES attributes and thermal stability of the resultant PCMs are evaluated by thermo‐analytical techniques such as Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA). A thermal reliability test has been conducted to determine the phase change efficiency of the prepared PCMs. PUPCMs have been found to be suitable for TES applications, such as passive space heating or cooling building applications.

Metal hydride reactor for output temperature control

Precise temperature control of fuel cell stacks is crucial to avoid degradation mechanisms and thus to extend lifetime. The present study demonstrates that metal hydride based modules can assist in controlling this temperature by serving as active thermal energy storage. The reversible reaction of metals with hydrogen is characterized by a temperature pressure correlation. Therefore, the temperature as well as the thermal power at which the thermal energy is released can be defined by the gas pressure. This enables the design of temperature controllers using the pressure as actuating variable on the metal hydride device. In this study, an existing metal hydride reactor based on LaNi4.85Al0.15 is integrated into a hydrogen and cooling fluid testing infrastructure. Based on experiments indicating the step response behaviour, the parameters for a 0D model have been identified. A PI controller was designed based on simulations and implemented on the test rig hardware. First experimental results show that it is possible to stabilize the temperature of the cooling fluid circuit to 70°C ± 1.5 K while a thermal power of +755 W/kgMH and 140 W/kgMH is provided for absorption and desorption, respectively.

Effect on the Thermal Properties of Building Mortars with Microencapsulated Phase Change Materials for Radiant Floors

The use of slag silicate cement mortar as a thermal mass layer for radiant floor heating systems holds significant potential for active thermal energy storage systems in buildings. The main objective of this article is to experimentally test the thermal performance of slag silicate cement mortar thermal storage blocks after the addition of phase change materials. The present study focuses on investigating the thermal performance of thermal storage blocks made of slag silicate cement mortar that incorporates a microencapsulated phase change material (mPCM). The mPCM consists of particles of paraffin-coated resin, which are uniformly distributed in the mortar. The analysis revealed that the introduction of mPCM particles into the mortar decreases the bulk density by approximately 9.4% for every 5% increase in mPCM particles ranging from 0% to 20%. The results obtained utilizing the Hot Disk characterization method demonstrate that the mPCM particles significantly affect the thermal properties of the mortar. Particularly, the thermal conductivity and thermal diffusion coefficient of the SSC30 mortar with a 17.31 wt.% mass of mPCM particles decreased by 59% and 69%, respectively. The results of this study provide a basis for the application of RFHS end-use thermal storage layers.

Reinforcement learning-based demand response strategy for thermal energy storage air-conditioning system considering room temperature and humidity setpoints

Heating, ventilation, and air-conditioning systems significantly influence demand response (DR) actions. In the literature, many studies have investigated the reduction of sensible loads through DR events by increasing temperature setpoints while satisfying indoor comfort levels. However, less consideration has been given to latent loads. Therefore, proximal policy optimization reinforcement learning (RL) algorithms are adopted to simultaneously control indoor temperature and humidity (T&H) setpoints to reduce sensible and latent loads. An EnergyPlus-Python joint simulation platform was created for the temperature-humidity independent control system. DR strategies based on RL, active thermal energy storage, and time-of-use electricity prices are formulated to find the optimal indoor T&H setpoints, considering environmental constraints, comfort levels, and energy consumption. The results showed that in the optimal experimental strategies implemented based on negative feedback regulation, savings of 29.15 % on electricity consumption were achieved during DR periods and 46.11 % savings on the daily operating costs. Moreover, a simulation strategy based on feed-forward regulation was developed and found to reduce electricity consumption by 6.92 % during DR periods. Compared with a fixed T&H setpoint, the developed strategy meets the preferences of different groups of people for thermal comfort and increases the system's flexibility.

Experimental and numerical study of epoxy resin-based composite phase change material in packed-bed thermal energy storage system for ventilation

Air heating of ventilation system occupies large energy consumption of buildings. Using phase change material (PCM) in active thermal energy storage system (TES) has practical significant to avoid unstable input and improve energy capacity. This work proposed a packed-bed TES system with solar collector for ventilation, by employing the novel epoxy resin-based composite PCM without capsule shell as fillers. In experiment, two types composites were first prepared and compared: the epoxy resin-based composite with Al additives has loading capacity up to 65% and a latent heat of 116.7 J kg−1, which shows better thermal performance than that of the Al-based composite (maximum 25%, 46.92 J kg−1). Then, a numerical model of TES system was built, validated, and then used to simulate the outlet temperature variation of air. Simulation results shows the designed TES system is beneficial to stabilize output temperature. Therefore, when the energy storage process is from 7:30 am to 17:30 pm, the utilization efficiency for composite filler system of 51.1 % is higher than that of pure PCM filler system of 49.7 %. At last, determining the phase change temperature of the composite at around 25 °C leads to an efficiency improvement around 22 %.

Generalizable Thermal Performance of Ventilated Block Walls and Energy Implication of Substitution for Wood-Frame Walls in Cold-Climate Buildings

Space heating and cooling of buildings is a major contributor to the ascending trend of global energy consumption and greenhouse gas (GHG) emissions. A potential solution to reduce the space heating and cooling is to use buildings’ mass for active thermal energy storage (TES). Having air circulation between an active TES and its associated zones can significantly enhance their thermal coupling; however, reported research studies have not focused on this kind of active TES. To that end, this study aimed to evaluate the thermal performance of a ventilated block wall (VBW) in reducing space heating and cooling loads in cold-climate buildings. In this system, air is circulated between a zone and the voided cores of a VBW, where the air exchanges heat with the wall before returning to the zone. To have a generalizable assessment of the system’s performance, typical-day and annual energy analyses were conducted under various boundary conditions and air circulation speeds. The study found that for a typical day with significant temperature fluctuation, a VBW with a 2 m/s air circulation speed throughout the day can lead to 67% more net energy exchange (the sum of thermal energy storage and release) when compared to having no air circulation. The annual analysis compared the energy performance between a VBW and a traditional wood-frame wall in three different cold climates. The results showed that substituting a wood-frame wall with a VBW can reduce space heating and cooling loads by 35.1 kWh/m2 (wall surface area) for a mixed dry–cold climate throughout the year. Having cement plaster as interior finishing can lead to 9% more net energy exchange than having drywall, on average, for all zone air temperature profiles.

Analysis of performance and suitable users of CCHP systems with active thermal energy storage

Introducing a thermal energy storage (TES) unit in the combined cooling, heating, and power (CCHP) system can help alleviate the supply–demand energy mismatch and improve the system performance. However, the active regulation function of TES has not been fully exerted and the applicable user scenarios of the CCHP-TES system are not clear due to the limitations of case studies. In this work, based on the supply–demand energy matching principle, an improved design of active thermal energy storage is proposed to further develop the role of TES units. Meanwhile, to overcome the limitations of case studies and obtain the universal applicable user scenarios, dimensionless user load characteristic parameters are used to analyse the correlation between the user characteristics and the system performances. The system thermodynamic model and integrated design methodology are developed. The results indicate that the average energy-saving rates of the proposed active CCHP-TES system are higher than 14.63%, and the maximum differences compared with those of the conventional CCHP system and passive CCHP-TES system are 22.27 and 29.20 percentage points, respectively. Moreover, among the user load characteristic parameters, the cooling/heating-to-electricity ratio has the greatest impact on the performances of the proposed system, the duration time of the peak electricity load the next, and the peak-to-valley ratio of electricity load the least. In most cases, the larger the cooling/heating-to-electricity ratio and the duration time of the peak electricity load are and the smaller the peak-to-valley ratio of electricity load is, the better the system performance will be. Finally, the applicable user scenarios of the proposed system are given quantitatively and the system is applicable once the peak-to-valley ratio of electricity load is less than 2 and the cooling/heating-to-electricity ratio is greater than 0.5.

A demand response method for an active thermal energy storage air-conditioning system using improved transactive control: On-site experiments

Transactive control (TC) and active thermal energy storage (ATES) strategies can effectively achieve a supply–demand balance across energy sources in the power grid. However, past research mainly focused on one of these demand response (DR) strategies, and integrated DR strategies that combine TC and ATES are unavailable. Therefore, to fully utilize both TC and ATES strategies in the DR of air-conditioning systems and thus enhance power grid stability, a comprehensive DR strategy that combines an improved version of TC with ATES (ITC + ATES) was developed for energy storage air-conditioning systems. The ITC + ATES strategy utilizes the automatic feedback regulation effect of the real-time electricity price on the grid side to the air-conditioning users. Then, the system bids the cooling or heating demand on the user side in the power market, thus adjusting the dynamic temperature set-point in real time. The ATES system's automatic energy storage and release process is then regulated accordingly. Thus, this strategy not only allows the air-conditioning system to participate in the power DR, but ensures that the electricity consumption of the air-conditioning system conforms to the power market law and realizes the optimal social value of the power commodity. On-site test experiments of space cooling and heating conditions were conducted on a full-scale intelligent centralized air-conditioning experimental platform with an ATES system to test the effect of this strategy. Comparing the ITC + ATES strategy with the conventional control strategy, ATES strategy, and ITC strategy reveals that the ITC + ATES strategy is most effective in ensuring the thermal comfort of occupants, particularly during the DR period. Moreover, the maximum total electricity consumption reduction rate, the maximum total operating cost reduction rate, the maximum electricity consumption reduction rate during the DR period, and the maximum total operating cost reduction rate during the DR period of the ITC + ATES strategy were 33.20%, 49.69%, 76.14%, and 76.16%, respectively. These results suggest that the ITC + ATES strategy is an efficient and integrated DR approach.

Second law performance prediction of heat pump integrated stratified thermal energy storage system using long short-term memory neural networks

Thermal energy storages (TES) are transient state energy devices. These devices are used in renewable energy systems as a buffer for non-coincidence in heat supply and demand. TESs use thermal stratification to ensure high efficiency in heat storage and acquisition. This article is focused on predicting the performance of thermal energy storage (TES) integrated with heat pump using neural networks. In addition, exergy and entropy equations were derived for the calculation and prediction of the stratification efficiency in storage systems and of the performance factor (PF) of renewable energy systems (RES). As for data analytics, real time data-streaming edge devices were customized. The model fitting and prediction were done directly on the edge devices. The key objectives and findings are:•To demonstrate stream-data processing framework which can graphically represent the stratification decay of an active Thermal Energy Storage (TES) charge/discharge process in real time.•Derivation of a custom exergy equation for stratification efficiency and streaming it graphically in real time. The optimized key performance index (KPI) at the heat pump end i.e. coefficient of performance (COP) or performance of factor (PF) was 3.3, and at charge and discharge end, in terms of efficiency was 83 % and 84 % respectively.•A deep neuronal network applying a long short-term memory (LSTM) architecture for predicting stratification deterioration in the charge/discharge cycle with a prediction error below 5 %.

Nanofibrous films embedded with phase change material: Flexible and photo-responsive dressings for thermal compress therapy

The medical dressings with temperature regulating function can promote wound recovery and cure chronic diseases. To address the outdoor heat source of thermal compress film, Kevlar nanofibrous film (AKD) containing n-Docosane (DE) and silver nanowires (AgNWs) with active photo-thermal conversion and thermal energy storage properties was developed. The results shown that DE and AgNWs were wrapped in Kevlar nanosheets through entanglement and extensive hydrogen-bonding interaction, and AKD film presented the layered structure. Due to the formation of non-melting layer, the melting latent heat of AKD film containing 58 wt% of DE without leakage is 105.1 J·g−1. The as-prepared film also presents enviable photothermal conversion efficiency of 88.5 %, impressive tensile strength of 26.15 MPa and high flexibility even at temperatures below the melting point of DE. Moreover, the pilot-scale film with the thickness of 1.54 mm can rapidly rise to 50 °C in 402 s and maintain the temperature from 50 °C to 37 °C for about 18 min. The excellent heat storage capacity, impressive mechanical and flexible properties, rapid photo-thermal response and stable heat release performance make it promising to be used as thermal compress therapy.

Demand response with active phase change material based thermal energy storage in buildings

A large potential to shift the electricity consumption to adapt to the stochastic renewable electricity generation is identified through the utilisation of a combination of Heat Pumps (HP) and local Thermal Energy Storage (TES) devices in building heating systems. In this paper, a building heating system coupled with an active Phase Change Material (PCM) TES device and a HP is simulated to characterise its potential for Demand Response (DR) applications. A control-oriented numerical model for the PCM TES is developed and previously validated numerical models of the building, HP and hot water radiators are integrated to simulate the dynamics of the coupled building heating system. A Genetic Algorithm (GA) based control strategy is designed to optimise the building heating energy consumption and operational cost with respect to time-varying electricity price signals. The developed control strategy is successfully implemented to utilise the TES capability of the PCM and the thermal inertia of the building to intelligently shift the electrical load of the HP to low price periods, while satisfying the specified indoor comfort requirements. In comparison to a reference case utilising a sensible TES, cost savings and consumption reductions of more than 40% and 30%, respectively, are attained with the active PCM TES. Simulation results indicate that utilising an active PCM TES over a sensible TES offers significant advantages for DR applications in building heating systems in terms of load shift flexibility, energy costs and consumption.

Revolutionizing the Building Envelope: A Comprehensive Scientific Review of Innovative Technologies for Reduced Emissions

Abstract This paper reviews innovative building envelope technologies that can improve total building energy efficiency and reduce greenhouse gas emissions. The building envelope has a significant impact on energy and thermal performance, making various technologies like phase change materials, aerogel, and active and adaptive systems essential for enhancing the building envelope’s energy efficiency. Phase change materials reduce energy consumption by lowering peak heating and cooling loads, improving thermal comfort. Paraffin wax is considered the most dependable phase change material for building envelopes, as it can decrease energy consumption for heating and cooling. Study shows that active PCM thermal energy storage system can reduce room temperature by 6.8 °C in summer. Aerogel, in comparison, is recognized for its exceptional insulation capabilities and low density that minimize heat transfer and boost thermal insulation in buildings. Silica aerogel’s outstanding feature is its capacity to offer thermal performance that surpasses traditional insulation materials by 2–4 times, thereby delivering substantial energy savings of up to 35 %. Active and adaptive systems, such as smart windows and kinetic facades, enable real-time control of building envelope performance, improving energy efficiency and indoor comfort. Smart windows can lead to annual energy savings up to 35.9 kWh/m2 compared to traditional windows, and kinetic facades can reduce cooling loads for buildings up to 21 %. The review assesses various adaptive facade solutions for their suitability in diverse climate zones, versatility in application and energy efficiency. Despite the existence of some limitations and challenges, such as high costs and insufficient understanding of their long-term performance, the continuous development and deployment of these technologies can still yield a significant contribution to improving building energy efficiency and mitigating greenhouse gas emissions.

Performance evaluation of a dynamic wall integrated with active insulation and thermal energy storage systems

Thermal energy storage systems in buildings can store cooling/heating energy during non-peak load hours or when renewable energy sources are available for later use when demanded. Using the building envelope as a thermal battery can help to shave the peak load, reduce the burden in electric grids, and enhance the occupant's thermal comfort. Sensible energy storage on wall systems such as thermally activated building systems can provide an active thermal storage strategy. However, most of the stored energy is used through passive means directed by the thermal lag, which can impede the on-demand release of the stored energy. Integrating active insulation systems with building thermal storage systems can increase the flexibility of charging and discharging time and duration. In this study, a wall system equipped with an active insulation system and thermally activated storage system was designed, and its performance on active cooling energy contribution was studied. The performances of energy storage (charging), release (discharging) of the thermal energy storage energy, and the active insulation system were studied separately and together as an integrated system. Results showed that the thermal properties of the thermal energy storage core material and the pipe spacing of both embedded pipes in the thermal energy storage and hydronic pipes used in the active insulation system affected the wall performance the most out of all the tested wall system parameters. During discharging, the heat flux into the wall was as high as 81.92 W/m2. The active insulation system's dynamic R-value varied between less than 1ft2•°F•h/BTU (0.18 m2•K/W) to 98% of the equally thick foam insulation's R-value.

Effect of Copper Electrode Geometry on Electrofreezing of the Phase-Change Material CaCl2·6H2O

Abstract The development of effective active thermal energy storage systems requires an understanding of how electrode geometry affects the electrofreezing process. This study aimed to observe the nucleation behavior of an inorganic phase-change material, CaCl2·6H2O, using a DC electric field and various copper electrode geometries. The effects of both the electrode diameter ( d = 0.5 d=0.5 and 0.7 mm) and the tip shape (flat and sharp end surfaces) were investigated. Data analysis was performed to reveal the nucleation temperature, freezing temperature, supercooling degree, supercooling time, and crystallization time period. The copper electrode with the larger diameter was found to result in a higher nucleation temperature, a smaller supercooling degree, faster nucleation, and a shorter crystallization time period. Moreover, changing from a flat tip to a sharp tip decreased the nucleation temperature and increased the supercooling degree. This study showed that the electrode geometry plays an important role in the phase-change behavior of CaCl2·6H2O.

Parametric analysis and design optimisation of PCM thermal energy storage system for space cooling of buildings

The paper presents a parametric analysis and design optimisation of an active PCM thermal energy storage system for space cooling of nearly zero-energy buildings. The design for a new space cooling system proposes a TES system composed of stand-alone PCM storage units incorporated into the building interior under the ceiling slab. Active control of the thermal energy storage is realized by night cooling of a phase change material by means of cold water flowing within a capillary pipe system. The objective of the study is to examine the effect of the system’s main parameters such as PCM layer thickness, number of parallel pipes, diameter of pipes, night cooling duration, cooling water inlet temperature and velocity under Baltic summer weather conditions. Parametric analysis is performed by using computational fluid dynamics software Ansys Fluent. Optimisation of the main parameters is carried out to determine the best combination of their values ensuring the system is able to absorb the daily thermal loads within the room and operates with minimum energy consumption for the night cooling of PCM.

Performance Evaluation of an Active PCM Thermal Energy Storage System for Space Cooling in Residential Buildings

Abstract This paper presents a numerical simulation-based study that evaluates the potential of an active phase change material (PCM) incorporated thermal energy storage (TES) system for space cooling in residential buildings. In the proposed concept, TES system is composed of stand-alone PCM storage units which are installed between the concrete ceiling slab and the ceiling finishing layer. Active control of the thermal energy storage is achieved by night cooling of a phase change material by means of cold water flowing within a capillary pipe system. Effectiveness of the system under the typical summer conditions of the Baltic States is analysed by using computational fluid dynamics (CFD) software Ansys Fluent. Results showed that installation of the active TES system has a positive effect on thermal comfort, reducing the average indoor air temperature by 6.8 °C. The outcome of this investigation would be helpful in selecting the key characteristics of the system in order to achieve the optimum performance of an active TES system for space cooling of buildings in similar climates.

Comparison of Direct and Indirect Active Thermal Energy Storage Strategies for Large-Scale Solar Heating Systems

Large-scale solar heating for the building sector requires an adequate Thermal Energy Storage (TES) strategy. TES plays the role of load shifting between the energy demand and the solar irradiance and thus makes the annual production optimal. In this study, we report a simplified algorithm uniquely based on energy flux, to evaluate the role of active TES on the annual performance of a large-scale solar heating for residential thermal energy supply. The program considers different types of TES, i.e., direct and indirect, as well as their specifications in terms of capacity, storage density, charging/discharging limits, etc. Our result confirms the auto-regulation ability of indirect (latent using Phase Change Material (PCM), or Borehole thermal storage (BTES) in soil) TES which makes the annual performance comparable to that of direct (sensible with hot water) TES. The charging and discharging restrictions of the latent TES, until now considered as a weak point, could retard the achievement of fully-charged situation and prolong the charging process. With its compact volume, the indirect TES turns to be promising for large-scale solar thermal application.

Development and characterization of novel and stable silicon nanoparticles-embedded PCM-in-water emulsions for thermal energy storage

A phase change material (PCM) emulsion was prepared by dispersing the PCM in water with the aid of emulsifiers, which had higher fluidity, higher thermal conductivity, and more flexible volume change than the solid-liquid PCMs. However, a critical issue for their large-scale applications is phase instability due to aggregation and precipitation of the PCM droplets. This work was to develop stable PCM emulsions prepared with n-hexadecane by manipulating the key factors and analyzing the emulsion properties including emulsifier combinations and process conditions, interfacial film properties, droplet size distribution, and rheology characteristics. The stability was further improved with the addition of SiO2 nano-particles. The SiO2 nano-particles also acted as an effective nucleating agent to reduce the degree of supercooling. The thermal performance for potential application in thermal energy storage systems was also examined. Eventually, a novel and highly stable PCM-in-water nano-emulsions with droplets on a scale of tens of nanometers and a transparent appearance was developed for potential application in active thermal energy storage systems.

Thermal energy storage (TES) technology for active and passive cooling in buildings: A Review

Thermal energy storage (TES) system is one of the outstanding technologies available contributes for achieving sustainable energy demand. The energy storage system has been proven capable of narrowing down the energy mismatch between energy supply and demand. The thermal energy storage (TES) - buildings integration is expected to minimize the energy demand shortage and also offers for better energy management in building sector. This paper presents a state of art of the active and passive TES technologies integrated in the building sector. The integration method, advantages and disadvantages of both techniques were discussed. The TES for low energy building is inevitably needed. This study prescribes that the integration of TES system for both active and passive cooling techniques are proven to be beneficial towards a better energy management in buildings.