Thermal Storage Devices Research Articles

Numerical analysis and improvement of the thermal performance in a latent heat thermal energy storage device with spiderweb-like fins

• A bionic spiderweb-like fin is designed and optimized for latent heat storage. • A solidification model with consideration of free convection is developed. • Free convection is conducive to the overall energy discharging performance. • Spiderweb-like fin with a bifurcation number of 8 maximizes thermal performance. • Solidification time reduces by 47.9% and discharging rate increases by 1.44 times. Solidification enhancement is an essential topic for sustainable energy development. An innovative latent heat thermal energy storage (LHTES) device employing spiderweb-like fins is designed to improve solidification efficiency. The numerical solidification process of the current LHTES device is performed and compared to that with plate fins occupying an identical fin volume. Moreover, the solidification strengthening mechanism is revealed as well as the role of free convection in the heat release performance. Finally, the configuration of the spiderweb-like fin is improved, and the solidification efficiency of LHTES devices is evaluated. The results imply that the LHTES device with spiderweb-like fins improves the solidification performance by eliminating the heat transfer hysteresis zone. The dominant heat transfer mechanism of the solidification process shifts from convective heat transfer in the early stages to heat conduction in the later stages. More significantly, the free convection strengthens the solidification performance of the LHTES device with plate fins but has little effect on that with spiderweb-like fins. A spiderweb-like fin with the bifurcation number of 8 is suggested for maximizing the discharging performance. Compared to the plate fin, the full solidification time of the improved LHTES device with spiderweb-like fins is shortened by 47.9%, and its corresponding time-averaged heat release rate increases by 1.44 times.

Effect of cycle-induced crack formation on the hydration behaviour of K2CO3 particles: Experiments and modelling

Thermochemical energy storage using salt hydrates is a promising concept to bridge the gap between supply and demand for solar thermal energy in residential buildings. Using a suitable thermochemical material such as a salt hydrate, a thermal energy storage device, also known as a heat battery, can be created to supply low-temperature thermal energy during colder periods. To generate adequate power from a heat battery for the production of domestic hot tap water or space heating, the hydration rate of the salt hydrate needs to be sufficiently fast. It is hypothesized that the hydration rate of the material increases over multiple charge and discharge cycles due to crack formation and volume increase of the salt hydrate particles. This hypothesis is tested by performing two kinds of experiments: optical microscopy experiments using a micro-climate chamber to evaluate the particle size, and Thermo Gravimetric Analysis (TGA) experiments to determine the hydration rate of the particles. The hydration rate and particle size are input for a nucleation and growth model that takes into account crack formation and particle growth. Optical microscopy experiments show a particle expansion of approximately 30 % over 12 cycles. Typical hydration rates are increased by a factor 15 comparing the first and the 12th TGA cycle. It is shown that particle growth and crack formation significantly contribute to the improvement of the hydration rate. Finally, taking into account crack formation and particle growth in the numerical model results in a good agreement between model and experiments. Such a numerical model can be used for heat battery design.

Synthesis of nanoscale lambda-Ti3O5 via a PEG assisted sol-gel method

λ-Ti3O5 with unique phase transition properties is prosperous functional materials for application in data storage, thermal storage and optoelectronic devices. However, the synthesis of λ-Ti3O5 nanocrystal has long been a great challenge. In this work, the λ-Ti3O5 nanocrystals were obtained by a PEG assisted sol-gel method, in which Polyethylene glycol 600 (PEG-600) was used as a reducing agent. Subsequently, the as-prepared λ-Ti3O5 nanocrystals were characterized by X-ray diffraction, scanning electron microscope, transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectrum. We also investigated the thermal-induced phase transition characteristics of λ-Ti3O5 by in-situ temperature-dependent XRD. This work provides a facile method to fabricate λ-Ti3O5 and would be significant for the further investigation and application of λ-Ti3O5.

Numerical and experimental investigations of latent thermal energy storage device based on a flat micro-heat pipe array–metal foam composite structure

Latent heat thermal energy storage (LHTES) is crucial in the application of renewable energy and waste heat recovery. A novel LHTES device with a flat micro-heat pipe array (FMHPA)–metal foam composite structure is designed in this study to obtain excellent heat transfer performance. An evaluation standard called integrated power is proposed to assess and compare the structural advantages of the LHTES device with others. Performances of FMHPA, temperature distribution, effort of inlet temperature and velocity of heat transfer fluid (HTF) are also studied in the experiments. A three-dimensional numerical model is developed to investigate the effort of porosity and pore density of metal foam on the charging process. Results show that the FMHPA–copper foam composite structure improves the performance of the LHTES device. This structure exhibits stronger heat transfer performance than that of other devices. The increasing inlet temperature of HTF has a better promotion effect on power than raising HTF velocity. High porosity is conducive to natural convection but detrimental to heat conduction. High pore density is disadvantageous to natural convection and does not affect heat conduction. • Strengthen of flat micro-heat pipe arrays and metal foam are studied. • The temperature distribution of PCM are experimentally studied. • The effect of temperature and velocity of HTF are experimentally studied. • The porosity and pore density of copper foam are numerically studied. • An evaluation standard is proposed to evaluate the structural advantages.

Assessment and optimisation of energy consumption in building communities using an innovative co-simulation tool

Energy efficiency in building sector is attracting an increasing interest in the scientific community, due to its strong impact in terms of greenhouse gas emissions. In this context, the REMOURBAN H2020 project has carried out a pilot deep refurbishing work on a small cluster of 10 homes, implementing energy saving measures and a hybrid energy-supply system to satisfy the heating and domestic hot water demand. The system aims to achieve near-zero-energy homes level of performance at reasonable cost by offsetting the energy consumption with local energy microgeneration. It is designed as a local low temperature district heating system and includes ground source heat pumps, photovoltaic panels, electric and thermal energy storage devices. The management of the complex hybrid system requires a suitable control strategy to optimise the energy consumption and consequently running cost. With this purpose a co-simulation tool has been developed, coupling a model of the energy system built using Dymola-Modelica and the EnergyPlus model of the buildings. This allows to develop different control strategies aiming to reduce the energy consumption from the grid, maximize the self-consumption of photovoltaic energy and ultimately move away from fossil fuel to sustainable energy resources.

Form-stable phase change materials from mesoporous balsa after selective removal of lignin

We produce balsa-based structures by selective removal of lignin. The changes that occur in the main components of balsa upon delignification, including tracheids, closed pits and tylosis vessels, allow the development of mesopores and a substantial increase in fluid permeability. Such system is ideally suited as a support of phase change materials, PCM. Vacuum-assisted impregnation with polyethylene glycol (PEG, a PCM), results in a form-stable PCM system (FPCM). The FPCM displays a high encapsulating capacity (83.5%) at temperatures above the melting PEG transition, with a latent heat of 134 J/g and low supercooling (12 °C). The results are rationalized by the affinity between the unidirectional mesoporous structure and the PCM polymer, involving capillary forces and hydrogen bonding. The leakage-proof FPCM outperforms available systems (based on PEG or other PCMs) supported on minerals or other wood species. Compared to the latter group, the results obtained with balsa relate to its morphology and the effect of residual hemicelluloses in hierarchically-aligned cellulose nano- and microfibrils. The FPCMs resist compressive loads and performs stably for at least 200 cycles of heating and cooling. An insignificant loss in latent heat is observed compared to that of pure PEG. The phase transition temperature fluctuation and non-leaking characteristics under load make the balsa-based FPCM a superior alternative for passive heating/cooling, especially for uses at high ambient temperatures. The reversible thermoregulatory capacity, low cost, high efficiency, renewability, and operability of the balsa-supported FPCM, indicate an excellent option for thermal energy storage and conversion devices.

Optimal operation modeling and synergetic dispatch of source-load-storage for high renewable energy penetration

ABSTRACT With the increase in clean energy penetration, its power curtailment during operation is made increasingly evident. In this circumstance, it is imperative to address the conflict between inflexible combined heat and power (CHP) units and variable wind and solar power. With regard to the district energy system with high penetration renewable energy, a new source-load-storage synergetic optimization operation dispatch mode is explored in this paper. It demonstrates such characteristics as the full consumption of clean energy, the CHP units with lower power output and better capability of deep peak regulation, and the small installed capacity of electric energy storage (EES) devices. Besides, this model is capable to balance heat and power demands at any time throughout the day with a variety of different energy sources, including CHP, wind, photovoltaic (PV), electrical boilers (EBs), EES, and thermal energy storage (TES) devices. Moreover, with the competitive swarm optimization (CSO) algorithm applied to solve this model, the convergence performance is improved and the computation time is reduced compared to other heuristic algorithms. Relative to the other three simulation results, the single cost of simulation result can be reduced by as much as 29.3%-34.0% at most by applying the model proposed in this paper. Moreover, this model is verified as feasible.

Optimal rule-based control for the management of thermal energy storage in a Canadian solar district heating system

Solar district heating systems are a promising solution to facilitate the adoption of large-scale, solar energy-based technologies. Thermal energy storage devices could also play an instrumental role to manage the mismatch between solar energy resources and community heating loads; however, controlling such systems is no easy task. In this paper, a novel heuristic reactive control strategy is developed to properly manage the thermal energy storage capacity, at both the short-term and the long-term, in a Canadian solar district heating system (the Drake Landing Solar Community). This strategy targets the optimization of: (a) the temperature difference across solar collectors and (b) the flow rate modulation of the variable speed pump linking the short-term and the seasonal thermal energy storage devices. A simulation is used to compare this new control approach with the conventional control strategy currently in place. Results are assessed for the year 2017–18 in terms of energy, cost and greenhouse gas emissions. Despite a 10% increase in natural gas consumption, electricity use was decreased by 43%; reductions of 34% and 29% were respectively achieved in terms of energy cost and GHG emissions.

Investigation on charging enhancement of a latent thermal energy storage device with uneven tree-like fins

The charging intensification of latent thermal energy storage (LTES) devices has an important meaning for solar energy applications. For a more uniform temperature and faster melting rate of LTES devices, uneven tree-like fins are applied and optimized here. Numerical research of melting behaviors in tree-like finned LTES devices is performed with a particular focus on fin layouts. The melting interface transformation and transient temperature distribution in four LTES devices are examined. Moreover, the roles of fill angle and central-angle gradient in melting performance are studied for the optimization of uneven tree-like fins. The results indicate that uneven tree-like fins considerably improve energy charging performance since the heat conduction enhancement exceeds the free convection suppression. Compared with uniform tree-like fins, the uneven tree-like fins lead to a more uniform temperature distribution and consistently faster melting rate by the synergetic improvement of free convection and heat conduction in LTES devices. More significantly, the optimal fill angle and the central-angle gradient are suggested to be 300° and 8° for maximizing the melting performance. Correspondingly, the complete melting time of the optimal LTES devices with heterogeneous and gradient tree-like fins respectively decrease by 49% and 46% compared to LTES devices with uniform tree-like fins.

Virtual testbed for model predictive control development in district cooling systems

Recently, with increasing cooling demands, district cooling has assumed an important role as it is more efficient than stand-alone cooling systems. District cooling reduces the environmental impact and promotes the use of renewable sources. Earlier studies to optimise the production plants of district cooling systems were focused primarily on plants with compressor chillers and thermal energy storage devices. Although absorption chillers are crucial for integrating renewable sources into these systems, very few studies have considered them from the cooling perspective. In this regard, this paper presents the progress and results of the implementation of a virtual testbed based on a digital twin of a district cooling production plant with both compressor and absorption chillers. The aim of this study, carried out within the framework of INDIGO, a European Union-funded project, was (i) to develop a reliable model that can be used in a model predictive controller and (ii) to simulate the plant using this controller. The production plant components, which included absorption and compressor chillers, as well as cooling towers, were built using the equation-based Modelica programming language, and were calibrated using information from the manufacturer, together with real operation data. The remainder of the plant was modelled in Python. To integrate the Modelica models into the Python environment, a combination of machine learning techniques and state-space representation models was used. With these techniques, models with a high computational speed were obtained, which were suitable for real-time applications. These models were then used to build a model predictive control for the production plant to minimise the primary energy usage. The improvements in the control and the resultant energy savings achieved were compared with a baseline case working on a standard cascade control. Energy savings up to 50% were obtained in the simulation-based experiments.

Discharging process and performance of a portable cold thermal energy storage panel driven by embedded heat pipes

By recent interest in overcoming heat transfer limitations of phase change materials (PCMs), latent thermal energy storage devices that assisted by embedded heat pipes (HPs) have received much attentions. Due to coupling with HPs, phase change process of PCMs can be enhanced and be better managed. In this study, a cold thermal energy storage panel for portable applications that embedded HPs into PCM is investigated experimentally. An effective way to control the cold energy supply of the panel is to control air temperature and air velocity. However, air properties varied with temperature as well as layout of HPs in the airside have influence on energy supply, which is implicit to the control of the panel. Therefore, the average effectiveness-NTU method is utilized to indicate the connection between air properties to the cold energy supply performance. The model validation results show that the effectiveness-NTU technique can describe the variation of refrigeration capacity with inlet air temperature and air velocity. The calculation error of the model is below ±9.0%. This method can be used as a design tool and is expected to provide a framework for designing portable cold thermal energy storage device with manageable cold energy supply.

Study of Heat Transfer in Energy Storage System Using Phase Change Material

The usage of phase change materials to store heat in the form of latent heat is increased, because large quantity of thermal energy is stored in smaller volumes. A cylindrical thermal storage device is composed of an inner aluminum pipe having inner/outer diameter of 20/30 mm and length of 1000 mm. The pipe passes through a cylindrical storage device have a radius of 300 mm, fill with paraffin wax. In the present work, a numerical study of the effects of convection, number of fins and heating surface temperature on the storage characteristics of annular latent heat energy storage system (LHESS) using the CFD software FLUENT6.3.26. The results show that the effect of convection cannot be neglected, but it decreases when the space between fins decreases which occurs when numbers of fins increase. The heat transfer increases as the number fins increases and found that after 12 hours the rate of energy stored increased to 116.5% and 89.6% and 89.6% for 3 fins, 8 fins and 13 fins respectively. The temperature of heating surface is not required only to be greater than the melting point of PCM (paraffin wax) When increased to (T= 375K, 400K) found that it reduces the storage time energy to three and four hours while it is six and a half hours in the case of heating surface temperature of (T= 350 K) and increases the stored energy in the PCM to (114.4%)

Perspective of concentrating solar power

In this perspective paper, the present status and development tendency of concentrating solar power (CSP) are analyzed from two aspects: (1) Potential pathways to efficient CSP through improving operation temperature to above 700 °C; (2) Technologies for efficient solar collection, thermal storage, and power generation at >700 °C. Based on the analyses, barriers on the way to the high-temperature CSP are summarized. They are: (1) the lack of methodology for heliostat design and field layout optimization, (2) significant performance degradations of solar-thermal conversion, heat storage and transfer in receiver and thermal energy storage due to high temperature, (3) the lack of suitable supercritical CO2(S–CO2) Brayton cycle for CSP and mature design methods for S–CO2 components. To overcome these issues, perspectives on following three aspects are proposed. Firstly, optimization approaches for optimal heliostat size and layout, and game-changing techniques for heliostat structure design should be brainstormed. Secondly, receivers and thermal storage devices designed through efficiency-improving approaches and fabricated by durable materials should be developed to maintain efficient and reliable operation. Thirdly, the developments of novel S–CO2 cycle and corresponding key components are eagerly desired to achieve efficient thermal-electric conversion. Perspectives from this paper would present possible approaches to efficient CSP.

High power density thermal energy storage using additively manufactured heat exchangers and phase change material

Thermal energy storage using phase change materials (PCMs) is an effective way to store thermal energy. PCMs store thermal energy in the form of latent heat, a promising thermal management methodology for intermittent heat loads. Because the thermal conductivity of many PCMs is relatively low (~0.1 W/(m⋅K)), high-power thermal storage is possible only when the PCM is integrated with a high thermal conductivity matrix. Enabled by recent advances in metal additive manufacturing (AM), we develop an ultra-compact high-power PCM heat exchanger and demonstrate its performance. The thermal storage device absorbs heat from, or rejects heat to, a flowing liquid coolant. Numerical simulations of heat transfer and phase change within the PCM were used to predict the device performance and evaluate potential designs. AM enabled three-dimensional (3D) metal structures were designed to serve as a matrix that conducts heat into the PCM or as extended surfaces that enhance convection to the liquid coolant. The 3D metal structures reduce conduction thermal resistance by 17X and convection thermal resistance by 3X compared to conventional designs. We fabricated three devices made of an aluminum silicon alloy (AlSi10Mg) and tested these devices with paraffin (CnH2n+2) PCM. Measurements validated the simulations and showed a 4X improvement in power density (0.58 W/cm3) compared to conventional designs. This work demonstrates AM as a powerful technique for the development of PCM-based thermal storage systems and suggests design methods that aid the development of heat exchangers.

Development of reduced order thermal dynamic models for building load flexibility of an electrically-heated high temperature thermal storage device

A control-oriented model of an Electrically heated Thermal Energy storage device (ETS) is presented. The ETS consists of bricks heated up to 871 °C with electric coils; heat is discharged with an airflow passing through channels in the bricks. The device stores energy during off-peak periods to meet future on-peak building heating loads. While its rationale is understood, improvement regarding operation strategies is needed. The charging/discharging could be adjusted according to predicted heating loads to reduce bills, and enhance building energy flexibility in its interaction with the grid. The model, to facilitate implementation of model predictive control (MPC), is based on the following features: (a) calibration of grey-box resistance-capacitance models with a heat exchanger model; (b) periodic state updates from sensors; (c) “effective” brick conductivity. A 1-capacitance model with state updates and “effective” brick conductivity was comparable to a detailed 140-capacitance model. Therefore, a simple model with modifications gives adequate predictions and thus can aid with MPC. Finally, strategies are evaluated to study energy flexibility potential. Using the ETS during the morning critical demand hours and anticipating the rise in setpoint temperature from nighttime setback, peak reduction 11% is obtained. A reduction of 73% is found when HVAC heating coil is limited.

Experimental Work on Cold Starting Emission of CI Engine using Engine Waste Heat Energy

In the present day, emission by internal combustion engines causes several problems like acid rain, depletion of ozone layer, and global warming for which it has become a prior concern. The hazards further increases during cold weather conditions. In this paper, emission analysis has been carried out using single cylinder, 4 stroke, direct injection and water cooled variable compression ratio diesel engine. A thermal energy storage device (TESD) containing phase changing material (PCM) has been designed and tested for storing the waste energy of cooling water from engine and reutilizing it for pre-heating. The working principle of TESD is based upon the principle of absorbing and rejecting heat during phase change of PCM material. The test condition is 15° C and 1 atm pressure at which, the experiments are carried out using TESD. A significant reduction in CO (23.72%), HC (2.03%) and smoke opacity (6.05%) after 900secs and an increase in engine temperature upto 61ºC after 840secs of engine running is observed.

Performance enhancement of a phase-change-material based thermal energy storage device for air-conditioning applications

This work concerns performance enhancement of phase change material (PCM) based thermal energy storage (TES) devices for air-conditioning applications. Such devices have numerous potential applications in the building environment. The TES device often uses air as the heat transfer fluid and, as a result, its performance is often limited by heat transfer in either or both of the PCM and the air sides. This paper aims to overcome the heat transfer limitations through intensifying heat transfer using two methods of extending heat transfer surfaces (fins) in both the PCM and air sides and adding heat transfer enhancement materials in the PCM. First, a TES device with different configurations (no fins; offset strip fins on the air side only; straight fins on the PCM side only; offset strip fins on air side and straight fins on PCM side) and PCM with different thermal conductivities were modelled and compared. The comparison leads to an advantageous utilization of fins instead of adding thermal conductive particles. The results also indicated a significant extent of performance enhancement of the TES device due to the use of fins with the charging and discharging times reduced respectively by ~85% and ~74%. The airside fins were found to be more effective than the PCM side fins. The results also showed that the heat transfer enhancement due to the PCM side fins could be achieved by increasing PCM thermal conductivity. The modelling results were then validated by experimental data. Finally, a PCM-based TES device was designed and manufactured based on the above results. The device was integrated into an air-conditioning system and experimentally tested. The results showed that, for both charge and discharge processes, the stabilization of the outlet temperature of the air conditioning system was significantly improved, leading to an increased extent of thermal comfort and decreased outlet temperature fluctuations. The obtained results can be used as the design guideline of the compact TES device for air conditioning.

Development of a heat transfer coefficient based design method of a thermal energy storage device for transport air-conditioning applications

We studied the heat transfer characteristics of a phase change material (PCM) based thermal energy storage (TES) device for transport air conditioning applications. The charging and discharging times and the transient heat flux of the TES device were measured. These data allow quantification of the transient and overall heat transfer coefficients, thus a quantitative comparison between the overall heat transfer coefficients of the charging and the discharging processes. An attempt was made to process the overall heat transfer coefficient data to give the Nusselt number, the Reynolds number, and the Prandtl numbers for both the discharging and charging processes. An empirical relationship between these non-dimensionless numbers was then obtained, which was validated by further experiments. It was shown that the empirical expressions agree with the further experimental data within 2.33% and 1.65% respectively for the charge and discharge processes. To our best knowledge, this study represents the first effort to measure the transient behavior of a PCM based TES device, leading to a set of experimentally validated empirical expressions for the overall heat transfer coefficients.

Methanol production using hydrogen from concentrated solar energy

Concentrated solar thermal technology is considered a very promising renewable energy technology due to its capability of producing heat and electricity and of its straightforward coupling to thermal storage devices. Conventionally, this approach is mostly used for power generation. When coupled with the right conversion process, it can be also used to produce methanol. Indeed methanol is a good alternative fuel for high compression ratio engines. Its high burning velocity and the large expansion occurring during combustion leads to higher efficiency compared to operation with conventional fuels. This study is focused on the system level modeling of methanol production using hydrogen and carbon monoxide produced with cerium oxide solar thermochemical cycle which is expected to be CO2 free. A techno-economic assessment of the overall process is done for the first time. The thermochemical redox cycle is operated in a solar receiver-reactor with concentrated solar heat to produce hydrogen and carbon monoxide as the main constituents of synthesis gas. Afterwards, the synthesis gas is turned into methanol whereas the methanol production process is CO2 free. The production pathway was modeled and simulations were carried out using process simulation software for MW-scale methanol production plant. The methanol production from synthesis gas utilizes plug-flow reactor. Optimum parameters of reactors are calculated. The solar methanol production plant is designed for the location Almeria, Spain. To assess the plant, economic analysis has been carried out. The results of the simulation show that it is possible to produce 27.81 million liter methanol with a 350 MWth solar tower plant. It is found out that to operate this plant at base case scenario, 880685 m2 of mirror's facets are needed with a solar tower height of 220 m. In this scenario a production cost of 1.14 €/l Methanol is predicted.

Two-stage scheduling for island CPHH IES considering plateau climate

In this paper, an island integrated energy system (IES) combining power, heat and hydrogen (CPHH) is built, and the optimal power-heat-hydrogen scheduling is studied under actual plateau meteorological data. Considering the comprehensive demands of residents, an IES including renewable energy sources (RESs), fuel cell, electrolyzer, electric boiler, thermal energy storage device and new energy vehicles is established and operated. In the first stage of operation, according to the prediction of RESs and demands, the day-ahead optimization is carried out which aims to minimize the operation cost, while the electric vehicles (EVs) are considered as special demand response (DR) loads. As for the second stage of operation, the ultra-short-term prediction is implemented to provide prediction data for model predictive control (MPC), realizes the real-time operation. As CPHH system, fuel cell and electrolyzer work to meet various needs of the plateau residential area with high efficiency. In addition, the method and IES structure proposed in this paper is compared with other options.