Thermal Storage Devices Research Articles

Numerical study on the effect of the location of the phase change material in a concentric double pipe latent heat thermal energy storage unit

Latent heat thermal energy storage (LHTS) represents an intriguing solution to the problem of variability of supply that exists in solar thermal power systems. One such storage system consists of a double pipe, where a phase change material (PCM) is enclosed in either the central pipe, or the annulus surrounding it. The heat transfer fluid fills the other void in the system. Whether the PCM is used in the central pipe or the annulus could, potentially, significantly alter the thermal performance of the system. Thus, a comparison between the PCM mounted in the annulus (case A) and the inner tube (case B) was conducted numerically, to investigate the advantages and disadvantages of each case with regard to the melting and solidification performance. A horizontal double pipe latent heat thermal energy storage device was considered. The numerical simulation solved the transient balance equations in a two-dimensional system. The enthalpy-porosity method was used to simulate the phase change and the Boussinesq approximation, which accounts for the small changes in density that drive natural convection, was applied. The effect of the initial temperature of the heat transfer surface (HTS) on the sensible and latent heat changes of the model PCM, RT-50, was tested for both the melting and solidification processes. Aiming to assess the differences in the storage performance, the average PCM temperature, the liquid fraction, and the average velocity of both the melting and solidification processes were examined. The results of the simulation showed that for both cases, convection was dominant after only a short period of the melting process. The melting time was significantly different in the two cases, i.e. it was shorter in case B than case A by almost 50%. Furthermore, an increase in the temperature of the HTS by 5 °C notably affected the melting time of both cases by as much as 20%. This effect was more pronounced in case B, which had a melting time which was 41% shorter than case A. Finally, the results revealed that the solidification process in case A was more rapid than case B with the total solidification time of case A being lower by 43.4%.

Influence of the heat capacity of the storage material on the efficiency of thermal regenerators in liquid air energy storage systems

Abstract Liquid air energy storage is an innovative technology for electricity storage, using liquefied air as storage medium. Due to the high energy density of liquid air, the storage volume is smaller than that of similar storage technologies like compressed air or pumped hydro energy storage systems. Air is liquefied by means of a modified Claude-cycle. In the discharging process, liquid air is energy-efficiently compressed (compression of a liquid), vaporized, superheated, and finally expanded from high to ambient pressure using an air expander for power generation. In between, a thermal energy storage device at cryogenic temperature level is used to improve the round-trip efficiency of the system. One possible design is a packed bed thermal energy storage device, consisting of a cylinder and a packed bed of storage material. Investigations on thermodynamic properties show that especially the temperature-dependence of the heat capacity has a major influence on the performance of the thermal energy storage system. In this paper, nine real and further hypothetical storage materials are investigated. The influence of the heat capacity at cryogenic temperature is systematically analyzed and a general formulation in terms of summarizing key figures is developed. It turns out that especially the temperature-dependence is a significant parameter in this context, which is not considered within the majority of investigations in this field.

Effect of Acidity Levels and Feed Rate on the Porosity of Aerogel Extracted from Rice Husk under Ambient Pressure.

Silica aerogels have attracted tremendous interest due to their high specific surface area and the physical, chemical, and mechanical properties as promising materials for thermal insulation, chemical sensors, and energy storage devices. However, large-scale production of silica aerogels remains a challenge due to costly alkoxide precursors and energy-intensive supercritical drying processes. This paper analyzes the effect of acidity levels and feed rate on the porosity of rice husk aerogels with high specific surface area under ambient pressure. This synthetic approach is cost-effective, eco-friendly, and facilitates recycling. Rice husk ash, which consists of 92% amorphous pure silica, was produced by combustion. A process of solvent exchange and surface modification under ambient pressure at different pH levels was conducted for synthesis of the aerogel. The specific surface area of rice husk aerogel was confirmed as ranging from 385 to 861 m2/g under pH 1 to pH 9 and acid feed rate of 0.5 to 5.0 mL/min. The optimized aerogel had a specific surface area of 861 m2/g, a pore volume of 3.33 cm3/g, and an average pore diameter of 12 nm when synthesized at pH 1 and an acid feed rate of 2.5 mL/min. The aerogel was found to be highly hydrophobic, with a water contact angle of 156° up to about 340 °C.

An integrated model predictive control approach for optimal HVAC and energy storage operation in large-scale buildings

This paper deals with the problem of cost-optimal operation of smart buildings that integrate a centralized HVAC system, photovoltaic generation and both thermal and electrical storage devices. Building participation in a Demand-Response program is also considered. The proposed solution is based on a specialized Model Predictive Control strategy to optimally manage the HVAC system and the storage devices under thermal comfort and technological constraints. The related optimization problems turn out to be computationally appealing, even for large-scale problem instances. Performance evaluation, also in the presence of uncertainties and disturbances, is carried out using a realistic simulation framework.

Investigation of a liquid air energy storage (LAES) system with different cryogenic heat storage devices

Liquid air energy storage (LAES) is a large-scale storage technology, which is using liquefied air as storage medium. Comparable to pumped hydro (PHES) and compressed air energy storage (CAES), LAES is charged with excess electricity from the grid and discharged, when the electricity demand is high. Working as a buffer for the electric grid, the availability and integrability of fluctuating renewable energy sources can be improved by LAES.In the charging process, ambient air is liquefied with an adopted Claude respectively Kapitza process. Compression heat is stored in a hot thermal energy storage device (HTES); a cold thermal energy storage device (CTES) is used as heat sink at cryogenic temperature to significantly improve the efficiency of the liquefaction. In the discharging process, liquid air is pressurized, heated up to ambient temperature by the CTES, superheated by the HTES, and expanded in an air expander for electricity generation. The CTES is used to recycle an exergy flow at cryogenic temperature from the discharging to the charging process. Since the round trip efficiency of the LAES strongly depends on this exergy flow, two different types of CTES are compared within this work. The liquid cold thermal energy storage device (LCTES) is based on a multi-tank storage system using propane and methanol, the direct cold thermal energy storage device (DCTES) is a packed bed storage system with direct contact between the fluid and the solid storage material. In this work, a comparison and an exergetic investigation of both systems is presented. The significant influence of the exergetic efficiency of the CTES and other technical aspects are worked out. Additionally, the influence of the pressure on the liquefaction and discharging process, and on the round trip efficiency is investigated.

Enhanced Ground Source Heat Pump System With Thermal Storage System

The short-cycling operation of a heat pump decreases energy consumption efficiency. Short-cycling operations of ground source heat pump system (GSHP) occur when the ON/OFF control of a heat pump is used in a partial load condition. It is considered effective that GSHP with capacity controls installs to suppress short-cycling operations. However, there is no report on any continuous operations by capacity control GSHP in actual operations. We confirmed that GSHP (water to water) with capacity control operates short-cycling in the residence. Short-cycling operations occurred with a sudden load fluctuation due to opening or closing of the valves. We conducted effective verification experiments of the thermal storage device at the artificial heat load fluctuations condition. When the thermal storage device installed upstream brine circulation line of the heat pump with the capacity control, continuous operations are performed. It was under the condition at the heating heat load of 5 kW is turned ON/OFF every 20 min. In this case, energy consumption efficiency of a heat pump is 13% higher than the efficiency without the thermal storage device.

INVESTIGATION ON THE START-UP FEATURES OF SECONDARY-SIDE PASSIVE RESIDUAL HEAT REMOVAL SYSTEM

In order to study the start-up characteristics of passive residual heat removal (PRHR) system, a numerical model is established to calculate the transient characteristics of secondary-side PRHR system under station blackout (SBO) accident. The results show that two-phase flow oscillation occurs during the establishment of natural circulation. This is caused by the mismatch between the variation of reactor residual heat power and the PRHR system operation power. In this paper, a thermal energy storage device is introduced to remove the high residual heat power of the reactor at the initial stage of shutdown. The latent thermal storage device using phase change materials (PCMs) as heat storage medium has the advantage of high potential heat storage density and almost constant phase change temperature. This new configuration can eliminate the flow oscillation phenomenon and reduce the maximum pressure of the steam generator.

Dispatch Model for CHP With Pipeline and Building Thermal Energy Storage Considering Heat Transfer Process

Utilizing the flexibility provided by the thermal system components, for example, pipelines in the district heating network (DHN), building envelopes as well as thermal energy storage (TES) devices, can be an effective way for power system to solve the wind curtailment problem which closely relates to the limited flexibility of combined heat and power. However, almost all the previous studies ignored heat transfer (HT) constraints when modeling these abovementioned thermal system components. An HT constraint is one of the most important constraints in thermal system analysis, without considering it may result in infeasibility for the dispatch results. Hence, in this study, we propose a detailed model for these three components, namely DHN, building envelopes, and TES devices, with consideration of HT constraints. This model can be further simplified to consider different combinations of these components by setting relevant model parameters. An iteration solution to deal with such nonlinear HT constraints is put forward. The effectiveness of the proposed model is verified by case studies. The simulation results demonstrate that considering HT constraints is necessary when try to exploit the flexibility provided by these thermal system components, or we may overestimate their benefit.

A new concept of Al-Si alloy with core-shell structure as phase change materials for thermal energy storage

A novel phase change material based on the clad Al-Si composite ingot with eutectic Al-Si/Si-rich core-shell structure was designed for the high temperature thermal storage. The constructed Al-Si composite with core-shell structure exhibits a high thermal conductivity, large phase change enthalpy, and excellent cycling performance. Due to the spheroidized primary Si particles and continuous α-Al matrix in the Si-rich layer, the Al-Si composite ingot holds a good shape-stable property at 650 °C, which prevents the leakage of eutectic Al-Si melt during thermal cycling. Therefore, this novel clad Al-Si composite is a promising material for high temperature thermal storage, which may be directly used as an actual thermal storage device without containers.

Numerical investigation of the thermal performance enhancement of latent heat thermal energy storage using longitudinal rectangular fins and flat micro-heat pipe arrays

The performance of a new type of latent heat thermal energy storage (LHTS) device based on flat micro-heat pipe arrays (FMHPAs) with longitudinal rectangular fins is numerically studied by enthalpy–porosity technique based on finite volume method (FVM) in this research. The numerical model is verified correct. The temperature distribution and phase transition process in different directions of the interior of a thermal storage tank and the effects of fin height, spacing, and thickness on charging power and thermal storage capacity are also analyzed numerically. Results show that phase interface is presented in U type in the horizontal direction. In the vertical direction, the phase change material (PCM) among fins melts from up to down when the fin spacing is larger than 6 mm, and the opposite occurs when the fin spacing is less than 6 mm. The thermal storage capacity of the LHTS device is reduced drastically when the fin spacing is less than 4.14 mm. For fin height, the structure of multiple rows of FMHPAs with dwarf fins is recommended because it exhibits large power and exerts a small negative effect on thermal storage capacity. Fin thickness has a minimal effect on charging power and thermal storage capacity. The study results provide optimization design guidance for LHTS devices based on FMHPAs under various application backgrounds, such as solving the contradiction between energy supply and demand in solar thermal systems and the peak load shifting of electricity in heat pump systems.

Numerical simulation analysis on the thermal performance of a building walls incorporating Phase Change Material (PCM) for thermal management

Phase Change Material (PCM) plays an essential role in a thermal energy storage device, mainly, by utilizing its relatively high storage density, and latent heat property. PCM is potentially applicable as buildings material as it is possible to incorporate in the building envelope for energy conservation. Energy Plus is the chosen software to simulate phase change material in the building envelope. This paper will discuss in detail the numerical analysis of building thermal characteristics using PCM materials on the walls for climatic conditions of Banda Aceh City. The PCM used is beeswax. The weather data used in the simulation was customized for the year 2015 from the National Renewable Energy Laboratory (NREL) website. Some separate parametric studies, i.e., a variation of PCM thermal conductivity, and temperature range, and results are presented. It was discovered that a PCM with a melting point from 21 to 23 °C led to maximum energy savings and greater peak load time shift, and is more suitable than other PCM temperature ranges for lightweight building construction.

Influence of thermal energy storage system on flow and performance parameters of solar updraft tower power plant: A three dimensional numerical analysis

A 3D numerical model is developed for solar updraft tower to identify the effect of thermal energy storage system. Flow parameters such as temperature, velocity, pressure, and density are estimated, analyzed and compared for 2 different models, model - 1 (without thermal energy storage) and model −2 (with thermal storage). The collector diameter of 3.5 m, chimney height of 6 m, chimney diameter of 0.6 m, inlet gap of 0.15 m, slope of the collector 30° are the dimensions considered for model - 1. Model – 2 consists of the same dimensions of model – 1 with additional thermal energy storage system. Sand-rock mixture is considered as energy storage material and it has the thickness of 0.15 m and diameter of 3.5 m. Numerical simulations are performed under steady state condition for both the models at various times (10.00 a.m., noon, 1.00 p.m., 2.00 p.m. and 4.00 p.m.) in a day and for model – 2 up to 8.00 p.m. Results are compared with experimental and theoretical data from literature and found that the results are matching. Performance parameters such as power output, collector efficiency and overall efficiency of system were estimated and it was found that the values of model – 2 are lower because of heat stored by thermal storage device. Maximum power output for both models (79.92 W for model – 1 and 63.8 W for model – 2) was obtained at 1.00 p.m. The addition of thermal storage system enhanced the plant operation time till 8.00 p.m.

Encapsulation of high-temperature inorganic phase change materials using graphite as heat transfer enhancer

Abstract A quasi-isostatic pressing technique for encapsulating spherical high-temperature inorganic phase change materials (PCMs) was presented in this work. To enhance the thermal conductivity of PCMs, graphite powder was dispersed into the PCM pellets. Meanwhile, cellulose particles, as sacrificial particles, were mixed with the PCM pellets to relieve the volumetric expansion of the PCMs during the phase change process. The effects of the latent heat, thermal conductivity, and thermal expansion behavior of the PCM/graphite capsule were investigated. The PCM capsules were able to work at temperatures up to 900 °C, including undergoing a solid-liquid phase change at 803 °C with a latent heat of 159.6 J g−1, and survived more than 300 thermal cycles as thermal energy storage devices. The thermal performance of the PCM capsules between 500 and 900 °C was numerically investigated using a modified one-dimensional (1-D) enthalpy-based model. The results indicated that the average heat transfer of the PCM capsules with graphite-dispersed core was significantly elevated during both the charging (by 92.5% for 99% charge) and discharging (by 168.4% for 99% discharge) processes, compared to the pure core PCMs.

Study on Thermal Performance of a Phase Change Thermal Storage Device by Utilizing Off-peak Power

Based on the background of peak load shifting, this paper proposes a phase change thermal storage device by utilizing off-peak power. Experimental investigations on its thermal performance is conducted, it shows that the regenerator has high density of thermal storage and good thermal storage efficiency of 91.3%. At the same time, numerical simulations of the heat transfer enhancement by using fins are performed on the exothermic process, the results show that the addition of fins can effectively improve the exothermic efficiency to 80%. The experimental and simulation results have some reference value for exploring the application of latent heat thermal storage in real life.

Experimental study on random heat transfer of phase change heat storage device

In the study of the heat conduction of phase change materials, due to the inherent properties, the deviation of the measured data and the processing technology of the phase change material, the uncertainty of the parameters of the material properties and geometric properties of the phase change materials exist inevitably. But at the present stage, there are very few experimental researches content of uncertain heat transfer in phase change thermal storage devices. The purpose of this paper is to study the uncertain heat transfer process of phase change heat transfer experimentally. The influence of latent heat of phase transition on the heat transfer process is mainly analyzed. The latent heat of different phase transitions is transformed into internal heat sources, and simulated by electric heating. The thermal response of phase-change heat transfer is analyzed by constructing the electric heating quantity under different probability distribution and uncertainty. The experimental results show that when the uncertainty is the same as 1/7, the difference of the standard deviation between two mean values (560 W and 630 W) is not large. The standard deviation and the mean correlation of the experimental values are not significant under the same uncertainty.

Fast and experimentally validated model of a latent thermal energy storage device for system level simulations

Latent storages utilising phase change materials (PCM) to store thermal energy offer a considerably higher energy density at a nearly constant temperature level in comparison to sensible storage systems. Despite this advantage, only a few latent storage technologies have been integrated successfully to the market. This may be due several engineering challenges and in particular the lack of a computationally fast and accurate mathematical model to facilitate the optimal incorporation of latent heat storages into an energy system. The presented study fills this gap and proposes a new, fast and experimentally validated mathematical modelling approach for latent heat storage units. The numerical model proposed combines high accuracy, low computational effort and numerical stability. The validation was performed with two different commercial latent storage units supplied by Sunamp Ltd. with a nominal phase change temperatures of 34 °C and of 58 °C. Both units use a salt hydrate based phase change material in combination with a fin-tube heat exchanger. The proposed model may be used for both fast system level performance investigations as well as latent storage design for a given application. It may therefore be implemented in commercial software packages such as TRNSYS [1] or Simulink [2].

Modeling and optimal operation of community integrated energy systems: A case study from China

The operation of community integrated energy systems (CIESs) is challenging because it is necessary to couple energy production, conversion and consumption on the end-user side. Multiple energy demands need to be fed simultaneously with the goal of economy and reliability. This paper formulates the optimal scheduling problem based on a real community integrated energy system in China. The system has multiple chillers (i.e. three ground source heat pumps, two conventional water-cooled chillers and two double-duty chillers) and two types of thermal storage devices (i.e. two cold water tanks and an ice-storage tank). Based on detailed device modeling, a community integrated energy system operation strategy considering unit commitment is proposed, and then is transformed into a mixed-integer linear programming model by linearization of nonlinear items. Case studies are conducted based on the data for a typical day in summer. The results show that the proposed strategy can utilize the flexibility of the energy storage devices and realize an economic and reliable operation of the community integrated energy system by coordinating various energy devices. The strategy can also reduce the startup/shutdown frequency of chillers significantly.

Experimental and Numerical Study of an Electrical Thermal Storage Device for Space Heating

An electrical thermal storage (ETS) device for space heating is designed in this study. The proposed device is charged by the off-peak electricity and releases its thermal energy to warm the space all day long. The natural convection occurring in the flow channel drives the flowing of air to be heated up and warm the space. The dynamic process of a fully charging/discharging cycle of the device is tested. Meanwhile, a three-dimensional model is developed to simulate the transient thermal behavior. It is found that the experimental and numerical results agree with each other which indicates the validation of the proposed numerical model. The results show that the temperatures of the bricks and the outlet air can be as high as 1002 K and 835 K, respectively. The natural convection occurring in the flow channel transfers 40.4% of the total electrical heating energy to the space for the charging process and 26.9% for the discharging process. Heat losses to the space through the adiabatic material shares 13.3% of the total heating energy for the charging process and 7.2% for the discharging process. Based on those findings, three methods are recommended to improve the device in the successive research.

Effects of different thermal storage tank structures on temperature stratification and thermal efficiency during charging

In solar water heating systems, the structures of thermal storage devices have played essential roles in the improvement of thermal charging efficiency and system performance. This article was focused on the optimization of thermal storage tanks, as well as the influences of thermal storage tank structures on the temperature stratification and heat storage capacity. The thermal characteristics of three shapes of thermal storage tanks were investigated and analyzed through CFD (Computational Fluid Dynamics) simulation, which were: (a) Cylindrical tank, (b) Circular truncated cone tank and (c) Spherical tank. It was observed that among the three thermal storage tanks, the spherical tank had significant symmetrical temperature stratification, while the truncated circular cone tank had the best temperature stratification and thermal charging efficiency. For the circular truncated cone tank, it was found that when the radius ratio (n) of the water tank was 0.554, it could approach the excellent characteristics of temperature stratification and thermal storage performance. This study could provide a theoretical basis for the structure optimization of the solar energy storage devices, as well as proving to be beneficial to the enhancement of thermal charging efficiency.

Robust optimization for day-ahead market participation of smart-home aggregators

This paper proposes an optimization model to participate in day-ahead energy markets when PV generation, thermal and electro-chemical storage devices are aggregated at the residential level. The model includes uncertainty in energy prices, PV and load; and adjustable robust optimization is used to determine a tractable counterpart of the problem. By means of robust control parameters, solutions with different levels of conservatism can be found and analyzed. In addition, the presented model includes explicit representation of battery degradation by means of special ordered sets. This equivalent cycling aging calculation takes into account the non-linear relation between depth of discharge and total life cycles of the battery by piecewise linearization. Performance analysis shows the advantage of the proposed approach when compared to the deterministic solution in terms of average cost and risk. For the analyzed real-life test system, the robust formulation achieves cost reduction of up to 5.7% and standard deviation decreases as much as 36.4%.