High-temperature Thermal Energy Storage Systems Research Articles

Fabrication of heat storage pellets composed of microencapsulated phase change material for high-temperature applications

Latent heat storage using a metallic phase change material (PCM) is an attractive option for high-temperature thermal energy storage. However, there are serious technical barriers to applying a metallic PCM to practical applications, mainly caused by its high corrosivity. This study reports the fabrication of a pellet-type PCM composite, mainly composed of a microencapsulated metallic PCM, as a potentially viable solution to this issue. This microencapsulated PCM (MEPCM) has a core composed of Al-25 mass% Si, which acts as a PCM. Its shell is composed of Al2O3 (or a precursor of Al2O3), and grass frit was used as a sinter agent. The composites were fabricated by mixing the MEPCM with a sintering agent, pelletizing, and sintering. The PCM composites exhibited a melting point of ~577 °C and a latent heat of 108–122 J g−1. The thermal conductivities of the PCM composites were in the range of 2.16–3.20 W m−1 K−1. The cycling performance demonstrated the good durability of the composites. There were no significant changes in the shape and chemical composition of the pellets, even after 300 cycles of melting and freezing tests. These results indicate that the concept of pellet-type composites using MEPCM developed in this study will overcome the technical barriers to utilizing metallic PCMs. Thermal energy storage structures in various shapes could be fabricated via the method for fabricating PCM composites proposed in this study. This concept therefore shows substantial promise for application in high-temperature thermal energy storage systems.

Concrete based high temperature thermal energy storage system: Experimental and numerical studies

This paper presents the concept of developing a cost-effective Concrete based Thermal Energy Storage (CTES) system by performing extensive experimental studies and numerical simulations. A stand-alone experiment facility to study the performance of high-temperature thermal energy storage system which operates up to 500 °C using air as the heat transfer fluid has been developed. The CTES module is made of shell and tube configuration, where concrete is filled in the shell side, and 22 air passages are provided on the tube side. The inlet air temperature and velocity are the decision parameters used for analyzing the thermal behaviour of the CTES module. From the spatial variations of temperature, it is observed that the heat transfer rate is uniform and faster along all radial planes, whereas, the heat transfer rate drops gradually along the length of the CTES module due to drop in Heat Transfer Fluid (HTF) temperature. The parametric investigation conducted shows that the charging and discharging times were reduced by approximately 48% and 29%, respectively, with a change in inlet temperature of 40 °C at a fixed air velocity of 2 m/s. A 3-D model for CTES module developed using the finite element method has been validated with experimental results. The temperature contours plotted from 3-D simulation describes the spatial variation of CTES temperature at different inlet air temperatures. Further, a 1-D dynamic model has been developed, which is fast and accurate with a maximum error of ±4.9 °C with reference to real-time experiments and also provides a substantial scope of integrating the CTES with industrial applications. The outcomes of the present studies conclude that the developed CTES module is highly recommended for the high-temperature applications.

Thermal performance evaluation of lining materials used in thermal energy storage for a falling particle receiver based CSP system

In this study, experiments were conducted on a cylindrical-shaped Thermal Energy Storage (TES) system for a novel Concentrating Solar Power (CSP) falling particle receiver system. The TES is to be used for storing heated solid particles. The main objective of the study was to evaluate the thermal performance of some candidate materials of construction for the walls of TES Hot-bin. In particular, emphasis was on the calculation of the rate of heat loss over a prescribed period of time, since this parameter was critical to the overall performance of a high-temperature solar energy plant. The construction materials were selected in order to have low thermal conductivity and the ability to withstand cyclic high temperatures without failure. An experiment was continuously run in which the temperature was maintained at about 300 °C, 500 °C, and 700 °C respectively, allowing steady state conditions to be achieved for sustained period of time. To simulate the presence of a high-temperature storage material, a heater inserted in the centerline of the interior of the Hot-bin, and the thermocouples were installed to measure the temperatures at various locations throughout the composite wall. Results show that the thermal conductivity of insulating firebrick remains low (approximately 0.22 W/m·K) at an average layer temperature as high as 640 °C, but it was evident that the addition of mortar had an impact on its effective thermal conductivity. Results also show that the thermal conductivity of perlite concrete is very low, approximately (0.15 W/m·K) at an average layer temperature of 360 °C. This is evident by the large temperature drop that occurs across the perlite concrete layer. Due to the large daily ambient temperature swing, a measurement of the thermal conductivity of reinforced concrete was estimated by applying a 3D model of the reinforced concrete layer. Thermal conductivity of the reinforced concrete layer is about 1.936 W/m·K at an average temperature 52.5 °C. Based on the estimated thermal conductivity values of layers, heat loss was found to be 1.39%, 2.58% and 4.59% at 300 °C, 500 °C and 700 °C respectively. Furthermore, inspection of the materials used to construct the TES system showed that they remained intact and did not show signs of cracking or wearing. These results showed that the high-temperature TES systems can be constructed of readily available materials and yet meet the heat loss requirements for a falling particle receiver system, thereby contributing to reducing the overall cost of concentrating solar power systems. An important secondary outcome of this study is building a database for the thermal conductivity of some masonry materials at high temperatures. These results should be useful for future studies, especially those that focus on numerical modelling of TES bins.

A novel process for preparing molten salt/expanded graphite composite phase change blocks with good uniformity and small volume expansion

Herein a novel process was explored for preparing molten salt/expanded graphite (EG) composite phase change material (PCM) blocks, which involves mixing a solid molten salt with EG thoroughly, compressing the mixture into a block with a designed shape and then heating the block to a temperature above the melting point of the molten salt followed by cooling. An MgCl2-KCl eutectic salt was used as the PCM, and its EG-based composite PCM block was prepared by this process, in which the optimal mass fraction of the eutectic was determined to be around 85%. The microstructure of the MgCl2-KCl/EG composite PCM block containing 85% the eutectic shows a uniform distribution of the molten salt. The composite PCM block has a melting point of 424.14°C and a solidification point of 418.39°C, and its latent heat values are 161.37J/g for melting and 160.28J/g for solidification. Compared with the eutectic salt, the composite PCM block exhibits a reduction in supercooling by 3.7°C and an enhancement in thermal conductivity by 11-fold. It has been verified that the composite PCM block possesses excellent thermal reliability. Furthermore, the composite PCM block has been compared with the one prepared by the conventional method (first adsorption and then compression). It is found that the composite PCM block fabricated by the novel process exhibits better uniformity and smaller volume expansion than the one obtained from the conventional method. The MgCl2-KCl/EG composite block shows great promise in high-temperature thermal energy storage systems, and this novel process is very suitable for preparing molten salt/EG composite PCM blocks.

Lithium manganese oxides as high-temperature thermal energy storage system

The reversible oxidation of LiMnO2 to LiMn2O4 and Li2MnO3 coexisting phases has been investigated in view of its possible application as high temperature energy storage system. By means of thermoanalytical techniques information regarding the heat exchanged during both oxidation and reduction reactions have been collected and the temperature range for cyclic reduction–oxidation has been explored. Initial tests regarding the cyclical operation highlight the optimal thermal stability of the mixture confirming the exploitability of the system for high temperature thermal energy storage applications. Preliminary information concerning the kinetic behavior of the powder during the reduction stage evidences a two-step mechanism. The reaction can be described through a first stage characterized by oxygen depletion from the spinel phase, followed by a major oxygen loss concomitant with the depleted LiMn2O4 and Li2MnO3 solid state transformation into LiMnO2.

Numerical investigation of hydrodynamics and thermal performance of a specially configured heat pipe for high-temperature thermal energy storage systems

In the present study, a complex geometry, high temperature heat pipe was investigated numerically. The heat pipe was designed specifically to improve the performance of the thermal energy storage system incorporated in a concentrated solar power system. Due to the complexity of the heat pipe geometry, a new numerical approach was developed. The heat conduction in the wall and wick regions was included in the two-dimensional axisymmetric model. The compressibility of the working fluid and viscous dissipation were taken into account. Due to the high expansion ratio from adiabatic section to condenser, high speed vapor flow impinges on the condenser wall. The combined vapor jet impingement and condensation on the condenser wall was modeled by attaching a porous layer. This porous layer forces the impinging vapor jet to split out radially while it allows the wall suction. The heat rejection via the vapor condensation was estimated from the mass flux by energy balance at vapor–liquid interface. The heat transfer limits due to heat pipe geometry, working fluid, wick structure, and operational temperature were calculated. The vapor–wick interface temperature predicted by the current simulation agrees very well with the available experimental data. The performance of the heat pipe was evaluated by calculating the corresponding thermal resistance. The results showed that the thermal resistance decreases with the increase of the operating temperature and vapor radius while increases with the increase of the heat input. It was also shown that, for the cases studied, adiabatic section radius of 15 mm is sufficient. Further increase of the radius does not reduce the thermal resistance of the heat pipe.

Status of the Development of a New High Temperature Thermal Energy Storage System (HTTESS) for CSP-power Plants

Thermal Energy Storage-Systems (TES-System or TESS) are already acknowledged to be highly needed to optimize the electricity yield of CSP-power plants and to improve the economics of CSP-projects and to reduce electricity prices. Furthermore for many electricity grid operators it is important to have spinning reserves in the grid and dispatchable power available.It has been the task of enolcon to develop a new TES-System realizing a step change which is by its principle simple, robust with regard to execution and operation, and which is reducing the electricity costs of CSP-power plants. Furthermore such system shall be open to future developments of CSP-systems with regard to increasing steam temperatures and steam pressure. Such TES-system shall be commercially available for large scale application already in year 2014/2015.As a result enolcon has invented in year 2010/2011 a TES-System using ambient air as heat transfer fluid and stones/rock as storage medium. The key elements of the enolcon-TES are the open cycle using always ambient air with an air-air-heat exchanger and the arrangement of the storage material in such way to minimize the pressure losses and the own electricity consumption. The development is progressing in a structured way by studies, engineering works, TES-pilot plants, isothermal air flow test plant for the verification of the CFD-calculations, and since end of 2012 by the operation of a high temperature TES- module with all features of the large scale modules. The actual status of the TESS-development and the key results of the charging and discharging of the high temperature TES-module TESS002 is presented following.

Heat transfer of phase change materials (PCMs) in porous materials

In this paper, the feasibility of using metal foams to enhance the heat transfer capability of phase change materials (PCMs) in low- and high-temperature thermal energy storage systems was assessed. Heat transfer in solid/liquid phase change of porous materials (metal foams and expanded graphite) at low and high temperatures was investigated. Organic commercial paraffin wax and inorganic calcium chloride hydrate were employed as the low-temperature materials, whereas sodium nitrate was used as the high-temperature material in the experiment. Heat transfer characteristics of these PCMs embedded with open-cell metal foams were studied. Composites of paraffin and expanded graphite with a graphite mass ratio of 3%, 6%, and 9% were developed. The heat transfer performances of these composites were tested and compared with metal foams. The results indicate that metal foams have better heat transfer performance due to their continuous inter-connected structures than expanded graphite. However, porous materials can suppress the effects of natural convection in liquid zone, particularly for PCMs with low viscosities, thereby leading to different heat transfer performances at different regimes (solid, solid/liquid, and liquid regions). This implies that porous materials do not always enhance heat transfer in every regime.

Computer Simulation of a High-Temperature Thermal Energy Storage System Employing Multiple Families of Phase-Change Storage Materials

Previous work by one of the authors entailed modeling of a packed bed thermal energy storage system utilizing phase-change materials (PCM). A principal conclusion reached is that the use of a single family of phase-change storage material may not in fact produce a thermodynamically superior system relative to one utilizing sensible heat storage material. This paper describes the model constructed for the high-temperature thermal energy storage system utilizing multiple families of phasechange materials and presents results obtained in the exercise of the model. Other factors investigated include the effect on system performance due to the thermal mass of the containment vessel wall and variable temperature of the flue gas entering the packed bed during the storage process. The results obtained indicate efficiencies for the system utilizing the five PCM families exceeding those for the single PCM family by as much as 13 to 26 percent. It was also found that the heat transfer to the containment vessel wall could have a significant detrimental effect on system performance.

Further notes on sampling analysis, with application to coal

Herein a novel process was explored for preparing molten salt/expanded graphite (EG) composite phase change material (PCM) blocks, which involves mixing a solid molten salt with EG thoroughly, compressing the mixture into a block with a designed shape and then heating the block to a temperature above the melting point of the molten salt followed by cooling. An MgCl2-KCl eutectic salt was used as the PCM, and its EG-based composite PCM block was prepared by this process, in which the optimal mass fraction of the eutectic was determined to be around 85%. The microstructure of the MgCl2-KCl/EG composite PCM block containing 85% the eutectic shows a uniform distribution of the molten salt. The composite PCM block has a melting point of 424.14 °C and a solidification point of 418.39 °C, and its latent heat values are 161.37 J/g for melting and 160.28 J/g for solidification. Compared with the eutectic salt, the composite PCM block exhibits a reduction in supercooling by 3.7 °C and an enhancement in thermal conductivity by 11-fold. It has been verified that the composite PCM block possesses excellent thermal reliability. Furthermore, the composite PCM block has been compared with the one prepared by the conventional method (first adsorption and then compression). It is found that the composite PCM block fabricated by the novel process exhibits better uniformity and smaller volume expansion than the one obtained from the conventional method. The MgCl2-KCl/EG composite block shows great promise in high-temperature thermal energy storage systems, and this novel process is very suitable for preparing molten salt/EG composite PCM blocks.