Temperature Latent Heat Thermal Energy Research Articles

Study of the solidification behaviour of phase change materials by in-situ X-ray computed tomography

Calcium chloride hexahydrate (CaCl2.6H2O) is a stablished inorganic salt hydrate-based phase change material (PCM) with interesting properties for low temperature latent heat thermal energy storage (LHTES) applications. However, its phase transition during melting and solidification exhibits a complex behaviour with supercooling and phase segregation effects, which critically limit their practical applications. Experimental approaches enabling tracking of the solidification process in this PCM are thus crucial for a deeper understanding of these adverse effects and to design efficient LHTES. In this study, a novel method based on dynamic X-ray computed tomography (XCT) is used to monitor the solidification process in CaCl2.6H2O through sequences of XCT images. The method was tested with a sample of 60g of CaCl2.6H2O with density of 1.71 g/cm3 and purity higher than 98%. The results reveal that solidification of CaCl2.6H2O occurs through the fusion of crystalline structures initially appearing at the side and the bottom surfaces of the sample which grow over time. In the experiment performed, the solidification process is found to be incomplete, which is frustrated after 385 min of starting the solidification. This frustration was preceded by a change in the total solidification rate which is here estimated from the transient liquid volume fraction curve computed via image processing analysis. Quantitative estimation for the crystals size is also reported.

Microstructural characteristics and mechanical properties of Si-B alloys for functional and structural ultra-high temperature applications

Silicon boron alloys have been recognized as important materials for e.g. a direct usage in ultra-high temperature latent heat thermal energy storage systems or as a batch materials for processing boron enhanced silicide-based composites. In this work, we put new experimentally driven insights on a structure of selected Si-B binary alloys. For this reason, four Si-xB alloys (x = 2.5; 8, 13.5; 86 (at%)) were fabricated from pure elements by using a crucible-less electric arc melting technique. Nominal compositions of the alloys were selected in accordance to Si-B phase diagram as hypoeutectic, eutectic, hypereutectic and corresponding to the stoichiometric composition of SiB6 intermetallic phase, respectively. The fabricated Si-xB alloys were subjected to a detailed structural characterization. Additionally, a hardness, elastic moduli and fracture toughness of each phase constituent were examined through a micro-indentation technique. A co-existence of various silicon borides, namely SiB4, SiB6 and SiBn, has been experimentally documented. Obtained results allows receiving a description of each alloy in terms of microstructure, crystal structure and mechanical properties of involved phases. It was found that mechanical properties (both microhardness and Young modulus) increases with raising boron content in recognized Si-B phases. The fracture toughness of silicon borides assessed through indentation experiments was found to be in the rage of 2.13–3.29 MPa·m1/2.

Cost-effective ultra-high temperature latent heat thermal energy storage systems

The availability of cost-effective energy storage technologies with durations from 10 to 100 h is key for intermittent renewable energies, like wind or solar, to become a large share of the electrical grid power. Battery prices forecasted for the upcoming years are still too expensive; and storing the energy as heat instead of electricity, arises as a promising cheaper solution. Even if there is an efficiency penalty when converting heat back to electricity, the low cost of thermal energy storage (TES) systems is an important advantage. Besides, not always the heat stored in a TES system needs to be converted to electricity, as heat corresponds to about 50% of the global energy demand. In this work, the potential of Ultra-High Temperature Latent Heat Thermal Energy Storage (UH-LHTES), which can reach energy capacity costs below 10 €/kWh by storing heat at temperatures well beyond 1000 °C, is presented with the help of a Computational Fluid Dynamics (CFD) model. Modelling results, together with real materials performance and cost data, allow to estimate cost-efficiency boundaries for UH-LHTES systems for a range of sizes from 10 kWhth to 10 MWhth. The CFD model has been validated reproducing the real size and materials of a UH-LHTES prototype developed in the framework of the AMADEUS project. The CFD model is proven to be a reliable tool to reproduce the operation of a UH-LHTES system and it predicts quite precisely UH-LHTES prototype discharge rates and heat losses.

Isopropyl palmitate integrated with plasterboard for low temperature latent heat thermal energy storage

In recent years, the latent heat thermal energy storage using phase change materials (PCMs) has become a hot topic; it has attracted much attention in the utilization of renewable energy sources to improve the energy efficiency of buildings. Isopropyl palmitate (ISOP) with phase transition temperature of 10°C to 13°C and heat of fusion of ~113 J/g was selected to be used as a PCM in building applications. ISOP, 15%, was mixed with plasterboard (PB) as a building material in two techniques, immersion and direct incorporation, in order to retain the ISOP into the porous structure of PB. Scanning electron microscope (SEM) demonstrated that the layer structure of the PB particles was uniformly absorbed by ISOP. Phase transition temperature of ISOP in both techniques determined using differential scanning calorimeter (DSC) is still similar to that of ISOP accompanied with much reduction in the latent heat, particularly in the direct incorporation technique. TCA 300 thermal conductivity analyzer addressed that the thermal conductivity of the PB has been slightly increased with the addition of ISOP. The stored thermal energy performance in a small test room is experimentally investigated. These results indicated that applying ISOP to the south wall of the testing room had the potential to reduce the fluctuation of the indoor air temperature. The effect of ISOP on the mechanical properties of PB has been investigated in the flexural mode using three-point bending test. Moreover, the fire risk of ISOP has been assessed using cone calorimetric. The results confirmed that the novel composite ISOP has a significant opportunity in building applications for thermal energy storage.

Fatty acids based eutectic phase change system for thermal energy storage applications

Multiple fatty acid based eutectic phase change materials are prepared for low to moderate temperature latent heat thermal energy storage applications. In particular, palmitic acid, myristic acid, stearic acid, lauric acid and commercial PureTemp68 are used for making eutectic mixtures. The eutectic point of each eutectic mixture is determined using Schrader equation along with its thermophysical properties. Ten different eutectic mixtures are prepared in accordance with eutectic point obtained from the phase diagrams by melt blending followed by ultra-sonication. The latent heat, melting point as well as the specific heat capacity of the eutectic phase change material are determined by Differential Scanning Calorimetry (DSC). The results revealed that the melting point of these organic phase change materials ranges from ∼27–75 °C, with the latent heat from ∼127 to 210 kJ⋅kg−1 respectively. Chemical structure of these phase change materials is determined by using Fourier Transformation Infrared Spectroscopy (FT-IR) and the presence of carboxylic group in obtained spectra is in accordance with the fatty acids. It is evident that these eutectic phase change materials possess promising characteristics for thermal energy storage ranging from the room temperature to ∼75 °C.

Compatibility study between aluminium alloys and alternative recycled ceramics for thermal energy storage applications

Recycled ceramics from industrial wastes, compared with traditional high-purity ceramics, present high market potential as refractory materials due to their low cost production process with very low environmental impacts. However, there still exists a research gap of how recycled ceramics behave while in contact with liquid metal. In order to study the feasibility of using recycled ceramics as the encapsulation material in the application of high temperature Latent Heat Thermal Energy Storage system, this paper investigates the compatibility of recycled ceramics with three kinds of aluminium-based alloys at high temperature, with a comparison to the corrosion resistant behaviour of alumina. The recycled ceramics explored include Cofalit from asbestos containing waste, blast furnace slags from steel production, and coal fly ashes sintered ceramics from incineration plants. The study consists of a steady state thermal treatment of ceramic samples in contact with the three different alloys at 1000 °C for 100 h, and a post instrumental characterization of ceramic samples by Environmental Scanning Electron Microscopy, Energy Dispersive Spectrometry and X-ray diffractometer, to understand the chemical and structural transformation of the ceramics. Results demonstrate that Cofalit shows chemical stability with Al99% but instability with AlSi5% and AlSi12%. Blast furnace slag presents quite good thermochemical stability towards molten AlSi5% and AlSi12%. Coal fly ashes sintered ceramics are highly interactive towards all three aluminium alloys. In conclusion, besides alumina, Cofalit is recommended as alternative encapsulation material for molten Al99%, while blast furnace slag being recommended for molten AlSi5% and AlSi12%.

Compatibility tests between molten Aluminium alloys and recycled ceramics from inorganic industrial wastes

Recycled ceramics from industrial wastes, compared with traditional high-purity ceramics, present high market potential as refractory materials due to their low cost production process with very friendly environmental impacts. In order to study the feasibility of using recycled ceramics as the encapsulation material in the application of high temperature Latent Heat Thermal Energy Storage system, this paper investigates the compatibility of recycled ceramics with three kinds of aluminium-silicon alloys at high temperature. The recycled ceramics explored are Cofalit from asbestos containing waste and steel slags. The study consists of a steady state thermal treatment of both ceramics in contact with the three different alloys at 1000°C for 100 hours, and a post instrumental characterization of ceramic samples by Environmental Scanning Electron Microscopy and Energy Dispersive Spectrometry, to understand the chemical and structural transformation of each ceramics. Results show that Cofalit has chemical interactions with aluminium alloys, while steel slag presents an excellent corrosion resistance to molten aluminium alloys and is further recommended as an encapsulation material for molten aluminium alloys at high temperature.

The Role of Soya Oil Ester in Water-Based PCM for Low Temperature Cool Energy Storage

This study focuses on the preparation of the water-based phase change material (PCM) with very small soya oil solution for low temperature latent heat thermal energy storage (LHTES). Soya oil ester is soluble very well in water and acts as nucleating agent for a novel solid-liquid PCM candidate that is suitable for low temperature cool storage in the range between −9°C and −6°C. Thermal energy storage properties of the water with very small soya oil ester solution were measured by T-history method. The experimental results show that very small amount of soya oil ester in water can lower the freezing point and trigger ice nucleation for elimination of the supercooling degree. The phase transition temperatures of the water-based PCMs with soya oil as nucleate agent were lower than those of individual water. The thermal properties make it potential PCM for LHTES systems used in low temperature cool energy storage applications.

Techno-economic heat transfer optimization of large scale latent heat energy storage systems in solar thermal power plants

Concentrated solar power plants with integrated storage systems are key technologies for sustainable energy supply systems and reduced anthropogenic CO2-emissions. Developing technologies include direct steam generation in parabolic trough systems, which offer benefits due to higher steam temperatures and, thus, higher electrical efficiencies. However, no large scale energy storage technology is available yet. A promising option is a combined system consisting of a state-of-the art sensible molten salt storage system and a high temperature latent heat thermal energy storage system (LHTESS). This paper discusses the systematic development and optimization of heat transfer structures in LHTESS from a technological and economic point of view. Two evaluation parameters are developed in order to minimize the specific investment costs. First, the specific product costs determine the optimum equipment of the latent heat storage module, i.e. the finned tube. The second parameter reflects the interacting behavior of the LHTESS and the steam turbine during discharge. This behavior is described with a simplified power block model that couples both components.

Three-dimensional simulation of high temperature latent heat thermal energy storage system assisted by finned heat pipes

The startup process of a high temperature latent heat thermal energy storage system assisted by finned heat pipes was studied numerically. A transient three-dimensional finite volume based model was developed to simulate the charging process of phase change material with different configuration of embedded heat pipes. The melting of the phase change material was modeled by employing enthalpy-porosity technique. A eutectic mixture of sodium nitrate and potassium nitrate with melting temperature of 220°C enclosed by a vertical cylindrical container was used as the phase change material. The effects of different heat pipe arrangement and the heat pipe quantities as well as the influence of natural convection on the thermal behavior of the latent heat thermal energy storage system were studied. The results indicate that the heat pipe configurations and the quantities of heat pipes integrated in a thermal energy storage system have a profound effect on the thermal response of the system. Employing more heat pipes decreases the thermal resistance within the system, leading to the acceleration of charging process and the decrease of container base wall temperature. It was also found that the inclusion of natural convection heat transfer in the charging process of the system renders to higher melting rate and lower base wall temperature.

High Temperature Latent Heat Thermal Energy Storage Integration in a Co-gen Plant

Abstract Within the framework of the project TESIN funded by the German Ministry of Economic Affairs and Energy, a high temperature latent heat thermal energy storage unit is being developed and will be designed, built, commissioned and tested in an operating cogeneration plant in Saarland, Germany. This plant provides superheated steam to industrial process customers from a gas turbine with a heat recovery steam generator. Currently, a secondary boiler is operated at minimal load, from which it can be heated to full load in 2 minutes. With the integration of the thermal energy storage into the plant, the secondary boiler will be reduced from minimal to warm load operation. In case of a failure of the gas turbine, the storage will produce steam for 15 min. while the secondary boiler is heated from a warm to a hot operating load. This standby load reduction in the secondary boiler will reduce the use of fossil fuels. The steam demand from the thermal storage is for 8 t/h at around 25 bar and a minimum temperature of 300 °C. This results in a high power level of about 6 MWth and a necessary storage capacity of 1.5 MWh. At this pressure level, the steam is superheated about at least 75 K. The combination of the superheating and the required power level has led to a smaller tube distance than in previous storage units as well as a new axial fin design. The basic storage as well as the fin design combined with nitrate salts as the storage material have been analyzed with simulation tools. Detailed design planning, permitting and build of the system are the coming steps in this part of the project. The design of these fins, storage unit design and planned integration in the cogeneration plant are presented here.

A numerical and experimental study of solidification around axially finned heat pipes for high temperature latent heat thermal energy storage units

A numerical and experimental investigation was conducted on the thermal performance of latent heat thermal energy storage (LHTES) systems which use heat pipes (HPs) for solar thermal power generation. The aim of this study was to quantify the advantages of utilising axially finned HPs rather than bare HPs in LHTES systems. The numerical model uses the effective heat capacity formulation to simulate the solidification process in the phase change material (PCM) and adopts the thermal resistance network approach simulate the heat transfer phenomena through the HPs. The experimental measurements were conducted on a bare heat pipe and on an identical heat pipe with four axial fins. The numerical predictions and the experimental measurements were found to be in good agreement. The benefits of finning the HPs can be seen by considering the enhancement of the rate of energy extraction from the PCM as well as the HP effectiveness. The results have shown the energy extracted increased by 86% and the heat pipes effectiveness increased by 24%.

Numerical analysis on the thermal behavior of high temperature latent heat thermal energy storage system

High temperature latent heat thermal energy storage technology is a promising option for future cost reduction in parabolic trough or tower power plant. However, low thermal conductivity of phase-change material (PCM) is the major shortage of latent heat thermal energy storage. This paper proposed a new thermal energy storage system (TESS) that metal foam and fins were used to enhance the effective conductivity of PCM. Three-dimensional physical model was established for representative element extracted from TESS. Considering the natural convection in the liquid part of PCM, volume-averaged mass and momentum equations were employed with the Brinkman–Forchheimer extension to Darcy law to simulate the porous resistance. A local thermal equilibrium model was developed to obtain temperature field. The governing equations were solved with finite-volume approach and enthalpy method was employed to account for phase change. The model was firstly validated against low temperature experiments from the literature and then used to predict the charging and discharging behavior of the present TESS. The position of solid/liquid interface was explored and the effects of design parameters, including that of metal foam pore density and porosity, configuration of fin and Rayleigh number, on melting and solidifying rate and energy stored in each time step were revealed and discussed. The results indicate that metal foam and fins can effectively improve the heat transfer performance for thermal storage system and decrease charging and discharging time.

High temperature latent heat thermal energy storage using heat pipes

A thermal network model is developed and used to analyze heat transfer in a high temperature latent heat thermal energy storage unit for solar thermal electricity generation. Specifically, the benefits of inserting multiple heat pipes between a heat transfer fluid and a phase change material (PCM) are of interest. Two storage configurations are considered; one with PCM surrounding a tube that conveys the heat transfer fluid, and the second with the PCM contained within a tube over which the heat transfer fluid flows. Both melting and solidification are simulated. It is demonstrated that adding heat pipes enhances thermal performance, which is quantified in terms of dimensionless heat pipe effectiveness.

Numerical simulation and parametric study on new type of high temperature latent heat thermal energy storage system

Commercial acceptance and the economics of solar power generation using direct steam technology in parabolic troughs require the design and development of efficient, cost effective high temperature latent heat thermal energy storage systems (HTLHTES). Here, the heat transfer enhancement in the HTLHTES by using aluminium foils is presented. The phase change material (PCM) is initially liquid, and fills the space around the tube, foil and shell, while the heat transfer fluid (water) flows inside the tube. Transient two dimensional heat conduction problems are solved using the Fluent 6.2 software when heat is extracted from the PCM during the discharging process, yielding the phase interface position, temperature distribution, liquid fraction and surface heat flux as functions of time and the time of complete discharge. Parametric studies are conducted to assess how the performance of the storage is affected by the geometry, thermal and boundary conditions, which leads to correlations for the time of complete discharge. The computation results show that adding aluminium foils is an efficient way to enhance the heat transfer in the HTLHTES, and with the help of the correlations presented, the performance and optimum design of the HTLHTES can be investigated under different conditions.

Preparation and thermal properties of capric acid/palmitic acid eutectic mixture as a phase change energy storage material

This study focuses on the preparation, thermal properties and thermal reliability of capric acid (CA)/palmitic acid (PA) mixture as phase change material (PCM) for low temperature latent heat thermal energy storage (LHTES). The differential scanning calorimetry (DSC) results indicated that the CA/PA mixture with eutectic composition (76.5/23.5 wt.%) was suitable PCM for low temperature LHTES applications in terms of melting and freezing temperatures (Tm=21.85 °C; Tf=22.15 °C) and latent heats of melting and freezing (ΔHm=171.22 J/g; ΔHf=173.16 J/g). The thermal properties make it potential PCM for LHTES systems used in heating, ventilation, and air conditioning applications. Accelerated thermal cycling tests showed that the eutectic mixture as a PCM has good long-term thermal reliability. The probable reasons of the changes occurred in thermal properties of the PCM during accelerated thermal cycling were also investigated using Fourier Transform Infrared (FT-IR) spectroscopy method.