Comparison of thermal storage characteristics of phase change materials encapsulated in different capsule configurations (original) (raw)
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Springer Proceedings in Energy, 2018
This paper presents the thermal modeling and performance comparison of sensible and latent heat based thermal energy storage (TES) systems using concrete and phase change materials (PCMs) encapsulated in containers of different geometrical configurations. The sensible heat storage (SHS) and latent heat storage (LHS) module considered here is a capsule containing concrete or sodium nitrate which exchanges heat with the source material. SHS capsule is modeled using the energy conservation equation. Effective heat capacity method is employed to account the latent heat of the PCM. Boussinesq approximation and Darcy law's source term are added in the momentum equation to incorporate the natural con-vection of molten PCM and nullify the velocities of solid PCM. The equations of the 2D axisymmetric model are solved using COMSOL Multiphysics. Charging time of capsules in four different configurations viz., spherical, cylindrical (H = D, H = 4D) and novel cylindrical configurations are compared. The thermal characteristics are compared using isothermal contour plots and temperature-time curves.
Renewable Energy and Environmental Sustainability, 2017
Solar energy has been considered as one of the promising solutions to replace the fossil fuels. To generate electricity beyond normal daylight hours, thermal energy storage systems (TES) play a vital role in concentrated solar power (CSP) plants. Thus, a significant focus has been given on the improvement of TES systems from the past few decades. In this study, a numerical model is developed to obtain the detailed heat transfer characteristics of lab-scale latent thermal energy storage system, which consists of molten salt encapsulated spherical capsules and air. The melting process and the corresponding temperature and velocity distributions in every capsule of the system are predicted. The enthalpy-porosity approach is used to model the phase change region. The model is validated with the reported experimental results. Influence of initial condition on the thermal performance of the TES system is predicted.
Scientific Reports
Heat storage efficiency is required to maximize the potential of combined heat and power generation or renewable energy sources for heating. Using a phase change material (PCM) could be an attractive choice in several instances. Commercially available paraffin-based PCM was investigated using T-history method with sufficient agreement with the data from the manufacturer. The introduced LHTES with cylindrical capsules is simple and scalable in capacity, charging/discharging time, and temperature level. The overall stored energy density is 9% higher than the previously proposed design of similar design complexity. The discharging process of the designed latent heat thermal energy storage (LHTES) was evaluated for two different flow rates. The PCM inside the capsules and heat transfer fluid (HTF) temperature, as well as the HTF flow rate, were measured. The lumped parameter numerical model was developed and validated successfully. The advantage of the proposed model is its computationa...
International Journal of Heat and Mass Transfer, 2014
Thermal analysis of high temperature phase change materials (PCM) is conducted with the consideration of a 20% void and buoyancy-driven convection in a stainless steel capsule. The effects of the thermal expansion and the volume expansion due to phase change on the energy storage and retrieval process are investigated. Sodium nitrate is considered as a potential PCM for concentrated solar power applications. The charging and discharging into and from the capsule wall is simulated for different boundary conditions and is applied with both laminar and turbulent flow conditions. Computational models are conducted by applying an enthalpy-porosity method and volume of fluid method (VOF) to calculate the transport phenomena within the PCM capsule, including an internal air void. Energy storage and retrieval in different sized capsules is simulated. A cylindrical shaped EPCM capsule or tube is considered in simulations using both gas (air) and liquid (Therminol/VP-1) as the heat transfer fluid in a cross flow arrangement. Additionally a spherical shaped EPCM is considered with a constant wall temperature boundary condition to study the three-dimensional heat transfer effects. The presence of the void has profound effects on the thermal response of the EPCM during both energy storage and retrieval process. Melting and solidification per unit mass of the PCM takes longer when the void is present. Additionally, due to material properties and the lack of convective effects, the solidification process is much slower than the melting process.
Latent heat storage with tubular-encapsulated phase change materials (PCMs)
Energy, 2014
Heat capture and storage is important in both solar energy projects and in the recovery of waste heat from industrial processes. Whereas heat capture will mostly rely on the use of a heat carrier, the high efficiency heat storage needs to combine sensible and latent heat storage with phase change materials (PCMs) to provide a high energy density storage. The present paper briefly reviews energy developments and storage techniques, with special emphasis on thermal energy storage and the use of PCM. It thereafter illustrates first results obtained when encapsulating NaNO 3 /KNO 3-PCM in an AISI 321 tube, as example of a storage application using a multi-tubular exchanger filled with PCM. To increase the effective thermal conductivity of the PCM, 2 inserts i.e. metallic foam and metallic sponge are also tested. Experimental discharging (cooling) rates are interpreted by both solving the unsteady-state conduction equation, and by using Comsol Multiphysics. Predictions and experimental temperature evolutions are in fair agreement, and the effect of the inserts is clearly reflected by the increased effective thermal conductivity of the insert-PCM composite. Application of Comsol to predict the mechanical behavior of the system, when melting and associated expansion increase the internal pressure, demonstrates that the pressure build-up is far below the Young's modulus of the AISI 321 encapsulation and that this shell will not crack.
International Journal of Energy Research, 2018
The capability of an encapsulated phase change material (EPCM)-based thermal energy storage (TES) system to store a large fraction of latent energy at high temperatures was examined. A 3-dimensional simulation of a prototype heat exchanger was conducted employing sodium nitrate as the phase change material (PCM). The k-ω SST model was used to capture the turbulent flow of the HTF, while the melting front was tracked using the enthalpy-porosity method. The results show that the use of metal deflectors yields a nearly constant heat transfer coefficient over the capsule's surface. Despite this, the presence of the void in the capsule and natural convection within the molten PCM influenced the storage characteristics of the system affecting the shape of the isotherms and melting front. Furthermore, the EPCM capsules consecutively undergo the same heat transfer starting from the capsule closest to the inlet. The EPCM capsules store 80% of the energy lost by the HTF. The 17.7 kg of sodium nitrate stores 14.5 MJ of energy where 20% of the energy stored is via latent heat. Of the energy released by the heat transfer fluid, 80% was absorbed by the EPCM capsules with the remaining energy going into the test section walls. A total of 14.5 MJ of energy was stored by the 17.7 kg of NaNO 3 , of which 20% is attributed to the latent heat. The fraction of energy stored as latent heat would be larger if a smaller operating temperature range was used. Thus, an EPCM-based latent heat TES system is capable of storing a large fraction of the supplied energy and presents efficient means of storing thermal energy for hightemperature applications. Additionally, the strong agreement between the numerical and experimental works demonstrates that the numerical methods employed can predict the behavior of an EPCM capsule not only within a single capsule but on the system scale as well. Therefore, the applied numerical methods can be used for further design and optimization of EPCM-based latent heat TES systems.
Renewable Energy, 2015
The effect of an internal air void on the heat transfer phenomenon within encapsulated phase change material (EPCM) is examined. Heat transfer simulations are conducted on a two dimensional cylindrical capsule using sodium nitrate as the high temperature phase change material (PCM). The effects of thermal expansion of the PCM and the buoyancy driven convection within the fluid media are considered in the present thermal analysis. The melting time of three different initial locations of an internal 20% air void within the EPCM capsule are compared. Latent heat is stored within an EPCM capsule, in addition to sensible heat storage. In general, the solid/liquid interface propagates radially inward during the melting process. The shape of the solid liquid interface as well as the rate at which it moves is affected by the location of the internal air void. The case of an initial void located at the center of the EPCM capsule has the highest heat transfer rate and thus fastest melting time. An EPCM capsule with a void located at the top has the longest melting time. Since the inclusion of a void space is necessary to accommodate the thermal expansion of a PCM upon melting, understanding its effect on the heat transfer within an EPCM capsule is necessary.
Journal of Energy Storage
This work presents the design, experimental and numerical results related to a pilot-scale one-tank (thermocline) thermal energy storage (TES) combining latent and sensible heat storage materials with synthetic oil as heat transfer fluid (HTF). The layer of phase change material (PCM) is used to enhance the thermal performance of the TES. Alumina spheres are used as sensible heat storage materials, while the PCM is NaNO 3 (sodium nitrate). The PCM is encapsulated inside 140 stainless steel tubes. The volume of the PCM represents 5.5% of the storage volume. NaNO 3 was found safe to use with the synthetic oil at the designated operating conditions in case of leaks in the tank. The influence of the phase change material on HTF temperatures were observed experimentally during charge, discharge and stand-by. A numerical model that couples two one-dimensional (1D) physical methods is developed. It simulates natural convection within the PCM capsules by modifying the thermal conductivity of the material. The enthalpy porosity method is applied to simulate the phase changing behavior and a single equation estimates the liquid fraction of the PCM at each time step. The model is validated from the experimental results. This validation reveals that there is still a high fraction of unsolidified PCM in the tubes during discharge, which indicates that the performance of the combined TES solution is limited by heat transfer within the encapsulation tubes.
Heat transfer analysis of encapsulated phase change materials
Applied Thermal Engineering, 2013
h i g h l i g h t s < Heat transfer analysis is conducted for encapsulated phase change materials. < This thermal energy storage is applicable for concentrated solar power systems. < Zinc and mixture of NaCl and MgCl 2 salts are used as phase change materials. < Nickel and stainless steel are used as encapsulation materials. < Energy storage into capsules is predicted for gas and liquid heat transfer fluids. a b s t r a c t Solar energy is receiving a lot of attention recently since it is a clean, renewable, and sustainable energy. Solar energy is used for space heating, power generation and other applications. A major limitation however is that it is available for only about 2000 h a year in many places. Therefore it is critical to find ways to store solar thermal energy for the off hours. Sensible heat of material has been used for storing thermal energy but due to material properties this type of thermal storage has limitations. Using encapsulated phase change materials is potentially a better way to store thermal energy with the associated reversible heat transfer. The present work deals with certain aspects of storing solar thermal energy in high temperature phase change materials with melting points above 400 C. The objective is the storage of large amounts of solar energy (w600 MWh). Two kinds of encapsulated capsules are considered; zinc encapsulated in nickel and eutectic salt mixtures (57 mol% NaCl and 43 mol% MgCl 2 ) in stainless steel encapsulation. Diffusion and phase change computations are reported here in the form of temperature profiles of the phase changing and encapsulated materials for spherical capsules. The time for heating and melting during charging (storage of thermal energy into capsulated phase change material) and the time for cooling and solidification during discharging (retrieval of thermal energy) are presented for both zincenickel and saltestainless steel systems. As per expectations, the time for heat transfer is much shorter for liquid heat transfer media compared to those for gases. Moreover, the heat transfer times are shorter with smaller sizes of capsules.
Thermal Performance of the Thermal Storage Energy with Phase Change Material
Acta Mechanica et Automatica
Values of energy supply and demand vary within the same timeframe and are not equal. Consequently, to minimise the amount of energy wasted, there is a need to use various types of energy storing systems. Recently, one can observe a trend in which phase change materials (PCM) have gained popularity as materials that can store an excess of heat energy. In this research, the authors analysed paraffin wax (cheese wax)’s capability as a PCM energy storing material for a low temperature energy-storage device. Due to the relatively low thermal conductivity of wax, the authors also analysed open-cell ceramic Al2O3/SiC composite foams’ (in which the PCM was dispersed) influence on heat exchange process. Thermal analysis on paraffin wax was performed, determining its specific heat in liquid and solid state, latent heat (LH) of melting, melting temperature and thermal conductivity. Thermal tests were also performed on thermal energy container (with built-in PCM and ceramic foams) for transient...