Numerical investigation of a packed bed thermal energy storage system for solar cooking using encapsulated phase change material (original) (raw)
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In this paper, thermal performance of various phase change materials (PCMs) used as thermal energy storage in a solar cooker has been investigated numerically. Heat conduction equations in cylindrical domain are used to model heat transfer of the PCMs. Mathematical model of phase change problem in the PCM storage encompasses heat conduction equations in solid and liquid region separated by moving solid-liquid interface. The phase change problem is solved by reformulating heat conduction equations with emergence of moving boundary into an enthalpy equation. Numerical solution of the enthalpy equation is obtained by implementing Godunov method and verified by analytical solution of one-dimensional case. Stability condition of the numerical scheme is also discussed. Thermal performance of various PCMs is evaluated via the stored energy and temperature history. The simulation results show that phase change material with the best thermal performance during the first 2.5 hours of energy extraction is shown by erythritol. Moreover, magnesium chloride hexahydrate can maintain temperature of the PCM storage in the range of 110-116.7°C for more than 4 hours while magnesium nitrate hexahydrate is effective only for one hour with the PCM storage temperature around 121-128°C. Among the PCMs that have been tested, it is only erythritol that can cook 10 kg of the loaded water until it reaches 100°C for about 3.5 hours.
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.
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.
Numerical modeling of thermal energy storage system
Thermal energy storage in the form of latent heat of fusion of phase change material gained considerable attention in solar energy applications since it significantly increases the energy density and reduces the storage tank size compared to the sensible heat storage system. Several numerical and experimental studies have been conducted to enhance the performance of the system. In this study, 2-D continuous solid phase and effective packed bed models are developed to study the behavior and performance of a thermal energy storage system for high temperature applications, which is composed of spherical capsules encapsulated by phase change material (Sodium nitrate) and high temperature synthetic oil (Therminol 66) as heat transfer fluid. Temperature distribution, fluid flow, melting, solidification and thermocline behavior of the system are predicted and the influence of capsule size on the performance of the system is studied.
Journal of Zhejiang University Science, 2006
In thermal systems such as solar thermal and waste heat recovery systems, the available energy supply does not usually coincide in time with the process demand. Hence some form of thermal energy storage (TES) is necessary for the most effective utilization of the energy source. This study deals with the experimental evaluation of thermal performance of a packed bed latent heat TES unit integrated with solar flat plate collector. The TES unit contains paraffin as phase change material (PCM) filled in spherical capsules, which are packed in an insulated cylindrical storage tank. The water used as heat transfer fluid (HTF) to transfer heat from the solar collector to the storage tank also acts as sensible heat storage material. Charging experiments were carried out at varying inlet fluid temperatures to examine the effects of porosity and HTF flow rate on the storage unit performance. The performance parameters such as instantaneous heat stored, cumulative heat stored, charging rate and system efficiency are studied. Discharging experiments were carried out by both continuous and batchwise processes to recover the stored heat, and the results are presented.
Modeling and Numerical Simulation of Solar Cooker with PCM as Thermal Energy Storage
The 3rd International Conference on Information and Communication Technology (ICoICT )
In this article, mathematical modeling and numerical simulation of phase change materials (PCMs) used as latent heat thermal energy storage in a solar cooker have been conducted. The PCM to store thermal energy is packed in many small hollow cylinders and placed in a larger cylinder tank. Heat transfer fluid (HTF) which flows parallel to the PCM cylinder is used to distribute heat from the solar collector to the PCM storage unit and vice versa. A mathematical model describing the behaviour of temperature in the PCM and HTF is used. Numerical solutions are obtained by transforming heat conduction equations of the PCM and HTF into enthalpy equation and solving it by using the Godunov method. Thermal performance during charging and discharging process of several selected PCMs is investigated. The simulation results showed that magnesium nitrate hexahydrate has the highest capacity to store solar thermal energy whereas erythritol can achieve the highest temperature history during charging time and at the first 54 minutes of discharging time. The results provide an important information to design a solar cooker prototype equipped with thermal energy storage that has a good thermal performance.