Effective Thermal Conductivity Of Phase Change Material Research Articles

An experimental study of employing organic phase change material for thermal management of metal hydride hydrogen storage

This paper presents the details of an experimental study of using a phase change material (PCM), made of 4.56 kg RT28HC paraffin wax, for self-contained thermal management of an 800 NL metal hydride (MH) hydrogen canister (MmNiMnCo). Thermal behaviour and charging/discharging performances of this innovative arrangement were investigated by considering the impacts of the operating parameters including hydrogen pressure, hydrogen flow rates, and effective thermal conductivity of PCM. Introducing PCM for thermal management of MH offered a possibility to store the heat released during hydrogen charging mode and transfer it back to the canister during discharging. The test results suggest that the total amount of hydrogen being utilised, while MH absorption/release rates being maintained at desired levels, is improved by two times after employing PCM for its thermal management. However, the maximum accumulative amount of hydrogen absorbed/released by the MH was still only up to 26 % of its total storage capacity with hydrogen charging rate dropped gradually after being started from 1.1 slpm. Recognising the low thermally conductivity of the PCM as the root cause of this shortfall, 10 PPI copper metal foam (MF) with 96 % porosity was embedded in the PCM. By introducing this MF, the full capacity of the MH canister could be utilised at continuous charging and discharging rates of 1.1 slpm and 1.5 slpm, respectively.

Thermal energy storage characteristics of packed bed encapsulating spherical capsules with composite phase change materials

• Experiments were conducted on a solar heating system with a PBTESD. • The PA/EG/CF composite phase change material were prepared. • Models were developed for the dynamic processes of the packed bed. • Temperature field and dimensionless parameter analysis was conducted. Packed bed thermal energy storage technology is one of the valid methods to coordinate the balance of energy sources supply and demand and settle the matter of the time and space mismatching of renewable energy in the future. In this paper, the thermal energy storage characteristics of a packed bed thermal energy storage device (PBTESD) filled with spherical phase change capsules are analyzed. The PA/EG/CF composite phase change material (CPCM) was used as an encapsulation material, and water was used as heat transfer fluid (HTF). Based on Schumann’s model, a one-dimensional two-energy equation two-phase mathematical model of PBTESD was established. The heat transfer resistance between the spherical phase change capsules and HTF and the heat loss between the heat storage tank and the surrounding environment were considered in the model. MATLAB was used for numerical simulation calculation of the equation, and the results obtained were compared with the experimental results and literature results. The effects of Reynolds number, Stefan number, dimensionless diameter and effective thermal conductivity of phase change material (PCM) on the temperature evolution trend and thermal performance of PBTESD were investigated by single factor analysis. The results showed that the increase of Stefan number leaded to the increase of thermal energy storage density in the charging process. Increasing Reynolds number and Stefan number and decreasing dimensionless diameter increased the average energy storage rate and average exergy storage rate. The reduction of Stefan number and dimensionless diameter could improve exergy efficiency, while the effective thermal conductivity of PCM had little effect on the thermal characteristics of PBTESD. This paper elaborated on the transient thermal characteristics of PBTESD, which laid a foundation for subsequent research.

Improvement of Thermal Conductivity and Energy Stored by Paraffin with Different Metallic Additives

Phase change thermal storage is an innovative and promising technology for saving energy. It is one of the new areas of research because it provides the solution to problems related between the provided and the required energies. Paraffin is a common phase change material (PCM) that used in many applications in thermal energy storage (TES) systems. However, the main disadvantage is their low thermal conductivities. However, using metallic additives to improve effective thermal conductivity of PCM can lead to decreasing effective heat capacity and the thermal energy stored. An experimental study is carried out to analyze the thermal behavior of the paraffin melting in a thermal cavity integrating different metals (zamak, aluminum and copper) with different configuration. The originality of study is to try to predict the best duo that respects both the improvement of thermal conductivity and energy stored. The experiments show that adding aluminum perforated plate in paraffin accelerates the melting process by about 19% and increases the energy stored by 5.18%.

Experimental and numerical study of latent heat thermal energy storage with high porosity metal matrix under intermittent heat loads

The potential mismatch between the ever-increasing demand for usable energy and characteristically intermittent supply from the renewable energy sources can be effectively reduced by latent heat thermal energy storage system (LHTES) using phase change material (PCM). The low thermal conductivity of PCM leads to thermal stratification in LHTES. In the present study, a lab-scale shell and tube LHTES with the metal matrix as thermal conductivity enhancer is designed for medium temperature solar applications (~200 °C) and the thermal performance of LHTES is evaluated for different operating conditions. A commercial-grade organic PCM and metal matrix are placed in the annulus, while a commercially available thermic oil used as a heat transfer fluid (HTF), flows through the finned internal tube. A simplified dynamic numerical model comprising of two energy equations is developed considering the absence of local thermal equilibrium between PCM and HTF. A newly developed correlation for interfacial heat transfer coefficient and the effective thermal conductivity of PCM and metal matrix are introduced in the model, which is validated against the experiment. The model is extended to study the effect of porosity of metal matrix on the dynamic performance of LHTES under several cycles of charging and discharging operations. It is found that at a porosity of 0.85, the fluctuation in HTF outlet temperature is less with an improvement in cumulative energy fraction. A significant saving of computational time can be achieved for large scale simulations of LHTES with metal matrix using the simplified dynamic model with reasonable accuracy.

Numerical analysis for maximizing effective energy storage capacity of thermal energy storage systems by enhancing heat transfer in PCM

Stratified water storage (SWS) is of high energy efficiency, but of low energy density as it relies on the sensible heat of water. Latent heat thermal energy storage (LHTES) system using phase change material (PCM) is of higher energy density, and might be an alternative to the bulky SWS system. However, the utilization of latent heat in PCM has been hindered by the low thermal conductivity of PCM. Previous research has demonstrated that high thermal conductivity components, such as metal foam, can be used to enhance the effective thermal conductivity of PCM, keff. In this work, we proposed an index of effective energy storage ratio, Est, to characterize the effective energy storage capacity of an LHTES system with reference to an ideal SWS system of the same volume. A conjugate numerical fluid flow and heat transfer method was first validated and then used to characterize the Est of a tube-in-tank design. The effect of keff on Est was demonstrated. It is shown that there is an optimum compactness factor (CF) for PCM at any finite keff, and that the optimum CF and the corresponding maximum Est are higher for PCM at a higher keff. It is also shown that, even at the optimal CF for PCM at any keff, quite a percentage of the theoretical energy storage capacity is not utilized, indicating that heat transfer enhancement in the fluid side may further improve the Est. This analysis provides implications on an optimal design of LHTES systems.

Thermal management analysis of a Li-ion battery cell using phase change material loaded with carbon fibers

High latent heat of PCMs (phase change materials) has made them as one of the most important materials for thermal management purposes. However, PCMs’ low thermal diffusivities could limit their use in applications which require fast thermal response. The goal of this study is to simulate thermal performance of a lithium ion battery cell in the presence of carbon fiber-PCM composites. The effect of carbon fiber loading within the PCM on thermal performance is studied and the results are compared with the experimental data. The results showed that the presence of carbon fibers increases the effective thermal conductivity of PCM and hence influences temperature distribution within the cell. PCM composites containing higher percentages of carbon fibers present a more uniform temperature distribution. The results showed that the minimum and maximum thermal conductivity enhancement of 85% and 155% respectively (105% on average). A reasonable agreement is obtained between the simulation results and the experimental data.

THERMAL ENERGY STORAGE FOR CONCENTRATING SOLAR POWER PLANTS

Thermal energy storage for concentrating solar thermal power (CSP) plants can help in overcoming the intermittency of the solar resource and also reduce the levelized cost of energy (LCOE) by utilizing the power block for extended periods of time. In general, heat can be stored in the form of sensible heat, latent heat, and thermochemical reactions. This article describes the development of a costeffective latent heat storage TES at the University of South Florida (USF). Latent heat storage systems have higher energy density compared to sensible heat storage systems. However, most phase change materials (PCMs) have low thermal conductivity that leads to slow charging and discharging rates. The effective thermal conductivity of PCMs can be improved by forming small macrocapsules of PCM and enhancing convective heat transfer by submerging them in a liquid. A novel encapsulation procedure for high-temperature PCMs that can be used for thermal energy storage (TES) systems in CSP plants is being developed at USF. When incorporated in a TES system, these PCMs can reduce the system costs to much lower rates than currently used systems. Economical encapsulation is achieved by using a novel electroless deposition technique. Preliminary results are presented and the factors that are being considered for process optimization are discussed.