Bed Of Phase Change Material Research Articles

An experimental investigation of porosity gradient and staggered arrangements in packed bed thermal energy storage

AbstractOwing to the intermittent nature of solar energy, the thermal energy from the sun or waste heat can be stored in the form of latent heat in a packed bed of phase change material (PCM). In this article, PCM‐based latent heat thermal energy storage is experimentally investigated, and the effects of porosity and specific surface area (ratio of surface area to the volume of the encapsulation) on the charging and discharging times are evaluated in packed bed thermal energy storage. Locally available paraffin wax was utilized as PCM. Differential scanning calorimetry (DSC) was used to investigate the thermal properties of paraffin wax. Enthalpy of phase change, melting point, and specific heat capacity of the paraffin wax were obtained by using DSC. Experimental results revealed that there was a 17% decrease in the charging time when porosity was reduced from 0.568 to 0.485, and the corresponding surface area was 2.581 and 3.4546 m2/m3, respectively.

Cooking performance assessment of a phase change material integrated hot box cooker.

Previous research on solar box cookers focusing on the bulk usage of energy storage materials is a costly technique for performance enhancement. Bulk energy storage materials take much time to charge and, thus, result in a low rate of cooking at the start. Therefore, a hot box solar cooker has been developed and experimentally studied for thermal performance enhancement in a hilly region of Uttarakhand, India. A bed of phase change material (paraffin wax) filled with small capsule-shaped containers was prepared (detachable) and placed over the cooking tray of the tested cooker. These containers were vertically positioned over the bed to enhance the heat transfer rate inside the cooker to attain a fast-cooking response. Notably, the combined effect of extended geometry with PCM is an excellent method to increase the efficiency of a solar cooker. As per the author's knowledge, likely techniques have not been studied for a box cooker to achieve a fast-cooking rate in any hilly region up to date. The results of cooking tests show that the cooking plate attained a maximum temperature of about 150°C. It is because of the combined effect of extended fins (vertical capsules) and PCM filled inside them. The results of the experimental study show that the thermal efficiency of the cooker was found to be about 45.7%, the cooking power was calculated about 54.71W, the heat transfer coefficient was estimated about 311W/m2 °C, and the overall heat loss coefficient was computed about 5.71W/m2 °C. This modified cooker costs about $48.19, and the payback period is about 03years and 11months. Cooking trials also showed that the present SBC could cook almost all the dishes commonly cooked in Uttarakhand.

Numerical study on stratification performance of cascaded three-layered packed-bed in the thermal storage process

Packed-bed thermal storage tank (PBTST) has broad application prospects in solar water heating systems, thermoelectric decoupling in power plants, and industrial waste heat recovery. In the present work, a three-dimensional PBTST model with three-stage phase change material (PCM) and three-layered diameter capsules was proposed, its thermal energy storage behavior and thermal stratification performance were studied by numerical simulation. The effects of PCM bed height ratio, fluid flow rate, and inlet temperature on the stratification performance of the new tank were investigated. The results reported that due to the increased heat transfer area and expanded heat-transfer temperature difference: (1) Compared with the conventional non-cascaded single-layered diameter capsule PBTST, the charging efficiency of the cascaded three-layered diameter capsule PBTST increased from 24% to 61% at the end of charging; (2) the cascaded three-layered diameter capsule PBTST shows significantly improved stratification properties compared to the conventional ones, with the Richardson number increased from 20.78 to 49.47; (3) increasing the height of the bottom PCM bed, raising the inlet temperature, and reducing the flow rate all can improve the stratification performance.

Analytical modeling of conjugate heat transfer between a bed of phase change material and laminar convective flow

Latent heat of a phase change material (PCM) is commonly used for energy storage and thermal management. A flat bed configuration is commonly used in latent energy storage systems, in which, fluid flow over a PCM bed results in heat transfer to/from and melting/solidification of the PCM. Understanding the nature of heat transfer in such systems is critical for design and performance optimization. This paper presents an analytical model for conjugate heat transfer in a flat bed latent energy storage system. The model solves the governing energy conservation equations by utilizing the interfacial conditions to iterate between the PCM and fluid flow. Good convergence of the iterative technique is demonstrated. The model is used to study phase change propagation and energy storage as functions of various problem parameters such as imposed temperature difference, flow velocity and thermal properties of PCM and fluid flow. It is found that the convective heat transfer coefficient between the PCM and fluid flow is not constant, but, rather, is a function of space and time, and is affected by flow speed and properties, as well as thermal properties of the PCM. The general methodology presented here can be adapted for annular and other latent energy storage configurations. This work contributes towards a fundamental understanding of heat transfer processes in an important energy storage system that is commonly encountered in practical applications.

Cyclic behaviors of a novel design of a metal hydride reactor encircled by cascaded phase change materials

The integration of a phase change material (PCM) with a metal hydride (MH) reactor has received considerable attention recently. In such a system, the exothermic and endothermic processes of the MH reactor can be utilized effectively by enhancing the thermal exchange between the MH reactor and the PCM bed. In this study, a novel design that integrates the MH reactor with cascaded PCM beds is proposed. Magnesium nickel (Mg2Ni) alloy is used as the hydride reactor. Two different types of PCMs with different melting temperatures and enthalpies are arranged in series. A parametric study is carried out to identify the optimum distribution of the different PCMs. The results indicate that the proposed cascaded MH-PCM sandwich design improves the heat transfer rate which consequently shortens the time duration of the hydrogenation and dehydrogenation processes by 26% and 51%, respectively, compared to an MH-PCM sandwich design that includes only a single PCM.

Thermal performance analysis of a metal hydride reactor encircled by a phase change material sandwich bed

Phase change material (PCM) is integrated with a metal hydride (MH) storage reactor to enhance its performance. The main function of the PCM is to store the heat released by the MH reactor during the hydrogenation process and reuse it later during the dehydrogenation process. In such a system, the exothermic/endothermic reactions are highly dependent on the heat transfer rate between the MH reactor and the PCM bed. To increase the heat transfer rate, a new configuration of the MH-PCM storage system is proposed. The new configuration consists of an MH reactor encircled by a cylindrical sandwich bed packed with PCM. The results indicate that the proposed configuration of the MH-PCM system improved the heat transfer rate which accordingly reduced the time duration of the hydrogenation and dehydrogenation processes by 81.5% and 73%, respectively, compared to a conventional MH-PCM system that includes only a single PCM bed.

Study on phase change material thermal characteristics during air charging/discharging for energy saving of air-conditioner

In this study, RT-18 HC phase change material (PCM) having a melting point at 18 °C in a form of packed bed was used to reduce the power consumption of a 1 TR inverter air conditioner. A set of spherical balls was packed and integrated with the air conditioner for reducing the air temperature before entering the evaporator coil. The enthalpy method was developed for calculating the PCM temperatures and verified by testing results. The operating parameters such as bed thickness, air mass flow rate, inlet air temperature, operating time and properties of air and PCM during charging and discharging modes were also correlated in dimensionless forms. With the correlations, the charging and the discharging modes for three PCM bed thicknesses: 0.08 m, 0.16 m, and 0.24 m were evaluated, and it could be found that the results from the correlations agreed well with those from the experimental data within ±6% deviation. In addition, the electrical power consumptions of the air conditioner with these integrated PCM beds were 4.96 kWh/d, 4.7 kWh/d and 4.34 kWh/d compared with 5.04 kWh/d of the normal unit or 1.58, 6.80 and 13.84% of electrical energy could be saved, respectively.

Effect of varying extent of PCM capsule filling on thermal stratification performance of a storage tank

Among the different pathways of improving the energy storage and energy utilization in a thermal energy storage system, the formation of thermal stratification in hot water tanks is a promising technology. In this study, we developed a novel numerical model to assess the thermal stratification performance in a hot water tank due to addition of encapsulated phase change material (PCM) by varying the bed height, bed porosity and the encapsulation diameter. The formulation of the present numerical model is devoid of complicated momentum and energy equations. A set of simplified energy balance equations is developed to account for heat transfer between the heat transfer fluid (HTF) and the PCM considering local thermal non-equilibrium. The temperature profile at the outlet of the storage tank during charging process demonstrates an enhancement in the Richardson number by 58.3%, at the end of the charging process, upon doubling the PCM bed height, and the corresponding improvement in the charging efficiency is found to be 54.6%. The melting point of PCM plays a pivotal role on the extent of stratification and storage efficiency, as it dictates the amount of heat diffusion towards the bottom section of TES. During discharge phase, if the HTF flow rate is doubled from the nominal value of 2 L/min, the extraction efficiency is enhanced by ∼16.7%.

Thermal characteristics on melting/solidification of low temperature PCM balls packed bed with air charging/discharging

Melting and solidification phenomena of low-temperature phase change material (PCM) balls packed bed with air flowing during charging and discharging were tested and modeled by numerical enthalpy method. In this study, a commercial paraffin RT-42 (melting point 38°C-43 °C) contained in a set of celluloid balls each having 0.04 m diameter. The experiments were performed with various bed thicknesses, air mass flow rates, and inlet air temperatures. It could be found that the simulated results on the PCM temperature and the outlet air temperature of the PCM bed agreed well with those of the experimental data. With these data, correlations were developed in dimensionless terms which could be used to estimate the air temperature leaving the bed at any operating period and any PCM bed thickness for both melting and solidification processes.

Energy and economic analysis of a building air-conditioner with a phase change material (PCM)

In this study, a concept of using phase change material (PCM) for improving cooling efficiency of an air-conditioner had been presented under Thailand climate. Rubitherm20 (RT-20) was selected to evaluate the thermal performance by reducing the air temperature entering the evaporating coil. The model of PCM celluloid balls had been performed with the air-conditioner. For the experiment, 2 TR of R-134a air-conditioner was chosen to test a pack bed of PCM balls with thickness 40cm. The pressure drops of the air flowing through the bed were considered with and without a set of by-pass tubes along the height of the storage bed. The mathematical model of the air-conditioner with the PCM storage was developed and verified with the testing results. From the study results, it could be seen that pressure drops of the bed with and without bypass tubes were nearly the same results. Thus, PCM ball pack bed using RT-20 without bypass tubes was used to improve the cooling efficiency of the air-conditioner. The experimental result of the modified unit was compared and verified with the mathematical model, which agreed quite well with the simulation result. Finally, the model was used to analyze the economic result, which found that the electrical consumption of the modified air-conditioner could be decreased around 3.09kWh/d. The saving cost from the PCM bed could be 9.10% of 170.03USD/y and the payback period was around 4.15y.

Energy reduction of building air-conditioner with phase change material in Thailand

In this study, a concept of using phase change material (PCM) for improving cooling efficiency of an air-conditioner had been presented under Thai climate. Paraffin waxes melting point at around 20°C was selected to evaluate the thermal performance by reducing the air temperature entering the evaporating coil. The model of PCM celluloid balls had been performed with the air-conditioner. Moreover, the mathematical model of the air-conditioner with the PCM storage was developed and verified with the testing results. From the study results, it could be seen that the simulated data agreed quite well with the experimental result at the discrepant around 2–4%. Finally, the model was used to analyze the economic result which was found that the electrical consumption of the modified air-conditioner could be decreased 3.09kWh/d. The electrical power consumption of the modified unit was 36.27kWh/d at the operating time 15h/d compared with 39.36kWh/d of the normal unit at the operating time 12h/d. The saving cost of the PCM bed could be 9.10% or 170.03 USD and the payback period was 4.15y.

Thermal energy storage in a fluidized bed of PCM

The objective of the present work was to research the storage behavior of a fluidized bed filled with a granular phase change material (PCM) with a small particle diameter (dp¯=0.54mm). The performance of the fluidized bed was compared to that of well-known storage methods such as fluidized beds with sand and packed beds based of sand and PCM. For this purpose, heating experiments were conducted in a cylindrical bed with air as the working fluid.The influence of the bed height and flow rate on the storage and recovery efficiencies of the fluidized bed of PCM was analyzed. Additionally, the stability of the PCM during various charging–discharging cycles was studied.The results indicate that this PCM is an alternative material that can be used in fluidized bed systems to increase the efficiency of storing thermal energy in the form of latent heat. Under the experimental conditions tested in this study, higher charging efficiencies were observed for fixed and fluidized beds based on PCM than those of sand. High gas velocity and low bed height shorten the charging time but also reduce the charging efficiency. The cycling test shows that the PCM is stable under bubbling conditions up to 15 cycles, which corresponds to approximately 75h of continuous operation.