Simultaneous charging and discharging performance for a latent thermal energy storage system with a microencapsulated phase change material

The cold thermal energy storage (CTES) is of great significance for the sake of decline of fossil energy usage. However, there exhibits a low energy charging and discharging efficiency, due to the poor thermal conductivity of phase change material (PCM). In this context, the metal foam is utilized to improve the ice storage performance of the CTES device in this paper. The solidification performance of PCM in a horizontal shell-tube CTES system with fins is numerically studied with consideration of natural convection. The ice front evolutions and the dynamic temperature distribution for the case with metal foam is analyzed and compared with these without metal foam. Moreover, the influence of the porosity on the ice storage performance for the CTES system with metal foams is analyzed as well. The results imply that the addition of metal foam effectively improves the thermal performance of the CTES system, the distribution of the temperature in the shell-tube CTES system is more uniform and the freezing speed of PCM is faster due to coupled effects of thermal conduction enhancement and the suppression of the natural convection. The porosity of the metal foam plays an essential role in the ice storage performance of the CTES system. The smaller porosity of the metal foam induces faster freezing speed and more uniform temperature distribution. Therefore, a smaller porosity is recommended for faster energy discharging rate in practical engineering applications.

Role of metal foam on ice storage performance for a cold thermal energy storage (CTES) system

Solar energy is one of the unlimited sustainable energy resources that can be stored for different applications by using latent heat thermal energy storage systems. These systems utilize the phase change process (melting/solidification) of a phase change material (PCM) for the store and release energy. In the present study, an annulus with an inner ellipse as a thermal energy storage system was numerically investigated which the outer wall is considered as a isolation wall, and the inner wall is employed at a constant temperature. This study investigated the effects of various aspect ratios of the inner ellipse diameters (1, ¾, ½ and 1/3) as well as the angular position of the inner ellipse (90°, 60°, 45°, 30° and 0°) during both melting and solidification processes. The results show that the lowest aspect ratio (W = 1/3H) has the best performance during charging (minimum melting time); however, the aspect ratio has no considerable effect on the solidification process. On the study of the ellipse angle, the results reveal that lower melting time is achieved for a higher angle while the solidification time is not varied significantly. The melting time reduces by almost 61% and the efficiency enhances by 26% for the case of W = 1/3H with the angle of 90o compared with the case of aspect ratio=1 (circular case).

Numerical study of circular-elliptical double-pipe thermal energy storage systems

For the reduction of the electricity cost for cooling buildings in hot climate zones, thermal energy storage at low temperature using phase change materials is a potential solution. The storage medium can be cooled during periods of low electricity cost and then absorb the cooling load any time of the day. In this study, an experimental cold thermal energy storage system using organic phase change materials (A9 and A14) with melting temperature of 9 or 14 °C is investigated. The system is designed for real application conditions coupled with a heat pump which cools down the organic material through the circulation of water in the pipes and fan coil units which dissipate their thermal load in the initially cooled storage medium. It comprises of a thermal energy storage tank, inside which is positioned a staggered heat exchanger with finned tubes immersed in phase change material. The results from the experiments yielded shorter process duration (for both melting and solidification) and higher heat transfer rate when the feed rate was higher. Moreover, the temperature of the water entering and exiting the tank is more than 5 °C for A9 organic material for more than half the capacity of the tank thus, using A9, the fan coil units can operate efficiently for a long time. The heat transfer rate exceeds 5 kW for 32 and 24 min using A9 and A14 respectively and the thermal load that the tank can absorb from the fan coils is above 6 kWh per tank. During the heat pump operation, A9 results to a more constant water outlet temperature over the time compared to A14 which in addition is not affected significantly by the water flow rate. Moreover, the staggered heat exchanger transfers less than 10 kW of heat when the water outlet temperature ranges between 5 and 10°C, a power value which can easily be provided by a typical commercial heat pump.

Investigating the performance of a thermal energy storage unit with paraffin as phase change material, targeting buildings’ cooling needs: an experimental approach

Worldwide, the energy consumption of refrigeration systems increased by 50% in the last 20 years. Currently, active refrigeration systems are often used to maintain cold chains in industry. However, there are remarkable drawbacks in the operation of active systems such as susceptibility to blackouts in the power supply and vibrations during their operation. Therefore, to overcome the aforementioned problems, passive cold chain transport using latent thermal energy storage systems arose as a potential solution. However, these systems require long charging times due to the low thermal conductivity of most phase change materials. In that sense, this paper presents a novel design of a cold storage battery with metal foam enhanced phase change material. The peak efflux of energy and solidification time of the battery is correlated as a function of the inlet temperature and mass flow rate of the heat transfer fluid with a root mean square deviation of 11.4%. The solidification time prediction allows determining the geometry which results in the maximum efflux of energy density for a given energy density. Moreover, the cold battery is placed in an insulated container to analyse its performance during transport. Results show that the tested refrigeration battery can act as a standalone refrigeration system during 15 h. However, improvements in the design of the insulated container are suggested to increase the performance of the system along the discharging cycle.

Experimental analysis of a latent thermal energy storage system enhanced with metal foam

In this work, phase change material (PCM) is considered as thermal energy backup system for solar cold storage applications when there is peak power demand or power failure or no sun shines situations. A numerical study of solidification (charging) and melting (discharging) of PCM validated by experimental data is performed to explore the performance of a unique latent heat thermal energy storage (LHTES) system. The LHTES unit (PCM pack) occupied with PCM acts as a heat exchanger made up from evaporator tube along with rectangular metal fins which enhance the heat transfer during phase changing of PCM. Evaporator with 5, 8, 10, and 12 longitudinal aluminum fins, and without fin inside the PCM pack of six different thicknesses (4.5 cm, 5.0 cm 5.5 cm, 6.0 cm, 6.5 cm, and 7.0 cm) are considered to investigate the charging and discharging characteristics of PCM in our present study. To obtain the best performance from the PCM pack, heat transfer characteristics and temperature distributions of PCM, and the effect of variation of thickness of the PCM pack are studied through computational fluid dynamics (CFD) simulation. Solidification and melting model of ANSYS Fluent 15.0 package employing enthalpy-porosity technique is used to develop 3-D simulation models. Faster solidification, as well as a higher energy storage capacity and heat flux during melting is found for the PCM pack of 6.5 cm thickness with higher number of fins. A lab scale experimental set-up is fabricated to compare the present simulation results which shows reasonably well validation during both the charging and discharging.

Heat transfer enhancement of charging and discharging of phase change materials and size optimization of a latent thermal energy storage system for solar cold storage application

In hot climatic regions, some kind of cooling system is necessary to avoid warmth and humidity. Many of the available cooling systems are not economic and sustainable. In this study, sustainable and feasible space/room cooling systems have been experimentally analyzed. A solar operated cooling system with two options have been designed and their performances are compared. Phase Change Material (PCM) is proposed to store thermal energy instead of a costly battery. A 1200-watt compressor and fin-type condenser are used to construct the vapor compression system. When the incoming air is passed through the cooling coil, it gets cool. For this cooling coil, 50 feet copper tube is used. The front side copper tube diameter of the fan is 3/8 inch and the backside tube diameter is 1/2 inch. It took about 35 minutes and 5 minutes to minimize the room temperature at the desired level in the case of the stand fan and duct fan, respectively. Furthermore, the stand fan and duct fan systems reduced 3 ? and 6 ? of the outside temperature, respectively.

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Development and performance analysis of a PCM based cooling fan for hot climate of Bangladesh

This study is an experimental investigation of melting and solidification characteristics of phase change materials (PCM). In the present study, PCM is used as back-up providing energy storage in the form of latent heat in solar cold storage application when there is peak power demand or power failure or no sun shines situations. Heat transfer and temperature distribution of PCM during melting and solidification is analyzed to acquire the better performance of PCM pack. The PCM pack is a rectangular (100cm × 76cm × 5.5cm) container made of stainless steel sheet which contains the PCM like water, salt solution, etc. along with the evaporator coil. Heat transfer enhancement for both solidification and melting phenomena is observed by inserting 10 rectangular aluminum fins in a cross-wise direction with the evaporator coil. Results found a faster solidification and melting of 40kg of water. Compared to the evaporator coil arrangement without fin the solidification time is reduced by 60min when the arrangement is with 10 fins. Salt solution (Brine solution) of 10% NaCl by weight is used as PCM for experimentation which shows phase change during melting at varying temperature rather than at constant temperature like pure water. It takes higher time for solidification and melting than water. For low temperature application eutectic brine solution would be a better option of PCM to maintain constant temperature phase change.

Md. Abdul Aziz

Papers

Heat transfer enhancement of charging and discharging of phase change materials and size optimization of a latent thermal energy storage system for solar cold storage application

Development and performance analysis of a PCM based cooling fan for hot climate of Bangladesh

Experimental study of solidification and melting characteristics of thermal energy storage (TES) system