Phase change material (PCM) based thermal energy storage (TES) systems are gaining increasing importance in recent years in order to reduce the gap between energy supply and demand in solar thermal applications. The present work investigates the effect of PCM capsule material on the performance of TES system during charging and discharging processes. The TES unit contains paraffin as PCM filled in spherical capsules and is integrated with flat plate solar collector. Water is used as sensible heat material as well as heat transfer fluid (HTF). The PCM capsules are of 68 mm diameter and are made using three different materials, namely, (i) high density polyethylene (HDPE), (ii) aluminum (Al), and (iii) mild steel (MS). The experimental investigation showed that the charging and recovery of stored energy are less affected by the spherical capsules material. The variables, like charging time and discharging quantity, are varied around 5% for the different capsule materials. Even though aluminum thermal conductivity is much higher than HDPE and mild steel, its influence on the performance of TES system is very low due to the very high internal heat resistance of PCM material stored in the spherical capsules.
Solar energy is the most promising inexhaustible heat energy source for the present and future needs of mankind. Compared to the power generation from the solar energy, utilization of solar energy for moderate temperature heat applications is more efficient and economical. The increasing cost of fossil fuels in the recent years is making solar energy utilization more economical for heating applications. One of the major problems with the solar energy is its intermittent nature. So to balance the energy supply and demands, a heat energy storage system is necessary. There are three main methods of thermal energy storing systems, that is, sensible, latent, and combined sensible and latent heat storage systems. The thermal energy storage systems using both sensible and latent heat storage methods are gaining a lot of importance now a days, due to their high thermal energy storage capacity per unit volume and isothermal behavior during charging and discharging processes.
In this direction, a lot of research is going on throughout the world for improving the performance of TES systems in the recent years. Some of the important contributions related to TES system using sensible and latent heat are presented. Weislogel and Chung [
The objective of the present work is to predict the best material for spherical capsules among three different materials (HDPE, Al, and MS) for better efficiency of sensible and latent heat thermal energy storage unit integrated with varying (solar) heat source. Parametric studies are carried out to examine the effects of the material and HTF flow rates on the performance of the storage unit for varying inlet fluid temperatures. For both the energy storage and recovery processes, water is used as the heat transfer fluid.
Figures
Schematic of experimental setup (1) solar flat plate collector, (2) pump, (3) and (4) flow control valves, (5) flow meter, (6) TES tank, (7) PCM capsules, and (8) temperature indicator,
Photographic view of TES tank coupled with solar collector.
A flow meter with an accuracy of ±2% is used to measure the flow rate of HTF and a centrifugal pump (500 lit/hour) is employed to circulate the HTF through the storage tank.
The performance of the charging of TES is studied using 2 lit/min, 4 lit/min, and 6 lit/min flow rates with varying inlet HTF temperatures. Initially, the energy is stored inside the capsules as sensible heat until the PCM reaches its melting temperature. As the charging process proceeds, energy storage is achieved by melting the PCM at a constant temperature. Finally, the PCM becomes superheated. The energy is then stored as sensible heat in liquid PCM. Temperatures of the PCM and HTF are recorded at an interval of 12 minutes. The charging process is continued until the PCM temperature reaches the value of 70°C.
Batchwise discharging of TES is studied with different discharge flow rates, that is, 2 lit/min, 4 lit/min, and 6 lit/min, keeping the constant cold water inlet, that is, 2 lit/min and 30°C. A certain quantity of hot water (20 lit) is withdrawn from TES tank and the tank is again filled with cold water of quantity equal to the amount of water withdrawn. Again after a time interval of 20 minutes allowing transfer of energy from PCM to HTF, another 20 lit of water is withdrawn from the TES tank. This process is continued until the water (HTF) outlet temperature reaches 34°C.
The temperature distributions of HTF and the PCM in the storage tank for different mass flow rates and different materials of capsules are recorded during charging and discharging processes. Table
Thermophysical properties of PCM.
Paraffin wax type II* | |
---|---|
Melting temperature (°C) | 61 |
Latent heat of fusion (kJ/kg) | 213 |
Density (kg/m3) solid | 861 |
Density (kg/m3) liquid | 778 |
Specific heat (J/kg °C) solid | 1850 |
Specific heat (J/kg °C) liquid | 2384 |
Thermal conductivity (W/m °C) solid | 0.40 |
Thermal conductivity (W/m °C) liquid | 0.15 |
The charging experiments are conducted for the combination of various parameters of mass flow rates, various materials of the spherical capsules, and HTF inlet temperature (Figure
Photograph view of different materials of spherical capsules.
Figure
Temperature histories during charging process (
Figure
Figure
Effect of mass flow rate of HTF on charging time for varying HTF inlet temperature.
Figure
Effect of PCM spherical capsule materials on charging time for varying HTF inlet temperature.
The graphs show that the temperature difference between high thermal conductivity aluminium capsules and low thermal conductivity HDPE capsules is very low because the overall heat transfer coefficient between HTF and PCM is much influenced by the thermal conductivity of the PCM and diameter of the PCM spherical capsule but not by the thermal conductivity of PCM spherical capsule material. In the process of heat transfer from HTF to PCM, the thickness of the PCM capsule material (around 1.0 mm) influences heat transfer coefficient only by around 5%, because more thickness of the low thermal conductivity PCM within the capsule of diameter 68 mm has lot of influence on the heat transfer rate. With these results, we can conclude that the change of low cost HDPE PCM capsule material to high cost aluminium/MS PCM capsule material is not improving the performance of TES tank. Even though the thermal conductivity of aluminium is very high compared to HDPE/MS, because of the very low thickness of the capsule material, it only improves the heat transfer rate of the inner adjacent layer of PCM material, not the overall heat transfer rate.
The discharging experiments are carried out by batchwise method. This method of discharge permits the complete utilization of heat in the storage tank. In the case of batchwise discharging process, a certain quantity (20 lit) of hot water is withdrawn from the storage tank and the same amount of cold water is filled in the storage tank. Withdrawn hot water is stored in the bucket having capacity of 20 lit and the average temperature of the hot water in the bucket is measured. The optimum retention period is 20 minutes between batches. The optimum retention time 20 minutes between batches is arrived by conducting a number of experiments (with different retention times like 10, 15, 20, 25, and 30 minutes). The batches of withdrawn hot water are continuous till the outlet temperature reaches 34°C. The average temperature of the total withdrawn hot water is approximately 45 ± 2°C.
Figure
Variation of output (lit) for different flow rates for 68 mm diameter spherical capsule.
Figure
Variation of output (lit) for different spherical capsule materials.
A thermal energy storage system has been developed for the use of hot water at an average temperature of 45°C for domestic applications using combined sensible and latent heat storage concept. Charging experiments are conducted on the TES unit to study its performance by integrating it with varying (solar) heat source. The temperature histories of HTF and PCM are studied during charging the process for paraffin (type II). Mass flow rate has significant effect on charging time. It is seen from the figure that the charging time is decreased by 24% when the mass flow rate is increased from 2 to 6 kg/min.
Also the investigation of the effect of PCM capsule material on the performance of TES tank while charging and discharging for variable (solar) heat source concludes that there is no appreciable performance improvement in the system by changing the capsule materials varying from low thermal conductivity (HDPE = 0.52 w/m°C) to very high thermal conductivity (aluminium = 240 w/m°C).
The authors declare that there is no conflict of interests regarding the publication of this paper.