Experimental and Numerical Investigation of Macroencapsulated Phase Change Materials for Thermal Energy Storage

Abstract

Among the different types of phase change materials, paraffin is known to be the most widely used type due to its advantages. However, paraffin's low thermal conductivity, its limited operating temperature range, and leakage and stabilization problems are the main barriers to its use in applications. In this research, a thermal energy storage unit (TESU) was designed using a cylindrical macroencapsulation technique to minimize these problems. Experimental and numerical analyses of the storage unit using a tubular heat exchanger were carried out. The Ansys 18.2-Fluent software was used for the numerical analysis. Two types of paraffins with different thermophysical properties were used in the TESU, including both encapsulated and non-encapsulated forms, and their thermal energy storage performances were compared. The influence of the heat transfer fluid (HTF) inlet conditions on the charging performance (melting) was investigated. The findings demonstrated that the heat transfer rate is highly influenced by the HTF intake temperature. When the effect of paraffin encapsulation on heat transfer was examined, a significant decrease in the total melting time was observed as the heat transfer surface and thermal conductivity increased. Therefore, the energy stored simultaneously increased by 60.5% with the encapsulation of paraffin-1 (melting temperature range of 52.9-60.4 °C) and by 50.7% with the encapsulation of paraffin-2 (melting temperature range of 32.2-46.1 °C), thus increasing the charging rate.

Keywords: aluminum encapsulation; heat exchanger; heat storage; paraffin; phase change material.

Figure 1

A general overview of the experimental setup.

Figure 2

(a) Drain tap mounted on TESU; (b) draining process of melted PCM.

Figure 3

Locations of the thermocouples in the PCM along (a) the XZ-axis and (b) the XY-axis.

Figure 4

Phase change processes in TESU: (a) melting of P2; (b) solidification of P2; (c) melting of EP2; and (d) solidification of EP2.

Figure 5

(a) Thermal energy storage unit and heat exchanger; (b) phase change material and heat transfer fluid; and (c) numerical model of the numerically analyzed thermal energy storage unit.

Figure 6

Variation in total PCM melting time depends on number of meshes.

Figure 7

Mesh structure of the thermal energy storage unit.

Figure 8

Drawings of PCM capsules: (a) PCM with nine capsules; (b) numerical model of the encapsulated PCM; and (c) mesh structure of the encapsulated PCM.

Figure 9

Variation in total encapsulated PCM melting time depends on number of meshes.

Figure 10

Comparison of numerical analysis results and experimental results of (a) P1 and (b) EP1.

Figure 11

Temperature and liquid fraction contours of P1 during the melting (charging) process.

Figure 12

Temperature and liquid fraction contours of P2 during the melting (charging) process.

Figure 13

Liquid fraction and temperature contours of EP1 during the melting (charging) process.

Figure 13

Liquid fraction and temperature contours of EP1 during the melting (charging) process.

Figure 14

Liquid fraction and temperature contours of EP2 during the melting (charging) process.

Figure 14

Liquid fraction and temperature contours of EP2 during the melting (charging) process.

Figure 15

Time-dependent temperature variations on the same plane during melting (P1, $T_{HTF}$ = 80 °C ve ${\dot{Q}}_{HTF}$ = 0.0333 lt/s).

Figure 16

Time-dependent axial and radial temperature variations for (a) the X-axis, (b) the Y-axis, and (c) the Z-axis during melting (P1, $T_{HTF}$ = 80 °C ve ${\dot{Q}}_{ITA}$ = 0.0333 lt/s).

Figure 17

Effect of HTF temperature on the charging performance (P1 ve ${\dot{Q}}_{ITA}$ = 0.0333 lt/s).

Figure 18

Effect of the HTF flow rate on the charging performance of the TESU (P2 ve $T_{HTF}$ = 60 °C): (a) temperature–time and (b) flow–time.

Figure 19

Effect of PCM encapsulation on the charging performance of the TESU ($T_{HTF}$ = 70 °C ${\dot{Q}}_{ITA}$ = 0.0666 lt/s).

Figure 20

The total energy stored at the same time for P1 and EP1 ($T_{HTF}$ = 70 °C ve ${\dot{Q}}_{ITA}$ = 0.0666 lt/s).

Figure 21

The total energy stored at the same time for P2 and EP2 ($T_{HTF}$ = 70 °C and ${\dot{Q}}_{ITA}$= 0.0666 lt/s).