Experimental Study on Heat Transfer Enhancement of Phase Change Material using Embedded Oscillating Heat Pipe for Thermal Energy Storage

Abstract

Phase change materials (PCM) have great application potential in the field of thermal energy storage (TES). However, most PCM have low thermal conductivity, which limits the TES efficiency. In this paper, a novel method employing oscillating heat pipe (OHP) to enhance the heat transfer performance of latent heat TES system was investigated. OHPs with different turns, inner dimeters and wall thickness were designed, manufactured and tested. Thermal performance of TES system enhanced by embedded OHPs were tested. The influence of turn number, inner dimeters and wall thickness of OHPs on the heat transfer performance of paraffin were investigated. The results show that the prepared OHPs have high heat transfer performance. The time of the thermal storage process for the paraffin based TES coupled with OHP (Di=3 mm, Do=4 mm, 4 turns) is 38.45% shorter than that without OHP. Therefore, the OHPs can effectively improve the heat transfer rate inside paraffin in the thermal storage process.

1. Introduction

PCM can absorb or release a large amount of latent heat when undergoing phase transition, which could be utilized for TES application.¹⁾ Because of this characteristic, PCM plays an important role in energy conservation and emission reduction.^2,3) Up to now, PCM have showed great application potential in many fields, such as solar energy utilization,^4,5,6) intelligent building,^7,8,9) waste heat recovery,^10,11,12) thermal management^13,14,15) and so on. In particular, phase change materials have great application value in waste heat recovery. The waste heat produced by steelmaking plant has high temperature and large emissions. If this part of the waste heat is directly discharged into the environment, it will cause a lot of waste of energy. Therefore, the use of phase change materials to recycle and reuse this waste heat can effectively improve energy efficiency.¹⁶⁾

Most of the organic PCM have advantages of large latent heat, chemical stability, no phase separation, negligible supercooling and low cost,^17,18,19) which have been widely used in TES.²⁰⁾ However, the low thermal conductivity limits the heat transfer efficiency of the TES system.²¹⁾ Therefore, the heat transfer enhancement of organic PCM is of great significance for improving the TES efficiency.²²⁾

In recent years, various methods have been investigated in heat transfer enhancement of PCM, such as dispersing high thermal conductivity materials,^23,24,25) inserting porous materials^26,27) or inserting metal fins.^28,29) As metal powders and carbon materials are mainly used as high thermal conductivity materials for modification, they have also showed great effects on heat transfer enhancement of PCM.^30,31) However, the dispersed high thermal conductivity materials in PCM are easy to precipitate which is not stable for practical application. Although inserting porous materials or metal fins could avoid this problem, the size of the porous materials or metal fins needed to reach the target thermal conductivity is large, which could greatly reduce heat storage quantity of the system with certain volume.

Heat pipe is a highly effective passive device used for heat transfer, which makes full use of the principle of heat transfer and the rapid heat transfer properties of phase change media.^32,33) The heat pipe consists of a closed container, a wick structure and amount of a working fluid. The working fluid in heat pipe evaporates at the evaporation section and condenses at the condensation section.³⁴⁾ Driven by the heat source, the working fluid starts to evaporate at the evaporation section and flows to the condensation section. After cooling at the condensation section, the working fluid flows back to the evaporation section based on gravity or capillary force.³⁵⁾ Thereby the heat at the evaporation section is quickly transferred to the condensation section. Up to now, there have been few reports on heat pipes used for heat transfer enhancement in heat storage process.^36,37) The heat transfer enhancement properties of vertically-oriented heat pipes, solid rods, or hollow tubes for PCM were compared by Sharifi et al.³⁸⁾ Among all the experiments, the heat transfer enhancement for PCM carried by heat pipe is the most significant compared with either the rod or tube. Tiari et al.³⁹⁾ numerically investigated the effects of the finned heat pipes on heat transfer performance in charging process of the latent heat TES system. The results indicated that the finned heat pipes accelerated the charging process and reduced the total charging time by approximately 30%. Motahar et al.⁴⁰⁾ experimentally studied the effect of using a heat pipe on melting and solidification behavior of PCM. The results indicated a 15°C increased in reservoir temperature in melting experiment with heat pipe, which almost decreased the melting time by 53% and a 10°C decreased in temperature in solidification, also reduced the solidification time by 49%.

Oscillating heat pipe (OHP) is one type of heat pipe, which composed of turns of one continuous capillary tube without wick structure.³²⁾ The basic principle of OHP is that when the evaporation section of the heat pipe is heated, the liquid plug at the evaporation section will be pushed to the condensation section. Condensation at the condensation section will further increase the pressure difference at both sections. The movement of the liquid-vapor plug in the tube toward the condensation section also causes the next segment of the liquid-vapor plug to move toward the evaporating end. Thus the transfer of heat within the heat pipe is achieved. Compared with the traditional heat pipe, OHP has the advantages of simple structure, low cost, small volume, random bending. In this paper, the OHP was used in heat transfer enhancement of paraffin based TES. Considering the thermal performance of OHP could directly affect the heat transfer enhancement of TES, the OHP should reach the optimal thermal performance. Thermal performance of OHP is highly depended on many parameters such as size, turning number and tube diameter et al. For the specific utilization in TES, the OHP is set as a certain size, limited parameters could be varied to optimize the thermal performance of OHP. In this case, OHPs with different turn number, inner diameters (Di), outside diameter (Do) and wall thickness were designed, manufactured and tested. Then, the heat storage rate of paraffin coupled with OHPs were tested. The influence of turn number, inner dimeters and wall thickness of OHPs on the heat transfer performance of paraffin were investigated.

2. Experimental and Procedure

The injection-cleaning platform is shown in Fig. 1(a). The platform consists of a vacuum pump, a vacuum meter, a dryer, a steam water separator, an injection syringe and a three-way valve. The OHPs were fabricated by copper. The width and height of the OHP are 200 mm and 220 mm, respectively. The OHP (Di=3 mm, Do=4 mm) with three turns, four turns and five turns were prepared. Three turns OHPs with different inner and outer diameter (Di=2 mm, Do=3 mm and Di=2 mm, Do=4 mm) were prepared. Water was selected as the working fluid. The filing ratio of the OHP was 50±5%.

Fig. 1.

Schematic of the experimental setup. (Online version in color.)

The test diagram for heat transfer performance of the OHPs is shown in Fig. 1(b). It consists of a DC power supply, a fan, K-type thermocouples, heating wires and a data acquisition system. The heating wires were twined around the evaporation section of the OHPs and heated by the DC power supply. The condensation section of the OHPs was cooled by the fan. The heat transfer performance of the OHPs were tested under different heating power. Thermocouples were placed on the bottom (evaporation section) and top (condensation section) of the OHPs for temperature measurement. The temperature of the OHPs were recorded by the data acquisition system.

The system diagram of phase change TES system is shown in Fig. 1(c). The system includes a TES unit, a high-temperature water bath and a data acquisition system. The TES unit consists of a container, PCM, OHPs and K-type thermocouples. The container was made of acrylic plate with thickness of 6 mm, and its interior dimensions were 220 mm (length)×22 mm (width)×290 mm (height). The outside of the container was thermally insulated by adiabatic cotton with thickness of 10 mm in order to reduce heat loss. The OHP was vertically positioned in the middle position of the container. The container was filled with 1000 g of paraffin (The DSC curve of paraffin is shown in Fig. 2, and the melting temperature and latent heat are 56.91°C and 225.31 J/g, respectively). During heat storage process, the bottom of the container immersed in the water of the high-temperature water bath. The temperature of the high-temperature water bath was set to 90°C to heat the container. In the heat storage process, the temperature of the PCM in the container was recorded by the acquisition system.

Fig. 2.

DSC curve of the paraffin.

3. Results and Discussion

3.1. Start-up Characteristics of OHPs

3.1.1. Start-up Characteristics of OHPs with Different Turn Number

The temperature variations of vertically placed OHPs (Di=3 mm, Do=4 mm) with different turn number are shown in Figs. 3(a)–3(c). It can be seen that the temperature of evaporation and condensation sections of the OHPs is increased with the increasing of heating power. The temperature of the evaporation and condensation sections of the OHPs with three and four turns started-up when the heating power is 10 W, while the OHPs with five turns could not start-up until 20 W. The start-up temperature of the OHPs with three, four and five turns is about 44°C, 49°C and 54°C, respectively. The start-up temperature of the OHPs increased with increasing of turn number.

Fig. 3.

Temperature oscillation curves of the evaporation and condensation sections of the OHPs (Di=3 mm, Do=4 mm) with (a) three, (b) four, (c) five turns and (d) equivalent thermal conductivity. (Online version in color.)

The equivalent thermal conductivity is an important index to characterize the heat transfer performance of the OHPs. The equivalent thermal conductivity of the OHP can be calculated through the following formula:   

$$\lambda =\frac{QL}{A\left(T_e-T_c\right)}$$(1)

Where Q is the heat load, L is effective length of the OHPs, A is the area of the cross section of the OHPs, T_e is the average temperature of the evaporation section and T_c is the average temperature of the condensation section. The equivalent thermal conductivities of the OHPs with different turn number are shown in Fig. 3(d). It can be found that the equivalent thermal conductivities of the OHPs increased with increasing of heating power and decreasing of turn number. The reason for these phenomena probably is that more turns the OHPs have, greater flow resistance of working fluid would gain in the pipe. The thermal driven force of gas-liquid phase transition produced by evaporation section is not strong enough to make the gas liquid slug oscillate in the pulsating heat pipe when the temperature of evaporation section is low. Therefore, the three turns OHPs have better heat transfer performance than four or five turns under the similar working condition.

3.1.2. Start-up Characteristics of OHPs with Different Inner Dimeters and Wall Thickness

The temperature variation of vertically placed OHPs (three turns) with different inner dimeters and wall thickness are shown in Figs. 4(a)–4(c). As can be seen in Figs. 4(a) and 4(c), the temperature of the evaporation and condensation sections of the OHPs with Di=2 mm, Do=3 mm and Di=3 mm, Do=4 mm started-up when the heating power is 10 W, while the OHP with Di=2 mm, Do=4 mm did not start-up until 20 W. The start-up temperature of the OHPs (Di=2 mm) with Do=3 mm and Do=4 mm is about 48°C and 55°C, respectively. It can also be found that the temperature of evaporation and condensation sections of the OHPs is increased with the increasing of heating power. Figure 4(d) shows the equivalent thermal conductivities of the OHPs with different inner dimeters and wall thickness. It is obvious that the OHPs with Di=2 mm, Do=3 mm has higher equivalent thermal conductivity than others. The reason might be that the thickness of this group is the lowest comparing with others. Meanwhile, a much smaller Di could drive the liquid slug oscillating significantly.

Fig. 4.

Temperature oscillation curves of the evaporation and condensation sections and equivalent thermal conductivity of OHPs (three turns) with different inner dimeters and wall thickness: (a) Di=2 mm, Do=3 mm, (b) Di=2 mm, Do=4 mm, (c) Di=3 mm, Do=4 mm and (d) equivalent thermal conductivity. (Online version in color.)

3.2. The Heat Storage Performance of PCM Coupled with OHPs

3.2.1. The Heat Storage Performance of PCM/OHPs with Different Turn Number

The temperature variation curves of paraffin and paraffin/OHPs (Di=3 mm, Do=4 mm) with different turn number are shown in Fig. 5. As can be seen in Fig. 5(a), the temperature of the PCM rise rapidly due to the larger temperature difference between heating water and PCM at the beginning of heating stage. When the temperature reached the melting temperature, the PCM began to absorb a large amount of latent heat and the temperature rising rate went down. When the PCM melted completely, its temperature continued to rise until stable. It should be noticed that a temperature drop could be observed at about 8000–24000 s for pure paraffin case. This phenomenon is because that the volume change of paraffin after phase change. The heat input for this heat storage unit is at the bottom, which lead to the paraffin melting from bottom to top. For same amount of paraffin, the liquid state has a bigger volume than the solid state. With the phase change process, the top part may be move upward because of the volume expansion, especially around T12. Because the thermocouple was fixed on a wood stick, once the liquid state reach T12 along the stick, the temperature increased to phase change temperature. And then, the liquid state transfer heat to solid phase paraffin around T12, lead to temperature drop. For heat storage with OHP inside, the phase process would be more uniform along the vertical direction and less volume change effect was observed. Therefore, the temperature of paraffin near T12 increased first and then decreased. In the heat storage process, the temperature fluctuation of the paraffin/OHPs with different turn number also occurred, but the degree of fluctuation is much smaller than that of paraffin.

Fig. 5.

Temperature variation curves of (a) paraffin and paraffin/OHPs (Di=3 mm, Do=4 mm) with (b) three, (c) four and (d) five turns in the heat storage process. (Online version in color.)

As can be seen in Fig. 6, the duration of T12 temperature from 15°C up to 62.5°C of the heat storage process of paraffin is 47123 s. The duration of T12 temperature from 15°C up to 62.5°C of the heat storage process of PCM/OHPs with three, four and five turns is 36440 s, 29006 s and 42516 s, respectively. It is obvious that the time required for heat storage process of the PCM/OHPs is shorter than that of paraffin. The time of the heat storage process of PCM/OHPs is 22.67%, 38.45% and 9.78% shorter than that of paraffin when the turn number of OHPs was three, four and five, respectively. The results indicated that the TES efficiency increased by OHPs which has high equivalent thermal conductivity.

Fig. 6.

T12 Temperature variation curves of (a) paraffin and paraffin/OHPs (Di=3 mm, Do=4 mm) with (b) three, (c) four and (d) five turns in the heat storage process. (Online version in color.)

The temperature oscillation curves of the evaporation and condensation sections of OHPs in TES unit are shown in Fig. 7. During the heat storage process, the start-up temperature of the OHPs is about 50°C, 55°C and 55°C when the turn number of the OHPs is three, four and five, respectively. It is obvious that the temperature amplitude of the OHPs decreased after coupling with paraffin. In addition, the temperature amplitude of the OHPs gradually decreased and disappeared eventually with the decreasing of temperature difference between the evaporation and condensation sections of the OHPs.

Fig. 7.

Temperature oscillation curves of the evaporation and condensation sections of OHPs (Di=3 mm, Do=4 mm) with (a) three, (b) four and (c) five turns. (Online version in color.)

3.2.2. The TES Performance of Paraffin/OHPs with Different Inner Dimeters and Wall Thickness

The temperature variation curves of paraffin/OHPs (three turns) with different inner dimeters and wall thickness are shown in Fig. 8. It can also be found that the temperature of the paraffin rose rapidly at the beginning of heating stage, and the temperature rise rate went down when the temperature reached the melting temperature of paraffin. The temperature of PCM was continued to rise after paraffin completely melting. Finally, the temperature of paraffin is gradually stabilized with the decreasing temperature difference between heating water and paraffin. Figure 9 shows the T12 temperature of paraffin and paraffin/OHPs with different inner dimeters and wall thickness. The duration of T12 temperature from 15°C up to 62.5°C of the heat storage process of PCM/OHPs (Di=2 mm, three turns) with wall thickness of 0.5 mm and 1 mm is 37322 s and 40209 s, respectively. That indicated the time of the heat storage process of PCM/OHPs is 20.8% and 14.67% shorter than that of paraffin when the wall thickness of OHPs (Di=2 mm, three turns) is 0.5 mm and 1 mm, respectively. The time of the heat storage process of PCM/OHPs (Di=3 mm, Do=4 mm, three turns) is 22.11% shorter than that of paraffin. The result indicates that the OHPs (Di=3 mm, Do=4 mm, three turns) improved the TES efficiency of paraffin better than other two kinds of OHPs. The result indicates that the thickness of the wall is the priority to improve the efficiency of the system, while a larger Do could provide a much sufficient contact between the OHP and paraffin which can also improve the efficiency of the system.

Fig. 8.

Temperature variation curves of paraffin/OHPs (Di=3 mm, Do=4 mm) with different inner dimeters and wall thickness in the heat storage process. (Online version in color.)

Fig. 9.

T12 Temperature variation curves of (a) paraffin and paraffin/OHPs (three turn) with different inner dimeters and wall thickness in the heat storage process. (Online version in color.)

Figure 10 shows the temperature oscillation curves of the evaporation and condensation sections of OHPs with different inner dimeters and wall thickness in TES unit. The start-up temperature of the OHPs (Di=2 mm, three turns) is about 49°C and 60°C when the wall thickness of the OHPs is 0.5 mm and 1 mm, respectively. The tendency of temperature variation in Fig. 10 is the same as Fig. 7. The temperature oscillation degree of the OHPs decreased after coupling with paraffin, and the oscillation amplitude of temperature in the OHP gradually decreased until disappeared with the decreasing of temperature difference between the evaporation section and condensation section.

Fig. 10.

The temperature curve of OHPs with different inner dimeters and wall thickness in phase change TES. (Online version in color.)

4. Conclusions

In order to enhance heat transfer performance of latent heat energy storage system, a novel method coupling PCM and oscillating heat pipe (OHP) was investigated. Firstly, OHPs with different turns, inner dimeters and wall thickness were designed and manufactured. The thermal performance of the OHPs were tested. And then, the thermal energy performance of paraffin coupled with OHPs were tested. The influence of turn number, inner dimeters and wall thickness of OHPs on the heat transfer performance of paraffin were investigated. The results indicated that the prepared OHPs have high heat transfer performance. The time of the heat storage process of paraffin coupled with OHP (Di=3 mm, Do=4 mm, 4 turns) is 38.45% shorter than that of pure paraffin. The OHPs can effectively improve the heat transfer efficiency of the paraffin in the heat storage process.

Acknowledgements

This work was supported by “the Fundamental Research Funds for the Central Universities” (NO. 2018QNA08).

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