The enhancement of heat transfer with mechanical aids includes rotating the LHTES system or stirring the PCM. Classically, rotation is used in heat transfer enhancement of heat exchangers for industrial applications [
23]; however, it has recently been applied to LHTES systems. A subset of rotation is scraped surface enhancement, where a surface scraper removes solidified PCM from the outer wall of the storage unit. Scraped surface heat exchangers are applied in the food [
24] and chemical [
25] processing industries, which can be studied to improve LHTES systems.
2.1. System Rotation
Rotation of the LHTES system works on the principle of forced convection heat transfer, which has been adapted into new techniques for heat transfer enhancement within PCMs. One of the major heat transfer mechanisms during the melting process of PCM is natural convection [
26]. Without rotation, natural convection is not significant in the solidification process [
27]; however, the addition of rotation may have a major impact. Huang et al. [
28] numerically studied the solidification of n-eicosane and RT-82 PCM in a horizontal triplex-tube LHTES system, as shown in
Figure 1. The system was enhanced using four fins with optimized lengths, widths, and angles along with constant system rotational speed at 0.05, 0.1, 0.2, 0.5, and 0.1 rpm. It was found that as rotational speed increases, the solidification time decreases and the heat transfer rate increases. The results showed that rotation at 0.05 and 1 rpm reduced solidification time by 45.99% and 83.85%, respectively, compared to no rotation. Natural convection has little impact on the solidification of PCM, but the solidification rate with rotation is higher than that of PCM with only natural convection. Although the study optimized the design of the fins, there was no comparison to the system with no fins.
Soltani et al. [
29] numerically studied both the charging and discharging processes in a horizontal shell-and-tube LHTES system with n-eicosane PCM. Four cases of copper radial fins were employed with 0, 4, 8, and 16 fins at rotational speeds of 0.1, 0.5, and 1 rpm. The greatest reduction in charging and discharging time was achieved by the case with four fins at a speed of 1 rpm, with a 73.13% and 83.21% reduction, respectively. Increasing the rotational speed decreased the charging times, but increasing the number of fins decreased the effect of natural convection and the rotational speed. In a following numerical study, Soltani et al. [
30] analyzed the enhancement of fins, nanoparticles, and rotation on a horizontal shell-and-tube LHTES system with n-eicosane PCM. Cases were analyzed with four, six, and eight copper fins; copper nanoparticles volumetric fractions of 0.02 and 0.05; and rotational speeds of 0.1, 0.3, and 0.5 rpm. The case with eight fins, 0.5 rpm, and 0.05 volume fraction of nanoparticles had the fastest charging and discharging times, with 78.49% and 76.88% reduction in time compared to the unenhanced case; however, it decreased energy storage and release capacity by 5.1% and 6.5%, respectively. An optimized case was found with six fins, 0.02 nanoparticle volume fraction, and charging and discharging rotational speeds of 0.42 and 0.4 rpm. The optimal case decreased charging and discharging times by 73.67% and 69.73%, respectively, compared to the unenhanced system, with only 2.5% and 4.1% reduction in energy storage and release capacity, respectively. It was found that the enhancements with lower charging and discharging times had lower amounts of energy stored and released.
Based on the previous studies analyzed, the use of constant rotation has a significant impact on the charging and discharging process of LHTES systems. Other studies have analyzed alternative methods to constant rotation to reduce the additional energy consumption. Yu et al. [
31] numerically studied the melting time of D-mannitol PCM in a rotating horizontal shell-and-tube LHTES system. Constant rotation speed at 0.01667, 0.08333, 0.16667, 0.5, and 1 rpm was compared to a case with no rotation. The authors also proposed a “single rotation” method where the system was rotated 180° at specific liquid fractions to enhance heat transfer equivalent to constant rotation. It was shown that increasing the rotational speed results in a higher melting speed, but continuing to increase the speed will result in insignificant reductions in melting time. “Single rotation” was found to have similar effect as the highest rotation speed at 0.5 liquid fraction, which decreased melting time by 811% compared to the unenhanced system. The “single rotation” was also found to reduce melting time by 116% compared to a static case with three radial fins. During charging, the PCM in the upper half of the system is dominated by natural convection, which melts faster than the lower half, which is dominated by conduction. By flipping the container at a specified liquid fraction, the cooler PCM at the bottom of the container can move to the top, where greater heat transfer takes place. A similar process was analyzed by Khoroshahi and Hossainpour [
32], who numerically studied “step by step” rotation in a horizontal shell-and-tube LHTES system with RT-82 PCM. Eight cases of constant rotation were investigated between 0 and 0.0036 rpm. A novel “step by step” rotation included one scenario with counterclockwise turns with different numbers of stops and another scenario where the system is rotated once counterclockwise and then turned clockwise back to the initial position. The study determined that constant rotation does not have a significant impact on melting time because of the lack of natural convection vortex formation. An optimized “step by step” case was found, where the container was not rotated for a duration of 190 min and then was rotated 180°. This process was then repeated two more times, which resulted in a 15.5% reduction in melting time compared to the fixed case. This study may have concluded that there was no significant increase in melting time from constant rotation because the rotational speeds chosen were slow compared to other studies. The same authors, Khoroshahi and Hossainpour [
33], further analyzed the “step by step” rotation method in the same system enhanced with two, three, and four aluminum fins compared to a no-fin case. The fins designed in the study fully section off the PCM in the container, which decreased the melting time by not allowing hot PCM to exchange energy with colder regions during rotation. An optimal case with four fins at 90° and two vertical stops at an angle difference of 180° was determined. The optimal case decreased melting time by 72% and increased the energy stored after 167.67 min by 115% compared to the fixed and no-fin storage. The previous study by the authors used constant rotation, but constant rotation was not used with full-scale fins. The effect of full-scale fins with constant rotation should be further investigated. Zheng et al. [
34] numerically analyzed the melting of paraffin PCM in a rotating horizontal shell-and-tube LHTES system. A static case was compared to rotational cases of 1, 2, 4, 8, and 16 rpm. Rotation was started only after the PCM around the fins reached melting temperature to prevent the fins from hitting the solid PCM. As determined with other studies, increasing the rotational speed increases the melting rate. At 16 rpm, a 46.96% reduction in melting time and 79.02% increase in thermal energy storage rate was achieved compared to the static case. The study recommends that 2 rpm should be the lowest rotational speed used. A complimentary experimental validation showed 1.5 W was required for rotation of the system.
There are many computational studies on rotation in LHTES systems; however, due to the complexity of the systems, there is little attention towards experimental work. Fathi and Mussa [
35] experimentally analyzed the rotation of a horizontal shell-and-tube LHTES system with paraffin wax PCM. Rotational speeds of 3, 6, and 9 rpm both clockwise and counterclockwise were studied. No significant difference in the melting time was found when comparing the still case to the rotating cases. After 10.75 h the 9-rpm case had an 8% increased liquid fraction compared to the fixed case. The authors, however, did not fully analyze the melting time and did not fully charge the system. Another experimental study conducted by Yang et al. [
36] analyzed rotation in a horizontal shell-and-tube LHTES system with paraffin PCM. Three cases were studied including static, one flip 180° after 120 min, and 0.13 rpm. The flipping time was then optimized using dimensionless flipping times of 0.25, 0.375, 0.625, and 0.750. The constant rotation case decreased melting time by 19.35% compared to the static case. Flipping the system at an optimal point of 0.375 decreased melting time by 37.50% compared to the static case. A further energy analysis showed that flipping the system takes an additional 5 J of energy, whereas the constant rotation is an additional 1500 J. The energy required in both cases was considered negligible by the authors. The experimental methods and apparatus, shown in
Figure 2, provide a thorough explanation of the components used to build the system, which can be used for future research.
Essa et al. [
37] experimentally studied the enhancement of a tubular solar still with a rotating drum and paraffin wax PCM. The solar still was analyzed with copper oxide nanoparticle paint and rotation speed between 0.05 and 0.7 rpm. Without PCM and nanoparticle paint, the production of water from the solar still was increased by 121% at 0.1 rpm compared to the conventional system. When the system was enhanced with PCM, nanoparticle coating, and 0.3 rpm rotation, the productivity was increased by 218%.
The flipping method shows promising results when compared to constant rotation in both thermal performance and energy consumption. The flipping of a horizontal PCM container can be further improved by using eccentric HTF tubes. Modi et al. [
38] numerically investigated eccentric tube locations within a horizontal shell-and-tube LHTES system with RT-50 PCM. The study found that eccentric position of the tube moving towards gravity improves melting performance but does not significantly improve after a dimensionless position of 0.75. Moving the tube away from gravity improves solidification but, as with melting, reaches an optimized position at −0.30. The opposite behavior of melting and solidification led to a novel 180-degree rotation method where the eccentric tube could be located at the bottom of the container during charging and flipped to the top during discharging. The concept for the eccentric tube rotational LHTES system is shown in
Figure 3. An optimized case for rotation was found to be 0.30, which had a total melting and solidification time of 12.46 h compared to the concentric configuration, which had a combined time of 16 h.
A similar study by Soltani et al. [
39] numerically analyzed eccentric tube locations in a rotational horizontal shell-and-tube LHTES system with n-eicosane PCM. In addition to eccentric tube locations, various diameter ratios of the container were compared, as well as constant rotation. Using the eccentric tube location while flipping the system between charging and discharging processes, the thermal performance of the system was increased compared to the central and constant rotational cases. The optimal case of 0.05 eccentricity with 180° rotation between charging and discharging achieved the fastest combined melting and solidification time for all the cases.
The studies focusing on rotational enhancement of LHTES systems are summarized in
Table 1. The following are suggestions for future enhancements of rotational LHTES systems. To the authors’ knowledge, there has only been combined passive techniques and rotation using fins. Future studies should combine porous material and nanoparticle dispersion with rotational enhancement. In addition, rotation should be more thoroughly compared to passive techniques to see if the energy expenditure is cost effective. Additional experimental analysis on single rotation or flipping should be carried out. All of the studies reviewed used horizontal shell-and-tube LHTES systems. Rotation in vertical LHTES systems or other geometries/orientations should be examined.
2.2. Rotating Cylinder within PCM
Another rotational technique applied to LHTES systems is the use of a rotating cylinder located directly within the PCM. Selimefendigil and Öztop [
40] numerically studied the fluid flow and heat transfer characteristics of RT-25 in a rectangular enclosure LHTES system with the addition of a rotating cylinder. The system, which is similar to those in the following studies, is shown in
Figure 4. The system was analyzed in the melting process with angular rotational speeds of −7.5 to 7.5 rad/s, vertical locations of the cylinder from 0.25H to 0.75H, and two different sizes of cylinders, 0.05H and 0.1H. The study determined that the average Nusselt number increases by 22.50% for an angular rotational speed of 7.5 rad/s compared to the no-rotation case. In addition, spatial average heat transfer is 19.8% higher for the stationary cylinder and increases 10% for clockwise rotation of the larger cylinder. The study did not include an analysis of the rotating cylinder on discharging.
A similar study was conducted by Farahani et al. [
41] on a rectangular enclosure LHTES system with lauric acid PCM enhanced with rotating and oscillating cylinders. The system was analyzed with zero to four cylinders in different arrangements in the system with rotational speeds from −10 to 10 rad/s and oscillating frequencies of 0.1, 0.5, 1, 5, and 50 Hz. It was found that rotation of the cylinders decreased melting time most effectively at −3 and 10 rad/s compared to the stationary case. Increasing the cylinder oscillation range increased melting time, and increasing frequency decreased melting time. The most effective case was two cylinders horizontally placed in the system. In a numerical analysis by Qasem et al. [
42], the effect of a rotating cylinder and wavy-walled rectangular enclosure LHTES system was analyzed on nano-PCM. Rotational speeds of −5 (clockwise) to 5 rad/s as well as number of undulations in the wall from one to four were studied. It was determined that the cylinder should rotate in the same flow direction as natural convection (clockwise). At −5 rad/s, the melting speed increases by 88% compared to no rotation. The undulation number was also found to be most efficient at 1. Al-Kouz et al. [
43] numerically studied a rectangular enclosure LHTES system enhanced with a rotating cylinder and external magnetic field. The cylinder was rotated at speeds of −10, 5, and 10 rad/s at vertical locations of 0.025, 0.05, and 0.075 within the enclosure. An external magnetic field was applied with Hartmann numbers of 0, 5, and 10. The rotation of the cylinder was found to increase the heat transfer rate up to 21.2% compared to a static case; however, the external magnetic field was found to decrease heat transfer. The authors determined that vertical placement of the cylinder can be used to optimize heat transfer.
A similar heat transfer enhancement technique has been applied to the HTF in PCM-packed bed systems. This can be seen by introducing rotating cylinders [
44,
45], surfaces [
46,
47], and disks [
48] to the HTF flow before reaching the PCM. Another study analyzed the mixed convection of water and nano-encapsulated PCM with a rotating cylinder [
49].
Table 2 summarizes the studies on PCM with rotating cylinders. To the authors’ knowledge, there have been no experimental studies on this enhancement technique, which suggests that the findings should be verified through experimental testing.
2.3. Scraped Surface
The concept of rotation can be applied to scraped surface heat exchangers for LHTES systems. During the solidification process, PCM begins to solidify on the heat transfer surface. The low thermal conductivity of the PCM in its solid state acts as insulation between the molten PCM and the heat transfer surface. By actively scraping the solidified PCM from the heat transfer wall, the thermal resistance between the HTF and molten PCM is decreased, which increases the discharging rate. This technique has been applied in the following studies. Maruoka et al. [
50] experimentally studied the solidification of sodium acetate tri-hydrate PCM in a vertical shell-and-tube LHTES system with a rotating HTF tube and fixed blade. During the discharging process, the HTF tube was rotated at 100, 300, and 500 rpm, which allowed for the solidified PCM to be scraped off the HTF tube. The proposed schematic of the system is shown in
Figure 5. The study showed that as rotation rate increases, the overall heat transfer coefficient increases, reaching more than 2000 W/m
2K at 500 rpm. The heat transfer rate decreased at a heat release ratio of more than 80–90% due to the PCM around the HTF tube solidifying.
A similar study with a unique LHTES system was conducted by Egea et al. [
51]. The authors experimentally analyzed the heat release rate of RT-44HC PCM in a vertical cylindrical LHTES system with the HTF surrounding the PCM container. Three sets of rotating blades were connected to a pipe, which rotated to scrape PCM off the outer wall of the container. The schematic of the system is shown in
Figure 6. Rotating speeds of 3, 5, and 7.5 rpm were utilized and compared to a non-scraping mode at 0 rpm. It was determined that the heat release rate is not significantly impacted by an increase in rotational speed. The scraping mode resulted in a 2–3 times higher heat release rate regardless of rotational speed.
Another application of the scraped surface technique is a rotating drum heat exchanger, which is proposed by Tombrink et al. [
52]. The proposed design is a horizontal rotating drum partially submerged in a liquid PCM bath. An HTF is passed through a cavity within the rotating drum to extract thermal energy, which solidifies the PCM on the outer surface of the drum. The PCM is scraped off the outer surface of the drum before rotating back into the PCM bath. Tombrink et al. [
52] experimentally analyzed this concept using decanoic acid PCM. Three different operational modes between 0.25 and 25 rpm were studied, which are shown in
Figure 7. It was determined that minimizing the thickness of the solid PCM layer on the surface of the drum increases the heat transfer rate. In addition, the heat transfer rate is 142% higher for constant temperature difference between the PCM and HTF and constant rotational speed when compared to pure forced convection of PCM without solidification. By changing the rotational speed, the thermal output is able to be controlled, but the increase of heat transfer flattens out at higher rotational speeds.
Tombrink and Bauer [
53] numerically confirmed the results of the previous experimental study. The results show only an 8% deviation in the heat transfer rate from the experiments for the transient numerical simulations above 4 rpm. Additionally, the authors analyzed high-temperature sodium nitrate PCM for steam generation at 0 to 300 rpm. When simulated with 300 rpm and a 150 K temperature difference between the HTF and the melting point of the PCM, results show that a surface-specific heat flux of up to 500 kW/m
2 is achieved. This shows that the system is acceptable for steam generation. Utilizing the information from the experimental and numerical studies, Tombrink and Bauer [
54] numerically optimized the horizontal rotating drum heat exchanger for use in steam generation and co-generation of electricity and heat using renewable energy. Utilizing the high-temperature PCM, water within the rotating drum can be converted to steam. The authors determined that the design parameters of the system are freely scalable for usage in industrial applications.
The studies focusing on scraped surface enhancement of LHTES systems are summarized in
Table 3. The following are suggestions for future enhancements of scraped surface LHTES systems. The scraped surface technique has only been applied to the discharging process of PCM; however, the charging process needs to be enhanced as well to store thermal energy. A future study could perhaps employ scrapers that act as fins during charging. The optimization of energy input for rotation should be further investigated.