Solar Hot Water Systems Using Latent Heat Thermal Energy Storage: Perspectives and Challenges
Abstract
:1. Introduction
2. Solar Hot Water Systems Using LHTES
2.1. Heat-Pipe-Assisted LHTES System
2.2. LHTES Modules Integrated into the Water Storage Tank
2.3. Water Storage Tank with a Separate LHTES
3. Current Research Activities (Last 5 Years)
4. Perspectives and Challenges
- (i)
- The heat-pipe-assisted LHTES system appears to be a promising solution for domestic hot water supply. Its ability to provide hot water at a temperature of 40–45 °C, at a flow rate of 50 L/h, for an extended period of 3 to 4 h, along with a thermal efficiency between 30% and 50%, and its ability to solve thermal stratification and overheat issues are significant advantages. Furthermore, replacing the conventional hot water tank also reduces the space requirement. These features make the system an attractive option for those looking to integrate sustainable energy solutions into their buildings. However, the high cost of the heat-pipe-based solar collector and a lack of experimental work to prove the system’s effectiveness and economics are challenges that need to be addressed. Further research and optimization studies are needed to justify the LHTES applicability and bring down the overall cost of the system.
- (ii)
- The second type of system, which utilizes LHTES modules inside a hot water tank, has the potential to maintain a temperature of 40–55 °C for extended periods, i.e., of 6 to 12 h. However, most studies have not accounted for the hot water withdrawal during the testing. As of now, there is no evidence to suggest that this type of system is superior to the first type, but it does exhibit higher thermal efficiency (50–70%), theoretically. While it cannot fully replace a conventional tank, it has the potential to reduce the size of the tank. One of the biggest challenges in implementing this type of system is the risk of PCM leakage from the small LHTES modules if not thermally cycled before use. Additionally, there are a variety of parameters that govern the system’s performance, such as the type of PCM, aspect ratio of the storage tank, position of PCM modules (top, medium, and bottom), storage tank volume, and the number of storage tanks, which make optimization a complex process. Therefore, more on-demand performance studies are needed to optimize hot water production and address the challenges associated with this type of system.
- (iii)
- The third type of system, which utilizes a separate LHTES tank, has not yet reached maturity. Studies have shown that these systems are currently unable to provide hot water at temperatures of 40 °C for even a short period (3–4 h). The major challenges are the formation of a solid layer around the inner tube of the heat exchanger during discharging and the low thermal conductivity of the PCM, which reduces the system’s ability to provide hot water at a desired temperature. In addition, the design, orientation, and position of the heat exchanger significantly affect the phase-changing phenomenon and impact the system’s performance. Hence, more research is needed to improve the heat transfer rate during energy storage and recovery, optimize the heat exchanger’s design, and find ways to increase the thermal conductivity of the PCM used in the system.
5. Conclusions and Future Work
Author Contributions
Funding
Data Availability Statement
Conflicts of Interest
Abbreviations
CFD | computational fluid dynamics |
ETHPSC | evacuated tube heat pipe solar collector |
HTF | heat transfer fluid |
LHTES | latent heat thermal energy storage |
MEPCM | microencapsulated phase-change material |
NEPCM | nano-enhanced phase-change material |
PCM | phase-change material |
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---|---|---|---|
Abhat [10] | A finned-heat-pipe-assisted LHTES system. | Experimental | The system was able to operate within smaller temperature gradients (<10 °C). |
Liu et al. [11] | A heat-pipe heat exchanger with latent heat storage. | Experimental | The system was able to perform simultaneous charging/discharging for the continuous operation of the system. |
Naghavi et al. [12] | The ETHPSC assisted LHTES system. | Numerical | The system was able to control the overloading of the heat pipe and prevent overheating of the water supply during peak solar radiation hours. |
Lee et al. [13] | A two-phase closed thermosyphon system with LHTES. | Experimental | The usage of PCM could make the storage tank lighter than traditional heating systems. |
Brahim et al. [15] | A plate-screen-meshes-heat-pipe-assisted solar water heater. | Numerical and Experimental | It achieved a collector efficiency of 60% by adding fins to the condenser region of the heat pipes. |
Tiari et al. [16] | A finned-heat-pipe-assisted LHTES system. | Numerical | An increasing number of heat pipes improved the thermal performance by increasing the melting rate. |
Robak et al. [17] | Different combinations of the heat pipe and fins in the LHTES system. | Experimental | Fins were not as effective as heat pipes in improving thermal performance. |
Naghavi et al. [14] | The ETHPSC-assisted LHTES system. | Numerical | Extended the operating time for 3 to 4 h with an outlet water temperature of 39 °C. |
Bazri et al. [18] | The ETHPSC-assisted LHTES system. | Numerical | The system was able to provide hot water at a temperature of 46 °C for 4 h, with a flow rate of 50 L/h. |
Naghavi et al. [19] | The ETHPSC-assisted LHTES system. | Experimental | It achieved a thermal efficiency of 38–42% on sunny days and 34–36% on cloudy/rainy days. |
Naghavi et al. [20] | On-demand performance study of the ETHPSC-assisted LHTES system. | Experimental | The system was able to deliver a minimum of 112–170 L of hot water per day in the worst weather conditions. |
Reference | Examined System/Scope of the Study | Type of Study | Observations |
---|---|---|---|
Canbazoğlu et al. [25] | Placed cylindrical LHTES modules inside the hot water tank. | Experimental | The water temperature remained constant at 45 °C for approximately 10 h after the solar radiation decreased. |
Mazman et al. [26] | Added cylindrical-shaped PCM units at the top of the storage tank. | Experimental | It achieved a thermal efficiency of 74%. During discharge, the average temperature of the storage tank dropped below the PCM melting temperature range (49–53 °C) within 6–12 h. |
Al-Hinti et al. [27] | Placed the PCM-filled aluminium bottles inside the hot water tank. | Experimental | The water temperature was maintained at 13–14 °C higher than the system without PCM. |
Fazilati and Alemrajabi [29] | Utilized PCM-contained spherical capsules in the jacketed shell-type storage tank. | Experimental | The system was able to supply hot water at a specified temperature for a 25% longer time. |
Fang et al. [31] | Designed a MEPCM-based LHTES system. | Experimental | The system exhibited a stable operation and a high heat transfer rate, indicating that it is practical for use in domestic hot water systems. |
Nkwetta et al. [32] | Studied different positions of PCM inside the storage tank. | Numerical | The top position of the PCM was better than the middle position. |
Teamah et al. [34] | Studied the combination of different storage tanks with different PCMs. | Numerical | The storage tank volume was reduced by more than 50% by using multiple hybrid storage tanks. |
Afshan et al. [35] | Studied different aspect ratios of the hybrid water storage tank. | Experimental | A lower aspect ratio (1:1) was recommended for hybrid thermal storage with PCM spheres. |
Elbahjaoui and Qarnia [37] | Rectangular-shaped LHTES with different PCMs. | Numerical | The outlet water temperature was observed in the range of 43.6–24 °C, 51.7–24 °C, and 62.86–24 °C, respectively, for RT42, RT50, and RT60 during discharging. |
Kılıçkap et al. [39] | The PCM was filled inside the annulus of the hot water tank. | Experimental | It achieved the highest thermal efficiency of 58% by using PCM. Moreover, the system with PCM was able to transfer stored heat to water at night, providing hot water for an additional 1–1.5 h. |
Reference | Examined System/Scope of the Study | Type of Study | Observations |
---|---|---|---|
Mahfuz et al. [44] | A shell-and-tube heat exchanger as a separate LHTES system. | Experimental | For the lowest flow rate (0.033 L/min), the outlet water temperature remained above 40 °C for just 30 min. |
Luu et al. [45] | A shell-and-tube-type tankless latent heat battery. | Numerical | Improved the fossil fuel saving by 15.7% more than a conventional system. |
Luu et al. [46] | A shell-and-tube-type tankless latent heat battery. | Numerical | Achieved the discharge average temperature of 40 °C. |
Lamrani et al. [47] | Parabolic-trough-collector-assisted rectangular shell-and-tube-type separate LHTES system. | Numerical | The PCM with a low melting temperature was unable to provide hot water at the desired temperature, while a PCM with a high melting temperature was not able to fully utilize available solar energy for storage. |
Lu et al. [48] | A spiral-finned heat-exchanger-type separate LHTES system. | Experimental | The system has provided hot water of 40 °C with a flow rate of 0.5 L/min for just 2000 s (0.55 h). |
Gao et al. [49] | A tube-in-tank-type separate LHTES system. | Experimental | The system was able to provide hot water of 40 °C with a flow rate of 0.6 L/min for just 19.3 min. |
Dogkas et al. [50] | A staggered finned heat exchanger as a separate LHTES system. | Experimental | The system was able to produce 106 L of hot water instantly at a temperature above 40 °C during discharging. |
Osman et al. [51] | A multi-tube heat exchanger as a separate LHTES system. | Numerical and Experimental | The LHTES unit was able to increase hot water temperature by 7–12 °C and maintained a constant hot water supply for extended periods of about 2–3 h. |
Shalaby et al. [53] | Rectangular shell-and-finned tube-bank-type heat exchanger as a separate LHTES system. | Experimental | The configuration was able to provide hot water at a consistent temperature range of 50–60.4 °C for 24 h. |
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Modi, N.; Wang, X.; Negnevitsky, M. Solar Hot Water Systems Using Latent Heat Thermal Energy Storage: Perspectives and Challenges. Energies 2023, 16, 1969. https://doi.org/10.3390/en16041969
Modi N, Wang X, Negnevitsky M. Solar Hot Water Systems Using Latent Heat Thermal Energy Storage: Perspectives and Challenges. Energies. 2023; 16(4):1969. https://doi.org/10.3390/en16041969
Chicago/Turabian StyleModi, Nishant, Xiaolin Wang, and Michael Negnevitsky. 2023. "Solar Hot Water Systems Using Latent Heat Thermal Energy Storage: Perspectives and Challenges" Energies 16, no. 4: 1969. https://doi.org/10.3390/en16041969