Performance evaluation of single stand and hybrid solar water heaters: a comprehensive review

FP-SWHs are categorized based on their absorber characteristics, flow inserts, vortex generators, and insulation material. Conventional and once-through designs have been developed. Water is continually cycled through the panels in the conventional design, while in the once-through type, the water passes once through the solar panel and then enters the hot water storage tank (Karwa et al. 2018). It has been tried to enhance the heat transfer coefficient, decrease the pressure drop and heat loss, and overcome the drawback of the conventional SWHs by saving solar energy for the early morning, late evening, and nighttime applications (Vengadesan and Senthil 2020). The double-glassed FP-SWH performs better than the single-glassed type due to a lower heat loss to the atmosphere (Nirmala 2020). The schematic of the double-glassed and single-glassed FPCs is presented in Fig. 5.

The performance study of the FP-SWH with three types of twisted tape experimented by Almeshaal et al. (2020) confirmed the positive effect of using twisted tape inside the riser tubes as a turbulator. The geometric profile of a riser tube in an FP-SWH was modified by creating the dimples inside the riser tube (Arun et al. 2020). It caused turbulence and swirl flow throughout the riser to improve the heat transfer. The experimental and computational fluid dynamics (CFD) results were compared. The hot water output increased at the low water flow rates, while at higher flow rates, it was moderately constant. Twist ratio variation from 3 to 12 led to heat transfer rate improvement by 18–70%, though the pressure drop augmented by about 87–132%, as compared to a plane collector. Due to the lower plate temperature, the heat loss decreased (Kumar and Prasad 2000). The thermal efficiency of an FP-SWH with pebble stones as a porous medium and the riser tube containing an agitator was developed by Kanimozhi et al. (2019). Placing an agitator and the pebble stones increased the convective heat transfer and reduced thermal loss leading to a slow increase in energy efficiency. The maximum energy efficiency of 86.4% and 59.59% was obtained in the case of SWH without and with an agitator, respectively. Using a hindrance promoter was another option to improve the thermal performance of an FP-SWH (Rohit Khargotra and Kumar 2021). At the optimal values of the parameters, the hindrance promoter improved the Nusselt number by 4.56 times higher than the smooth tube. The rectangular-winglet- and delta-winglet-type vortex generators within the circular tube of an FPC were simulated (Da Silva 2019). The vortex generators were coupled to augment the heat transfer rate between tube walls and working fluid. The primary and corner vortexes were recognized on the flow, which increased the heat transfer rate, even though the corner vortex was more obvious for the rectangular-winglet generator (independent of the attack angle and Reynolds number). The secondary flow produced by the inserts through the corner vortex was observed via the rectangular-winglet generator. A maximum heat transfer rate resulted in 45° for vortex generators. However, the optimum ratio between the rate of heat transfer and the drop in pressure penalty was obtained at 30° for the delta-winglet generator. Modifications of the risers are illustrated in Fig. 6.

The absorber is the heart of an SWH, which absorbs the sunlight and warms up the water. The efficiency of a collector depends on the absorptance and emittance of the absorber. There is an urgent need to develop highly efficient, structurally, thermally, optically, and mechanically stable solar absorber coating. The emittance is comparatively less significant than the solar absorptance. Most solar collectors use spectrally selective absorber coating with low emittance and high absorptance at the infrared spectrum and in the solar range, respectively (Bello and Shanmugan 2020). The collector with the electrodeposited coating had a better performance than the one coated with CuO. At temperatures ˃50 °C, it performed similarly to a TiO₂-based collector. This was due to lower heat losses from the backside of the Cu flat plates with a Ni coating (Lizama-Tzec et al. 2019). Coating the absorber with black chrome and nickel–cobalt coating on copper and using the reflectors improved the heat absorption. The system’s thermal efficiency with a nickel–cobalt absorber was higher due to its higher emittance than the black chrome panel. By increasing the flow rate, more heat was absorbed leading to higher thermal efficiency while a further increase in flow rate decreased the time of heat absorption and led to a lower thermal efficiency (Ramesh et al. 2022). Furthermore, free convection took place inside FPCs with boosted flow rates. Hence, the water flow rate should be boosted according to the solar radiation intensity, collector area, and temperature difference between the hot water leaving the collector and the cold water entering it. The maximum outlet temperature of about 96 °C was obtained as the CuO/H₂O mixture absorbed more heat due to more heat transfer coefficient at high temperatures. Due to the short contact time of the nanoparticle inside the riser tubes, a lower water temperature was obtained in forced circulation. The copper, copper oxide, aluminum oxide, and titanium oxide nanoparticles in water were used to improve the thermal performance of an FP-T-SWH (Darbari and Rashidi 2021). Among the introduced nanoparticles, copper oxide had the best thermal efficiency. The useful energy and energy efficiency increased by increasing the flow rate (higher Reynolds number and the heat removal factor, leading to an improvement in the convective heat transfer rate inside the tubes and a reduction in the thermal resistance between the fluid inside the absorber and the tube) and volume fraction of nanoparticles (due to a reduction in the average temperature of the absorber, hence, a lower heat loss). The maximum energy efficiency of 80% was obtained at a solid volume fraction of 0.05 for the solar intensity of 200 W/m². The ambient temperature variation from 20 to 40 °C enhanced the energy efficiency by 5.5%, while, when the water temperature increased from 30 to 55 °C, it reduced the energy efficiency by 15%. Using a mixture of CuO–MgO (0.1–0.1%) in an FP-SWH also showed better performance compared to pure water (Janardhana et al. 2022).

In SWH containing corrugated-surface-based absorber without flow rate, the highest water temperature reached 58 °C and 78 °C during winter and spring, respectively. In the case of the absorber with flow rate, the daily energy efficiency of the solar collector increased from 59 to 67%, when the mass flow rate increased from 0.005 to 0.013 kg/s (Yassen et al. 2019). A comparative analysis of the performance of circular and a trapezoidal surface with absorber tubes indicated better performance of the trapezoidal surface over the circular absorber by energy and exergy efficiency improvement by about 4% and 6%, respectively, due to the reduction in the heat loss (Sharma, 2022). The sinusoidally folded absorber sheet also ensured a higher water temperature, hence a higher flow rate compared to a flat plate absorber. It means the collector's useful gain is linked with the flow rate and by increasing the gain, the flow rate also increases via the collector (Yehualashet et al. 2022). Different types of absorber plates are presented in Fig. 7.

The effect of the tracking orientations such as vertical-axis (V), north–south (NS), east–west (EW), and inclined east–west (IEW) on SWH’s performance was numerically investigated. The Perez diffuse model with the hourly weather data was utilized. The study provided a comprehensive outlook to adopt the FPC incorporated with various single-axis and dual-axis trackers in the northern hemisphere (Bahrami et al. 2022). The collector installation at an improper angle reduced the received solar radiation.

An ICS-SWH is a water heating system that alchemizes solar radiation directly into heat (Rao and Somwanshi 2022). The highest collection efficiency can be increased by 36.17%. Despite many advantages of these systems, high heat losses during the night decreased the efficiency of the system. Introducing baffle plate structure, insulation of tank, connecting tanks in series, using PCMs and reverse thermosyphon valve minimized the heat losses from the system.

Designing a C-SWH with a high heat collection rate is complex. Many investigations have been performed to improve its design and thermal performance. High-throughput screening (HTS) technique using machine learning estimates the probable combinations of extrinsic properties of these heaters. ET-SWHs have the disadvantages of slow water temperature increase as well as a slow response to loading, which can be rectified using an optimum closed-loop pulsating HPI (Siritan et al. 2022). The solar radiation intensity is the main parameter affecting the SWH performance. At higher solar radiation intensity, water viscosity decreases, and the density gradient becomes more significant for similar temperature differences, leading to a more significant driving force and producing natural circulation. The ET-SWH containing argon in the space between the cover and the absorber is also recommended due to the lower Prandtl number of argon in comparison with the air (Sadeghi et al. 2019).

In SWH with a low tilt angle, the cold water of the storage tank moves down along the lower wall of tubes and then returns to the storage tank along the upper wall of tubes, whereas, for the SWH with a high tilt angle, the situation in the morning is the same as the SWH with a low tilt angle (Fig. 2B). In the afternoon, the cold water from the storage tank moves down (fully or partially mixed with the hot water) and then returns to the water storage tank. The shorter tube distance leads to worse thermal performance due to more sun shading between tubes.

The performance of T-ET-SWH and ET-SWH incorporating 20 wickless HPIs containing methanol was compared. The inner glass tube was coated with Al–N/Al. Daily efficiency improvement of the system by HPI over the thermosyphon system was 22.5%, 32.4%, and 42.5% for no loading, continuous loading, and intermittent loading, respectively (Al-Joboory 2019). The T-ETC modeling showed the explicit finite difference method (EFDM) makes a better accuracy than the thermal resistance technique (Wannagosit et al. 2018). The effect of working fluid (e.g., ammonia, methanol, acetone, pentane, and water) on the technical performance of an ET-SWH investigated by Arab and Abbas (2013) resulted in the superiority of the ammonia against its counterpart owing to a sudden decrease in the thermal resistance between 9 and 15 h.

The performance of two designs of a WGET-SWH (220-L tank and 2.9 m² collector area; in-tank boost: Design A and preheater with boost tank: Design B) was compared with an FPC (300-L tank and 3.7 m² collector area) by TRNSYS (Budihardjo and Morrison, 2009). The thermal performance of two FPCs was better than that of a WGET-SWH. The performance of ETC was less sensitive to tank size. Annual energy savings of FPC, Design A, and Design B were 77%, 70.9%, and 66.2%, respectively. Coupling a larger tank with Design B resulted in a minor reduction in annual solar fraction, representing the improved collector performance. Two ET-SWHs with the same area were modeled. One utilized internal HPI condensers (Case 1), whereas the other used external ones (Case 2) (Redpath 2012). Case 1 was 17% more efficient than Case 2. These systems were feasible for hot water production in northern maritime climates.

The thermal performance of an ET-SWH at six ETC tilt angles (15°, 20°, 25°, 30°, 35°, and 40°) was evaluated (Kumar et al. 2023). The highest thermal efficiency was 75.04% at the tilt angle of 15°, which was 5.3%, 11.6%, 12.8%, 17.5%, and 18% more than the efficiency at tilt angles of 20°, 25°, 30°, 35°, and 40°, respectively. The lowest energy payback time was 1.14 years at this tilt angle. The payback time was from 1.67 to 2.26 years at other tilt angles. The energy payback time was minimum for ETC installed at a tilt angle of 15°. WGET-SWH produced high levels of stratification at low tilt angles (β < 27°), which was comparable to those obtained in systems with stratification baffles or promoters (Bracamonte 2017). This result was confirmed by an 18% higher thermal efficiency at a tilt angle of 15° compared to a tilt angle of 40°. Using a shorter tube was recommended due to better thermal performance than a longer one and less probable to be damaged during transportation. It was found that compact concrete absorbers in SWH with a spiral tube arrangement are more cost-effective than traditional flat plate collectors. In this arrangement at the lowest mass flow rate (0.005 kg/s) inside the spiral tube, the maximum water temperature in typical sand and concrete absorbers reached 75 °C and 70 °C, respectively (Edwin et al. 2023).

A major disadvantage of the ETCs is the weak circulation of water at their lower end. It makes them inactive most of the time and decreases the effective absorber area. Using turbulators and the flat and wavy diffuse reflectors can overcome this disadvantage. The incorporation of turbulators significantly improves the performance of the ET-SWH by introducing turbulent flow within the tubes. The investigations presented that the water temperature in the tank reached 75 °C, 79 °C, and 81 °C for plain (without twisted tapes) ETC, ETC equipped with flat and wavy diffuse reflectors, respectively. The wavy diffuse reflector had better performance due to the extra energy saving in the range of 12.2–16.7% at different zones. An optimized ETC consisting of 10 tubes with a reflector showed the importance of the distance between tubes as well as the distance between the reflector and collector (Arun et al. 2020; Kong et al. 2020a, b, c). Experiments on ETC with matrix turbulator (MT-ETC), ETC with coiled turbulator (CT-ETC), and ETC without turbulator (simple ETC) revealed that the turbulator-equipped designs could significantly enhance the tube mean temperature (Vasanthaseelan et al. 2021). The plain ET-SWH, ET-SWH with a twist ratio of 2 (Twist 2), and the ET-SWH with a twist ratio of 3 (Twist 3) were experimented with by Gunasekaran et al. (2021). The inserts reduced velocity and provided a uniform temperature distribution. They augmented the heat transfer at moderately high temperatures. The inserts avoided conducive heat transfer at a low temperature. The daily thermal efficiency of the plain ET-SWH, Twists 2 and 3 were 68.4%, 75.2%, and 73%, respectively. The photograph of CT, MT, and twisted tape designs is shown in Fig. 8.

FPCs and ETCs are widely used for water heating systems; however, incorporating other types of more efficient solar collectors based on the concentration concept in water heating applications can enhance energy savings and environmental impacts. CPCs are one of the promising solar collectors for domestic hot water applications. The concentrating collectors are equipped with mirrored surfaces to concentrate the solar energy on its receiver which is positioned at the focal point of the surface. Water flows into the receiver and absorbs the heat. These collectors reach much more temperatures than FPCs and ETCs. A good concentrator should be designed to improve the energy efficiency of the SWHs. Performance comparison of the empty tube PTC and PTC-equipped rings attached to different twisted tapes presented that the attachment of the ring improves the heat transfer rate; however, it augments the pressure drop. To overcome the pressure drop, modification should be made in the rings. It has been concluded that the Nusselt number increases up to 101% for the modified rings leading to a thermal performance improvement of 24% (Isravel et al. 2020). The feasibility of utilization of CPC in several locations worldwide (latitudes of 0, 15, 30, 45, and 60°N) by solar radiation range of 2509–852 kWh/m² and electricity cost range of 0.04–0.28 USD/kWh was investigated. In locations with high electricity costs, even in cases of low solar radiation, investment in CPC had the highest profits (Gilani and Hoseinzadeh, 2021). Preheating of boiler feed water (Pathak et al. 2021), generating steam for use in the textile industries (Eskin 2000), water treatment (Li et al. 2022), and desalination plants (Rahimi-Ahar and Hatamipour 2020) are the applications of CPC technology. CPC by providing hot water of > 60 °C can also be used in medium-sized food industries (Ktistis et al. 2021; Yılmaz et al. 2023).

ICS-SWHs convert solar radiation into heat at a considerable conversion rate. It consists of the main components of a vessel and a reflector. Figure 9 presents the schematic of an ICS-PT-SWH. Due to the inherent freeze protection of an ICS-SWH, it offers a promising technique for water heating in colder climates. These compact systems are of low cost with a high solar collection efficiency. The thermal analysis on an ICS-CPC showed that mirror, steel sheet, and aluminum foil have the maximum daily efficiencies of 66.7%, 47.6%, and 43.7%, respectively (Panahi et al. 2019). Selecting a proper concentrator, controlling hot water withdrawal, coupling the absorber to the storage tank, and developing an ICS-SWH system increase the thermal efficiency of SWHs. The position of the absorber pipe relative to the PTC substantially affected the system's efficiency by 70% while for an absorber pipe positioned at the focal line of the collectors, it was less than 60%. An increase in the solar radiation intensity decreased the thermal efficiency of the proposed systems. The lowest and the highest efficiencies were in July and April, respectively. It means the highest efficiency was obtained on colder days or in colder areas (Avargani et al. 2020).

The thermal performance of SWH was evaluated for transparent baffles positioned at various parts in the collector cavity. The baffles positioned at the upper part of the exit aperture of the CPC decreased the thermal losses by convection suppression without a meaningful increase in the optical losses (Smyth, et al. 2005). The CPC using a plastic substrate permitted the heater to reach an optical efficiency of nearly 65%. The energy efficiency of 55–35% was obtained in the proposed design depending on the solar radiation intensity (Smyth et al. 2004). An ICS-SWH equipped with a linear PTC with a reflector made of rectangular and rotating mirrors was designed by Harmim et al. (2019). The setup was effective during winter with a daily efficiency of 36.4–51.6%. Under a clear sky with an initial water temperature of 22 °C, the water temperature reached 49 °C. The schematic of an ICS-SWH containing a PTC with rotating mirrors is shown in Fig. 10.

An ICS-SWH was theoretically studied to evaluate the thermal and optical behaviors of the heater and clarify the effect of the truncation. The heater consisted of the upper part with two parabolic sections and the lower part with three involute reflectors. The concentration ratio and the height of the upper concentrating stage were important parameters. A maximum temperature of about 60 °C was obtained for full and truncated CPC. For the full CPC, the direct solar ray falling on the aperture plane provided more reflections before reaching the absorber, while for truncated CPC, less reflection was made. It means a reduction in the mean number of reflections had a positive influence on the optical performance of the studied heater. A reduction in the size of the storage tank was suggested to maintain an acceptable level of the mean temperature (Benrejeb et al. 2016). The reflection of the solar radiation on the CPC reflectors was simulated using a ray-tracing model in an ICS-SWH consisting of two symmetrical parabolic and three involute reflectors (Kong et al. 2020a, b, c). The design of the lower stage concentrator showed a meaningful enhancement in optical efficiency. The proposed system reached the maximum water temperature of 65 °C. The reflector shape was modified, in which the old reflector had three parabolic sections, and the modified one had two involute parts and two parabolic sections; hence, the system size was reduced while keeping an acceptable level of the mean water temperature. This truncation reduced the manufacturing cost and facilitated its installation.

An FPC was equipped with a controllable sun tracker PV unit to increase the absorbed solar radiation. The heater produced hot water of 50 °C and electricity for the households in winter. The maximum overall average energy efficiency was 62% (Touaba et al. 2020). When the economic, environmental, and energy benefits were considered, the solar thermal system was more favorable compared with PV and PV/T systems. An ICS-SWH combined with a PV/T module offered a promising approach for water heating and electricity generation. A high tank water mass and the solar cell packing factor led to a high PV/T efficiency. It was due to the high temperatures of the solar cell and the water in the tank. The planar and tubular thermoelectric generators also could increase the quantity of the produced hot water (Faddouli et al. 2020).

A direct-expansion solar-assisted heat pump water heater (DX-SAHP-WH) comprised of the electronic expansion valve, compressor, solar collector/evaporator, and condenser enhances the thermal performance of the heater by hybridization technology. Remote buildings in cold areas benefit from the winter heating system of air source HP coupled with an ET-SWH (Li et al. 2023).

The schematic of a typical DX-SAHP-WH is presented in Fig. 11.

A quasi-dynamic mathematical model of a DX-SAHP-WH was developed at different compressor speeds. The mathematical equations governing lumped elements were established to model the compressor and expansion valve. The uniform flow distributed parameter methods were used to model the unglazed FPC/evaporator and condenser. A minor impact of the compressor speed on the average coefficient of performance (COP) was noticed. Under distinctive working conditions, the COP reached above 4.6 for various compressor speeds, when the water temperature changed from 28.2 to 61 °C. When the mean compressor speed was reduced from 2910 to 2650 rpm, the COP increased by 7.5%. By increasing the compressor speed, the evaporating temperature stabilized while slowing down the compressor speed stabilized the compressor power (Kong et al. 2020a, b, c).

The multi-criteria optimization of a DX-SAHP-WH using R-134a by genetic algorithm (GA) was performed by the trial and error technique (Cao et al. 2020). The comparison of single- and bi-criteria optimization results confirmed the superiority of this optimization method. The bi-criteria optimization results revealed that collector efficiency was reduced by 1.6% compared to the primary model, while the COP improved by about 20%. The performance of the optimized heater was enhanced so that the working hours of the heater decreased to 109 h. In a DX-SAHP-WH, as the gas cooler outlet pressure and temperature increased, the water inlet temperature also increased; hence, the COP decreased. The relative humidity variation from 31.6% to 55.8% and solar radiation intensity increase from 30.17 to 876.9 W m⁻² increased the COP by 6% and 30%, respectively. The increase in solar radiation led to an increase in compressor suction and discharge temperatures, evaporating pressure, and water flow rate, while the reduction in the collector efficiency was concluded (Duarte et al. 2021). A DX-SAHP-WH comprised an expansion valve, a compressor, a micro-channel solar collector/evaporator, and a condenser was designed (Kong et al. 2020a, b, c). R290 with a refrigerant charge of 410 g or 350 g were used, and the average COP ranged from 2.12 to 4.43. A DX-SAHP-WH charged with 800 g R134a produced water with a maximum temperature ranging from 55 to 60 °C, while the COP of the system was more than 3.0 (Kong et al. 2018). By increasing the ambient temperature or solar radiation intensity, the COP increased, while the heating time decreased. Modeling of DX-SAHP-WH, including the collector/evaporator under typical frosting conditions, presented the superiority in delaying frosting compared to the traditional evaporator (Kong et al. 2020a, b, c). The collector efficiency enhanced effectually during the growth of frost crystals.

SWHs incorporating PCMs as thermal energy storage (TES) have been developed since the 1970s (Shalaby et al. 2020). A schematic of SWH containing PCM is illustrated in Fig. 12.

PCMs are classified as inorganic, organic, and eutectic materials. They are encapsulated within the side surface of the storage tank. The remaining energy of PCMs helps in any sudden thermal loads during the night. PCMs categorize into solid–gas, solid–liquid, liquid–gas, and solid–solid states (Kee et al. 2018). The solid–liquid PCMs are commonly used in SWHs due to the limitations of the three other types of PCMs. The solid–solid types have relatively low heat transformation, significant volume variation, and high pressure in the existence of gas. During the daytime, the temperature of PCM increases, and PCM absorbs heat; it reaches the melting point and absorbs heat at a certain temperature. The PCM absorbs thermal energy and stores heat without a significant increase in temperature until all PCM melts. During the night, the surrounding temperature of the liquid PCM decreases and solidifies the material, consequently releasing the stored latent heat to the surrounding environment. Adding high thermal conductive fillers to a pure PCM improves thermal conductivity and stability, hence speeding up the heat storing and releasing rates during phase change. Forming a stable composite by polymerization, physical blending, or encapsulation is recommended. Notably, during the melting, the PCM is confined in the layered and the movement of the PCM is limited; hence, using stable composite PCM is strengthened. The integration of PCM increases the outlet temperature and extends the operating hours due to a thermal lag caused by energy storage, hence a delay in the temperature response (Tamuli and Nath 2023).

A proper PCM has a demanded melting/freezing temperature within the desired usage temperature range, high values of latent heat of fusion, thermal conductivity, specific heat, and density, a small difference between densities of solid and liquid phases, congruent melting of the materials, insignificant supercooling, high chemical stability, non-corrosivity, non-poisonous, nonflammability, non-explosivity, and abundant supply at a low cost (Liu et al. 2012). Adding nanofillers in lower concentration improves these characteristics and provides more hot water for the next day morning as compared to a conventional SWH. Several attempts have been made to increase the heat exchange between the water and PCM. Different PCMs used in SWHs, their characteristics, and their effect on the thermal efficiency of the SWH are tabulated in Table 1. Aluminum oxide nanofillers could augment the energy storing capacity of an SWH (Dhinakaran et al. 2020). PCM-containing nanofillers absorbed more heat energy and released the heat for a longer time, leading to a 33% increase in water temperature. The low thermal conductivity of the PCM was compensated by dividing the storage system into thin slices (Shalaby et al. 2020). Shell and finned tube latent heat storage system integrated with the FP-SWH design provided a large heat transfer area between the storage material and the working fluid. The maximum energy efficiency of 65% was achieved when a paraffin wax/water mixture was used. This heater was able to provide hot water at a maximum temperature of 60 °C. A double-walled tank by embedding the PCM in the outer part of the tank with two positive functionalities of thermal insulation and thermal storage led to the highest water temperature of 80.3 °C and the maximum thermal efficiency of 74% at a water flow rate of 1.75 l/min. Moreover, the maximum thermal stratification occurred at a water flow rate of 1.25 l/min, and it weakened by increasing the flow rate (Yari et al. 2023). The thermal efficiency in an ET-SWH in stagnation mode (without water flow) was improved from 66 to 82% using PCM. By flowing mode variation from 10 to 27 l/h, no considerable reduction in the heat gain of the collector was observed; however, 40 l/h plunged the discharge efficiency (Sadeghi et al. 2022).

A mixture of paraffin wax and nano-CuO was used as a thermal energy reservoir (Mandal et al. 2019). An increase in the nano-CuO concentration in the CuO-PCM mixture decreased the outlet temperature of the water. This was because of the large volume/surface area of nano-CuO leading to transferring the maximum thermal energy to the water instead of absorbing it as well as the low heat capacity of CuO-PCM nanocomposite over the paraffin. A maximum temperature of about 58 °C was stored by base paraffin in the gap of 2 cm between the glazing and the absorber. It was concluded that the nano-CuO was not effective during the night. A solar storage tank coupled to a PCM (NaOAc, 3H₂O) module was simulated and showed that increasing the PCM amount reduced the melting and enhanced the heat losses to the surrounding (Bouhal et al. 2018). A PCM was prepared by modification of coconut shell charcoal (CSC) by H₂O₂ and then was stabilized by stearic acid (SA) (Xie et al. 2020). The thermal conductivity of the SA/CSC was about 2.9 times more than that of pure SA. The modified CSC appeared more super macropores contributed to the impregnation of SA than non-modified CSC. SA/CSC composite had excellent thermal stability at high working temperatures (180 °C). The composite was stable during phase change and had good thermophysical properties as well as more heat transfer efficiency than pure SA. The maximum temperature of the water leaving the FPC, CPC, and ETC was about 115, 105, and 104 °C, respectively.

The performance of ET-SWH was studied without PCM and using the paraffin and nano-CeO₂ diffused in paraffin with mass fractions of 0.5%, 1.0%, and 2.0%. PCM-containing nanoparticles was well performed because of the nonlinear increase in thermal conductivity by increasing the nanoparticle concentration. The PCM-containing 1.0% of nanoparticles used in thermosyphon-based WGET-SWH led to the maximum first and second law efficiencies of 89.15% and 8.59%, respectively (Kumar and Mylsamy 2020). It was due to a low reduction in latent heat and proper shrinking of the gap between solidification and melting points. A set of ET-HP arrays coupled to a tank filled with three types of PCM based on paraffin wax was modeled (Bazri et al. 2019). The maximum energy efficiency of the PCM coupled with SWH on a sunny day was 54%, while this efficiency increased to 58% on a cloudy or rainy day. The average efficiencies of the heater containing three used PCMs were increased from 10 to 58% compared with the design without PCM.

A transparent glass cover was placed at 0.02 m from the absorber, polyurethane insulation, and using a copper pipe as a heat exchanger provided hot water of 75 °C. Paraffin wax, a paraffin wax nanocomposite with 1.0 wt% SCi and CuO, used as the PCMs in an ET-T-SWH (Manirathnam et al. 2021). The thermal conductivity of paraffin wax by the diffusion of CuO and SCi nanoparticles was meaningfully augmented (22.53%). The water temperature in the cases of PCM and nanocomposite PCM was 53 °C and 56 °C, respectively, while for the heater without PCM, it was 45 °C. The energy efficiencies were found to be approximately 34%, 38%, and 42%, respectively, for ET-T-SWH without PCM, with PCM, and with nanocomposite PCM (Mandal et al. 2020). A double-pipe helical coil heat exchanger containing PCM (RT-50) in the annulus SWHs (creating a turbulent mode of water flow in the inner pipe) was simulated. The k-ε RNG turbulence model well described the turbulent flow. Among helical pitch, outer and inner pipe diameters, Reynolds number, and fluid inlet temperature, the outer and inner pipe diameters and fluid inlet temperature were the main factors influencing the storage system's design. The 1.5% and 42% increases in the inlet temperature and the inner pipe diameter enhanced the melting rate by 27% and 92%, respectively, while a 20% increase in the outer pipe diameter led to a 52% reduction in the melting rate (Najafabadi et al. 2022). Adding PCM into a mantled hot water tank increased the outlet water temperature in the tank compared to without a PCM case. Furthermore, placing the mantle resulted in rapid maximization of temperature within the tank. In addition, replacing the PCM tubes on the top section of the tank resulted in an enhancement of 19.6% in the outlet water temperature compared to the case without PCM (İzgi and Arslan 2022).