Experimental Enhancement of Solar Water Heater Performance Using Carbon Nanotube Coatings and Enclosure Modifications

The technique of photo-thermal technology extracts solar radiation power through components that absorb solar light before transforming it into heat energy, which gets transferred to water or air as a working fluid. The generated heat has applications for residential, industrial, and agricultural purposes through direct usage or storage for later retrieval. The system utilizes surfaces that capture broad solar radiation while achieving effective reduction of thermal losses by conduction, convection, and radiation. Photo-thermal technologies have become a worldwide standard to produce thermal energy because their applications extend to water heating, space heating, and industrial process requirements [1]. The solar thermal collector constitutes a fundamental part of photo-thermal systems because of its specific characteristics described in the study of Xia and Chen [2]. Evacuated tube solar collectors (ETSCs) demonstrate superior performance across different environmental settings. These systems are becoming more and more popular due to their minimal impact on global warming, long duration, and low cost [3].

Solar water heating systems are a good alternative to the use of fossil fuels and greenhouse gas emissions in homes. Etihad ETSCs are the most suitable and efficient technology among other technologies in terms of thermal efficiency, especially in cool and low-radiation weather, because the vacuum provides them with insulating properties. Nevertheless, convective loss, non-optimal absorber materials, and an inefficient reflector integration still hold back the system performance. Recent studies have been carried out on how to improve ETSC, which can be done by nanofluids, alteration of the absorber geometry, integration of a reflector, and optimization by computer. Research has demonstrated that nanofluids have the potential to enhance the properties of heat transfer, and a reflector system can enhance solar concentration under a controlled environment. Another wide use of computational fluid dynamics (CFD) has been in predicting thermal performance as well as in optimizing collector designs. In spite of such developments, the majority of available studies consider single techniques of enhancement separately or use numerical modeling without further outdoor confirmation. Further, there has been very little literature that has experimentally tested the concomitant impact of surface modification on absorbers, enclosure inspired insulation, and manipulable reflectors in the actual climatic setup. Specifically, deploying carbon nanotube (CNT) coatings as solid layers (instead of nanofluid additives) in the layer of solid absorbers is an area that has not been fully explored in practice with regard to enhancing solar water heating. Additionally, reflector angle positioning has been found to affect shading losses in outdoor operating conditions, which has to be empirically confirmed. To deal with these gaps, the current research experimentally examines the nine modular designs incorporating:

(1) CNT-coated aluminum plates of absorbent,

(2) insulated structures of enclosure boxes to minimize the convective losses, and

(3) 45 degree and 90 degree reflective mirrors that can be adjusted.

The system was operated in the actual outdoor conditions with multi-point temperature monitoring to check the daily thermal performance and heat retention properties of the system. The aim is to establish the best combination of material and geometry changes to enhance domestic solar water heating, to employ applied and least expensive solutions.

Recently, the enhancement of solar thermal collector performance has been the focus of several studies. In order to prevent solar collectors from overheating [4] has been developed two vacuum tube models by raising their infrared emissivity and heat convection coefficient, respectively. Found that the first model can negatively influence the collector's efficiency by at least 26%, whereas there was no effect on the second model. Fertahi et al. [5] proposed an improved model of a horizontal hot water storage tank of a solar water heater (SWH), and showed that there is a direct relationship between the horizontal tank's temperature on average and the number of heat pipes. Alfaro-Ayala et al. [6] modified the arrangement of ETSC, water in glass, and low temperature to forecast their ideal efficiency. Found that the thermal performance of the best form had increased in all running and environmental variables, with 14 fewer tubes and 38.9% lower cost compared to the industrial model. Salvi et al. [7] showed that the adjustments of the absorber’s, and utilization of the thermal mirror films and appropriate nanoparticles can improve the performance of solar collectors. Teles et al. [8] proposed a numerical model of ETSC with low concentration. Showed that decreasing heat generated stress and maintaining a consistent temperature for the absorber were the two benefits of the reflective film, with a significant increase in thermal efficiency. Yurddaş [9] showed that the utilization of Cu-Water nano-fluid provides an improvement in heat transfer for ETSC. In addition, Cucumo et al. [10] studied the effectiveness of a solar thermal generator. Found that the largest losses were caused by the poor reflectance and distance of the mirrors, with around 38% wastage. Otherwise, about 11% of losses were due to the poor absorption of sunlight. In the study of Abo-Elfadl et al. [11], the integration of reflectors with ETSC-HP (evacuated tube solar collector-heat pipe) system on its thermal energy storage showed its capacity to increase the collector system's storage of energy in comparison with previous research. Furthermore, Madadi Avargani et al. [12] had analyzed an array of parabolic trough collectors (PTCs) for SWH as a new kind of technology. This system showed better thermal efficiency compared with the traditional systems. According to the results of studies published over the past 10 years that used nano-fluids in ETSCs, Olfian et al. [13] showed that ETSCs were the most suitable choice among other solar collectors. The use of Single-Walled Carbon Nanotubes (SWCNTs) nanofluids provided beneficial effects for improving the heat pipe model. Nokhosteen and Sobhansarbandi [14] aimed to analyze the heat transfer mechanisms occurring in ETCs using a hybrid numerical tool. Accurate, quick forecasting came from the proposed Reduced-Order Proper Orthogonal Decomposition–Lattice Boltzmann Method (RNPOD-LBM) models according to Du et al. [15]. Solar thermal research extensively utilizes CFD as a fundamental tool because it enables thorough simulation of fluid movement coupled with thermal distribution and pressure differences across the collector framework. The thermal response of solar collectors running under different conditions becomes predictable through Navier-Stokes equation solutions alongside energy conservation models in CFD simulations. CFD processes represent an essential simulation method for system optimization that enhances design quality and reaches maximum thermal efficiency by reducing operational losses before prototype development. Kumar et al. [16] showed that the rise in solar radiation intensity, changes in tilt angles, rating of heat transfer forced and natural circulation, number of evacuated tubes, heat storage materials, depth of water in still, material of glass cover, and its mass flow rate and temperature had an impact on the evacuated tube's performance. Dinesh et al. [17] used a wavy and flat diffuse reflector in order to enhance the efficiency of the ETC- SWH. According to the results, using wavy and flat diffuse reflectors enhances the temperature of tank waters by 4 ℃ and 6 ℃, respectively, throughout the day. Altaher et al. [18] showed that the utilization of aluminum foil as a collector surface cover of both main and secondary vertical reflectors resulted in a better thermal performance of the ETC. Rahimi-Ahar et al. [19] analyzed the thermal performance of several SWH types. Found that concentrated SWHs performance outperforms other types of heaters, as well. Combining these systems with heat pumps, phase change materials (PCMs), or energy storage components provides outperformed performance than traditional SWHs. The work discussed by Mokhlif et al. [20] showed that the thermal efficacy of integrated collector storage (ICS) with SWH used together with insulated reflectors as a cover was higher than that of the ICS-SWH without insulated reflectors by 23%. Al-Mamun et al. [21] showed that the application of base liquids and nanofluids, selection of the collectors’ types, within the operating temperature range. Moreover, PCM integration with solar collectors or storage tanks could enhance the thermal properties of SWH systems in comparison to the conventional types. Furthermore, Abu-Zeid et al. [22] used a flat plate solar collector (FPSC), still, FPSC nanofluids, a reflecting mirror, and a v-corrugated basin in a conventional solar still (CSS), which significantly increased thermal performance and productivity (Pd) by increasing water temperature and the gradient between the hot and cold inner glass cover surfaces. Aljabair et al. [23] aimed to investigate the effect of different diameters and depths of the parabolic dish collectors on the focal point's location. Showed that the dish's diameter and the degree of angle of inclination can determine the focus's placement from the dish. The performance of thermal energy storage was examined experimentally by Abed et al. [24] found that, the maximum thermal storage of 110 minutes is achieved in hot flow rate qh = 4 LPM, cold flow rate qc = 5 LPM, and h = 20 cm, in present of porous medium [25] investigated that, there was a direct relationship between the porous media's height level and hot water supply. As well, when the rate of solar radiation intensity increases and the average variation in temperature reduces, the collector's efficiency improves.

The technique mentioned is the ETSCs, which are becoming prominent as the technology of harnessing solar energy, especially in cold regions where there may not receive much sunlight. Studies are done to improve ETSC efficacy by means of nanofluids and algorithms optimization of systems, and to discover CNT nanotechnology coating to promote heat entry. Nevertheless, there are still some challenges, such as the absence of real-life testing of the optimizing elements and the lack of innovative ideas for providing affordable solar power to the community with limited resources. Available information about the performances of reflectors is not many especially on the angle and position of reflectors. The paper suggests the experiment with different design improvements of solar collectors, the use of CNT-buttered absorbers in insulated envelopes, and the importance of a comprehensive study of reflector angles to reduce the losses of shading. The almost optimal design was proven to be the one that had an experimental increase of maximum temperatures of 64.24 ℃.

Figure 5. Temperature increase in Cases 1–6

The data demonstrated by Figure 5 and Table 2 show that the baseline system has a moderate growth in temperatures during its daily working cycle. The longitudinal temperature profile over the storage tank represents a fairly homogeneous heating profile with little stratification effects across the top to bottom. Although there is this consistency, the general increase in temperature is low, which is a pointer to inefficiency in terms of performance. It is the moderate effectiveness of the baseline configuration due to a number of factors. Firstly, the enclosure of the heat is a major concern, also, the evacuated tubes are to operate under a vacuum situation that the internal convective and conductive heat losses are to be minimized through the vacuum enclosure the external factors like ambient wind are able to cool down the system, this is due to the fact that there is no surrounding structure to reduce the external heat losses through convection. The second factor contributing to the suboptimal performance is that there is no further plate of solar absorber, which increases the amount of solar radiation that is entrapped, and a fraction of the solar energy is reflected back or does not trap any solar energy effectively. In addition, the uncovered areas of the collector also carry energy dissipation effects by radiative and convective losses, particularly on elements such as the collector manifold and the structural elements.

3.3 Effect of reflector angle and shading mechanism

This paragraph talks about the physical phenomenon of the shading effect of reflector panels that are used to enhance incident sun radiation. It shows that in some designs of reflectors, the performance can even be worse than expected in theory, mostly because of self-shading. Reflector panels at a 45° angle will be able to prevent direct solar radiation at peak solar altitudes, thereby casting a shadow on the absorber surface, resulting in lower net radiation gain than expected. On the other hand, at 90° angle, there is some redirection of low-angle radiations, but at the same time, vertical panels are able to block the lateral incidence of the sun, thereby providing performance inferior to that of a system employing a single enclosure. The results demonstrate one design reality according to which a geometry that optimizes reflective geometry should be adopted to consider the changes in the altitude of the sun. The paper underlines the fact that improper angular position may result in the net solar gain reduction, regardless of the theoretical benefits of the reflectors. This empirical evidence has culminated in the need to take into consideration the seasonal trajectory of the sun in the integration of a reflector design instead of working with the definite angle arrangement. Figure 6 and Table 3 investigated reflector integration at 45° and 90° angles for Cases 7–9.

Figure 6. ΔT for no reflector (Case 6), 45° reflector (Case 7), 90° reflector (Cases 8–9)

Table 3. The numerical values

3.4 Nighttime thermal retention performance

Study data demonstrated that the system maintained thermal energy efficiently during evening hours using enclosure boxes and high-absorptivity coatings that included CNTs. Vacuum insulation in the tube design prevents thermal losses due to both convection and conduction. An enclosure box created a thermal barrier to protect from wind-driven cooling and ambient air. The nighttime temperature decrease was minimal, because of which the system managed to sustain warm water temperatures throughout the early morning. The system improves thermal energy storage during nighttime, which benefits domestic water heating because users need hot water after dark. These design modifications improved daytime performance while extending the accessible stored thermal energy beyond the direct solar period, which gave the system better practical usability.

The paper discusses ways of solar water heating systems by the use of CNTs, reflective materials, and outside boxes. Combining the experimental data with theoretical constructions, researchers today strive to enhance efficiency and create applications with the help of advanced materials. Results indicate that the developed performance criteria may be employed to make policy inferences on the use of renewable energy and provide incentives to adopt solar thermal in schools to present possible cost savings and higher feasibility in cold climates.