Performance Comparison and Light Reflectance of Al, Cu, and Fe Metals in Direct Contact Flat Solar Heating Systems

Abstract

The Sun is a huge and clean energy source that must be relied upon to reduce greenhouse gases and promote the renewable and sustainable energy transition. In this paper, the testing of Al, Cu, and Fe metals with different thicknesses, both bare and painted matte black, was investigated for solar water heating systems. The used technique was a direct contact flat solar heating system (DCFSHS). Many experiments were run to assess this system in terms of metals’ thicknesses and their thermal conductivities as well. Thicknesses of around 0.35 mm and 1 mm of Cu gave almost similar feedback. Maximum temperatures in the range of 93–97 °C were achieved during the autumn season in Amman, Jordan, while it was approximately 80 °C in winter. It has been confirmed that high water temperatures can be obtained in all used metals, regardless of their thermal conductivities. It was also found that a white color of the solar heater case inner wall leads to an increase in water temperature of approximately 4 °C in comparison to a black color. Furthermore, a light reflectance % test in the wavelength range of 240–840 nm for the studied metals, with both bare and black-painted surfaces, gave a superb result that was in line with the obtained results of the DCFSHS. Our innovative system design for solar water heating is due to improvements in many aspects, such as design, production costs, environment, and weight.

1. Introduction

Solar energy can be used directly to produce green electricity and green thermal energy for domestic and industrial uses, thus promoting green energy sources, reducing greenhouse gases to improve the environment, and reducing energy bills over the long term. Often, conductive metals are used in manufacturing solar water heating systems (SWHS), such as copper, aluminum, nickel, and their alloys. The reason is due to their high thermal conductivity and good corrosion resistance. Generally, there are three types of SWHS, namely the flat plate, concentrated heating technique and evacuated tube technology. The following heating systems have been discussed and reviewed by Vengadesan et al. and Dehghan et al. [1,2]. The authors encourage further investigation in exploiting solar energy to face the greatest challenge of the greenhouse effect and the shortage of fossil energy sources. Moreover, they give essential details about the different types of SWHS and their use to obtain better ranges of water temperatures as required.

Sethupathi et al. and Hohne et al. have reported significant information about the efficiency of many types of SWHS, depending on many parameters, such as the surface area of collectors, the received solar intensity, the specific heat capacity of the heated fluid mass and its mass, and the system design [3,4]. The cheapest and easiest solar heating system is the flat plate, because it does not need highly sophisticated technology. Most of these systems start with plane sheets of Cu, Al, Ni, and their alloys, and stainless steel, followed by surface treatment to produce highly absorbent solar surfaces [5,6,7,8,9]. Some metal particles, oxides, and compounds are normally added into a black pigment during the manufacture of solar selective surfaces to increase their absorption of solar radiation, reduce emissivity, and thus increase efficiencies, and probably improve the corrosion resistance [10,11,12,13].

Furthermore, in solar heating systems, nanoparticles play a vital role in enhancing the absorptivity of selective surfaces when mixed with black paint. Moreover, these particles will increase the thermal conductivity and heat transfer fluids that are used in indirect solar heating systems. This crucial subject has been reported by many workers in this field, supporting their results with TEM and SEM images to show the range of nanoparticle sizes of several pure metals and metal oxides [14,15,16,17,18]. Generally, the significant properties of any selective surface for solar heating applications are its absorptivity for visible light and infrared radiation, corrosion resistance, design simplicity, low cost, and material availability. Mostly, these specifications are attainable with some non-ferrous metals such as aluminum, copper, and nickel, which are black-painted or coated with good black solar absorbance materials [19,20].

Practically, the thermal conductivity of metals used in the manufacturing of solar heaters is not an effective factor in obtaining high water temperatures when direct contact is made between the heated fluids and the solar absorbent surfaces [21]. However, practical information is needed on this subject, as well as on the effect of the metal sheet thicknesses of SWHS, which impacts the manufacturing cost. In addition, the light reflectance % (R%) of metal surfaces, painted and non-painted, that are used in solar heating production needs to be investigated to elucidate the gained results. Moreover, the interior color of the box containing the parts of the solar heater may affect the temperatures of the heated fluid.

Therefore, the research goals of the current study are to assess the performance of direct contact flat solar heating systems (DCFSHS) of Al, Cu, and Fe metals, both non-painted and painted matte black. The effect of metal types and their thicknesses will be examined. Water temperatures, global solar intensity, and ambient temperatures will be measured instantaneously to interpret the results in Amman, Jordan. This attempt at research also focuses on the effect of the thermal conductivities of the used metals in the construction of the DCFSHS, as well as its inside case wall color. Light reflectance % (R%) will be measured for all surfaces, non-painted and painted, to support the attained results.

2. Heat Conduction

It is very necessary to give an idea about the factors affecting the process of heat transfer in materials. Predominantly, metals and their alloys, such as Cu, Al, Fe, and Ni, are commonly used in the manufacturing of solar water heating systems (SWHS). The reason for this selection is that these materials are characterized by high thermal conductivities. In addition, their surfaces are black-painted, coated, chemically treated, and oxidized to increase their absorption of solar radiation. The heat conduction of the used materials is very important to harvest the available solar energy in a short time and thus obtain high water temperatures pertaining to the desired applications.

Generally, good conductors of electricity are also good conductors of heat due to the presence of a vast number of free electrons per unit volume in conductive materials. As an example, the density of the free electrons (conduction electrons) of high-purity Cu is approximately 8.5 × 10²⁸ m⁻³ [22]. Thus, it is classified as a good conductor of electricity with very high thermal conductivity (~400 W/m·°C) [22]. The process of heat conduction in any solid substance can be explained due to the following:

• The vibration of molecules or atoms of solid materials, as a result of heating, causes micro-mechanical waves (phonons) because of their lattice structure vibration. Thus, they transmit thermal energy toward the temperature gradient from one point to another in the heated substance. This process depends on the lattice structure and temperatures of heated materials [22].
• Free electrons in solid materials play a vital role in heat transfer. When the substance is heated up, the kinetic energy of its free electrons increases. The free electrons then move away from the region of higher temperature to the region of lower temperature to lose their thermal energy, and then return to the higher-temperature region, and so on. Consequently, the formation of a structure resembling a continuous electronic vortex transfers thermal energy if a temperature difference exists in the used material. The thermal conductivities and specific heat capacities of Al, Cu, and Fe are reported in

  Table 1. Thermal conductivities and specific heat capacities of Al, Cu, and Fe at 25 °C and 1 atm [22].

  Metals	Thermal Conductivity W/m. °C, Page 609	Specific Heat Capacity J/kg. °C, Page 594
  Aluminum	238	900
  Copper	397	387
  Iron	79.5	448

  . Essential details about heat transfer in materials can be found in the literature [22].

3. Brief Solar Energy Information about Jordan

It is imperative to give an idea about Jordan’s solar energy, as the topic of the research is in Jordan. The geographical location of Jordan is between latitude 32° North and longitude 36° East. This region receives a global solar energy intensity within the limit of 4–8 kWh/m² as an annual daily mean value, and this is equivalent to the energy intensity of 1400–2300 kWh/m² a year [23]. The literature also confirms that Jordan is classified as a country that has few fossil sources, which fulfil only a minor part of the national energy needs. Therefore, the government relies on importing fossil fuels to meet the energy demands of the country. The sunny days in Jordan are in the range of 300–330 per year, which is expected to continually change according to the global weather conditions [24,25,26,27]. The global irradiation distribution map of Jordan according to solar information from the International Renewable Energy Agency (IRENA), and the curves of sunshine hours and global solar energy intensity, are shown in Figure 1a,b. It can be concluded that most of Jordan’s territory is suitable for the generation of sustainable clean electricity and thermal energy.

Accordingly, the search for renewable, clean energy sources that are locally available is one of the main tasks of Jordanian society to provide the required energy to reduce its expenditures. Meanwhile, improving the environment is a national and global contribution. The foregoing essential solar details lead us to think seriously about the use of solar energy at a product level, as it is the most available source, to keep pace with the inevitable global shift towards renewable energy projects, consequently supporting the environmental aspect and the country’s economy.

Electric energy consumption for water heating is very high and it costs huge sums on a national level. Currently, the price of electricity in Jordan is the highest among the countries of the Middle East (~0.1 USD/kWh); approximately 1000 MJ monthly (~28 USD) per system of hot water is required in the city of Amman [2]. As solar energy is sufficiently available in Jordan during most days of the year, it must be considered to provide hot water for domestic and industrial uses. Our research project was conducted in the School of Science, at the University of Jordan in Amman, the capital of Jordan. Figure 2 shows the mean values of daily sunny hours against the different months in Amman, and the cost of hot water per month in Amman.

4. Materials and Methods

4.1. Experimental Apparatus and Materials

Three identical homemade wooden boxes were constructed with outside dimensions of length 65 cm, height 35 cm, and 15 cm depth, with double glazing on the front side. Each box had an air gap of around 1 cm between glasses of a thickness of 3 mm. The inside color of the boxes was white. Each box was divided into two rooms by a wooden piece of thickness 3.5 cm for good thermal insulation. The thickness of the used wood to construct the boxes was around 1.7 cm. The air gap between double glazing was used to achieve better insulation to reduce solar energy losses. In addition, one room of one box was painted with a matte black color, while the second room was left white to determine the effect of the interior color on the water temperature during solar heating.

Eleven water metal containers were made with dimensions of 8 cm front wide, 4 cm in depth, and 28 cm in height and could be fixed in the wooden boxes. Three of them were of commercial anodized aluminum with a thickness of around 1 mm; two of them were painted with matte black paint (B. Al) and one was left as it was (Al). Four containers of commercial copper (Cu), with two thicknesses, 0.35 mm and 1 mm, were used; one of each thickness was painted with matte black paint. We also used four containers of commercial iron (Fe) with two thicknesses, 1 mm and 2 mm; again, one of each thickness was painted with matte black paint. The non-painted water metal containers are denoted as Al, Cu, and Fe, while the matte-black-painted containers are denoted as B. Al, B. Cu, and B. Fe. All water containers are specified in

Table 2. Details of the used water containers of different metals and thicknesses, non-painted and matte-black-painted.

Used Metals	Container Type	Metal Thickness (mm)	Symbolize
Commercial anodized Al	As it is Al Matte-Black-Painted Al	1	Al B. Al
Commercial Cu	As it is Cu Matte-Black-Painted Cu	0.35 0.35	Cu, 0.35 mm B. Cu, 0.35 mm
	As it is Cu Matte-Black-Painted Cu	1 1	Cu, 1 mm B. Cu, 1 mm
Commercial Fe	As it is Fe Matte-Black-Painted Fe	1 1	Fe, 1 mm B. Fe, 1 mm
	As it is Fe Matte-Black-Painted Fe	2 2	Fe, 2 mm B. Fe, 2 mm

.

An LED digital light meter, type MASTECH, MS6612T series, range 0~200,000 LUX, accuracy ±3%, China, was used to measure the global solar intensity (GSI). A matte black spray paint (Glance 20), UAE., was used to paint some of the designed containers of Al, Cu, and Fe as required. Two digital thermometers, type JR-1, −50–300 °C, accuracy ±0.1 °C, were used for water and ambient temperature measurement (T_wat. and T_amb.). A micrometer, 0–25 mm, accuracy ±0.01 mm, was used for thickness measurement. A Filmtek-3000 instrument, US-made, was used for the light reflectance % test. Figure 3a shows a schematic design of the twin wooden box with two Al water containers, silvery and black. Meanwhile, Figure 3b shows photographs of the three twin containers of Cu, Al, and Fe of around 1 mm thickness, bare and matte-black-painted. The three boxes received Sun exposure at the same time and this is illustrated in Figure 3c.

4.2. Experimental Procedure

The conducted method of solar water heating in this research was the DCFSHS. This route of water heating is very effective to obtain solar energy regardless of the thermal conductivities of the used selective solar surfaces [21]. Three identical wooden boxes with removable glazing facilities were constructed for easy water container housing. The designed water containers were fixed in wooden boxes according to the required experiments. The slope angle of the DCFSHS was around 42° {the site latitude + (10° -15°)} [28], as Amman’s location is at 32 latitude. The wooden boxes together with the water-filled containers were exposed to solar radiation. In the morning, the boxes were south-facing, and then they were always manually rotated to face the solar radiation during the measurement period. The rotation angle was around 90° during the measurement day, so the boxes at the end were facing westward. Water and ambient air temperatures and global solar intensity were measured on different dates and times. We used an instrument that only measured the global solar intensity and not direct solar intensity. Several experiments were run to test the design system for long periods as follows.

4.2.1. Al and B. Al Containers Solar Heating

A run of solar water heating was carried out using two water containers, Al and B. Al of 1 mm thickness. The experiment was run as mentioned above; the water and ambient temperatures (T_wat. and T_amb.) and global solar intensity (GSI) were recorded every hour from 8:00 a.m. to 5:00 p.m.

4.2.2. Effect of Metal Sheet Thickness of DCFSHS on Water Temperature

An experiment was conducted to distinguish the effect of the sheet thickness used to manufacture the solar water heating system. For this purpose, commercial iron (Fe) and commercial copper (Cu) of two thicknesses were tested, non-painted and matte-black-painted. Four containers of Fe were filled with tap water at room temperature. Two of them were of approximately 1 mm thickness, Fe and B. Fe, whereas the other two were of approximately 2 mm thickness, also Fe and B. Fe, as detailed in Table 2. After this, the four containers were exposed to solar radiation on 11/10/2021, and we measured the water and ambient temperatures and global solar intensity hourly. Likewise, on 14/11/2021, the same described procedure was repeated using four commercial Cu containers, Cu and B. Cu, with two thicknesses of around 0.35 mm and 1 mm, to determine the effect of Cu metal thickness on the water temperature.

4.2.3. Three Metal Types of DCFSHS with a Unified Thickness of Approximately 1 mm

An experiment on solar water heating was carried out by using six containers of three different metals, Al, Cu, and Fe, with a unified thickness of approximately 1 mm. This attempt aimed to determine the effect of the metal type on water temperatures when using DCFSHS. The containers were three pairs, as follows: Al and B. Al, Cu and B. Cu, and Fe and B. Fe. The experiment started at 7:00 and ended at 17:00 on a sunny day on 07/11/2021. Another attempt of the same experiment was conducted on 08/01/2022 to determine the limits of the water temperatures in the winter season as well. We observed the formation of black iron oxide in the heated water in the iron containers only, as presented in the Results and Discussion.

4.2.4. Effect of Inside Wooden Box Color on Water Temperature of Solar Heater

An experiment was performed to determine the effect of the interior color of the designed solar heater on the temperature of heated water. For this trial, two B. Al water containers were filled with tap water and then inserted into the wooden box with black and white interior rooms, as described in the Experimental Apparatus. The experiment was conducted on 12/02/2022, and its feedback and the experimental settings are given in the Results and Discussion.

4.2.5. Percent Reflectance (Reflectance %) (R%) Measurement

The percent reflectance (Reflectance%) (R%) as a function of wavelength within a spectrum range of 240–840 nm was measured using the Filmtek-3000 instrument. The test was carried out for the used metals, Al, Cu, and Fe, both bare and matte-black-painted. Two samples of each metal with dimensions 1.5 cm × 1 cm were cut and ground with 500-grade emery paper. One of each metal was painted matte black in the same conditions as that used for the water containers, whilst the other three were left as they were to obtain a reference R% measurement.

5. Results and Discussion

5.1. Al and B. Al Containers Solar Heating

It can be seen from Figure 4 that maximum water temperatures of around 74 °C and 93 °C were achieved with Al and B. Al, respectively. The mean value of T_amb. on the experimental sunny day was around 24.5 °C, and the maximum value of the global solar intensity was recorded at around 1:00 p.m., which was approximately 780 W/m² during the experimental insolation period.

The water temperature continued to increase until 4:00 p.m. and then started to decrease. The reason for this result is that the absorption of solar energy continued until the container’s radiation became higher than its absorption when the solar radiation decreased with time after reaching the maximum value.

5.2. Effect of Metal Sheet Thickness of DCFSHS on Water Temperatures

It seems from Figure 5 that the trend of all curves was almost similar. The water temperatures of the B. Fe containers were very close to each other; although they differed in thickness by two times, the thicker bare one had a higher temperature. This result may be due to its greater mass, as the absorbed energy was proportional to the mass of the heated body to increase its temperature (Q = m c ∆T).

After a 9 h insolation period (at 05:00 p.m.), all water temperatures were close to each other. After this, the temperature of the B. Fe container started to decrease faster than that of the Fe containers, because, normally, the radiation of black objects is higher than that of objects of other colors. The result of this simply designed experiment using the DCFSHS showed that high water temperatures were reached in all Fe containers, as shown in

Table 3. The maximum water temperatures, according to the used Fe, B. Fe, Cu, and B. Cu containers, with two thicknesses each, 11/10/2021 and 14/10/2021, respectively, Amman, Jordan.

Container Type	Container Sheet Thickness (mm)	~ Max. Water Temp. (°C)	Mean Values of Tamb. and GSI
Fe B. Fe Fe B. Fe	1 1 2 2	91 96 94 97	26 °C, 414 W/m2 Sunny day
Cu B. Cu	0.35 0.35	76 88	27 °C, 368 W/m2 Partially cloudy day
Cu B. Cu	1 1	75 88

.

It can be seen from Figure 6 that the curves of both Cu and B. Cu at 0.35 mm and 1 mm thicknesses are close to each other from the beginning to the end. This attempt confirmed that the thickness is not an effective factor to control the water temperatures of the DCFSHS, because, in the end, almost similar water temperatures will be attained. This result can be explained as depending on the idea of heat transfer by free electrons in metals, and due to phonons’ activity during solar heating, as discussed in Section 2. This finding is very important for reducing the price of solar heating system manufacturing while achieving almost the same yield. Therefore, choosing a minimum sheet thickness, which achieves sufficient durability for the solar heater, is the best choice in terms of production costs and system weight. Moreover, a maximum water temperature of around 88 °C was obtained in the case of matte-black-painted Cu containers, whereas the weather during the experimental period was partially cloudy, and the mean value of the global solar intensity was approximately 356 W/m².

5.3. Three Metal Types of DCFSHS with a Unified Thickness of Approximately 1 mm

Figure 7 shows six curves of DCFSHS for three metals, Al. Cu, and Fe, both bare and matte-black-painted. A water temperature of approximately 72 °C was reached after three hours of solar exposure of the B. Al container. In addition, maximum water temperatures of 93.5 °C, 92.8 °C, and 90.5 °C were reached in the cases of B. Al, B. Cu, and B. Fe containers, respectively, where the global solar intensity mean value was around 383 W/m² on a sunny day.

Furthermore, the rise in water temperatures for the three cases was almost linear up to 65 °C, and it then showed a nonlinear increase until reaching the maximum temperature, and then started decreasing at around 02:00 p.m. The water temperature of the non-painted Fe container was greater than those of non-painted Al and Cu. The maximum temperatures of the non-painted containers were around 90 °C, 74 °C, and 77 °C for Fe, Al, and Cu, respectively. This result can be explained by the fact that the absorbance and emittance coefficients of these metals depend on their purity, color, the roughness of their surfaces, and the working temperatures. It is clear from the curves that all water temperatures of the Fe non-painted container were greater than those of Al and Cu. This case can be justified by the container’s colors, as the color of the non-painted Fe container was almost grey, and its solar absorption was higher than that of the non-painted Al and Cu. The most important finding in Figure 7 is that the result is in good agreement with Bent et al. [21]. They reported that the thermal conductance of the used metal is not a very influential factor to obtain a large difference in water temperatures when the fluid is in direct contact with the selective surfaces of the solar heating system. As confirmation of this, it is noticed that although the thermal conductivities of the used metals strongly differed, as seen in Table 1, very similar high water temperatures were reached in all cases in the DCFSHS.

Generally, the water temperature in the Fe containers was slightly higher than that of Al and Cu, but its corrosion resistance was not comparable with the Cu and Al metals. Thus, choosing Al to manufacture solar heaters is the best option to achieve the following characteristics: a high water temperature, the overcoming of corrosion matter, a lightweight system because of its low density, and availability at a reasonable price [29]. Corrosion resistance in 3.5% NaCl solution of different Al surfaces, i.e., bare Al, silver anodized Al, and blackened anodized Al, has been reported by AlShamaileh et al. [30]. Meanwhile, when the water was emptied after a solar heating round, a black Fe oxide was formed in the Fe containers, and there was nothing in the water of the Al and Cu containers. This result is expected because the corrosion resistance of Fe with tap water is much lower than that of Al and Cu. Figure 8 shows a part of the suspended Fe black oxide of the drained water that was heated by solar energy in Fe containers. Practically, the issue of Fe corrosion resistance can be overcome by coating the container with a thin layer of Ni or Ni-P alloy, using chemical electroplating for Ni, or via a chemical electroless route for a Ni-P alloy [31].

It can be observed from Figure 9 that high water temperatures can be attained in the range of 87–89 °C, even in the cold winter, with approximately a 14 °C mean value of T_wat. and low global solar intensity of approximately 293 W/m² during the measurement duration.

5.4. Effect of Inside Wooden Box Color on Water Temperature of Solar Heater

Figure 10 illustrates the effect of the interior color of the used wooden box on the water temperature. It can be noticed that the maximum water temperatures were around 86.6 °C and 82.3 °C for B. Al containers that were inserted in the rooms with white and black walls, respectively. The difference was around 4 °C in favor of the water temperature in the white-painted room. The reason for this effect is due to the multiple reflections of solar radiation from the white walls, in addition to the reflection of the double-glazed front side. Meanwhile, the black walls will absorb the most incident solar radiation, which affects the water temperature as an inevitable result, as there is almost no light reflection. The mean values of the global solar intensity and the T_wat. during the experimental day were around 368 W/m² and 11 °C.

Therefore, it is recommended to paint the inside walls of any solar heating system case with glossy white paint to achieve high solar radiation reflection, so as to increase the water temperatures significantly. Figure 11 shows the experimental setting of the black and white twin room boxes with black-painted Al water containers. We believe that using different metal types for containers or different weather conditions will lead to the same trend as in the obtained results for the effect of the wooden box’s interior color on the measured water temperature. This effect is caused by the multiple reflections of the light falling on the water containers, in addition to the direct exposure to sunlight. The effect, obviously, may change upon changing any of the experimental variables.

5.5. Percent Reflectance (Reflectance %) (R%) Measurement

It can be determined from Figure 12 that the R% of the bare Fe surface was higher than that of anodized Al in the range of 240–400 nm, and lower than the anodized Al surface in the rest of the measured wavelength range. The bare Cu sample showed a noticeable increase in R% above the 600 nm wavelength. The variations in R% for different metal surfaces may be related to differences in grain sizes existing on the exposed metal surfaces, and the observed surface colors of the different metal surfaces mainly depend on the absorption and reemission of the incident light. Our result is in agreement with the results published by Oliva et al. on Cu, bare and black-painted, as they worked on Cu only [5]. In addition, the test confirmed that all matte-black-painted surfaces had almost the same R%, at approximately zero percent, and they were superimposed on each other. In general, the result of the R% measurement is in good agreement with the obtained solar heating measurements from the measured temperature point of view. Thus, the water temperatures of all matte-black-painted containers were very close, as seen in Figure 9, as an example, in which their R% is almost zero. As for the three non-painted containers, the curve of the Al is the lowest one because its R% is higher than that of the Fe and Cu.

6. Conclusions

Reliance on solar energy is a major factor in accelerating the transition to renewable energy. The use of renewable energy can reduce the emissions of harmful greenhouse gases, and it is gaining increasing attention as an economic alternative to fulfil the ever-increasing energy demand. The results of this research project can be summarized as follows:

• The obtained results confirmed the feasibility of using the DCFSHS technique as an efficient water heating mechanism, regardless of the thermal conductivities of the metals used in solar heating system production.
• Water temperatures higher than 90 °C were possible with Al, Cu, and Fe metals colored with matte black paint, whereas water temperatures in the range of 65–70 °C were possible using stock metals without any paint. These temperatures are well suited for domestic home usage.
• The thickness of the selected metals used in the construction of DCFSHS is irrelevant and high water temperatures can be easily reached in all cases. This was confirmed by using 0.35 mm and 1 mm thicknesses in the Cu containers. The almost identical heating profiles of both of these thicknesses allow for a reduction in the cost of solar heating system construction.
• Matte-black-painted anodized Al is an excellent choice for the construction of water heaters due to its good corrosion resistance, availability as a lightweight metal, and high thermal conductivity.
• It was also found that white inner walls in the solar heater can lead to an increase in the water temperature by approximately 4 °C compared to the black color. This improved the heat trapping process in the container.
• The R% test in the wavelength range of 240–840 nm for Al, Cu, and Fe metals showed tangible differences. The matte-black-painted metals gave almost zero R%, which makes them a better choice for solar-radiation-absorbing surfaces.
• The above-mentioned points highlight the innovative design of our solar water heating system due to the improvements in many aspects, such as design, production costs, environment, and weight.