Effect of Carbon Nanoparticles on the Performance Efficiency of a Solar Water Heater

Carbon nanoparticles are prepared by sonication using carbon black powder. The surface morphology of carbon black (CB) and carbon nanoparticles (CNPs) is investigated using scanning electron microscopy (SEM). The particles size ranges from 100 nm to 400 nm for CB and from 10 nm to 100 nm for CNPs. CNPs and CB are mixed with silicon glue of different ratios of 0.025, 0.2, 0.05, and 0.1 to synthesis films. The optical properties of the prepared films are investigated through reflectance and absorbance analyses. The ratio of 0.05 for CNPs and CB is the best for solar paint because of its higher solar water heater efficiency and is then added to the silicon glue. The temperature of cold water and temperature of hot water in the storage tank were tabulated on hourly basis with the help of an Arduino device. The atmospheric temperature was also noted. It was observed that the outlet temperature of the water was attained up to 75°C as compared to the inlet water temperature of 23°C for the tank applied with CNP-based paint. The tank applied with CNP-based paint has 4 °C higher water temperature than that coated with CB-based paint after 1 month of the test under sun irradiation. Based on the results, the efficiency of a solar water heater depends on the difference in temperature of inlet water and outlet of heater. The efficiency of the solar water heater coated with CNPs is around 77% but the solar water heater coated with CB has an efficiency of 67%.


Introduction
The increase in global energy demand and the use of non-renewable energy sources, such as fossil fuels, have reduced the availability of these sources, which will not be available once consumed completely [1,2]. With the depletion of fossil fuels worldwide, alternative energy sources have become important [3]. Renewable energy sources are a suitable alternative because they provide clean energy and reduced strong negative environmental effects, such as air pollution and global warming as well as it being a low-cost source of energy [4][5][6][7][8]. Renewable energy is derived from natural processes that are replenished constantly. In its various forms, renewable energy is derived directly from the sun, wind, rain, tides of the ocean, biomass, and geothermal resources from heat generated deep within the earth [9,10].
Solar energy is one of the most important forms of renewable energy the radiation from the sun and carries it with them heat and light sources; this energy form can take advantage of the sun in many applications [11][12][13][14]. Solar energy can be used for water heating; a solar heater can be used to invest sun irradiation to obtain hot water [15].
83 As an auxiliary system, solar water heaters are used to reduce the use of conventional fuel by preheating water in industrial and domestic sectors [16]. Nowadays, solar heater is used in many countries worldwide to provide hot water in homes and buildings as well as for industrial applications to generate electricity [17,18]. Solar energy is one of the important clean sources of energy therefore, conversion methods of this power to useful energy have attracted a great deal of attention in the scientific publishing [19]. A solar heater generally consists of glass-covered box encompassing painted steel tubes, through which water is heated as it circulates by thermo-gravity in the hot water tank [18,20]. The surface that absorbs solar energy is a major component of the solar heater and is coated with different absorbent materials. Solar energy is easily obtained using a surface coated with a thin layer of black paint [21]. Nanomaterials have also found applications in solar cells [22] as well as many other applications such as photosensors [23], and photocatalyst [24], …etc. This is because of a very important property of nanomaterials which is their very high surface-to-volume ratio.
Different nanomaterials, such as carbon nanotube (CNT) [21,24] and carbon nanoparticles in NiO, ZnO, and SiO2 matrices [25], are used to coat solar water heaters. The use of CNPs increases the absorption efficiency of solar collectors. CNPs have drawn considerable interest over the last years due to their high chemical stability, conductivity, and broadband optical absorbance [26,27]. The development of new materials for solar absorbers remains a challenge. Researchers aim to obtain coatings with a higher coefficient of absorption of solar energy at the lowest production cost.
In the current work, CNPs were prepared by sonication. CNPs-based coating was tested and easily applied to investigate its optical properties and evaluate its thermal performance.

Materials and equipment
Carbon black, ethanol with purity of 99.9%, silicone glue (R.T.V, made in IRI). and temperature sensor (Arduino, China) were used. Optical absorption and reflectance were investigated using UV-Vis. spectrophotometer (ShimadzuUv-1800). The surface morphology of the prepared samples was investigated with a Scanning Electron Microscope (SEM) (Nano Nova 450). Ultrasonicator (type TF-1000) was used to prepare CNPs.

Preparation of carbon nanoparticles (CNPs)
Carbon nanoparticles were prepared through sonication by adding 5 g of carbon black powder to 60 mL of ethanol. The mixture was placed in a sonicator for 30 min. at room temperature. After that, it was left for 24 h until ethanol evaporated and leaving the CNPs.

Coating
A 10 g of silicon glue was mixed with 0.05 g of carbon black and the same ratio of CNPs was added to silicon glue to produce a homogeneous paint. The components were manually mixed and used to paint the hot water tank and condenser with a brush. Ddifferent ratios of 0.025, 0.2, 0.05, and 0.1 were used to prepare CB/silicon glue and CNPs/silicon glue films. An experiment was conducted on two solar water heaters filled with water and fitted with a thermometer to compare the thermal performance. The temperature was monitored for 1 month.

Solar water heating system (SWHS)
The experimental setup is shown in Fig. 1. The system consisted of two identical simple SWHSs, where each was made of a condenser and a storage tank. The condenser was made from a copper tube with a diameter of 1. 5 cm and a length of 2.5 m. The cylindrical storage tank was made of galvanized iron and had a height of 32 cm and a base diameter of 20 cm. The components were placed in a wooden box and covered with a 5 mm-thick glass plate. The collectors were thermally insulated at the back by glass wool to reduce heat loss. The first system (T1) was painted with carbon black paint, and the other system (T2) was painted with CNPs paint. The two experimental systems were mounted on a building rooftop in the city of Basrah (30° 30' 03" North, 47° 48' 59" East). They were oriented to face the south direction with a tilt angle of around 45° with the horizon. Fig. 2 shows the UV-Vis. absorbance spectra for the prepared CB/silicon glue and CNPs/silicon glue films with different ratios (0.025, 0.2, 0.05, and 0.1) deposited on glass substrates. An absorbance peak was found at 320 nm in the spectra of the prepared samples which is around the same wavelengths of the peaks of the spectra of CB and CNPs. This is due to the π-π* transition in the ultraviolet frequency region. The long tail extended into the visible range is because of the n-π* transition, in contrast to the data obtained by Tan et al. [28]. Moreover, the 0.05 ratio of carbon nanoparticles film had the highest absorbance compared with the same ratio of carbon black. Carbon nanoparticles have higher absorbance because the structure and size of each nanoparticle can influence the position of π-π* absorbance [29]. Using carbon nanoparticles as coating paint can enhance the efficiency of the solar water heater. Fig. 3 shows reflectance spectra of the prepared samples of CB/silicon glue and CNPs/silicon glue films of different ratios (0.025, 0.2, 0.05, and 0.1). The reflectance spectra show two peaks for CB and CNPs at wavelengths of 400 nm and 850 nm, respectively. The first reflectance peak arises because of π-plasmon bands (collective oscillations of π electrons) in the ultraviolet frequency region, and the second reflectance peak is due to π-plasmon bands (collective oscillations of π electrons) in the (IR) frequency region, leading to tall-lived electronic oscillations. The reflectance for CB/silicon glue films of the 0.05 ratio was 0.15% at the wavelength of 400 nm and 0.23% at the wavelength of 850 nm.

Figure 3: Reflectance spectra of different ratios of: (a) carbon black, and (b) carbon nanoparticles with silicon glue.
While the reflectance of CNPs at the same ratio were 0.45% and 0.99% at the same wavelengths, respectively. However, the reflectance value for all the prepared samples was less than 1% making them suitable to coat solar heater to obtain high efficiency. Fig. 4 shows the FESEM images and histogram plot of the particle size distribution of CB/silicon glue film and CNPs/silicon glue film. The FESEM image of CB/silicon glue in Fig. 4a show white individual particles of a nearly spherical shape with size ranging from (100-400) nm. Comparing with the FESEM image in Fig. 4b, CNPs/silicon glue films have smaller particles size. Moreover, CNPs are homogeneous and spherical, with particle size ranging from (10 -100) nm. Fig. 5 shows the change of temperature of the water stored in the heater and the water in the cylinder with time for uncoated heater. The temperature is low in the first hours of the morning and gradually increases reaching a maximum value at the hour 14:00; after which, it gradually decreases in the last hours of the day due to the influence of the sun's ray and its angle of incidence. This finding could be due to the fall of the sun's ray in the early hours of the morning on the solar heater at a certain angle and that most of these rays are reflected away from the heater, while the other part is absorbed by the heater and so heats the water to a certain temperature.

SWHS performance
At midday, the sun's rays fall perpendicular on the solar heater, thereby increasing the water temperature. In the late hours of the day, solar radiation begins to decrease, leading to a decrease in the temperature of the water. The highest temperature recorded at 14:00 was 46 °C, while the temperature of the water in the cylinder at the same time was 23 °C. The difference is due to the solar heater, which slightly increases the temperature of the water inside it because it works without coating.  Fig. 6 shows the satisfactory thermal performance of the two heaters. T1 heater recorded a temperature of 65.25°C and T2 heater recorded 69.94 °C at exactly 13:00 in the middle of the day. The performance of the two heaters began to increase from 8:00 until it reached the maximum value (72ᵒC for T1 and 75ᵒC for T2) at 15:00. The figure shows that T2 heater exhibits superior performance. The two heaters were operated under the same conditions and received the same amount of radiation throughout the day. The properties of CNPs characterized by small particle size increase the surface area, thereby increasing their effectiveness for absorbing solar energy radiation. After 15:00, the T1 heater has superior performance because nanomaterials have a large surface area, which increases heat loss in the absence of solar radiation. The solar water heater with the CNPs new paint exhibited better thermal performance than that with carbon black paint, with an average increase in water temperature of 4 °C.

Thermal efficiency
Thermal efficiency of the fabricated solar water heater was calculated at some specified hours during the day using Eq. (1) [30]: (1) where m is the mass of water in kg (which was 10Kg), CP is the specific heat of water = 4186 J/kg.°C, t2 is outlet water temperature in °C, t₁ is inlet water temperature in °C, I is the intensity of the incident solar radiation in W.h/m 2 , which was previously calculated in the Solar Energy Laboratory at the College of Education-Department of Physics with a solar integrator device, and Ac is the surface area of the collector in m 2 .
The surface area of the collector was calculated using Eq. (2) [31]: where r is the radius of the cylinder in m, and h is the height of the cylinder in m. Table 1 summarize the calculated thermal efficiency of the fabricated solar heaters T1 and T2 for 10 days, and for specific hours. The table demonstrate that the thermal efficiency of the designed solar water heater painted with CNP-based paint was better than that of the solar water heater painted with CB. The reason of the different in the thermal efficiency could be due to increase of light absorption of CNPs.

Conclusions
Carbon nanoparticles were prepared using ultrasound waves because this method is easy to conduct, economical, and inexpensive. Moreover, this method can be used for large-scale production of CNPs within a short period of time. When CNPs are distributed in the solution, a smooth surface is obtained, in which the particles are homogeneously distributed and has good nano volume. The solar water heater with carbon nanoparticles coating collected thermal energy more efficiently than the heater coated with carbon black paint. The results of the outdoor experimental study show that the nano-coating produced hotter water.