Water heater employing granular coal photothermal absorber and solar thermal collector combination

Abstract

Keywords: Differential scanning calorimetry (DSC); Renewable energy; Solar energy; Spectrophotometry; Thermogravimetric analysis (TGA); Thermodynamic.

Introduction

Coal is one of the most abundant natural rocks and is currently the most widely used energy source worldwide. In 2023, the coal supply reached 156,636,851 TJ, which is equivalent to 27% of the world’s energy supply, and its dominant supply is only below oil at 29% [1]. Currently, the most popular method for utilizing the energy contained in coal is combustion [2]. However, coal combustion has the negative effect of releasing carbon emissions into the atmosphere, damaging the environment [3]. In addition, coal combustion also faces concerns due to its limited resources and non-renewable (non-renewable). In response to these problems, utilizing coal without burning is considered an appropriate option. It includes the photothermal process, where coal is used as a light absorber to convert solar radiation into heat. This concept is promising because solar energy is renewable with abundant availability, although its utilization is still very low. The potential of solar energy reaching Earth reaches about 440,000 TWh [4]. However, the use of solar energy is still low because, by 2022, it will only be 1,322.62 TWh or less than 1% of the global energy mix and its potential [5], [6]. Coal is a sedimentary rock whose primary composition element is carbon. He et al. [7] have classified various carbon-based materials as photothermal absorbers. Therefore, converting solar radiation energy into heat via a photothermal method is more prospective for sustainability and protecting the environment than just burning it. Moreover, several previous studies have proven that natural rocks with

a granular structure are useful as photothermal absorbers in various types of solar thermal collectors and applications. Natural materials with dark colors and pores have the potential to produce reliable photothermal performance for low and medium temperatures such as solar water heaters [8]. The granular structure is a type of light trap structure that can increase light absorbance because it recaptures the reflected light [9]. Table 1 presents the latest research developments regarding solar photothermal absorbers with granular structures from natural rock materials. Solar thermal performance is also greatly influenced by the type of its collector [21]. Most of the previous research presented in Table 1 used a flat plate type. Therefore, the temperature generation performance was relatively low. Based on the classification carried out by Bie et al. [22], solar thermal collector (STC) temperatures are classified into low (<353.15 K), intermediate (353.15 K to 523.15 K), and high (>523.15K). The low-temperature generation by flat plate is because the concentration ratio is ≤1 times [23]. The experiment conducted by Elmaadawy et al. [18] using a parabolic reflector tracking system (PRTC) could be classified as an intermediate temperature. It is because the PRTF’s temperature could reach a temperature of 375.15 K [16]. This happens because the concentration of the parabolic reflector is 15-40 times higher than the flat plate type [24]. As shown in Table 1, it can be understood that there has been no research using coal as a photothermal absorber material for solar thermal collectors. The latest research conducted by Sarifudin et al. [25], who compared natural

Table 1. Development of research on solar photothermal absorbers with granular structures from natural rock materials Materials

*Note: α is the light absorbance of the photothermal absorber, η is the efficiency of the solar collector system, and T is the temperature of the absorber or the material heated by the absorber.

rocks with the granular structure of andesite, coal, and pumice using controlled laboratory experimental methods, found that coal material was the best. Furthermore, tests using concentrated xenon light radiation in laboratory scale condition showed better heat generation performance than direct radiation [26]. Therefore, the next urgency is to know coal material as a photothermal absorber which is installed in concentrated solar collectors. Moreover, this research also tested the optimal granular size and operational limits. The novelty of this research is in the utilizing a different design type of STC. A type of STC with an equivalent concentration level is necessary to obtain a temperature generation comparable to the results of the research findings by Elmaadawy et al. [18]. Fresnel Lens is a type of solar collector with a concentration equivalent to a value of 15-40 times [27]. This research uses a stationary system to consider system simplification. The tracker system is more complicated because it requires an electrification device to control the automation system and drive motor. However, the stationary system has a weakness due to the sun’s movement, so the light’s focus cannot always be precisely on the absorber. Therefore, the Fresnel lens collector is combined with a compound parabolic reflector to reflect the unfocused light

to the absorber. A compound parabolic reflector is a collector with an intermediate performance because its concentration reaches 1-5 times with an operational temperature range of 333.15 to 513.15 K [28]. Moreover, Fresnel lens collectors have the advantages of using little material, cost-effectiveness, and high optical efficiency [29]. Based on the background of the problems above and the solutions offered, this research aims to determine the morphology and element composition of a granular coal photothermal absorber (GCPA). Considering the research findings of Attia et al. [15] and Mohamed et al. [19], granular size influences performance. Therefore, this study also optimizes size for temperature generation and light absorbance. Furthermore, this study also wants to know the operating temperature limit of GCPA without damage and its enthalpies. Finally, this study calculates the efficiency of energy and exergy for the fundamental component of GCPA and the systems of STC and SWH.

Materials And Methods

The main objectives of this research lay in the development of GCPA materials and the STC-SWH system. Therefore,

Figure 1. Granular coal in different sizes: (a) chunk, (b) 5−10 mesh, (c) 10−20 mesh, (d) 20−60 mesh, and (e) 60−100 mesh.

below is presented the process of making GCPA and the STCSWH specifications that were researched and developed. GCPA Preparation The raw materials in this research are coal chunks with random sizes and dirty, as shown in Figure 1 (a). GCPA in various sizes that are ready to be tested are presented in Figure 1 (b)−(e). Specimens are prepared from coal chunks, which are then crushed [26]. The crushed materials are then sieved to pass through a big mesh and retained on a small mesh. Next, the sieve material is washed using water to obtain clean natural material. Washing not only removes dirt from the material but also removes fine grains that cover the pores of the material. The material that has been cleaned is then dried in an oven at 423.15 K for 3 h. After baking, the material is left in the open space for 48 h to make the temperature return to room temperature. The material preparation procedure is illustrated in Figure 2. STC-SWH Specification The STC-SWH tested in this study is presented in Figure 3(a), and its main components are presented in Figure 3(b). Moreover, the detailed information about each component is listed in Table 2. Analytical Method This study was conducted through a tiered experiment with a workflow shown in Figure 4. More detailed information regarding the analytical methods used is explained in each subchapter. Scanning Electron Microscopy and Energy Dispersive Microscopy (SEM-EDS) SEM test was carried out to determine the morphological properties of the material. Furthermore, the EDS test

was used to determine the composition of the elements contained. Preparation is carried out with gold plating to ensure the GCPA structure does not burn easily during testing. Temperature Generation in Simulator The dried material was then tested for its temperature generation performance using a Fresnel lens collector under one solar illumination (1000 W/m2). The power used by the xenon lamp is 100 W and 12 V DC voltage. The temperature generation test was conducted in a controlled room at a temperature of 298.15±1 K and humidity of 51.6 to 82.2 for 1200 s. The thermometer used has an accuracy of 0.25 K with a maximum error of ±2 K. The hygrometer used has an accuracy of 0.1% with a maximum error of ±3%. The 20 g of GCPA was tested in dry conditions. Optical Analysis Optical absorbance testing uses a spectrophotometer with a wavelength of 360–1100 nm (Vis-Infrared). Tests were carried out in dry conditions with all size variations. The purpose is to determine the optical properties of absorbance and transmittance of the material. Transmittance (%T) is defined as the amount of transmitted light (I) at each incident light wavelength (Io), formulated in Eq. (1) [31], [32]. (1) Meanwhile, absorbance (αs) is defined as a logarithmic function formulated in Eq. (2) [33] or (3). (2)

Figure 3. Developed STC-SWH: (a) real view, and (b) main components illustration.

Tga-Dsc

Thermogravimetric analysis (TGA) testing is carried out to determine mass changes in the material because

of thermal transition and degradation in the material. In this way, the working temperature operational limit of the photothermal absorber material will be known without significant damage. Furthermore, a differential scanning calorimetry (DSC) test is carried out to determine changes in material enthalpy due to physical and chemical degradation with changes in temperature at a certain time. These tests used granular coal material with optimal temperature generation and light absorbance performance. Apart from the fact that only material with the optimal size is selected in testing at STC-SWH, it is also because thermal degradation in coal is not influenced by particle size [35, 36]. Testing

was carried out, increasing the temperature by 1 K/min from 290.31 to 1,273.31 K.

focused. Therefore, the focus value of the light radiation (F) is required, formulated in Eq. (8).

Thermodynamics Analysis The thermodynamic analysis is carried out in two conditions: unloaded and loaded conditions. Each test is carried out to determine the performance of the components (absorber) and main systems (STC and SWH). Unloaded condition testing is used to determine the GCPA performance in converting light to heat by temperature generation, energy efficiency, and exergy efficiency. The test was carried out on August 15-17 without being used to heat water. Meanwhile, the loaded condition test was carried out on August 12-14 to heat the water in the system. The position of each temperature sensor and the specifications of the components on the receiver tube are presented in Figure 5. The loaded condition instrument was used to analyze the performance of the STC-SWH system. The sensor installation position in the system is presented in Figure 6. The energy conversion efficiency (ηece.abs) in the photothermal absorber is formulated by Yan and Li (2023) in Eq. (7). (7) where m is the mass of the absorber, ∆HT is the change in enthalpy of the absorber at (T), I is the light illumination intensity, Aabs the surface area of the absorber exposed to radiation, and ∆t is the time required to reach (T). The measurements were carried out by Yan and Li (2023) in direct radiation conditions without being focused, whereas in this experimental case, the light was

Figure 6. Sensor installation position for loaded condition test.

Figure 5. Installation position of the temperature sensor in the absorber and receiver components.

(8) The radiation focus value (F) comes from the combined material transmittance (%T) and concentration ratio (C), formulated in Eq. (9). (9) The concentration ratio factor (C) is the ratio of the lens surface area (Alens) to the absorber surface area (Aabsorber), formulated in Eq. (10) [37]. (10) The lens surface area in the simulation is 0.09 m2 and the lens surface area in the SWH is 0.44 m2. The absorber surface area in the simulator is 8.10 × 10-3 m2 and the absorber surface area in SWH is 1.76 × 10-1 m2. The light that passes through a lens cannot be completely transmitted. Therefore, the transmittance of the system is the combination of several lenses’ transmittance, which through the light passes, as formulated in Eq. (11). (11) The transmittance for the lens in the simulator is 92%, while the combined transmittance in the SWH is 0.73%. The transmittance of SWH is smaller than that of the simulator transmittance because the lens on the SWH is thicker and composed of two layers of sandwich (acrylic lens, Frensnel lens, and glass tube). The lens on the simulator only consists of a Fresnel lens. In addition, the SWH testing conditions are in a real environment, so there is dust that can reduce transmittance, while the simulator is placed in a closed room. The transmittance value is calculated using equation 11 and measured by comparing the intensity of light illumination transmitted by the lens to the intensity of light illumination on the lens surface. Eqs. (7) to (11) can be simplified and Eq. (12) can be obtained. (12)

The heat conversion exergy by a photothermal absorber was formulated by Li et al. (2023) and Hu et al. (2023) using the maximum possible limit approach for heat energy that can be utilized in the Carnot formula written in Eq. (14). (14) where Q̇abs is the heat flow in the absorber, Ta is the ambient temperature, and Tabs is the absorber temperature. The heat flow rate of the photothermal absorber is formulated in Eq. (15). (15) where ηopt.abs is the optimal energy efficiency and Ġsources is the rate of solar radiation energy. Exergy produced from light radiation sources (Ėxsources) is formulated using the Petela equation which is formulated in Eq. (16) [39]. (16) where Aabs is the surface area of the absorber, Isources is the intensity of the light radiation source, Ta is the ambient temperature, and Tsources is the temperature of the light radiation source. The temperature of the arc of light in a xenon lamp is 1280.15 K [40], and the surface temperature of the sun is 6,273.15 to 20,273.15 K [41]. Based on the explanation of Eqs. (13) to (16), the exergy efficiency of the photothermal absorber can be simplified to Eq. (17).

Solar thermal collector energy efficiency (ηenergy STC) is the ratio of the energy produced to the energy required to operate the system in Eq. (18) [42]. Where the energy produced is in the form of heat [43] and the energy input is in the form of solar radiation [44]. (18)

The exergy efficiency of the photothermal absorber is the ratio between the heat exergy converted by the photothermal absorber to the maximum potential exergy obtained from the radiation source, formulated in Eq. (13).

Useful energy in the heat form (Ėheat) [45] or heat floẇ flow) [46] is formulated in Eq. ing in the pipe of STC (Qheat (19).

In the thermosiphon mechanism, the mass flow rate of water (ṁ) is very slow, so it is difficult to measure. However, by the second law of the thermodynamics approach, which states that heat flows from high potential to low potential, the mass flow rate can be calculated. Evidence of mass flow in the system is a change in temperature increase in the storage even though the connecting hose between the STC outlet and storage inlet is an insulator, so the heat flow is convection. Finally, the system will go to the Zeroth law of thermodynamics (energy balance), which is formulated in Eq. (20).

For application in this case study, Eq. 26 can be written as Eq. (27).

The heat loss rate at the collector is also formulated similarly in Eq. (29).

The water heat change rate in the tank is formulated in Eqs. (21) and (22) [47]. (21)

(22) The water heat change rate in the STC is formulated in Eqs. (23) and (24).

(27) When solar radiation power has a value of 0, no heat energy from solar radiation conversion enters the water tank, so the heat energy change rate in the tank is the heat loss rate, as formulated in Eq. (28). (28)

(29) Based on calculations of the temperature data of the system in Figure 13 using Eqs. (28) and (29), the k value in the water tank was found to be -3,844 J/s·K while at the collector, it was -0,086 J/s·K. The K value validation is carried out using the Zeroth law of thermodynamics (energy balance between systems), which applies Eq. (30).

The validation test of each calculation in Eq. (30) using the coefficient of variation shows that the error level is 0%. The calculation results are presented in Figure 14. Energy input (Ėinput) per unit time is formulated in Eq. (31) [49], where Is is the solar intensity and ASTC is the STC surface area. The conservation of energy in terms of the first law of thermodynamics (energy cannot be created or destroyed but can change form) is formulated in Eq. (31). Where the input energy (Ėinput) turns into useful energy in the form of heat (Ėuseful energy (heat)) and useless energy (Ėuseless energy).

where Ttn is the temperature in the nth period and Ttn-1 is the temperature in the previous period. Considering that the value of Cpwater changes for changes in temperature, the average value in the range of temperature changes is used, formulated in Eq. (25).

(25) The water mass in the full tank and STC is 23.325 and 0.094 kg, respectively. Evaporation and condensation that occur do not change the mass significantly because the installed measuring instrument does not detect any changes in water volume. Heat flow loss in the water tank (Q̇heat flow lost.wt) is calculated using Newton’s law of cooling in Eq. 26, where k is the heat transfer coefficient and Tt is the temperature at the time (t) [48]. (26)

Useless energy is formed due to the apparent motion of the sun and errors in collector geometry, which result in the focus of light not being able to fall precisely on the absorber. Furthermore, useless energy occurs because there is light that is reflected and transmitted by the collector and absorber. Useless energy is also caused by absorber conversion inefficiencies. Therefore, the absorber cannot convert all the sunlight obtained as formulated in Eq. (32).

The contribution of useless energy (%Ėuseless energy) to STC is formulated in Eq. (33).

Exergy input (Ėxinput) from solar radiation is formulated in Eq. 41 [54].

Eqs. (32) and (33) are too complicated because it requires measuring many parameters, so %Ėuseless energy is calculated using the simpler formula in Eq. (34).

̅ where I solar is the solar intensity, Acollector is the collector surface area, and Ts is the solar surface temperature. Next, the sustainability index (SI) is calculated in Eq. (42), which is a crucial exergy-based effectiveness parameter [55].

(34) The exergy balance at STC is formulated in Eq. 35 [50], [51]. (35) where Ėxinlet is the exergy rate at the inlet, Eẋ destruction is the exergy rate of destruction, and Ėxoutlet is the exergy rate at the outlet. Next, each exergy rate is formulated following Eqs. (36) and (37) [52].

(42) SWH energy efficiency (ηSWH) [56] or charging efficiency [57] is defined as the ratio of heat energy stored as hot water in the tank to solar radiation energy formulated in Eq. (43). Therefore, SWH energy efficiency can be interpreted as the efficiency of storing heat energy in the SWH system. (43)

where the amount of heat in the tank (Qwater in tank) is formulated in Eq. (44) [58], [59], [60] and Gcumulative is formulated in Eq. (45) [61].

where ṁ is water’s mass flow rate, Cp is water’s specific heat coefficient, Tinlet is the temperature at the inlet, T0 is the ambient temperature, and Toutlet is the temperature at the outlet. The mass flow rate of water in the thermosiphon mechanism is difficult to measure because it is very slow. The solution, mass flow rate is calculated using Eqs. (38) and (39). (38)

(45) where cp is the specific heat coefficient of water, m is the mass of water, Ttn is the temperature at time t, and T0 is the initial temperature. Then, Acollector is the collector’s surface area, Is is the solar intensity, and t is the duration of exposure. As the first law of thermodynamics, energy conservation in SWH satisfies Eq. (46), where the total energy input is converted to the total saved and unsaved energy.

(39) (46) Exergy efficiency in STC is formulated as the increase in exergy between the inlet and outlet flow relative to the input exergy [53]. Therefore, exergy efficiency is formulated as useful exergy relative to exergy from solar radiation in Eq. 40 [54].

The total energy that cannot be stored is the accumulated useless energy and heat loss, formulated in Eq. (47). (47)

Exergy SWH compares exergy outcome to exergy input, formulated in Eq. (48) [62].

Considering that there are many variations in the sample in research, the formulation of the average CoV is for simplification in Eq. (55).

(48) The outcome exergy is formulated using the Jeter equation because it comes from the heat source (photothermal absorber), as written in Eq. (49) [63]. (49) where Qwater in tank is the increase in heat stored in the water tank, Ta is the ambient temperature, and Tsources is the temperature of the source (absorber). Furthermore, the exergy input is the accumulated exergy of solar energy received by the STC, formulated using the Petela formula [39], which can be written as Eq. (50). (50) Where Gcumulative is the accumulated solar energy received by STC, Ta is the ambient temperature, and Ts is the source temperature (the sun’s surface). Uncertainty Analysis To analyze uncertainty in experimental measurement data using Eq. (51), population deviation (σ), as used by Gitan and Al-Kayiem (2023). (51)

Meanwhile, the sample deviation (SD) can be calculated by using Eq. (52). (52)

where xi is the i value, x̅ is the population mean, µ is the sample mean, and N is the population number. Next, the uncertainty during measurement is represented as a coefficient of variation (CV), which is formulated in Eqs. (53) and (54). (53)

(55) where n is the number of sample variations. Uncertainty in performance is calculated using the error propagation equation (δl) formulated in Eq. (56) [65], [66], [67]. (56)

Sem-Eds

The SEM test with a magnification of 7,800 times in Figure 7 (a) shows that the material has a rough and crack gap structure like porous. The light trapping effect of the photothermal absorber to increase absorbance is caused by the presence of specific structures, including rough [68] and porous [69]. Therefore, the rough and crack gap structure of GCPA has a positive effect on temperature generation. These findings support the statement of Tuncer et al. [70] in his review study on carbon nanomaterial-based photothermal absorbers, that differences in particle size, shape, and concentration directly affect the thermal performance. The EDS graph at three points is presented in Figure 7 (b), (c) and (d). This GCPA specimen is dominated by non-metal elements of carbon (C) and oxygen (O). The absence of the hydrogen element in the specimen indicates that the structure has changed to char. The release of H element bonds in coal is caused by baking during material preparation. The H element is released from the chemical structure of Carboxylic acid (C(=O)OH) because its decomposition temperature is only around 373.15 to 473.15 K [71]. However, it is also possible that hydrogen element is not found due to the inability of EDX to detect elements that have only one shell of K atoms, such as hydrogen and helium. The metal group elements aluminum (Al), iron (Fe), and niobium (Nb) contribute less than 5% based on mass concentration and less than 1% based on atomic concentration. The concentration contribution of each element in GCPA is presented in Table 3. The different elements composition of each coal do not indicate test inconsistencies, but different geological conditions of coal formation produce different element compositions [72]. Temperature Generation The temperature generation test bar diagram in Figure 8 shows that a granular size of 10 to 20 mesh is optimal.

Figure 7. (a) SEM, (b) EDS graph spot 1, (c) EDS graph spot 2, and (d) EDS graph spot 3.

There is a tendency that materials with higher absorbance will produce higher temperatures. The absorbance value of the material can be seen in Figure 9 (b) and Figure

10. However, other material properties such as thermal

resistance, latent and sensible heat, hydrophilic–hydrophobic, and light-trapping capability can affect the temperature performance of photothermal absorbers [9]. The optimal granular photothermal absorber size and scale for each

material can vary. Attia et al. [73], using granular gravel material from 4 to 16 mm, found the optimal size to be 16 mm. Meanwhile, Mohamed [19] used basalt stone 1 to 2 cm and found the optimal size to be 2 cm. Furthermore, the test used Au nanoparticles of 25 to 40 nm; the optimal size was 25 nm [74].

However, the transmittance increases after a wavelength of 910, and the absorbance decreases. Figure 10 shows that the transmittance trend is inversely proportional to the temperature generation while the absorbance is directly proportional. Other research comparing different concentrations shows that materials with higher absorbance have higher temperature generation [75]. Anomalies appear in the size ranges of 40–60 and 60–100 but remain insignificant as they fall within overlapping measurement uncertainty limits. Therefore, determining the optimal material for a photothermal absorber is insufficient, as with one of the absorbance spectrophotometry or temperature generation tests using a lamp simulator. Based on the highest temperature generation test and the highest absorbance spectrophotometry test, it is known that the optimal granular size is mesh 10 to 20. The optimal mesh size confirms that the contact gaps between granules can trap light optimally and show a trend of higher absorbance, resulting in higher temperatures. The elemental concentrations in natural materials are generally less uniform than in purified materials, leading to more significant uncertainty in test results. Furthermore, the material’s transmittance and absorbance values may be inconsistent due to the random arrangement of its granular structure, which leads to variations in the size of gaps acting as light traps. The SEMEDS test data results in Figure 7 confirm the condition of non-uniformity of elemental composition and random gap size.

Optical Analysis The transmittance and absorbance of each wavelength of light are presented in Figure 9. The light absorbance for all granular sizes tends to increase with an increasing wavelength of up to 910 nm. The optimal absorbance is in the infrared spectrum, with wavelengths from 700 to 910 nm.

Tga-Dsc

TGA-DSC testing on GCPA, which has a mesh size of 5-10, is presented in Figure 11 and is divided into four stages. The first stage is dehydration and desorption, starting from the beginning of the test, characterized by an endothermic reaction at a temperature of 290–336.31 K. In

Figure 9. Optical properties (a) transmittance and (b) absorbance.

Figure 10. Optical properties throughout the wavelength (a) transmittance and (b) absorbance.

the first stage, an endothermic reaction occurs due to the removal of water vapor and gas desorption [76]. Next, the second stage begins when an exothermic reaction occurs at a temperature of 336.31-684.31 K with a mass change of 11.95%. In the second stage, this is the ideal working temperature for the photothermal absorber because GCPA releases heat. Furthermore, there was no significant damage to the material in this area, as indicated by the lack of substantial change in mass. In the oxidation phase, the mass can increase or decrease depending on the major functional groups contained in the coal [76]. The increase in mass in coal samples is due to the presence of carboxyl groups

(-COOH), which contribute to the increase in CO2 gas and carbonyl groups (-C==O), which contribute to the increase in CO gas. Meanwhile, mass loss is caused by methyl groups (-CH3) and methylene (-CH2-). The third stage (degradation) begins when the exothermic oxidation reaction ends, characterized by rapid mass changes due to pyrolysis and combustion reactions, depending on the availability of oxygen during the reaction. Mandal et al. [77] reported the degradation of combustion in Indian coal, and Kellali et al. [78] reported that the degradation of pyrolysis in lignite coal occurs exothermically and endothermically. In the degradation stage, the mass release is permanent; therefore, at this stage, the coal is considered

the wavelength spectrum of sunlight is more diverse, the power intensity is higher, and the test duration is longer. Moreover, the direct test was carried out in a closed system (the absorber was in the receiver tube), while in the simulator, the absorber was tested in an open system (the absorber was on a petri dish). However, there is a possibility that the temperature generation in the simulator is higher than in the real test, depending on the different parameter conditions of each test. Simulator testing and real testing are not intended to compare directly the two methods. Still, simulator testing is carried out as a guide to selecting the most optimal parameters (granular size) that will be tested in real conditions. This consideration is made because, in real testing, natural conditions cannot be controlled, and it can result in unreliable and biased data. As an alternative, comparative testing in real conditions must be carried out simultaneously using several collectors. However, the simultaneous method would be too expensive and impractical.

Temperature generation of GCPA The results of testing the temperature generation of GCPA under real sunlight are presented in Figure 12. The highest light intensity during the test occurred at noon. Meanwhile, the highest GCPA temperature is at 1:00 PM for all sensor points. The maximum temperature in real testing was 399.25 K, so the results were higher than using a xenon lamp simulator of 378.15 K. This happens because

Energy and exergy efficiency of GCPA The results of energy efficiency measurements for all GCPA size specimens and test conditions are presented in Table 4. A photothermal absorber with a higher temperature means a higher energy conversion efficiency. The calculation results from Eq. (17) presented in Table 5 show that exergy efficiency is lower than energy efficiency. Exergy can have a higher or lower value than

to have experienced structural damage as a photothermal absorber. The TGA test shows that the degradation stage starts at a temperature of around 684.31K, in Figure 11, while in the photothermal absorber test in the collector, the highest temperature is 399.25K, in Figure 12. These results show that the collector design is safe and does not cause damage because the generation temperature is still below the degradation temperature. The degradation stage ends when the change in specimen mass begins to slope and begins to enter the burnout stage. In the burnout stage, the mass change is insignificant compared to the temperature change. The graphic data shows that heating with a temperature change reaching 296 K for 296 min only experienced a mass change of 0.14%, and ash contained in the GCPA is 24.11%. Furthermore, the visual characteristic that can be observed is that the specimen has turned to ash.

Figure 12. Temperature generation of absorbers in tube collector under unload condition.

Table 4. Energy conversion efficiency at all GPA sizes and test conditions Radiation Source

Table 5. Efficiency Exergy Absorber Radiation Sources Xenon lamp

energy because exergy analysis reviews energy quality based on reference sources and the environment. Temperature of system The loaded condition test, in Figure 13, shows the highest intensity of solar radiation at 12.00. Meanwhile, the optimal temperature in the collector is reached at 2:00 PM, and the water in the storage is reached at 3:00 PM.This finding

follows the trend conducted by Yanbolagh et al. [79], where the highest ambient temperature and water temperature in the collector occurred at 2:00 PM.The time shift to reach the optimal temperature in each component is closely related to the flow of energy and exergy in the system. Further discussion regarding this phenomenon is presented in the energy and exergy analysis of each system. Overall, the water storage temperature performance of SWH has met the needs

Figure 13. Temperature generation of STC-SWH system in load condition.

for warm water bathing in the range of 309.15 K to 318.15 K and generally ranges in 313.15 K [80]. Energy and exergy efficiency of STC Figure 14 shows the energy flow rate in the STC and water tank (WT) systems. The positive heat change rate indicates that the system has increased storing heat.

Meanwhile, the heat change rate is negative, indicating a decrease in stored heat which occurs starting between 3:00 PM. This condition occurs because the cooling heat rate release is faster than the heat that can be transferred from the photothermal absorber to the water working fluid. Figure 16 shows that the highest STC energy efficiency occurs at 11:00 AM at 33.27%. After 11:00 AM, the

efficiency cannot increase anymore because the increase in useless energy continues until 2:00 PM at 87.83%, but the rate of useful energy decreases. The rate of useless energy in percentage increases because the energy conversion efficiency of the photothermal absorber tends to decrease as the light intensity is too low (yellow graph line). However, cumulatively useless energy increases as the intensity of solar radiation entering the system increases (grey graph line). The average STC energy efficiency is 22.84%, and the average useless energy contribution is 77.16%. Figure 16 and Figure 16 shows that exergy increases drastically as the mass flow rate increases. The mass flow rate will flow faster if the temperature difference is more significant. In the thermosiphon mechanism, the fluid flows faster due to the difference in the density of the water being higher, where a higher temperature will reduce the density and flow upwards. This finding aligns with research conducted by Bhakta and Singh using a parabolic concentrating solar water heater where a drastic increase in mass flow rate resulted in an instant rise in energy efficiency [81, 82].

Figure 17 shows that the highest exergy efficiency was achieved at 11:00 AM at 5.51%, and the average was 3.44%. Furthermore, the highest SI was 1.06, with an average of

Conclusion

This study comprehensively examines the performance of granular coal photothermal absorber in the novel design of a solar thermal collector, a combination of Fresnel lens-compound parabolic reflector, applied to solar water heater. Based on experimental data and comprehensive analysis, it is concluded that: • Granular coal photothermal absorber has a light trap structure, that is, the gap between granules’ contact, fracture porosity, and rough surfaces. • The main composition of granular coal photothermal absorber is carbon and oxygen , with minority elements niobium , iron , and aluminum.

The temperature generation test using a xenon lamp and the absorbance test using spectrophotometry found that granular coal photothermal absorber size 10 to 20 was the best. The ideal operational limit of granular coal photothermal absorber in the oxidation stage region, at a temperature of 336.31 K to 684.31 K, with a maximum mass change of 11.95%. In the unloaded condition test (real environment test using solar thermal collector), the maximum generation absorber temperature was 399.25 K, with an energy conversion efficiency of 86% and an exergy efficiency of 21.29%. In loaded condition testing (real environment test using solar water heater), it was found that the optimal temperature of the solar thermal collector outlet and water in the solar water heater storage was 317.88 K and 316.75 K. The stored water temperature performance has met the ideal for warm bathing. The maximum solar thermal collector energy efficiency is 33.06%, and the average is 22.90%, while the maximum exergy efficiency is 5.51%, and the average is 3.44%. The maximum solar water heater energy efficiency is 27.85%, and the average is 20%, while the maximum solar water heater exergy efficiency is 5.90%, and the average is 3.09%.

Limitations And Future Improvements

This study shows that granular coal has good potential as a photothermal absorber in thermal solar collectors, especially for low to medium temperature applications. This material can absorb heat well, but some aspects can still be further developed. One of the main challenges is to improve efficiency and exergy. Furthermore, expanding the operational temperature range allows this technology to be applied in various applications. In addition, although material properties such as size, shape, type, and absorbance are known to affect temperature and heat performance, this relationship has not been formulated mathematically with high accuracy. Further studies are also needed to evaluate the material’s durability in long-term use and analyze the economic aspects in commercial-scale applications. In the future, innovation is needed in the development of absorber materials with higher thermal stability and longer durability so that the efficiency and service life of the system can be improved. More innovative collector designs, such as integrating tracking and geometry engineering, can potentially significantly improve energy the efficiency of energy and exergy. In addition, exploring combinations of other photothermal materials can help improve overall system performance. The application of more complex numerical models and computational simulations is also needed to formulate the mathematical relationship between

material properties and performance. This approach can improve the accuracy of predicting collector performance under various environmental conditions. Therefore, this technology can be applied more widely and effectively.

Data Availability Statement

The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.

Conflict Of Interest

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethics

There are no ethical issues with the publication of this manuscript.

Statement On The Use Of Artificial Intelligence

Artificial intelligence was not used in the preparation of the article.

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