Experimental evaluation of hybrid electric-heat operation for reliable residential water heating tec

Abstract

Keywords: Amperage; Coolant; Flowrate; PVT; Sand; Thermal

Introduction

acts to mitigate severe climate change. The engineering pro-

Despite population growth on global scale, utilization of fossil fuel transforms into cleaner energy. It shows serious

cess for utilizing biomass and renewable sources [1–3] indicates positive achievement on higher portion of renewable

*Corresponding author. *E-mail address: reza.a@univpancasila.ac.id This paper was recommended for publication in revised form by Editor-in-Chief Ahmet Selim Dalkilic Published by Yıldız Technical University Press, İstanbul, Turkey Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

sources in energy mix. However, global projection reveals that annual energy consumption tends to increase [4], so further effort is required to maximize the potential of clean energy sources, particularly for building sector. The sector is ideal for being role model in the application and utilization of clean energy technology to meet the local demand, such water heating. Moreover, residential heating consumes enormous amounts of energy [5]. It makes additional improvements in supplying clean energy for the sector is essential. Solar energy is essential for supplying hot water in residential housing through solar thermal technology [6]. It uses specific thermal collector to enhance the energy intake from the sun [7]. The system reliability is enhanced by coupling the collector with thermal storage, which utilizes latent and sensible heat of the material [8–10], making the system operate at longer duration with higher capacity. Moreover, recent trend indicates alternative option to operate photovoltaic (PV) as input energy for heating purposes. The work [11] assessed the potential of PV for the heating sector, implying that around 48% of the required energy for heating purposes can be supplied by PV. The concept offers one significant advantage regarding novel configuration to store the excess energy production from PV. Different assessment was taken [12] for the idea, emphasizing the positive contribution of PV for supplying heating demand in residential sector under suitable control process. Thus, it adds a new option for applying PV system specifically designed for hot water generation. PV operates using photoelectric, which converts sunlight into electrical energy. The process increases the surface temperature (SUT) of the PV. It is considered waste heat for the system operation [13]. It disrupts the conversion process, significantly decreasing the PV efficiency. The system requires additional cooling system to maintain the optimum working temperature of PV. The basic method is passive cooling, which operates without working fluid [14]. It is located at backsheet (BS) assembly of PV module. M. B. Elsheniti et al. developed heat management under passive mode operation for PV modules, resulting in a higher power output around 5.75% compared to the reference module [15]. Increasing the cooling capacity was conducted using dual model cooling system, achieving a better thermal and electrical efficiency at 24.3% and 0.16% [16]. It shows that heat management is essential to prevent inefficient output from PV modules. Another method is developed specifically concerned with the operating condition of PV cooling system. H. Chen et al. evaluated intermittent cooling for PV modules with nanofluids, indicating that electric generation efficiency increases 19.7% [17]. Q. Wang et al. employed heat pipes and iron-oxide nanofluids for PV cooling system, showing the SUT reduces around 20.4 °C [18]. Adjustment of the type of nanofluid and operating system contributes notably to enhancing the capability of the system, as summarized in this work [19]. The heat exchange is the key

aspect to ensuring the effective cooling process. Installing additional fin at BS enhances the heat exchange, particularly under suitable fin orientation [20]. Also, different cooling mechanism is proposed by employing magnetic field. Numerical optimization was performed here [21] by introducing an additional magnetic field for the system, revealing a better heat distribution corresponding to higher cell efficiency. It demonstrates the urgency of performing a cooling mechanism to maintain the effective output of PV modules. Despite that, the usability of nanofluids and extensive equipment may result in high installment cost, reducing the net energy balance and potentially bringing negative drawbacks at night [22]. The accumulated waste heat from PV operation is another form of solar energy potentially harvested as additional energy input. Thus, several works focus on utilizing the energy through thermoelectric (TE). This study [23] conducted numerical analysis for PV cooling system with TE, demonstrating the potential of cell temperature decrement about 2.9 °C with increment of power generated by TE. Combined PV-TE was evaluated numerically [24], which showed the potential of cell temperature decrement for the proposed model and the ability to increase PV efficiency slightly (1.82%). Different work [25] conducted experimental process for TE concept as PV cooling method, showing the cell temperature drops by 3% with improvement in the system efficiency by 13.9%. Comprehensive numerical and experimental assessment [26] indicated that cascaded PV-TE offers higher cooling function with suitable power production, making the system ideal for alternative PV-TE assembly. Nevertheless, employing TE is still in development phase since the net produced power is insignificant (less than 5%) [27]. Therefore, effort to maximize the potential of waste heat from PV while simultaneously reducing the system temperature is still developed to achieve a more feasible method. The produced heat from PV can be harvested and stored to material with heat storage capability such phase change materials (PCM). It can store the heat under certain working temperature [28–30]. A. Hamada et al. investigated PV-thermal (PVT) with PCM, indicating the cumulative achievement of system efficiency using passive PVT-PCM reached 74.1%, which was higher than system with no PVT at 34.6% [31]. Advance modification was taken with nano-PCM, which improved thermal efficiency and electrical power by 83.3% and 44.5% [32]. Optimization for the PVT-PCM was done by incorporating composite material, showing reliable improvement for continuous operation of the system for suitable regions [33]. Moreover, a different model was studied numerically to accommodate different heat transfer from the surface. This work [34] employed dual PCM as passive heat management for PV modules. Numerical modeling demonstrated a significant achievement in the efficiency (35.8%). In general, positive achievement is obtained by integrating system with PCM.

Employing PCM offers reliability of the application PVT. The PCM is used as alternative storage medium. However, the system demands additional equipment to harvest the stored heat, which requires supplementary equipment for the operation. Also, thermodynamic limits come as serious barrier due to maximum temperature generated by PV surface. Thus, waste heat from PV might be used directly rather than stored in different mediums. It is also suitable for the current trend of utilizing PV heaters for heat generation, particularly for rich electricity regions [35]. A. Szajding et al. analyzed high capacity PV-heating system, indicating simplification operation for heat generation purpose using PV [36]. Optimization was done to adjust the suitable operating condition for hot water supply from PV system based on specific household demands [37]. Therefore, it is advisable to harvest heat from PVT, which can be combined with the PV-heating concept. The possibility of employing PV-heating with waste heat accumulation as one combined system offers a new option for residential water heating development. Previous works imply that the system operates independently and is not considered one compact system. Thus, the present study proposes to evaluate the idea of combined PV-heating and waste heat accumulation. It is the main novelty of this work, which differs from previous studies. The work here focuses on harvesting waste heat from PV modules for hot water production by placing thermal collector (TC) at the BS area and simultaneously connecting the PV output to electric heater. As a result, compact PVTC (photovoltaic-thermal collector) is obtained as one robust assembly. Therefore, the objective is designed to introduce and provide preliminary evaluation of the operation of PVTC for hot water production. The mass flowrate (MF) and filler material (FM) of the TC are varied to understand the impact of the given aspect on the accumulated energy of the system. The temperature, electrical output, and accumulated energy are presented and discussed briefly to provide new perspective on the

development of this concept. We believe it offers significant contribution for the development of residential water heating and potentially maximizing the utilization solar energy for the sector.

Materials And Methods

Manufacturing PVTC Unit The specification of PV module is presented in Table 1. TC unit is an additional component placed at the BS surface of PV assembly. Locating the TC in the given region is aimed to absorb heat and transferring it to the cold water. The process causes cooling effect for PV cells at the surface area. In this work, TC unit was equipped with serpentine copper tube (OD/ID: 6.35 mm/5.23 mm) as coolant path. The serpentine form is chosen for its suitable performance [38] and technical simplicity. Figure 1a shows the photograph of the serpentine tube. There is empty space between each pass which makes the assembly required FM. Considering actual implementation, the FM must be widely available, have excellent technical properties, and be economically feasible. Thus, this work employed sand and grease as FM for the TC assembly. Sand is taken as solid filler with an excellent nature conductivity compared to water [39–41], making it favorable to fill the

50. Wp monocrystalline PV

Figure 1. a) Photograph of serpentine tube and b) Dimension of the PVTC unit.

Figure 2. a) Schematic of the assessment and b) Photograph of the process.

empty space of the TC assembly. Sand with average mesh size 18 was used to maximize the filling ratio. Another FM for this work was thermal grease, which offers suitable contact area between each tube and BS layer [42]. In this work, grease class NLGI grade 2 was employed for the FM. The detailed dimension of PVTC assembly is presented in Figure 1b. All component is placed in specifically designed frame made of acrylic. The tube was installed precisely to ensure direct contact with BS layer. The inlet/outlet tube was extended to accommodate the installation of additional hose for the working fluid. The system was examined for potential leakage and then installed at the BS layer to obtain one compact component as a PVTC. Experimental Test The assessment was conducted to observe the power output from PV and the amount of collected heat from TC. The schematic evaluation process is illustrated in Figure 2a. Cold water was distributed through TC assembly and collected heat from BS surface. Cold water temperature was maintained at 31 °C (± 1 °C). The temperature increment of the water after passing TC assembly was measured to

determine the input energy from TC. The SUT and BS temperature (BST) were recorded simultaneously during experiment. The PV was connected to heater, and the amount of produced electrical energy was recorded by I-V meter. The heater was placed in insulated storage with 7 liters of fresh water. The heated water was also measured throughout the process. Figure 2b shows the pictorial view of experimental process. The evaluation was performed according to the variation of FM and MF of cold water. The designed name for each model is summarized in Table 2. Two systems were assessed simultaneously during each case. It was performed on October 2024 in West Java, Indonesia. The evaluated model, as presented in Table 2, is expected to provide additional information regarding the effect of FM and MF on the accumulated energy of system. Additionally, PVTC-SG was designed as mixture of sand/grease with ratio 75/25 (by volume) to observe the potential of the combined version of the two fillers. On-field measurement was performed based on the schematic presented in Figure 2. The process was conducted

Table 2. Specific test model for evaluated system System Name

Pvtc-Sh

Table 3. Measurement properties of weather and apparatus Aspect

from 08 AM to 03 PM (local time). As seen in Table 2, each test was taken at intervals one day due to weather condition on subsequent days. The local weather condition and all measurement equipment used in this work are summarized in Table 2. Each component has range error to indicate the uncertainty range from the measurement.

Results And Discussion

Weather Conditions The weather measurement was performed for each case since it affects the obtained result for evaluating the system (Table 2). The solar radiation profile (Fig. 3a) shows that peak radiation above 1000 W/m2 occurs between 10:50–12:30. The average radiation for all cases is obtained between 769 – 827.2 W/m2 with maximum radiation at 1273, 1178.5 and 1267 W/m2 for evaluation at case 1, case 2 and case 3, respectively. The radiation values demonstrate the average condition for clear sky with intensity above 1000 W/m2 [43]. The recorded air speed is considerably low during the test, with an average air speed less than 1 m/s.

Another weather parameter is ambient air temperature and RH (Fig. 3b). The average temperature for all cases ranges around 35.9 – 37.7 °C, with peak temperature appearing in the midday along with peak solar intensity. The air was relatively dry during the experiment, according to the recorded RH. Despite the variation, the lowest RH is 29 – 31%, with average value between 34.4 – 38.5%. The recorded weather condition provides essential information to understand the obtained performance for the tested PV and PVTC. Moreover, the dynamic condition emphasizes that the actual measurement shows significant variation, which is more relevant for actual evaluation than in lab measurement. The error measurement from the metering device presented in Table 3 is provided to give additional consideration regarding the value obtained from this assessment. PV Module Temperature The direct exposure from the sun to PV module elevates its SUT notably. Figure 4 shows the recorded SUT, which clearly emphasizes the high temperature condition of PV module during the operation. The accumulated heat

Figure 3. Weather condition on the experimental test: a) Solar radiation and air speed, and b) Air temperature and relative humidity.

increases the SUT of PV module up to 55.5 °C with an average 49.1 °C. The specific increment in the initial operation is obtained at 4.7 °C/hour, then continues to increase steadily until the peak point at midday. The weather affects this high SUT, corresponding to the average ambient air temperature and low RH (Fig. 3b). The cooling effect from the ambient air is insufficient due to low air speed (Fig. 3a), making the average SUT higher (around 11.3 °C) than ambient temperature. The presence of TC for the PVTC contributes positively to managing extreme SUT. The recorded SUT for PV modules decreases between 12.7-17.3% with TC. It confirms the benefit of using TC for PV, as it can achieve a lower SUT. Moreover, the FM plays different cooling effect for the PVTC system. PVTC with sand offers a higher heat dissipation from the PV module. It decreases the SUT by 8.8 °C (PVTC-S) and 9.6 °C (PVTC-SH). Lowering the MF for the same FM reduces the average heat dissipation around 8.1 °C. Moreover, the combined S/G for PVTC-SG has drawbacks due to the highest SUT (48.5 °C) among the other PVTC. In contrast, PVTC-G offers a relatively better SUT around 46.6 °C. The designed variation in this work shows relevant result to choose suitable FM and operating condition, making the aspects crucial to maximize the heat dissipation rate from PV modules. The accumulated heat from the surface is transferred to the backsheet layer. As seen in Figure 5, the backsheet zone has remarkably high temperature, which is affected by the relatively thin layer assembly of typical PV modules [44]. Specifically for PV, the BST has identic temperature with the SUT, with average and maximum temperature of 46.7 °C and 55 °C. Adding additional TC effectively reduces the backsheet temperature, indicating a suitable cooling effect from the component. As observed, all cases show an optimum heat dissipation, which reduces the BST at lower temperature around 11.28–16.52% than PV.

Using different PVTC assembly directly affects the achievement of cooling effect for the BST. For example, PVTC with sand demonstrates suitable temperature decrement around 15.87% for PVTC-S. Modifying the MF for the same assembly (PVTC-SH) boosts the heat dissipation rate, reducing the temperature at relatively higher value (16.52%). It achieves the lowest recorded BST at 38.4 °C. Unfortunately, changing the filler with grease makes unfavorable cooling effect where the temperature at BST reduces only 11.3°C relative to PV. It is minimized by adding sand (PVTC-SG), which can provide a higher heat dissipation, reducing the BST up to 14.2 °C even though it operates at lower MF. Regardless of the result, PVTC-G remains better at reducing the BST than PV without TC. The ability to provide heat dissipation for the PVTC is observed according to the temperature deviation between SUT/BST (Fig. 6). PV has a lower gradient, indicating the heat transferred rapidly from surface to backsheet region.

The low cooling effect for PVTC-G indicates the same behavior with average deviation of 3 °C, which varies insignificantly from PV (2.3 °C). Accordingly, it confirms the unsuitable heat dissipation rate, which might be affected by low conductivity of the grease. All PVTC with sand shows an excellent result, removing the heat effectively, which enhanced the temperature gradient between SUT/BST. The peak gradient is achieved by PVTC-SH (8.3 °C) and PVTC-S (6.9 °C). The low MF for PVTC-SL and PVTC-SG has similar value around 7.6 °C. The profile demonstrates the capability of the PVTC to reduce the BST, which eventually decreases the SUT (Fig. 4). Generated Power from PV Module The electric generation from PV occurs as unsteady process, as observed for the obtained V-curve in Figure 7a. It needs auxiliary component (charge controller) to maintain the steady voltage output. However, it comes with additional cost and potential power decrement [45]. Thus, the output PV is connected directly to the load to prevent additional power consumption and installation cost. Moreover, the heater is designed as the sole component that neglects voltage variation when generating heat. The plotted V-curve indicates the positive contribution of the addition of suitable PVTC configuration. The minimum voltage for PVTC increases around 7.4% (PVTC-S), 7.3% (PVTC-G), and 6.3% (PVTC-SH). According to [46], temperature increment results in a higher voltage drop. Thus, the obtained higher voltage for the PVTC demonstrates the positive contribution to temperature decrement of the module. However, exceptional result is observed for PVTC-SL and PVTC-SG, which indicates relatively identic value with PV. It is related to poor cooling effect for both systems. Additionally, external factors from weather conditions are also considered for this justification. The I-curve is plotted in Figure 7b, showing more apparent implications on the impact of cooling effect on

the PV module. The obtained current for PV throughout the evaluation is 1.93 Ampere. It increases by 15.1% for PVTC-SH. Both systems were tested on the same day (Table 2), neglecting the weather factor and proving the positive achievement for implementing suitable PVTC. In addition, increment in the current is also achieved by PVTC-S (6.3%) and PVTC-SG (8.1%). It improves at relatively higher value for the PVTC-SL and PVTC-SG, approximately 10.6% and 12.3%. The general representation of the conversion rate of the PV is observed according to the power map (Fig. 8). It is clear that PV is unable to achieve the designed maximum power capacity, which is affected by several conditions, such as soiling factor and weather condition [47]. Regardless of

Figure 7. (a) V and (b) I curve for the tested PV and PVTC assembly.

the issue, positive achievement is generally demonstrated by the PVTC system, which consistently indicates a higher peak power. It is the main purpose to maintain the suitable SUT of PV module to ensure the effective power generation of the system [48]. Therefore, the evaluated PVTC in this work shows positive achievement in improving the power output of PV module. Coolant and Heated Water Temperature The specific role of employing different PVTC is shown according to the coolant outlet temperature (COT). Figure 9 presents that each model has variative COT profile, implying the specific role of the FM and operating condition based on the designed MF. PVTC-S shows a lower COT compared to PVTC-G. The average COT is 36.9 °C, while it increases about 3 °C for PVTC-G. It shows that PVTC-S has a more suitable heat dissipation behavior than PVTC-G, which supports the higher temperature gradient

for the PVTC-S (Fig. 6). In contrast, reducing the MF for PVTC-SG demonstrates steady profile, showing certain benefit for utilizing grease/sand mixture as FM for TC. It makes the COT maintain relatively constant value around

11. The electrical energy from PV modules is lower than

its designed maximum capacity, confirming the dynamic condition for on-field measurement, which is significantly affected by the weather conditions. Regardless of the condition, positive tendency is observed for PVTC system, which shows a higher electrical gain than PV. The maximum improvement is 14.2% and 13.1% for PVTC-SL and PVTC-SH. In the case of this condition, using grease as FM is less favorable due to insufficient heat dissipation, making the obtained electrical power relatively low (Fig. 8). It demonstrates suitable cooling behavior PV module promotes a higher power output, which makes the usability of sand as FM considered advantageous based on this finding. One interesting result is that the system harvests more heat energy than the output electricity from PV module. It is the main challenge for implementing large PV system since the produced waste heat is higher than the generated electric power [53]. From practical perspective, it comes as a serious problem. However, if the system is also intended

for heat generation for water heating, the problem may be taken as additional advantage. According to TC operation, MF influences notably to harvest more heat energy from PV module, as observed for PVTC-SH, PVTC-SG, and PVTC-S. In general, all PVTC positively contribute to collecting the accumulated waste heat from PV module and taken as additional input energy from the assembly. Figure 12 presents the gained energy from PV module as single component and system (PVTC). The value is obtained from the theoretical PV module output (50 Watts), which operated for 7 hours during the experimental process. PV module only generates electric energy with ratio 80.4%. All PVTC shows a higher value for the generated electricity, confirming the benefit of providing additional cooling mechanism for the PV module. Besides, PVTC also produces additional energy (heat) collected by the coolant. It promotes a positive contribution since the system offers a higher energy input from PV combined with TC, obtaining a higher cumulative energy harvested as a system.

Conclusion

The work assesses the PV system operation integrated by active cooling system with thermal collector. The system is operated for specific heating application, accumulating the output from PV and the collected heat from cooling system. The finding reveals that the concept is applicable with two benefits: improving the PV output by reducing the cell temperature and simultaneously harvesting waste heat as input energy for heating system. The cooling system reduces the PV temperature to 40.7 °C and offers maximum coolant output 48.7 °C. The collector is filled with sand and grease, showing significant contribution regarding the cooling ability which corresponds to their physical properties. Also, coolant flowrate plays crucial role in obtaining maximum cooling capacity and temperature output, making additional adjustment can be set according the requirement. PV and thermal collector produce higher cumulative energy around 1.23 kWh, while PV as single component only generates 0.28 kWh. Therefore, this point shows a notable energy output from the combined system. The system (PV and thermal collector) offers a reliable energy output specifically designed for water heating. It allows the system to reach energy output 350.4% higher than the designed power output of the PV module in this work. However, this preliminary study is designed as an introductory for combined PV-thermal collector water heating. Thus, further work is required to assess in detail about reliability of the system for long term operation, including the possibility of introducing different filler material. Another crucial aspect is net energy balance for operating the pump and detailed techno-economy consideration. The finding from this work is suitable for taking as relevant baseline for further development of the PV-thermal collector for heating purpose, hoping for more advanced technology to maximize solar energy as a clean source for residential heating.

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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