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Scrutiny of Solar Water Heating System Employing Supercritical Fluid
This paper proposes a solar collector that utilizes supercritical CO2 as the working fluid to detect implicit water heating and boost the collector’s heating rate efficiency. Solar water heating system efficiency, cost, and environmental friendliness all depend on the working fluid used. CO2 is a possible natural refrigerant replacement. Even a little increase in temperature or pressure may have a big impact on the density of CO2 at the critical point. Because of this, solar heating can readily generate a spontaneous convection flow of supercritical carbon dioxide. The most basic collector characteristics, such as CO2 pressure and temperature, were determined by building and testing an experimental setup using a CO2-based solar collector. Due to solar radiation, liquid, gas, or supercritical CO2 pressures and temperatures change throughout the test. There was a 50% time average collector efficiency (ηcol) and a 30% heat recovery efficiency (ηRE). Solar thermal collectors based on supercritical CO2 have now been shown in this paper. Since the results show that even though the solar energy is low, the CO2 temperature, pressure, and supercritical stress remain constant, this is distinct from conventional liquid-based solar collectors.
The creation of advanced green technologies has been sparked by environmental concerns and the possibility of an energy crisis. A broad range of commercially available systems for solar water heating are accessible, and the technology is widely utilized all over the Earth . The increasing popularity of solar water heating (SWH) systems can be attributed to their low operating and maintenance expenses relative to those of traditional water heaters. The SWH system’s thermal performance has been explored extensively theoretically and experimentally using a variety of collectors, working fluids, and storage tanks . Because water-based collectors are prone to freezing, this is the case. Fluorocarbon and hydrofluorocarbon refrigerants have been attempted as alternative working fluids by experts. As long as these propellants do not harm the ozone , they may be used without worrying about their impact on climate change. When it comes to greenhouse warming potential (GWP) R134a, for example, it is 1300 times more than CO2. Natural refrigerants like CO2, ammonia, and propane might be a solution to this problem [4, 5].
To build an optimal, expense, and ecologically friendly SWH system, it is critical to choose a working fluid that can operate even at low ambient temperatures. Carbon dioxide (CO2) will be utilized as the system’s working fluid due to its low ozone depletion and global warming potential. In addition to not freezing, not igniting, not corroding, and not being poisonous, CO2 is also not volatile . There is also no requirement for recovery and reclamation when fixing and discarding equipment, and it may just be discharged into the environment with little ramifications. Furthermore, thermodynamic and transport characteristics of CO2 make it a good heat conductor [2, 3].
1.1. Properties of Carbon Dioxide
For an efficient, cost-effective, and environmentally sustainable SWH system to operate even at low ambient temperatures with weak solar radiation, the refrigerant characteristics are critical. Because it has characteristics that differ significantly from those of typical refrigerants, carbon dioxide was chosen as the working fluid for this analysis. The low critical point of CO2 (31.11°C at 7.3 MPa) sets it apart from other refrigerants [4, 5]. As shown in Figure 1, in the CO2 phase diagram, the triple point is −56.6°C and 0.52 MPa, whereas the saturation pressure is 3.5 MPa when the temperature is 0°C. Because CO2 has a lower critical temperature and a higher decreased pressure than typical refrigerants, the low-side circumstances are considerably closer to the pivotal stage .
As a nonflammable, noncorrosive, and environmentally benign alternative organic refrigerant, carbon dioxide has great potential. In the case of mending or discarding the apparatus, it does not need to be collected or salvaged and has a minor impact on the environment when emitted into the air . A natural refrigerant, CO2 has little impact on global warming, yet it does not deplete the ozone layer.
The paper’s goal is to improve public awareness about renewable energy sources by showcasing how solar energy may be used to heat water. The possibility of an energy crisis will be reduced as a result of the availability of a reliable energy source to meet residential demands that are presently covered by power and biomass. The following are the paper’s major objectives:(1)A modified evacuated tube solar collector using CO2 as the working fluid was evaluated for its efficiency in harvesting solar energy in the solar adverse zone in this paper.(2)We create a thermosyphon system with an evacuated tube solar collector that is dependable and economical. It is predicted that this technology will continue to supply hot water even if the temperature at which it collects is below the acceptable level. First, the system was designed for heat pump support, which necessitated the purchase of a pump and compressor.(3)We create an energy alternative that is more cost-effective than traditional sources, such as electricity and firewood, while still meeting residential demand.
3. Literature Review
Sadhishkumar et al.  discussed that phase change materials (PCMs) were used to explore the feasibility of storing energy from the sun and then utilizing them to heat water for residential usage at night. Using three different approaches (i.e., no reflector, reflector, and reflector cum PCM), pipes for solar receivers that use glass-filled evacuated tubes are examined for efficiency, as well as water flow is examined through the tubes. The simulation results were combined with the test results to arrive at conclusions. Over lengthy periods, the adoption of the recommended configuration can result in rise of 5 to 7 degrees Celsius in the retained warm water temperature, according to the findings.
Chaudhary et al.  suggested CFD analysis and an evacuated tube heat pipe; this study shows how solar energy may be used to generate useful heat. Using water nanofluid to increase heat transmission in solar energy is a controversial practice. Two heat pipes make up the topology. Water and Al2O3 are the operating fluids in heat pipes. Closed tube solar water heaters with nanofluid thermal output are more efficient than ordinary evacuated tube solar water heaters (SWHs).
Vasudeva Karanth et al.  examined the effects on heat productivity using varying absorber plate pipe diameters and shapes. When compared to alternative configurations, the Nusselt number for the collector’s circular cross-section tube with a flattened contact surface when compared to that of the absorber plate is substantially higher in the CFD study. The mathematical analysis demonstrates that the changes in heat efficiency are essential, while various forms and sizes are taken into consideration for the solar panel pipes according to the criteria of a constant cross-section area and constant perimeter. The triangular pipe design has relatively large pressure reductions and absolute temperature increases, according to the requirement of a constant cross-sectional region.
Patel et al.  discussed there will be a comparison of thermal efficiency between an actual experimental setup and the suggested circular pattern tube solar heater, which contains a spiral-shaped copper tube, flat plate collectors, and K temperature sensors and try to adjust the maximum temperature. This comparison is addressed. Thermal efficiency analyses may already be done with the current design. Using a 16-degree difference in intake and output temperatures, the solar water heater spiral pipe’s efficiency was assessed. May was a taken as the month for research on thermal performance. Throughout the experiment, the efficiency of the curved solar air heater improved by 47.63 percent.
Soni et al.  implemented SWH’s rectangular design which was improved with the addition of a baffle mechanism. It was designed with baffles to split the vessel into two compartments: a storage volume on the inside and a collection volume on the outside. Baffle plates incorporated in rectangular and triangular ICSSWH systems were the subject of the first experimental and numerical tests.
Extensive research of Dabra et al.  showed a V-trough SWH system with direct airflow that was both more efficient and cheaper. The combination of a solar absorber and a V-shaped trough reflector increased the SWH’s efficiency considerably. Various insulating materials, as well as glass, were tested on the system throughout the testing phase. The prototype was reported to have an optical efficiency of 71% at 82°C, with and without encapsulation, and this was attained at 67°C.
4. The Use of Supercritical Carbon Dioxide as a Working Fluid in the SWH System
The solar collector is the heart of an SWH system, and numerous works have been done to improve collector performance, primarily by altering the collector’s size, structure, shape, and material, to reduce large losses or enhance the coating’s absorbent properties . A solar water heating system installed in an unfavorable solar radiation environment cannot employ water as a working fluid. Because water-based collectors might freeze, this is a common occurrence . When developing an environmentally friendly, efficient, and low-cost solar water heating system, the choice of a working fluid is critical [7, 8]. This fluid must also perform well at low temperatures. As a result of the need to maintain the ozone layer while also limiting global warming, there is a growing market for environmentally friendly working fluids .
Due to the general immediate issue of chlorofluorocarbons (CFCs), there has been a rise in attention in using CO2 as a working fluid in recent years. High CO2 emissions into the environment are thought to have the potential to set off the greenhouse effect. The development of energy systems that stop global warming and stabilize or recycle CO2 as a working fluid is thus essential . This can also provide meaning to CO2 collection and storage technologies. When utilized in low-sun-radiation and zero-temperature regions, CO2’s low pivotal factor makes it a suitable working fluid option. At a pressure and temperature of 7.38 MPa and 31.1°C, CO2 can reach its critical point [11, 12].
5. Different Types of the SWH System
5.1. Flat Plate Collector SWH System
In this case, the flat plate collector is constructed of an evacuated metal box with a glass substrate mounted on top of it. Water-carrying channels or riser tubes are inserted into metallic absorber (selectively coated) plates that are incorporated into the device. The absorber collects solar energy and transmits it to the running water . Figure 2 depicts the architecture of a conventional flat plate collector SWH system.
5.2. Evacuated Tube Collector SWH System
An evacuated tube collector (ETC) is used because it uses two inner tubes, one of which has a U-shaped end and is separated from the other by a spacer as shown in Figure 3. To reduce the amount of heat lost through radiation, the tubes’ mouths are sealed and the area between them is drained. Because the tubes are spherical and the sun’s rays hit them at a straight angle , ETC has great absorption (over 93 percent) and low energy loss (less than 6 percent). Some evacuated tubes include parabolic reflectors at the bottom to increase solar energy efficiency. An automated control panel oversees and manages the whole system .
6. Proposed System for the SWH System Using Supercritical CO2
The solar water heater (SWH) experimental setup is shown in Figure 4. For this system, a heat exchanger, a tank of hot water to store the hot water collected with the evacuated solar collector, and two valves to regulate the amount of heat entering the system are required. In addition, the system can be split into two separate heat exchanger-coupled loops . The heat exchanger is constructed using two tubes. Water in the outside tube receives heat from CO2 circulating within. The inner tube measures 12.7 mm in diameter, while the outside tube is 34 mm in diameter. The surface of the heat exchanger is 1.0 m2 [13, 15]. There is a maximum permissible working pressure of 12 MPa for evacuated solar collectors, and they can withstand working temperatures of up to 250°C . While the evacuated solar collector has a surface area of 1.97 square meters, it only has an effective heat-collecting area of 1.5 square meters. Except for the evacuated solar collector, the other elements of the CO2-based circulation loop have a maximum permitted operating pressure of 10 MPa apiece. Figure 4 depicts a CO2-based loop that does not make use of the feed pumps [6, 7]. Controlling the flow rate of CO2 over 60°C requires the use of valve 1. Valve 2 regulates the flow rate to keep it at 10 kg/h for the cooling water. A magnetic gear pump drives the flow of water in the water heating loop. The mass flow in the two loops is monitored by two Coriolis effect mass flow meters .
A working fluid of CO2 is used in the CO2-based loop, which is advantageous owing to its lower thermal temperature and high specific heat capacity in supercritical conditions. To begin with, the liquid CO2 is purged to a pressure of 6 MPa and a temperature of 20°C [17, 18]. Solar energy is used to heat CO2, which then becomes a high-temperature supercritical fluid in the solar collectors while the system is in operation . The CO2 is then cooled to a liquid form using a heat exchanger. This method generates thermal energy that may be used to generate hot water. At the end of the process, the liquefied CO2 returns to the solar collectors.
6.1. Storage Tank
The SWH system’s storage tank is critical, as it has a significant impact on how well the system works. Energy from the sun is often stored in a storage tank to provide hot water to end-users at a suitable temperature . Steel, concrete, plastic, fiberglass, and other appropriate materials are commonly used to build hot water storage tanks. Steel tanks are the most popular since they are the easiest to build of all the types described [15, 16].
Heat loss from the combination of hot and cold water is a major issue connected with the storage tank. Several storage tank systems have been proposed to reduce mixing and promote thermal boundary layer thickness. Sedimentation is critical for reducing mixing and, as a result, maximizing the amount of energy that can be harvested from the collector . Theoretical and experimental analyses for various geometric patterns and operating situations were carried out to assess the efficiency of stratification storage tanks. There are a number of variables to take into account while doing operations in the tank, including heat transmission, climate, and fluid flow rates [11, 13].
6.2. Heat Exchanger
Heat is transferred from the working fluid to the storage tank by means of an exchanger in a hypothetical SWH system. Aluminum, steel, and bronze are the most often utilized metals in the manufacture of HXs. Because of its high heat conductivity and corrosion resistance, copper is widely used in SWH systems . There are several types of heat exchangers used in passive heating water storage facilities, the most common of which are hidden coils in tanks, shells, and tubes and volcanic heat exchangers .
6.3. Heat Transfer Fluid (HTF)
It is necessary to utilize a heat transfer fluid to transfer heat from the collector to the solar panel. Thermal energy must be transferred from the heating element to the water in the storage tank by means of a heat transfer fluid [8, 12].
Carbon dioxide (CO2) is a potential natural fluid since it is nonflammable, noncorrosive, and environmentally safe. Because of its low critical point, CO2 may be utilized as a refrigerant in a transcritical heat pump cycle (31.1°C at 73.7 bars) . Analysis of CO2 has accelerated recently, with the primary goal of determining whether or not CO2 can be used in heat pump SWH systems and how effective a transcritical CO2 heat pump is. The main energy usage of a CO2 heat pump water heater may be decreased by over 75% when compared to electrical systems, and hot water temperatures up to 90°C can be produced without any operating issues .
6.4. Hot Water Storage Tank
The SWH system would be incomplete without the storage tank. It also has a significant impact on how fast the system runs. It is necessary to store energy from the sun that has been gathered to increase the mechanical properties of the water stored inside the storage tank since this is great for the environment . Tanks for hot water are generally made of a variety of materials, including steel, concrete, plastic, fiberglass, and several other types of appropriate materials. Steel tanks are the most often used among the above varieties because they are simple to build and typically free of corrosion in the appropriate sizes .
6.5. Governing Equations
To further examine the solar water heater, the following characteristics are developed based on measurements taken throughout the testing process .
The useable heat gain (Hg) of a solar collector is expressed as a function of the following:
Here, represents input from the sun’s rays, is the total heat loss coefficient, is the working fluid temperature, and C′ denotes collector efficiency which is shown in the following equation:
For instance, the heat transfer coefficient between the fluid and tube wall is given by . symbolizes the bond conductance, while “d” represents the diameter of the U-tube. The circumferential distance between U-tubes is given by = p/2, where “p” is the perimeter of the cross-sectional area of the tube .
A solar collector’s instantaneous collector efficiency () may be defined as the ratio of the useable heat gain supplied by a solar collector to the available solar energy at the solar collector at any moment in time .
Heat quantity recovered by the storage tank can be expressed in the form of the following equation:where denotes the mass of water contained within the storage tank, and denote the temperatures of the water entering and exiting the storage tank , and represents the specific heat of the water .
Finally, the heat recovery efficiency () of the system may be calculated by evaluating the amounts of heat retrieved by the flow of water via the heat exchanger () to the amount of useable heat obtained by the system throughout its operation .
7. Process Flow of Solar Water Heaters (SWHs)
Figure 5 shows how the solar water heater works using open-loop principles. The system flow chart clearly shows that the sun is responsible for heating the water. The hot water tank is used to store this heated water. This process will make use of a tiny solar panel and pump controller circuit . The logic circuit, which controls the pump, automatically regulates the system. To determine the temperature, the S1 and S2 sensors are used . Sensing logic states that when S1 surpasses S2 (S1°>°S2), the pump turns on and S1 is executed (on) and the pump then shuts off when S2 is less than or equal to S1 (off). If the logic works, then the water is sent to the absorber from the cold water reservoir . The procedure will then terminate when the heated water has been stored in the solar collector.
8. Results and Discussion
8.1. Performance of an ETC While Using Supercritical CO2
It is evaluated how the effectiveness of a heat exchanger collector changes over time in response to solar radiation intensity when supercritical CO2 is used as the heat transfer fluid. Specific aspects such as CO2 pressure, CO2 temperature, useful heat gain, heat retrieved by water, storage water temperature, collector efficiency, and heat recovery efficiency are all evaluated.
There are a variety of factors affecting the effectiveness of a solar device, such as the amount of direct sunlight and the amount of greenhouse gas present.
Monitoring and analysis of the solar radiation dynamics are therefore conducted on a certain day. Figure 6 illustrates how the temperature of the air changes throughout the day as the intensity of solar radiation increases, with the temperature of the air rising in the morning and dropping in the afternoon, as indicated. Because of the clear sky, radiation from the sun and the air temperature were both 750 W/m2 and 23 degrees Celsius on the test day.
8.2. CO2 Pressure and CO2 Temperature Changing with Time
Experimentally, CO2 pressure and temperature are monitored with pressure gauges and thermometer probes, respectively. The system was charged with CO2 and subjected to sunlight at a pressure of 8 MPa. During the testing period, it was discovered that sun radiation had an impact on the CO2 collector’s temperature as well as pressure. Experiment results track the changes in CO2 pressure and temperature. Figure 7 shows the pressure and temperature of CO2 at the collector exit as a function of time.
The temperature and pressure of CO2 increased steadily over the first several hours of exposure. Small pressure increases lead to significant changes in pressure once the supercritical pressure is reached (8.5 MPa). Even a slight change in pressure or temperature causes dynamic changes in the thermophysical characteristics of CO2 when it is close to its supercritical state. As the sun sets at about 12:00 p.m., the intensity of its radiation diminishes. This lowers pressure, but even as it does, a tiny temperature rise is noted.
8.3. Storage Tank Water Temperature
During the test, the storage tank’s water temperature was monitored and is shown in Figure 8. Increased energy from the sun increases CO2 temperature, which increases the temperature of storage tank water.
The water temperature in the proposed model increases until 13:00, after which it drops considerably. This is because CO2 temperature reduces with lowering solar radiation intensity, insulation drops, compressor design decreases, and the wind is the cause of the low water temperature. Heat exchanger design and correct insulation selection are critical when the water temperature drops from 34.98°C to 30.01°C. Along with insulation, wind speed is an important factor to be considered. On the test day, the wind blew at 2.5 m/s at 13:00 and a whopping speed of 3.4 m/s at 14:00.
8.4. Collector and Water Storage Tank Heat Gain
When the CO2 temperature increases, as shown in Figure 9, the collector’s useable heat gains and its ability to recover heat via water rise until 12:00 p.m. and then drops as the CO2 temperature falls. The storage tank absorbs heat and losses it when the CO2 temperature decreases.
Heat recovery from storage tank water during the incorporation of a CO2 close loop was found to have a long range compared to that of collectors. There was a mistake in the heat transfer, which is why this happened. The storage tank and exchanger designs, as well as the use of sufficient insulation, must be changed to increase heat recovery efficiency.
8.5. Collector Efficiency and Heat Recovery Efficiency
The time-averaged collector efficiency (ηcol) and heat recovery efficiency (ηRE) are computed at 50 percent and 30 percent, respectively, based on the data collected in Figure 10, which is displayed. The blue color line in Figure 10 depicts the collector efficiency at 8:00, which was reported at 43.5%. The suggested ETC likewise performed well in the initial test. Solar radiation intensity, collector area, working fluid circulation, and insulation all affect collector efficiency. By taking these aspects into account while designing a flat plate collector, the collector’s efficiency may be improved.
It is anticipated that induced convection would improve collector efficiency in the proposed method because of the natural turbulent character of the supercritical CO2 flow. Figure 8 shows the heat recovery efficiency range as 53% with an average daytime efficiency of 39%. This is due to the heat exchanger and condenser design restriction that was discussed previously, which results in a low average efficiency. When solar intensity drops below a certain threshold, heat recovery efficiency decreases to 20%, but after that point, it starts to increase again. As solar radiation intensity falls, the efficiency of heat recovery reduces as well. This is because heat recovery efficiency is inversely related to useable heat gain. Furthermore, the effectiveness of heat recovery increases after 12:00 hours since it is inversely related to the amount of useable heat gained during the day.
Figure 11 shows that the evacuated tube collector covers more energy requirements for water heating than flat plate collectors.
This paper proposes the use of supercritical CO2 as a working fluid for heating water, and the analytical analysis of the solar collector is performed to establish the solar collector’s essential properties. The evacuated tube collector covers more energy requirements for water heating than flat plate collectors. According to the collected data, the temperature and pressure of CO2 rise in response to an increase in solar energy. As a result of these findings, the time-averaged collector efficiency (ηcol) for an evacuated tube collector is around 50%. But while it is true that the recorded evacuation tube collector efficiency remained constant throughout the experimental test, it is also true that it may be improved by increasing collector area, tube number, and tube diameter for increasing the amount of working fluid circulation. Changes in the heat exchanger design can enhance the time-averaged heat recovery efficiency (ηRE) which is currently about 30%. Even though evacuated tube solar heat pump systems with U-pipe assistance have a greater initial cost, the ecologically beneficial and economically inexpensive CO2 has a stronger future in solar thermal applications than other heat pumps.
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
Conflicts of Interest
The authors declare that there are no conflicts of interest.
The authors gratefully acknowledge the Deanship of Scientific Research, King Khalid University (KKU), Abha 61421, Asir, Kingdom of Saudi Arabia, for funding this research work under grant number RGP.2/9/43.
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