Water mist system application in solar collector system to increase clean water production

The shortage of clean fresh water in remote areas such as coastal/sea and riverside/estuary areas is still faced by the majority of people living in these areas. The community needs fresh water for bathing, washing toilets, and drinking every day. To meet the need for freshwater, some coastal communities build wells and apply pumps to drain the water. The taste of freshwater around the coast is generally typical of brackish water. Apart from wells, some people also use river water for bathing and washing toilets. Water with a low level of hygiene is almost ignored as long as it can still be used to meet daily needs.

To overcome this problem, this research proposes a method to convert feed water or brackish water into clean fresh water. Some inexpensive technology-based tools such as solar collectors can be used to convert feed water into clean water even at low production rates. Although this tool has been proven to be able to produce clean water, due to its low production rate, it is not suitable to be applied to marine coastal communities. For this reason, further research on solar collectors is needed to increase their productivity. Modification is by adding a water mist system in the solar collector chamber. Fine water droplets in the form of a mist are sprayed on the hot surface of the solar collector so that more evaporation occurs. A higher evaporation rate is expected to produce more freshwater production. The water purification method through evaporation carried out on the solar collector combined with a water mist system may improve the evaporation rate. Clean water production, in this case, is correlated with the number of mist nozzles and feedwater flow rate, which will be studied further.

The purpose of this study is to investigate an evaporation method to increase clean water production by adding a water mist system to the solar collector. This research is distinguished by the evaporation method, which employs a water mist system in the solar collector. In addition, this research will produce a mathematical equation that relates the clean water production with number of nozzles and feed water flow rate. With the discovery of a method to increase clean water production from the purification of feed water, this research has a small contribution to improving public health, especially communities around the sea coast.

Studies on evaporation in solar collector systems have been carried out by many researchers. In this section, a description of previous research will describe research and patents on solar collector systems and their performance. A parameter study on the comprehensive characteristics and performance of a parabolic solar collector system was carried out by Cheng et al. (2014). In his research, Cheng describes the results of a theoretical analysis of the relationship between the reflector geometry parameters of a parabolic solar collector system and the focal shape formed by the defocusing phenomenon of non-parallel sunlight. Then the effect of the designed parameters and the obscuration phenomenon focused on the comprehensive characteristics and performance of the whole photo-thermal conversion process in the PTC system was studied and numerically optimized.

Not only Cheng, but research on parabolic solar collector systems was also carried out by Wang et al. (2014) and Wu et al. (2015). Wang et al. (2014) carried out a three-dimensional simulation of a parabolic solar collector system using molten salt as a heat transfer fluid. In his research, Wang describes three-dimensional simulations based on the finite element method to solve complex problems related to radiation, heat conduction, and convection in parabolic solar collector systems. The performance of the solar system using molten salt as the heat transfer fluid was numerically studied, and the influence of the main operating parameters on the solar collector system was investigated. Another time, Wu et al. (2015) conducted an experimental study on the heat transfer characteristics of low melting point salts in parabolic solar collector systems. The melting point of the new salt is 86 °C and the upper limit of the working temperature is about 550 °C. Experiments were carried out to obtain the heat loss of the heat collecting element, the total heat transfer coefficient from the water to the salt heat exchanger, and the convective heat transfer coefficient for molten salt in a circular tube.

The investigation of the solar collector system by Chen & Zhang (2014) was experimentally carried out with the natural convection of supercritical CO₂. This study deals with the thermal conversion of solar and water heating systems achieved by the natural circulation of supercritical CO₂. The experimental system was created and tested in Zhejiang Province (approximately. N 30.0°, east 120.6°) in southeastern China. The system is designed to operate in the supercritical region so that the system can be compactly fabricated and achieve high levels of natural convective flow. Another solar collector system in the form of a flat plate was studied by Zhou et al. (2019). In his research, Zhou proposed a flat plate solar collector system with antifreeze characteristics that use PCM material to store a certain amount of heat energy during the day and release energy at night. The daytime and nighttime antifreeze performance of the system was evaluated. The results showed that the conventional flat plate solar collector system will freeze when the daily average temperature is less than 5 °C. The system can work if the daily average temperature is in the range of 0–5 °C.

The use of hybrid solar collector systems in medical plant drying technology was investigated by Čiplienė et al. (2015). In the temperature under natural conditions, obtaining dried medicinal plants to a moisture content of 8–12% and the preparation of quality medicinal plant raw materials is a complex task. In many cases, the drying process of medicinal plant raw materials, especially those rich in volatile compounds, requires an optimal drying temperature of 30–45 °C and relative humidity of not more than 50–60%.

Other research on solar collector systems was carried out by Dikmen et al. (2014). The research was conducted to estimate and optimize the thermal performance of the evacuated tube solar collector system. In this research, an artificial neural network (ANN) and adaptive neuro-fuzzy inference system (ANFIS) have been used to predict the thermal performance of the evacuation tube solar collector system. ANN results were compared with ANFIS using the same data set. The value of R² for the collector thermal performance value of 0.81 can be considered satisfactory.

Furthermore, Owolabi et al. (2017) investigated the performance of an integrated thermal energy storage solar collector system using nanofluids. The effect of Fe nanofluid on improving the performance of a solar water heater integrated with a thermal energy storage system was experimentally investigated. The nanoparticle fraction of 0.5% Fe (Wt) was synthesized with a mixture of water/propylene-glycol base liquid. Experimental implementation using 40 nm Fe nanoparticles, 15° collector tilt angle, and circulating heat transfer fluid with a mass flow rate of 1.5 kg/min.

Through an experiment, Shehata et al. (2019) also studied the conversion of saltwater to freshwater. The proposed desalination system uses solar radiation, hot water spray, and a humid-dehumidification system in a channel, as opposed to the evaporation and condensation methods developed in this study. As part of their research, Shehata et al. utilized an ultrasonic fogger to increase the humidity of the channel's air, allowing for the production of more pure water.

In addition to submitting research reports, several researchers have also patented solar collector systems, including Robertson (1976), Mills (2000), Natter (1981), and Nydahl & Carlson (2002). Robertson (1976) reported a patent on a solar collector system. From this invention, a solar collector system comprises a multi-element collector coupled with an optical radiation concentrator system, the system of which is designed to enable optimization of the energy collection process by a precise sequential flow of the heat transfer fluid through the multi-element.

Natter (1981) reports another patent describing a solar energy collector system having an array of solar concentrators mounted at intervals along with a network of channels through which the heat transfer fluid is circulated. The concentrator includes an arcuate duct that provides a heat absorption surface and a saddle member for attaching the duct section to the duct. An insulating sheath surrounds the conduit and conduit members. An elongated lens panel can be positioned above the absorption surface to focus on incident solar radiation. The angle of inclination of the lens panel can be varied by the manual rotation of the solar concentrator around the longitudinal axis of the channel.

Mills (2000) published a patent describing a solar collector system consisting of several reflectors and a receiver system representing a surface that absorbs solar radiation reflected by the reflector. The receiving system is raised relative to the reflector and the reflector is pivotally mounted to support the structure in such a way that it can be positioned at an angle to reflect radiation against one of the receiving systems. Later in 2002, Nydahl & Carlson (2002) also reported a patent on a solar collector system. The solar collector system consists of a frame member and a top plate supported by a frame member with a top plate transparent to solar energy.

The experiment was started by heating the collector plate inside the solar collector chamber. Heating the plate with an electric heater is an analogy of the plate being heated by the sun because in this experiment the collector plate does not receive direct heat from the sun. The collector plate temperature is monitored by a K-type thermocouple with MAX 6675 module. This module works at 12-bit and 0.25 °C resolution. The end of the thermocouple wire is sticked at collector plate surface with aluminum foil tape. The collector plate as a vaporizer component receives a feed water supply in the form of water mist from the nozzle and evaporates it into the solar collector chamber so that water vapor fills the chamber with a certain density. The feed water flow starts from the water container and is directed to the nozzle by a water pump with a certain pressure where the feed water pressure is not specifically varied. Feed water flow measured by a YF-S201 digital sensor, which has flow range from 1 to 30 l/min. The performance of the evaporation process in the solar collector chamber was studied by varying the use of single and multiple nozzles and varying the feed water flow rate through the ball valve adjustment. Finally, the water vapor drifts in the solar collector are condensed in the condenser pipes to produce clean water. The volume of clean water is accommodated in the measuring cup after one experimental batch is completed. As described, the tested variables in this experiment are the number of mist nozzles, feed water flow rate and pressure, and solar collector plate temperature as input variables, and clean water production as output variable.

From Figures 4,⁵⁶–7 for the various numbers of nozzles, feed water flow rate at steady state is summarized in Table 1, while the measured clean water production for each experimental batch is presented in Table 2. The presented data in Tables 1 and 2 are the mean values and their uncertainties. The uncertainty value of the measurement results is calculated from the difference between the maximum value and the minimum value divided by two.

The uncertainty value may express the fluctuation of the flow rate measurement results relative to the mean value. The use of single and multiple nozzles in the water mist system shows that the feed water flow rate tolerance is quite varied. However, the entire percentage of uncertainty is still below 10%, which means that the fluctuation of the measured flow rate is still within reasonable limits.

The clean water production presented in Table 2 is the product of evaporation that occurs in the modified solar collector. Evaporation occurs when the mist water touches the collector plate. As soon as the mist water touches the collector plate, the fine droplets enter the boiling phase and then evaporate. The tendency of evaporation is further enlarged after the mist water touches the hot collector plate at a temperature of 100 °C.

From this correlation, it can be seen that the clean water production was very small compared to the supplied feed water through the nozzle, which is indicated by the gradient value of the curve of 0.0079. In other meaning, the ratio of clean water production to feed water flow rate is 7.9%. Some of the contributing factors include the nozzle hole size, the feed water pressure, and the condenser geometry. The nozzle used in this experiment has a hole diameter of 0.3 mm but can be replaced by a diameter of 0.5 mm to increase the flow rate. Likewise, the feed water pressure can be increased from 2 bar.g to refine the mist water sprayed by the nozzle. The finer the mist water, the easier it to evaporate when it touches the collector plate surface.

The third factor that affects clean water production is the condenser geometry. The condenser has a large enough gap between the pipes that allows water vapor to escape from the chamber, thereby reducing the condensation rate or the clean water production.

From the results of this experiment, the evaporation method that applies a water mist system to the solar collector system has been proven to be able to produce clean water even though the percentage is relatively small. However, when compared to the traditional evaporation method, which evaporates water directly from the collector plate surface, as done by Khan & Mustafa (2019), the production of clean water from this method is still higher. The maximum clean water produced by Khan‘s research is 230 g/hour or 3.83 ml/minute, while the method using a water mist system can reach a maximum of 5.78 ml/minute.

Then research conducted by Puja et al. (2016) distillation of solar energy water with energy recovery using the capillarity method, produces distilled water of 4.38 l/m².day or equivalent to 3.04 ml/minute for a collector cross-sectional area of 1 m². From the comparison of the two other research products, it has been proved that the water purification method that applies the water mist system can produce higher clean water. Compared to the research of Shehata et al. (2019), they produce 11 times more clean water, which is approximately 45 kg per day or 62.5 ml per minute. Based on the findings of this study, there is still an opportunity to increase the production rate of clean water, such as by replacing a nozzle with one that has a diameter between 0.5 and 0.8 mm.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.