IoT-based performance analysis of hybrid solar heater-double slope solar still

Water and energy are the first need for living in any locations in the world. The major quantities of water are available in the oceans rather than in the earth. So, only a little amount of fresh water is available in the earth as lakes, rivers and ground water. Demand for water is increasing in the world due to the increase of population, agricultural production and the industrial sector. The impure water has extreme effects on the health of humanity. So, due to the small amount of fresh water available, it is primordial to extract fresh water from impure water to meet people's needs for fresh water. For this reason, industries focus on getting fresh water from impure water using many industrial processes.

There are two processes, which are direct and indirect desalination process. In direct systems, solar energy produces thermal energy needed for the water desalination process (Solar Still Systems) while in indirect systems, we produce the electricity that is needed to power the membrane processes. In indirect desalination systems, there are two units to produce distilled water, which are the desalination process and solar collector. Desalination of seawater has been done using an experimental solar still system in an arid region. Adaptive Neuro-Fuzzy Inference System (ANFIS) models have been generated using a hybrid learning algorithm with different functions where data has been distributed randomly (training, testing and validation) (Mashaly & Alazba 2018).

Another experimental work proposed the use of lenses and a mirror attached to the basin of a single slope solar still to enhance the concentration effect for solar radiation. The productivity of distilled water has been monitored (Fang et al. 2018).

A recent study has been done to improve the heat transfer inside an evaporator, which allows increase of the yield of distilled water. It has been found that using energy storage considerably enhances the average daily output of distilled water (Tigrine et al. 2021).

Another recent study has been done which found that using a hybrid solar distiller gives a production of 6.5 l/day compared to the traditional solar production (3.4 l/day) in the same environment. This increase in productivity represents almost an increase of 91% compared to the classical system (Randha et al. 2021).

One of the low-cost processes is using the solar still desalination system. In fact, the availability of solar radiation over the year encourages use of the solar desalination system based on solar heat energy. The principle of the solar water desalination system consists of taking out dissolved salts from water. The different components of this type of solar still are: Basin, Black Liner, Transparent Cover (glass in general), Output channels and Insulation.

The glass cover is an essential element in the still. It has been concluded (Panchal & Shah 2011, 2012) that to improve the performance of the still, it is recommended to choose the lower glass cover thickness. An analysis of different kinds of solar still has been done by Kaushal & Varun (2010). Different designs have been presented to show the impact of these parameters including the meteorological conditions, on the performance of a solar still. Experimental and theoretical investigation has been done, by Badran & Abu-Khader (2007), to study the effect of water depth on a single slope solar still. The results showed that the yield in output water increases by almost 26% if water depth decreases from 3.5 to 2 cm. They observed also that maximum solar radiation intensity allows an increase in the output of the still.

The performance of the double slope solar still has been investigated by Kandasamy et al. (2013), the authors made an experimental test on water depth. Another investigation about the effect of condensing cover material has been given by Dimri et al. (2008). Due to the high thermal conductivity of copper compared to plastic and glass, the copper gives a high yield. Also, in this same study, it was found that the lowest thickness of glass cover gives the best yield for passive and active solar still. Another study has been investigated by Tiwari & Tiwari (2007) about the inclination angle of solar still slope. It was found that 15° is the best angle to give the best annual performance of a solar still.

The study of the performance of single slope solar still has been experimented by Badran (2007) using asphalt basin liner and sprinkler. The effect of absorbing materials has been studied by Abdallaha et al. (2009) on four solar still units with same size. Another experimental study was carried out, by El-Bialy (2014), for studying different floating absorber plate in a single slope solar still. Also, an experimental and theoretical results have been done for a double slope active still by Dwivedi & Tiwari (2010). Boukar & Harmin (2001) have investigated the effect of desert climatic condition in Algerian Sahara, on the efficiency of both a single solar still and a solar still attached to a flat plate collector.

An experimental and theoretical research have been done by Rajaseenivasan & Kalidasa Murugavel (2013), to study the different basin conditions on double basin double slope solar still.

The impact of depth under insulated and uninsulated conditions has also been investigated by Elango & Kalidasa Murugavel (2015) to study the performance of such solar still. Experimental results were done by Panchal (2015) to study the impact of connecting evacuated tubes to the basin of solar still using absorbing material such as black granite or without. Another investigation has been done by Khalifa & Ali (2015) to study the effect of wind speed on the performance of a solar still. In addition, the impact of water salinity was studied by Akash et al. (2000). It was concluded that the yield of output water decreases with increasing salinity. The high solar irradiation gives the maximum yield in experimental results done by Safwat Nafey et al. (2000). A recent study has developed a new solar still device using a new design formed by a dome slope solar still which cover on the walls of the solar still. The goal of this work is to increase the area of the glass cover and due to this new geometry, more sunrays are concentrated inside the basin. It was found, that the yield of distilled water for this new design is 36% higher that four slope still (Sun et al. 2021).

Desalination and water recycle is considered one of the scientific research priorities in Saudi Arabia. We have designed an experimental solar still called two slopes solar still. A two slopes basin type solar still is fabricated with inner dimensions of 1 m × 1 m (effective area is 1 m²) and the tilt of the glass cover is 25 °C and which correspond to the latitude of Madinah city location. A solar heater has also been added to the solar still designed, in order to preheat the salted water before entering the basin of solar still.

The objective of this research is to design a smart hybrid solar heater – solar still system. A monitoring system based on a low-cost microcontroller and Internet of Things (IoT) technique. The monitoring system permits to control and study the performance of the proposed hybrid solar still remotely. The system has been implemented and experimentally validated at Madinah city, Saudi Arabia (Benghanem et al. 2021a).

Numerous designs of solar still have been developed in different locations in the world to improve the water productivity per day (Watercone 2001; Nassar et al. 2007; Panchal & Shah 2012; Dsilva et al. 2014; Mamouri et al. 2014; Suneesh et al. 2014; Harris Samuel et al. 2016; Panchal & Patel 2016; Chandrashekara & Yadav 2017; Randha et al. 2021; Tigrine et al. 2021). Our present work contributes in this field by presenting a new design based on monitoring of a hybrid solar heater-solar still using the IoT technique. A comparison between the outcome of present work with average daily production with others research works is given in this manuscript.

This paper is organized as follows: The principal of solar still is given by the next Section. The effect of some parameters on the performance of solar still are reported in Section 3. In Section 4 provides the discussion and performance analysis of the proposed solar still, while the final Section 5 gives some concluding remarks and perspectives about the designed prototype.

The principal of a simple solar still is presented in Figure 1. It is constituted of a basin containing the salted water, a transparent cover which should be in general glass. Due to the incident solar radiation and due to the design of the solar still, which acts like a greenhouse effect, the water is evaporated and condensed on the inner side of the glass cover and then cooled down due to the tilt of the glass until the output channel. The black liner of the basin absorbs more solar irradiation and then improves the heating of the solar still and allows more evaporation of salt water. There are two kinds of solar stills: Active and Passive solar stills. The active solar still needs an external energy source in order to upgrade the heating inside the still by using the collector to give distilled water while the passive solar still is based only on solar thermal, needs only a suitable design and uses thermal evaporation to get distilled water.

The performance of the solar stills system is affected by numerous operating factors and design configuration (Safwat Nafey et al. 2000). Thus, to get the best performances from the investigated solar still system, some steps should be taken in consideration for this work, which can be summarized as follows: best absorption of solar radiation; great surface area; using a heat recovery system to increase the condensing surface temperature (Dimri et al. 2008). The two slopes solar still is designed with inner dimensions of 1 m×1 m and the glass cover is tilted at 25° (latitude of the location). We have studied the effect of water capacity in the basin on the internal and external heat transfer. The average yield of distilled water produced by the proposed solar still is 9.73 L/m²/day. So, to get the best performance, we have combined the two slopes still and solar heater in order to preheat the salted water before injecting it into the basin, as shown in Figure 2.

This proposed hybrid system (Solar Heater – Solar Still) allows the fast evaporation of water vapor. The average yield of distilled water produced is around 12 L/m²/day. In order to test and control the proposed prototype, we have designed a smart data acquisition system based on a microcontroller (Arduino Mega 2560). The different data are as follows: solar radiation, ambient temperature, relative humidity, input water temperature to the heater, output water temperature from the heater, temperature inside the heater, water temperature inside the basin, relative humidity inside the basin, the amount (liter/hours) of output distilled water, salinity of the desalinated water and the daily average yield of distilled water (Liters/day).

Figure 3 illustrates the schematic diagram showing the viability of the IoT technique in a water desalination unit. In addition, the integration of a small photovoltaic system has contributed to power the designed circuits corresponding to the different sensors. The salty water is introduced first in a solar heater also designed in this work. The heated water is introduced on a black plate of the solar still. The solar irradiance allows the fast evaporation of the heated water to produce distilled droplets of water. The droplets slide down, due to gravity, until a special channel for collecting water (Benghanem et al. 2021a).

Figure 4 shows the photos of different experiments of the proposed prototype hybrid solar heather – solar still. During the test phase, we have done different measurements under different conditions to measure the hourly and daily output distilled water (Benghanem et al. 2021a).

A low-cost smart water desalination system powered by solar energy has been designed. The solar module inserted between the solar heater and solar still in Figure 4(b) allows powering the designed data acquisition system-based Arduino mega board, which collects different data described above.

The monitoring system has been integrated to the designed solar still using the Internet of Things (IoT) in order to control the evolution of the system in real time. The main steps of the control process, implemented into the microcontroller, are summarized as follows:

• 1.

  Initially the tank (containing salt water) is considered full (L3=500 L) as well the basin (100 L).

• 2.

  Checking the water position in the basin, which should be between two levels (L1=2 cm and L2=10 cm), if the position sensor indicates the measured level is less than L1, the microcontroller activates the reply and the valve will be opened till water level L2 is reached, then the relay is set off automatically by sending a signal from the microcontroller.

• 3.

  Verification of the water level in the tank if the level is less than L3, the microcontroller sends a signal to start the water pump till L3 is reached.

As an example, the basic electronic circuit connection is shown in Figure 5.

The control process is presented in Figure 6.

The controller circuit is designed and simulated by using the Proteus simulator. The corresponding developed circuit is shown in Figure 7. A basic Arduino code for measuring air temperature (T) and relative humidity (RH) is shown in Appendix 1.

As an example, Figure 8 shows the collected RH and T on the webpage using the ThingSpeak platform of MathWorks.

The detailed code that permits the connection to the ThingSpeak platform via the ESP8266 Wi-Fi module is given in Appendix 2.

The performance of solar still depends on the choice of some parameters, which can contribute to get the best amount of distilled water per day. The most influential parameters are the meteorological factors and the design of different constituents of the solar still. So, we can classify the factors affecting the performance of solar still in three categories:

• Environmental factors.

• Design factors.

• Operation factors.

The environmental factors influencing the performance of the solar still are:

• ✓ Solar irradiation,

• ✓ Ambient temperature,

• ✓ Wind velocity.

The design factors influencing the productivity of solar still are:

• ✓ Cover material,

• ✓ Thickness and tilt of glass,

• ✓ Water depth,

• ✓ Evaporation area,

• ✓ Insulation and salinity of water.

The operation factors affecting the performance of the solar still are:

• 1

  Maintenance,

• 2

  Dust accumulation,

• 3

  Water feeding,

• 4

  Position of the solar still.

In this work, we have studied the performance of the proposed solar still and the effect of climatic factors such as Air temperature and solar radiation; and design factor such as water depth, on the daily output yield of distilled water.

Some researches work (Safwat Nafey et al. 2000; Elango & Kalidasa Murugavel 2015) concluded that the maximum output yield of distilled water produced by different types of solar still corresponds to the minimum water depth. In fact, the rate of evaporation is affected by water depth. In this present work, we have chosen different depth of water at each experiment and we confirm also that minimum depth chosen of 2 cm give us the maximum output. Figure 9 presents the hourly evolution of yield for different water depth. Figure 10 indicates the amount evolution of output distilled water for different depth. From this curve, it can be concluded that maximum output water corresponds to a minimum water depth. This is due to fast evaporation of salted water.

So, it is found that the productivity of water increases with the decrease of basin water depth.

During our experimental work, it has been observed that the increase in air temperature, still productivity is increasing. Figure 11 presents the impact of ambient temperature on the output distilled water. 1.35 L/h has been obtained when the air temperature reaches the value of 31 °C (Benghanem et al. 2021a).

The measured parameters are: Air temperature, Vapor Temperature, Glass Temperature and Water Temperature as shown in Figure 12. Temperature measurement of the hybrid system are given by Figure 13.

It has been concluded that the productivity of distilled water is improving when the temperature difference between the water and glass cover plate is high

Figure 14 shows the amount of output water of double slope distiller connected with solar preheating water. The impact of a solar heater on the yield was carried out as indicated in Figure 14.

With reference to the experimental results, a higher amount of distilled output from the hybrid solar heater-solar still compared to the output of double solar still is obtained (Benghanem et al. 2021a).

It has been found that the efficiency of the hybrid solar heater-solar still is greater than the standard double slope solar still, for the same size of basin. The maximum daily yield produced by double slope solar still in this sudy is 9.73 L/m²/day while the hybrid solar heater-solar still produce the maximum yield of 12.165 L/m²/day.

Figure 15 represents the hourly solar irradiation recorded during a clear sky on 18 February 2021. We note that Madinah city has a great potential for solar energy (Benghanem & Joraid 2007) and almost all days are sunny and enjoy a clear sky. This is an advantage to make different applications in solar energy such as solar desalination systems.

In this part, we have studied the influence of incident solar irradiation, on the productivity of the proposed hybrid solar still. In fact, the evaporation of salted water inside the still is due to the incident solar irradiation, which allows the heating of water. So, distillation requires daily high intensity of solar irradiation to get best daily yield from the distiller. Figure 16 shows that the maximum productivity is reached when maximum incident solar irradiation is obtained in the time period of 12 to 2 pm.

Figure 16 shows that solar radiation intensity recorded daily at Madinah city during the period February–April 2021, is influencing the distilled water productivity. It has been found that the maximum yield, on the day of 25 March 2021, is obtained at midday due to high solar intensity, which was around 1,100 w/m². Thus, it can be concluded that the productivity of the solar still increases with the increase of solar radiation intensity.

The measurements were made during the period February–April 2021. After a large experimentation, we obtained good results. In fact, the proposed solar still unit gives a good amount of distilled water production in comparison with the previous solar still systems. Table 1 shows the outcomes of research work with average daily production. The proposed prototype allows fast evaporation of water inside the still and the average yield of water production is around 12 L/m²/day.

Figure 17 shows the average water production for different designs of solar still corresponding to hourly daylight in different countries including our present design (Type No. 9 in the graph).

From Figure 17, it is shown that the best productivity of solar still design is given respectively in decreasing order: Elliptical solar still design (Nassar et al. 2007); Solar water purifier (Chandrashekara & Yadav 2017), (Manual of low-cost solar water purifier for rural households, http://www.nariphaltan.org/swpmanual.pdf); hybrid design (present work); Still coupled with ETC (Mamouri et al. 2014); Stepped type (Dsilva et al. 2014); Still in V type design (Suneesh et al. 2014); Pyramidal type (Panchal & Patel 2016); Cone type design (Watercone 2001) and triangular type (Harris Samuel et al. 2016).

We conclude the following outcomes:

• To increase the output of distilled water, we should use minimum water depth.

• It is recommended to choose a glass cover with lower thickness.

• It is suggested a tilt angle of glass cover around the latitude angle of the site.

• Integrating the solar preheating with the still enhances the amount of output of distilled water.

• Still productivity is increased by 44% by decreasing the water depth from 5 to 2 cm.

• In this work, we got the maximum productivity by using a minimum water depth which is equal to 2 cm.

• A low thickness of glass cover is recommended, which is 2 cm in this work.

• It is observed that high intensity of solar irradiation gives greater yield.

Table 2 shows the chemical parameters (before and after desalination) of a ground water from a palm farm at Madinah city. A comparison has been made with mineral water (Nestle) and ordinary home drinking water. The results show that the ground water after desalination has almost the same values of chemical parameters of mineral water (Nestle) and ordinary home drinking water. The limit values (SASO 2017) of the Saudi Standard Organization (SASO) have been in good agreement with the produced water after desalination (Benghanem et al. 2021a).

In this study, the water sample analyzed has a concentration of hydrogen ions (pH) within the recommended values of 6.5 to 8.5 standard values (Benghanem & Joraid 2007). Freshwater is usually between 0 and 1,500 uS/cm (SASO 2017; Mary River Catchment Coordinating Committee 2013). The electrical conductivity (EC) of the used sample before desalination is 7,550 μS/cm which is higher than the standard value and after desalination, the electrical conductivity is 384 μS/cm, which is considered as acceptable value. Before treatment the Total Dissolved Solids (TDS) value was 3,770 (mg/L) and after desalination was 210 (mg/L). This TDS value of water after desalination is within the permissible value (SASO 2017). The salinity of the tested ground water is 0.39%, corresponding to 3.9 g/kg. After desalination, the salinity is 0.01% corresponding to 0.1 g/kg, which is lower than 0.5 g/kg (<0.5 ppt, where ppt is parts per thousand).

As a scientific discussion about using the IoT technique in solar desalination plants, we can say that the IoT technique allows easy control of the performance of solar desalinations units. The IoT-based performance analysis of solar still units will populate a dedicated web server-based database with real time data that will improve the decision-making process of the considered solar desalination unit. The generation of monitoring, fault detection of the solar desalination units in real time will be available on the website and then data such as hourly and daily productivity of distilled water, temperature inside and outside the still and relative humidity can be used for historical analysis. Thus, using the IoT technique will enhance the decision-making process for large-scale solar desalination units. In this present work, we proposed an IoT-based performance analysis of a hybrid solar heater-double slope solar still. The realized prototype of the solar desalination unit was studied, implemented and successfully achieved the online supervision of collected data which have been posted on a webpage via the Internet of Things (IoT) technique using the ThingSpeak platform of MathWorks.

It has been shown that the proposed solar still preheated by a solar heater system gives a good amount of distilled water production, in comparison with the previous solar still systems. It allows fast evaporation of water inside the still and the average output of distilled water is around 12 L/m²/day.

The physical-chemical parameters of the produced distilled water have been found in a good agreement with the limit values of the Saudi Standard Organization (SASO). The monitoring and control of the proposed hybrid still has been done using a low-cost microcontroller (Arduino mega board).

Thanks to the IoT technique and ThingSpeak platform, the user could easily monitor diffrent parameters like temperature, relative humidity, daily yield of distilled water, pH, EC, TDS and salinity. In addition, based on the measured data, the user can check if the system works correctly or not by analyzing the obtained data.

Further extension to this work is to design an application that permits the user to be notified by an email about the status of the system, in this case another board such as Raspberry Pi will be suitable.

The authors extend their appreciation to the Deputyship for Research & Innovation, Ministry of Education in Saudi Arabia for funding this research work through the project number 20/1.

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