Experimental and modeling study to estimate the productivity of inclined passive solar still using ANN methodology in arid conditions

Solar still is a simple solar device used for converting seawater or brackish water into potable water. It can be fabricated easily using locally available materials and can be maintained economically without the need for skilled labor. Solar still can be a suitable solution to solve the drinking water problem. However, it is not widely used because of its low productivity (Kabeel et al. 2015; Mashaly & Alazba 2017a). Consequently, several techniques and researches were introduced to investigate and enhance the design and productivity of solar stills, such as the addition of heat through a flat-plate collector (Riffat et al. 2005); the addition of dye in the feed water (Badran 2007); investigation of the effects of climatic, design, and operational variables on the performance of solar stills (Yeh & Chen 1986); and investigation of the effect of water flowing over the glass cover (Dhiman & Tiwari 1990). Radhwan (2004) investigated the transient performance of a stepped solar still with built-in latent heat thermal energy storage. Badran et al. (2004) investigated the simulation and experimentation of an inverted tickle solar still. Aybar et al. (2005) investigated the experimentation of an inclined solar water distillation system.

Much research has been reported in the literature focusing on experimental investigations to find better design and improved productivity for solar desalination systems. This experimental research is time-consuming and expensive. Mathematical modeling can be the best method to find better designs and operational variables for solar stills. Many numerical techniques have been used to model and predict solar still productivity, such as computer simulation (Cooper 1969), thermic circuit and Sankey diagrams (Frick 1970), periodic and transient analysis (Sodha et al. 1980; Tiwari & Rao 1984), and iteration methods (Toure & Meukam 1997). These methods are dependent on internal heat and mass transfer processes. Owing to the large amount of data required to validate the heat and mass transfer model, the ability to predict the solar still productivity is restricted by the capability to determine the parameters required to evaluate the model. On the other hand, artificial neural networks (ANNs) have a potential advantage in forecasting the productivity of solar stills because they use fewer parameters, less time, and are more accurate compared to the heat and mass transfer models.

ANNs attempt to mirror the brain functions in a computerized method by restoring the learning mechanism as the basis of human behavior. They are adaptive systems that change their structure based on external or internal information that flows through the network during the learning phase. ANNs can reflect the complex relationship between inputs and outputs successfully. Their predictions can lead to a closer fit with the data than the predictions of the classical modeling techniques, which usually results in more precise predictions (Benli 2013). ANNs have been used in a wide variety of thermal engineering applications, particularly on renewable energy engineering, such as solar energy engineering. ANNs were used to model the layer temperatures in a storage tank of a solar thermal system (Géczy-Víg & Farkas 2010), predict the in-situ daily performance of the solar collectors (Lecoeuche & Lalot 2005), determine the thermal performance of different types of solar collectors (Benli 2013), model solar still distillate production using local weather, analyze the performance of a solar powered membrane distillation system (Porrazzo et al. 2013), estimate the thermal performances of solar collectors (Caner et al. 2011), evaluate the performance of solar photovoltaic technologies (Velilla et al. 2014), and model the thermal performance of solar still (Mashaly & Alazba 2016a, 2016b, 2017b).

However, solar still production has not been clearly elucidated, and no previous research has addressed this topic, particularly in arid conditions. Therefore, the objectives of this study are: (1) to develop mathematical models to estimate the productivity of the solar still using ANNs; (2) to evaluate the performance of ANNs using a statistical comparison between the productivity obtained from the model and experimental results; and (3) to compare the ANN models with the multiple linear regression (MLR) models in terms of their applicability, suitability, and accuracy in forecasting the productivity of the solar still.

The experiments were conducted at the Agricultural Research and Experiment Station, Department of Agricultural Engineering, King Saud University, Riyadh, Saudi Arabia (24°44′10.90″N, 46°37′13.77″E), during the period from February to April 2013, and the weather data were obtained from a weather station (model: Vantage Pro2, Davis, USA) located close to the experimental site (24°44′12.15″N, 46°37′14.97″E). The solar still system utilized in the experiments consists of one stage of C6000 panel (F cubed, Ltd, Carocell Solar Panel, Australia) with an area of 6 m². The solar still is manufactured as a panel using modern cost-effective materials, such as coated polycarbonate plastic. The panel heats and distills a film of water flowing over the absorber mat of the panel. The panel was fixed at an angle of 29° to horizontal. The basic construction materials were galvanized steel legs, aluminum frame, and polycarbonate covers. The transparent polycarbonate was coated from the inside by special coating material to prevent fogging (patent for F cubed, Australia). The front and cross-sectional views of the solar still are presented in Figure 1. The operational concept of the available system is summarized in the following paragraphs.

Water was fed to the panel using a centrifugal pump (model: PKm 60, 0.5 HP, Pedrollo, Italy) with a constant flow rate of 10.74 L/h. Eight drippers/nozzles drip the feed resulting in a film flowing over the absorbent mat. Under the absorbent mat, an aluminum screen helps to distribute the dripping water over the absorbent mat. An aluminum plate is also placed beneath the aluminum screen. Aluminum was selected for the manufacturing process because it is a hydrophilic material, which assists in the even distribution of the dripping water. The water flows through and over the absorbent mat and the solar energy is absorbed and partially collected inside the panel, which heats the water resulting in hot air that naturally circulates within the panel. The hot air flows in the upper part toward the top, and then reverses direction toward the bottom. With this circulation, the humid air is in contact with the cooled surfaces of the transparent polycarbonate cover and the bottom polycarbonate layer; therefore, the water condenses and flows down the panel to be collected as distilled steam. Seawater was used as a feed water input to the system. The solar still system was run during the period from 23/02/2013 to 23/04/2013. Raw seawater was obtained from the Gulf, Dammam, East of Saudi Arabia (26°26′24.19″ N, 50°10′20.38″ E). The initial concentration of the total dissolved solids (TDS), pH, density (⁠⁠, and electrical conductivity (EC) of the raw seawater were 41.4 ppt, 8.02, 1.04 g.cm⁻³, and 66.34 mS.cm⁻¹, respectively. The productivity or the amount of distilled water produced (MD) during a time period by the system was obtained by collecting the cumulative amount of water produced over time. The temperature of the feed water (T_F) was measured using thermocouples (T-type, UK). The temperature data for feed brine water was recorded on a data logger (model: 177-T4, Testo, Inc., UK) at 1 min intervals. The amount of feed water (M_F) was measured by a calibrated digital flow meter mounted on the feed water line (micro-flo, Blue-White, USA). The amount of brine water and distilled water were measured by a graduated cylinder. TDS and EC were checked using a calibrated (TDS) meter (Cole-Parmer Instrument, Vernon Hills, USA). A pH meter (model: 3,510 pH meter, Jenway, UK) was utilized to determine the acidity. ρ was measured by a digital density meter (model: DMA 35_N, Anton Paar, USA). The seawater was fed separately to the panel using the pump mentioned previously. The residence time for the water to pass through the panel was about 20 min. Consequently, the flow rate for feed water, distilled water, and brine water was measured every 20 min. Furthermore, the total dissolved solids of feed water (TDS_F) were measured every 20 min. The weather data, such as air temperature (T_o), relative humidity (RH), wind speed (U), and solar radiation (Rs), were obtained from the weather station mentioned previously. Here, the production (MD) of solar desalination/still system is a dependent variable, whereas seven variables are independent, namely, T_o, RH, U, Rs, TDS_F, M_F, and T_F.

The ANN model was developed using Qnet2000 software. The modeling process includes three stages, namely, training, testing, and validation. The available data set, which is composed of 160 data points obtained from the experimental work, was divided randomly into training (70%), testing (20%), and validation (10%) subsets. Therefore, the training, testing, and validation sets have 112, 32, and 16 data points, respectively. Trial and error is the best method to find the optimal number of neurons in the hidden layer (Abutaleb 1991). Consequently, the trial and error method was used to determine the optimum neurons in the hidden layer of the network. Before the modeling process, the data is automatically normalized between 0.15 and 0.85. The normalization accelerates the training process and enhances the network's generalization capabilities. The iteration was fixed to 200,000. The learning rate and momentum factor were fixed at 0.01 and 0.8, respectively.

The statistical data analysis was carried out using the data analysis tool in Microsoft Excel (MS Excel). Table 1 presents the statistical parameters of the experimental data. The statistical parameters were minimum (MIN), maximum (MAX), mean (AVG), standard deviation (SD), and coefficient of variation (CV). According to the findings of the field experiments, the average MD for the solar still system was 0.50 L/m²/h (approximately 5 L/m²/day), which is consistent with the findings of Radhwan (2004) and Kabeel et al. (2012). The results from the experiments have also revealed that the most dominant meteorological parameter affecting the MD was the Rs. The increase in T_o and U tends to increase the MD. The effect of increasing the U on the MD is more significant than the effect of increasing the T_o because increasing the U causes an increase in the convective heat transfer coefficient from the cover to the atmosphere. This leads to the decrease in the cover temperature and increase in evaporation and condensation rates inside the solar still, including the MD. RH was inversely proportional to the MD because low RH (drier air) is likely to increase and enhance the evaporation rate. The evaporation rate also increases with the increase of the T_F and thereby higher MD. It was found that with the increase of the M_F, the MD decreases. With the decrease in the TDS_F, the MD increased where the evaporation rate increased, which may be attributed to the weakness of ionic bonds for the low TDS_F. A more complete illustration of these experimental data is given by Mashaly et al. (2016).

Table 2 shows the correlation matrix of the experimental data (all input parameters). The last row in Table 2 lists the correlation coefficient (CC) between the input parameters (T_O, RH, U, Rs, T_F, M_F, and TDS_F) and the output parameter (MD). This table displays that the linear correlation between Rs and MD is 73%. Therefore, any model that employs Rs should be able to estimate the MD satisfactorily. The model's performance can be augmented by considering other parameters that have aerodynamic behaviors on MD, such as RH, U, and T_o. However, the T_o, RH, U, T_F, M_F, and TDS_F are not well correlated with MD. Instead, these parameters are included in the modeling process for better accuracy of MD estimation. Additionally, some of these parameters are correlated to others, and yet these were included in the modeling process because their presence was found to advance the model accuracy. However, the sign of the CC (+,−) is used to denote the correlation, which is either positive or negative. The last row in Table 2 shows that most of these linear correlations were very weak in reflecting the non-linearity of the dominant processes, which supports the use of ANNs. Moreover, the solar distillation process is considered to be highly nonlinear.

Equation (12) shows that the T_o, U, and TDS_F were inversely proportional to MD. Furthermore, the RH, Rs, T_F, and M_F were directly proportional to MD. Table 5 illustrates the standard error (SE) of the regression, probability (p-value), and t statistic (t-stat) of the MLR model parameters. The significance of each coefficient in Equation (12) was determined by t-stat and p-value, which are presented in Table 5. Larger t-stat and smaller p-value indicate greater significance of the corresponding coefficient. Table 5 also shows the meaningfulness degrees of the input variables. This degree of meaningfulness is determined via the p-value less than 0.05. By reviewing the p-values from Table 5, a significant relationship was found between independent variables (RH, U, Rs, M_F, and TDS_F) and dependent variable (MD) at a statistical significance level of 0.05. This finding can be attributed to the p-value of these variables, which is less than 0.05. The T_o and T_F were not statistically significant as their p-value is greater than 0.05. Thus, the significance ranking of the input variables is determined as Rs, U, M_F, TDS_F,_, and RH.

Figure 4 indicates the comparison between the predicted versus observed MD using ANN and MLR models during the training process. For the ANN model, the data points were mostly evenly and tightly distributed around the 1:1 line. There was a very close visual agreement between the observed MD and the results obtained by the ANN model. Furthermore, the figure indicates that many points given by the MLR model during the training process are located above and below the 1:1 line for the output. Thus, the figure shows that the ANN model gives an excellent match between the observed and predicted values. The overall performance of the ANN model and the MLR model was assessed using the statistical analyses shown in Table 6, which supports the better performance of the ANN model compared to the MLR model. From Table 6 and using the training data set, the MLR model had a DC value that was about 8% less accurate than that from the ANN model. The RMSE value for the MLR model (0.119 L/m²/h) was almost five times the value for the ANN model (0.024 L/m²/h). Meanwhile, the ANN model had an OI value that was about 17% more accurate than that from the MLR model. The CRM value for the ANN model was closer to zero than its value for the MLR model. Figure 5 shows the relative errors of the predicted MD values for the ANN and MLR models during the training phase. The relative errors of the predicted MD values for the ANN model were mostly around +10 to −10%, except for a few data points. For the MLR model, the figure shows more relative errors than the ANN model.

Figure 4 presents a comparison of the observed and predicted values for both the ANN and MLR models using the testing data set. The figure shows the fit of the trained ANN between the predicted and observed MD values. The tight banding around the 1:1 line demonstrates the remarkable agreement between the predicted and observed data. From Table 6 and using the testing data set, the MLR model had a DC value that was about 5% less accurate than from the ANN model. The RMSE value for the MLR model (0.128 L/m²/h) was almost double that of the value for the ANN model, and the OI value for the ANN model was approximately 16% more accurate than the MLR model. The CRM value for the ANN model was closer to zero than that of the MLR model. Furthermore, the CRM value for the MLR was nearly 7.5 times the value of the ANN model. Figure 5 shows the relative errors for the ANN and MLR models during the testing process. Figure 5 indicated that the relative errors of the predicted MD values are not considerable and most of it falls in the domain of +10 to −10% for the ANN model.

Figure 4 describes the relationship between the observed and predicted values of MD using the ANN and MLR models during the validation process. Similar to the training and testing processes, the ANN model provides better agreement between the observed and predicted values than the MLR model. The figure illustrates that the ANN model provides an excellent match between the observed and predicted values. From Table 6 and using the validation data set, the MLR model had a DC value that was approximately 3% less accurate than from the ANN model. The RMSE value for the MLR model (0.142 L/m²/h) was almost three times the value of the ANN model. The OI value for the ANN model was closer to one than for the MLR model. Moreover, the OI value for the ANN model was approximately 20.1% more accurate than the MLR model. The CRM value for the ANN model was closer to zero than for the MLR model. Additionally, the CRM value for the MLR model was almost 7.5 times the value of the ANN model. Moreover, Figure 5 displays the relative errors of the predicted MD values using the validation data set for the ANN and MLR models. The relative errors of the predicted MD for the ANN model were mostly in the vicinity of ±10%. The low relative errors demonstrate the strength of the ANN model.

Figures 4 and 5 demonstrate the inaccuracy of some values predicted and obtained from the MLR model, while most of the predictions were highly precise when using the ANN model. This shows that the MLR model is not an accurate predicting technique for MD. Thus, the ANN model produced a better fit with the observed data during the training, testing, and validation processes. Table 6 shows that better agreement between the observed and predicted MD values is obtained using the ANN model. This agreement is reflected in the DC, RMSE, OI, and CRM results, as mentioned previously. This result agrees with the findings of Şahin et al. (2013), El Badaoui et al. (2013), and Mashaly & Alazba (2016c, 2017c).

The capability of solar stills to yield water is highly beneficial for small communities under hyper-arid environments. Consequently, solar stills should be optimally designed and operated, and the prediction of solar still production or water being distilled (MD) is one of the important parameters that should be precisely determined. The prediction of MD helped in determining the amount of potential distilled water attainable by the solar still and to ensure the adequacy of productivity; that is, that sufficient water quantities are achieved. This will help in decision making that will support various development plans. One technique of predicting the MD is the use of the ANN model. Seven variables were used as inputs to the ANN model in the input layer, namely, To, RH, U, Rs, T_F, TDS_F, and M_F. One neuron in the output layer represents the output (MD). A feed-forward back-propagation algorithm was used to train the ANN model. Several neural network architectures with different numbers of neurons in the hidden layer were trained and tested to determine the architecture that gave the minimal error and best performance. Eight neurons were the best number of neurons in the hidden layer. The 7-8-1 architecture was the optimal ANN architecture. TANH was used as the activation function in the hidden and output layers and was better than the SIG function. The findings from the developed ANN model were compared with those from the MLR. The performance of the models was evaluated by DC, RMSE, OI, and CRM. From the results, the ANN model demonstrated better prediction performance than the MLR model and revealed adequate precision in the forecasting of MD. The results revealed that the developed ANN model has a very high DC and OI between the predicted and the observed values of MD. Furthermore, the developed ANN model has a very low RMSE and CRM between the predicted and the observed values of MD. These results support the applicability of the developed ANN model. The MLR model results were also satisfactory for predicting MD, but it was less accurate compared to the ANN model. In this study, the ANN model was proven to be a sufficient, accurate, and successful tool for modeling the MD without the need for comprehensive experimental investigations. Therefore, this investigation allows a preliminary decision on the usability under conditions in which the solar still is required.

The project was financially supported by King Saud University, Vice Deanship of Research Chairs.