Experimental Evaluation and Theoretical Analysis of the Long Stills Covered with the GI, Cu and Al Sheets

Electrical energy is mostly converted from thermal energy drown from the thermal or nuclear power plant for large scale and Diesel generator (DG) sets are used as a backup device. In these, we mostly used water as a working substance and cooling agent for maintaining the operating temperature of the devices. Experimental and theoretical evaluation of three identical reduced scales long still prototypes covered with the different metallic sheets (viz. GI, Cu, and Al) are reported in this manuscript. Hot wastewater at 70±1 oC is fed to the distiller unit as a discharge of the thermal unit is ranges between 65 to 75 oC. The theoretical result obtained from the regression analysis gives good agreement with the result obtained from the experimentation. Long still covered with the Cu sheet material is gives better performance as compared with the others (viz. 55.03 and 34.69% higher than the GI and Al sheet cover). It has been observed that the still covered with the Cu sheet gives better distillate yield as compared with the still covered with the Al or GI sheet material.


INTRODUCTION
The demand for potable water is increasing day by day due to the depletion of potable water mainly due to the rapid enhancement in the population and industrialization. As most of the industries are utilizing water as a cooling agent for the Internal combustion engines. A conceptual model for the performance of a solar still in two different modes(viz. (i) hot wastewater from the thermal power plant to solar still at a constant flow rate and (ii) Hot wastewater obtained from thermal power plant fed once in a day to the solar still) have been reported by Sodha et al. [1]. Mishra et al. [2] have reported experimental evaluation of long still covered with the glass material for utilization of hot wastewater will pave the way for utilization of this waste energy. Sharma et al. [3] have reported the effect of different flow rates on the performance of long still covered with the GI sheet. The effect of the hot wastewater flow through the still in the hot day has been reported by Tiwari et al. [4]. Single basin solar still integrated with the earth have been reported by Sodha et al. [5]. Dumka and Mishra [6,7] have reported Experimental and theoretical evaluation and the way for utilization of ground energy for the desalination using single slope solar still. Elminshawy et al. [8]have utilized low-grade energy for the desalination of water. Fathi et al. [9] have reported enhancement in an overall efficiency of a nuclear power plant by utilization of waste heat. The relation between performance and depth of the water within the solar still has been reported by Tiwari and Tiwari [10] and the influence of water depth on the heat and mass transfer has reported by Tripathi and Tiwari [11]. The effect of different salt concentrations on the performance of the solar still has been reported by Dumka et al. [12]. Experimental evaluation of hybrid solar still integrated with the air compressor has been reported by Hidouri et al. [13]. Exergo-enviro-economic performance evaluation of a single slope distiller unit has been reported by Joshi and Tiwari [14]. Different geometry new absorber configuration (flat, grooved, and fin-shaped absorbers) of single basin solar still for partial utilization of hot wastewater has tested and reported by Hansen and Murugavel [15].
Dumka & Mishra [16] and Dumka et al. [17] have reported the use of ultrasonic fogger without and with cotton cloth tent to enhance the distillate output of solar still. They have reported an increase of 33.26 and 53.12% in the distillate output of modified still in comparison to CSS. Chauhan et al. [18] have reported the use of ANN trained with the LM algorithm to predict the thermophysical properties of moist air between the condensing and evaporating surface of a still. Dubey and Mishra [19] have theoretically and experimentally examined the impact of glass cover angle on the performance of single basin double slope solar stills. For this sake, they have used 3 cover angles viz. 15, 30, and 45°. They have reported that the maximum yield is obtained from still with 15° inclination with a contribution of 48 and 52% form west and east sides, respectively. Gojak et al. [20] have reported a stepwise simulation of a water heating system that works on solar energy. An extensive review of different solar concentrator designs has been reported by Stojicevic et al. [21].
Detailed reviews on the performance of single slope solar stills have been reported by Sharma et al. [22]. A water augmentation technique [23], review on pyramidal still [24], geometric and fin design, and its deployment for enhancing the energy storage capacity [25] and application of nano energy [26] have been reported by the various research groups. A detailed review of different glass cover cooling techniques has been reported by Omara et al. [27]. A comprehensive study on PCM material used in solar still has been reported by Kabeel et al. [28]. A theoretical and empirical study of heat and mass transfer inside a basin type solar still has been reported by Madhlopa [29]. Utilization of water as a cooling agent for maintaining the operating temperature of the domestic and industrial thermal equipment is a normal practice worldwide. Cooling water is used to excavate more than 40% of generated heat energy and inject into the ambient with the help of a radiator, Chiller unit, or cooling towers. The utilization of this energy may boost the overall efficiency of thermal units.
Experimental (in control environment) and theoretical evaluation of the reduced scale model after 8-hour experimentation at a one-hour interval is recorded and reported. The proposed model can be utilized in thermal power-based industries; thermal power plants and different capacity of diesel greater units were water is used as a cooling agent in an enlarged scale between the discharge and cooling unit for gating distillate yield by utilizing waste energy. The application of the proposed device will enhance the overall efficiency of the plant or unit.

EXPERIMENTAL SETUP
The schematic agreement of the experimental setup and the actual photograph is shown in Fig. 1(a), (b), and 2 respectively. Three identical stills with basin area (1m × 0.1m) of long are made with the help of Fiber reinforcement plastic (FRP) material. 10 K-Type thermocouples are deployed at equidistance within the basin area of each still for recoding the fed hot wastewater temperature along the length of the distiller unit during the experimentation. The top covers of the distiller units are made with the Cu, Al, and GI metallic sheets. The top cover of each still contains 10 K Type thermocouples at equidistance in each stills for recording condensing cover temperature behavior. Cover materials are sealed with the basin using epoxy resin to ensure it leak proof. Inlet and outlet of the still as are made with the help of PVC pipe nipples as its bad conductor to heat. An insulated water heater tank with 50 l capacity was used for getting hot water which has fed to the distiller unit at 70 ±1°C with flow rate of 230 l/h from one end and discharges from the other end recirculated through the water heater with the help of a pump. The recirculating valve was used to regulate the flow rate of hot wastewater. All the experiments are carried out within the control environment of the laboratory at a temperature 22±1°C. Following data were recorded while the experimentation for the theoretical evaluation of the distillate output: • Ten equidistance condensing cover & basin water and room temperatures. • Distillate yield from all ten points. •Still is leak proof.
•Metallic cover heat capacity is neglected.
•The still is in steady-state.
•There are no side losses from still. the energy balance equation for water and metallic cover one can get: where, evaporative convective and radiative heat transfer rates from water to condensing cover are: where, 1 Evaluation of Pr and Gr, using humid air physical properties relations are given by Tsilingiris [30] and theoretical model based on linear regression analysis proposed by Kumar and Tiwari which uses experimental yield to generate the values of C and n of a still is used. The relations of C and n from the model can be written as: exp Convective heat transfer coefficient from water to condensing cover can be evaluated as: by knowing the value of h cw , h ew can be evaluated as: whereas radiative heat transfer rate from water to the inner condensing surface is calculated as [31,32]: The total heat transfer rate from water to the inner condensing surface is given by:  to predict the strength of individual mode of the heat transfer within the distiller unit.

RESULTS AND DISCUSSIONS
Variation in evaluated partial pressure at condensing covers of long stills covered with the Cu, Al, and GI sheet cover material and on the basin water surface from the entrance to exit is shown in Fig.4. Partial pressure on Cu condensing sheet cover has maintained its lead throughout the length of still in comparison with the other two stills, whereas, it goes on reducing throughout the length of the long stills. Partial pressure on the water surface also shows a reduction from the entrance to the exit in all three cases. Long still which covered with the Cu sheet material shows higher pressure difference (i.e. the pressure difference between water and condensing surface) as compared to the other two identical long distiller units. The higher gradient will lead to better performance and gives higher distillate output from the long still covered with the Cu Variation of heat transfer coefficient of evaporative, radiative and conductive heat transfer rate along the length of stills are shown in Fig.5.  Evaporative mode of heat transfer shows dominance over the convective and radiative as coefficients for the evaporative heat transfer is higher as compared to the radiative and convective heat transfer rate at all the stages of long still along its length. The evaporative heat transfer coefficient of still covered with the Cu sheet has maintained its lead throughout the experimentation along the length of the still in comparison with the still covered with the GI and Al sheets. Variation of the influence of the different meta-llic covers is used as a condensing cover in long stills on the total heat transfer rate is shown in Fig.6. The total heat rate in all the cases decreases along the length of the long still. Long still covered with the Cu sheet maintains its lead throughout the experimentation in comparison with stills covered with the Al and GI sheet. Maximum   Overall heat transfer rate at the entrance (0.1 m distance) of the long still having a condensing cover of Cu sheet is 61.25 and 48.43% higher than the still covered with the GI and Al sheets. At intermediate position long still covered with Cu sheet maintain its lead in terms of overall heat transfer rate by 56.35 and 41.49% with the long stills covered with the GI and Al sheets. At the exit (viz. 1.0 m distance) GI and Al sheet covered long stills are lagged by 34.06 and 19.76% in comparison with the long still covered with the Cu sheet, as overall heat transfer has great influence of thermal conductivity. Heat transfer rate goes on reducing along the length of long still.
Variation of theoretical and actual observed distillate output from the still units in control environmental conditions from reduced scale identical models covered with the GI, Al, and Cu sheet materials along the length are shown in Fig.7. Long still utilized Cu sheet as a condensing cover maintain its leads along the length of the still, whereas still covered with the GI sheet gives lowers output and still covered with the Al sheet gives an intermittent performance in comparison to the other two stills. The measured distillate output from the long still covered with the Cu sheet gives 39.78 and 58.53 % higher distillate output in comparison with the long stills covered with the Al and GI sheet materials respectively. At 0.5 m away from the entrance, it is 51.47 and 35.52 % higher than the GI and Al sheet condensing cover long still. At exit section long still covered with the Cu sheet leads by 40.91 and 16.98 % in comparison with the GI and Al sheet cover material of the long stills respectively. Total distillate output obtained from the long still covered with the Cu sheet material gives 55.03 and 34.69 % higher distillate output as compared with the long stills covered with the GI and Al sheets respectively.  Highest and lowest distillate outputs are recorded and evaluated in all the cases at their entrance and exit respectively. Total distillate evaluated for the Cu, Al, and GI sheet cover long still is 0.07 % higher, 2.53% lower, and 0.43% higher respectively, which will give credence to the results obtained from the theoretical evaluation and shows good agreement with the experimental results. Theoretical models use the regression model for evaluation Gr and Pr numbers for each section. Varation of the fraction of the overall heat transfer coefficient through the different modes of heat transfer took place while the experimentation in long stills covered with the GI, Al, and Cu sheet cover material is shown in Fig.8. Evaporative mode of heat transfer in all the stills dominating to the heat transfer which took place in radiative and convective mode and it goes on decreasing along the length of still. Convection mode shows its least contribution whereas radiative mode has an intermittent stage. A fraction of energy utilization in radiative mode is increasing along the length of the long stills covered with the GI, Al, and Cu sheet material.