Water Distillation Using Solar Energy System with Lauric Acid as Storage Medium

An experimental investigation on a solar still with lauric acid as phase change material (PCM) is carried out to examine the effect of both the mass of PCM and basin water on the daily distillate productivity and efficiency of the system under outdoor condition. Basic energy balance equations are written to predict the water and glass temperatures, daily distillate productivity and instantaneous efficiency of the single slope solar distillation system with PCM. It is found that the higher mass of PCM with lower mass of water in solar still basin significantly increases the daily productivity and the efficiency. Therefore, the distillate productivity at night and on day for solar still with PCM increased by 127% and 30-35% respectively than without PCM one. Shukla et al. approach of the use of inner glass cover temperature for productivity prediction which has also been investigated, and the prediction shows relatively better agreement with the experimental data than outer glass cover temperature.


Introduction
Portable water is indispensible for life and its scarcity is growing across the world, particularly in dry regions, such as deserts and modern industrial areas. The similar problem exists in remote areas and islands where fresh water supply through any transportation means is expensive. On the other hand, seawater exists in plenty but it necessitates the removal of the salinity with additional expense of energy which is another scarce resource. Survey reveals that approximately 79% of available water is salty, 20% is brackish, and only 1% is fresh. Therefore, solar energy being free and in abundance is being utilized for the purpose of desalination of brackish water to produce drinking water [1,2].
Extensive research work on parametric studies and methods to improve the productivity of passive solar stills has been reported in literature. Passive solar stills directly use solar energy to produce distillate water, and are self-operating, simple in construction and relatively free of maintenance [3] whereas, active stills comprise of energy concentrator to add more energy to the system for enhancing the distillate productivity. The double slope solar still did not prove superior to single slope solar still [4,5]. However, single slope solar still with a condenser in the shaded region slopped increases the still efficiency by 45% [6], bla-ck dye injection in basin water enhance the productivity by 29% [7], the maximum of the annual productivity attained when the condensing glass cover inclination is equal to the latitude of the place [8,9] were found that a lower water depth gives better efficiency, they were dependent on instantaneous gain and loss efficiency curves to give a better understanding of the performance of solar stills.
Yet, the solar stills have not found commercial utility due to low productivity, and the reason associates to the heat loss to the surroundings [10][11][12] Options like use of energy storage system for post sunset operation are being investigated involving phase change materials (PCM) that exploit latent heat capacity [13] to absorb the large amount of energy.
They [14,15] were used phase change material (PCM) as a energy storage medium to study the performance of stepped solar still and a single basin solar still, respectively. The productivity of solar still with PCM storage medium were found to increase during post sunset operation, which occurred due to higher temperature difference between water and glass cover at relatively lower ambient temperature. They [15] analysed the transient performance of a single slope solar still with stearic acid as a storage material and observed increased productivity with increasing the mass of the PCM, however, they mentioned that the work on solar still with PCM storage medium is still scanty. In the present study, performance of a single slope solar still with Lauric acid (99% purity) as PCM has been investigated for various masses under outdoor conditions at day in summer season. The energy balance equations for different components of the still during charging and discharging modes have been written and solved analytically. Calculations have been carried out for daily productivity and instantaneous efficiency of the still.

Experimental Procedure
A photograph of both the solar stills, with and without phase change material (PCM) as a storage medium, is given in Figure. 1. The basin, fabricated from a black painted mild steel sheet of thickness 2mm, has an area of 1m 2 each. A vertical gap (0.05m) beneath the horizontal portion of the basin liner is provided to upload and/or unload the PCM through a pipe which takes care of the volumetric expansion of the melting PCM as well. The operational and melting temperature of PCM, in fact, governs the applicability of different types of phase change materials. The lauric acid relates to the class of fatty acids that have superior properties such as melting congruency, good chemical stability and non-toxicity, good thermal reliability [16,17]over many other PCMs.   Therefore, lauric acid has been used as a latent heat storage material due to its low cost and easy availability. Table  1 summarizes the thermo-physical properties of lauric acid [18,19] used in the experimentation.
The measurement of property of PCM was done in Netzsch Technologies India Pvt Ltd. By using the differential scanning calorimetry (DSC) technique as revealed in Figures 2 and 3.
The bottom and sides of the basin are insulated by 5 cm thick layer of rock wool contained in an aluminium tray. The top cover of the still is made up of 4mm thick window glass which inclines at an angle of 25° with horizontal, and has an average transmissivity (τ) of value 0.88. A U-shaped channel is used to collect the condensate from the lower edge of glass cover and carry it to storage. Figure.   A temperature scanner (Altop Industries ltd, Sn.1005164, model ADT 5003) with resolution 0.1 o C and HTC DT-8811Infrared thermometer range: -20 o C -450 o C Spectral response: 6-14µm CE ASCO products had been used to record the temperature with k-type thermocouples in solar stills. The solar radiation measure by using the daystar meter Watts/m2 ASCO products, the wind speed was observed by HTC Instruments AVM-07 Anemometer vane probe CE.
The variation of the typical observed ambient temperature and calculated solar radiation have been shown in Fig

Mathematical Models
Assumptions have been made:-1.The solar still distillation unit is vapor-leakage proof.
2.The conduction heat transfer mode between PCM and basin liner.
3.The temperature gradient through PCM negligible. 4.The experiments were carried out under atmospheric pressure.
.  Energy balance of the glass cover of the inner surface α g ' I(t) + h 1 (T w -T gin )=k g /L g (T gin -T go ) (1) Energy balance of the glass cover of the outer surface k g /L g (T gin -T go ) = h 2 (T go -T a ) (2) Energy balance of the water mass α' w I(t) + h 3  M equ = m pcm C s,pcm for T_b<T_mt , M equ = m pcm L s,pcm for T_b<T_mt+δ , M equ = m pcm C l,pcm for T b >T mt . The resultant was T go ,T gin and T b from eqs. (1), (2) and (4): T go = (k g /L g T gin +h 2 Ta)/(h 2 + k g /L g ) (6) T gin = (α' g I(t) +h 1 T w + U ga Ta )/(U ga +h 1 ) We got eq (9) by placing the value of eq (7) and eq (8) in the variants of the eq (3) dT w /dt + a c T w = fc(t) (9) Eq(6) was solved by using the integral factor with the boundary condition, when t=0 that led to Tw = Two thus : T w =fc(t) /a c [1-exp(-a c t) ]+ T wo exp(-a c t) (10) To get Tpcm from eq (11) and eq (8) T pcm = fc1(t) /a c1 [1-exp(-a c1 t) ]+ T pcm exp(-a c1 t) (11) 3.1.2 . The still with the PCM discharging mode The energy balance equation for the PCM at the interval time Δt may be written as: m pcm L pcm /A b ∆t=h bd (T pcm -T b )+hb (T pcm -T a ) (12) For T pcm =T mt h bd (T pcm -T b )+ hb (T pcm -T a )=(M equ / A b )(dT pcm / dt) (13) For T pcm ≠T mt where M equ = m pcm C l,pcm for T pcm > T mt , M equ = m pcm C s,pcm for T pcm < T mt , h bd = k pcm /x pcm that is the conductive heat transfer coefficient from the PCM to the basin liner.

Numerical Computation
Two C language programs were developed to find the predicted theoretical values for equations , first one had been to calculate parameters c and n for equation (A-3) appendix A , by using of Tw and Tgin observed data, then these values it used with the second developed program to calculate the productivity of solar still with and without PCM. The programme executed by using parameter design of solar still with and without PCM as shown in Table 2.

Results and Discussion
The experimentation started every day at 7:00 AM, and the temperatures of ambience vapor in vicinity of glass, at outer and inner surfaces of glass cover, basin liner, stored water and PCM were recorded at an interval of one hour till 6:00 P.M. in the evening. Simultaneously, the values of solar radiation, and distillate productivity were also recorded. Figure 7 for solar still without PCM at summer day shows that the temperatures of basin water, vapor of vicinity, outer and inner surfaces of glass cover, basin liner, stored water and the temperature increased with increased (I(t). Ab) and decreases thereafter following the similar trend to that of solar radiation given in Figure 7   The latent energy stored in the lauric acid keeps the system operational during the night to deliver distillate productivity. However, the decreasing trend of all above temperatures shows that the quantity of distillate productivity reduces (see  due to lower difference in the operating temperature of glass cover and ambience that continues till next day morning. The productivity due to solar still without PCM has also been shown in Figure 8 for the comparative purpose. It is observed that the PCM enhances the productivity of the system by 30% due to PCM being acting as heat source under discharge mode during night. Further, the productivity, for given mass of water in the basin of the solar still with PCM has also been plotted for daytime and night hours, separately to understand the operational characteristics of the system with time. Figure 9 (A) shows that the productivity decreases with the increase in water mass during daytime, and it is due to the fact that higher quantity of water relatively needs more energy to raise its temperature before being evaporated. However, the productivity increases with the increasing water mass and PCM mass during night hours (see Figure 9 B) due to relatively larger storage of energy in PCM as latent heat and sensible heat in higher water mass, thus for 30 kg PCM the percentage of enhancement of productivity was 127% compare with solar still without PCM.   The variation of total distillate productivity with time, for different water masses, has been plotted for 24 hrs duration (see Figure 10). The decreasing trend of the curve in Figure  10 reveals that the thicker water depth in solar still basin yields lower distillate productivity over 24 hours, and the reason relates to the conversion of major portion of energy to sensible heat due to higher heat capacity of water mass. Further the quantity of the solar radiation is very important reason to increase the day time productivity that it has been explained in Table 3.In this table the solar radiation for 20kg PCM was lower quantity than 10kg ,therefore 10kg of PCM was given more productivity, other reason the 10kg absorbed low heat during day.

Figures 9 A and B also
show that the productivity of solar still for different water masses during day and night hours, respectively. It is observed that the trend of the curves for three different water masses remains diminishing during day hours and converse during night.
It is interesting to note that the lower water mass in solar still with PCM yields more distillate productivity during day hours than that due to higher water mass (see Figure 9 A), whereas, the higher water mass in solar still basin gives relatively higher productivity during night hours. The reason lies in the fact that the lower heat capacity of thinner water depth during day hours allows more energy to be utilized for evaporation purpose whereas the system with thicker water depth stored relatively more energy to yield higher productivity during night hours.
The variation of temperature for solar still with PCM is shown in Figure 11 for 24 hours. However, the difference in the insulation temperature in relation to that of still without PCM is more significant for solar still having higher mass of PCM (see Figures 12A, B and C).    The daily productivity for a given water mass for three different masses of PCM has been shown in Figure 13. It is observed that the overall productivity does not change significantly for 24 hours time but the productivities day and night at differ significantly as revealed by Figure 6 plotted for a given mass of PCM. The productivity of the solar still without PCM has also been compared see Figure 14 with the data of [4,9]. The observed data show the similar trend, however, the reason for the difference in the productivity may be attributed to the difference in the climatic conditions of two cities of INDIA: Varanasi and NewDelhi.  Further, the energy balance equations were applied to predict the distillate productivity due to stills, with and without PCM, for the comparison with the observed data. Figures 15 and 16 show that the predictions due to analytical method reasonably agree with the observed data and deviation falls within ±28% for solar still without PCM, and ± 31,31 and 20% for solar still with PCM mass of 10kg with water mass of 30 ,40 and 50kg, respectively. The increased/ decreased deviation, i.e., 32, 35, 26% and 24, 30 and 26% between predicted and observed data were found for the solar still at other two different masses of PCM for water masses 30, 40 and 50 respectively.  The variation of the instantaneous efficiency for solar stills, with and without PCM, has also been plotted in Figure 17. It is observed that the instantaneous efficiency for solar still with PCM is always higher than that due to traditional still; however, the increasing trend of both the curves with time indicates that the instantaneous efficiency keeps increasing due to increased temperature difference between basin water and ambience.

Conclusions
The integrated PCM with conventional solar still will increase enhancement productivity 30-35%.
1. Reduce the loss of heat to surrounding when use sswpcm.
2. By considering the inner glass cover temperature there is reasonable agreement between the experimental and predicted theoretical 3. It was found that the highest productivity rate is the least water depth.
4. The productivity on night increased significantly with increased PCM.
5. The productivity of observed solar still give good agreement with another observed work [4].