Elsevier

Journal of Energy Storage

Volume 32, December 2020, 101824
Journal of Energy Storage

Sensible desalting: Investigation of sensible thermal storage materials in solar stills

https://doi.org/10.1016/j.est.2020.101824Get rights and content

Highlights

  • PVA sponges, spherical clay balls, pebbles and CuO plates are tested in the solar still.

  • These materials absorb solar influx and enhance the heat transfer to the water.

  • Water quality, cost analysis, heat transfer and efficiency are discussed.

Abstract

The present investigation elucidates the effect of different absorbing materials in the single slope solar still (SSSS). The said absorbing materials are Polyvinyl Alcohol (PVA) sponges, pebbles, spherical clay balls (SCB) and CuO nano-coated absorber plates (CuO-NCAP). The experiment has been conducted in the following ways: (i) SSSS with PVA sponges, (ii) SSSS with pebbles, (iii) SSSS with SCB, and (iv) SSSS with CuO-NCAP. This research also investigated the night water collection of all individual experiments. Four identical SSSS were designed with 0.50 m2 collector area each and the four absorber types were tested in the same climatic conditions of Chennai (13.0821°N Latitude & 80.2702°E Longitude) during the period of April to June 2018. Physical and chemical water quality tests were conducted for the feed water (well water) and treated water from the SSSS. The productivity of SSSS-PVA sponges, SSSS-pebbles, SSSS-SCB and SSSS with CuO-NCAP are 1.9 L/m2.day, 2.8 L/m2.day, 2.6 L/m2.day, and 2.9 L/m2.day respectively. The cost of water distilled by SSSS with PVA sponges was approximately $0.0120/L. Similarly, it was $0.0073/L for SSSS with pebbles, $0.0080/L for SSSS with SCB and $0.0077/L for SSSS with CuO-NCAP.

Introduction

Scientists are searching for Earth-like planets in the galaxy that humanity might be able to move to. In the near term, we do not have the technology to move to planets outside the solar system, and the other planets in our solar system are not very hospitable. Basically, we are in our comfort zone, because Earth is the only planet that has sufficient O2 and H2O available for living organisms. We are slowly losing our comfort on Earth due to high levels of pollution. Conservation of groundwater is essential because during the summer, it is the only option to provide water for many places. Among all the uses of water, domestic use of water has prime importance. Solar stills are one sustainable way of producing drinking water, which involves water evaporating from a basin and condensing on the glass above. One way of improving the efficiency of solar stills is placing different materials in the basin.

Sharshir et al. [1] experimentally studied graphite flake nanoparticles, phase change material (PCM) and top cover cooling in relation to the solar still. The broadband absorption of flake graphite nanoparticle was higher (99.5%) than normal black basin (90%). The PCM in the solar still stored the incoming solar radiation and increased the temperature of the solar still later in the day by releasing latent heat. The inclusion of graphite flake nanoparticles, PCM and film cooling increased the productivity of the solar still by 73.8%. Estahbanati et al. [2] constructed a solar still with and without internal reflectors (IR) to enhance the solar still's efficiency. The study was conducted in the following ways (i) conventional solar still, (ii) IR on the front wall, (iii) IR on the back wall, and (iv) IRs on all four sides. The solar still with IRs on all four sides yielded 5.22 kg/m2/day. The productivity enhancement of solar still with IR on the front wall, IR on the back wall and IR on both the sides were 18%, 22% and 65%, respectively. Feilizadeh et al. [3] demonstrated a multi-stage solar still integrated with a flat plate collector (FPC) to increase the basin temperature. The effective absorber area of the multi-stage solar still and the FPC are 0.47 m2 and 1.57 m2, respectively. The broadband absorptivity of the FPC was about 96%. The test was conducted in the summer and winter to understand more about the solar distiller. One FPC was not sufficient to deliver the energy to the multi-stage solar still (1–4 stages) effectively. Three FPCs would produce a significantly better result than one or two FPCs.

Zhang et al. [4] fabricated a molybdenum di-sulphide (MoS2) loaded carbon foam (CF) for solar steam generation. The experiment was conducted in two ways, (1) MoS2 loaded carbon foam (CM) and (2) carbon foam (CF) only. The absorption spectrum of CM was better than CF. The test was conducted in a solar still under natural sunlight. The CM foam produced 0.9 g of water versus 0.79 g for the CF. Higgins et al. [5] coated titanium dioxide (TiO2) nano-rods onto CF for solar steam generation. The TiO2 nano-rods were synthesized by a hydrothermal process. The evaporation rate and the solar thermal conversion efficiency of TiO2-CF were 0.9 kg/m2/h and 60.2%, respectively. In addition, the TiO2-CF was tested in the solar still for a wastewater treatment application. The TiO2-CF was able to effectively remove the contaminants from the saline water as well as industrial wastewater. Lin et al. [6] obtained a carbonized melamine foam (MF) in a one-step calcination process for solar steam generation. The broadband absorption (α) of MF was about 95% in the solar spectrum. The low thermal conductivity (0.002 Wm−1K−1) and appropriate pore diameter was the reason for efficient solar evaporation. The South China Sea water was evaporated under natural solar radiation. The concentration of ions in the seawater (Ca2+, Mg2+, Cr, SO42+) was reduced 3–5 orders of magnitude after evaporation.

Many researchers have focused on efficient solar still designs such as tubular solar stills [7,8], pyramid solar stills [9], concentrator assisted solar stills [10], multi-effect active solar stills [11], PV/T hybrid solar stills [12], and solar stills with energy storage materials [13], [14], [15], [16], [17], [18] to augment the distillate productivity. Further, researchers have performed their experiments with sensible energy storage materials including pebbles [19], sponges [20], iron scraps [21], magnets [22], and sand [23] to improve the evaporation.

Herein four different absorbing materials [Polyvinyl Alcohol (PVA) sponges, pebbles, spherical clay balls (SCBs), and CuO-NCAP] are investigated. Further, the unused well water was fed into the solar still and analyzed the water quality before and after evaporation. For the first time, spherical shaped clay balls were used as a sensible energy storage material in a single slope solar still (SSSS). The PVA sponges (each 11.5 g) are hydrophilic and hold up to 12 times their dry weight in water. Four different colored PVA sponges are used in this experiment. PVA sponges have more pores than normal sponges, making the PVA sponges better absorbers of water. Finally, the night water production of all the four experiments was measured.

Section snippets

Materials and methods

Four identical SSSSs (0.71 m × 0.71 m) were fabricated and tested during the months of March to June, 2018. Galvanized iron (GI) sheet was used to construct the SSSSs and it was coated with black paint to absorb the sun more strongly. The cover material (3 mm thick) was plain glass which prevents distilled water from falling back into the basin (hydrophilic property). The outer sides and bottom of the SSSSs were properly wrapped with bubble wrap (BW) of thickness 30 mm [24,25]. Table 1

Recent advances in sensible energy storage materials

Sensible heat storage occurs when the temperature of a material increases; sensible heat release occurs when the temperature decreases. The heat storage material should possess high specific heat and density [26]. The amount of energy stored is determined by the following equation [27].Q=m.Cp.ΔTwhere, Q is the amount of heat stored in the material (J), m is the mass of the storage material (kg), Cp is the specific heat capacity of the material (J/kg.K), and ΔT is the change in temperature (K).

Mechanism of distilled water production by solar stills with sensible energy storage materials

Solar radiation is transmitted through the glass cover of the distillation unit and absorbed by the blackened basin; hence, its temperature increases. When no solids are placed in the basin, the heat is convected to the water. Sensible materials including rock-bed, pebbles, gravel or bricks, if present, absorb some of the heat without change of phase. Afterwards, absorbed heat is transferred from the sensible materials to the water surface via convection. This results in the vaporization of

Results and discussion

The pictorial views of the SSSSs with different absorber materials are shown in Fig. 4 (a–d). Fig. 5 (a-b) shows photographs of the SSSS outdoor units with researchers for scale.

Conclusion

Four absorbing materials including Polyvinyl Alcohol (PVA) sponges, pebbles, spherical clay balls (SCBs) and CuO nano-coated absorber plates (CuO-NCAPs) were tested in SSSSs. The CuO-NCAPs were prepared by the thermal evaporation technique. The findings from the experiments are:

  • The yields of SSSS-PVA sponges, pebbles, SCB and CuO NCAP are 1.9 L/m2.day, 2.8 L/m2.day, 2.6 L/m2.day, and 2.9 L/m2.day, respectively.

  • The calculated efficiency of SSSS-PVA sponges, pebbles, SCB and CuO NCAP are 32%,

Future work

To increase the evaporation rate of the distiller, natural hydrophilic porous materials such as wood and leaves could be carbonized. The carbonized wood and leaves would be capable of absorbing the incoming solar radiation (broadband absorption) perhaps better than conventional absorbers. These materials could also act as sensible heat storage.

Author statement

T. Arunkumar: Experimental design & analysis, Jiaqiang Wang: Supervision, D. Dsilva Winfred Rufuss: Data verification and experimental calculations, David Denkenberger: Reviewing and editing, and A.E. Kabeel: Helped supervise the project.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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