Solar distillation meets the real world: a review of solar stills purifying real wastewater and seawater

Abstract

Solar energy-driven evaporation-based freshwater production is one of the sustainable ways to purify contaminated/salty water. Recent advances in solar absorbers’ assemblies, design modifications, and integrations with heating sources improved the rate of freshwater productivity. However, the type of feed water affects the evaporation rate in a solar desalination system (SDS). Many studies used tap water with added contaminants to test the performance of a SDS and studied the water quality improvement. As a typical result, pH, total dissolved solids (TDS), and electrical conductivity (µS/cm) are reduced after solar evaporation. The performance of SDSs for real wastewaters are also important to understand, e.g., the reduction of high organic pollutants after solar evaporation. In this aspect, the main objective of the present work is to review solar distillation of real wastewaters and seawater by using SDSs. Further, the mechanism of a solar distiller with heat transfer principles, parameters affecting evaporation process, real wastewaters and seawaters purified in a solar distillation system, improvement of various parameters before and after solar evaporation, pathways of handling wastewaters, challenges, and future perspectives are discussed. Conclusively, SDSs are found to remove pollutants effectively after solar evaporation. The evaporation rate is relatively slower due to high concentration of pollutants that reduce vapor pressure. The COD removal of various real wastewaters, including sludge, kitchen, textile, palm oil, petroleum, water plant, and municipal wastewaters, was 98.13%, 97.85%, 96.84%, 96.71%, 87.99%, 86.99%, and 85.67%, respectively. The reduction rate of salt concentration in real seawater after evaporation in the solar distiller was 99.99%.

Appendix

Appendix 1 Heat transfer in solar desalination system

In general, heat transfer means the flow of heat from one place to another due to temperature differences between them. The utilization of input heat energy and the loss coefficients are determined with an aid of heat transfer calculations. In a solar desalination system, the flow of heat is further classified as internal heat transfer and external heat transfer.

• (i) Internal heat transfer

Energy is transferred within the system by convection, evaporation, and radiation. Here, the heat transfer occurs due the temperature difference between the any two segments in the system. Convection is driven by the temperature difference between the water and the glass cover (Agrawal et al. 2017).

$$q_{cwg} = \,h_{cwg} \,(T_{w} - T_{g} )$$

(1)

where q_c is referred as convective heat transfer, h_c is the convective heat transfer co-efficient (W/m² K), and (T_w − T_g) is the temperature difference between water and the glass surface.

$$h_{cwg} = \,0.884\,\left[ {(T_{w} - T_{g} )\, + \frac{{(P_{w} - P_{g} )(T_{w} + 273)}}{{(268.9 \times 10^{3} - P_{w} )}}} \right]^{{{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 3}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$3$}}}}$$

(2)

where P_w and P_g are the partial saturated pressure of water and the glass (N/m²).

Further, the evaporative heat transfer occurs when the flow of vapor from the water liner to internal glass cover (Agrawal et al. 2017).

$$q_{ewg} = h_{ewg} \,(T_{w} - T_{g} )$$

(3)

where q_e is the evaporative heat transfer rate and h_e is the evaporation heat transfer co-efficient (W/m² K).

$$h_{ewg} \, = \,(16.28\, \times 10^{ - 3} )\,h_{cwg} \,(p_{w} - p_{g} )/(T_{w} - T_{g} )$$

(4)

The radiative heat transfer occurs due to the temperature difference between water liner to the glass cover (Sharma and Mullick 1992).

$$q_{rwg} \, = \,h_{rwg} \,(T_{w} - T_{g} )$$

(5)

$$h_{rwg} \, = \,0.9\,\sigma \,\left( {T_{w}^{2} + T_{g}^{2} } \right)\,\left( {T_{w} + T_{g} } \right)$$

(6)

where q_r is the radiative heat transfer rate, h_r is the radiative heat transfer co-efficient, and σ is the Stefan-Boltzmann constant (5.67 × 10⁻⁸ W/m² K⁴).

• (ii) External heat transfer

The radiative heat transfer between top condensing cover to the sky is given by (Agrawal et al. 2017),

$$h_{rgs} \, = \,\varepsilon_{g} \,\sigma \left[ {\left( {T_{g} + 273} \right)^{4} - (T_{sky} + 273)^{4} } \right]/\left( {T_{g} - T_{sky} } \right)$$

(7)

where ε is the emittance of glass cover.

T_sky can be elaborated from (Agrawal et al. 2017),

$${T}_{sky}=0.0552\times {{T}_{amb}}^{1.5}$$

(8)

where T_amb is the ambient air temperature (K).

The evaporation efficiency of the solar desalination system can be calculated by (Arunkumar et al. 2012),

$$\eta = \frac{{M\, \times \,h_{fg} }}{I\, \times \,A \times \,\Delta t}$$

(9)

where M is the mass of the distillate output, h_fg is referred as latent heat of vaporization (2260 kJ/kg or 40.8 kJ/mol) (Kuhle 2011), I is the incident solar irradiation, surface area of the collector is named as A, and time period of experiments in denoted in Δt.