Enhanced solar evaporation using a photo-thermal umbrella for wastewater management


Zero-liquid discharge is an emerging wastewater management strategy that maximizes water recovery for reuse and produces a solid waste, thereby lowering the environmental impact of wastewater disposal. Evaporation ponds harvest solar energy as heat for zero-liquid discharge, but require large land areas due to low evaporation rates. Here, we demonstrate a passive and non-contact approach to enhance evaporation by more than 100% using a photo-thermal device that converts sunlight into mid-infrared radiation where water is strongly absorbing. As a result, heat is localized at the water’s surface through radiative coupling, resulting in better utilization of solar energy with a conversion efficiency of 43%. The non-contact nature of the device makes it uniquely suited to treat a wide range of wastewater without contamination, and the use of commercial materials enables a potentially low-cost and highly scalable technology for sustainable wastewater management, with the added benefit of salt recovery.

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Fig. 1: Surface heating using a photo-thermal (solar) umbrella.
Fig. 2: Lab-scale experimental results.
Fig. 3: Outdoor testing of the solar umbrella.
Fig. 4: Thermal model validation and performance prediction for the solar umbrella.
Fig. 5: Simulated performance of evaporation ponds for ZLD.

Data availability

The data that support the findings of this study are available in the Supplementary Information. Additional data are available from the corresponding author on request.


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This work was supported by the Laboratory Directed Research and Development Program at Lawrence Berkeley National Laboratory under contract number DE-AC02-05CH11231. The authors thank Z. Huang and S. Mohammed for assistance with thermal and mass transport modelling, and gratefully acknowledge Almeco for providing selective absorber samples. A.K.M. acknowledges funding support from the ITRI-Rosenfeld Fellowship from the Energy Technologies Area at Lawrence Berkeley National Laboratory. I.H. acknowledges funding support from the Zeno Karl Schindler Foundation.

Author information




The idea of non-contact radiative heating was conceptualized by R.S.P. and developed by S.L., S.K. and I.H. A.K.M. and I.H. conducted the experiments, developed the models and analysed the results. A.K.M., I.H., S.K. and R.S.P. wrote the paper. R.S.P. supervised the research.

Corresponding author

Correspondence to Ravi S. Prasher.

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Extended data

Extended Data Fig. 1 Pathways towards zero-liquid discharge.

Flow charts showing three different schemes for ZLD using membrane-based and thermal systems; the final step involves the use of either a brine crystallizer or an evaporation pond to extract remaining water and yields a solid waste for disposal.

Extended Data Fig. 2 Radiative heat localization.

(a) Absorption coefficient of electromagnetic radiation in water with penetration depth of 30 m in visible wavelengths and 20 μm at mid-IR wavelengths. (b) Blackbody emissive power of the sun (T= 5505 °C) that emits in the visible and near-IR, and a blackbody at 100 °C that emits in mid-IR where water is strongly absorbing.

Extended Data Fig. 3 Optical properties of the solar umbrella.

(a) Reflectance of the selective absorber (TiNOXenergy, Almeco) and black paint emitter measured using an FTIR. (b) Structure of the solar umbrella comprising a selective solar absorber coated on the top surface of an aluminum substrate and a black paint sprayed on to the bottom surface.

Extended Data Fig. 4 Lab-scale experimental setup.

The solar umbrella comprises a selective solar absorber and black emitter which is placed on an acrylic tank containing water. The temperature profile and evaporation rate are monitored using thermocouples and a mass balance, respectively. Holes are drilled into the umbrella to serve as vapor escape pathways.

Extended Data Fig. 5 Surface heating for enhanced evaporation.

(a) Temperature of the absorber-emitter when exposed to a solar flux showing a fast thermal transient response. (b) Mass change over time due to evaporation under dark conditions, and under one sun illumination with and without the absorber-emitter.

Extended Data Fig. 6 Performance under optical concentration.

(a) Temperatures of the absorber-emitter, water vapor (measured 2 mm below absorber) and the water surface as a function of low optical concentrations. (b) Mass change over time due to evaporation under different solar fluxes.

Extended Data Fig. 7 Brine evaporation experiments.

Images of the emitter surface and water tank after brine evaporation experiments with a 25 wt% NaCl solution over a five-day period. Salts are deposited on the walls of the tank and there is no evidence of fouling of the emitter surface owing to the non-contact device design.

Extended Data Fig. 8 Energy balance for the system.

Schematic showing the heat transfer modes and thermal losses for the solar umbrella and acrylic water tank.

Extended Data Fig. 9 Effect of view factor.

Radiation view factor, F for radiation exchange between the solar umbrella and water surface as a function of distance between the two surfaces, L and the length of the evaporation pond, X. The dashed line represents a view factor of 0.8 required for efficient radiation heat transfer between the surfaces.

Extended Data Fig. 10 Comparison between the lab-scale prototype and an evaporation pond.

Mass transport resistance network for (a) lab-scale prototype and (b) large-scale evaporation pond where the distance between the umbrella and the pond can be very large depending on the dimensions of the pond (1 m is shown here as an example).

Supplementary information

Supplementary Information

Supplementary Figs. 1 and 2, Notes 1–7, Table 1 and references.

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Menon, A.K., Haechler, I., Kaur, S. et al. Enhanced solar evaporation using a photo-thermal umbrella for wastewater management. Nat Sustain 3, 144–151 (2020). https://doi.org/10.1038/s41893-019-0445-5

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