Maximizing Freshwater Yield: A Critical Review of Solar Still Enhancement Techniques

Abstract

Global freshwater scarcity is among the most pressing challenges of the twenty-first century, yet approximately 2.2 billion people still lack access to safely managed drinking water. Solar stills offer a compelling decentralised response to this need — they require no grid electricity, operate from incident solar radiation alone, and yield distillate of drinking-water quality from saline, brackish, or contaminated feedwater. Their principal shortcoming, however, is inherently low productivity: a conventional passive design typically delivers only 1–3 L/m2/day, far short of the 2–5 L/person/day minimum for tropical climates. This paper presents a systematic critical review of 118 peer-reviewed studies published between 2016 and 2026, spanning five principal enhancement strategies — phase change material (PCM) integration, fin-based heat-transfer augmentation, nanoparticle and nanofluid dispersion, external solar heating and concentration, and condensation-surface engineering — as well as multi-technique synergistic combinations and the emerging class of interfacial solar evaporators. Quantitative performance data are synthesised across nine solar still geometries and novel floating-absorber platforms. Documented improvements range from +14% for simple fin additions to +300% for integrated flat-plate-collector systems, and exceed 9 kg/m2/h for interfacial nanostructured evaporators. The comparative analysis identifies multi-technique combinations and interfacial evaporator architectures as the highest-gain frontiers, while techno-economic and lifecycle assessments show that bio-derived and locally sourced materials are steadily closing the gap between performance and affordability. Five critical research gaps are identified — long-term durability under real saline conditions, nanoparticle lifecycle analysis, scale-up pathway characterisation, standardised testing protocols, and AI-driven multi-objective optimisation — and a corresponding future-scope roadmap is proposed.

Introduction

This text is a review-style overview of advances in solar still-based desalination aimed at addressing global freshwater scarcity using low-cost, solar-driven technology.

At its core, it explains that conventional solar stills—devices that evaporate and condense saline water using sunlight—are simple and sustainable but suffer from low efficiency due to heat losses and limited temperature gradients, typically producing only 1–3 L/m²/day, which is insufficient for daily needs. To overcome this, recent research (2016–2026, 118 studies) has explored several enhancement strategies: integrating phase change materials (PCMs) to store and release heat, using extended surfaces (fins, wicks, stepped basins) to increase evaporation area, adding nanoparticles/nanofluids to improve heat absorption, coupling with external solar collectors for preheating, and improving condensation surfaces to boost vapor recovery.

The review highlights that combining techniques often produces the best results, with some systems more than doubling or even tripling productivity in experimental setups. PCM-based systems are especially prominent, but their effectiveness improves significantly when paired with materials like carbon or nanoparticles that enhance thermal conductivity. Similarly, fin designs, wick structures, and hybrid systems (including photovoltaic-thermal and multi-stage setups) consistently show large gains by improving heat transfer and reducing energy losses.

Conclusion

This review has systematically classified and synthesised the findings of 118 peer-reviewed solar still enhancement studies published between 2016 and 2026, spanning single-slope, double-slope, pyramidal, hemispherical, conical, tubular, stepped, twin-wedge, shadow-assisted, and interfacial-evaporator platforms across five principal modification strategies and their multi-technique combinations. The following conclusions are drawn: 1) Multi-technique integration consistently delivers the highest productivity gains, as demonstrated quantitatively in Fig. 3. The highest experimentally validated improvements — 205.17% for double-slope wick-sponge-nanofluid [85], 300% for integrated FPC-condenser pyramid [79], 116.7% for packed-bed-cotton pyramid [77], 123% for double-finned nano-PCM single slope [38] — all arise from combinations of two or more techniques addressing complementary thermal loss mechanisms simultaneously. 2) PCM integration and nanoparticle augmentation are the most widely adopted strategies (each appearing in ~40% of classified studies). As shown in Figs. 1 and 2, their combination with geometric modifications routinely delivers super-additive gains through simultaneous improvement of thermal storage capacity, effective thermal conductivity, and evaporation surface area. The optimal nanofluid concentration of 0.04–0.10 wt% (400–1000 ppm) and the need for conductivity-enhancing PCM dopants are the key practical design parameters identified by the classified database. 3) Interfacial solar evaporators represent a paradigm shift. Evaporation rates of 2–9.3 kg/m2/h reported for CNT hydrogels [89], fibre aerogels [90], and plasmonic evaporators [91] — one to two orders of magnitude above conventional basin still values — establish the interfacial evaporator as the performance frontier for solar desalination, provided scale-up and durability challenges can be resolved. 4) Externally coupled solar heating systems deliver step-change productivity improvements (+288% for evacuated-tube coupling [34], +300% for FPC integration [79]), substantially exceeding the gains achievable through passive basin modifications alone, but at higher capital cost and operational complexity. The comparative ranking in Fig. 4 confirms that external heating and multi-technique combined configurations carry High cost complexity designations that restrict their applicability to well-resourced deployment contexts. 5) Bio-derived and locally available materials are closing the performance-affordability gap. Green-synthesised nanoparticles from jackfruit peel [21] and sugarcane juice [40], carbonised biomass PCM substitutes [49], custard apple seed nanoparticles [98], natural fibre wicks [71], and locally quarried stones [73] have delivered performance competitive with commercial materials at a fraction of the cost, strengthening the economic case for solar still deployment in low-income, water-stressed communities. 6) Critical gaps remain. Long-term durability, nanoparticle ecotoxicology, standardised testing protocols, scale-up engineering, and multi-objective AI-driven optimisation are the five priority research areas that must be addressed before laboratory-scale performance advances can be reliably translated into community-scale freshwater production systems. Meeting this challenge will require sustained interdisciplinary collaboration spanning thermal engineering, materials science, environmental chemistry, economics, and public health.

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Copyright © 2026 Manish Prajapati, Shemal Parmar, Dhairya Raval. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.