Benchmarking and performance evaluation of solar thermal evaporation and atmospheric water harvesting technologies

In view of this, Professor Tan Swee Ching of the National University of Singapore and others recently published a review article entitled "Best practices for solar water production technologies" on Nature Sustainability. Problems and misunderstandings in the process of atmospheric water harvesting technology testing, characterization and reporting, the best practices for standardized testing and performance evaluation to deal with the above problems are proposed and discussed in detail, and industry norms and standards for related testing are formulated. This strategy provides new ideas for the development of solar water production technology.

a) Solar photothermal interface evaporation technology, b) Schematic diagram of the working principle of adsorption atmospheric water harvesting technology.

For solar photothermal interface evaporation technology: The article first emphasizes the importance of a stable and uniform output of AM 1.5 to simulate sunlight. To suppress additional heat input from the light source and ambient, the authors recommend the use of a reticle for light testing and further recommend the necessary pre-test encapsulation of the photothermal evaporator to reduce heat between non-exposed areas and the air quality exchange. A windless environment is crucial for the validity and comparability of test data. In order to dilute the interference of the above factors as much as possible and maintain the accuracy of the test data, the article recommends the use of large-sized samples for the test of light evaporation. In addition, the authors subsequently emphasize the importance of simulation tools in ensuring and validating the reasonable validity of the test method.

In the field of photothermal evaporation, one of the most representative performance parameters is the evaporation rate, but this parameter cannot truly reflect the water yield of the evaporator used per unit area and time. This is because the evaporation rate is measured by observing the mass loss of the system, ignoring the condensation process of the system, and the truly meaningful water production capacity, the water collection rate, should be tested for mass gains (mass gains) . In the article, the authors highlight the importance of reporting water collection rates and discuss in detail a near-term reference strategy for efficient photothermal evaporative condensation.

In addition to the concentration of salt ions, the article points out that organic and microbial testing is also an indispensable link in the complete water quality testing process, which should also be paid attention to by the academic community. The authors further formulate the regulation mechanism and evaluation criteria of the evaporation enthalpy at the photothermal interface under the micro-nano structure, which provides a theoretical basis for clarifying the photothermal evaporation mechanism.

For solar-driven adsorption-type atmospheric water harvesting technology: The article first emphasizes the importance of full-humidity isotherm adsorption tests, and focuses on the exploration of the 0-20% humidity range, because elucidating the adsorption behavior at low humidity can better It helps to understand the solid-gas interaction process and the orientation of adsorption sites, and is beneficial to guide the design of high-performance adsorbent materials suitable for arid climates. The author also recommends a multi-temperature isothermal adsorption test and a multi-pressure isobaric desorption test to simulate and predict the operating characteristics of atmospheric water catchment materials under different working conditions. It is worth noting that the article points out that the kinetics of atmospheric water adsorption and desorption are more suitable for experimental evaluation using large-scale devices, and it is not recommended to use small-scale samples such as particles and powders for testing, because the former can more realistically restore real-life operating scenarios. Heat and mass transfer within a material.

Another current situation that hinders the comparison between different materials in the field of atmospheric water harvesting is that the literature often uses different core performance parameters for selective reporting, and the main contradiction lies in the mass yield (litre/kg·day) and area yield (litre /m2 day). The authors believe that the above two parameters have important reference value for the evaluation of atmospheric water-harvesting materials and are worth reporting simultaneously, because it is crucial to achieve high mass yield and area yield at the same time. This is because, in the future, ideal Atmospheric water harvesting materials/devices should have the characteristics of miniaturization, light weight, and high water production rate at the same time.

In addition, limited by different working modes and cycle times, it is difficult to accurately compare the daily water production rates of atmospheric water-harvesting materials and devices. In view of this, the authors propose a quantitative assessment of the desorption energy requirements of the adsorbent material, i.e. reporting the specific water yield: litre/kWh of the adsorbent material per unit energy input. In the case of a certain total insolation, the water production rate limit of different materials can be effectively estimated, so as to get rid of the limitations of different working modes and cycle conditions.

The solar-driven clean water production technology itself has the advantage of being green and sustainable, so the sustainability of its application stage is largely determined by the material itself. However, the preparation of kilogram-scale high-performance materials with scale-up potential remains a major challenge.