Solar Thermal Source for Passive Water Desalination and Cooling Driven by Capillarity and Salinity Gradients
Nowadays energy-intensive and expensive processes are mainly used to desalinate contaminated water and supply cooling capacity. Developing more sustainable, robust and cost-effective technologies (e.g. powered by renewable energy or waste heat) for generating clean water and cooling effect is a crucial need. This necessity is particularly important and severe in rural countries due to the lack of infrastructure and investments.
The abundant solar thermal source, which is now mainly employed to provide domestic hot water, could have a significant impact in achieving such targets, thanks to a wider engineering exploitation of innovative materials and technologies. [1,2,3] In this respect, passive distillation and cooling technologies – where all processes occur without mechanical moving parts – can make an important contribution. Unfortunately, state-of-the-art passive devices operating under one sun (that is, below 1 kW m-2) still require efficiency enhancement, durability increase, and cost reduction to scale from lab to fab. [4,5,6] The passive technologies for solar water desalination and cooling developed at the Multi-Scale Modeling Laboratory of Politecnico di Torino (www.polito.it/small) are presented.
First, a fully passive, multistage and low-cost solar thermal device able to produce freshwater from seawater is conceived, prototyped and tested under both laboratory and outdoor conditions. The device does not require any mechanical moving parts by exploiting the capillarity of hydrophilic materials. Under realistic conditions, a distillate flow rate of almost 3 L m−2 h−1 from seawater at less than one sun is demonstrated. Theoretical models, under the same conditions, also suggested that the concept has the potential to further doubling the observed distillate rate, namely up to 6 L m-2 h-1.  Long-term operations of the passive distiller are then improved by optimizing the structure of evaporators towards enhanced salt-rejecting performance.  Finally, the passive distiller is optimized to recovery the low-temperature heat, to co-generate electricity and distilled water at the same time. 
Second, a passive device exploiting similar phenomena as i) capillarity of hydrophilic materials and the ii) salinity difference between multiple evaporators and condensers is experimentally demonstrated to provide cooling capacity up to 100 W m-2.  The implemented prototype works at ambient pressure, employs environmentally friendly water solutions, and consists of several identical stages where the cooling effect comes from a cascade of evaporation-condensation processes driven by vapor pressure gradient. The experimentally validated model predicts that cooling capacities above 200 W m-2 could be possibly achieved with a proper design optimization of the current lab-scale prototype.
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