Harvesting the Sun's energy 

Scientists review some of the research behind a technology that could cauterise a growing global drinking water crisis.

Nearly 1.1 billion people worldwide lack access to fresh water and another 2.4 billion suffer from diseases borne by unclean drinking water. This is because while science has yielded advanced water treatment methods such as membrane distillation and reverse osmosis, these are often difficult to implement in developing countries owing to their high cost and low productivity.

A nascent but promising solution to the world’s water scarcity problems could be water purification via direct solar steam generation technology. But while researchers are well on the path to making this technology practically applicable, the finish line remains distant. Their study involves device design strategies to optimise the steam generation process.

A new, developing technology shows promise as an alternative for such regions of the world: direct solar steam generation (DSSG). This involves harvesting the heat from the sun to convert water into vapour, thereby desalinating it or removing other soluble impurities. The vapour is then cooled and collected as clean water for use.

This is a simple technology, but a key step – evaporation – presents roadblocks for its commercialisation. With existing technology, evaporation performance has hit the theoretical limit but it is not sufficient for practical implementation. Measures to improve device design to minimise solar heat loss before it reaches bulk water, recycle latent heat in the water, absorb and utilise energy from the surroundings as well and so on, have been taken to improve the evaporation performance beyond the theoretical limit and make this technology viable.

Professor Lei Miao from Shibaura Institute of Technology, in Japan and colleagues from China, review strategies formulated in the last two years to surpass this theoretical limit in their latest paper. “Our aim is to summarise the story of the development of new evaporation strategies, point out current deficiencies and challenges, and lay out future research directions to hasten the practical application of the DSSG purification technology,” says Prof Miao.

A pioneering strategy with which this evolutionary saga begins is the volumetric system, which uses a suspension of noble metals or carbon nanoparticles, as opposed to bulk heating, to absorb the sun’s energy and then transfer heat to the water surrounding these particles and generate steam. While this increases the absorbed energy of the system, there is much heat loss.

The stages of development of the technology to address this issue include:

The direct contact type system in which a double-layer structure with pores of different sizes covers the bulk water. The top layer with larger pores serves as a heat absorber and vapour escape route and the bottom layer with smaller pores is used to transport water up from the bulk to the top layer. In this system heat loss is reduced to about 15 per cent.
The 2D water path or indirect contact type system which further lowered heat loss by avoiding contact between the solar energy absorber and bulk water.
The eventual development of the 1D water path system inspired by the natural capillary-action-based water transport process in plants. This system displays an impressive evaporation rate of almost three times the theoretical limit and only seven per cent heat loss.
The injection-control technique in which the controlled sprinkling of water as rain on the solar energy absorber allows its absorption in a manner mimicking that in soil. This results in a conversion efficiency of 99 per cent from solar energy to water vapour.

In parallel, strategies are being developed to improve the evaporation rate by gaining additional energy from the environment or from the bulk water itself and recovering the latent heat from high-temperature steam. Techniques to reduce the energy required for evaporation in the first place are also being developed, such as hydratable and light-absorbing aerogels, polyurethane sponge with carbon black nanoparticles and carbon dot (CD) coated wood to hold the sun’s energy and the water to be evaporated.

Several other such design strategies exist and several more are to come. Many pertinent issues such as the collection of the condensed water, durability of the materials and stability during outdoor applications under fluctuating wind and weather conditions, remain to be addressed.

Yet the pace at which work on this technology is progressing makes it one to look forward to. “The path to the practical implementation of DSSG is riddled with problems,” says Prof Miao. “But given its advantages, there is a chance that it will be one of the front-running solutions to our growing drinking water scarcity problem.”

The paper, Strategies for breaking theoretical evaporation limitation in direct solar steam generation, is published in Solar Energy Materials and Solar Cells

Photo courtesy: Lei Miao from SIT