A study conducted at Rice University, Houston, Texas, developed a method for desalinating (purifying) salt water using sunlight and nanoparticles. Apparently, this method is even more effective than theoretical calculations estimated it would be.

Researchers at the Rice Lab have shown that the efficiency of solar-powered desalination systems can be increased by more than 50% by adding plastic lenses that centered the sunlight’s power. The lenses created “hot spots” using the same amount of light and increased the radiation efficiency.

In “normal” membrane distillation experiment, when warm and salty water is held on one side of a membrane, while cool, filtered water is held on the other side, the temperature gap creates a vapor pressure difference, which drives water vapor from the heated side through the membrane toward the cooler side. Thus, the cooler side receives more clean water.

This is easy to test in a small lab environment. When the environment is larger and the membrane size increases, the temperature difference along the membrane decreases and accordingly so does the purified water output. However, when wrapping the top layer of the membranes with nanoparticle designed to convert more than 80% of sunlight energy into heat, particle heating reduces the energy costs of production making the process more efficient.

The improvement resulting from the concentration of sunlight to small spots is similar to the use of magnifying glass on a sunny day to produce a hotspot capable of burning a branch. The concentration of light at a small precise point on the membrane results in an increase in temperature and an increase in vapor pressure. The increased pressure forces more water vapor to pass through the membrane in less time.

Improving efficiency in this process is important because water scarcity is an everyday reality for about half of the world’s population. Efficient and cheap solar refining can change that. Beyond water purification, this method may also improve the technologies that use solar heating to drive chemical processes such as photo catalysis (acceleration by light energy). Already today, copper-based nanoparticles have been developed, capable of converting ammonia to hydrogen fuel.


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