Revolutionary nanoparticle discovery converts solar energy to steam at 80% efficiency

Tea in Antarctica (source)

Researchers from Rice University (Naomi Halas, Oara Neumann and colleagues) have developed nanoparticles (sand, silica, grains coated with gold or carbon, they say) that transform solar energy into heat at 80% efficiency.

The nano-device is described as using focused sunlight to generate local heat that creates steam even from near-freezing water (heat is so local that the overall temperature of the liquid does not change, at least not quickly).

Oddly enough the discoverers seem unaware of the apparent potential of their invention, suggesting that it could be used to make tea in the Arctic or wherever other forms of energy production are not available, emphasis in the developing World.

Therefore I wonder if there is some sort of hidden caveat: the materials of which the particles are made (even if it'd be gold but they say "or carbon", which is actually very cheap) don't seem particularly costly and if it can be used in the developing world, as they insist, then it can't be something expensive, really.

The process is described at the Washington Post as follows:

In the Rice experiment, the researchers stirred a small amount of nanoparticles into water and put the mixture into a glass vessel. They then focused sunlight on the mixture with a lens.

The nanoparticles — either carbon or gold-coated silicon dioxide beads — have a diameter shorter than the wavelength of visible light. That allows them to absorb most of a wave of light’s energy. If they had been larger, the particles would have scattered much of the light.

In the focused light, a nanoparticle rapidly becomes hot enough to vaporize the layer of water around it. It then becomes enveloped in a bubble of steam. That, in turn, insulates it from the mass of water that, an instant before the steam formed, was bathing and cooling it.

Insulated in that fashion, the particle heats up further and forms more steam. It eventually becomes buoyant enough to rise. As it floats toward the surface, it hits and merges with other bubbles.

At the surface, the nanoparticles-in-bubbles release their steam into the air. They then sink back toward the bottom of the vessel. When they encounter the focused light, the process begins again. All of this occurs within seconds.

Everything seems simple and cheap. And, as Ah, Mephistophelis comments, steam is the main source of energy in our world, being what nuclear power plants (or also thermal power plants and even the cleaner thermal solar plants) use to generate electricity. 

And the efficiency of sunlight-to-heat conversion is simply put brutal. The Solar Daily compares that efficiency with that of regular photovoltaic solar panels, which is of only 15%...  

However this rate would have to be compensated by the efficiency of the turbine (pressure-to-electricity conversion), which may be of 60% (combined cycle), so the overall result may be of some 50% rate. 

But we are talking of free clean sunlight as the only input anyhow. 

And we are talking of an energy source that can be almost as small as needed. The main constraint being the turbine generator and the need of sunlight.

So the potential of this discovery is unimaginable. And yet the authors talk about a mere cup of tea in Antarctica? What is the problem with them?!