permaculture

[Lawrence London <lfljvenaura@gmail.com>]

http://newsoffice.mit.edu/2014/new-spongelike-structure-converts-solar-energy-into-steam-0721

 Steam from the sun

New spongelike structure converts solar energy into steam.

Jennifer Chu | MIT News Office
July 21, 2014
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Comment

A new material structure developed at MIT generates steam by soaking up the
sun.

The structure — a layer of graphite flakes and an underlying carbon foam —
is a porous, insulating material structure that floats on water. When
sunlight hits the structure’s surface, it creates a hotspot in the
graphite, drawing water up through the material’s pores, where it
evaporates as steam. The brighter the light, the more steam is generated.

The new material is able to convert 85 percent of incoming solar energy
into steam — a significant improvement over recent approaches to
solar-powered steam generation. What’s more, the setup loses very little
heat in the process, and can produce steam at relatively low solar
intensity. This would mean that, if scaled up, the setup would likely not
require complex, costly systems to highly concentrate sunlight.

Hadi Ghasemi, a postdoc in MIT’s Department of Mechanical Engineering, says
the spongelike structure can be made from relatively inexpensive materials
— a particular advantage for a variety of compact, steam-powered
applications.

“Steam is important for desalination, hygiene systems, and sterilization,”
says Ghasemi, who led the development of the structure. “Especially in
remote areas where the sun is the only source of energy, if you can
generate steam with solar energy, it would be very useful.”

Ghasemi and mechanical engineering department head Gang Chen, along with
five others at MIT, report on the details of the new steam-generating
structure in the journal *Nature Communications*.

*Cutting the optical concentration*

Today, solar-powered steam generation involves vast fields of mirrors or
lenses that concentrate incoming sunlight, heating large volumes of liquid
to high enough temperatures to produce steam. However, these complex
systems can experience significant heat loss, leading to inefficient steam
generation.

Recently, scientists have explored ways to improve the efficiency of
solar-thermal harvesting by developing new solar receivers and by working
with nanofluids. The latter approach involves mixing water with
nanoparticles that heat up quickly when exposed to sunlight, vaporizing the
surrounding water molecules as steam. But initiating this reaction requires
very intense solar energy — about 1,000 times that of an average sunny day.

By contrast, the MIT approach generates steam at a solar intensity about 10
times that of a sunny day — the lowest optical concentration reported thus
far. The implication, the researchers say, is that steam-generating
applications can function with lower sunlight concentration and
less-expensive tracking systems.

“This is a huge advantage in cost-reduction,” Ghasemi says. “That’s
exciting for us because we’ve come up with a new approach to solar steam
generation.”

*From sun to steam*

The approach itself is relatively simple: Since steam is generated at the
surface of a liquid, Ghasemi looked for a material that could both
efficiently absorb sunlight and generate steam at a liquid’s surface.

After trials with multiple materials, he settled on a thin, double-layered,
disc-shaped structure. Its top layer is made from graphite that the
researchers exfoliated by placing the material in a microwave. The effect,
Chen says, is “just like popcorn”: The graphite bubbles up, forming a nest
of flakes. The result is a highly porous material that can better absorb
and retain solar energy.

The structure’s bottom layer is a carbon foam that contains pockets of air
to keep the foam afloat and act as an insulator, preventing heat from
escaping to the underlying liquid. The foam also contains very small pores
that allow water to creep up through the structure via capillary action.

As sunlight hits the structure, it creates a hotspot in the graphite layer,
generating a pressure gradient that draws water up through the carbon foam.
As water seeps into the graphite layer, the heat concentrated in the
graphite turns the water into steam. The structure works much like a sponge
that, when placed in water on a hot, sunny day, can continuously absorb and
evaporate liquid.

The researchers tested the structure by placing it in a chamber of water
and exposing it to a solar simulator — a light source that simulates
various intensities of solar radiation. They found they were able to
convert 85 percent of solar energy into steam at a solar intensity 10 times
that of a typical sunny day.

Ghasemi says the structure may be designed to be even more efficient,
depending on the type of materials used.

“There can be different combinations of materials that can be used in these
two layers that can lead to higher efficiencies at lower concentrations,”
Ghasemi says. “There is still a lot of research that can be done on
implementing this in larger systems.”