The porous, insulating material structure floats on water and is made up of layer of graphite flakes and an underlying carbon foam. Picture credits: Massachusetts Institute of Technology (MIT).

By Jason Deign 

Researchers say a Massachusetts Institute of Technology (MIT) breakthrough in steam generation research poses no challenge to traditional CSP.

“I think it has very limited applications in the field of CSP,” says Dr Eduardo Zarza Moya of the Spanish Centre for Energy, Environmental and Technology Investigation (Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas or CIEMAT in Spanish).

The process, unveiled in July, involves using a novel material structure to produce steam from sunlight at ambient temperature and pressure. The porous, insulating material structure floats on water and is made up of layer of graphite flakes and an underlying carbon foam.

When sunlight hits the structure’s surface, it causes a hotspot in the graphite. This draws up water through the material’s pores, which then evaporates into steam. The steam generation increases with the brightness of the light.

The MIT research team claims the material can convert 85% of incoming solar energy into steam. This is “a significant improvement over recent approaches to solar-powered steam generation,” said MIT in a press release.

“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.”

Furthermore, the research team points out, the sponge-like structure can be made from relatively inexpensive materials.

MIT’s press release highlights desalination, hygiene systems and sterilisation as possible applications, and points out: “Today, solar-powered steam generation involves vast fields of mirrors or lenses that concentrate incoming sunlight, heating large volumes of liquid.

Heat loss

“However, these complex systems can experience significant heat loss, leading to inefficient steam generation. 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.”

MIT post-doctoral researcher Hadi Ghasemi said in a press statement: “This is a huge advantage in cost-reduction. That’s exciting for us because we’ve come up with a new approach to solar steam generation.”

Speaking to CSP Today, Dr Gang Chen, Carl Richard Soderberg Professor of Power Engineering and head of the Mechanical Engineering Department at MIT, adds: “The double-layer structures are easy to make and do not require coatings as in current solar trough CSP.

“We hope by reducing optical concentration, we can reduce the tracking cost or completely eliminate tracking.

“I have the feeling if we can achieve these, we will be able to reduce the cost of solar thermal to electricity production, although I should acknowledge that we need to do detailed cost analysis after getting more data.”

He says the team will be looking to evaluate potential commercialisation strategies after carrying out further research between now and 2016.

“We need to pressurise the system to see what are the highest temperature and pressure we can reach at each optical concentration, and evaluate different system configurations for efficiency and cost for power generation, desalination and other applications,” he comments.

“We would like to have some answers to these questions within next two years. I hope we can get to even higher temperature and reduce further optical concentration by optimising the structures and also further look for ways to reduce the cost.”

Boiling point

For now, though, the system is not pressurised and the steam produced is merely at boiling point, with a temperature of 100ºC. It is also unclear whether this temperature can be significantly increased using the process.

“It can certainly be used with concentrated light, as we show in the paper the steam gets to higher temperature as we increase concentration from one to 10,” Chen says.

But, he adds: “As one uses higher optical concentration, there are questions on whether focusing heat locally will be more efficient than current ways of surface heating using selective absorbers, as used in trough, or volumetric absorbers, as some used in towers. I do not have all the answers yet.”

For CIEMAT’s Zarza, this current temperature range limits the extent to which the MIT process might compete with current CSP technologies, which are largely concerned with the production of superheated steam for power production and industrial processes.

“I’m aware of MIT’s discovery,” he says. “In the field of solar thermal plants, the application for this would be very reduced. I could see it being used for desalination”.

“But if we want to feed an industrial process that requires thermal energy, it is usually with mid- to high-temperature steam, say between 10 and 15 bar and 200ºC plus. For electricity production, you need 100 bar”.

“And even if you increase the concentration of light, the pressure is still ambient. The steam has a very low energy content.”

With CSP in need of cost reduction, this is actually a shame, he suggests. “I wish this were a real step forward, but it’s not really an alternative.”