A water coating for a greener life

Applying a novel coating to part of the machinery in power plants could significantly reduce carbon dioxide emissions. Applying it at just one coal plant would reduce yearly emissions as much as taking 4,000 cars off the road would, said Kripa Varanasi, a professor of mechanical engineering at MIT who helped develop the new coating, which is being commercialised by a startup called DropWise.

The coating improves the efficiency of a key part of a power plant, the steam
condenser. In power plants, fuel is burned to produce steam that spins a turbine. As the steam emerges from the turbine, it needs to be cooled down and condensed back into water – doing so creates a suction force that helps spin the turbine.

The coating helps increase that suction force. The condenser is a series of pipes, and when steam hits them, it turns to water. Ordinarily, water builds up on the walls of the pipes and slows down the cooling process. The new coating repels water, keeping it from building up.

Researchers have been attempting to develop such coatings for decades, but
existing methods for depositing them – such as spraying – have trouble producing the correct thickness. Depending on the method and material, they were either so thick that they themselves slowed cooling, or too thin to withstand the harsh steam, said Jonathan Boreyko, a Virginia Polytechnic Institute professor and expert on heat transfer.

To get the right thickness, MIT researchers invented a new process that involves flowing two gases past heated filaments. The gases react and form a polymer coating that is “just thin enough to still be much more efficient, but thick enough to be durable,” Boreyko said.

So far the coating technology has been tested only in the lab. DropWise is working on deals to test the technology in power plants. While the technology could help with emissions, the main incentive for power plants to use the technology would likely be fuel savings – power plant operators could save nearly half a million dollars per year.

Rachel Becker

Surf zones warmed from within

For humans, wading into the ocean shallows is generally a good way to cool off. But researchers have found that energy from breaking waves actually warms near-shore waters – in some places, may have a greater effect than sunlight does. This effect adds a wrinkle to scientists’ understanding of shallow coastal environments, ranging from coral reefs to eroding Arctic shorelines to beaches popular with vacationers.Up to now,
researchers have assumed that radiation from the Sun was the main source of heat in surf, with a small contribution from the warm air. The water loses heat through radiation or evaporation.

But few scientists had considered the effect of the energy that dissipates each time a wave breaks. Some of that energy takes the form of sound and erosion as the water pounds the shore. But most of it is converted to heat from the forces of friction within the swirling waters. Physical oceanographers Gregory Sinnett and Falk Feddersen measured each of those factors in the coastal heat budget and calculated the amount of energy in the waves by measuring their height and speed from buoys offshore.

The analysis showed that waves were packing much more heat than the team had expected: roughly one-quarter the amount coming from the southern California sunlight. Furthermore, they calculate that in places with stronger waves and cloudier skies – such as the U.S. Pacific Northwest – wave heating could be nearly three times stronger than the energy imparted by the Sun. The work was originally intended as a pilot study to supplement a grant proposal, says Feddersen.

Other scientists say that the notion of waves heating coastal waters is a simple idea that has been overlooked until now. “It’s obvious when you think about it,” says Peter Wadhams, a physical oceanographer at the University of Cambridge, U.K. “No one had suggested it before, but on first principles, of course it makes sense,” says Stephen Monismith, a physical oceanographer at Stanford University in California.

Wadhams suspects that the effect may be contributing to coastal erosion in Arctic Alaska. There, the loss of sea ice has created open water that is generating waves that are claiming coastlines at rates of tens of metres per year, forcing entire villages to relocate.

The heat, Wadhams says, is probably helping to melt ice trapped in the former permafrost. Monismith says that the fine spray created by breaking waves evaporates faster than water from the ocean surface, which could partially offset their heat. But he thinks the heating effect could be significant at coral reefs in shallow waters, and plans to re-examine some data he has taken from French Polynesia to find out.

Mark Zastrow
The New York Times

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