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Charged Superhydrophobic Condenser Surface May Make Power Plants More Efficient
New submitter _0xd0ad sends this news from the CS Monitor:
"The activities of bantam water droplets in just one region of a power plant could make a significant difference in the output of power plants, scientists say. ... When a water droplet forms on a sheet of metal coated with a superhydrophobe, the droplet can camp there only so long as it does not merge with another droplet. As soon as it weds with another droplet, the energy produced is so great that the two will 'jump' away from that surface, as if in urgent deference to the surface's severe water phobia. Scientists have proposed that this 'jumping' could be incorporated into power plant design. ... 'To have the most efficient condensing surface, you want to remove the droplets as early as possible,' says Dr. Nenad Miljkovic, [postdoctoral associate at MIT and co-author on 'Electrostatic charging of jumping droplets']. But, in prototypes, this 'jumping' design is not as efficient as engineers believe it could be. Some of the droplets will just fall back to the condenser's surface, recoating it and slowing the process down. ... But a newly discovered component to the 'jumping' process might allow scientists to eliminate this fall back. In an accidental find, the MIT team found that droplets don't just spring from the surface — they also rebound from each other ... because an electrical charge forms on the droplets as they flee the hydrophobic surface. So, if a charge is applied to the condenser system, the water droplets can be electrically prevented from returning to the surface, he said.
The efficiency that is mentioned is water recovery/usage efficiency, not electrical efficiency. In this case a power plant would use more electrical energy to produce the condensation. This is still good news in that it could reduce water usage which is a big issue with power plants in the water starved west.
Yea, the OP did make some assumptions about the understanding of the reader and basic heat engine operations.
Most power plants have a boiler that converts the working fluid to a gas. This gas is piped at high pressure though a turbine which drives a generator. This lowers the temperature and pressure of the gas which is then sent to a condenser where the gas is converted back to a liquid. This liquid is pumped into the boiler and the cycle starts over.
What they are talking about is the place where they take the working fluid from a low pressure gas to a liquid by removing heat. This takes place in a condenser. A condenser has cold surfaces that are exposed to the low pressure vapor. These cold surfaces have liquid condense on them that then runs off to be pumped back into the boiler. Apparently being able to get the liquid off the cold surfaces quickly makes the transfer of heat more efficient and faster.
Of course, the real question about their theory here is if the process they claim can be engineered to happen and provide more energy savings than it consumes. So far they have not been very successful doing this on an industrial scale.
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I just got to tour a power plant - very interesting! The running units were all combined cycle (close to 60% thermal efficiency), and the volume difference on steam in/steam out is really tremendous! The steam was vacuumed out of the LP turbine. And they were complaining about their cooling/condensing system not being efficient enough in the summer to run the plant at full power. Interesting too that the vast majority of the plant was about steam generation/handling/reheating/processing. The turbines and generators are a tiny, albeit critical, piece of the plant.
Also interesting is the sheer quantity of backup and emergency systems. And the generators being cooled with hydrogen and therefore hydrogen lines running all over the plant.
The Monitor is actually a pretty good news source, for the most part. They typically run a single CS article per issue, and are otherwise pretty non-religious. You should take a look before being too critical. IANACS
Super-hydrophobic coatings are now easily available. They work very well when new, but customers complain about the coating wearing off rapidly. Something with a more durable bond will be needed.
This is yet another of those materials science articles which jumps from "minor discovery in materials science" to "huge commercial breakthrough Real Soon Now." It's bad for MIT's reputation that they put out so much hype.
For any particular pressure (or vacuum) there is an associated dewpoint temperature. In this case, it is where the liquid water condenses from the steam. Condensers use cooling water to remove the heat of condensation and subcooling. Cooling water is often cooled by evaporating some of the cooling water in cooling towers, so that fresh makeup water is needed. The steam condensate is recycled to the boiler to be heated and vaporized back to steam to power the generator turbine.
However, the condensed water adhering to the condenser tubes is further sub-cooled below its dewpoint. This means that more cooling water is needed, more condenser surface area, and more energy to reheat and vaporize condensate back to steam.
I speculate that the technology described reduces the amount of condensate subcooling, leading to less cooling and heating duty, improving overall efficiency.
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Smaller area means less water means smaller pump means less wasted electricity means higher plant efficiency.
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Another small detail to add to your comment. The faster you turn the vapor into liquid, the lowest is the pressure at the condenser, reducing the work of the pump that creates vacuum after the turbine* and increasing the overall efficiency of the system.
Also, if you have a bigger heat conductivity, you can apply a smaller temperature gradient into the vapour. That could theoreticaly improve the efficiency, but I don't know how that part works in practice.
* Yes, the vapor goes from the turbine into a pump. Seems counterintuitive, but you want to condense it, what is easier to do in a highter pressure, but the turbine works much better with vacuum... In the end big plants gain efficiency by putting some work back into the steam.
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You know what kills heat transfer? Scale. This system will die (i.e. drop to not so fancy smancy levels) as soon as the scale builds up. - a chem. eng.
Will scale accumulate on superhydrophobic surfaces? I would think the water should move off the surface too fast to evaporate.
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