Hygroelectricity

No, that’s not a typo, but spellcheck seems to think so. Everyone is familiar with hydroelectricity, electricity generated from the power of falling water. But what we’ll be looking at today is something totally different. Hygroelectricity is the generation of electricity by the power from moving humid air.

When people talk about “renewable energy” they are referring to a source of energy present in Nature that can be harvested essentially indefinitely with no fear of ever running out (as is the case with fossil fuels). Another feature of renewable energy is that, once the system is built, which comes with an inherent carbon footprint, no or minimal carbon is generated by maintenance during the system’s operating lifetime. Examples:

  • solar photovoltaic: rooftop panels and land-based farms
  • solar thermal: rooftop water heaters, air heating, and focused solar boilers
  • wind: turbine generators and marine propulsion
  • hydroelectric: both falling and flowing water drives turbines or waterwheels
  • wave: mechanical systems that convert the oscillating motion of water into electricity
  • tidal: placing generator turbines underwater to capture the tidal ebb and flow
  • geothermal: for space heating or electricity generation
  • fusion: theoretical renewable energy source, but not yet demonstrated at scale

Conspicuously absent from this list are breeder reactors, which can produce more nuclear fuel than they consume. Although they technically meet the definition of “renewable” in the sense that the energy source is essentially inexhaustible, they still produce radioactive waste, and they have an extremely high carbon footprint for construction and operation.

For years these seven sources of renewable energy seemed like the only real choices. There just didn’t appear to be any other possibilities. That’s why I was excited to learn about hygroelectricity —  the idea that there are renewables yet to be discovered by scientists who “think outside the box” makes me wonder what others we might be missing.

Before I get into how hygroelectricity works, I’d like to note a matter of linguistics. Although the term “alternative energy” is still in frequent use, environmentalists decided decades ago that the term “renewable energy” was a better label. The word “alternative” can carry negative connotations compared to “renewable”, and it’s already an uphill fight to replace fossil fuels.

Which brings me to hygroelectricity. Truth be told, there is still some discussion about just how it works, and the researchers working on it haven’t published much yet — most of the details are proprietary. But in a nutshell, it harnesses the same mechanism that scientists believe generates lightning: electrical charge exchanged between water droplets and other solid particles or molecules floating in the atmosphere.

Some history: In 1840 the engineer of a steam locomotive in Newcastle upon Tyne in northern England noticed he was getting static electricity shocks from the steam escaping a pressure relief valve. After further experimentation by several scientists, it was determined that the moving steam was somehow generating an electric charge. This subsequently became known as the Armstrong Effect, named for William Armstrong, one of the investigators. The discovery languished as a scientific curiosity with no real practical applications until just recently.

In 2018, a team of researchers at the University of Massachusetts Amherst were working on a humidity sensor. The sensor was producing an electrical signal, as expected, but then they found that the student in charge had forgotten to plug in the experiment. Their serendipitous discovery showed that electrical energy could be produced directly from humid air, and that the only moving part required was the air itself!

Further research showed that the materials used were less important than the surface structure, and that incorporating nano-structures (which increase the surface area) enhanced the effect. Although not yet fully understood, it seems that the kinetic energy of moving humid air was being converted into electrical energy!

The top graphic shows the process schematically. When a water molecule in the air interacts with a water molecule adsorbed to the surface it can create either a hydroxide ion or a hydronium ion. Which of the two occurs depends on the charge and composition of the surface the adsorbed molecules are attached to.

Creation of these ions constitutes a transfer of charge, so the cathode and anode will build up a usable voltage that can serve as an energy source. They’ve been dubbed hygroelectric generators (HEGs). The most powerful built to date can only light a single LED, but the technology can be scaled up indefinitely. Some researchers speculate that in humid climates an HEG the size of a washing machine, incorporating hundreds of closely-spaced layers of HEGs, could power an entire home.

The first HEGs would only function well in a humidity of 60% or higher, but recent improvements have brought that requirement down to 20%. Even desert climates have some humidity, so further improvements could make HEGs useful pretty much everywhere.

If you’re curious about the exact mechanism of charge transfer in HEGs, good — because so am I. As I said, many of the details are still proprietary, but here’s how I think it goes down. The graphic shows two water molecules entering an HEG, the top one interacting with another water molecule adsorbed to the cathode, and the bottom water molecule interacting with one adsorbed to the anode:

The actual adsorption geometry of the water molecules is a bit “messy” compared to what is shown here, but this simplified geometry makes it easier to follow what I hypothesize to be the charge transfer mechanism. Note that the hydroxide and hydronium ions created by this process are short-lived, and are quickly neutralized by interactions with other water molecules and ions in the moving air.

The puzzle that remains for me is exactly where the energy is coming from. The air exiting an HEG has the same humidity as the air entering it. The First Law of Thermodynamic (conservation of energy) requires a full accounting of energy flow into and out of any system. I’d be curious to know if the air exiting the HEG is any cooler than when it enters. I’d also be curious to know how the exiting air speed compares to that entering. Alas, those details are not yet available.

I may return to this topic in a future post, once the research on HEGs has been published (and patents filed). Either way, it now appears we have another source of truly renewable energy that could help us move away away from harmful fossil fueled energy sources.

Next Week in Sky Lights ⇒ Pumped Storage at Apache Lake

Q&A: Persistence of Vision
Pumped Storage at Apache Lake
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