The efficiency demonstrated is a joke. There are better ways and more direct ways to extract power from humidity gradients[0]. In a dry environment, the maximum amount of energy extractable per unit water is fairly high, about the energy density of lead acid batteries[1]. There are even proposed large scale power plants for generating power from humidity gradients[2].
The catch? It needs fresh water and works best in hot dry areas where freshwater is at a premium. Also, the powerplant would make surrounding communities up to 100 km extremely humid. Trading freshwater for electricity is not a very attractive proposition...
Yes, dunking bird style evaporation engines are among the lowest efficiency kinds of evaporation engine. One of the big reasons for this is that the process is using evaporation to create a temperature difference where the cold is generated at the same place the evaporation occurs. Evaporation works better when it is hot. This is a negative feedback which reduces the effectiveness of the system.
Fresh water is not a strict requirement for most evaporation engines. It is the low efficiency which drives that. In theory, the osmotic energy difference between fresh water and sea water is much smaller than the energy obtainable from the evaporation process. After all, seawater is observed to evaporate! However, when your efficiency is under 1%, even a low additional cost becomes a big one.
IIRC, Carlson, from ref 1 in the energy tower article (I remember adding that reference to Wikipedia in 2006!) had a design which used a Stirling engine running off an evaporation generated temperature difference which had the same negative feedback problem. There are other designs, like the piston-based one which Barton worked on before pivoting to a co-generation system (Hmm, that seems to have fallen down the Wikipedia memory hole - https://handwiki.org/wiki/Physics:Barton_evaporation_engine ).
Efficiency is always the big challenge for evaporation engines, but the strict thermodynamic limits are much higher than any current design. This means that there is a lot of room to improve! One of these days I'll write a paper on that.
This is an article written by someone who has no idea what they're talking about:
> Until now, previous attempts to convert evaporation energy into electricity have suffered from a low-conversion efficiency. But using the drinking bird method, scientists have managed to generate an output of 100 volts using just 100 millilitrers of water, enough to power small electronic devices.
I am very skeptical how much usable power could be generated using the 'drinking bird' approach. Show me the engine or generator that the drinking bird powers . . .
My impression was that evaporation occurs because that's just the way liquid behaves. It's more important that water molecules are really small than that they experience sunlight. Temperature-based evaporation is "boiling", and sunlight doesn't make a meaningful difference there, as far as water goes...?
The phrase "breaking the bonds which hold water molecules together" can be interpreted two different ways.
1) Breaking the covalent bonds that hold individual water molecules together, preventing them from splitting into two hydrogen and one oxygen atom. That's clearly not true.
2) Breaking the hydrogen bonds that stick one water molecule to another, keeping the water in liquid form. That's true.
Is that right? Molecules with no polarity can still form liquids. It happens all the time; we call those liquids "hydrophobic" (or, most commonly, we call them "oils"). The hydrogen bonds aren't what's keeping the water in liquid form.
Cooking oil, notably, will not evaporate the way water will -- if you spill it somewhere, the oil will mostly just stay wherever you left it until you clean it up -- but my understanding was that this is because oil molecules are huge and heavy while water molecules are tiny.
There are a number of forces that allow molecules to form liquids/solids, the hydrogen bond just happens to be the one that acts on water. If my memory serves it's the London force for non-polar molecules.
Water evaporates because their energy (heat) causes the molecules to bounce around and sometimes escape the cohesive attraction of their friends and into the air. More energy, more evaporation.
This works well with small water molecules, less well with larger and heavier liquids.
This conversation is a very clear sign that the article is poorly worded. If it takes this much discussion to get at its meaning, it's probably not communicating clearly.
Hair dryers and clothes dryers both operate by adding a bunch of heat to the thing they are trying to dry. I am not a physics expert but that seems to suggest that heat helps speed up evaporation!
At least that confusion is a "normal" one in terms of pop-literature talking about energy. Every "new" energy scam talks about the voltage gradient generated as if it is meaningful, with zero information about the state of the circuit generating that voltage.
I will never understand how insanely ignorant the average person is about electricity when every single electrical object in the US comes with label that concisely and accurately describes how much voltage and current it uses, and you get billed by the kWh, and the equation that pulls all this together is literally a simple multiplication, and I've struggled to put two transistors together in a circuit every single time I've tried to learn analog electronics!
It feels like watching that early internet video of the lady that couldn't figure out that if you are driving 60 mph, you drive one mile a minute. I want to be clear, I'm saying this as someone who struggles with doing basic dimensional analysis in high school physics. I cannot comprehend the willful and purposeful ignorance that most people seem to carry throughout their lives.
I don't think people do enough recreational math or unit conversions. My STEM trained friend recently expressed surprised at how much chicken "2 kg" is, and I just don't get it. 1 pound is close enough to 500 grams as to not care about the actual conversion! Same as 1 meter = 1 yard, and 1 qt damn near equals 1 liter. I know my brain is broken compared to most, but how do you live without figuring these things out?
I think those are correct? I think I was made to memorize those at some point, but of note, I am also an immigrant to the US from a wacky metric system nation.
The laws of thermodynamics apply in closed systems, and our planet is not a closed system.
Solar power, geothermal power, hydro power, are effectively perpetual motion, for our purposes. Heat pumps kind of violate the "you can't get something for nothing" principle too, since you can get >100% return on the electricity you put into them. The more we look into renewables, the more we can find ways to "cheat" by using "preprocessed" goods where the sun or the earth already did the hard part for us.
There's no thermodynamic reason you couldn't have a small device on your desk that uses ambient heat, moisture, light, etc to store energy that you can use to do work, just engineering and chemical reasons it's hard to do this on a scale that's relevant. https://en.wikipedia.org/wiki/Crookes_radiometer is over 100 years old and, if kept in sunlight, can provide "perpetual motion" for "free".
> The laws of thermodynamics apply in closed systems, and our planet is not a closed system.
> Solar power, geothermal power, hydro power, are effectively perpetual motion, for our purposes.
I get solar power and hydro [also solar] power, but isn't geothermal power a closed part of the system? Where does solar input feature in that?
> The more we look into renewables, the more we can find ways to "cheat" by using "preprocessed" goods where the sun or the earth already did the hard part for us.
This is also a closed part of the system. If the argument you're making is "the planet is not a closed system", then you need to be using energy that comes from outside, not energy that is already contained in the system.
The sun is arguably ultimately responsible for geothermal. Even if geothermal is part of the closed system, it's still functionally perpetual motion for our purposes. The main argument I'm making is that perpetual motion* is, for all practical purposes, possible, as long as you're willing to be specific, and you're unconcerned with the heat death of the universe (none of us should be concerned with the heat death of the universe).
And this is an academic paper so even though the reported number is given as an "average" it is not necessarily going to be typical for application and it is fair to expect that "average" means "best average".
It's a good exercise to compare these numbers to a CR2032 coin cell - it takes about a 6 μW load to drain one over its shelf life of 10 years, assuming 20% derating for aging.
Have we improved the failsafes when using this for regulating toxic gas venting at nuclear reactors?
In one such incident, a synchronized pendul-bird lost angular momentum, failing to press a key that regulated the gas pressure. Further analysis revealed that friction and heat losses, combined with evaporation, led to a reduction of potential energy that fell below the Y/N Press-One threshold.
In the end, an overweight employee acted irresponsibly and dangerously on his own, using his body as a cork to prevent further casualties.
The commission produced two recommendations: use a liquid with a higher vapor point; and hire three additional manager birds, and a department head with three resource birds, to track metrics.
Offtopic but sort of related to ultra low power electronics:
Let's say we wanted to make a bunch of tiny dirigibles, to measure complex air currents. How small could we make a uniquely ID'd transmitter, temperature, air pressure, and attitude sensor? Something that could be tracked by multiple fixed receivers.
This isn't for anything real, I just like the thought of tiny dirigibles.
Low drain battery powered devices should be required to drain batteries to a state below 1% charge (varies by chemistry) rather than simply cutting out below a working voltage, because they are simply "razor blade" battery eaters otherwise.
My gut was that this was some sort of April 1 paper that released early, especially when the link to the paper from CNN was broken. Now that we found an actual paper, I'm still not convinced this wasn't done tongue-in-cheek.
Probably costs for a drinking bird machine are cubic function to size, while power generation is square or linear.
Funny to think about CNNs vision here, every coastline and backyard swimming pool dotted with drinking bird machines. Need to charge your phone at the restaurant? Pull out your pocket drinking bird and put it on top of your water glass.
Like first of all, we can already generate electricity from evaporation. They're called steam turbines and they're very old technology.
These work at STP, but so do steam turbines if you use a mirror dish to concentrate sunlight.
Secondly, this is just solar power. It's just photon -> heat -> movement -> electricity instead of directly photon -> electricity. My naive take is that it will never be as efficient as solar due to the inherent losses in converting energy.
Lastly, even if we did want to something like this, these birds seem like the worst way to do it.
Couldn't we tap into the same energy source by putting something lightweight and porous like aerogel or cheesecloth onto an "elevator" of sorts and moving it up into the clouds and back down? I.e. it starts off dry at ground level, weighing 10g or whatever. We hoist it up to the clouds where it soaks up water until it weighs 2kg or whatever, then we lower it and harvest the potential energy. Then we wait for the sun to dry it out and we do it again.
It weighs 10g on the way up and something much more on the way down, which I think should net us energy.
To be clear, I still think actual solar power makes more sense, but I don't think grid-scale deployment of this would involve literal drinking birds.
Evaporation engines like the drinking bird operate on the other half of the water cycle (surface water -> vapor -> cloud water). A 'cloud sponge elevator' would operate on essentially the same half of the water cycle that we already tap into with hydroelectric dams (cloud water -> rain -> surface water). We could get the same increase in energy per unit water by building dams almost as high as the clouds. Now, that would be a huge capital expense (but probably look pretty awesome), but I'm not sure it would be more of a capital expense per watt than a cloud sponge elevator.
One of the reasons why people are interested in evaporation engines is to try to tap into the energy in the water cycle, like dams do, without the high capital expense. Of course, dipping birds aren't efficient, but they also aren't the only possible design.
> m not sure it would be more of a capital expense per watt than a cloud sponge elevator.
If I were going to take a wild guess, the dam is probably cheaper lol.
> One of the reasons why people are interested in evaporation engines is to try to tap into the energy in the water cycle, like dams do, without the high capital expense. Of course, dipping birds aren't efficient, but they also aren't the only possible design.
You seem like you know more than me here, what's the theory behind capital costs being lower? Is it the lack of need to contain pressure?
An Electric Power Research Institute (EPRI) engineer used a drinking bird toy to defeat an IR motion sensor to keep automatic lights on without having to move or wave their hands around. It requires a large/insulated warm liquid container to work for ~4 hrs.
PS: Complex two phase fluid flow is an awesome subject onto itself.
The catch? It needs fresh water and works best in hot dry areas where freshwater is at a premium. Also, the powerplant would make surrounding communities up to 100 km extremely humid. Trading freshwater for electricity is not a very attractive proposition...
[0]https://www.researchgate.net/publication/279853268_Scaling_u...
[1]https://www.researchgate.net/publication/237493406_A_Dunking...
[2]https://en.wikipedia.org/wiki/Energy_tower_(downdraft)