A few people sent links to the announcement of a "revolutionary" advance for solar cells - namely to let these large areas that would be unproductive in a rainstorm, be productive. Numbers weren't mentioned, but it's worthwhile thinking about the problem.
The Sun heats evaporates water which rises up into the atmosphere and can condense and precipitate. A question is how much of the energy that went into evaporating the water can be recovered? If rain could fall without resistance it would be a lot.. also walking outside in a storm could be a lethal activity. The simple physics is the moisture in the sky has a considerable amount of potential energy relative to the ground it will fall on .. as it falls the potential energy turns into kinetic energy .. you can work out some numbers if you like.1 But that's not what happens. First think about it.. a sanity check .. are you feeling the force of rain when there's no wind? What does that say about power density?
Raindrops run into air resistance and their terminal velocity is low.. fast ones are traveling about 8 meters per second - about 18 miles per hour. You can throw a ball a lot faster than a raindrop. So now the problem is simple. You don't have to know how far it falls. So here's a sample calculation I did in my head when I heard about this. The kinetic energy for one centimeter of rain falling on a one square meter area is 320 joules.2 That isn't much .. about 0.09 watt-hours per square meter. One way to think about it. If you had a 100 square meter array (large for a house!) you'd average 9 watts during a one hour storm that put down a centimeter of rain .. an inch of rain would get you into the 25 watt range .. less as the converter won't be close to 100 percent efficient, but maybe enough to run a small LED lamp during a strong rainstorm.
On my rowing machine I regularly convert power from my muscles into mechanical power at a flywheel averaging about 150 watts or so. A good generator could convert about 90% of that into electricity. A photovoltaic in cloudy Germany averages about 40 watts per square meter across the year (including night) - that includes the conversion efficiency. Problem solved.. this is impractical for solving the general energy problem... no way would it average out periods where other forms of renewable power aren't working well.
So now for the more interesting question. Why were people excited? I think there may be several things going on.
First it appears a group did make an advance in turning mechanical energy into electrical energy with piezoelectrics. Traditionally this is very inefficient - one percent or so. They've made an advance and it could be very useful for niche applications like low power electronics located where it is impractical to run power cables or replace batteries. Aces for the team.
These announcements often fall into the hands of university or corporate PR people who don't understand what the invention or discovery is and don't ask questions or get sign off. This turns out to be a major problem even at name universities and often worse in the corporate world. I don't blame the PR people as getting this right is a non-trivial problem. The announcement is picked up by a tech writer who lacks a clear idea of the announcement, but thinks they understand. They write something right out of the telephone game. Meanwhile the latest version is picked up by social media or others who are too lazy to check things out. The reader needs to learn what sources are trust-worthy and, if the work is important to them, figure out how to drill down.
There's a lot of interesting physics thinking about rain and even liquid water, And water for energy storage can be inexpensive and enormously practical if you have the right location (a mountain lake for example). Denmark cleverly uses Norway as a storage battery for wind energy to even out the load. This may become less practical for the Danes as Norway is seeing a larger demand for electricity at night recharging electric cars so it will come down to economics.
1 gravitational potential energy mgh becomes kinetic energy mv2/2
2 E = 1/2 mv2 where the mass of the water is density times volume. The density is very close to 1000 kg per cubic meter.