The Stirling engine was invented in 1816. It produces power from heat. Since there is heat everywhere, it should be the answer to all of our problems. It doesn't seem to exist in any practical applications on any significant scale, with the possible exception of Swedish submarines. And that is only if you think that eight is a significant number of submarines. Wikipedia has several applications, each hedged by "may", "could", or recording historical experiments. Cryocoolers are cited as a use of stirling engines, but this doesn't really count as they are not being used as engines to convert heat into power, but are using power to create extreme cold. The largest use of Stirling engines is probably in classrooms, to demonstrate how Stirling engines work. This sounds a bit like perpetual motion.
We've all had ideas for generating energy that amount to engines connected to generators. They all fail before we start as we are bound by the second law of thermodynamics, and rather than the energy generating itself, it will fall short as some is sapped away by entropy. There are many ideas that aren't actually perpetual motion, but they are so impractical that they might as well be.
Hydrogen is one of the components of water. There are gazillions* of hydrogen atoms literally floating around in the ocean. They come in pairs bonded with oxygen atoms, so all you need to do to get the hydrogen out is pass a bit of electricity through the water. Wouldn't it be easy to store extra electricity by just converting it to hydrogen?
I used to do this when I was a kid, with a transformer and a couple of carbon electrodes. Hydrogen gas would bubble up from the anode, and oxygen gas from the cathode. A little bit of salt helped the process by making the water more conductive. The build-up of hydrochloric acid was only very slight, but the electrodes deteriorated and discoloured pretty quickly.
So don't hold your breath for hydrogen as a way to store all that excess solar energy.
I may have been trying to produce enough gas to fill a model airship, but that project was plagued by lack of materials, equipment, knowledge, experienced personnel and time. Without effective storage technology, all of my hydrogen was destined to leak away, or vanish with squeaky pops.
You can try this as home, but it's not a very practical way of storing electricity. Very little of the electricity going into electrolysis actually liberates hydrogen atoms. Once they have been liberated, the hydrogen molecules need to be stored, probably via a compressor which will use more power. When you have the compressed hydrogen, you need a special engine to use it as a fuel, unless you are just content with a squeaky-pop machine, or an explosion hazard.
The R100 airship started to develop engines that could run on either hydrogen or kerosene. (That was the airship that didn't explode in Picardy, Northern France.) It used hydrogen for buoyancy; the tanks of kerosene would get lighter as the journey went on, and they could then switch to hydrogen, which would make the ship heavier. In the end they didn't have enough time to develop the engines.
If electrolysis were a good way of producing hydrogen, they would be using that to produce hydrogen on a commercial scale. In fact 95% of hydrogen gas is produced by passing high-temperature steam through natural gas.
So don't hold your breath for hydrogen as a way to store all that excess solar energy.
* a million is one with six zeros, a billion has nine zeros, a trillion has twelve. A gazillion has more than that. The word is often used colloquially to describe large numbers that would more accurately be described as trillions, billions, millions, thousand, or even hundreds.
