Like I use energy to lift a stone, and I get 100% of that energy back if the stone drops.
No body is fully elastic. You will have infinitesimally small energy losses due to changing internal stresses in the two bodies. You get very very close to 100% recovery, but not 100% recovery. The energy is still there. It hasn't escaped your hypothetical closed system. It is simply not recoverable in a useful form. It is disordered. Reversible processes are those "special cases".
Doesn't matter much, it's the same people that drive their cars that also buy supplies that require trucking.
So what? You're artificially buoying up industries that perhaps shouldn't be. Nearly all of our shipping is done over the road, due to cost and convenience. Make roadway shipping pay to repair its fair share of damage done to the roadway. Initially, shipping costs will rise. Costs for all products would rise across the board as those increased operating costs trickle down to consumers. Over time, those companies will find new ways to reduce costs. Money would be pumped into the rail system, expanding and modernizing it to improve speed and throughput. Manufacturing would become more regionally diverse so less has to be shipped across the country. Fewer vehicles on the road means lower traffic congestion. Less roadway maintenance further means lower traffic congestion. Locomotives are more efficient per unit of shipped material are more easily managed in terms of emissions. Fixed, limited access railways can be more easily converted to electric.
The trucking industry would suffer, unquestionably, but it's a much more complicated issue than you give it credit for, and perhaps the advantages in other areas outweigh those effects.
Semis burn 5x more fuel yet cause 80x more damage.
Is that taking into account traffic volume, as a typical loaded tractor trailer is going to cause several thousand times the damage as a typical sedan.
Electrolysis has nothing to do with thermodynamics
By that, you actually mean everything has to do with thermodynamics. You're adding energy to disassociate a molecule. Thermodynamics dictates that you cannot recover that same amount of energy by letting the constituent elements recombine. Tyr07's uncertain belief coincides with one of the fundamental principles at play in any real world system.
Fuel cell efficiency varies greatly, it goes up to 85% for current marketed high temperature hydrogen fuel cells.
No it doesn't. The only way you could hope to achieve anywhere near that is through some combined cycle process that scavenges waste heat from the fuel cell. You might find some experimental units pushing 70%, but anything commercially available is going to be under 60%.
Total cycle efficiency is going to be under 25%. That is complete nonsense, you must be bad in math.
Assuming realistic values for electrolysis and fuel cells, you're already well under 40%. Depending on your compression ratio, you're only likely to recover 50-60% of the energy spent compressing the hydrogen for storage, so that's either higher losses, or higher capital costs for storage volume. Tack on a couple percent for leakage, and 25% is very reasonable.
conversion ratio wise I believe it takes more power to produce the hydrogen than it returns so there is a loss
Yes. Electrolysis does not violate the laws of thermodynamics. What I assume you were getting at, electrolysis usually runs around 50-60% efficiency and fuel cells range from 30-70% depending on the chemistry, and in practice since you have to store it, you also have to factor in compression losses, hydrogen leakage, and burners to bring your decompressed gas back up to the operating temperature of your fuel cell. Total cycle efficiency is going to be under 25%.
You can expect about 900kWh yearly per 1kW of typical solar panel installed in Denmark. Therefore it takes an installed capacity of 240GW of solar panels to cover Denmark.
That makes the assumption that your daily average production and consumption is anywhere close to your yearly average, or that you have an absolutely massive energy storage system capable of storing hundreds of petajoules to last the winter months.
Because it would be meaningless to "compensate" for the time difference between clocks moving and accelerating differently. Time literally moves at different rates in different reference frames. The clocks are correct; the problem is that the concept of similtaneity is fundamentally flawed.
I'll admit, I don't understand why the arbitrary reference time we use currently is any less valuable now that we have surface clocks whose real time is measurably changing due to relativistic effects.
So someone who straps a couple homemade AP motors to the side of an ultralight qualifies as well? (One could argue doing such a thing would preclude them classification as an ultralight...)
They're daring, sure, but they're not pioneering new territory, as we did this over 50 years ago; they're just making it cheaper. Even when they do get there and make it cheaper, there's no way to expand upon its capabilities without a complete from-scratch redesign. They're not enabling anything further. Unless they directly expand science, industry, or medicine, or indirectly provide the tools for others to do so, they're not benefiting society, just themselves personally. Being a hero implies acting for the benefit of someone other than one's self.