All good questions.
Describe what you think the hydrogen energy cycle is.
Almost all of the energy on the Earth comes from the sun. Plants photosynthesize sunlight to grow. Then they die, rot, are steamed and pressurized and become oil. We burn the oil to retrieve the energy for our use. BUT, there is only so much oil, and burning it is very bad for our environment. So this is a poor method catching, storing, and using the sun's energy.
An ideal method would be to somehow capture and use sunlight directly. But so far this has not proven a viable possibility, except for very low energy needs.
A good viable possibility, however, is a hydrogen energy cycle. Hydrogen is very abundant, It can easily be extracted from lots of other energy collectors (oil, gas, natural gas, coal, decomposing vegetation, algae, salt water, even from human garbage). It can be burned cleanly if necessary to extract it's energy, but does not have to be. It can be introduced into a hydrogen fuel cell where the electrons can be "borrowed" to do work while the hydrogen is recombined with oxygen to produce clean water as a byproduct.
So ideally a hydrogen energy cycle would work something like this: hydrogen is produced or extracted by one or several bio/electrical processes. For example, salt water could be pumped through a vast array of solar panels, which would use the photovoltaic energy from the sunlight to produce electrolysis in the salt water, thus separating it into hydrogen and oxygen molecules. The hydrogen we collect, and the oxygen we let go free into the atmosphere.
The hydrogen is now our storage mechanism (as oil is, today). Any machine that requires energy to operate, would carry a hydrogen fuel supply, and use the flow of electrons in the hydrogen as it recombines with oxygen (via a fuel cell membrane) to become water. Hydrogen and oxygen in, water and electricity out. No pollutants, no drilling and spilling, just sunlight and water going into the system to separate the water into hydrogen and oxygen, and then the recombination of these two elements to retrieve the energy and the water on the other end. Or if we don't use electrolysis to create hydrogen, we could use bio-methods, like algae farms, where the algae convert the sunlight into hydrogen for us.
We can do all these things, now. But we need to make the processes more efficient, and perhaps explore other methods, too. Hydrogen is very abundant, all we need to do is find ways of extracting and or producing a lot of it.
Describe the challenges needed to be overcome in order for it to be a viable replacement for petroleum.
We already have most of the technology. What we lack is the efficiency, and scale needed to produce enough hydrogen to satisfy our energy needs.
We also need to study safe containers for the hydrogen. We already have the "explosion" problem beat but we need some way of further compressing the hydrogen into smaller containers.
We also need a whole new energy distribution system. We need places where we can go to purchase hydrogen, and that can full up fuel tanks of many different sizes and designs.
And obviously, we will need to redesign many of our machines so that they can be powered by hydrogen fuel cells.
Describe what you think the ultimate energy cost will be once hydrogen is implemented, with regard to petroleum. For instance, will the same energy equivalent of hydrogen cost more or less than gasoline?
Well, the hydrogen is cheap. As I said, it's everywhere. So really the cost will be about the cost of production/extraction and distribution. These will all depend on the usual market forces. It's difficult to predict. But with proper competition, it shouldn't need to be particularly expensive.
