Fuel has many costs, the cost to produce the energy and the cost to use the energy. The cost to use can be seen in ways like how to get it to where it is used, refining or storing and in energy density issues where the cost of using the fuel includes things like carrying it with you for future use.
Energy is needed to continue our expansion and improvement of our society. Fuels have costs, either in generating them or getting them. Things like coal, oil, uranium were all created a long time ago either in plants by photosynthesis or in the explosion of a long dead star. This makes them very cheap for us now. In the other side of this coin we have solar, wind and fusion (and lets just make a leap here, anti-matter) and biofuels which are energies we are creating now and so have to pay for the fuel by paying to create it.
You would think that something like solar where you are getting the energy for free from the sun would be the perfect way to power things. Or wind which is again a way to use power that has hit the earth from the sun. The costs here come in a couple of ways, one being the cost of the technology to capture the energy, the cost of creating the devices, the cost of the the materials used and the irony that to make green energy products you have to use fossil fuels (from the carbon fiber that comes from oil, plastics and the delivery trucks that carry the products running on diesel.) Other costs are the space used to produce the power either hundreds of square miles for solar collection and wind farms or fields and fields of what used to be food producing farmland for bi-fuels.
Fuels like coal end up with huge amounts of land ripped up to mine coal and it spreads the results from burning the coal, including radioactive particles over even wider areas. Hydro electric floods huge areas and things like oil spills and their effects need to be calculated into the cost of those fuels.
The other problems are that solar and wind produce electrical energy which is nice if you want to use it right now but it has to be stored for later use and you get into the cost of carrying the batteries, the cost of batteries and all of that.
Energy density has a lot to do with how the energy is produced and released. Chemical energies mean carrying the chemicals needed and how to combine those chemicals to produce energy. So external fired engines, like steam, meant carrying a lot of coal and water in a container, then having a place to make fire, make steam, convert steam to movement then do something. Oils and gases mean tanks and motors like internal combustion engines and rocket motors. 95% of a rocket is the fuel to get to earth orbit and it’s chemical in nature. The fuel for a hydro dam is the water and gravity so it’s just pure potential energy, less efficient than chemical. A water fall will not get you into orbit and neither will batteries.
Hydrogen as a fuel is nothing more than an expensive way to store electricity, ironically it’s cheapest way to be made is by extracting it from oil and it’s dangerous but as a rocket fuel it beats gasoline and so far the best we have. If you want better energy to weight ratio you have to stop burning things and move to making things blow up, really small things, atoms. Uranium has an energy density that is 10,000 time higher than burning anything. The energy produce was stored into the uranium in the explosion of a supernova billions of years ago. That really big energy that slammed atoms together and so fused them is released when the atom is split. One atom becomes two smaller ones, 2 neutrons and energy from a small loss of weight, the direct conversion of mass to energy.
There are two other ways to do this same trick, mass into energy. Fusion, like what happens in a star is the first one. This requires forcing two hydrogen atoms to smush together and make a helium atom, losing some mass which is released as energy. At the moment there are two ways to do this other than in a star. One is lasers, a lot of them, and hugely powered. This is expensive and as of yet it uses more energy than it creates. Stars can do it because they have a huge advantage, gravity. The other way is to use a lot of electricity, again expensive and uses more power than it creates but it’s fun and dangerous.
The third way to do mass into energy is matter meets antimatter, where all of the mass of the two particles is converted directly to energy, gamma-rays. Of course making antimatter is super expensive and almost impossible but if the cost of energy density is dropped by as much as this suggests then making it to power a rocket may be cheaper than any other fuel. (Of course we can’t make antimatter in useful quantities yet or store any if we make it but we can’t make a solar collector that will pay for itself yet either.)
Cost of energy is dictated by what created the energy, how it is stored, transmitted and used (and the cleanup) and for mobility the density of the energy as it is carried. Few people who look at energy are prepared to even think of all of the costs of the energy they use. And then when you think of it what ones do you include in your costs and what to you put those costs as? The cost of clean up of fossil fuels is not included in the cost of your electricity. Heck when you talk about solar the salesmen never tell you that the electricity produced will not cover the cost of the panels you bought. And the land used to make a useful amount of power doesn’t include the cost of the food that could have been grown with that sunlight.
So when you pay your hydro bill or gas up your car, are you really paying what it should cost to use that energy?