Are renewables and batteries part of the power generation & storage solution?

How efficient are different types of batteries/energy sources? A key metric is 'energy density'.

Broadly, energy density is a measure of how much energy/power you can stuff into how small a weight/volume/area. This can be used to rank alternative sources of energy.

Converting matter into energy is best, as Einstein worked out (E=mc² and all that). Anti-matter is top of the pops because, combined with matter, all of both are converted to energy. But anti-matter is a bit like 'unobtainium' in the movie Avatar. We can't get it or use it, at least at scale.

For currently-obtainable terrestrial sources, nuclear energy is by far the most energy-dense source available. It converts a very small amount of matter into huge amounts of energy, as per Einstein. We're trying to develop cold fusion. But fission-based power currently is best (for peaceful purposes anyway). Depending on the metric used, this can be thousands of times (plus) more energy-dense than hydrogen and fossil fuels like oil and gas, coal, charcoal and wood.

Depending on the metric used, fossil fuels can be 40 to 50 times (plus) more energy-dense than man-made batteries. They use chemical reactions, usually oxidisation, to generate heat and other chemicals.

Man-made batteries are much less energy-dense than fossil fuels. Battery technology is improving, so lithium-based batteries can be more energy-dense than the old lead-acid batteries. They use chemical reactions to generate electrons, heat and other chemicals.

What about renewables like solar, wind and hydro?

These are the least energy-dense sources. Solar, wind and even hydro power energy sources are very diffuse, not concentrated. They require very large areas (solar and wind) and/or large volumes (hydro) for collection. (They are also intermittent, reducing their average energy density as well.) Measuring these differences involves a blizzard of different metrics. I'll let properly-qualified scientists pontificate on these.

But, to illustrate, I've read startling statements like the following:

• Gasoline is one billion times more energy dense than wind and water power, and ten quadrillion times more than solar radiation.
• To store the energy contained in 1 gallon of gasoline requires over 55,000 gallons of water to be pumped up 726 feet (assuming 90% recycling process efficiency).

Energy density 'bang for your buck' is maximised using nuclear power. Fossil fuels come a distant second, but still far ahead of renewables.

These very different energy densities have huge implications for practical energy policy (see below).