A Tower of Molten Salt Will Deliver Solar Power After Sunset

To your point about electricity being 1/3 of energy consumption, I still think it's important to celebrate and invest in technologies that store electricity. Even though electricity is just one part of the energy picture, electricity is far more expensive to store than other kinds of energy such as liquid fuel (which can be stored in barrels and tanks), natural gas (which can be stored in the ground or in tanks), or solid energy sources like coal or nuclear (which can just sit in a box).

1. Electricity substitutes for other energy uses. Electrically-supplied heat (heat exchange, direct resistance heating, induction heating), some motive power (personal EVs from bicycles to automobiles, catenary-supplied rail, possibly commutator-supplied intracity cargo freight). Cooking.

2. Much of present energy use combines storage + on-demand use. Coal, oil, and gas are _largely_ simply energy carriers (a non-negligible component is actually _chemical_ inputs -- 14% of coal, about 8% of oil, I'm unsure of the number for gas). If you can swap elecricity for those inputs (local on-demand power or heat), you're ahead of the game. Some options have poor substitutability at best, particularly marine and air transport, small handheld machinery away from mains power, and remote sites with generating needs. Take a look at small Alaskan towns above the Arctic Circle. The central feature of most is their fuel depot.

3. Generally, as fuels become more expensive (extraction costs, taxes, synthesis expenses), electricity will substitute for more of their uses.

Note that molten salt isn't a battery in the standard redox sense, but simply raw thermal storage capacity. Yes, there's the standard 60% Carnot loss, though actually that's _far_ better than the 65-85% PV losses for conventional single-layer PV technology (theoretical maximum efficiency about 40%, higher efficiencies are possible with multi-layer, and more expensive, technologies).

1. Far too few good sites for pumped hydro, the most cost-effective and round-trip efficient, as well as proven, option. Seawater-based pumped hydro exploiting oceans as a lower reservoir and areas of high coastal elevation gain may offer some additional option.

2. Biomass. Which is what coal, oil, and gas are. Simply, not at current scales. Biomass accounting for a few percent of total present energy use might work, but at present (or forecast) energy usage rates, simply not scalable.

3. Batteries face chemical barriers and substrate availability. Most offer only 1/100th the energy storage density of hydrocarbons (by weight or volume). At scales contemplated, lead and lithium are both critically constrained resources. Super-abundant salts, or organic compounds, might work. But you're still left with scale-of-operation.

4. Compressed air energy storage (CAES) is something my buddies at the US National Energy Labs tell me to keep an eye on, though I'm dubious. Boyle's Law and thermal losses through compression and expansion are problematic. But maybe.

5. Fuel synthesis. 50 years of research, no production systems. That worries me. A lot. But the basic science is sound. Problems: energy cost of liquid hydrocarbon fuels increase 80x over current, and your round-trip efficiencies are on the order of 17-22%. Good news is that storage stability is proven for millions of years.

6. Kinetic systems: flywheels. Engineering challenges (precession's a bitch), very low energy densities, very high costs. They're highly responsive, but other than short-term (sub-second to several minutes) of power conditioning, not an option.

Storing excess from wind would of course work, though you might choose other options as well. The advantage with direct CSP is you've got the heat and mechanism to start with.