Re the complementarity of wind and solar: the answer is highly dependent on location.
Thanks for that.
Ummmm..... all of it.... and I'm just gonna start reposting...
There isn't much to understand or argue about. The German data is real, and they have so much renewables that it covers the entire demand when conditions are favorable. Still, they have weeks of output well below 10 %.
There has been little to no incentive to develop large scale storage.
Yes, currently no business case for large scale storage exists, because they have very low capacity factors except when operating on a diurnal cycle. They might become economically attractive by making reliability a tradable commodity.
Some may only have a round trip efficiency of ~60-70% but energy that would have been curtailed if it wasn't stored is free in nearly every sense of the word.
I'll get back to that.
Yeah..... pretty..... pretty sure none of that implied that a week without wind wasn't impossible........... kinda why I keep repeating myself in terms the need for storage......................... what part of storage don't you understand?
When we start getting high levels of stranded energy... with no hope of future transmission lines providing a market... there are storage methods that could carry the grid for weeks cost-effectively that we don't use (on that scale) because there is currently no need.
We've barely scratched the surface on storage methods....
We won't need storage on the scale to carry the grid for a week for another ~15 years... predicting cost is about as productive as predicting todays solar cost 15 years ago... who thought solar would be ~$1.30/w today 15 years ago? No one. Many storage methods are incredibly simple... and very likely to be equally cheap.
Ok, time for some numbers.
Energy storage relies on the physical properties of materials and either gravity or electromagnetism. These things have been well understood for 100 years. Any huge improvements or surprises in this field would imply that we have completely misunderstood physics.
Both gravity and electromagnetism are weak forces, so they can't easily be used to store lots of energy.
In a compressed spring, energy is stored by forcing electrons a little bit away from their preferred positions. In chemical reactions, the electrons move much longer - the outer ones rearrange themselves and can even switch positions to a different atom. This makes systems based on chemical energy more energy dense than systems based on gravity, compression, heat, etc. Gravity is weaker than electromagnetism.
As an example, 1 kg of gasoline contains enough energy to lift five tons one kilometer up. Alternatively, it could heat one ton of steel by almost 100 degrees C.
Nuclear fission involves the strong nuclear force, and this is why 1 kg of uranium contains enough energy to launch a thousand tons into deep space. This is irrelevant for energy storage, naturally, as you would need a supernova to create uranium, and we don't want any supernovas around here. But it illustrates how which fundamental force we utilize changes the achievable energy density of the process.
There is no way around the fact that mechanical and gravitational storage methods are not energy dense. They cannot be. Chemical methods do better in this regard, but they often involve high conversion losses or materials of limited availability.
Also, remember that the electrification of society will cause electricity consumption to double in a CO2-free future. I'll ignore that for now, but keep it in mind.
Combined wind and solar output can fall below 10 % for a week. This happens often. Two weeks has happened. I'm guessing a 100-year event would be on the order of three to four weeks. However, when the storage capacity is empty, you don't know when the same thing will happen again, and electricity prices will be crazy. So you want to make sure you have some extra buffer - let's say we would ideally want six weeks of storage capacity. I estimated that we can get 20 % from other sources. Assume that wind/solar averages exactly 10 % during the period, so we only need the storage system to cover 70 % of the demand.
Scaling our hypothetical average country to the size of the US: US electricity consumption is about 4200 TWh per year, so six weeks worth is about 500 TWh. 70 % of this is 350 TWh, which is approximately 46 gigawatt-years. Total conventional generation capacity is typically about 1.6 times average demand. US average demand is 535 GW, installed conventional capacity about 900 GW. 70 % of this is 630 GW. 70 % of average demand is 375 GW.
Let's look at batteries first. Batteries store energy chemically, so they should be fairly energy dense. They are also very efficient, and this is a rare combination.
One of the more promising batteries for grid scale storage is the flow battery. These can store about 50 Wh/kg. We need to store 350 terawatt-hours. 350 TWh = 350,000,000,000,000 Wh. Convert from kg to tons by deleting three zeroes, then divide by 50. We would need 7 billion tons of batteries.
The Tesla Powerwall delivers 2 kW continuously for an installed price per kW of about $3000. 315 million powerwalls would be able to deliver the required power, but only for about 3.5 hours. 35 billion powerwalls have enough storage capacity (350 trillion Wh divided by 10,000 Wh unit capacity). They would weigh 3.5 billion tons, and the cost works out to $306,000 per kW of average demand that needs backing up. It would also require 31 million tons of lithium (assuming 44 kg of cells per unit, 2 % lithium content). Total worldwide lithium reserves are 13.5 million tons.
If we reduced our requirement to only backing up a single week instead of six, one would still need 5.9 billion powerwalls, at a cost of $51000 per kW of average demand. They would gobble up 38 % of the known lithium reserves.
Battery prices would have to fall by 99 % for this idea to even begin to approach the realms of possibility, and then only very abundant materials can be used.
