IntroductionIn an excellent blog post, Dr Tom Murphy examined whether it would be possible to power the entire USA using a combination of renewables and lead-acid batteries. He found that it would not be possible, because there is nowhere near enough lead in the earth's crust to make enough lead-acid batteries to compensate for the intermittency of renewables. Renewables occasionally don't produce power for 7 days in a row (during prolonged wind lulls, for example). As a result, it would be necessary to build enough lead acid batteries to power the country for 7 days to prevent the lights going off during those periods. However, the amount of lead in the earth's crust is not sufficient even to power just the USA for 1/3rd that long, to say nothing of the rest of the world. As a result, lead-acid batteries cannot compensate for the intermittency of renewables on a nationwide scale.
However, lead-acid batteries are not the only storage option available. We should investigate whether there are other storage options that have sufficient materials, not just whether lead-acid batteries have sufficient materials.
In this article I’ll investigate some other energy storage options. I'll try to determine if those options would be sufficient to power the entire USA during wind lulls. In all cases, I will assume that society needs 7 days of energy storage (or 336 billion kWh for the USA, as per Dr Murphy’s article) to prevent the lights from going out during occasional prolonged lulls in wind power.
Molten SiliconFirst I will examine the possibility of using molten silicon as an energy storage medium. Molten silicon would be stored in insulated tanks, and heated up whenever the wind is blowing. When the wind isn’t blowing, hot air is drawn over the molten silicon and used to drive a turbine. The silicon doesn’t actually change temperature; instead it changes phase from solid to liquid when heat is added, and back from liquid to solid when heat is taken out, so the temperature remains constant at 1414 degrees C. This technology is already being pursued by a startup; see here.
Let’s find out if there is enough silicon in the earth’s crust to provide energy storage for the USA for 7 days. We’ll start by calculating how much silicon metal would be required. Murphy’s article says that we’d need 336 billion kWh to power the entire country for 7 days. Silicon has a latent heat of melting of 1.926 MJ/kg, which is equivalent to 0.535 kWh/kg, or 0.278 kWh/kg after subtracting waste heat losses (discussed further below). As a result, we would need 1.2 billion tonnes of silicon (336 billion / 0.278), which is a cube of silicon that’s 0.78 kilometers on a side (at 2.5 g/cm^3 ). Silicon is the primary ingredient of dirt, so we could gather a cube of silicon that’s 0.78 kilometers on a side from within a 10 kilometer radius around my house. That would be enough silicon to power the entire USA for 7 days. Furthermore, the silicon is not being “used up” at any rate, but could be re-melted, over and over again, for millions of cycles, with no degradation.
Of course, we’d also need gas turbines, in order to convert the heat back into electricity. However, gas turbines have already been scaled up and already provide much of the electricity generation for the world. Those are natural gas turbines, not hot air turbines, but their construction would be similar. I presume we can continue building gas turbines on a wide scale.
As a result, it is clear that we have vastly more silicon than we need to meet our energy storage requirements for the entire USA for all purposes, and can also build the requisite turbines on a wide scale.
Power-to-methanePower-to-methane relies upon electrolyzing water to obtain hydrogen gas, then converting that hydrogen gas to methane using the Sabatier process. When the wind is blowing, methane gas is created. When the wind isn't blowing, that gas is converted back into electricity using existing natural gas turbines.
There is enough carbon in the Earth’s atmosphere to create the needed methane gas. We've been burning fossil fuels for more than a century now, so there is obviously enough carbon in the atmosphere to make a 7-day inventory of methane gas. Remember that carbon is not being “used up” during this process of synthesizing gas and burning it. This storage scheme is a closed cycle, in which carbon is taken from the atmosphere (actually, probably absorbed from the atmosphere into the oceans and then taken from there) and then re-released to the atmosphere. As a result, the maximum amount of carbon we would ever need is a 7 day inventory of methane, which obviously is a small fraction of the carbon we have emitted into the atmosphere over the last century.
There is also the question of how we could store 7 days worth of methane gas. However, that gas could be injected into the existing natural gas distribution network, which already is large enough to store months of gas. For example, California (where I live) can store 2 months of gas in the existing gas distribution network.
It is clear that power-to-methane could be scaled up to provide 7 days of storage when the wind isn’t blowing.
Compressed Air Energy StorageCompressed air energy storage relies upon compressing air when the wind is blowing. The compressed air can be stored in naturally-occurring underground caverns. When the wind stops blowing, the compressed air is released which powers a turbine and generates electricity.
There are enough caverns underground worldwide to hold the compressed air. The world is scattered with underground salt domes, salt caverns, porous rock formations, and aquifers. Here is a map of salt formations in Europe, for example.
There is one formation underneath Poland and Germany, for example, that appears to be 1,000 km long and 100 km wide. If it's 2 km deep, then it has a total volume of 200,000 km^3. Compressed air stores approximately 4 kWh/m^3 which is 800,000 billion kWh for the entire formation, whereas we need only 336 billion kWh for the USA according to Murphy's article. Obviously, it wouldn't be possible to convert an entire large underground salt region to a single compressed energy store. Still, if we could use even 0.1% of it, then we'd have more than enough energy storage for that region.
It is also possible to store compressed air in above-ground steel tanks in the regions which do not have suitable geography for underground storage.
ConclusionsThere are already many solutions for storing 7 days worth of electricity. Those solutions are fairly low-tech and do not rely upon any technological breakthroughs. Furthermore, they could all be scaled up and do not face any material constraints.
The only objection to the low-tech storage mechanisms listed above is that they have fairly low round-trip energy efficiency. For example, molten silicon storage relies upon heating silicon to 1414 degrees centigrade to power a turbine, which implies a round-trip efficiency of approximately 50%. This means that approximately half the energy placed into storage would be lost as waste heat. Power-to-gas would probably be somewhat less efficient, at 40% or so (60% lost as waste heat). Compressed air could be somewhat higher, at 60-70%. In all cases, however, there would be considerable round-trip energy losses.
However, that drawback is not as important as it would seem. Those energy losses are incurred only part of the time, because most renewable electricity is delivered directly to the grid without ever being placed into storage. As a result, storage losses would constitute only a fairly small fraction of the total electricity generated. For example, if solar panels could provide enough electricity to meet 40% of electricity demand directly, without storage, then only 60% of electricity would need to be drawn from storage. In which case, we would need to overbuild solar panels by only 60% (not 100%) to compensate for storage losses with 50% efficient storage (0.40 electricity delivered directly, 0.60 to storage, and 0.60 to waste heat, which implies 1.60/1.00, which is 60% overbuilding). Waste heat losses would constitute only 37.5% (0.6/1.6) of all energy obtained from solar panels, not 50%.
As a result, the round-trip losses from energy storage would be less important, because those losses are incurred only part of the time. This is quite different from waste heat losses from coal power plants, for example. Coal power plants lose 65% of their energy as waste heat, 100% of the time. On the other hand, the round-trip losses from energy storage from renewables are only occasional, and so would represent a fairly small fraction of all electricity generated.
As a result, it is clearly possible to build a “nation-sized” energy storage mechanism with tolerable energy losses and at reasonable expense. We do not face material constraints on energy storage. No technological breakthroughs are required. We could build an energy storage system that would provide continuous, dispatchable power at all times from renewable sources.
One more thing. There are also newer electricity storage mechanisms being developed. For example there are new flow batteries being developed which use abundant materials (such as the iron flow battery described here, or the organic flow battery described here). Those flow batteries would have higher round-trip efficiency (like 70% or more) and could store large amounts of energy at the same time. If higher round-trip efficiencies could be achieved, then less overbuilding would be required. Overbuilding is unfortunate, and we should try to reduce it. However, even if those new flow battery technologies never reach commercialization, we still have other, lower-tech options which are perfectly workable and which impose modest energy losses relative to all electricity generated.
*NOTE: I modified this article on Sept 21 and changed the overbuilding example from wind turbines to solar panels. I also re-worded the clumsy opening paragraph.