1.8: Variable renewables

Ideally these should be much more cost effective than batteries (even after a lot of learning curve progress) for storing very large loads, e.g., much more than a few hours of a significant fraction of the grid’s output:

Compressed air energy storage has some very appealing features, e.g., it can be very large-scale, and doesn’t require any exotic materials like lithium. Here is the basic issue. When you compress air quickly, you heat it. This heat has to go somewhere and then has to be recovered. Otherwise you have thermal losses. Ideally, you’d be able to do isothermal compression and expansion, shuttling heat slowly to and from the environment at a constant temperature with no energy permanently lost in doing so — any heat you give off, you get back for free. Unfortunately this is not easy to do for compressed air energy storage, e.g., you have to compress a lot of air to a high pressure quickly, and this is hard to do while maintaining a constant temperature and fast thermal contact with the outside world.

Innovative but now sadly defunct company LightSail tried to deal with this through a “quasi-isothermal” process in which water vapor was injected into the compressing air to absorb the heat and then release it back during expansion — because water has a very high specific heat, it can absorb a lot of heat with minimal change in temperature. Fortunately, there are published papers and patents and others taking related approaches.

How do the numbers work out for this? Using the formula for the energy associated with isothermal compression, we could store in theory about 50 kWh per cubic meter of compressed air at 300 atm pressure, whereas LightSail was trying to reach about 30 kWh per cubic meter.

Molten salt thermal energy storage is basically storing heat. You want to store a lot of heat, and for purely practical reasons melted salts are a good medium in which to store it. As this article explains: “The salt is a combination of sodium and potassium nitrate, with a melting temperature of 460°F. In the liquid state, molten salt has the viscosity and the appearance similar to water. “In solar applications, molten salt is used for a number of practical reasons,” says Terry Murphy, Chief Executive Officer for SolarReserve, who along with others helped develop the molten salt technology at Rocketdyne. “Molten salt is a heat storage medium that retains thermal energy very effectively over time and operates at temperatures greater than 1000°F, which matches well with the most efficient steam turbines. Second, it remains in a liquid state throughout the plant’s operating regime, which will improve long-term reliability and reduce O&M costs. And third, it’s totally ‘green,’ molten salt is a non-toxic, readily available material, similar to commercial fertilizers.” A primary advantage of molten salt central receiver technology is that the molten salt can be heated to 1050°F, which allows high energy steam to be generated at utility-standard temperatures (1650 psi minimum, 1025°F), achieving high thermodynamic cycle efficiencies of approximately 40 percent in modern steam turbine systems…” Laughlin has proposed a particular heat engine design using such molten salts.

Flow batteries are electrochemical energy storage. The “flow” refers to the fact that you don’t have to place the chemicals right in the path between the electrodes: the chemicals can be in separate tanks that can flow into that space as needed. Also, they can be based on materials more abundant than lithium, e.g., iron, and even largely on organic chemicals, not rare elements like lithium.

As this article explains: “Flow batteries store energy in chemical fluids contained in external tanks—as with fuel cells—instead of within the battery container itself. The two main components—the electrochemical conversion hardware through which the fluids are flowed (which sets the peak power capacity), and the chemical storage tanks (which set the energy capacity)—may be independently sized. Thus the amount of energy that can be stored is limited only by the size of the tanks. The design permits larger amounts of energy to be stored at lower cost than with traditional batteries.”