The Case for Li-ion Batteries
Energy storage is really important. Since the cheapest, cleanest, and most abundant sources of energy are intermittent, it is a well-established fact that we need a cheap and easy way to collect this energy when it is being produced and deliver it when we need it. Here are the options:
Kinetic energy storage
Kinetic energy is the energy that is associated with anything that is moving. Since an object in motion tends to stay in motion, it follows that making something move may be a good way to store energy. This is the principle behind the flywheel. Start it spinning, and it will continue to spin until the rotational kinetic energy is converted to electrical energy when we need it. Sounds cheap. But a complicating factor is air resistance. Since air will slow down the flywheel – wasting all of the energy that was stored – flywheels must be contained in a vacuum chamber. All of the air has to be pumped out. And creating a vacuum is expensive and costs energy. So…not so cheap and efficient anymore.
Heat is the kinetic energy associated with molecules moving. Converting energy into heat is the worst, most inefficient way to store energy. The only way to efficiently reclaim heat energy is by ensuring that the heat storage is really, really hot. And the problem with that is it’s not possible to keep something really hot. It cools. And as it cools, all of the valuable energy is wasted. If the storage medium is not really, really hot, but instead it is just warm, then consider it wasted. It can’t be used again.
Another way to store energy is by using compressed air storage. In this case, electrical energy can be converted to the kinetic energy of air molecules using a compressor and a storage tank. The energy is ideally stored as pressure – not heat! Even though the kinetic energy of individual air molecules is (ideally) not increased, the total kinetic energy stored in compressed air increases because the process involves cramming more molecules into the storage tank – each added molecule brings its own kinetic energy to party. The problem here is that the process of cramming molecules into the tank causes heat. And all the energy that is converted to heat is wasted, making this form of energy storage not very efficient. Some folks have been trying to solve this problem by spraying water droplets to absorb the heat and prevent the temperature of the air from increasing. The problem here is that the heat in the air can only be transferred through the surface of the droplets. So the drops would have to be very, very small, and very well-dispersed. And this takes energy! Good luck on that one.
Electrical energy storage
A device that stores electrical energy is a capacitor. Capacitors are ubiquitous in electric circuits for storing charge (and thus electrical energy). The principle is so simple. If you pump electrons from one conductor to another, the electrons will stay put until the conductors are connected together again, and then the electrons will rapidly go back to their original conductor. The amount of energy associated with transferring electrons from one conductor to another depends on the capacitance and the voltage. The capacitance refers to how many charges can be fit onto the conductor – and charges only sit on the surface of a conductor. So, more surface area means more charges can fit. The voltage refers to how hard it is to transfer electrons from one conductor to the other. If the conductors are in very close proximity to each other, it takes more energy to make the transfer. The designers of large capacitors for actual energy storage are fighting against the limits of how much charge can fit on a conductor surface. So they kind of cheat by storing some of the charges within the conductor – and this can only be done by forming a chemical bond. So really, the only way to make a capacitor work for energy storage, is to make it a battery….
Chemical energy storage
Now we are getting somewhere. The energy stored in a chemical bond is huge – far bigger than the energy associated with the rotating molecules that comprise a flywheel, or the air molecules moving around in a storage tank, or the electrons sitting on a conductor. After all, when we burn gasoline, we are harnessing the energy that is produced when the chemical bonds in the fuel, formed over millions of years, are broken. Batteries also store energy in chemical bonds. It is the next best thing, in terms of total energy. But, unlike burned fuel, the chemical bonds in a battery can be easily reformed – making batteries rechargeable. And Li-ion batteries hold more energy and can deliver more power than any other type of rechargeable battery. And that is why Li-ion batteries are the only game town for energy storage. Large Li-ion batteries will become far more common for off-grid and on-grid. It is only a matter of time.