“We have a technological problem; it demands a technological solution.”
So wrote Donald R. Sadoway, a materials chemistry professor at the Massachusetts Institute of Technology, and co-founder of the battery company Ambri, Inc., in a letter to the editor of The New York Times this past February. The topic of Dr. Sadoway’s letter was the “intermittency” of solar power generation ‚Äì that is, the fact that solar panels produce electricity when it’s light out, but not when it’s dark. Intermittency is the “technological problem” Dr. Sadoway identified. But, what is the “technological solution”?
For homeowners, conventional sealed lead and lithium battery storage systems are generally the most cost-effective way to store and release excess solar-generated energy (with the possible exception of solar-heated water storage tanks). But, commercial-scale solar utilities that generate electricity from solar on a large scale can take advantage of alternative energy storage systems to smooth out their power delivery to meet demand even when the sun isn’t shining. In this article, we discuss some of those existing and emerging alternative storage technologies. (More detailed discussions of each of these technologies can be found on the website of the Energy Storage Association, from which the information below is drawn.)
Pumped Hydroelectric Storage
Pumped hydroelectric storage has been in use in the U.S. since at least the 1920s. It relies on the basic principle that water sitting at a higher elevation releases its potential energy when it flows to a lower elevation. A solar installation that produces excess electricity during daytime hours can use that extra energy to power a pump that pushes water uphill. Some of that energy can then later be recovered by releasing the water to flow back downhill, powering a hydroelectric turbine. Picture a hydroelectric dam with a reservoir that’s being constantly refilled by water from below the dam. A drawback of pumped hydroelectric storage is that it requires an available water resource and plenty of space in which to move it back and forth. There can also be a fair amount of energy lost in this sort of system, both from evaporation and in running and maintaining the hydroelectric turbines.
Compressed Air Storage
The basic principle behind compressed air energy storage is similar to that of pumped hydroelectric storage. Excess solar energy is used to power a pump that compresses air in a pressurized storage vessel (large-scale compressed air facilities use caverns for storage). The energy can later be recovered by releasing the pressurized air to drive a turbine. In another similarity to pumped hydroelectric, the efficiency of these systems can be diminished by the “moving parts” associated with pressurizing air and running turbines from it, and in the need for a storage vessel large enough to store meaningful amounts of energy under pressure.
A flywheel is a form of solar energy storage that employs kinetic energy. The basic concept is that excess solar energy can be used to power a motor that spins an ultra-low-friction heavy rotor at a high rpm. The energy used to spin the rotor can later be recovered by connecting the rotor to a drive system that turns a generator. In some cases the motor that spins the flywheel can also act as the generator. Today’s flywheels are made of strong and light carbon materials and rest on almost frictionless bearings in a vacuum. Like pumped hydroelectric storage, flywheels represent a durable and time-tested technology, although meaningful improvements in efficiency through weight and friction reduction come with a steep price tag.
Another technology that’s been around for a while but is constantly improving, a flow battery is a hybrid of a conventional flooded battery and a fuel cell. Like a conventional battery, a flow battery stores and releases chemical energy through the interaction of a cathode and an anode via an electrolyte. But, in a flow battery, excess solar energy is used to dissolve cathode and anode materials in an electrolytic solution that can be pumped to and from larger storage tanks. That chemical energy can be recovered by circulating the fluids through a system that places them in proximity to each other, causing the electrochemical reaction that creates a discharge of electricity. The benefit of flow batteries is that they can be recharged instantaneously by circulating new fluid through the system. The storage capacity of flow batteries mostly depends upon the size of the tanks holding the electrolytic solutions, and anode and cathode material available for the chemical reaction.
There are various ways to store excess solar energy using temperature change to drive a generator. In one solution, electricity drives a heat pump to remove heat from one storage vessel and deliver it to a different storage vessel (similar to the way a refrigerator operates). To recover the energy, the pump is reversed and the heat is harnessed to generate electricity and/or re-heat the cold vessel. In another setup, excess solar electricity can be used to drive a pump that cools oxygen to its liquefied form, then recovered by allowing the liquid oxygen to warm and return to its expanded gaseous state, driving a turbine. A third technology can store energy by converting electricity to hydrogen and back through electrolysis. As with the technologies above, moving parts and energy conversions result in some energy loss.
Molten salt energy storage relies on the principle that salts heated to a molten (very hot liquid) state take a long time to cool. The solar energy employed to heat salts to their molten state can later be recovered by using the heat to power a steam generator. A distinguishing characteristic of molten salt systems is that they do not rely on photovoltaic technology to supply the energy to be stored. Instead, these systems capture the sun’s heat energy directly by using highly engineered mirrors to reflect and focus sunlight on a “concentrator” tower that contains the salt to be heated to its molten state. Once heated, the liquefied salt can be used to deliver power immediately or later depending upon demand. Molten salt facilities require fairly massive initial investment and a stable, dry, sunny climate conducive to making use of and maintaining hundreds of precision mirrors.
About Mountain View Solar
At Mountain View Solar, we stay abreast of storage technologies, in part, to be sure we’re up to date on the latest methods of energy storage on a small scale. Even though the storage methods described above are not currently practical for storing the excess electricity your residential solar array generates, we’re constantly looking for insight into improving existing storage solutions to ensure you get the most out of your system.
To learn more about your options for storing solar energy you generate at home or at your business, contact us today for a free zero-pressure consultation. mtvSolar is your licensed and experienced local contractor, specializing in quality installations expected to last decades.