Incorporating energy storage into a solar array is not as easy as just picking a battery off the shelf. Certain chemistries work better in certain environments, and storage capabilities are influenced by the solar application.
The U.S. Energy Information Administration (EIA) released a trends report on the U.S. storage market in May 2018. The report found that lithium-ion batteries represented more than 80% of the installed power and energy capacity of large-scale energy storage applications. Nickel- and sodium-based batteries represented around 10% while lead-acid and other chemistries rounded out large-scale battery representation.
Within small-scale battery installations (where commercial and industrial installs make up 90% of capacity), EIA was unable to pinpoint specific chemistry data, but it can be assumed that lithium-based batteries still reign supreme. Lead-acid batteries have been popular within off-grid installations for decades, but lithium-ion’s longer cycle life, lighter weight and decreased maintenance have made it the preferred choice for large-scale, EV and residential applications.
But lithium-ion is not the only—or best—choice out there for batteries used in solar+storage installations. Here’s a brief rundown of the common storage technologies used in the industry, and which chemistries some popular brand names use.
Lithium-based energy storage systems are overwhelmingly the most common storage technology used within the solar market. These batteries are characterized by the transfer of lithium ions between electrodes during charge and discharge reactions. Additional materials, such as cobalt, nickel and manganese, are inserted into the battery cells and can affect the battery’s performance, voltage and safety. Lithium-ion batteries are more expensive than other chemistries, mostly because of their need for battery management systems to monitor voltage and temperature. The benefits of lithium-ion, though, include long cycle life, high charge and discharge efficiency, lighter weight and no maintenance (lithium-ion batteries are solid and don’t require refills).
Lithium Cobalt Oxide (LCO) — LCO batteries are very stable and small, making them a popular choice for cell phones and laptops. Any battery with cobalt has a higher risk of thermal runaway and fire danger, which is why some phones and hoverboards were catching fire a few years back. Their short life spans and limited load capabilities do not make them a great choice for larger energy storage applications, but LCO batteries are a good introduction point to lithium-based storage.
Lithium Manganese Oxide (LMO) — LMO batteries have fast-charging properties and increased thermal stability since there is no cobalt. These batteries are often used in medical devices and power tools, although they’re entering the C&I market because they’re a safer alternative to cobalt batteries and can be optimized for longevity and high energy capacity.
Lithium Nickel Manganese Cobalt Oxide (NMC) — NMC batteries are a popular chemistry within the lithium-ion category. The combination of nickel and manganese provides these batteries with high specific energy and stability. Their use of cobalt, though, increases the risk of thermal runaway.
Lithium Nickel Cobalt Aluminum Oxide (NCA) — NCA batteries are a relatively new chemistry and act similarly to NMC-based systems. The addition of aluminum provides the batteries with more stability.
- NCA example: TrinaBESS range of systems
Lithium Iron Phosphate (LFP) — LFP batteries use iron phosphate to increase safety and thermal abilities while also experiencing a long cycle life. Since they generate little heat, these batteries don’t require ventilation or cooling, so they can be installed in more unique, indoor applications.
Nickel-based batteries, mainly nickel-cadmium (NiCd), are simple units without complex management systems. They are rugged and reliable. NiCd in particular has been used in large-scale energy storage because of its forgiving performance in extreme temperatures. These batteries are suited for demanding applications where reliable backup power is essential and maintenance can’t be regularly performed, but they do require ventilation.
Sodium-based batteries use salt—sometimes saltwater—to produce nontoxic, long-duration power. Salt-based cells can be completely drained to zero charge without damaging the system. Lithium-ion batteries, in comparison, always require some charge or they will fail. Sodium batteries are not flammable or explosive (as long as other materials are not added to the chemistry) and can function in a wide temperature range.
- Na example: Aquion Energy
Lead-acid chemistry is one of the oldest forms of energy storage and is widely used in vehicles. Lead-acid batteries are known for being dependable and inexpensive. These batteries use a lead-based grid submerged in an acidic electrolyte that may need replenishing for long, successful life. Lead-acid batteries are heavy because of their materials. They have a limited cycle life and are inefficient when it comes to charge and discharge when compared to other chemistries. But they’re cheap to manufacture and are reliable if the owner knows how to charge and discharge properly.
Flooded — Flooded lead-acid batteries must be flooded with a liquid. They are not resilient to damage and require significant care and maintenance. Flooded batteries need to be refilled regularly as the electrolytes evaporate during charging. These batteries must be housed in an enclosure with enough ventilation to keep off-gassing levels from reaching a dangerous point.
- Flooded examples: Trojan Battery’s Solar Premium line, Rolls’ Flooded 2 YS series, U.S. Battery’s RE series
Valve Regulated Lead-Acid (VRLA) — VRLA batteries can be “sealed” and use valves to regulate off-gassing. They require little to no maintenance when compared to flooded lead-acid batteries and can therefore be handled a little more aggressively or installed in hard-to-reach applications. VRLA can be further separated into two categories: absorbed glass mat (AGM) and gel. AGM batteries hold the electrolyte in its glass mats and use only enough liquid to keep the grid wet. Gel batteries use a thick silica-based gel as its electrolyte base. AGM batteries perform better in colder temperatures, while gel batteries work better in warmer temperatures when there’s less chance for the thick paste to freeze.
- AGM examples: Crown Battery’s Crown1, U.S. Battery’s Sealed AGM line
- Gel examples: MK Battery’s Deka Solar Gel Monobloc batteries, Trojan Battery’s Deep Cycle Gel series
Flow batteries use two chemical components dissolved in liquids separated by a membrane to provide a charge. Both chemical liquids circulate in their own space while the flow of electric current happens through the membrane. Flow batteries work like fuel cells, because the liquid energy sources are the elements creating the electricity. They can be instantly recharged by replacing the electrolyte liquids and store additional electrolytes externally, usually in tanks that are then pumped into the system. Flow batteries excel in long-duration storage applications and require little maintenance. Instead of adding more battery units to a storage system to increase capacity, flow battery systems just need more electrolyte liquid.
Redox flow batteries (RFB) — RFB systems use a chemical reduction and oxidation reaction to store energy in the liquid electrolyte solution. During discharge, an electron is released through an oxidation reaction and accepted via a reduction reaction on the other side of the membrane. Specific RFB types include iron flow batteries (IFB) and vanadium redox flow batteries (VRB).
- RFB example: ESS Energy Warehouse
Hybrid flow batteries — Hybrid flow batteries use RFB qualities but with a solid metal additive. Specifically, zinc bromine (ZNBR) flow batteries have zinc bromide salt dissolved in the electrolyte liquid.
- ZNBR example: Primus Power EnergyPod2