The integration of energy storage in a solar system is not as easy as just taking a battery off the shelf. Certain chemicals work better in certain environments and storage capabilities are affected by solar application.
The US Energy Information Administration (EIA) published one Trend report on the US 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 energy storage applications. Nickel- and sodium-based batteries made up about 10%, while lead acid and other chemicals completed the large-scale representation of batteries.
For small battery installations (where commercial and industrial installations account for 90% of the capacity), EIA was unable to determine specific chemical data, but it can be assumed that lithium-based batteries still prevail. Lead-acid batteries have been popular in off-grid installations for decades, but lithium-ion’s longer life, lower weight, and lower maintenance requirements have made them the preferred choice for large, electrical, and residential applications.
But lithium-ion is not the only – or best – choice for batteries used in solar-plus storage installations. Here’s a quick rundown of the common storage technologies used in the industry and what chemicals some popular brand names use.
Lithium-based energy storage systems are the most widely used storage technology in the solar market. These batteries are characterized by the transfer of lithium ions between the electrodes during charging and discharging reactions. Additional materials such as cobalt, nickel and manganese are introduced into the battery cells and can affect the performance, voltage and safety of the battery. Lithium-ion batteries are more expensive than other chemicals, mainly because of their need for battery management systems to monitor voltage and temperature. However, the advantages of lithium-ion include a long service life, high charging and discharging efficiency, lower weight and no maintenance (lithium-ion batteries are solid and do not need to be refilled).
Lithium cobalt oxide (LCO) – LCO batteries are very stable and small, which makes them a popular choice for cell phones and laptops. Any battery with cobalt carries a higher risk of thermal runaway and fire, which is why some phones and hoverboards caught fire a few years ago. Their short lifespan and limited load capacity don’t make them a good choice for larger energy storage applications, but LCO batteries are a good entry point into lithium-based storage.
Lithium manganese oxide (LMO) – LMO batteries have fast charging properties and increased thermal stability because there is no cobalt present. These batteries are widely used in medical devices and power tools, although they are entering the C&I market because they are 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 gives these batteries a high specific energy and stability. However, their use of cobalt increases the risk of thermal runaway.
Lithium Nickel Cobalt Aluminum Oxide (NCA) – NCA batteries are a relatively new chemistry and behave similarly to NMC-based systems. The addition of aluminum gives the batteries more stability.
Lithium iron phosphate (LFP) – LFP batteries use iron phosphate to increase safety and thermal capabilities while achieving long life. Because they generate little heat, these batteries do not require ventilation or cooling, so they can be installed in more unique indoor applications.
Nickel-based batteries, mainly nickel-cadmium (NiCd), are simple units with no complex management systems. They are sturdy and reliable. NiCd in particular has been used in large energy storage devices because of its forgiving performance at extreme temperatures. These batteries are suitable for demanding applications where reliable backup power is essential and maintenance cannot be performed regularly but ventilation is required.
Sodium-based batteries use salt – sometimes salt water – to create non-toxic, long-lasting energy. Salt-based cells can be completely discharged to zero without damaging the system. Lithium-ion batteries, on the other hand, always need a little charge or they fail. Sodium batteries are non-flammable or explosive (as long as no other materials are added to the chemistry) and can function over a wide range of temperatures.
Lead-acid chemistry is one of the oldest forms of energy storage and is widely used in vehicles. Lead-acid batteries are known to be reliable and inexpensive. These batteries use a lead-based grid immersed in an acidic electrolyte that may need refilling for a long, successful life. Lead-acid batteries are heavy because of their materials. They have a finite lifespan and are inefficient to load and unload when compared to other chemicals. But they’re cheap to make and 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 resistant to damage and require considerable care and maintenance. Flooded batteries need to be refilled regularly as the electrolytes evaporate during charging. These batteries must be housed in a case with adequate ventilation to prevent emissions from reaching dangerous levels.
Valve Regulated Lead Acid (VRLA) – VRLA batteries can be “sealed” and use valves to regulate degassing. Compared to flooded lead-acid batteries, they require little to no maintenance and can therefore be handled more aggressively or installed in applications that are difficult to access. VRLA can be further divided into two categories: absorbed glass mat (AGM) and gel. AGM batteries hold the electrolyte in their glass mats and only use enough liquid to keep the grid wet. Gel batteries use a thick silica-based gel as the electrolyte base. AGM batteries work better in colder temperatures, while gel batteries work better in warmer temperatures when there is less chance of the thick paste freezing.
Flow batteries use two chemical components dissolved in liquids separated by a membrane to provide charge. Both chemical liquids circulate in their own space as the electrical current flows through the membrane. Flow batteries work like fuel cells because the liquid energy carriers are the elements that generate the electricity. They can be recharged immediately by exchanging the electrolyte fluids and store additional electrolytes externally, usually in tanks, which are then pumped into the system. Flow batteries are characterized by long-term storage applications and require little maintenance. Instead of adding more battery units to a storage system to increase capacity, flow-through battery systems simply require more electrolyte fluid.
Redox flow batteries (RFB) – RFB systems use a chemical reduction and oxidation reaction to store energy in the liquid electrolyte solution. During the discharge, an electron is released through an oxidation reaction and taken up on the other side of the membrane through a reduction reaction. Specific types of RFB include iron flow batteries (IFB) and vanadium redox flow batteries (VRB).
Hybrid flow batteries – Hybrid flow batteries use RFB grades, but with a solid metal additive. In particular, zinc-bromine (ZNBR) flow batteries have zinc bromide salt dissolved in the electrolyte liquid.