What are the key advances in mini-grid energy storage?

A battery’s cycle life is an important factor in performance. Cycle life refers to the number of charge-discharge cycles the battery can endure before it fails. The depth of discharge (DOD)—how deeply the battery is discharged in each discharge/charge cycle—affects a battery’s cycle life. If lead-acid batteries are regularly discharged to 80 percent DOD, for example, they have a much shorter cycle life than they would if discharged to only 50 percent DOD. When deeply discharged, lead-acid batteries suffer irreversible damage.

To guard against damage, mini-grids planners typically choose larger lead-acid batteries than required so that the batteries never have to discharge more than 50 percent. Compared to lead-acid batteries, newer battery technologies can maintain longer cycle life at deeper discharges, resulting in smaller, longer-lasting batteries for mini-grids. Using these larger batteries, however, increases transportation and construction costs.

Temperature sensitivity refers to how a battery performs in high temperatures. Lead-acid batteries degrade more quickly in high-temperature environments. In hot climates like those of many developing countries, lead-acid batteries have a lower cycle life. Newer battery technologies can withstand higher temperatures.

Round-trip efficiency is the amount of electricity that a battery puts out compared to how much is put in. Batteries lose energy in the form of heat, for example, when their stored water breaks down into hydrogen and oxygen. Battery research is a key part of improving the success of mini-grids (and other renewable energy technologies). As battery efficiency improves, energy can be delivered more effectively to larger communities for lower cost. Many emerging battery technologies have higher round-trip efficiency.

A battery’s cost relative to its storage capacity is a key consideration. Initial cost is the purchase cost of a battery that can store 1 kWh of electricity, measured in dollars per kWh. Initial costs for many emerging battery technologies are high, but prices are likely to decrease over time.

Specific energy (in Wh/kg) is a measure of how much energy the battery can store per unit of mass. Batteries that are more energy-dense reduce transportation costs and power-house size.

Lithium ion batteries, widely used in consumer electronics and electric vehicles, were less common in mini-grids prior to 2016 due to their cost. Rather they were more common in off-grid applications like solar home system kits and solar lamps. Improved efficiency and decreasing cost of lithium-ion batteries now allow system designers to integrate them into mini-grids. Increased use of lithium-ion batteries will continue since the battery technology is far superior to traditional lead-acid batteries.

Lithium-ion batteries now have lower life cycle costs than lead-acid batteries. Compared to lead-acid batteries, lithium-ion batteries have significantly better cycle life, especially in hot climates, where lead-acid batteries have a shorter-than-usual cycle life. Lithium-ion batteries can also tolerate deep discharge better than lead-acid batteries, so they don’t need to be oversized in mini-grids.

Two promising variations of lithium batteries are lithium-sulfur and lithium-air. These technologies are not yet available for mini-grids, but laboratories and manufacturers are developing them for new applications. Lithium-air batteries have perhaps the best potential, but as of 2017, the technology is still years from commercial availability.

Lithium-ion batteries will become more affordable in coming years. The International Renewable Energy Agency (IRENA) predicts that lithium-ion battery costs will decrease from $700 per usable kWh in 2016 to $200–$500 per kWh by 2025, and to less than $200 per kWh by 2030–2035.

Lithium-sulfur batteries have twice the energy density of lithium ion batteries, and they use less expensive material. Lithium-sulfur batteries hold great promise for mini-grid applications. Technical advances have solved stability problems that result from expansion and contraction of materials during the charge/discharge process. As of 2017, lithium-sulfur cells are commercially available for limited applications, but have not yet been packaged in configurations suitable for mini-grids.

Metal-air batteries use atmospheric oxygen instead of an internal oxidizer, substantially increasing energy density. While primary (non-rechargeable) zinc-air batteries have long been commercially available for other products, secondary (rechargeable) metal-air batteries suitable for mini-grids emerged in 2013, partially due to support by Tesla.

The most promising metal-air technologies for mini-grids are zinc-air batteries. The batteries cost about the same as lead-acid batteries, survive many more charge/discharge cycles and perform well in warm climates. As of May 2016, U.S. company Fluidic Energy has deployed 75,000 zinc-air batteries at 1,200 sites around the world. In Indonesia, state-owned utility Perusahaan Listrik Negara plans to use Fluidic Energy batteries to bring power to 500 remote villages, providing electricity to 1.7 million people.

On the more distant horizon, lithium-air batteries show promise. Lithium-air batteries have specific energy levels greater than 11,000 Wh/kg, several times higher than lithium-sulfur, and approaching the energy density of gasoline. Developers will need to solve problems related to stability, however, before the batteries will be practical.

VRBs (also called flow batteries) store their chemical energy, or vanadium ions, in solution. It’s easy to increase capacity of and recharge VRBs since the ions are not stored on the battery plates themselves. To increase capacity, operators can simply increase the amount of vanadium solution, or electrolyte, and the size of the storage tanks. To recharge VRBs when no power source is available, operators replace the electrolyte. Unlike lead-acid batteries, VRBs can be left completely discharged for long periods of time with no ill effects. VRBs have a very high cycle life, with thousands of cycles at 80 percent DOD.

VRBs have some disadvantages, however, compared to lead-acid batteries. VRBs are heavy for the amount of electricity they can hold. VRBs have a relatively low specific energy of 10–20 Wh/kg, compared to 33–42 Wh/kg for lead-acid batteries. Moreover, VRBs use pumps to circulate the electrolyte; most chemical batteries do not require moving parts.

More than a dozen installations of VRBs have been deployed in capacity and power ranges similar to those used in mini-grids.

Zinc-bromine batteries have a higher specific energy than VRBs, similar to lead-acid batteries (33–42 Wh/kg). Energy is stored in the form of electroplated zinc. With higher life cycles, zinc-bromine batteries can discharge completely (to zero percent) without damage.

GE Global Research has identified high-temperature sodium batteries are appropriate for utility-scale storage. High-temperature sodium batteries are constructed from a molten salt of sodium and sulfur. Sodium batteries are very efficient (90 percent) and use fairly inexpensive materials. These batteries, however, require high operating temperatures (300–350 degrees C) that mini-grids can’t accommodate. High-temperature sodium batteries are also very large. Battery capacity is 1.5–34 MW, too large for all but the largest mini-grids. So far, high-temperature sodium batteries have only been used for large-scale grid storage and supplemental power to wind and solar farms.

FES stores energy in the form of a quickly rotating disk. Rotors made of strong carbon-fiber composites spin in a vacuum at speeds from 20,000 to more than 50,000 rpm. Steel is the most common rotor material, but researchers are exploring carbon nanotubes, which could increase energy density tenfold by increasing rotation speed.

Flywheels reach target speed in minutes, much faster than chemical batteries. They last a long time with little maintenance. Quoted cycle life varies from 100,000 cycles. Flywheels with magnetic bearings and high vacuums can reach an electrical charge/discharge efficiency of 81 percent. Flywheels, however, have low specific energy (about 11 Wh/kg).

Flywheels work best for high-power, short-duration applications—like stabilizing frequency or voltage—that require hundreds of kW in tens of seconds. A mini-grid installation in Flores Island, Portugal, for example, uses a 5 kWh FES system that can provide up to 350 kW of power for short periods (seconds or minutes, but not hours). The flywheel stabilizes frequency and voltage for a MW-scale hybrid wind/diesel/hydro-power system.

Flywheel prices are expected to drop. IRENA predicts that storage costs could decrease from $1,500 to $4,000 per usable kWh in 2017 to $1,000–$2,500 US per kWh by 2035.

Hybrid Optimization of Multiple Energy Resources (HOMER) software for mini-grid modeling and optimization is an excellent source of detailed performance information about commercially available, cutting-edge mini-grid battery technologies. In addition to conventional lead-acid batteries, HOMER Pro covers lithium-ion batteries and commercially available models of zinc-bromine batteries, VRBs and FES.