Energy storage systems play a crucial role in off-grid applications globally. Although various energy storage systems exist, this article considers different battery technologies for their suitability for off-grid and electric vehicle applications.
Off-grid electricity can be defined as a stand-alone power system or mini-grid to typically provide a small community with electricity without the support of a remote electric-grid infrastructure. Ever since the discovery of electrical energy, there has been a need to create a means of storing the electrical energy generated for use on demand.
Energy storage systems (ESSs) are required to store excess electricity when power production is greater than consumption. They are very useful as off-grid storage to backup renewable energy sources such as solar and wind power used by people who live or work in remote areas.
Electrical energy can be stored in different forms, including electrochemical (batteries); kinetic (flywheels); as potential energy (pumped hydro which works by pumping the water from a lower elevation reservoir to a higher elevation), or as compressed air (CAES). Fig. 1 illustrates the most important storage technologies for the power grid.
This article presents the battery technology options available commercially, which of these are being used most dominantly and suitable for off-grid and electric vehicle (EV) applications.
Battery-based electrical energy storage represents the most cost-efficient technology available today. According to the International Energy Agency (IEA), there will be 140-million electric vehicles on the world roads by 2030 . Studies indicate that the battery market will be worth US$120-billion by 2019.
Battery technology options
There are four types of mature battery technologies in use: lead-acid, nickel-based, sodium sulfur, and lithium-ion (Li-ion). Of these, high energy density Li-ion batteries are currently being used as stationary battery packs for off-grid applications and to power EVs.
The lead-acid battery is chronologically the first and oldest type of rechargeable battery, having been invented in 1859. It is widely-used and well established in industrial applications. Its electrochemistry mechanism is based on chemical reactions involving lead-dioxide as the cathode electrode, lead as the anode electrode and sulfuric acid as the electrolyte. See Fig 2.
Each lead-acid cell can provide a voltage of 2 V, energy density of around 30 Wh/kg, and a power density around 180 W/kg . Lead-acid batteries can provide high energy efficiencies (about 90%), are easy to install, require little maintenance and have a low investment cost. They can be used in applications varying from 1 kW (such as in uninterruptable power supplies (UPSs)) up to 10 MW (for transport and distribution systems) as shown in Fig. 1.
Lead-acid batteries are attractive for small-scale PV installations due to their low initial cost. However, in large-scale renewable energy storage installations the use of lead-based batteries is small compared to the use of Li-ion batteries.
Nickel-based (NiCd/NiMH) batteries
Nickel-cadmium (NiCd) rechargeable batteries were invented in 1899 and they are suitable to store energy in extreme climate, long-cycle or fast-charging conditions. Both NiCd and Nickel Metal Hydride (NiMH) batteries use nickel hydroxide for the cathode and an aqueous solution of potassium hydroxide with some lithium hydroxide as the electrolyte. However, for the anode, the NiCd type uses cadmium hydroxide; and the NiMH uses a metal alloy as shown in Fig. 3.
The NiMH battery was invented in 1967, only three years prior to the invention of the Li-ion battery. A NiMH battery can have two to three times the capacity of an equivalent size NiCd, and its energy density is comparable to that of a Li-ion battery. Although NiMH was used in portable equipment, it has been replaced in recent years as a result of the increase in manufacture of Li-ion batteries. The rated voltage of nickel-based batteries is 1,2 V per cell.
NiMH battery and Li-ion batteries are preferred due to their improved energy densities and higher specific energy, even though they are more expensive than lead-acid batteries.
Sodium-sulfur (NaS) batteries were introduced in the early 1970s. They are primarily manufactured and popular in Japan and with some usage in USA. An NaS battery consists of liquid (molten) sulfur at the positive electrode and liquid (molten) sodium at the negative electrode as active materials separated by a solid beta-alumina ceramic electrolyte as shown in Fig. 4.
NaS batteries are suited for applications with loads varying from approximately 0,8 to >10 MW. NaS batteries are suitable for on-grid energy storage because of their high efficiency, long cycle life, fast response and high energy density. However, they function at high temperature and are more costly than other battery technologies.
Li-ion batteries (LIBs)
The Li-ion battery was introduced in 1970. Since then the growth in its use in portable electronic products has been amazing. Li-ion batteries attracted the interest of the electronics industry due to their high energy and power density, compact size and weight, superior resilience and longer lifecycle.
LIBs are used in applications varying from 1 kW to 1 MW as indicated in Fig. 1. The use of Li-ion batteries for stationary off-grid installations operating worldwide is increasing exponentially.
In LIBs, the lithium intercalated transition metal oxide serves as cathode, while the anode is made from graphite carbon. A single Li-ion cell can produce a voltage as high as 4 V which makes it high energy density battery. It can also deliver high current and high power density.
Famous pioneering companies such as Tesla and Panasonic are demonstrating a ground-breaking achievement in the Li-ion battery industry. Tesla produced Li-ion batteries for off- grid and EV applications. When the company’s “Gigafactory 1” for Li-ion batteries opened in Nevada USA in 2016, it had the capacity to produce 50 GWh of Li-ion batteries – the same as that manufactured in the entire world at the time.
The ultimate stationary battery pack is expected to power 0,5-million (50 GWh) electric cars by 2018, growing to 1,5-million (150 GWh) cars per year when the factory is at full scale. In 2014, the total output of Li-ion batteries for all uses was 30 GWh according to Elon Musk, the CEO of Tesla, which shows how the company is investing in Li-ion battery technology.
Use of ESSs in Africa
Tesla’s stationary battery pack is called Powerwall. It is used as an energy storage backup system in solar PV and off-grid electrical micro-grid applications in lodges and game reserves in South Africa, Botswana, DRC, etc.
In conclusion, of all battery technologies, Li-ion is proving to be the favorite because of its superior energy and power density, smaller size and lower mass weight, improved resilience, and extended service life.
 Global EV Outlook 2018, IEA, https://www.iea.org/gevo2018/
 Investigation into storage technologies for intermittent renewable energies: evaluation and recommended R&D strategy, INVESTIRE-NETWORK (ENK5-CT-2000-20336), Storage Technology Reports (https://cordis.europa.eu/project/rcn/56929_en.html)
Contact Mesfin Kebede, Energy Centre CSIR, Tel (012) 841-3588, firstname.lastname@example.org
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