How Do Batteries Work?

Energy storage devices, a.k.a batteries, come in many shape and sizes. Energy storage enables multiple applications on the grid: energy management, backup power, load leveling, frequency regulation, voltage support, and grid stabilization. These storage options include pumped hydro, compressed air energy storage, flywheels, and electrochemical capacitors .

Tesla “skateboard” battery pack

Most of the batteries we used in our modern lifestyles – from laptops to electric vehicles – use chemical cells. They all have the same four components: a negative end (anode), positive end (cathode), a separator and an electrolyte. The electrolyte is a chemical medium that enables the flow of ions between the cathode and anode. The separator prevents the positive and negative end from mixing when you are not using the battery. When a load is attached to a battery, chemical energy is converted into electrical energy.

Batteries can be composed of an individual cell or a set of cells that come in various form factors: cylindrical, pouch, prismatic, and button. For example, in Tesla’s forthcoming Model 3 electric vehicle, engineers have designed a “skateboard” battery pack is actually composed of hundreds of 2170 cylindrical cells. These cells are named after their dimensions: 21mm x 70mm. They are larger and hold more charge than the industry standard 18650 cell that is used in laptop computers

What do you mean by charge or energy density?

Charge density is the amount of electric charge that can be stored in a given mass or volume. Batteries are usually rated by their charge capacity, which is in milli amp hours (mAh), which is a measure of charge, i.e. the number of hours a battery can supply a particular amount of current. For example, the iPhone 7 battery holds a charge of 2,900 milli ampere hours.

On the other hand, the energy density is the amount of electrical work a battery is capable of delivering per unit mass of the battery. One can determine the energy capacity of the battery by multiplying the charge capacity by the output voltage (V) of the battery. However, voltage output is rarely even for a number of reasons, including the memory effect that results from under or overcharging of the battery.

Different chemistries determine the energy density of a battery. However, engineers must contend with tradeoffs between the energy density, power density, and safety of the battery.

How long can a chemical battery last?

It depends.  Different chemistries and structures impact the life time of a battery. All chemical batteries expand when cycled, e.g. charged/discharged.  It this particularly problematic for chemical batteries contained in a pouch. The expansion and contraction during the life of a rechargeable battery can affect the battery’s various components and ability to keep the positive and negative end separated.  Over time, the packaging may fail requiring the replacement of the entire battery.

The formation of dendrites in the battery is also a major challenge. Dendrites are fibers of lithium that grow from one electrode to another. It spreads throughout the electrolyte during charging and discharging. Over time, dendrites may puncture the ultra thin separator causing a short circuit, causing it to rapidly overheat.  In some cases, the battery may rapidly overhead and catch fire. A major focus of research today is to understand how these microstructures form and how they can be controlled.

What kind of batteries technologies dominate today?

It depends on the scale and application. For the grid, pumped hydro is the most widely deployed options. For our mobile phones, laptops and even our cars, lithium-ion batteries are the dominant technologies.

In electric vehicles, there are five main chemistries used: nickel cobalt aluminum (NCA), lithium manganese oxide (LMO), nickel manganese cobalt (NMC), lithium iron phosphate (LFP) and lithium titanate (LTO). They each have their strengths and weaknesses. LMO might be the most cost effective today but has a relatively short lifetime (300 to 700 cycles) whereas LTO is expensive but offers much better safety and longer lifetimes (3000 to 7000 cycles).

Source: Bloomberg New Energy Finance

While there are many other different chemistries available, much of today’s research is focused on improving the performance and safety of lithium-ion batteries. As such the price of lithium-ion batteries has dropped such that they are being considered for grid applications.

Why is energy storage needed for the grid? What are the existing technologies?

Energy storage systems like, pumped hydro plays an important role in load shifting and store energy during low-demand hours (e.g., nighttime). There are at least 40 pumped storage plants in operation in the US, comprising more than 22 gigawatts (GW) of storage capacity (roughly 2% of U.S. generating capacity).  Europe and Japan have notably higher fractions of grid storage at 5% and 10%, respectively. Pumped storage plants uses excess electricity to pump water into storage tanks which can be used to power turbines.

Source: Aneke and Wang, 2016

To accommodate the introduction of more renewables into the grid, the state of California has mandated utilities add 1.32 gigawatts of storage by 2020. In February 2017, Southern California utility San Diego Gas & Electric (SDG&E) together with AES Energy Storage announced its brand new energy storage facility, a 30MW battery system capable of storing 120MWh of energy, which can serve 20,000 customers for four hours. The system, the largest grid scale battery in the world, consists of 400,000 Samsung 2170 cells, like the ones being used for Tesla’s Model 3. They were installed in nearly 20,000 modules and placed in 24 containers.  By 2021, this facility will be superseded by one in Long Beach, California that will be capable of running at 100MW for four hours.

Why is energy storage needed to enable higher renewable penetration on the grid?

Energy storage solves two problems with renewables: intermittency and surplus power. Solar energy output fluctuates during the course of the day or when clouds are out. The same goes for wind, wind can pick up or slow down. Energy storage allows utilities to smooth out the surge in power, especially when there is more power produced than needed at the moment. For example solar power produced to during the day peak power can stored and later released in the evening, when there is no sunlight.

As more renewable energy is installed, excessive supply results in curtailment, resulting in unused energy generation from wind and solar. This problem is particularly acute in California where the load profile is that of the “duck curve”. This represents the gap between the total load a utility serves and what that load looks like after wind and solar generation serve some of that load. The installment of storage helps to prevent such curtailment.

The rapid fall in cost of lithium-ion technology is also enabling the replacement of expensive peaker power plants, which typically run natural gas.  They are used to provide additional capacity to the grid during peak demand when generation from base load power plants and intermediate load-following plants are not enough. Because speakers are used less than 10% of the time throughout the year, they are enormously costly and account for between 10% and 20% of electricity costs in the U.S. They are also prone to natural gas supply disruptions. In comparison, energy released from a battery plant does not rely on a gas pipeline and is instantaneous.

In 2015, the natural gas leak at Aliso Canyon, California spurred concerns of energy storage. This led to the building of the 80 MWh Mira Loma battery substation that use a collection of Tesla Powerpacks.

What energy storage technologies are appropriate for renewable integration

As described above, pumped storage historically has been the prevailing technology for storage on the grid, however the focus of energy storage investments today is in lithium-ion battery and flow battery systems. The California projects in storage use lithium-ion technology but flow batteries are becoming viable alternative.

In Japan, Hokkaido Electric Power Company and Sumitomo Electric Industries are testing a 60 MWh redox flow battery with an output of 15 MW at the Minamihayakita Transformer Station in Hokkaido.  Flow batteries, which are already used by the US military, have advantages to lithium-ion batteries. The power output can be easily varied and do not have safety issues associated with lithium-ion batteries today.

Flow batteries store energy in liquid electrolyte solutions which flow across a special membrane that prevents them from mixing but allows ions to pass though creating electricity.

What are the challenges in battery research?

When designing batteries, engineers must trade off between energy density, power output, lifetime, and safety. While incremental advances have led to signifiant improvements in energy density and performance, battery science is still not well understood. For example, researchers are exploring new chemistries and structure, and materials:

  • Semi-solid battery technology, such as the one being developed at battery startup 24M, greatly simplifies the manufacturing and architecture of the cell.
  • The use of aqueous solvents and binders to reduce environmental impact of the batteries and expand end-of-life options.
  • Using silicon as the active anode material. Silicon can increase the energy density of the cell by as much as 40%.