Batteries: an important energy storage solution

How it works

Batteries are indispensable for our daily lives.  We need them to power our smart phones and notebooks, and they make sure our electric vehicle gets from A to B. They have also become a key for maintaining a stable power grid. 

Large-scale battery storage systems help balance out fluctuations in the power grid. They can rapidly store electricity and make it available quickly in order to keep production and consumption in balance in the power network.

Be it in a smart phone or an electric vehicle, or as a large-scale battery storage system: Batteries use electrochemical cells that chemically store energy that can be converted into electricity.

Lithium-ion batteries are the most commonly used electrochemical batteries. The advantage: They have a high energy density. Lots of energy can be stored in a minimal space.

As the term “lithium-ion” suggests, lithium is an important battery component. According to the globally identified reserves, lithium occurs most frequently in Chile, followed by Australia, Argentina and China. 

A lithium-ion battery consists of two electrodes, the cathode and the anode. They are separated by an electrolyte, a conductive material. While the cathode comprises storage material like lithium-nickel-manganese-cobalt-oxide, the anode is normally made of graphite.  

When the battery is discharged, the lithium ions – ions are positively charged load carriers – migrate from the anode to the cathode through the electrolytes. At the same time, the anode releases electrons that flow to the cathode by means of an external power circuit, and supply our smart phone or other devices with power. The lithium ions from the anode travel through the electrolytes to the cathode where they accumulate. In contrast, when charging, i.e. when the battery is supplied with power from an outside source, the electric energy is transformed into chemical energy. The lithium ions and the electrons flow in the reverse direction.

When cells are connected in series, also known as serial connection, operating voltage increases. If a higher storage capacity is required, the cells are connected in parallel. These two systems can also be combined.  

The most important parameters of a battery are the output (measured in kilowatts, kW, or megawatts, MW), and the energy that can be stored (measured in kilowatt-hours, kWh, or megawatt-hours, MWh).  

Axpo works with large-scale battery storage systems. The output and storage capacity of these batteries is in the megawatt or megawatt-hour range.

Batteries from A to Z

Large battery energy storage systems (BESS) are designed for high output and large storage capacity. A system consists of a battery system, the battery cell, and an integrated cooling and energy management system (EMS), as well as an energy conversion system comprising inverters and transformers.

The most important answers on the topic of batteries

Construction of a BESS takes about one year and, as a rule, the system can be operated for 20 years. Environmental impact is minimal in construction and system operation is environmentally friendly.

The majority of batteries are produced in Asia. The main battery components such as lithium and graphite are also mined and processed in this region of the world. 

Battery storage capacity decreases over time and through use. One charging cycle per day results in a capacity reduction of about 2% per year.

Once the battery has reached its sevice life, the lithium, aluminium and cobalt can be extracted and recycled.

Costs

It is expected that the investment costs for battery systems will decrease in the long term because the efficiency in implementing projects will increase, and more and more scaling effects will be generated. However, lithium shortages can result in a temporary price increase as was seen at the beginning of 2022.

Regulatory framework conditions

Battery storage systems are used in multiple markets and are both power consumers and power generators. It is therefore important that the regulatory authorities set a clear framework for the construction and operation of such systems.

Grid conditions

Historically seen, the transmission and distribution grids in most countries were designed with centralised production in mind. The battery capacity that can be installed is dependent on the feed-in point and the network topology at that specific location.

  • Battery storage capacity indicates the energy volume that can be stored (measured in megawatt-hours, MWh).
  • Output indicates the energy per time unit that can be charged or discharged (measured in megawatt-hours, MWh).
  • The C-Rate indicates how quickly a battery can be charged or discharged. C-Rate = output divided by storage capacity.
  • State-of-Health (SoH) describes the relationship between the storage capacity of a used battery and a new battery: Initially the SoH is 100%.
  • The State-of-Charge  (SoC) indicates the charging state of a battery: at 0% the battery is empty and at 100% SoC it is fully charged.
  • The number of cycles indicate how many equal, full charging and discharging cycles a battery carries out over a specific time period. As a rule, batteries carry out 1 to 2 equivalent full load cycles per day.
  • Round trip efficiency measures the energy that can be recovered in comparison to the energy stored (output divided by input). Round-trip efficiency is normally 85% or higher.
  • Battery service life depends on its operation. Frequent charging and discharging, as well as extreme charging states, decrease the service life of a battery.

Applications

Large battery energy storage systems (BESS) perform the following tasks:

  • Energy shifting: For example, power that was generated during the day from photovoltaic can be stored and fed into the grid at night
  • Peak shaving: Peak shaving refers to lowering and smoothing peaks in the power grid in order to save grid costs
  • Imbalance: The difference between the effective, generated energy in relation to traded energy
  • Day-ahead and intraday trading
  • Ancillary services: Primary and secondary control capacity
  • Reserve energy
  • Alleviating grid bottlenecks and postponing grid investments through peak energy flow management

In addition, battery storage systems contribute to ensuring reliable energy supply. Making capacities available can be remunerated.  

BESS can be connected in front-of-the-meter (FTM) or behind-the-meter (BTM). As a rule, large-scale battery storage system to support the grid are connected in front-of-the-meter (FTM).

A battery can be operated independently or together with photovoltaic or wind energy systems. The advantage of the second option is that infrastructure can be jointly used (land, grid connection). Operational restrictions are the disadvantage.

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