Utility-scale Battery Energy Storage Systems

Battery energy storage systems (BESS) market is divided into 3 main segments: residential, commercial and industrial (C&I) and utility-scale. Utility-scale BESS are connected to the electric grid. See the leading BESS manufacturers here.

Electric grid

• Know-what: the electric grid is a system that encompasses electricity generation, electricity transmission and electricity distribution.

• Analogy: Think of a trip from a nearby city to your home 100 km away. The city is the generation entity, the highway is the transmission system and the feeder roads are the distribution network.

Generation

• Know-what: electricity is generated at sites known as resource entities. Electricity generation involves transforming various forms of energy into electrical energy. This can be done through mechanical and non-mechanical means.

• Analogy: various modes of transport to get you from one major city to another

Mechanical means

• Know-what: involves converting energy forms such as potential and chemical first into kinetic energy (driving a turbine) then to electrical energy. Mechanical means can be further subdivided into kinetic (wind, water), chemical (natural gas, coal) and nuclear (fission, fusion) based on the nature of the initial source of energy. 

Non-mechanical means

• Know-what: conversion of a form of energy directly into electrical energy without conversion to kinetic energy first. Two examples are conversion of solar energy directly into electricity through photovoltaic (PV) panels and conversion of solar energy into heat energy through solar concentrators.

Transmission

• Know-what: this is transfer of electricity from the point of generation through high voltage lines. Electricity is typically transmitted at tens of thousands of volts to reduce transport losses (the technical term for losses due to electricity transfer is power losses). 

• Analogy: Just as highways have high speed limits, transmission lines transport electricity at high voltages.

Distribution

• Know-what: this involves the transfer of electricity from the high voltage to low voltage lines that go into your house.

• Analogy: distribution lines are similar to the low speed limit roads in your neighborhood.

Modern grid

Traditionally, electricity was consumed as it was generated thus grids had to match supply and demand. This was feasible as conventional generation was dispatchable, that is, could be ramped up and down depending on demand. The increase in the share of renewable generation in the grid mix introduces a challenge to the modern grids.

Energy storage

Enter energy storage. Energy storage enables smoother integration of solar and wind sources into the grid by storing surplus electricity production and discharging it to the grid when demand rises. There are various energy storage technologies with lithium-ion battery (LIB) energy storage systems leading in terms of number of installations and pumped storage hydropower (PSH) leading in terms of total installed capacity.

A battery energy storage system (BESS) is composed of a battery unit, control system, thermal management system, power conversion system and fire and explosion protection system. These components are typically housed in a container or a custom enclosure.

Battery unit

• Know-what: the battery unit consists of battery cells packaged into battery modules. Battery modules are then combined into strings. Several strings are typically housed in a container or a customized enclosure. The enclosure is connected to the grid via the power conversion system and a transformer.

• Know-why: this is the component that consumes electrical energy, converts it to chemical energy and releases it back to the grid as electrical energy.

Power conversion system

• Know-what: the main component is an electronic device called an inverter.

• Know-why: batteries store electrical energy in direct current (dc) form but the grid transports electricity in alternating current (ac) form. Inverters are used to transform the dc output of the battery to an ac form for the transmission and distribution lines.

Thermal management system

• Know-what: BESS are air-cooled, liquid-cooled or a combination of the two. Air cooling is provided via a heating, ventilation and air conditioning (HVAC) equipment while liquid cooling is delivered by chillers.

• Know-why: LIB cells operate within specified temperature ranges beyond which performance deteriorates and safety risks increase.

Controls system

• Know-what: consists of an energy management system (EMS) at system level and battery management system (BMS) at the battery module level. The controls consist of software and hardware (sensors). The software stipulates operational limits which the hardware implement. The parameters of interest are voltage, current and temperature.

• Know-why: this is the brain of a battery. It implements charge and discharge limits for safe operation of a battery.

Fire and explosion system

• Know-what: consists of fire alarms, smoke detectors, deflagration panels and exhaust systems.

• Know-why: this system monitors and mitigates BESS safety events such as gas venting and thermal runaway. The main gases produced in a safety incident includes hydrogen, carbon monoxide and various hydrocarbons.

Application and use case 

Application refers to the service the BESS renders to the grid. Typical applications include energy arbitrage, peak shaving, ancillary services (frequency regulation and voltage support) and deferral of transmission and distribution upgrades. BESS can be designed to offer a combination of services - a phenomenon known as value stacking.

On the other hand, use case refers to the number of cycles per day, depth of discharge and environmental conditions (temperature and humidity) required to be maintained within the enclosure. Use case determines performance (capacity and roundtrip efficiency) degradation rate. Performance degradation rate is directly proportional to the number of cycles per day and dependent on operating temperature and humidity.

Summary

The impacts of climate change are driving the transition to generation resources with lower carbon footprints. These resources include intermittent renewable sources such as solar and wind. BESS reduce the fluctuations in solar and wind production thereby improving grid reliability.