Battery Energy Storage System (BESS) is a technology developed for storing electric charge by using specially developed batteries. The underlying idea being that such stored energy can be utilized at a later time. Enormous amount of research has led to battery advances that has shaped the concept of Battery Energy Storage System into a commercial reality.

Battery Energy Storage Systems (BESSs) are a sub-set of Energy Storage Systems (ESSs). Energy Storage System is a general term for the ability of a system to store energy using thermal, electro-mechanical or electro-chemical solutions. A BESS typically utilizes an electro-chemical solution.

Essentially, all Energy Storage Systems capture energy and store it for use at a later time or date. Examples of these systems include pumped hydro, compressed air storage, mechanical flywheels, and now BESSs. These systems complement intermittent sources of energy such as wind, tidal and solar power in an attempt to balance energy production and consumption.

Energy storage results in a reduction in peak electrical system demand and ESS owners are often compensated through regional grid market programs. Regulators also offer incentives (and in some cases mandates) to encourage participation.

BESS has advantage over other Storage technologies as it has small footprint and no restrictions on on geographical locations that it could be located in. Other Storage technologies like Pumped hydro storage (PHS) and Compressed air energy storage (CAES) are only suitable for limited number of locations, considering water and siting-related restrictions and transmission constraints. Energy and power densities of some technologies

Accordingly BESS utilizing Lithium Ion technology offer high energy and power densities that are suitable for utilizing at distribution transformer level. The available space at the distribution transformer setup can be used to locate the BESS.

The night peak that that needs to be managed is about 4 hours maximum and hence the discharging time required for a particular BESS is less than 4 hours. Further the rated apparent power of distribution transformers are in the range of 160 kVA, 400kVA up to 1 MVA (for rural, urban and metropolitan respectively).

Therefore BESS only needs to supply a part of that capacity during maximum of 4 hours of peak time.

Following figure illustrates the places different technologies have in the space having the power, energy and discharge time as dimensions.

Characteristics of a Battery Energy Storage System

Round-trip Efficiency — Indicates the amount of usable energy that can be discharged from a storage system relative to the amount of energy that was put in. This accounts for the energy lost during each charge and discharge cycle. Typical values range from 60% to 95%.

Response Time — Amount of time required for a storage system to go from standby mode to full output. This performance criterion is one important indicator of the flexibility of storage as a grid resource relative to alternatives. Most storage systems have a rapid response time, typically less than a minute. Pumped hydroelectric storage and compressed air energy storage tend to be relatively slow as compared with batteries.

Ramp Rate — Ramp rate indicates the rate at which storage power can be varied. A ramp rate for batteries can be faster than 100% variation in one to a few seconds. The ramp rate for pumped hydroelectric storage and for compressed air energy storage is similar to the ramp rate of conventional generation facilities.

Energy Retention or Standby Losses — Energy retention time is the amount of time that a storage system retains its charge. The concept of energy retention is important because of the tendency for some types of storage to self-discharge or to dissipate energy while the storage is not in use.

Energy Density — The amount of energy that can be stored for a given amount of area, volume, or mass. This criterion is important in applications where area is a limiting factor, for example, in an urban substation where space could be a limiting constraint to site energy storage.

Power Density — Power density indicates the amount of power that can be delivered for a given amount of area, volume, or mass. In addition, like energy density, power density varies significantly among storage types. Again, power density is important if area and/or space are limited or if weight is an issue.

Safety — Safety is related to both electricity and to the specific materials and processes involved in storage systems. The chemicals and reactions used in batteries can pose safety or fire concerns.

Life span — measured in cycles.

Depth of Discharge (DoD) — Refers to the amount of the battery’s capacity that has been utilized. It is expressed as a percentage of the battery’s full energy capacity. The deeper a battery’s discharge, the shorter the expected life time. Deep cycle is often defined as 80% or more DoD.

Ambient temperature — Has an important effect on battery performance. High ambient temperatures cause internal reactions to occur, and many batteries lose capacity more rapidly in hotter climates.

BESS by Battery Types

BESSs intrinsically use electro-chemical solutions which manifest in some of the following Battery Types:

1. Lithium-ion — these offer good energy storage for their size and can be charged/ discharged many times in their lifetime. They are used in a wide variety of consumer electronics such as smartphones, tablets, laptops, electronic cigarettes and digital cameras. They are also used in electric cars and some aircraft.

2. Lead-acid — these are traditional rechargeable batteries and are inexpensive compared to newer types of batteries. Uses include protection and control systems, back-up power supplies, and grid energy storage.

3. Sodium Sulfur — uses include storing energy from renewable sources such as solar or wind.

4. Zinc bromine — uses include storing energy from renewable sources such as solar or wind.

5. Flow — flow batteries are quite large and are generally used to store energy from renewable sources.