A 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.
Why BESS over other storage technologies
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 are as follows:
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.
Important Considerations for Battery Selection
Many criterion play an important part in selection of the battery for BESS as depicted above. These range from regulatory issues to cost and technology dimensions. However biggest deterministic factor for battery selection is the application which the BESS is required to service along with performance requirement management.
Aspects of Battery Energy Storage Systems’ Economics:
The optimization of BESS Economics lies in closely triangulating the market parameters , consumer parameters and storage system parameters. Each of these lumped parameters have multiple sub parameters which play a significant role in overall economics of the system.
Classification of 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.
Why is BESS gaining popularity?
All types of BESS offer pros and cons in terms of capacity, discharge duration, energy density, safety, environmental risk, and overall cost. However, Li Ion batteries are by far the most widely used in BESS systems these days.
A major factor in the rapid increase in the use of BESS technology has been a 50% decrease in costs of energy storage over the last two years. While costs are still high compared to grid electricity, the cost of energy storage has actually been plummeting for the last 20 years.
Storage systems at the utility customer level can also result in significant savings to businesses through smart grid and Distributed Energy Resource (DER) initiatives, where cars, homes and businesses are potential stores, suppliers and users of electricity.
Security of supply
Storage technologies are also popular because they improve energy security by optimizing energy supply and demand, reducing the need to import electricity via inter-connectors, and also reducing the need to continuously adjust generation unit output.
In addition, BESS can provide system security by supplying energy during electricity outages, minimizing the disruption and costs associated with power cuts.
Many governments and utility regulators are actively encouraging the development of battery storage systems with financial incentives, which is likely to lead to further growth.
Risks involved in using BESS
While the use of batteries is nothing new, what is new is the size, complexity, energy density of the systems and the Li-ion battery chemistry involved — which can lead to significant fire risks.
‘Thermal runaway’ — a cycle in which excessive heat keeps creating more heat — is the major risk for Li-ion battery technology. It can be caused by a battery having internal cell defects, mechanical failures/damage or over voltage. These lead to high temperatures, gas build-up and potential explosive rupture of the battery cell, resulting in fire and/or explosion. Without disconnection, thermal runaway can also spread from one cell to the next, causing further damage.
Difficulty of fighting battery fires
Battery fires are often very intense and difficult to control. They can take days or even weeks to extinguish properly, and may seem fully extinguished when they are not.
Failure of control systems
Another issue can be failure of protection and control systems. For example, a Battery Management System (BMS) failure can lead to overcharging and an inability to monitor the operating environment, such as temperature or cell voltage.
Sensitivity of batteries to mechanical damage and electrical transients
Contrary to existing conventional battery technology, some batteries are very sensitive to mechanical damage and electrical surges. This type of damage can result in internal battery short circuits which lead to internal battery heating, battery explosions and fires. The loss of an individual battery can rapidly cascade to surrounding batteries, resulting in a larger scale fire.
Unique experiment: South Australia’s Battery Farm
Hornsdale Power Reserve (HPR), the 129MWh battery energy storage system (BESS) deployed by Tesla and developer Neoen in South Australia in 100 days duration was found to have had a positive impact on the local network.
The system went into operation in November 2017. Aurecon, an independent engineering and infrastructure advisory company, issued a case study report based on HPR’s performance on its first 365 days of service.
Description: It is the world’s largest lithium battery system in existence to date, although it is rapidly being caught up in size, mostly by big solar-plus-storage and gas peaker replacement projects around the world. HPR has a 100MW discharge capacity and shares a 275kV network connection point with Hornsdale Wind Farm (300MW).
Of the 100MW capacity, 70MW is reserved for system security services contracted to the South Australia government, while Neoen can use the remaining 30MW and 119MWh of storage capacity to participate in market opportunities.
Aurecon’s experts found HPR to have supplied numerous benefits to the system throughout the year, including removing the need for network upgrades and additional capacity. It also responded effectively to stress events on the local network.
System Operation: The first year of operation included a “large system security event” , when an inter-connector between the grids of Queensland and New South Wales was tripped, islanding the Queensland region. Using fast frequency response (FFR) services, HPR was able to stabilize grid frequency to within the accepted range of 0.15Hz either side of 50Hz. Aurecon said HPR “performed as required”, providing significant frequency support to all connected National Electricity Market regions.
HPR also participates in six further Frequency Control Ancillary Services (FCAS) markets for AEMO, including managing total deviation of frequency and Rate of Change of Frequency (RoCoF). From operational data provided, Aurecon said the system is providing “very rapid and precise response to FCAS regulation signals”. This is in particularly stark contrast to the much slower and less precise responses provided by steam turbines, which have been the traditional means of balancing this part of the network.
Aurecon said HPR has greatly increased the competitive dynamic of the FCAS markets, highlighting that in 2016 and 2017 FCAS constraints meant 35MW of capacity had to be kept aside, adding close to AU$40 million in regulation costs in that time.
In addition to frequency regulation, the system also participates in AEMO and ElectraNet’s System Integrity Protection Scheme (SIPS), which offers protection against the loss of the locally-sited Heywood Interconnector if multiple generators fail. It’s a direct response to the 2016 blackout which eventually inspired the deployment of HPR, and Aurecon acknowledged that HPR plays a significant role in injecting energy into the network. It is required to discharge up to 100MW in under 150ms. If this step fails, the system is then used to help provide load shedding to the network.
Traditionally, the power industry has contended that energy business is differentiated from every other entities and markets since it can’t be stored.
This has been essentially right, yet future advancements can possibly expel this notion and can consolidate storage with other grid matrix technologies to make a new Paradigm!
Electric batteries may offer the best potential as a smart grid enabler. Latest batteries are right now being conveyed to serve different forms of transmission and distribution applications, with different advantages possibly spilling out of a solitary establishment.
Research continues to look for new technologies that will give the advancement in storage medium which will give a breakthrough in low cost storage medium for future utilities.
· National Energy Technology Laboratory, USA
· Lithium-ion Battery Energy Storage Systems, AIG Energy Industry Group