Battery Energy Storage - Key Points

Credits: NREL, SANDIA and CIGRE Research

What is grid-scale battery energy storage system?

Battery storage is a technology that enables power system operators and utilities to store energy for later use. A battery energy storage system (BESS) is an electrochemical device that charges (or collects energy) from the grid or a power plant and then discharges that energy at a later time to provide electricity or other grid services when needed. Several battery chemistries are available or under investigation for grid-scale applications,?including lithium-ion, lead-acid, redox flow, and molten salt (including?sodium-based chemistries).

Need for ESS / BESS

The electrical energy obtained from renewable resources like solar energy through photo-voltaic (PV) module, or electricity generated by wind energy through wind turbines is intermittent due to the unpredictability of nature. Furthermore, the large volume of renewables being integrated pose a challenge to the system which may affect or change the flow pattern of power systems, for example, the reverse power, power variation, etc.

A popular option to minimise the impacts from renewable energy in recent years is implementing the application of Energy Storage Systems (ESS). ESS can absorb the excessive power from grid and inject the required power to system within a quite short time. Various ESSs as shown in Figure 1 are available for electric power system application depending on factors considered by the user such as location limit, response time, technologies, etc.

Figure 1 - Type of Energy Storage Systems

Among all types of ESS, the Battery Energy Storage System (BESS) is a particularly attractive technology due to wide variety of options which provide high energy density, high efficiency, fast response, modularity, less geographical limitation, small footprint, low maintenance, ease of erection and installation. The size of battery can be implemented into a wide range of systems from some kWh for household use up to GWh for the grid system.

In terms of a working principle, BESS transforms electrical energy into electro-chemical energy and vice versa. The energy can be stored in batteries for later use. A Battery Management System (BMS) and other auxiliary systems facilitate operation and protection of batteries. This basic structure and working mechanism of BESS has enabled it to output active and reactive power in a relatively fast response timeframe compared to traditional generation sources such as a coal power plant or a hydro power plant. At present, BESS`s implementation is not only limited to steady state balancing of the active power between RE generation and load demand (RE smoothing), it is also used in various applications like frequency and voltage regulation, peak shifting and many more benefits.

What are the key characteristics of battery storage systems?

• Rated power capacity?is the total possible instantaneous discharge capability (in kilowatts [kW] or megawatts [MW]) of the BESS, or the maximum rate of discharge that the BESS can achieve, starting from a fully charged state.
• Energy capacity?is the maximum amount of stored energy (in kilowatt-hours [kWh] or megawatt-hours [MWh])
• Storage duration?is the amount of time storage can discharge at its power capacity before depleting its energy capacity. For example, a battery with 1 MW of power capacity and 4 MWh of usable energy capacity will have a storage duration of four hours.
• Cycle life/lifetime?is the amount of time or cycles a battery storage system can provide regular charging and discharging before failure or?significant degradation.
• Self-discharge?occurs when the stored charge (or energy) of the battery is reduced through internal chemical reactions, or without being discharged to perform work for the grid or a customer. Self-discharge, expressed as a percentage of charge lost over a certain period, reduces the amount of energy available for discharge and is an important parameter to consider in batteries intended for longer-dura- tion applications.
• State of charge,?expressed as a percentage, represents the battery’s present level of charge and ranges from completely discharged to?fully charged. The state of charge influences a battery’s ability to?provide energy or ancillary services to the grid at any given time.
• Round-trip efficiency, measured as a percentage, is a ratio of the energy charged to the battery to the energy discharged from the?battery. It can represent the total DC-DC or AC-AC efficiency of the battery system, including losses from self-discharge and other electrical losses. Although battery manufacturers often refer to the?DC-DC efficiency, AC-AC efficiency is typically more important to?utilities, as they only see the battery’s charging and discharging from the point of interconnection to the power system, which uses AC.

What services can batteries provide?

Below are the key services that BESS cater for:

1. Arbitrage:?Arbitrage involves charging the battery when energy prices are low and discharging during more expensive peak hours. For the BESS operator, this practice can provide a source of income by taking advantage of electricity prices that may vary throughout the day. One extension of the energy arbitrage service is?reducing renewable energy curtailment. System operators and project developers have an interest in using as much low-cost, emissions-free renewable energy generation as possible; however, in systems with a growing share of VRE, limited?flexibility of conventional generators and temporal mismatches between?renewable energy supply and electricity demand (e.g., excess wind generation in the middle of the night) may require renewable generators to curtail their output. By charging the battery with low-cost energy during periods of excess renewable generation and discharging during periods of high demand, BESS can both reduce renewable energy curtailment and maximize the value of the energy developers can sell to the market. Another extension of arbitrage in power systems without electricity markets is?load-leveling. With load-levelling, system opera- tors charge batteries during periods of excess generation and discharge?batteries during periods of excess demand to more efficiently coordinate?the dispatch of generating resources.
2. Firm Capacity or Peaking Capacity:?System operators must ensure they have an adequate supply of generation capacity to reliably meet demand during the highest-demand periods in a given year, or the peak demand. This peak demand is typically met with higher-cost generators, such as gas plants; however, depending on the shape of the load curve, BESS can also be used to ensure adequate peaking generation capacity. While VRE resources can also be used to meet this requirement, these?resources do not typically fully count toward firm capacity, as their generation relies on the availability of fluctuating resources and may not?always coincide with peak demand. But system operators can improve?VRE’s ability to contribute to firm capacity requirements through pairing?with BESS. Pairing VRE resources with BESS can enable these resources to shift their generation to be coincident with peak demand, improving their capacity value (see text box below) and system reliability.
3. Operating Reserves and Ancillary Services:?To maintain reliable power system operations, generation must exactly match electricity demand at all times. There are various categories of operating reserves?and ancillary services that function on different timescales, from subsec- onds to several hours, all of which are needed to ensure grid reliability. BESS can rapidly charge or discharge in a fraction of a second, faster than conventional thermal plants, making them a suitable resource for short-term reliability services, such as Primary Frequency Response (PFR) and Regulation. Appropriately sized BESS can also provide longer-duration services, such as?load-following and ramping?services, to ensure supply meets demand.
4. Transmission and Distribution Upgrade Deferrals:?The electricity grid’s transmission and distribution infrastructure must be sized to meet peak demand, which may only occur over a few hours of the year. When anticipated growth in peak electricity demand exceeds the existing grid’s capacity, costly investments are needed to upgrade equipment and develop new infrastructure. Deploying BESS can help defer or circum- vent the need for new grid investments by meeting peak demand with energy stored from lower-demand periods, thereby reducing congestion and improving overall transmission and distribution asset utilization. Also, unlike traditional transmission or distribution investments, mobile BESS installations can be relocated to new areas when no longer needed in the original location, increasing their overall value to the grid.
5. Black Start:?When starting up, large generators need an external source of electricity to perform key functions before they can begin generating electricity for the grid. During normal system conditions, this external electricity can be provided by the grid. After a system failure, however, the grid can no longer provide this power, and generators must be started through an on-site source of electricity, such as a diesel generator, a process known as black start. An on-site BESS can also provide this service, avoiding fuel costs and emissions from conventional black-start generators. As system-wide outages are rare, an on-site BESS can provide additional services when not performing black starts.

System Studies

The owners and the BESS designers must study the wider power system requirements and constraints before determining which technology and capacity is the most suitable for the specific location and specific grid issues.

The study from CIGRE’s Working Group C1.30 TB 666 “Technical risks and solutions from periodic,?large surpluses or deficits of available renewable generation” summarises some common impacts which arise from Renewable Energy Source (RES) integration. Some common grid problems and risks are specified with possible resolutions:

A. Power Fluctuation

Intermittent electricity output in modern power system usually refers to non-continuous electrical energy generation due to uncontrollable external factors such as RES including solar power, wind power, etc. RE itself is unstable and is hard to predict in both long term and short-term perspective. In the long-term perspective, seasonality is playing a bigger role. For instance, solar PV output in summer can produce up to three times more power than the power generated in winter due to longer day time and higher solar irradiation. On the other hand, a wind turbine in winter can produce as much as twice the power produced in summer.

From the short-term perspective, an unpredictable weather pattern is concerning. Although solar power routinely produces power in a bell shape almost every day, a solar PV farm can be randomly shaded by thick cloud. This effect causes a sharp loss of power generation which could result in sudden system frequency fluctuation. On the other hand, wind turbines have no production pattern, it may produce a lot of power or nothing. Power generation from wind turbines is proportional to the cube of wind speed. Hence even a small change in wind speed can change power generation significantly. Hence, wind power prediction can be inaccurate and lead to difficulty in balancing power supply and demand making the fluctuations in frequency inevitable. If RES penetration is small, it could be neglected, and the conventional counter measures could cover all its impact. But when the penetration grows, this interruption comes along when RES is significant as compared to conventional sources and this could increase the cost of power production and reduce security and reliability of the power grid.

In the system study for application of BESS to alleviate RES impact on the grid, BESS are generally introduced to smooth the output of wind power or solar power plants or to store excessive generated power. So, some indexes could be introduced to evaluate the actual needs to approach such a solution. For example, in some countries, the active power fluctuation at the PCC needs to be controlled to within certain range or a rate of change to meet grid operation stability needs for both wind and solar power plant. The difference between the required range and the actual performance of specific wind and solar power plant could be used as input to configure BESS.

Lack of System Inertia

Modern power systems are integrated with a high portion of inverter-based RES. These inverter-based sources provide little fault-level contribution to the system which will impact the sensitivity and reliability of the system. In other words, the lower the short-circuit ratio,?the faster the system’s frequency will?change with the same amount of change in active power. Most power systems are designed by assuming that they have enough system inertia from the standard thermal generation, hence, with the increase in inverter-based RES, conventional designed countermeasures for the partial loss of power supply might not be enough. This may lead to the unwanted power system risk of forced automatic load- shedding (ALS) which cause a utility to lose revenue and get penalties from the regulators. In conventional power plants, rotating machines such as generators have their rotating speed related to power system frequency, which is generally 50 Hz or 60 Hz. The stored rotating kinetic energy of these machines,?called “system inertia”, acts as a buffer against rapid change of active power and relates to the frequency dynamics. In other words, the rate of the frequency fall is inversely proportional to the system inertia.

Hence, a system with large inertia will be less sensitive and more resilient when facing frequency problems than a system with lower levels of inertia. However, inverter-based RES produces no inertia, therefore, in a system with high penetration of RES, frequency may change more rapidly. If RES continues to grow its proportion of power generation it may reach the point where the change is too fast to be countered by conventional methods, considering that some countries are now shutting down or decommissioning conventional power plants to use RES as energy supply. This is especially the case during the peak of RES output which will be seen by the grid as light load where some power-producing units could be spinning as reserve or even be shutdown (reducing system inertia due to less spinning machine). If large RES units are lost during this duration because of an electrical fault or there is a large change in power generation due to intermittent effects, the system will face difficulty in returning to acceptable operational frequency without some load shedding. Moreover, large generation units typically trip out if the rate of change of frequency (ROCOF) is 2-2.5 Hz per second (depending on the national grid code) which could lead to a major black out.

The evaluation needed to configure BESS in this application might consider the current system inertia characteristics and what contingencies that system together with BESS needs to handle. This application is very similar to frequency regulation in which BESS also provides active power in accordance with dispatched signals.

Change of System Load Profile

In the power system, to supply adequate customer demand, utilities plan power plant construction, profile the daily load curve, and control power plants to match the load curve. However, high RES integration could drastically change the daily load curve. The characteristic bell curve power output from solar PV reduces only the day-time load curve but during sunset, the load curve can ramp up at concerning rates. A steep ramp rate that occurs in the afternoon is very challenging for power management due to the limitation of conventional generation and affects both the power cost and reliability of the power system.

In this case, BESS might be asked to operate together with RES to perform a required generation waveform required by the local electrical power dispatching agent. The specific waveform is to consider local generation and load profile by statistics and prediction, with certain tolerance allowed.

Power Grid Congestion

AC power grid management can be complicated due to its characteristic complexity in controlling of the power flow. A lack of transmission line capacity to deliver electricity without exceeding thermal, voltage and stability limits which are required to ensure system reliability may lead to the system operating under constraints. Bottlenecks could occur in the power system and decrease system efficiency and capacity. Moreover, in fast-growing areas the power demand could outgrow the capacity of transmission system. The congestion could also occur in the event of a natural disaster leading to unavailability or rolling blackouts. New construction or rebuilding the line may also put the stress on the power system.

So far, it is a rare application for BESS worldwide since it is hard to predict the load growth that might make transmission line more attractive than BESS. To evaluate the possibility of using BESS to postpone the need to upgrade power supply and transmission facilities, the current load characteristics and future prediction, congestion duration and frequency, and the cost comparison of implementing BESS and upgrading the facilities need to be carefully analysed.

BESS Functions

After the system study is performed, the designers should know the system requirements and the necessary BESS functions. The functionality of the BESS will frame its component ratings which will be suitable to the specific function. The functional performance requirement will determine the charge and discharge pattern, response time and ramp rate, etc. Also, to consider overall economic benefits of BESS, factors such as efficiency, life cycle, and land occupation also need to be considered

The purpose of storage applications can be categorised based on the nature and duration of events that take place in the grid.

A battery system can theoretically respond to a system change instantaneously, but it is limited by the nature of the interface with the grid. It also has a small land footprint (depending on the type of battery) and has limited installation constraints. These advantages of a BESS make it possible to apply it for various applications to serve as the solution for many grid problems.

IEC 62933-2-1 categorises BESS applications into three classes:

1. Class A for the short-duration power-focused application. BESS is designed to repeatedly inject or absorb up to the specific amount of power for a short period of time (BESS can charge and discharge at rated output power for not more than 1 hour).
2. Class B for the long-duration energy-focused application. BESS is designed to routinely provide or take power from the power system over a long duration period (BESS can charge and discharge at rated output power for more than 1 hour).
3. Class C for emergency or ancillary services.

Consequently, two key points from this part of IEC62933-2-1 are the charging and discharging time and the combination of the application.

The explanation of each application is categorised and described as follows:

Table 1 Example of typical applications for BESS classification (IEC 62933-2-1)