The figures below compare utility-scale batteries used to support solar photovoltaic (PV) energy to board-level energy storage components for filtering, bootstrap, (near) lossless energy commutation, resonance, and other circuit-level applications.
What is Energy Storage?
2022. 11. 4
Energy storage is "the capture of energy produced at one time for use at a later time to reduce imbalances between energy demand and energy production."[1] This applies across the full spectrum of timescales and energy density, meaning energy can be stored for nanoseconds or years and in applications ranging from picowatts to gigawatts of power.
Table of contents
Most discussions around energy storage focus on battery storage solutions, but there are numerous other forms. From an electronics perspective, energy can be stored as electrochemical potential in rechargeable batteries, voltage in capacitor and supercapacitor devices, or current in magnetic devices (i.e., inductors and transformers).
Dynamic energy can be stored in both kinetic forms (i.e., flywheels) and potential forms (i.e., water reservoirs on mountaintops). Energy is stored in a given form for convenience, efficiency, or cost, even if eventually converted into electrical energy. Thermal energy storage in heated salts, used to boil water for turbines, is a type of phase-change material energy storage. Compressed air energy storage is also common, even at utility scale [2].
Some forms of direct energy reuse include repurposing "waste" heat from data centers to heat nearby homes during winter. Given energy storage’s wide range of applications, it’s critical to evaluate each solution’s performance expectations and design considerations to maximize performance and safety. This article compares multiple storage technologies, but first defines important terms and concepts. Some aspects of energy storage are universal, others are highly component- or application-specific. There is no equivalent of Moore’s Law for energy storage—energy density doubles roughly every decade. Meanwhile, power system loads advance more rapidly, requiring designers to navigate unsynchronized development timelines.
Load demand only increases over time, placing higher demands on storage density. However, storage strategies vary greatly by application. For example, backup power for a data center differs greatly from safety capacitors in EMI filters. Even within power supplies, there’s a range of storage methods and design implications. Energy storage devices in power supplies may also function as safety components, subject to derating, testing, and thermal lifetime standards.
Dynamic energy can be stored in both kinetic forms (i.e., flywheels) and potential forms (i.e., water reservoirs on mountaintops). Energy is stored in a given form for convenience, efficiency, or cost, even if eventually converted into electrical energy. Thermal energy storage in heated salts, used to boil water for turbines, is a type of phase-change material energy storage. Compressed air energy storage is also common, even at utility scale [2].
Some forms of direct energy reuse include repurposing "waste" heat from data centers to heat nearby homes during winter. Given energy storage’s wide range of applications, it’s critical to evaluate each solution’s performance expectations and design considerations to maximize performance and safety. This article compares multiple storage technologies, but first defines important terms and concepts. Some aspects of energy storage are universal, others are highly component- or application-specific. There is no equivalent of Moore’s Law for energy storage—energy density doubles roughly every decade. Meanwhile, power system loads advance more rapidly, requiring designers to navigate unsynchronized development timelines.
Load demand only increases over time, placing higher demands on storage density. However, storage strategies vary greatly by application. For example, backup power for a data center differs greatly from safety capacitors in EMI filters. Even within power supplies, there’s a range of storage methods and design implications. Energy storage devices in power supplies may also function as safety components, subject to derating, testing, and thermal lifetime standards.
Batteries vs. Capacitors
Electrical energy storage devices fall into two categories: capacitors or batteries, though hybrid designs exist. Any two parallel plates with a polarizable dielectric form a capacitor, storing charge as an electric field. In contrast, batteries store charge electrochemically via an ion-conductive electrolyte.
Batteries are either "wet" (liquid electrolyte) or "dry" (solid-state electrolyte). Dry cells last longer and fail less catastrophically, as the solid-state electrolyte resists thermal degradation and shorting. Batteries are also categorized as primary (single-use) or secondary (rechargeable) storage. Secondary batteries dominate energy storage solutions for reuse and waste mitigation. However, primary batteries remain common in cost-sensitive, low-maintenance applications, especially in IoT and IIoT devices—where billions of deployments raise sustainability concerns.
Batteries are either "wet" (liquid electrolyte) or "dry" (solid-state electrolyte). Dry cells last longer and fail less catastrophically, as the solid-state electrolyte resists thermal degradation and shorting. Batteries are also categorized as primary (single-use) or secondary (rechargeable) storage. Secondary batteries dominate energy storage solutions for reuse and waste mitigation. However, primary batteries remain common in cost-sensitive, low-maintenance applications, especially in IoT and IIoT devices—where billions of deployments raise sustainability concerns.
Do Not Underestimate Rechargeable Batteries (Pun Intended!)
Batteries are often viewed as simple 2-terminal DC sources, but in reality, they have complex characteristics. Secondary batteries especially require consideration of parameters such as capacity, impedance, cycle life, and safety. Some of these items are listed/defined in the table below.
| TERM/FIGURE OF MERIT | DEFINITION | IMPACT |
|---|---|---|
| STATE OF CHARGE (SOC) | Battery charge level relative to capacity (based on open-circuit terminal voltage), 0–100% |
|
| C-RATE | Battery charge or discharge rate, typically a min/max spec given by battery’s spec sheet and expressed as a ratio of the battery’s capacity (e.g., a 2.0 C max discharge rating for a 40 mAh-rated cell means the max discharge rate is 80 mA) |
|
| FAST CHARGE RATE | Current limit (typically set by the battery management system [BMS]) for the constant current portion of the charge cycle |
|
| DEPTH OF DISCHARGE (DOD) | Battery discharge level relative to capacity, 0–100% (opposite of SOC) |
|
| CYCLES | Number of charge/discharge cycles supported before battery is considered out of spec (minimum capacity) |
|
| EQUIVALENT SERIES RESISTANCE (ESR) | Intrinsic, internal resistance (typically ac or frequency-dependent resistance) of cell as measured at terminals |
|
| CONSTANT-VOLTAGE CHARGING | The BMS controller applies a constant voltage to the battery, while the cell organically draws current based on charge transfer |
|
| CONSTANT-CURRENT CHARGING | BMS controller applies a constant current to the battery, while the cell charges to end-of-charge target potential |
|
| CELL BALANCING | Battery packs (even dual-cell supercaps) may require cell terminal voltages to be within certain range of adjacent cells, even if there is variability in part-to-part capacity |
|
Table 1: Common Terms Related to Secondary Batteries & Typical Usage
Battery behavior is chemistry-specific. Charge/discharge curves, voltages, current thresholds, and thermal characteristics differ across chemistries like Li-Ion, LiFePO4, NiMH, and SLA. A Battery Management System (BMS) or Power Management IC (PMIC) regulates these states—e.g., transitioning from constant-current to constant-voltage mode during charging.
Many batteries require initial conditioning cycles before reaching full capacity. Designers often miscalculate battery life by ignoring environmental, chemical, and aging variables. Simple average current divided by rated capacity is not sufficient. Temperature variations and charge inefficiencies introduce large margins of error.
Many batteries require initial conditioning cycles before reaching full capacity. Designers often miscalculate battery life by ignoring environmental, chemical, and aging variables. Simple average current divided by rated capacity is not sufficient. Temperature variations and charge inefficiencies introduce large margins of error.
Scales of Energy Storage
Energy storage design must account for both quantity and delivery speed of stored energy. Faster response times often correlate with smaller energy quantities and higher costs. Data centers illustrate layered storage systems: fast-response devices handle immediate needs, while long-term systems provide sustained backup. Cost ($/Wh) inversely relates to energy capacity and response time.
Sometimes, energy storage policies are driven by regulation. For example, California Senate Bill 431 [5] mandates 72-hour backup for telecom base stations. While well-intentioned, such blanket mandates may not align with practical system-level constraints. Grid-tied inverters and home PV storage regulations vary by region. These policies often treat energy storage as a quick fix, without addressing application-specific requirements or minimizing unnecessary cost.
Sometimes, energy storage policies are driven by regulation. For example, California Senate Bill 431 [5] mandates 72-hour backup for telecom base stations. While well-intentioned, such blanket mandates may not align with practical system-level constraints. Grid-tied inverters and home PV storage regulations vary by region. These policies often treat energy storage as a quick fix, without addressing application-specific requirements or minimizing unnecessary cost.
Different Applications within Energy Storage
Energy storage has diverse use cases. Backup power is the most recognized, but storage is also used in filtering, biasing control ICs, and resonant applications. Peak shaving is increasingly important—localized storage (e.g., Li-Ion or supercaps) meets short-term spikes, allowing downsizing of base infrastructure.
Battery backup units (BBUs) are modular solutions that attach to system buses, delivering near-instant hold-up time and reducing internal capacitor size. This improves power supply density and reliability. In residential PV, energy storage’s economic value can surpass its technical role. Stored energy can be sold to the grid when prices are high. Originally used in off-grid systems, PV with storage now offsets utility bills. Dynamic pricing models reward strategic energy dispatch.
Battery backup units (BBUs) are modular solutions that attach to system buses, delivering near-instant hold-up time and reducing internal capacitor size. This improves power supply density and reliability. In residential PV, energy storage’s economic value can surpass its technical role. Stored energy can be sold to the grid when prices are high. Originally used in off-grid systems, PV with storage now offsets utility bills. Dynamic pricing models reward strategic energy dispatch.
Energy Storage for Sustainability
Energy storage is critical to most sustainability strategies. It supports renewable integration, stabilizes intermittent generation, and enables efficient energy redistribution.
Industries leading sustainability—such as electrified transportation and smart factories—depend on high-performance storage. Smart factories with robotics. Electrified vehicles, and high computation needs benefit from dense, responsive energy systems with peak-shaving capabilities. However, most current storage tech relies on hazardous or resource-limited materials. Manufacturing processes are often energy-intensive and involve harmful chemicals. A complete sustainability assessment must include embodied energy and environmental impact throughout the lifecycle.
Industries leading sustainability—such as electrified transportation and smart factories—depend on high-performance storage. Smart factories with robotics. Electrified vehicles, and high computation needs benefit from dense, responsive energy systems with peak-shaving capabilities. However, most current storage tech relies on hazardous or resource-limited materials. Manufacturing processes are often energy-intensive and involve harmful chemicals. A complete sustainability assessment must include embodied energy and environmental impact throughout the lifecycle.
References
[1] Wikipedia contributors, "Energy storage," Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Energy_storage&oldid=1086908512 (accessed May 11, 2022).
[2] H. Lund, G. Salgi, "The role of compressed air energy storage (CAES) in future sustainable energy systems". Energy Conversion and Management. (May 1, 2009). 50 (5): 1172–1179. doi:10.1016/j.enconman.2009.01.032.
[3] Qorvo ACT2801 Spec Sheet. [Online]. Available: https://www.qorvo.com/products/d/da006751.
[4] B. Zahnstecher, “Module PR-2: Data Center Structure Overview,” PowerRox Training Module. Last updated 1/25/19.
[5] M. McGuire, S. Glazer, "SB-431 Telecommunications service: backup electrical supply rules." CALIFORNIA LEGISLATURE—2019–2020 REGULAR SESSION. [Online]. Available: https://leginfo.legislature.ca.gov/faces/billTextClient.xhtml?bill_id=201920200SB431.
[2] H. Lund, G. Salgi, "The role of compressed air energy storage (CAES) in future sustainable energy systems". Energy Conversion and Management. (May 1, 2009). 50 (5): 1172–1179. doi:10.1016/j.enconman.2009.01.032.
[3] Qorvo ACT2801 Spec Sheet. [Online]. Available: https://www.qorvo.com/products/d/da006751.
[4] B. Zahnstecher, “Module PR-2: Data Center Structure Overview,” PowerRox Training Module. Last updated 1/25/19.
[5] M. McGuire, S. Glazer, "SB-431 Telecommunications service: backup electrical supply rules." CALIFORNIA LEGISLATURE—2019–2020 REGULAR SESSION. [Online]. Available: https://leginfo.legislature.ca.gov/faces/billTextClient.xhtml?bill_id=201920200SB431.
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