## PNNL study outlines requirements for grid storage, reviews four electrochemical energy storage systems: vanadium redox flow, Na-beta, Li-ion and lead-carbon

##### 15 March 2011
 Classification of potential electrical storage for stationary applications. Credit: ACS, Yang et al. Click to enlarge.

There appears to be general agreement on the needs of electrical energy storage (EES) for the electrical grid, given the current trend toward increasing penetration of renewable energy, demands to improve power reliability and quality, along with implementation of smart grids, according to a new report by researchers at the US Department of Energy’s Pacific Northwest National Laboratory.

However, while a number of potential technologies for EES exist, and some have been applied or demonstrated, they face either challenges in meeting the performance and economic matrix for the stationary applications, or limits in environment, site selection, and so on, Yang et al. note.

In addition to outlining requirements for ESS and general guidelines for investigating technologies, the paper by Yang et al., published in the ACS journal Chemical Reviews, reviews in detail four stationary storage systems considered the most promising candidates for electrochemical energy storage: vanadium redox flow; sodium-beta alumina membrane; lithium-ion; and lead-carbon batteries.

In their study, Yang et al. note the potential of each technology and explain what advances must occur with each if they’re ultimately to be deployed. To be successful, systems will need to evolve—in some cases, considerably—to compete financially with the cost of natural gas production. And besides technical improvements, the systems will need to be built to last, using materials that are safe and durable so that batteries could operate more than 15 years and require very little maintenance over their lifetime.

The need for storage. EES is already an established, valuable approach for improving the reliability and overall use of the entire power system (generation, transmission, and distribution [T&D]). EES can be employed for services such as (1) frequency regulation and load following; (2) cold start services; (3) contingency reserves; and (4) energy services that shift generation from peak to off-peak periods. In addition, it can provide services to solve more localized power quality issues and reactive power support.

However, the authors note, to date EES (almost exclusively pumped hydroelectric storage) contributes to only about 2% of the installed generation capacity in the United States. (The percentages are higher in Europe and Japan, at 10% and 15%, respectively, largely because of favorable economics and government policies.) The future grid will face significant challenges by providing clean power from intermittent resources to a much more dynamic load. These challenges will not only be faced in the United States, but also internationally.

To smooth out the intermittency of renewable energy production, low-cost electrical energy storage (EES) will become necessary. EES has been considered as a key enabler of the smart grid or future grid, which is expected to integrate a significant amount of renewable energy resources while providing fuel (i.e., electricity) to hybrid and electrical vehicles, although the cost of implementing EES is of great concern.

—Yang et al.

Issues for potential ESS systems include:

• Performance requirements. These can vary based on the application market. For example, they note, to regulate frequency, the energy storage capacity may not need to be long-lasting—minutes can be sufficient—but it must have a long cycle life because the system is likely to encounter multiple daily discharge events. High discharge rates or high current densities are important, although the state-of-charge (SOC) of the storage system typically will not move over a wide range.

In comparison, energy management, such as load shifting, requires systems of up to MWh or even GWh levels that are capable of discharge durations up to a few hours or more at designated power. This type of application requires high round-trip energy efficiency and a long deep-cycle life, along with low operation and maintenance costs.

Unlike vehicle applications that have constraints on weight and volume, high-energy densities may not be strictly required for stationary applications. Also, the grid and renewable applications often require a quick response from the storage that can bring the grid up to full power in a matter of a second.

• Cost. This, the authors note, is probably the most important and fundamental issue of EES for a broad market penetration. Among the most important factors are capital cost and life-cycle cost.

• Reliability, durability, and safety. EES must have a long calendar life (e.g., >15 years) and a long cycle life (e.g., >4,000 deep cycles for energy applications) as well as minimum maintenance and safety requirements for utility assets and for a low life-cycle cost.

 Approximation of capital cost per cycle of different ESS technologies. Carrying charges, operation and maintenances (O&M), and replacement costs are not included. Credit: ACS, Yang et al. Click to enlarge.

ESS technologies can be broadly classified into two groups: those that store electricity directly in electrical charges (e.g., capacitors or supercapacitors that are highly efficient (close to 100%) but have a low energy density and discharge typically in a short period of time; and those that convert electrical energy to another form of energy that can be kinetic, potential, or chemical energy.

The most commonly used systems today work by converting electricity to kinetic (e.g., flywheel) or potential energy (e.g., pumped hydro (PHS) or compressed air (CAES) and then discharging that energy back to the grid when needed. These systems have limiting factors such as lack of portability. Electrochemical energy storage systems, on the other hand, can efficiently store electricity in chemicals and then release it upon demand.

Overall, however, most existing technologies cannot meet the economic requirements for most, if not all, utility markets even without accounting for carrying charges, operation/maintenance (O&M), and replacement costs. Particularly, ESSs for energy applications compared to power applications are probably facing even more challenges to meet economic targets for broad market penetration. With the exception of PHS and possible CAES, all others are higher than the target for broad market penetration. The high cost is related to unsatisfactory performance and the high cost of raw materials and fabrication as well as the scale of production. Thus, cost reduction must rely on advances in technology to improve reliability, cycle life, efficiency, use of less expensive materials, etc.

To help the research community obtain a better understanding of stationary EESs, a relatively new area, and accelerate development efforts, we offer a comprehensive review on electrochemical energy storage technologies or batteries. This includes principles of operation as well as the status and challenges in materials, chemistries, and technologies. While there is no intention to cover all potential technologies, this paper focuses on redox flow, Na-solid oxide electrolyte, and Li-ion and lead-carbon batteries to illustrate the needs of research and development.

—Yang et al.

 Schematic of an all-vanadium redox flow battery as an example of redox flow batteries (or regenerative fuel cells). Credit: ACS, Yang et al. Click to enlarge.

Vanadium redox flow battery. A redox flow battery (RFB) stores electrical energy typically in two soluble redox couples contained in external electrolyte tanks sized in accordance with application requirements. Liquid electrolytes are pumped from storage tanks to flow-through electrodes where chemical energy is converted to electrical energy (discharge) or vice versa (charge). The electrolytes flowing through the cathode and anode are often different and are referred to as anolyte and catholyte, respectively.

Unlike traditional batteries that store energy in electrode materials, the authors note, RFBs are more like regenerative fuel cells in which the chemical energy in the incoming fuels is converted into electricity at the electrodes. As such, the power and energy capacity of an RFB system can be designed separately.

All-vanadium RFBs (VRBs) exploit the capability of vanadium to exist in solution in four different oxidation states and use this property to make a flow battery that has only one active element in both anolyte and catholyte. As such, the cross-contamination of the anolyte and catholyte in VRBs is significantly diminished.

With all the stated advantages and the successful demonstration of systems up to MWh levels, all RFB technologies have, however, not seen broad market penetration. The current technologies are still expensive in capital cost and life-cycle cost...The high cost is attributed to the high cost of materials/components and performance parameters, including reliability, cycle/calendar life, energy efficiency, system energy capacity, etc.

—Yang et al.

 Single-cell and tubular design of a Na-beta battery. Credit: ACS, Yang et al. Click to enlarge.

Sodium-beta alumina membrane battery. Sodium-beta alumina batteries (SBBs) reversibly charge and discharge electricity via sodium ion transport across a β"-Al2O3 solid electrolyte (or BASE) that is doped with Li+ or Mg2+. To minimize electrical resistance and achieve satisfactory electrochemical activities, SBBs typically operate at moderate temperatures (300-350 °C). The anode is metallic sodium in a molten state during battery operation. The cathode can be either molten S/Na2Sx, which is known as a sodium-sulfur (Na-S) battery, or solid metal halides or Zeolite Battery Research Africa (ZEBRA) batteries. The Na-beta batteries are commonly built in tubular designs.

Researchers say the battery’s high energy density and rapid rate of charge and discharge make it a candidate for powering electric vehicles and for other applications that require short, potent bursts of energy.

However, materials are expensive and there are safety concerns with the high operating temperature of the battery. PNNL researchers say modifying the shape of the battery can improve efficiency, lower the operating temperature and cost. PNNL and EaglePicher Technologies, LLC, are studying these improvements as part of DOE’s ARPA-E program.

Lithium-ion battery. Li-ion batteries store electrical energy in various compounds, comprising layers of different elements, such as lithium, manganese and cobalt. High energy and power capacity over other technologies has made Li-ion batteries the most promising option for transportation applications like electric vehicles.

Researchers say bringing down materials cost and improving safety could vastly improve Li-ion batteries and could help accelerate the penetration of electric vehicles, which themselves could serve as back-up storage on the grid.

In a traditional Li-ion battery cell, positively charged lithium ions migrate through a liquid electrolyte, while electrons flow through an external circuit, both moving back and forth from one side to the other. This movement creates and stores energy. Li-ion batteries have been a success for small, mobile electronics such as cell phones and laptop computers, but making them larger is difficult because they are expensive, prone to overheating and can lend themselves to electrical shorting. Scientists say while substantial progress has been made over past years to improve the technology, more work must be done to extend life, improve safety and reduce materials cost for the stationary applications.

Lead-carbon battery. Lead-carbon batteries are an evolving technology born from the traditional lead-acid battery, commonly used for traditional automobiles and back-up generators. Scientists have found by adding a bit of carbon to traditional lead-acid batteries they can significantly increase the lifespan of the battery. Researchers say lead-carbon batteries could serve as a viable back-up source for wind and solar power because of their concentrated power.

During discharge in a traditional lead-acid battery, sulfuric acid reacts with the lead anode and cathode to create lead sulfate. The process reverses during charge. This conversion produces a short, powerful burst of energy, such as needed to jump start a vehicle. But over time, a lead-acid battery can lose its charge due to the gradual crystallization and buildup of lead sulfate within the battery's core. The corrosive acid also can eat away at a battery’s core.

Adding carbon to the battery seems to minimize or prevent this crystallization from occurring, improving the cycle life and overall lifespan of the battery. Researchers say this technology has potential for storing renewable energy but that more field work is needed to understand the limitations—and to find ways to bring down the cost. The capital cost of the technology remains at $500 per-kilowatt hour and they believe it needs to be reduced to between$150-\$200 per-kilowatt hour to be viable.

While there has been an increased interest and attempts to improve stationary storage, investment in the area is still limited in comparison with that of Li-ion batteries for vehicle applications. Currently, there are only a few government-funded programs worldwide with limited funding for developing electricity storage technologies for stationary applications. There is a general public and political lack of awareness of the need for new technologies for these applications. Even renewable energy industries are reluctant to lend support because of concerns about adding extra cost to renewable power systems as they struggle to reduce system cost. Furthermore, wide deployment of stationary storage will not happen without changes in regulations. Government support, such as tax incentives, can also be critical to the market entrance at early stages.

—Yang et al.

The Department of Energy’s Offices of Electricity Delivery and Energy Reliability, Energy Efficiency and Renewable Energy, and Advanced Research Projects Agency-Energy (ARPA-E), as well as PNNL’s Laboratory Directed Research and Development fund, supported this work.

Resources

• Zhenguo Yang, Jianlu Zhang, Michael Kintner-Meyer, Xiaochuan Lu, Daiwon Choi, John Lemmon, Jun Liu (2011) Electrochemical Energy Storage for Green Grid, March 4, 2011, Chem. Reviews) doi: 10.1021/cr100290v