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Comment PDF Heat Transfer

Transporting Energy Through Time

By Chemical Engineering |

Modernizing the world’s power grids represents a key infrastructure goal for economic growth. Large-scale, energy-storage technologies offer the opportunity to optimize the efficiency of the next-generation power grid and extend its capabilities. Renewed attention on energy storage systems for the grid has given rise to a host of emerging technologies aimed at more effectively transmitting electricity not only across space, but also through time.

Several factors have fostered development of new electrochemical and mechanical strategies for large-scale energy storage systems that can collect generated electricity and store it for discharging to the grid when needed. Among the major drivers are the high capital costs associated with managing peak power demand, as well as the increase in grid modernization initiatives. Meanwhile, deployment of wind and solar power are increasing, making the grid more sensitive to the variability inherent in nature, and thus strengthening the need for energy storage systems.

Currently, 99% of existing grid-energy storage is achieved through “pumped hydro,” a term used to describe the strategy of pumping water to higher elevations at times of low demand, and releasing the water through turbines to produce electricity when demand is higher. Pumped hydro has been relatively effective for years, but its use has specific site requirements, and plants are highly capital-intensive and may have environmental drawbacks.

These drawbacks, coupled with grid modernization and renewable energy growth, have spurred considerable effort by engineers in many disciplines to commercialize newer storage technologies — such as flow batteries, lithium-ion batteries, compressed-air energy-storage systems and flywheels — that are more broadly applicable and offer higher performance for specific applications. Meanwhile, questions remain about which storage technologies can be deployed cost-effectively, how best to match technologies to a variety of grid applications, and about how the economic and regulatory environment can be adjusted to allow storage technologies to fulfill their potentials (for more, see the online version of this article at www.chemengonline.com).

 Figure 1. Integration of wind power is a major application for grid energy-storage technologies, such as the planned Li-ion battery facility shown here (image courtesy of A123 Systems)
 

Grid-storage needs are varied

Energy storage systems have the potential to provide a range of benefits to the entire electrical system, from power generation to transmission and distribution, an ultimately to the end-user. According to U.S. Dept. of Energy (DOE; Washington, D.C.; www.energy.gov) and independent analysts, the size of the grid-energy storage market may be over $1.5 billion today, and has been forecasted to grow to more than $35 billion by 2020.

The evolution of the modern power grid, as well as the degree to which renewable energy technologies are deployed, is highly intertwined with the development of economical grid-energy storage systems. Among the most frequently discussed applications of energy storage involving power generation is off- to on-peak shifting, where storage devices would be charged at the site of intermittent renewable energy sources, and discharged into the grid during on-peak periods. This would enable increased penetration of wind and solar power into the power grid. Another key generation-related application is in grid frequency regulation, where short-term fluctuations in energy supply and demand are smoothed by storing energy when supply exceeds demand, and discharging when the reverse is true. This allows traditional fossil-fuel power generators to operate at more consistent levels, where they are most efficient, and produce lower emissions. Other potential applications of energy storage include energy arbitrage, support of electricity transmission and distribution systems, and backup power. With the large number of applications possible for maximizing grid reliability and asset utilization, it is likely that multiple energy-storage technologies that meet cost, performance and durability requirements will find significant uses.

The needs of a particular application can often be characterized by the power and energy matrix, an assessment of the level of power required over a period of time. A significant engineering challenge for energy storage technology developers is broadening the capabilities of a technology to handle a wide range of energy versus power demands. Extending the versatility, while reducing costs for energy storage technologies, represent the main goals for developers.

Until costs for deploying energy storage technologies come down, technologies that can perform well for several applications may have the inside track on commercial uptake. “Energy-storage technologies will play a critical buffering role in the next-generation power grid,” but costs remain high, says Chris Kuhl, an applications engineer at ZBB Energy Corp. (Menomonee Falls, Wisc.; www.zbbenergy.com). “The industry still has a long way to go to get down to $500 and $250 per kWh installed,” the cost points put forth by engineers at Sandia National Laboratory and DOE as the target for economic competitiveness of commercially sited and grid-scale (utility-owned) energy storage assets, respectively.

New compressed air approach

After pumped hydro, the next most common energy-storage technology is compressed-air energy-storage (CAES). Classical CAES uses electric motors to pump air into underground caverns adiabatically. When electricity is needed, air is released, driving a turbine to generate power. Disadvantages of classical CAES include its requirement for specific geologic sites, and its heat losses during the compression and expansion.

A new CAES technology developed by SustainX Inc. (Seabrook, N.H.; www.sustainx.com) uses isothermal compression and expansion to increase the efficiency of the operation. SustainX overcomes the difficulties of classical CAES by using a reciprocating engine, as opposed to a turbomachine, and sealed-pipe storage tanks, rather than underground caverns or aquifers, for the compressed air. An electric motor compresses air isothermally using a mechanical crankshaft to store air at 3,000 psi. When electricity is needed, the compressed air is expanded isothermally, running the same crankshaft, which in turn, drives a generator.

The company has engineered a proprietary method to control the temperature of the air during compression and expansion, so that its temperature never deviates by more than 10 to 15ºF. The result is a highly efficient cycle that minimizes heat loss. To achieve the isothermal compression and expansion cycle, SustainX engineers had to devise an effective method to transfer heat to and from the air. To accomplish this task, the company developed a system to efficiently aerosolize water. The high surface area of the droplets, and their proximity to air molecules, enables efficient heat transfer during compression and expansion. Company cofounder Dax Kepshire says the company’s aerosolization method consumes little energy, helping to make the technology economically viable.

SustainX has demonstrated a 40-kW CAES system at its New Hampshire facility, and plans to build (with power company AES) a megawatt-scale system next year. SustainX is pursuing grid applications that require electricity for durations of 1–6 h, although Kepshire says the power and energy are independently scalable, allowing the technology to potentially be applied elsewhere. For now, SustainX sees its technology as a more efficient solution than the combustion-engine “peakers” that are commonly used at power plants for balancing supply and demand. 

Flow batteries advance

Flow batteries employ pumps to move electrolyte solutions past electrode plates where reduction-oxidation reactions occur. Several flow-battery electrochemistries are under investigation in an effort to capitalize on their advantages — including the ability to store electricity in a modular design, fast response rates and the ability to vary durations of discharge.

An example in this area is ZBB Energy, which has developed a zinc-bromine flow battery that is currently in commercial production. The electrochemical cell consists of a zinc anode and a bromine cathode, separated by a microporous membrane. Aqueous electrolyte solutions of zinc and bromide ions are circulated through the cell stacks. The battery design employs a replaceable cell stack of high-density polypropylene spacers, and 60 bipolar electrodes. As the battery charges, zinc metal is plated onto the anode in the cell stacks, and bromide is converted to bromine, which is stored in a phase-separated manner.

ZBB’s technology targets applications that require power discharge in the 2–8 h range. “The ZBB flow battery is ideal for time-shifting renewable energy,” says ZBB’s Chris Kuhl. “It is also useful as a ramp-control device to stabilize power demand peaks,” he says. Among the advantages of the ZBB flow battery is its ability to operate at full output over a wide temperature range (–30 to 50°C). The electrolytes never need replacing, and although the high-surface-area anode will wear out, it can be changed out with the stack after five years.

In a DOE-funded project, flow batteries using a different chemistry are under development by United Technologies Corp. (UTC; East Hartford, Conn.; www.utc.com), along with partners including the University of Texas (Austin; www.utexas.edu). The team of scientists has designed a unique flow-battery cell that offers power densities an order of magnitude higher than those published so far for flow batteries. “The electrochemical cell stacks dominate the cost of batteries,” says UTC engineer Craig Walker, so if we can increase power output, the batteries can produce the same power at smaller cell sizes, which lowers costs.

The flow battery, which is targeted at improving power quality from wind farms and time-shifting off-peak power, has been demonstrated at small scale, and could be described as a hybrid between a fuel cell and a conventional battery. Liquid reactants flow over electrodes separated by an ion-exchange membrane. Leveraging UTC’s expertise in fuel cells, the cell stack was designed for higher performance and optimized efficiency. The team plans to complete a 20-kW prototype of the battery in late 2012.

Primus Power (Hayward, Calif.; www.primuspower.com) is also developing a flow battery — one based on zinc-halogen chemistry. Key differentiators for the Primus technology include high-conductivity electrodes, which increase power capacity, and a carefully designed flow architecture to reduce capital cost. Primus cells are housed in 20-kW, 60-kWh units that can be linked together for utility-scale installations. Primus plans a demonstration-scale system in 2012, and a commercial-scale project in 2013.

 Figure 2. Flow battery chemistries, like the one shown here from ZBB Energy, use pumps to circulate electrolyte solutions within cell stacks (image courtesy of ZBB Energy)

Li-ion for storage

Li-ion batteries are common in portable electronic devices and newer electric vehicles, but they can also be a viable grid-energy storage technology. A123 Systems Inc. (Waltham, Mass.; www.a123systems.com) has been a player in the electric vehicle area, but is also utilizing its technology for grid-power applications, such as smoothing out the supply and demand variations as renewable energy is integrated into the power grid.

“With some grid-energy storage solutions, there are few [non-grid] applications, so it’s more difficult to reduce costs,” says Andy Chu, vice president for marketing and communications for A123 Systems. “With Li-ion technology, we can leverage the technology, manufacturing and logistics infrastructure that has been developed for Li-ion batteries in devices and vehicles, and apply that to power-grid applications.” A123’s Chu envisions costs coming down significantly compared to flow batteries because of the existing infrastructure.

Technology licensed and developed by A123 relies on nanoparticles of a doped form of lithium-iron-phosphate for its cathode material. The cathode structure allows lithium ions to move more easily into and out of the material than with other Li-ion materials, which reduces physical expansion and lengthens the lifetime of the battery and the number of charge-discharge cycles the battery can handle.

Among the advantages of A123 batteries used in grid-storage applications are the ability to site facilities anywhere, explains Chu. Also, Li-ion batteries don’t require water or emit pollutants, so there are fewer requirements for permitting and siting.

As part of a DOE demonstration program, A123 is building an 8-MW Li-ion battery for storing energy at a wind power site in Tehachapi, Calif. The project’s purpose is to demonstrate the flexibility of Li-ion batteries for grid storage, and to “test whether the same battery system could be appropriate for multiple grid storage functions in which several hours of power are needed,” says Chu.

Uptick may be coming in 2012

A recent report by the Electric Power Research Institute (EPRI; Palo Alto, Calif.; www.epri.com) remarked on the “large anticipated need for energy storage solutions,” and said that while relatively few grid-integrated storage demonstrations are operating currently, 2011 and 2012 will see a significant uptick, especially in the U.S., where projects funded by the economic stimulus bill (American Recovery and Reinvestment Act of 2009) come online.

In a report on deployed and announced energy storage products released last month by Pike Research (Boulder, Colo.; www.pikeresearch.com), energy analyst Anissa Dehamna found 600 projects, of which 323 are currently using technologies other than pumped hydro.

Also look out for the energy storage summit, coming in March 2012. Hosted by Messe Düsseldorf and Solarpaxis AG, the conference will be held in Düsseldorf on March 13–14.

Regulatory factors

In an analysis of energy storage applications published earlier this year, the Electric Power Research Institute (EPRI; Palo Alto, Calif.; www.epri.com) estimated the total market for energy storage technologies could be as large as 14 GW of capacity, if the technologies could be installed for around $700–750 per kWh, and if mechanisms were in place for owners and operators of energy storage systems to monetize their benefits.

Achieving lower costs and an improved regulatory environment will require not only advances by technology developers, but also adjustments to existing market regulations by policy makers to better allow the operators of energy storage facilities to help them integrate their systems into the evolving power-grid infrastructure.

“The value of energy storage remains difficult to commoditize,” Pike Research Inc.’s Dehamna says, and the rules for electricity markets haven’t caught up to the capabilities of energy storage technologies. While, in many ways, the existing economic and regulatory framework is not configured in a way that promotes rapid and widespread adoption of storage technologies, things may be changing.

On October 20 of this year, the Federal Energy Regulatory Commission (FERC; Washington, D.C.; www.ferc.gov) proposed a new method for compensating facilities that provide frequency regulation services for grid energy. The ruling, casually known as “pay for performance,” allows new energy storage technologies to compete on a level playing field with other types of resources that respond to demand on the grid. FERC Commissioner John Norris said in a statement that the commission’s aim with the rule was to “develop a mechanism to compensate resources providing frequency regulation in a way that rewards them for quickly responding to the needs of the system.” The ruling is expected to help new technologies that provide this quick response, while promoting efficiency in the grid.

Brad Roberts, executive director of the Energy Storage Assn. (ESA; Washington, D.C.; www.electricitystorage.org) agrees in general that the current energy regulations need to catch up with technology development. He suggests considering energy storage equipment as a separate asset class from other power generation, transmission and distribution equipment. In addition, Roberts advocates exploring tax credits for depolyment of energy storage technologies patterned after those for renewable energy facilities. 

Serving multiple applications

In EPRI’s recent analysis, Mark McGranaghan, vice president of power delivery and utilization, commented that when the potential benefits of energy storage technologies are aggregated, the cost of installing them is justified in many places. “Storage systems dedicated to a single application can be valuable, but the true value of storage appears when the same system serves multiple applications,” he says.

Sandia National Laboratories (Albuquerque, N.M.; www.sandia.gov) scientist Georgianne Huff, coauthor of a recent report assessing various grid-energy storage technologies, says the best energy storage technology depends squarely on the particulars of the application. For example, the ability to handle more than one storage application cost-effectively could make overall capital expense of Li-ion batteries more attractive than a storage system for a single application.

Other potential technology

In the area of flow batteries, a consortium of Fraunhofer Institutes (Munich, Germany; www.fraunhofer.de) is scaling-up vanadium-based flow batteries, among other energy-related projects. Vanadium flow batteries have two vanadium-based electrolytes separated by a proton exchange membrane, and the rechargeable batteries store electrical potential in the different oxidation states of vanadium ions. Earlier this year, Fraunhofer scientists demonstrated a system capable of generating 2 kW of electricity, and are working on a 20-kW version to be completed next year. Advantages of the vanadium batteries include their robustness and durability.

• In the area of flywheels, the future of Beacon Power Corp. (Tyngsboro, Mass.; www.beaconpower.com) was muddied somewhat because the company filed for Chapter 11 Bankruptcy at the end of October. Beacon has used DOE money through the Advanced Research Projects Agency-Energy (ARPA-e) for the development of a long-duration flywheel capable of producing 100 kWh of extractable energy. Beacon currently markets a 25 kWh flywheel that is sealed in a vacuum chamber and spins at speeds between 8,000 and 16,000 rpm. The larger-size flywheel had been slated to be completed next year.

• Batteries based on sodium-sulfur electrochemistry are experiencing intense work due to its advantages in cost and environmental friendliness. Japanese company NGK Insulator Ltd. (Nagoya, Japan; www.ngk.co.jp) is a leader here, and a host of research, at places such as the DoE’s Pacific Northwest National Laboratory (PNNL; Richland, Wash.; www.pnl.gov), is ongoing. PNNL has had success in improving the performance of electrodes for sodium-ion recharcheable batteries.

Another grid-storage demonstration for Li-ion batteries is being set up in Dietikon, Switzerland, where ABB is installing a 1-MW Li-ion battery facility with partner EKZ to provide balancing of peak power loads and intermittent power supply. Like the A123 project in California, the ABB/EKZ installation will seek to evaluate the behavior of grid-linked battery storage.

• Metal-air batteries, which have been used in small, low-power devices such as hearing aids, may be developed into grid-scale uses. Nanoscale engineering is likely to play a large role in the future, as research groups across the globe explore nanostructured electrodes for batteries, and supercapacitors.

Scott Jenkins

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