Mitigating the Unpredictable and Rapid Changes of Renewable Generation

By Jeff Brunings, Director Strategic Marketing, Altairnano

Energy storage isn’t just about storage capacity. It’s about power. And a new breed of fast response, high-power energy storage systems, like those introduced by energy and power management company Altairnano, is helping solve some tough challenges.

Take the Renewable Portfolio Standards for example. Twenty-nine states, including the District of Columbia, have adopted aggressive mandates for integrating renewable generation. Illinois, Minnesota, Nevada, Ohio, and Oregon, each with relatively low levels of renewable generation, are all shooting to have 25% renewable integration with the grid by 2025.

Those are aggressive goals, especially considering that renewable generation currently represents a little more than two percent of total U.S. electricity generation.

As the penetration of wind and solar generation deepens, energy storage systems like the Altairnano Energy Storage System (ALTI-ESS) will be needed to help stabilize the electric grid and mitigate the unpredictability associated with renewable generation.

Altairnano is the first company to replace traditional graphite materials used in conventional lithium-ion batteries with a proprietary, nano-structured lithium titanate oxide (nLTO), an achievement which produces distinctive performance attributes, including high power, fast charge and discharge rates, and long life.

The ALTI-ESS improves power quality by releasing powerful multi-megawatt bursts of energy to the grid and quickly absorbing energy off the grid during generation curtailment. The applications range from mitigating the uncertainty associated with wind and photovoltaic generation, providing active and reactive power, to frequency regulation.

In the U.S today, the relatively small amount of non-dispatchable energy in a balancing area rarely has significant impact on system operations. This is because the natural variability of the load is still greater than the uncertainty of utility-connected renewable energy resources.

But this is changing. And it's changing fast.

Among federal and state legislatures and policy makers, including FERC and NERC, and industry stakeholders (utility operators, renewable developers, and Independent System Operators) there is growing concern that a dramatic increase in renewable generation will cause severe reliability issues. In fact, FERC recently announced a $500,000 study to determine whether the existing U.S. power grid can handle the addition of substantial amounts of new renewable resources by 2012. According to Joseph McClelland, director of FERC's Office of Electric Reliability, a primary goal of the study is to use the 60 hertz regulation metric to determine how much renewable generation could be reliably integrated to the grid without causing significant instability.

The study marks the first time a frequency tolerance metric will be used for evaluating the impact of renewable generation on the reliability of electric system. FERC will test various renewable generation scenarios and the impact these scenarios would likely have on interconnection systems. Results are expected to be released in late 2009.

The unpredictability associated with renewable generation, primarily caused by unanticipated weather conditions, such as clouds or sudden shifts in wind velocity, poses an important question. Does a "tipping point" exist and if so, when will this "tipping point" occur?

The tipping point concept is defined as the moment within a load balancing area when the combination of renewable generation uncertainty, percentage of renewable penetration, and traditional generation crosses a threshold, tips, and the potential for grid instability spikes dramatically. Conventional generation assets, such as gas and diesel turbines, will not respond fast or effectively enough to compensate for sudden, unanticipated shifts in output.

The exact timing of renewable integration's tipping point in the U.S. is debatable. But most industry experts agree the tipping point concept is dependent upon load balancing areas and is influenced by a number of factors, including the types of conventional generation assets available for managing grid stability.

For example, "electricity networks largely dependent upon coal and nuclear generation are less equipped to handle intermittent renewable energy," says Nadav Enbar, research manager for IDC Energy Insights' Distributed and Renewable Energy Strategies consulting division. "Those (networks) that rely more heavily on natural gas are better positioned to manage a higher threshold of renewable energy, because natural gas generators can ramp more quickly and adequately back up wind generation."

Other factors include the extent of weather variability within a load balancing area, total megawatt capacity, and number of generation units. (The fewer the generation units the more difficult it is to maintain grid regulation standards.)

The tipping point will vary widely among load balancing areas and across diverse geographies. Countries like Denmark, Spain, and Germany may already be approaching the upper limits of the threshold. With nearly 25% renewable integration, these countries have achieved massive integration, in part, by an ability to "lean on" a highly interconnected transmission system.

Other countries, including the U.S., may not be as fortunate. The FERC study will emphasize the impact of highly variable energy resources on each of the three North American interconnections, the eastern, western, and Electric Reliability Council of Texas (ERCOT.) Since these interconnections have very few transmission lines connecting them to one another, the impact of potential renewable scenarios will not be supported by a utility's ability to "lean on" neighboring systems.

However, it may be island grids and developing countries that are first to reach the tipping point threshold. Ireland, an island roughly the size of the state of Indiana, has already achieved nearly 20% renewable integration, primarily from wind. The country is experiencing significant challenges in mitigating the uncertainty associated with wind generation and reducing costly wear-and-tear on conventional gas and diesel turbines dedicated to managing grid stability. The Falkland Islands, New Zealand, and Central American countries are anticipating similar challenges and are actively seeking more efficient, effective, and economical solutions for combating grid instability.

"Island grids, like those in Hawaii, tend to be the most fragile and the least capable of handling intermittent generation," says Enbar. "That's largely because, by virtue of their islanded state, they operate on a closed grid and do not have access to resources that can be shared from extended grid geographies. As a result, island grids are among those leading the way in integrating energy storage into their systems because it is much more of a necessity."

The U.S. isn't far behind. In October 2007, the California Independent System Operator (CAISO) reported quick generating start units would be required to mitigate the uncertainty associated with wind and solar generation. CAISO recognizes the current California electric system requires +/- 350 MW of regulation on an hourly basis. It is anticipated by 2012 regulation capacity could increase 250 MW for "up regulation" and up to 500 MW for "down regulation" for a total of 750 MW.

How Unpredictable is Renewable Generation?
Impacts of renewable integration are already beginning to emerge. For example, in February 2008, a sudden drop in wind generation in Texas coupled with colder weather, rising customer load, and inaccurate forecasts led to an electric emergency and the immediate curtailing of 1,100 megawatts to interruptible customers (customers who are paid to reduce power use when emergencies occur.)

According to the Electric Reliability Council of Texas (ERCOT), the grid's frequency suddenly dropped when wind production fell from 1,700 megawatts before the event, to 300 megawatts at the onset of the emergency. The event narrowly escaped widespread power losses. Fast response storage, to some degree, could have helped mitigate the emergency by injecting power to the grid until conventional generation resources could be ramped.  

The uncertainty associated with renewable generation isn't always as pronounced as the Texas wind event, yet despite advanced forecasting tools wind is often unpredictable. Unanticipated shifts in wind speed and direction will cause variable outputs. Modern blade designs featuring pitch control and variable speed drive generators will not sufficiently solve the problem.

"Pitch control and variable speed drives can't help address the need to balance real power measured in watts," says Alex Rojas, a technical director of asset management at Quanta Technology, a leading utility infrastructure consulting firm. "The prerequisite for maintaining a stable system frequency is to have continual and instantaneous balance between the demand and generation of real power. The difference in the sum of all real power sources and the sum of all real power sinks is reflected in the increase or decrease of system frequency, which is set nominally at 60 Hz or 50 Hz. Energy storage systems, such as those based on advanced lithium-titanate batteries, have the unique capability to act as a sink or a source of real power."

Of all renewable generation resources, wind and photovoltaic solar are most likely to contribute to grid instability. According to NERC, regulating reserves and ramping capabilities are "critical" attributes necessary in dealing with the short-term uncertainty of generation availability and demand forecasts. In a report published in April 2009 NERC predicted "at higher levels of variable generation, the operation and characteristics of the [system] can be significantly altered." The study cites the potential for wind generation to increase ramping requirements of conventional generation as much as 45% to maintain grid stability. Additionally, the report suggested that compared to errors in demand forecasting, which are relatively small, errors in a 12-hour wind forecasts could be as high as 20%.

Photovoltaic (PV) solar generation experiences similar uncertainty. Solar intermittency is more than the diurnal nature of the sun. It's the unanticipated shifts during the day, which can cause sudden, intense changes in output. Utility-scale PV systems have experienced substantial ramps during partly cloudy days. According to the NERC study, PV systems may experience variations in output of +/- 50% in a 30 to 90 second timeframe and +/- 70% in a five to ten minute time frame. "Furthermore," according to the study, "ramps of this magnitude can be experienced many times in a single day during certain weather conditions." This phenomenon has been observed at some of the largest PV arrays deployed in the U.S.

Annual PV solar installations in the U.S. are predicted to increase from 280 MW in 2008 to 1,515 MW by 2013, representing a nearly 50% annual compounded growth rate. The extension of the Investment Tax Credit (ITC) program and the option to accept an upfront ITC cash grant in lieu of the credit program are expected to further accelerate utility-scale adoption.

To maintain operational reliability and efficiency, operators use forecasts to anticipate requirements necessary to support demand and generation availability. This helps operators control the output of dispatchable, or traditional generation resources to follow changes in demand. According to utility consulting agency KEMA, frequency regulation using coal and natural gas-fired generation results in increased fuel consumption between .5% and 1.5%.

This additional generation responds to sudden and unanticipated changes in demand, which require responses in the second-to-minute timeframes. Bringing up a coal plant for frequency regulation is difficult. Gas turbines can ramp in 20 minutes, but the carbon footprint is high and the maintenance soars with variable use.

Energy storage systems by Altairnano have supplemented the use of traditional generation capacity by responding within milliseconds to the Automatic Generation Control (AGC) signal, which is dispatched every four seconds to meet moment-to-moment fluctuations in the electric system.

How the ALTI-ESS Works
Based on advanced lithium-titanate technologies, the ALTI-ESS responds within milliseconds to these fluctuations by releasing or absorbing power from the electricity grid. This helps improve equipment and capacity utilization, strengthen operational efficiencies and reduce carbon emissions. Current solutions in commercial operation include a 1 MW/250 kWh system at the PJM Interconnection, which runs nearly continuous operation throughout the day for The AES Corporation.

Fast response, multi-megawatt energy storage systems support a number of grid stability applications. To ensure maximum return on investment, utility companies must evaluate applications for advanced energy storage systems that include secondary benefits, such as providing frequency regulation services, responding to demand fluctuations, and providing "shouldering" of power during times of reduced variable generation output.

"There's more to energy storage systems than storage capacity," says Energy Insight's Enbar. "Technologies with high power capabilities can be used for a variety of applications. For instance, utilizing energy storage for ancillary services such as frequency regulation has many inherent advantages compared to traditional methods requiring the throttling of thermal power plants up and down. The response times are much faster and less wear and tear is placed on the plant. This would allow thermal generation earmarked for frequency regulation to instead be used to supply electricity to the grid and in turn generate revenue."

The problem with conventional generation is that it can not dispatch in the time required to respond to sudden variations in renewable generation output. Ramp rates for diesel engines may require up to three minutes to achieve full power. Gas engines require up to seven minutes. Industrial gas turbines may take up to 20 minutes.

The ALTI-ESS achieves full power in milliseconds.

The ALTI-ESS features a Power Module and Power Control System Module. The Power Module has a 1 MW lithium titanate battery stack and a battery management system. Power Modules can be added for multiple MW configurations. The Power Control System Module features necessary electronics to convert from DC to AC and communications software required for receiving and responding to grid signals, including PLC, SCADA, and Data Service Unit.

The PCS is sized for a 1.2 MVA, four-quadrant operation and features an overload capacity of 1.8 MVA for five seconds. The input/output voltage is 480 Vac, three-phase. And the system is designed to operate without scheduled off-line maintenance for five years.

"Buyers often mistake the up-front cost as the most important metric when evaluating energy storage technologies," says Enbar. "But it's important to look at the holistic value picture that a storage technology brings to bear. How many cycles will it be able to sustain during its useful lifetime? How many applications can it satisfy? After its useful lifetime for an application, does it still have a value another application? Those are the kinds of questions that have to be answered in order to get a better sense of the true economic value of a particular energy storage product."

The ALTI-ESS also provides voltage support. The Power Control System Module is designed for all four quadrants of the real-reactive power space, providing the capability to provide both active and reactive power to the grid. This helps reduce the dependency on traditional systems for providing reactive power. Transient voltage events can be dampened by injecting both real and reactive power.  

Additionally, the ALTI-ESS is transportable, scalable to multi-megawatt configurations, achieves greater than 90% roundtrip efficiency and has an expected calendar life exceeding 20 years. A basic configuration is a 1 MW/250kWh system, but it's capable of supplying different power and energy requirements depending on system.

As states move toward achieving Renewable Portfolio Standards no one knows the increased costs utilities will incur for managing grid stability. No one really knows the full mega-watt potential for fast response energy storage systems, either. But, if California is any indication, with an anticipated increase of 750 MW total regulation capacity required by 2012, both the cost and opportunity are significant.