By David Wagman, Chief Editor
More than 145,000 MW of new variable generation resources could be added to the North American bulk power system in the next decade, according to the North American Electric Reliability Corp.’s (NERC’s) 2008 Long-Term Reliability Assessment. Even if only half this capacity is built, NERC says it still will result in a 350 percent increase in variable resources over 2008. Driven in large part by new policies and environmental priorities, this growth in variable resource—read renewable energy—will mark what NERC says would be one of the largest new resource integration efforts in the electric industry’s history.
With variability comes uncertainty, however. A new NERC report (“Accommodating High Levels of Variable Generation,” April 2009) says that variable resources differ from conventional and fossil-fired resources in a fundamental way: their fuel source (primarily wind and sunlight, but also including run-of-river hydroelectric) cannot presently be controlled or stored. Unlike coal or natural gas, which can be sent to plants and stockpiled for use when needed, variable fuels must be used when and where they are available.
Adding to the complexity, variable resource availability may not always match electricity demand, either in terms of time of use/availability or geographic location. For example, wind resources tend to be best during overnight hours when demand is relatively low. What’s more, less than 10 percent of the U.S. population lives in the top 10 states for wind potential. And variable resource output is often marked by steep ramps as opposed to the more controlled, gradual ramp up or down generally experienced with electricity demand and traditional generation. Managing steep ramps can challenge system operators.
“The availability of the sun or wind can change quickly and cause a sudden drop in generation,” said Kurt Westermann, vice president and director of renewable energy for Black & Veatch. That variability can strain transmission and thermal generation sources.
More accurate and timely resource forecasting combined with advanced area control technologies and quick response reserve generation resources will be needed to optimize grid operation, he said.
Gas and diesel engines represent one form of quick response reserve generation. These machines are receiving growing attention from resource planners.
“With increasing amounts of renewable energy sources in a given control area, the existing amount of spinning reserves plays a more important role in determining if gas or diesel engines are needed to back up these variable generation resources,” Westermann said. After all, if wind displaces conventional thermal generation, a control area’s overall inertia may decline, since variable resources such as wind and solar lack this necessary quality. Quick-start gas and diesel engines (or even a combined cycle unit backed off from its peak load potential) can meet the gap.
“We believe that modern, high-efficiency reciprocating engines or gas turbines offer the optimum solution for enhancing stability as renewable source generators enter service,” said Akira Tsunoda, chief engineer, Strategic Planning Group at Mitsubishi Power Systems. “We also believe the opportunity exists to support these requirements using distributed generation in the form of smaller plants that can be integrated into the existing distribution and feeder system. We feel this approach will provide stabilization of system fluctuations, reduce voltage reliability ancillary support costs and afford faster grid restoration following a catastrophic service interruption.”
Engine sets in the range of 8 MW have proven to be the most popular size, said Kiah Harris, principal with Burns & McDonnell. He counted more than 40 units placed in service in the last two years to help balance variable resource load. His company is currently performing engneering studies for another 28 projects.
“Gas reciprocating engines are a great opportunity for that,” said Joel Puncochar, product marketing manager for Cummins Power Generation energy solutions business. “They are relatively efficient and can be pretty clean.” Compared to a gas turbine, the engines can up and running within a few minutes.
Cummins offers a range of 60 Hz power outputs through its QSV91 gensets, from 1,250 kW to 2 MW. Emissions are as low as 0.5 grams per horsepower hour. “They operate very well in parallel to the grid,” Puncochar said. Efficiency in the 2 MW machine is on the order of 40 percent.
Wärtsila North America has seen “quite a bit of success in U.S. markets” although the current recession means “all bets are off for 2009”, said Wayne Elmore, business division manager. The company markets 8.5 MW reciprocating engines that can be linked in tandem to provide fast response capacity to support variable renewable resources, peak demand and other ancillary service markets. “We compete well with gas engines where gas sets the marginal price,” he said. That includes markets such as Texas, California, Arizona the mountain west and southwest. “Wärtisla plants can operate at 40 percent load very efficiently” with the remaining capacity available as spinning load,” he said.
Last summer a 75 MW power plant using nine Wärtsila engines came online in western Kansas, dispatched primarily as an economic resource. Although the plant lacks a full year of operating experience it may be used to help support growing wind energy supplies in the region, Elmore said.
In Colorado, the Plains End power station has been operating since 2002, providing peak power and support for wind power resources within Xcel Energy’s market area, which has around a 15 percent windmill penetration. The first stage of Plains End included 111 MW of reciprocating engine capacity. That was followed last year with another 116 MW. The plant, owned by Congentrix and dispatched by Xcel, is used as a windmill integration resource, among other ancillary services, said Elmore.
“Plains End can go from a warm standby to full power in 10 minutes,” Elmore said. And the nature of a reciprocating engine means it experiences no power derate due to altitude, an important consideration for the plant, which sits at 6,100 feet in the foothills near Denver.
The South Texas Electric Cooperative is currently building a 203 MW project using Wartisla engines. Located near San Antonio, Elmore said the plant likely will be dispatched to help support wind resources. The first 100 MW are expected to come online this fall with the remainder becoming commercially available next summer.
Mitsubishi has two reciprocating engines ready to support the growing renewable energy support market. The first is a recently introduced 5.5 MW/60 Hz natural gas fired engine. The second is a 6 MW/60 Hz diesel fueled engine. Both offer rapid starting and ramping capabilities, going from a start command to full load in less than seven minutes, said Tsunoda. With up to four engines paired together at distributed generation sites Mitsubishi said it can provide considerable grid support while minimizing our emissions impact on the environment. As for O&M issues, Tsunoda said that with a multiple engine site, one engine can be rotated out of service for routine maintenance while continuing to generate power (and revenue) with the remaining units.
In addition to the rapid ramp up and ramp down capability, the gas engines offer the ability to shut down without penalty of long cool down periods, or the cost of “spinning” at inefficient heat rates between peaking cycles.
In general, no straight-line link exists between new variable resources and firm power from resources such as gas and diesel engines. “It’s difficult to see a direct link, but it’s strongly there,” said Clive Nickolay, global sales director for Caterpillar Power Generation Systems. The focus on renewable energy contained within the federal economic stimulus package likely means “significant demand” to add gas and diesel engines will result, he said. Overall demand in the coming years could run to “gigawatts” of reserve capacity, he said.
Caterpillar provided nine natural gas-fueled generator sets for Montana-based Basin Creek Power Services to provide electricity under a 20-year capacity and energy sales agreement with Northwestern Energy. The power plant consists of nine Caterpillar G16CM34 generator sets operating on natural gas rated at 5.75 MW, for a total plant output of 51.8MW. Caterpillar provided project management, plant systems design and engineering and full operation and maintenance services to the plant. The Basin Creek plant will supply reserve capacity to the Montana electric power grid and will also help support new wind energy resources being implemented in the state.
Caterpillar has two reciprocating engine products for the market, a 500 revolution per minute (RPM) engine and a 1,000 RPM engine. The choice of engine depends largely on market economics. “Shorter hours favor the higher speed machine due to the comparison of capital costs versus running costs,” he said. In terms of load variability, the ability to generate for VAR support and voltage support, the machines are virtually identical.
“We see it driving more opportunities, not just during peak times of the day but on hot days, for example, when the wind doesn’t produce as much energy,” he said.
Indeed, the NERC report said the California Independent System Operator (CAISO) tracked aggregate wind generation output over a 10-day heat wave in July 2006. CAISO found that aggregate wind generation output during the peak demand hours of each day typically ranged from 5 percent to 10 percent of nameplate capacity.
The recent NERC report points out that many different sources of system flexibility exist including ramping the variable generation, regulating and contingency reserves, reactive power reserves, quick start capability, low minimum generating levels and the ability to frequently cycle the resources’ output. Additional sources of system flexibility include operating structured markets, shorter scheduling intervals, demand-side management, reservoir hydro systems, gas storage and energy storage.
Based upon forecast growth in renewable sources, Mitsubishi believes the need for firming support can grow to as much as 400 MW a year.