Experts say reciprocating gas engines can respond faster to load changes than any other prime mover, can compete with small gas turbine systems and also help fill in an efficient way the gap left by the lack of energy storage in most grid systems, as Power Engineering reports.
In southern Minnesota, where wind turbines and ethanol plants are commonplace, two communities have turned to reciprocating engine technology to meet their future power generation needs.
The Fairmont Energy Station, a 25-MW project completed in 2014, and the Owatonna Energy Station, a 38-MW project now under construction, feature highly flexible, quick-starting, low-maintenance reciprocating engines suited for today’s market, which places a premium on rapid-cycling capabilities.
Minnesota is home to about 100 wind power projects and ranks No. 7 in net wind power production in the U.S. That means Minnesota power producers must have reliable backup power to fill the sudden gaps created by growing supplies of intermittent wind power. The $30 million Fairmont plant is well suited for the job, capable of reaching full capacity in just eight minutes. That’s significantly faster than power plants using the latest gas turbine technology.
“We required a more flexible and fast responding power source that could make up the difference,” said Peter Reinarts, manager of Generation and Operation at Southern Minnesota Municipal Power Agency (SMMPA), which owns the Fairmont and Owatonna plants. SMMPA either purchases or operates more than 100 MW of wind power capacity. The four 16-cylinder gas-fired engines provided by Caterpillar for the Fairmont project work in sync with the association’s portfolio of wind power. “The generator sets can be promptly put on or offline to fill in the holes of the current wind generation,” Reinarts said. “These two assets aren’t at odds with each other, but instead work in a dynamic tandem.”
The intermittent nature of renewable generation, low-priced natural gas and advancements in engine technology and flexibility have given reciprocating engines new life in the U.S. as a competitive form of reliable generation. Reciprocating engines are becoming increasingly popular for utility-scale power projects.
Gas engine power plants have several advantages over plants equipped with gas turbines. Perhaps the biggest advantage is flexibility. Gas-engine power plants with multiple modular units are better at scaling their output across a wide range of incremental load without sacrificing efficiency.
For example, 12 generator sets capable of generating up to 10 MW each can deliver output ranging from just a few MW to more than 100 MW in just minutes. By keeping a few units online, the other units can be deployed individually to offset sudden losses of wind power and bring balance to the grid.
“These new generator sets start quickly, like a car engine,” said Bruce Erickson, vice president of Ziegler Power Systems, which supplied the four Cat G16CM34 generator sets for the Fairmont project. “If the grid needs extra power 10 minutes from now – these generator sets can easily adjust to that need.”
In addition to speed and flexibility, gas-fired reciprocating engines can operate at part load – 25 percent or lower – without sacrificing fuel efficiency. Also, reciprocating engines have much lower maintenance costs versus the cost to maintain a sophisticated gas turbine.
What’s more, the output of a modern-day reciprocating engine now exceeds 20 MW, up from 10 MW a decade ago. This has led to the development of more engine-based power plants exceeding a capacity of 200 MW worldwide. Earlier this year, Sky Global One, a 51-MW gas-fired plant about 70 miles west of Houston in the Rock Island community of Colorado County, began commercial operation.
The plant features six 8.6 MW Jenbacher J920 FleXtra gas engines from GE and will supply power to the 18,000 members of the San Bernard Electric Cooperative. The plant can go from zero to full power in just five minutes, a useful feature in a state that leads the nation in wind power production. In addition to providing power on short notice, the power plant – and others like it – uses very little water.
“Reciprocating engines use no water as part of their cycle,” said Andreas M. Lippert, engineering leader for GE’s Distributed Power business. “Our Sky Global power plant in Texas basically uses no more water than an ordinary household.” The J920 Flextra, a two-stage turbocharged engine, has a maximum output of 10.4 MW and can achieve electrical efficiencies of 49.1 percent at 50 hertz.
The 60 hertz version has a capacity of 9.35 MW and can achieve electrical efficiencies of 49.9 percent. The two-stage turbocharging means the J920 can achieve a fuel efficiency rating of more than 90 percent when used in a combined heat and power (CHP) plant that produces hot water, GE said.