This article discusses integration of microturbines with energy storage systems and fuel cells for the development of new applications in distributed energy generation. Williams Energy Services integrated two Capstone Model 330 microturbines and one PowerBlock energy storage system to power an oil derrick near the Denver airport. A key microturbine component is its recuperator, which transfers heat from exhaust gas to air that is sent to the combustor. Preheating combustion air reduces the fuel consumption of the microturbine and increases its overall efficiency. One hybrid microturbine power plant now in commercial service was developed by Williams Distributed Power Services of Tulsa, Oklahoma. The Williams energy conversion unit (ECU) incorporates two Model 330-kilowatt microturbines developed by Capstone Turbine Corp. of Woodland Hills, California, and the PowerBlock energy storage device developed by Powercell Corp. of Burlington, MA. The ECU provides reliable 480-volt power when the grid power to the oil field is interrupted.
Despite their compactness, low emissions, and high reliability, relatively few microturbines have been sold so far. Most of the units have been purchased by utility and energy services companies studying retail market applications.
However, potential end users of microturbines are just beginning to become aware of them, so this market may grow significantly in the future, said Doug Herman, a mechanical engineer and manager of distributed resource technologies at the Electric Power Research Institute in Palo Alto, Calif.
These simpler versions of conventional gas turbines, about one-fifth their size, produce electricity at a cost of $600 to $1,200 per kilowatt, compared to the $300 to $500 per kilowatt for utility-scale gas turbines. However, microturbines may be closer to expanding their tiny niche with hybrid designs that integrate the pint-sized power plants with fuel cells or energy storage systems for distributed power generation.
Norbert König, a member of the group executive management of Siemens AG’s Power Generation Group, mentioned one of the combinations in his keynote address at the 45th annual ASME Turbo Expo, in Munich, Germany, in May. “Fuel cells will produce more power in the future,” König said. “Fuel cells coupled with microturbines may achieve 70 percent efficiency in lower end applications.” Most microturbines are typically single-shaft machines, with the compressor and turbine mounted on the same shaft as the electrical generator, which produces 25 to 300 kilowatts. Engineers equip the unit with a single rotating part to eliminate a gearbox and associated moving parts, thereby reducing maintenance.
The turbine and compressor wheels are welded to the rotating shaft, which supports the generator/alternator rotor. The shaft itself is mounted either on two oil-lubricated bearings or, in some cases, air bearings. Shaft speeds range from 60,000 to 100,000 rpm.
Microturbine alternator rotors are equipped with either two-pole or four-pole permanent magnets mounted on the rotor assembly. Engineers mount the stator into a common casing with the turbine and compressor.
Digital power controllers convert the high-frequency power produced by the generator into 50 to 60 hertz grid quality power. These electronic components also control all of the microturbines’ startup and operating functions, including full control of the turbine, signal conditioning, data logging, and diagnostics.
A key microturbine component is its recuperator, which transfers heat from exhaust gas to air that is sent to the combustor. Preheating combustion air reduces the fuel consumption of the microturbine and increases its overall efficiency. The relatively low inlet temperatures of microturbines, 1,600°F, and high air/fuel ratios in their combustor sections, enable the machines to keep their NOx emissions under 10 parts per million when burning natural gas. In addition, microturbines provide high operating efficiencies of 25 to 30 percent, high reliability, and perform as long as 50,000 hours without stopping for maintenance.
These are all valuable assets in distributed power generation applications where small-scale generation units are placed close to consumers, said Herman of EPRI. Utilities can use the relatively compact—about 12-cubic-foot—microturbines to increase service without building costly infrastructure, such as power lines and transformers. That’s an attractive alternative when facing the uncertainties of a deregulating energy market. Similarly, energy consumers can use microturbines to gain more control over their on-site power generation costs, power quality, and reliability, Herman explained.
Reliable Oil-Field Power
One hybrid microturbine power plant now in commercial service was developed by Williams Distributed Power Services of Tulsa, Okla.
Williams Distributed Power Services is a subsidiary of the Williams Co., an energy and communications firm based in Tulsa. Wilhams specializes in integrating energy and telecommunications systems to provide power and communications for oil and gas exploration, transportation, and refining facilities.
Williams believes that remote oil fields will be a major market for the hybrid power plant, which it calls the Energy Conversion Unit, or ECU. The first ECU was installed in December 1998, at a Colorado oil field owned by the city of Denver and operated by a local contractor. The energy system powers the electric motor of a pump derrick that extracts oil.
The Wilhams ECU incorporates two Model 330-kilowatt microturbines developed by Capstone Turbine Corp. of Woodland Hills, Calif., and the PowerBlock energy storage device developed by Powercell Corp. of Burlington, Mass. The ECU provides reliable 480-volt power when the grid power to the oil field is interrupted.
“Previously, the quality of electricity delivered by the grid fluctuated, causing spikes that hurt the induction motor on the pump derrick. Those problems disappeared with the installation of the ECU,” explained Hans Mertens, a civil engineer and business development director at Williams Distributed Power Services.
Mertens said that many exploratory oil patches like the Colorado field are 10 to 20 miles from the nearest major road. “When animals, lightning, or fire damage power lines to the oil field equipment, it takes the rural electric cooperative or utility a considerable amount of time to run a new power line to the field. This incurs costly downtime, as well as a typical cost of $20,000 to $30,000 per mile of line,” Mertens said.
The ECU is skid-mounted so it can be installed directly in the oil field. The Model 330 Capstone microturbine of the Energy Conversion Unit is equipped with fins to cool the generator section, eliminating the need for a dedicated cooling system.
The Capstone recuperator is housed in aircraft-grade stainless steel and preheats the air to about 600°F before sending it to the combustion chamber. Preheating to that temperature reduces fuel consumption by more than 50 percent.
When the expanding combustion gases drive through the blades, the compressor and generator can reach speeds of up to 96,000 revolutions per minute at full load. The generator, mounted on the same shaft as the turbine, produces 30 kW of electricity. Capstone uses patented air bearings to support its single shaft without contact, eliminating the need for lubrication.
Each microturbine’s turbogenerator is coupled with solid-state power electronics so it can be connected to a power grid. A digital power controller converts the high-frequency power into 50 or 60 Hz, grid quality power, and controls all turbine functions. The microturbine emits less than 9 parts per million of nitrogen oxides when burning natural gas, pressurized to 55 pounds per square inch gauge. Waste gas is removed from the recuperator through an exhaust outlet.
The Model 330 has a proven track record as a standalone unit. For example, a Capstone Model 330 microturbine has marked more than a full year of providing electricity to an office and service facility building owned by Southern Union Gas Co. in Galveston, Texas. The microturbine was installed in February 1999, and burns natural gas to provide up to 30 kW of electricity during the day to the Southern Union Co.’s office building, supplementing power from the main grid. At night, the Model 330 provides partial electrical load for security, heating, and air conditioning. The unit surpassed 10,000 hours of service in April, with availability exceeding 99 percent.
Eliminating Battery Equipment
The PowerBlock unit is housed in a 100-foot-long, 42-inch-square NEMA 4 steel container to serve indoors and outdoors. It holds zinc polybromide electrolyte, a brine solution, in two tanks, and horizontal plastic plates that form reactors. “This design eliminates equipment associated with battery systems, including HVAC, halon extinguishers, contaminant dikes, and levees needed to catch conventional lead acid battery spills,” explained John Slattery, vice president of Powercell.
A power control system, basically an inverter/converter device, manages both energy storage and energy supply to the local bus. Proprietary software embedded in this control module judges when, and how much, power is stored in the unit, and then delivered to correct power sags, surges, and harmonic disturbances.
The PowerBlock can deliver up to 100 kW of electricity for one hour. The manufacturer sees it as particularly applicable to situations where a loss of power for a second or less can damage equipment, services, or production runs, or possibly prove disastrous. These include semiconductor factories; textile, pulp, paper, or petrochemical plants; computer networks, and air traffic control towers.
The PowerBlock first proved itself as a partner of micro turbines in 1998-99, when the energy storage system was integrated with two 27-kW Capstone micro-turbines. This hybrid system served for 10 months at Williams Energy’s lipids pipeline facility in West Tulsa, Okla. “We learned lessons about working extreme heat and cold that we were able to apply to the Denver ECU installation,” said Slattery of Powercell.
The first commercial ECU was installed in a working wheat field near Denver International Airport in three days. The oil field is in a buffer zone of land separating the airport from residential areas, and provides a wheat crop as well as petroleum. “We integrated our power control system to run remotely off the Internet or telephone lines, and then simulated the loads the system would serve,” Slattery said.
Those loads are particularly challenging, because every 10 seconds, the upswing of the pump jack requires about 37 kW, but on the downswing, the jack generates 11 kW of electricity that are sent back to the PowerBlock.
“We redesigned the control system to accommodate this differential, as well as cold weather, which causes the oil to harden to the point where the pump jack requires 180 kW of power during startup until the oil warms up,” said Slattery.
Fuel for the energy system was right at hand—1,300-Btu gas tapped from the 8,760-foot-deep oil well. The fundamental question for Associated Electric Power Inc., the Denver-based energy services company that operates the ECU, was “could we inject essentially untreated, high-Btu gas directly into the ECU and provide reliable power to the oil pump jack,” according to company president David Burnett.
Adding to the challenge were difficulties posed by Mother Nature. “In the roughly nine months of operation, we had to contend with blizzards, lightning, extreme heat, high winds, birds nesting in the microturbines, and mice chewing through the communications wiring system,” Burnett said. “We overcame these various operating obstacles and now have a high-quality, reliable electric generation product.”
Associated Electric Power is testing a compressor with a stand-alone Capstone microturbine being operated by the New York State Electric and Gas Co. in Binghamton, N.Y. The purpose of this project is to provide a stable line pressure of 55 lbs. per square inch gauge for the microturbine and facilitate the delivery of gas from the well by reducing the gathering line pressure from 75 psig to less than 20 psig, explained Burnett.
Williams Energy Services sees opportunities for the ECU beyond oil and gas exploration. “The system will work equally well in other remote power applications, such as ski resorts, or an industrial facility on an island,” said Mertens, who added that the issue of reliable power quality represents a huge opportunity for the ECUs.
Williams Energy Services believes there will be a stronger market for the ECUs in the developing world than in the United States. “We believe overseas demand will be driven by industrializing countries’ greater need for power at any cost,” remarked Mertens.
Powercell is also convinced that there are strong opportunities for the ECU in rural electrification projects undertaken by industrializing countries. “We have a joint venture in Malaysia that is exploring using the ECU for this, as well as PowerBlock clients in the Philippines and Central America,” said Slattery.
Siemens Westinghouse Power Corp. in Orlando, Fla., is testing a proof-of-concept hybrid energy system incorporating its own solid oxide fuel cells and the Frame 3, 70kW microturbine designed by Ingersoll-Rand of Ports-mouth, N.H. Engineers factory-tested the hybrid energy system at a Siemens facility in Pittsburgh during April. The system produced 164 kW of direct current electricity without toxic emissions through the electrochemical reaction in the fuel cell module, and an additional 21 kW of alternating current from the microturbine. The total energy output was sufficient for a hotel or strip mall.
Siemens developed and manufactured the fuel cell stack for the project, while Ingersoll-Rand Energy Systems, formerly Northern Research and Engineering Corp. of Woburn, Mass., developed and built the micro turbine. Siemens is developing the hybrid fuel cell/microturbine as one of two major products in its solid oxide fuel cell program, the other product being stand-alone configurations of the SOFC modules.
The SOFC modules consist of tubular fuel cells that convert natural gas directly into electricity by electrochemical reaction, without combustion. As a result, SOFC power systems generate less than 0.5 part per million of nitrogen oxides, and no sulfur oxides, carbon monoxide, unburned hydrocarbons, or particulates.
The fuel cell operates at atmospheric pressure when operating as a stand-alone system. The hybrid SOFC/micro-turbine power generation system involves diverting air from a microturbine’s compressor stage, pressurized at three to four atmospheres, into the fuel cell module. The pressure increases the mass flow of gas through the fuel cells, thereby increasing the voltage they generate.
This arrangement also causes the fuel cells to produce a 1,600°F exhaust at three to four atmospheres that is piped into the microturbine’s expander stage, improving the system’s efficiency. A hybrid SOFC/microturbine system can convert 60 to 70 percent of the energy value of natural gas into electricity, while another 10 percent can be recovered as heat. Siemens Westinghouse is designing its hybrid fuel cell system to generate 300 to 350 kW for small industrial facilities, such as machine shops, commercial buildings, computer systems, or telephone exchanges.
The fuel cells Siemens used in the hybrid microturbine system are the same as those used in atmospheric pressure systems. These are tube-shaped cells made up of three layers. They begin with a substrate of lanthanum manganite, which serves as the cathode, covered with a layer of yttria-stabilized zirconia, which serves as the electrolyte, covered in turn by nickel, which serves as the anode.
“We lay an interconnecting skunk strip of lanthanum chromite on the cathode so when we sinter the tubes together into bundles, the fuel cells are connected anode to cathode, to build voltage,” said Chris Forbes, a mechanical engineer and manager of SOFC business development at Siemens. Gas runs outside the cells, while air moves through them. Oxygen ions in the gas migrate through the electrolyte, which oxidizes the fuel, releasing electrons that are captured by a copper wire circuit as electricity.
“When we began working on the hybrid system two years ago, we found that the NREC/Ingersoll-Rand microturbine was most closely matched to our temperature, pressure, and power requirements,” said Forbes.
Using Proven Designs
Unlike most microturbines, the Ingersoll-Rand model is equipped with two shafts, one to serve the power turbine and the other to serve the turbocompressor. “Using two stages enables us to reduce the speed at the blade tips, reducing stress on turbine parts, and reducing maintenance costs,” explained Jim Kesseli, a mechanical engineer and business unit manager at Ingersoll-Rand.
“In fact, our microturbines are basically modifications of proven industrial turbocharger designs. We intentionally use low-technology components to keep our microturbines’ stress, and overall costs, lower than more advanced components. For example, we use oil journal bearings rather than air bearings to support the shafts because they are reliable, the oil is an excellent coolant, and rarely needs to be replaced during the life of the turbine,” said Kesseli.
“The only adaptation the SOFCs required in the hybrid system was placing them in a steel pressure vessel,” Forbes said. Adapting the microturbine was harder. The major challenge Ingersoll-Rand engineers faced was designing its recuperator to interface with the fuel cell module.
They broke the microturbine between the turbine casing and the combustor, to pipe pressurized air to the fuel cell module. The Ingersoll-Rand designers also broke apart the microturbine inlet to pump fuel cell exhaust for the purpose of heating incoming air prior to combustion.
“We used high-performance turbine alloys, such as Hastelloy, in the piping that carries the hot exhaust, and redesigned the microturbine controls to accommodate the different operating conditions of the microturbine itself and the fuel cell module,” said Kesseli.
The proof-of-concept SOFC/microturbine system was installed at the University of California at Irvine in May for six months of testing. “We hope to establish all the operating parameters of the system, including startup and shutdown, simulating transients such as loss of gas and compressor air. With this information, we will build a prototype for testing within the next two years,” said Forbes.