Keeping It Flexible
Advancements in turbines allow plant managers to increase output incrementally in some power configurations, for optimal power utilization.
Tuesday, November 01, 2011
By Don Talend
In a Boston, MA, metropolitan area that is experiencing both considerable growth and an evolving regulatory energy environment, electric capacity will need to increase one way or another in the coming years. The Braintree Electric Light Department (BELD) is choosing to increase capacity in a forward-thinking way by taking advantage of the benefits that turbines can provide in terms of boosting grid output.
Flexibility and expandability are major potential benefits of installing turbines in power-production niches such as this. So are energy efficiency and low emissions. As utility managers try to balance an easing of the burden on regional electrical grids with finite financial resources, these prime movers are becoming a viable option in many cases.
In late 2008 the BELD installed a Rolls Royce genset consisting of two gas turbines as part of a $110 million combined cycle plant repowering project. The genset replaces a 32-year-old oil and natural gas-fueled combined-cycle gas turbine and is designed to meet the growing energy needs of the ISO-New England grid by supplying 116 MW of electrical power during peak demand. Montgomery Energy Partners LP, Houston, TX, operates the new Thomas A. Watson Generating Station in Braintree, MA, which made power commercially available starting in June 2009.
The two new Trent 60 units will provide to the ISO New England grid at times of peak demand by supplying electricity to BELD’s roughly 12,500 residential and 2,500 commercial customers. ISO New England had forecast that the region will need 4,030 MW of additional capacity by 2015. Not adding output capacity would mean that BELD would have to purchase power on the open market, likely causing rates to increase. A region-wide challenge is the fact that several antiquated plants in the New England area, including a nuclear plant, will be shut down in the next few years. So more output capacity will be needed in the New England region in coming years anyway.
BELD sought a prime mover source that, more than anything else, provided flexibility in varying peak-power situations. The Trent 60 that was eventually selected generates up to 64 MW in simple cycle service at 42% efficiency while adhering to 25 ppm nitrogen oxides emission limits. It is engineered to go from a cold start to full power in less than 10 minutes and thus suit peaking and flexible power generation markets. This design allows it add power to the grid very rapidly to compensate for the fluctuations and variability of various sources of power, including renewable sources. Trent 60-powered Wet Low Emissions (WLE) generating sets run on either gas or liquid fuel and generate power outputs of up to 58 MW each. Running either separately or in tandem, the turbines can produce 25 to 115 MW—a wide output range.
BELD considered other “frame unit” turbines that were designed for combined-cycle operating mode, but the Trent 60s—which are autoderivative, i.e., designed initially for use by aircraft—provided more flexibility, says Bill Bottiggi, BELD’s general manager. With frame units, “the whole envelope is more efficient than an autoderivative turbine, but also much more complex and less flexible,” he says. “The key to us was flexibility. ISO New England sets the rules for how plants are reimbursed, when plants run, capacity payments versus energy payments, and they’re always changing the rules, they’re always tinkering. We wanted something that would still have value regardless of what decisions the ISO New England makes. We could put a combined cycle plant in and get more efficiency, but then we’d lose flexibility. ISO New England could change the rules, and we’d make sure we have value regardless.”
Another highly sought-after attribute that BELD sought from additional capacity was redundancy. “That was important to us,” recalls Bottiggi. “With any power plant, you’re always going to have technical issues. We like the idea of having two machines in case one has an outage or trips or something like that—then we would still have half the output.” Obviously, having two identical turbine units that can operate independently of each other provides both redundancy and flexibility.
Bottiggi agrees that being the first to utilize a new turbine model in the United States normally would have caused a little bit of trepidation. But a visit to an early North American adopter of the Trent 60—Whitby Cogeneration in Whitby, Ontario, Canada, which had a unit installed to power a 51.2-MW baseload cogeneration plant in 1998—allowed BELD to determine how the model was performing and how well the manufacturer backed up its product. According to Bottiggi, the manufacturer provided Whitby Cogeneration with a great deal of support throughout the installation and early operational processes, and a strong warranty gave BELD a high level of confidence in the turbines.
Having operated the new genset for a couple of years, BELD management is very confident that the new genset is operating more cost-effectively than its existing combined-cycle plant. For example, BELD was able to lower its electricity rates by 3/4 of a cent per kilowatt-hour immediately. The fact that the Trent 60 units operate between 4 and 12 hours in a typical day allows BELD to reap substantial savings. “There are days when we have saved a significant amount of money on energy,” points out Bottiggi.
He explains that, on July 21 of this year when ambient temperatures exceeded 100°F, the new genset allowed BELD to produce its own power at about 1/10 of what it would have cost to buy it. “If we didn’t have our own power plant and we were left exposed to real-time market pricing, we would have been paying $700 a megawatt-hour for electricity—we were producing it at $75 per megawatt-hour,” he concludes.
New Turbines Suit Syracuse
A recent article in Distributed Energy (“Power Without Pause,” July/August) detailed the uninterruptible power supply system at Syracuse University’s 12,000-square-foot Green Data Center (GDC). Capstone Turbine Corporation—the supplier of the microturbines that power the system—and Syracuse provided the magazine with further insight into why Capstone’s Hybrid UPS MicroTurbines were the right fit for the $12.4 million project completed in November 2009.
Syracuse’s UPS is the first onsite power system to integrate “clean-and-green” C65 (65-kW) microturbines directly with a dual-conversion UPS to provide power for mission-critical loads. Substantial energy savings is the major reason why the GDC, which is on track for a Silver certification under the US Green Building Council’s Leadership in Energy and Environmental Design (LEED) program, is a landmark project in the use of microturbines as a distributed generation power source. Microturbines are a major factor in a UPS-combined cooling, heat, and power (CCHP) system designed to achieve a 50% energy reduction compared with a data center using traditional power sources.
The natural gas-fueled Capstone microturbines are at the heart of the data center’s electrical trigeneration system. Despite having patented the design years ago, this project is actually the first time that Capstone put the units into production form. Twelve units were installed in the facility and a maximum of 10 are used at any given time to power the servers and equipment. Kevin Noble, manager of engineering at Syracuse, points out that the units’ low-emissions design eliminated the need for a permit, although the emission sources are inventoried. Additionally, it was possible to mount the microturbines on concrete pads outside of the facility, and they produce virtually no vibration.
“We talked to Capstone and realized that they had an inverter and batteries in one version of the standard C65,” recalls Noble. “We said: ‘We are building a data center and need a UPS. It looks like this is most of one. Can we combine the functionality of trigeneration with a UPS?’ Capstone informed us that they already had the concept patented but had never developed it. In February 2009, they agreed to develop a product for this project, and delivered 12 units in September of that year. We just added batteries and avoided the cost of a discrete UPS.”
A key element in the facility’s energy efficiency is a separate “High Efficiency Mode” of operation. In this mode, the microturbine is turned on and supplies power to the critical alternating current (AC) bus through an inverter called a Load Control Module (LCM). In contrast to a typical data center that converts AC power from the utility’s electrical grid to direct current (DC) and then back to AC to power the servers, these microturbines can generate any combination of AC and DC power without a loss of power that typically occurs during transmission and conversion. The Hybrid UPS design does not require that the microturbine produce exactly the amount of power required by the critical load because a grid load control module (GLCM) allows power to flow out to the non-critical part of the distribution system. Alternatively, the GLCM can pull power in from the utility as needed to match critical load requirements.
The cooling component of the system is particularly efficient because it uses “a Double-Effect Absorption Chiller.” The microturbines produce very clean exhaust heat that is diverted to two 150-ton Thermax absorption chillers. More conventional absorption chillers would convert waste heat from the microturbines to hot water used to make chilled water. But this conventional process utilizes hot water that reaches a relatively low temperature of about 220°F. The Thermax chillers, in contrast, can directly utilize the microturbines’ clean combustion product, which is nearly two-and-a-half times hotter, and convert it to chilled water. The result is much higher overall efficiency of 85–90% than using utility power and a traditional cooling system with overall efficiency of around 50%.
The most consistent attribute of the Hybrid UPS system at the Syracuse GDC is flexibility. Depending on the facility’s fluctuating power needs, the number of microturbines producing dedicated power to it varies from 5 to 10. The remaining microturbines produce extra power that is shipped to the university power grid or the building next door.
The operation of the system is highly flexible. The Capstone turbine generator operates at very high speeds and produces a very high-frequency AC. One inverter, a generator control module (GCM) converts the high-frequency AC to DC using a 760-V DC Bus. Another converter, the LCM, then converts the 760 V of DC into a 50- or 60-Hz AC with a 400- to 480-V nominal three-phase voltage output for connection to the critical loads. The LCM maintains its frequency output in synchronism with the electric utility mode. A third converter, a load control module, connects to the utility and allows the facility to automatically either take power from, or inject power back to, the grid as conditions dictate. Due to this flexibility, the output of the turbine can be set to match the thermal load of the CCHP system for maximum efficiency. The GDC has the flexibility to produce 25 kW and get the other 25 kW from the local utility, for example.
Noble points out that some precautions have been taken to ensure the reliability of this first use of Capstone’s Hybrid UPS MicroTurbine. “We are deeply indebted to our chief information officer, Chris Sedore, for his encouragement of this project and his help in managing the risk associated with this level of innovation, which is quite radical in the data center world,” says Noble. “To deal with the potential for failure, and to facilitate maintenance, upgrades, and revisions to the systems, we have two strategies in place. The first is a completely redundant power system right down to every piece of IT equipment, as far as is practicable. Our racks all have white and black power cords for the a and b power systems, and we regularly test this system. We can completely drop one side, and no critical IT processes are significantly affected at all. The second builds on the fact that the load side inverters are synched to the grid. We have closed-transition transfer switches and can easily and seamlessly move either side from the microturbines to the grid at will.”
From Aircraft to Power Generation
Converting turbine engine technology that was originally designed for the aviation industry to non-aviation applications is inherently innovative. Marine Turbine Technologies, LLC (MTT) has recently developed several new applications for turbines.
The 250-C20B/J model turbine, the cornerstone of the Rolls-Royce Model 250 engine line, recently has been improved with a new first-stage turbine nozzle material designed to extend the life of the component while maintaining engine performance. Gearbox lip-seals and shaft journals have also been redesigned to reduce oil consumption.
MTT also recently entered the portable generator market by targeting small distributed energy applications with two lightweight portable generator products. A Rolls-Royce Model 250 helicopter gas turbine engine has been selected to power a lightweight high-speed alternator that provides 250 kW in a small footprint package. MTT is working with an international manufacturer that specializes in high-speed, lightweight alternators on the development of the product. Targeted first deliveries are forecasted for mid-2012. In addition, MTT has selected a Honeywell (Lycoming) T53-701/13B Helicopter Gas Turbine Engine to power a 1-MW portable power generator package. The program is in the early stages of development and a natural gas fuel system is currently the top priority. The targeted first delivery is mid- to late 2012.
In another application, the lightweight, portable 250-kW MTT Model 250 Series II can be transported by air, land or sea to bring reliable power supply to remote sites by incorporating its own fuel and oil supply and support systems. The generator is powered by Rolls Royce’s R-R Model 250 gas turbine engine. A modular aluminum enclosure allows the operator to change out the turbine engine and generator package modules individually, a design intended to cause less downtime and maintenance expense. The gas turbine engine has been successfully demonstrated using diesel fuels, jet fuels, and biodiesel fuels, and liquid gas and natural gas will be explored as alternative fuels.
Author's Bio:
Don Talend specializes in covering sustainability, technology, and innovation.
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