By Karst Postma and Mike Skov (The Industrial Co.) and Edward Wacek and Warren Ferguson (General Electric Co.)
Many GE Aeroderivative gas turbines are frequently needed to perform on the hottest days to provide peak power. Unfortunately, as the inlet air temperature to a turbine goes up, the power it can generate goes down. This has driven the need for inlet-chilling systems. Traditionally, there have only been two options available to customers: evaporative or mechanical coolers. In response to customers seeking more power with less variations due to ambient effects, GE Energy has teamed with Energy Concepts, Nooter/Eriksen and The Industrial Company (TIC) to design, build and supply an inlet air chilling unit that utilizes an ammonia-based absorption refrigeration cycle which recovers the exhaust heat from a gas turbine as the heat source.
In place of a mechanical chilling system, the ARCTIC unit enables the gas turbine to provide up to 5 percent more power on hot days, no requirement for 4,160V power or switchgear, improved heat rate and less maintenance requirements. The preengineered skid allows for less site civil work and improves system interconnection compared to existing systems today. The use of an absorption system minimizes the impact of parasitic loads at hotter ambient temperatures.
The use of inlet chilling on aeroderivative gas turbines provides a substantial improvement to a turbine’s power output and efficiency1. An innovative solution has been developed by a partnership to equip GE’s aeroderivative gas turbines with a more efficient and factory packaged inlet chilling alternative. The new system provides more hot-day power than other chilling systems available on the market today. The first commercial unit has been shipped to site with commissioning to occur in the third quarter of 2009, where it will be operating on an LM6000 PC Sprint unit serving the Electric Reliability Council of Texas (ERCOT) market. The system description, customer benefits and market potential are discussed below.
Turbine inlet chilling today is comprised of two primary technologies: mechanical chilling and evaporative cooling. Mechanical chilling uses mechanical compression to reduce the inlet air temperature to optimize the gas turbine’s output. It does so, however, at the cost of high parasitic loads, which reduce the net gains achieved by chilling the inlet air.
Evaporative cooling sprays water into the turbine inlet air stream where it evaporates, cooling the air. Evaporative cooling is not always as effective at increasing power output as mechanical chilling, but the capital costs associated with it are less than the costs of mechanical chilling, as are the parasitic loads.
The LM6000 has been among the most widely accepted aeroderivative gas turbine to serve the power generation segment2 since its commercial debut in 1992. The diversity and depth of the market experience gained has shown several key performance criteria sought by customers: Consistent net output, low parasitic load for a lower heat rate, 10-minute fast start and high reliability and low maintenance requirement.s
Neither traditional mechanical or evaporative cooling systems can support all of these needs, which established the design parameters for the new Absorption Refrigeration Cycle Turbine Inlet Chilling, or ARCTIC, system. This new system has the ability to provide more power on the hottest temperature days, which enables an even better heat rate than all other alternatives.
There are some key economic advantages ARCTIC provides customers, notably: more power and fewer support systems. The use of absorption chilling reduces the parasitic loads associated with mechanical chiller compressors, pumps and motors. In applications where selective catalytic reduction is needed for emissions abatement, the reduced temperature of the exhaust can also eliminate the need for tempering air fans.
The system can also produce chilled air during unit startup so that more power can be produced faster than existing mechanical systems. Also, the ARCTIC system does not require 4,160 volt transformer, switchgear and cabling, thus reducing the total number of systems to interconnect. These benefits have all been enabled in a system that can be packaged in a factory for faster site installation, improving the efficiency of the overall plant.
The employment of an ammonia-water refrigeration cycle has been used for many smaller applications over the past 100 years and its favorable properties have caused it to become the industrial refrigerant of choice3. The ARCTIC system is comprised of five simple core components: turbine inlet air coil, heat recovery vapor generator, evaporative condenser, absorber cooler and ARCTIC skid.
The turbine inlet air coil (TIAC) is placed in the same position within the air filter house as today’s mechanical chilling coils. The thermal energy of the gas turbine is extracted from the exhaust by the heat recovery vapor generator (HRVG) tubes. These tubes carry the high-pressure ammonia into the exhaust stream where it is heated to create the working temperature gradient needed for the ammonia water separation.
The working fluid is then passed through the skid-mounted Rectifier where the separation of the ammonia water solution occurs. The ammonia is then passed through a condenser to convert the fluid back into a liquid stage before going through the TIAC. The vapor is then blended with the water-ammonia mixture loop in the absorber cooler. The process is complete when the mixture is then pressurized and passed back through the rectifier in a closed capacity before reentering the HRVG. The elegance of the ARCTIC system is the ability to provide all of these systems in a skid-mounted design that facilitates plant flexibility along with unit operability.
This simplified summary of components underscores the robust engineering analysis performed on this refrigeration cycle. The system has been evaluated for its structural impact to the air filter, the airflow distribution to ensure adequate cooling and thorough reviews of the manufacturing and controls aspects as well. All applicable design practices by GE have been incorporated to the motors, controls, hazardous protection and detection systems, which the entire team has incorporated. In full, there have been over 100 drawings created and hundreds of engineering hours spent to ensure the ARCTIC system is reliable and capable of meeting or exceeding design targets.
The sound principles that constitute the basis of the ARCTIC system4 have enabled the first unit launch and has generated a robust market interest. The first unit began the manufacturing process at the end of 2008 and was shipped to site in August 2009. The unit began commercial service in October 2009, where it will be operating on a mid-merit basis with an estimated 4,000 hrs/yr.
The project economics for the ARCTIC system are driven by plant capacity factor, site ambient conditions and dispatch profile. Even though the launch unit in Texas has not started, the long hot conditions and nodal market justified the project economics. The viability of the ARCTIC system is further enhanced where CO2 legislation is viable, as it provides less CO2 per delivered MWe than alternative technology. Current customer interest has revealed a broad spectrum of needs that range from dry, remote South Pacific regions to the cooler climates of Canada.
Continuing the Innovation
The development of the ARCTIC system with the LM6000 thus far has led the team to realize several additional areas to take this technology. The applicability of inlet chilling spans the entire gas turbine product range and has been expanded to include the LMS100. Another area being explored is removing additional heat from the exhaust, resulting in the recovery of an important byproduct of combustion: water.
This water can be used to supply the Sprint and/or NOX systems to reduce the water requirement of the LM6000 package. Lastly, there is a potential to expand the system capabilities to include inter-stage chilling on the gas turbine to further optimize its performance as well.
This development could not have been accomplished if it were not for the close collaboration of several industry leaders. Energy Concepts has provided the process knowledge gained over decades of experience in the absorption refrigeration technology, which provided a solid cornerstone for the team to build upon. Nooter/Eriksen brought their industry leadership in exhaust, SCR and heat recovery steam generator engineering and fabrication to ensure sound manufacturing and design of the components was leveraged. Kiewit Power Engineers and The Industrial Company brought their extensive knowledge of designing and building aeroderivative gas turbine plants so that total plant constructability was addressed. GE provided not only its
manufacturing and process rigor to the development; it has also leveraged its commercial insight and packaging technology to ensure the success of the ARCTIC system.
1 “Power Generation Handbook”, McGraw Hill, Chapter 25; US Department of Energy Efficiency http://www.eere.energy.gov/de/thermally_activated/tech_basics.html
2 McCoy Power Report July 2009
3 Heat-Activated Dual –Function Absorption Cycle, ANSI 2004, Donald C. Erickson, G. Anand, Icksoo Kyung
4 “Industrial refrigeration handbook”, Wilbert F. Stoecker, 1998
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