By Steve Balek, Stellar Energy, and Tom McDonnell, Rockwell Automation
|Control technology plays an integral role in maximizing operational flexibility. Automated control technologies enable power producers to optimize operations and limit life cycle costs to protect their investments. Photo courtesy: Rockwell Automation|
Today, Operations Management in the power industry faces difficult challenges. How can power plants adhere to increasingly tighter compliance requirements while maintaining power generation to meet its growing demand? Stellar Energy, based in Jacksonville, Florida, is applying leading-edge control technology to help its power industry customers improve the efficiency of their turbines.
Gas turbines are constant volume air intake machines; consequently, air mass flow and resultant power output fall as ambient temperatures rise and air density falls. The loss of sellable power can be up to 30 percent of a turbine’s rated output. The purpose of Turbine Inlet Air Chilling (TIAC) is to restore the power output of a combustion turbine at elevated ambient temperatures to its rated capacity or better.
By cooling the ambient air with a TIAC system, the air density increases and the mass flow rate to the gas turbine increases, allowing the turbine to operate at peak performance and maximizing the profit of the power plant asset.
Increasing Power with Inlet Air Chilling
A TIAC system continues to be one of the most reliable and cost-effective ways for power producers to improve the performance of the combustion gas turbine, thereby increasing power production. TIAC is the use of a refrigeration system – mechanical chilling or absorption chilling – to supply chilled water in order to cool the inlet air to the gas turbines.
The major difference between evaporative and mechanical chilling is that an evaporative system cannot cool the air below the ambient wet bulb temperature, while mechanical chilling can cool the inlet air to the dew point and below. Mechanical TIAC gives the operator the ultimate control of the turbine’s inlet air temperature, thereby making output predictable, including the option of using heating/anti-icing to protect against ice formation on the compressor guide vanes. Adding thermal energy storage (TES) can further boost the benefits of inlet air chilling. With TES, plant operators can shift the power required to run the chilling system to off-peak times, when power is in less demand and the value of power sold from the power plant is at the lowest value. This increases the power available during peak periods, and improves the financial performance of the power plant asset.
Taking Control with Control Technology
As the critical ‘operating system’ of the TIAC enhancement, the control technology plays an integral role in maximizing operational flexibility. For example, the Allen Bradley CompactLogix control platform from Rockwell Automation enabled one Southwestern U.S.-based power plant to change functionality of its TIAC system in order to quickly respond to a new market opportunity.
In 2013, Stellar Energy designed and delivered a custom TIAC solution for this 500-MW combined cycle plant that includes a water-cooled chiller plant, thermal energy storage tank with stratified chilled water design, a unique coil design that simplifies the filter house modification, and an innovative freeze protection system for the coils that allows the system to remain in use during the warm days in the winter. The system features a 3 million -gallon thermal energy storage tank, two inlet filter house retrofits including cooling coils, and one nominal 6,780 TR water-cooled modular chilled-water plant to serve two GE 7FA turbines at guarantee case.
In consideration of the site conditions and the client’s objectives, Stellar Energy proposed N+1 redundancy configuration on both the chilled water and condenser pumps utilizing water-cooled mechanical chillers in a parallel configuration combined with the TES tank to deliver 12 hours of partial storage with a two-hour superpeak capacity. The parallel chiller design provides added system reliability – in the case of a single chiller loss, the entire plant capacity is not shutdown. The efficient system utilizes the TES tank’s chilled water to help the chillers supply 50°F inlet air to two turbine inlets during the day and then the chillers “charge” the tank at night.
For the system control technology, Stellar Energy relied on the CompactLogix controllers. The ControlLogix controller platform delivers the required processing and communication power to handle a system of this size, and it has proven its reliability by operating for two years with zero downtime related to hardware failure. The system incorporates Ethernet remote I/O, Modbus remote terminal unit (RTU) serial communications, and two human-machine interface (HMI) stations running Rockwell SoftwareFactoryTalkView Site Edition applications. Stellar Energy’s automation team configured the system so that the operator can switch from inlet chilling to charge mode and back with a couple clicks of a button and the system would seamlessly stage the chillers based on load of the system.
Recently, the power plant had an opportunity from a buyer who wanted to purchase a constant MW feed from one of the turbines but it required a guarantee of supply 24/7/365. The power plant contacted Stellar Energy to find out if it could run one turbine 24 hours a day at a higher inlet temperature and still be able to charge the tank at night. So instead of using the chillers to either chill the inlet coils or charge the TES tank, this system would need to be able to cool both inlet coils during the day (one at 50°F and one at a higher temperature) and also charge the TES tank while cooling the one turbine. Stellar Energy engineers determined that at an inlet temperature of 65°F on one of the turbines, the TIAC system would have enough cooling capacity from the two chillers to fully charge the TES tank during the night.
With Rockwell Software’s robust RSLogix5000 programming software and FactoryTalk View Site Edition HMI software, programmers quickly and easily changed the logic from the existing system to this new control method. The client was given a lot more flexibility. Although early in the re-design the client wanted to dedicate one turbine to run at this higher setpoint, during the course of the implementation they requested if the logic could be set up so that the operator can select the other turbine to run at the higher setpoint in the event that the first turbine needed to be shut down. Again, the control logic made it possible for this change to be quickly and seamlessly incorporated into the system.
Operational flexibility, quick changeover and simple operation are just a few of the benefits of a modular plant design. A skid-based approach allows the TIAC chiller equipment and automation control system to be assembled and Factory Acceptance Tested (FAT) prior to shipment to site, improving consistency and reducing time to market.
Previously traditional skid systems were built based on unique designs, which were closed and non-scalable. Typical skid designs offered a limited choice of optimal controllers for very large applications. In contrast, a modern control system for TIAC process skids are built for the needs of today’s power plants, providing wider ranges of architecture options and increased flexibility. A modern design with a scalable control platform provides the right-size control at the right cost, which eliminates the previous need to purchase expensive control capacity that isn’t needed.
This design is ideal for modular implementations, providing wider ranges of build options and increased flexibility. At the same time engineering costs are dramatically reduced when the same programming tools are used, regardless of system size or I/O capacity. Scalable HMI displays, alarms, data collection and alarm management requirements can be provided at a small scale. This skid-based automation system can also become part of the on-site process control system network, connecting with the site-wide HMI and historian servers.
As industrial power plants move toward gaining greater visibility into their operations and TIAC process skids, their need to establish a seamless flow of information from device to enterprise has become a requirement of modern industrial automation systems. A modern network design utilizes open standards such as EtherNet/IP, which is capable of handling the widest range of industrial applications
Because of its control system network capabilities, the modern TIAC process skid is able to easily integrate with devices such as variable frequency drives (VFD) and electronic overload on motor starters within motor control centers with IntelliCENTER software.
Reduced Total Cost of Ownership
Consolidated deployment tools greatly benefited the project’s engineering and configuration. They provide a single starting point for system configuration and tools for library management. Using predefined logic and HMI objects shorten development and testing time. This increased productivity saved 20% in engineering and at the factory witness test.
Defined system architectures minimize risk of control system errors and ensure system performance, which saved 30 percent in getting the TIAC process skid commissioned.
By use of IntelliCenter Motor Control Centers, these systems are simplified by using networks to reduce the large amount of control wiring from the control system to the VFD and motor starter controls. Reduction in system costs include: less I/O modules, smaller control panel size and wall space, reduced control wiring cost and installation labor by the electrical contractor, faster installation of the system, less time for system I/O check out at commissioning, less risk of wiring errors and time for corrections, less time in the fabrication shop and earlier process skid ship date, which saved on average 10 days.
Instrumentation integration using Endress+Hauser Faceplates provide directly from the instrument: Description, Message/Label, Engineering Units, Zero and Span, Analog fault status, Diagnostic Information, Device specific error codes, warnings, information, etc. This productivity saved an average of one day in engineering and at the fabrication shop floor.
These factors are all TIAC process skid control capabilities which help reduce the total cost of ownership (TCO). By taking in to account the lifecycle costs associated with engineering, inventory, training, system maintenance and support, and future system expansions, the savings can be even more substantial.
To further illustrate the impact of integrated control architecture of the TIAC process skid controls systems on the total cost of ownership, consider the following:
- The integration of devices and networks reduced the number of vendors needed to support the TIAC process skid, and enables on-site support in a timely manner.
- Reduced amount of vendor training required for engineers, operators, and maintenance professionals. Less training and simplified maintenance.
- Design provided a reduction of inventory/spare parts for the control systems.
- Do the disparate systems integrate easily with your skid equipment or are excessive integration time and resources necessary?
- Ability to transfer real-time information across the power plant facility from the TIAC process skid can affect optimization and productivity.
These items are often overlooked factors that result in higher costs over the lifetime of any control system. By utilizing a modern control system on the TIAC process skid, power plants can help reduce their total cost of ownership.
With 15 years’ experience in automation and controls engineering, Steve Balek serves as automation manager for Stellar Energy. Tom McDonnell is Rockwell Automation’s manager of Power Generation and Energy Industry Sales for North America.