By Mark Gillespie, P.E., and Ben Erickson
Dating back to the late 1980’s, the principle of inlet chilling has been utilized to increase power output of combustion gas turbines. Since gas turbines are constant volume machines, air mass flow through the unit increases as ambient temperature decreases, increasing power output at colder ambient temperatures. Chilling of combustion turbine inlet air is an effective means to augment existing plant output during hot periods when power demand is greatest.
Duke Energy teamed with Amec Foster Wheeler (now Wood) and installed the Chiller Uprate Project at the Hines Energy Complex, which increased the plant’s summer capacity by over 200 MW net. The Hines Energy Complex (located near Bartow, FL) consists of eight F-class combustion turbines arranged in 2×1 configuration for combined cycle operation with a total ISO site capacity of 1912 MWs. It is important to note that the four separate power blocks were commissioned at various times between 1999 through 2007, so each block is slightly different than the others.
While various technologies such as fogging, wet compression and evaporative cooling exist to chill inlet air, the Chiller Uprate Project chose to utilize a closed-loop chilled water system. The Chiller Uprate Project scope included the design and installation of a chiller plant, a thermal energy storage (TES) tank, a mechanical draft cooling tower, cooling coils into the combustion turbine inlet ducts, and a closed-loop chilled water distribution piping system. On hot ambient days, the chiller plant works in conjunction with the TES tank to supply chilled water to the combustion turbine inlets, reducing the inlet air temperature to 50°F, thereby recovering approximately 200 MW of “lost” power capacity.
Unique to the Chiller Uprate Project is the scale at which it takes advantage of off-peak power conditions at night to charge the TES tank. The stored energy is then utilized during daytime peak demand to boost power output, in effect creating energy storage similar to a battery. The TES tank, one of the largest in North America, stores over 17,000,000 gallons of chilled water (over 315,000 ton-hour capacity) and can support 12 hours of full-load cooling operation. During off-peak times (typically overnight), the tank is cooled to 36°F by the chiller plant, which consists of eight 3500-ton chiller units. The system is designed to charge the entire working volume of the TES tank in a 12-hour period. The tank maintains the chilled water temperature and holds a thermocline, which keeps the warm water from mixing with the cold water during charge and discharge periods. The stored chilled water, in conjunction with the chiller output, cools the combustion turbine inlet air to increase the output of each combustion turbine anytime the ambient temperature is above 65°F. The system can also be operated in ‘super peak’ mode for a limited time each day, during which the chiller plant is shut down and the TES tank supplies all the chilled water required to reach 50°F inlet air temperatures. This saves approximately 20 MW of parasitic load, further improving plant output during peak power demand conditions.
Chiller System Design at Hines
The chiller system is divided into two main loops – primary and secondary. The primary loop serves two functions depending on the mode of operation of the plant. During ‘charging mode’, the chilled water in the TES tank has been expended and the tank temperature is “hot”. Water is drawn through the diffuser at the top of the tank, cooled by the chiller plant, and pumped back to the bottom diffuser of the tank by the primary pumps. The chiller plant is sized to be able to cool the entire working volume of the TES tank in a 12-hour period. During ‘partial discharge’ mode, the TES tank and chiller plant work together to supply chilled water to the coils at the combustion turbine inlets. The function of the primary loop is to cool approximately half of the hot return water and pump it to the inlet of the secondary pump skid where it is blended with cold water from the TES tank and sent back to the cooling coils. Flow through the primary loop is controlled by a control valve at the outlet from the chiller plant.
The secondary loop serves one function: to pump cold water from the TES tank and chiller plant to the cooling coils at the combustion turbine inlets. Flow through this loop is controlled by eight dedicated control valves – one per combustion turbine. These control valves adjust chilled water flow to each cooling coil, controlling the turbine inlet air temperature (T2) to a desired set point (e.g., 50°F).
“The construction process for the thermal energy storage tank, while not groundbreaking in concept, was challenging in scale.”
The chiller plant consists of eight, centrifugal, duplex, Trane chillers, eight primary pumps (one per chiller), and a central electrical room (including HMI for system control). A four-cell mechanical draft cooling tower provides necessary cooling water to the chillers. The chiller plant equipment is housed in eleven pre-fabricated modules that were fully constructed at the Stellar Energy’s factory before being shipped to site for assembly. Interior walls were removed to allow greater access for maintenance activities, creating one unified chiller plant consisting of an east and west wing (four chillers and five primary pumps in each) as well as a central electrical room. The secondary pump skid houses nine large pumps in a standalone structure nearby.
Another innovative portion of this project is the inlet air condensate capture system. When cooling the hot and humid ambient air down to 50°F, a large amount of condensate is produced (up to 60 gpm per combustion turbine). Rather than wasting this clean water, the inlet coils were outfitted with a condensate capture system. The collected condensate is pumped from each turbine inlet house to a new, 200,000-gallon stainless steel collection tank where it is used for cooling tower makeup. The use of this clean water reduces both cooling tower chemical consumption and externally sourced makeup water.
The project is one of the largest combustion turbine inlet air cooling systems including a TES tank installed in the United States. Key challenges addressed in the design and installation of this project included providing a 17 million gallon TES tank with an internal diffuser system, coordination of combustion turbine inlet ductwork modification during planned plant outages, over a mile of underground large HDPE piping in an existing plant footprint, and the implementation of a control scheme to manage the turbine inlet air temperature to meet turbine vendor requirements.
TES Tank Design and Construction
The construction process for the thermal energy storage tank, while not groundbreaking in concept, was challenging in scale. The TES tank measures 215 feet in diameter and over 93 feet in height and includes a liner to inhibit leakage. The tank was constructed by CROM through a process that included erection of a metal diaphragm, application of shotcrete, installation of 700 miles of high strength carbon steel pre-stress wire, and finally the addition of insulation. Effective operation is dependent upon maintaining a thermocline between chilled water and warm return water. This was accomplished with a complex diffuser system designed to slow entrance and exit velocities of the water to reduce mixing effects creating a small region of the tank with a high temperature difference (typically 36°F on bottom and 70°F on top). The internal piping and diffuser required careful designing and planning for constructability. Performance testing demonstrated the diffuser system’s ability to maintain a thermocline thickness of approximately 4 feet.
Inlet Ductwork Modifications
One of the goals of the project was to pose minimal interruption to plant operation during construction. The biggest effort related to this goal was coordination and planning of the modifications of combustion turbine inlet ductwork during planned plant outages. Plant personnel and the construction team collaborated to ensure chiller work activities were integrated into the outage schedule to minimize impacts and delays. A total of eight gas turbine inlets were retrofitted with cooling coils during four separate outages in the spring and fall outage seasons. Retrofit kits were manufactured by GT ICE offsite and a mock fit-up was performed before disassembly and shipping. This helped ensure a smooth installation process.
Large Underground HDPE Piping
The project also included a substantial amount of underground high-density polyethylene (HDPE) piping used for cooling water, chilled water and cooling tower makeup. The main headers are as large as 54 inches and required substantial effort to install in an existing plant footprint. Using existing plant drawings and ground penetrating radar, the construction team successfully installed the new piping systems without impacting existing lines. The HDPE diameter piping was installed by utilizing a large field fusion welding machine to join long pre-manufactured portions delivered to site. One advantage of the use of HDPE is its natural insulative properties when installed underground. The large wall thickness of the pipe helps maintain the temperature of the chilled water delivered to the coils.
System Control Scheme
An additional hurdle the team had to overcome was designing the control system to limit rate of change for inlet air temperature within gas turbine OEM requirements. Per the turbine OEMs’ specifications, the inlet air temperature rate of change was limited to 3°F per minute. To satisfy this requirement, the chiller vendor implemented a control scheme to limit chilled water flow to each gas turbine during startup based on the air inlet temperature measured by the unit. The result is a smooth startup of inlet chilling providing a controlled ramp down to the desired set point of 50°F. The control algorithm also limits the number of chillers operating to save on parasitic load by prioritizing chilled water from the TES tank. During startup, chilled water flow from the TES tank is increased based on demand until it surpasses the TES tank flow set point. As flow demand continues to increase, chillers are cycled on appropriately.
Test Procedure Approach
A final challenge was to develop a unique, project-specific Performance Test procedure to ensure project performance guarantees were achieved. The system was generally tested under the guidelines of “ASME PTC 51-2011 – Gas Turbine Inlet Air-Conditioning Equipment” while ASHRAE guidelines were used to develop the portion of the performance test for the TES tank. A piecewise approach was used, breaking the system into two distinct parts: the TES tank and the chiller plant. Correction curves were derived from an empirical process during testing week to correct for off-design conditions. The TES tank capacity was calculated by running the chiller plant all night to charge the tank and then discharging without using the chiller plant the following day (‘superpeak mode’). To characterize the chiller plant capacity, the plant was run in ‘chillers only’ mode without supplemental chilled water from the TES tank. The cooling coils were easily characterized using the log-mean temperature difference (LMTD) method to calculate actual heat transfer coefficient (UA) values for each unit. Knowing the capacity of both the TES tank and chiller plant, as well as an empirically calculated model for the cooling coils, the system performance at design conditions was calculated. McHale & Associates, a third-party performance testing expert ,performed all testing and generated the required calculations to prove the system met all performance guarantees.
The Chiller Uprate Project increases the summer capacity of the existing Hines Energy Complex by 200 MW without requiring the installation of any new generation units. This project solidifies the Hines station as a leading power producer for Duke Energy in Florida and the additional capacity provided by the chillers helps offset the need to generate power from less efficient and more costly peaking units in the summer. Despite the challenges associated with installing a variety of new equipment within the footprint of an existing operating facility, the project team completed the project without any safety events and under strict environmental controls, with a focus put on minimal water and chemical spills. Inlet air chilling has proven to be an effective means of increasing combustion turbine power output without requiring the addition of new units to existing plants.
Mark Gillespie is plant general manager at Duke Energy. Ben Erickson is a power process engineer at Amec Foster Wheeler (now Wood).