THE APPLICATION OF THERMAL ENERGY STORAGE
`FOR DISTRICT COOLING AND COMBUSTION TURBINE INLET AIR COOLING
`
`KelUleth M. Clark, P .E.
`and
`Jerry A. Ebeling, P .E.
`Bums & McDonnell Engineering Company
`Kansas City, Missouri
`and
`Edward Godwin, P .E.
`Reedy Creek Energy Services
`Lake Buena Vista, Florida
`
`ABSTRACT
`
`The use of thermal energy storage is a proven and accepted technology for both district cooling
`and combustion turbine inlet air cooling. However, the two applications may never have been
`previously combined in a single project. Reedy Creek Improvement District is realizing the
`synergies available from this combination with their chilled water thermal storage project. This
`paper will discuss the modifications of an existing 20,000 ton district chilled water cooling
`system and 3 8 MW e combined cycle power plant to incorporate a chilled water thermal energy
`storage system for both comfort air-conditioning and gas turbine inlet air cooling. These
`modifications provide improvements to production, reliability, and economic performance. The
`conceptual and final design process will also be presented.
`
`KEYWORDS: Thermal Energy Storage, Turbine-Inlet Cooling, Chilled Water
`
`INTRODUCTION
`
`Reedy Creek Improvement District (RCID) is a municipal entity that services the utility needs of
`the Walt Disney Company in Florida. The RCID owns several utility systems including electric,
`gas, chilled water~ hot water, potable water, and sanitary sewer. The RCID Central Energy Plant
`serves the various needs of the Walt Disney World Resort. Reedy Creek Energy Services
`(RCES) is a private corporation providing design, build, operation and maintenance contract
`services to RCID for its utility systems.
`
`RCID and RCES recently modified the RCID Central Energy Plant. The modifications include
`the addition of a new chilled water thermal storage system and a new combustion turbine inlet air
`cooling system. The project also includes the addition of a new carbon monoxide (CO) catalyst
`
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`MAGIC KINGDOM
`Lake
`
`SCALE = 1" =2000'
`2000'
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`GRAPHIC SCALE
`
`TES SITE
`FIGURE 1
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`86
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`FIGURE 2
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`Page 3 of 13
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`87
`87
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`system on the turbine exhaust. These modifications will provide improvements to the
`production, reliability, and economic performance of the Central Energy Plant..
`
`EXISTING FACILITIES
`
`The RCID Central Energy Plant is an integrated energy production and distribution facility
`located one-half mile north of the Walt Disney World (WDW) Magic Kingdom Theme Park.
`(See Figure 1 and 2). The plant is a cogeneration facility consisting of a nominal32 MW
`General Electric LM5000 gas turbine generator, a three-pressure level 90,000 pph Vogt heat
`recovery steam generator (HRSG) and a nominal 8.5 MW Elliott extraction-condensing steam
`turbine generator. Steam from the HRSG is supplied to absorption chillers and a hot water
`system.
`
`A high temperature hot water district heating system provides 350 degree water to several WDW
`facilities. Production is normally supplied by steam from the HRSG by a steam-to-hot water heat
`exchanger. Back up supply capability is provided by a dual-fuel hot water generator.
`
`A chilled water district cooling system provides 40 to 44 degree F water to the WDW Magic
`Kingdom theme park and area resorts. The chiller plant consists of 2 absorption chiller and 9
`electric motor driven centrifugal chillers with a total deliverable capacity of 17,500 tons. Chilled
`water is distributed to customers by two piping loops.
`
`PROJECT OBJECTIVE
`
`RCID was in need of replacing two electric centrifugal chillers. In addition they were seeking to
`reduce cooling energy costs and improve the efficiency and capacity of their gas turbine
`generator. Therefore, within the scope of this project are the following objectives:
`
`*
`
`*
`
`*
`
`Eliminate the need of replacing two chillers with 3325 tons of capacity. The
`chillers use CFC refrigerants and operated at efficiency's that were low compared
`to today' s standards.
`
`Generate cooling during low energy cost periods and store the chilled water for
`peak load periods.
`
`Provide cooling to the existing gas fired turbine-generator to enhance the
`performance of the turbine.
`
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`To meet these objectives, the owner elected to develop the project in a design/build arrangement.
`The first step was to retain a consultant to develop the "scope documents" so that competitive
`design/build proposals could be obtained. The scope documents contained a description of the
`components, capacities and design criteria required for the project. A detailed description of the
`site development, control schemes, tank storage criteria and turbine-inlet-cooling requirements,
`component specifications were included in the scope documents.
`
`Competitive bids were obtained and the contract was awarded to the successful bidder. The
`bidders were given some latitude to develop the best, cost-effective design and proposal. The
`project was completed in April of 1998.
`
`CHILLED WATER SYSTEM
`
`Eleven chillers in the north Central Energy Plant (CEP) chiller building and two chillers in the
`Contemporary Hotel provide total sustainable cooling of 17,750 tons which supplies the Magic
`Kingdom and several hotel/resort complexes. The sustainable capacity is defined as the amount
`of tonnage the chillers are estimated to be capable of sustaining for more than one hour. The
`chillers in the CEP are separated into two groups, the West Chiller Group and the East Chiller
`Group. A cross connection with valving in the distribution system allows these chiller groups to
`operate together or supply their individual east and west chiller distribution systems. Either the
`East Group or the West Group of chillers can be isolated in case of a line break, leaving the other
`chiller group in service. The chillers are capable of operating at a ;::.. T of l5°F which provides for
`a chilled water flow of 1.6 gpm per ton. The chilled water supply temperature is normally near
`4JOF, but can be varied between 40°F and 44°F.
`
`The west chiller group consists of five chillers with a total cooling capacity of 10,000 tons. The
`east chiller group consists of six chillers with a total cooling capacity of 4,775 tons. The
`Contemporary Hotel chiller group consist of two chillers with a total capacity of2,975 tons. With
`the largest chiller out of operation, the total plant capacity is 14,950 tons. This total is still in
`excess of the peak load of 14,600 tons.
`
`The distribution system consists of two supply loops: the East Loop, supplying the Magic
`Kingdom, service areas, and several hotels; and the West Loop supplying additional hotels.
`These two loops are supplied from a single underground distribution header on the north side of
`~he chiller building. The two chillers at the Contemporary Hotel provide chilled water to the east
`loop of the system. These two chillers are used continuously as part of the chilled water supply
`system and are not normally cycled off during lower demand periods. Of the chilled water
`supplied to the cooling loops from the CEP chiller building, approximately 25% of the total
`cooling load is supplied to the west loop and 75% is supplied to the east loop.
`
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`UPGRADE TO CHILLED WATER SYSTEM
`
`Thermal Storage Planning for the Central Energy Plant
`
`Due to economic considerations resulting from the age of two of the existing chillers, with their
`high operating cost per ton, and their increasing maintenance costs, the decision has been made
`to provide a chilled water storage facility to take the place of the on-line capacity of these two
`chillers. This chilled water storage system also provides emergency chilled water backup
`capacity, allows for use of off-peak electricity at potentially lower rates, and reduces dependency
`on regulated "environmentally unfriendly" refrigerants. The chilled water storage system will
`also take into account the additional cooling load added to the chilled water system for providing
`turbine inlet cooling to the electric generation system. This turbine inlet cooling system will add
`electrical generating capacity to the generating plant, as well as increasing its efficiency at times,
`by cooling the inlet combustion air to the turbine generator. The chilled water storage facility
`will consist of a chilled water storage tank, charge I discharge pumps, and system controls.
`
`Planning for the Chilled Water Distribution System
`
`The chilled water distribution loop system is vulnerable to a partial shutdown in case of failure
`of, or required maintenance on, the W1derground distribution header located just north of the CEP
`chiller plant. A new East I West Loop Connector was provided to reroute chilled water from the
`chiller group remaining in operation after a distribution main shutdown. This new connector will
`allow chilled water to be rerouted and supplied to both east loop and west loop distribution
`systems. The new chilled water storage tank supply and return piping was tied into this loop
`connector to provide its cooling capacity to both the east and west loop distribution systems.
`
`Chilled Water Storage Tank and Supply Pump Sizing
`
`The load capacity of the chilled water storage system will take into account the removal of the
`on-line capacity of two chillers totaling 3,325 tons, as well as an increase of2,000 tons of
`cooling for the turbine inlet cooling system. It is assumed that the present peak cooling load of
`14,600 tons must also be met with the largest chiller out of service. The minimum size of the
`chilled water storage tank is determined as follows:
`
`PRESENT FIRM CAP A CITY
`Present peak chiller capacity
`Elimination of two chillers on-line capacity
`
`Total on-line tons available without old chillers
`
`Tonnage removed with largest chiller off-line
`
`Tons normally available without largest chiller
`
`90
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`17,750 tons
`-3.325 tons
`
`14,425 tons
`
`-2.800 tons
`
`11,625 tons
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`TANK CAP A CITY REQUIRED
`Peak system cooling load
`Less tons available from plant
`Plus additional tons for turbine inlet cooling
`
`Tons to be supplied by tank storage system
`Plus 10% contingency
`
`14,600 tons
`11,625 tons
`+ 2.000 tons
`
`4,975 tons
`500 tons
`
`Total required peak on-line capability to be supplied by tank
`
`5,500 tons
`
`Assuming that a 1 0-hour discharge time and 14-hour charge time are used, the volume of the
`storage tank required is determined by flow requirements of the system. As indicated above,
`2,000 tons of cooling is provided for turbine inlet cooling with the remaining 3,500 tons provided
`for comfort cooling. The comfort cooling system normally uses water supplied to the loop
`system near 40°F and returned 15° warmer. This combination requires water to be supplied for
`comfort cooling at the rate of 1.6 gpm/ton of cooling:
`
`Comfort Cooling Flow Rate= 3,500 tons x 1.6 gpm/ton = 5,600 gpm
`
`The turbine inlet cooling system normally uses water supplied near 40°F and returned 30°
`warmer. This requires water to be supplied for turbine inlet cooling at the rate of 0.8 gprn/ton of
`cooling:
`
`Turbine Inlet Cooling Flow Rate= 2,000 tons x 0.8 gpm/ton = 1,600 gpm
`
`The total flow rate from the tank is the sum of the comfort cooling flow rate (5,600 gpm) and the
`turbine inlet cooling flow rate (1,600 gpm) or 7,200 gpm. Using the 10 hour discharge time
`indicated above, the total usable volume of the tank must be:
`
`Minimum Usable Tank Volume= 7,200 gpm x 10 hours x 60 min./hour= 4,320,000 gal.
`
`With a storage efficiency (Figure of Merit) of approximately 0.9, the actual tank volume to be
`supplied is 5.000.000 gallons.
`
`The usable energy stored is as follows, assuming an averaget. T of 18.3°F and a figure of merit of
`0.9 minimum:
`
`Usable Energy = (5,000,000 ga1.)(0.9 FOM)(8.34 lb/gal)(l BTU/lb/°F)(18.3°F)
`
`= 6.87 x 108 BTU = 57,000 ton-hours usable at 18.3°F t. T
`
`The total flow rate from the chilled water storage tank as indicated above is 7,200 gpm. This
`water will be supplied by two chilled water pumps sized at 3,600 gpm each. An additional
`standby pump will be provided for emergency use and to allow quick storage tank recharge with
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`three operational pumps when the off-peak load allows. Pump speed and flow will be adjustable
`using variable frequency drive speed controllers on the pumps.
`
`Chiller Capacity for Chilled Water Storage Tank Recharging
`
`After a full usable capacity tank discharge, the chilled water storage tank is normally recharged in
`14 hours overnight. The pumping system is designed so that two pumps can pump the warm
`water from the top of the storage tank and into the chiller return piping system. The existing
`CEP chiller pumping system then circulates the water through the chillers,.cooling it, and
`returning it back to the bottom of the storage tank. As indicated above, one standby tank pump is
`provided for backup operation. In a quick charge situation, the third pump may be operated to
`decrease the recharge time from 14 hours to 6. 7 hours. The chilled water storage tank is
`recharged using chiller capacity that is not required for the normal night time operating cooling
`load on the east and west loops. The required extra chiller capacity during this recharge time is
`as follows:
`
`Extra chiller capacity for 14-hour charge cycle = 57.000 ton-hours
`14 hours
`
`= 4,100 tons excess capacity required.
`
`Minimum tank pump size = (1.6 gpm/ton)(4.100 tons) = 3,280 gpm per pump
`(for recharge)
`2 pumps
`
`Supply pumps sized at 3,600 gpm are required for the discharge cycle, so this
`determines actual pump sizes.
`
`Extra chiller capacity for maximum = 57.000 ton-hours = 8,500 tons
`quick charge using 3 pumps
`6.7 hours
`
`THERMALENERGYSTORAGETANK
`
`As mentioned above, the tank capacity is 5,000,000 gallons. The tank is an above ground steel
`tank. The tank is insulated with rigid insulation board and aluminum jacketing. The tank is
`required to have a maximum capacity of57,000 ton-hours at an 18.3° 6 T. The advantage of
`combining the turbine inlet cooling system (with its 30° 6 T) can be seen. If the cooling load was
`strictly comfort cooling with the 15° 6 T the capacity of the tank would be only 46,700 ton-hours.
`The tank and foundation was designed and provided by the tank manufacturer. The tank is 116
`feet in diameter and is 67 feet tall.
`
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`COMBUSTION TURBINE INLET AIR COOLING
`
`A combustion turbine's electrical generating capability is directly related to the mass flow of
`combustion air through the unit. Because combustion turbines have fairly constant volumetric
`inlet air flow, the mass flow of the combustion air increases as the density of the air increases.
`For most utilities, including Reedy Creek Energy Services, the requirements for peak capacity
`occur during times of highest ambient temperatures. These high air temperatures result in lower
`air density and reduced capacity from the combustion turbine generator at the time the capacity is
`needed the most. However, cooling the ambient air before it enters the combustion turbine
`increases the air density and, therefore, allows the unit to generate electricity at a substantially
`higher capacity. Capacity enhancements of up to 3 5 percent can be realized with inlet air cooling.
`Additionally, cooler combustion air improves the combustion turbine efficiency and lowers the
`exhaust temperature.
`
`Several combustion turbine inlet air cooling projects have been successfully completed
`throughout the world in the past several years. This new application of existing technologies has
`become widely accepted in the utility industry as an economical method of capacity
`enhancement.
`
`Based on the success of existing combustion turbine inlet air cooling projects, Reedy Creek
`Energy Services requested an evaluation to utilize the proposed chilled water thermal energy
`storage system to supply chilled water for inlet air cooling of their existing General Electric
`LMSOOO combustion turbine generator. It was determined that inlet air cooling would provide
`economical peaking capacity and energy, as well as providing a method of controiiing the
`temperature of the chilled water returning to the storage tank.
`
`The design parameters used for the design of the combustion turbine inlet air cooling system
`were as follows:
`
`•
`•
`
`•
`•
`
`•
`•
`•
`•
`•
`•
`•
`
`Air Flow
`Ambient Temperatures
`Dry Bulb
`Wet Bulb
`Inlet Air Cooled Temperature
`Chilled Water Temperatures
`Supply_
`Return
`Chilled Water Flow Rate
`Air Velocity Across Coils
`Air Pressure Drop Across Coils
`Tube Material
`Tube Wall Thickness
`Fin Material
`Fin Spacing
`
`219,200 scfm
`
`95 degrees F
`79 degrees F
`50 degrees F
`
`40-44 degrees F
`
`1600 gpm
`500 fpm
`1.2 inch H 20
`Copper
`0.049 inch
`Aluminum
`8 fins/inch
`
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`The existing inlet filter house is located on top on the existing combustion turbine building and
`has four separate horizontal inlet air openings that converge into a common plenum. This
`common plenum connects vertically into the compressor section of the LMSOOO combustion
`turbine. The new air cooling coils were located in four banks at the entrance to the existing
`combustion turbine air intake. The existing evaporative coolers, inertia separators and associated
`materials were removed from the inlet filter house. The exposed surfaces of the existing inlet
`filter house and plenum past the new air cooling coils were insulated and lagged for thermal
`efficiency and condensation protection.
`
`OPERATION
`
`Chilled Water Storage and Chiller System
`
`There are an infinite number of load conditions that the system may encounter depending on time
`of year, time of day, sky condition, humidity, park and hotel occupancy~ etc. A large number of
`different modes of operation were analyzed to ensure that the central plant, thermal storage
`system and new distribution system can function effectively under varying load conditions. The
`effect of chiller outages and distribution system breaks were also evaluated to develop means to
`keep the system in operation. See the attached Figure 3 for system schematic.
`
`The storage system has approximately 14 different control modes. However, it boils down to
`three primary modes.
`
`*
`*
`*
`
`Tank Recharge Mode
`Tank Discharge Mode with bias to the central plant chillers
`Tank Discharge Mode with bias to the chilled water storage tank
`
`The pumping schemes for the three primary modes are further described as follows:
`
`Tank Recharge: The tank distribution pumps draw water from the top of the tank and supply it to
`the chiller plant. The chilled water is then returned to the bottom pf the tank.
`
`Tank Discharge, Central Plant Bias: The central plant chillers and pumping system supply a
`preset flow level (base load) to the distribution system. The tank pumping system supplements
`this base load. The control system will vary the chilled water supply flow from the tank by
`adjusting the pump(s) VFDs to maintain system pressure.
`
`Tank Discharge. Tank Bias: Under this control scheme the tank pumping system is set to
`maintain a preset chilled water supply level to the distribution system. This level is maintained
`
`94
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`SCHEMA TIC DIAGRAM
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`

`and adjusted by flow transmitters in the chilled water supply to each distribution loop. The
`central plant pumps will adjust their flow to maintain system pressure.
`
`Combustion Turbine Inlet Air Coolinfl System
`
`The intended use of the inlet air cooling system is to cool the combustion air for the GE LMSOOO
`existing combustion turbine. Cooling provides more mass flow of air thus increasing the
`generating capacity of the unit. The air cooling coils will be used for up to 24 hours per day, but
`will primarily be used during the highest temperature periods of the day, typically 10 hours.
`
`The operation of the combustion turbine inlet air cooling system is accomplished by a new
`programmable logic controller (PLC), and was integrated with the existing Reedy Creek Energy
`Services, F omey and Woodward control systems. The plant operator has the ability to start and
`stop inlet air cooling as well as control the air cooling temperature from ambient temperature
`down to the design cooled temperature of 50 degrees F.
`
`CONCLUSIONS
`
`RCID has been able to realize significant energy and economic advantages of the Chilled Water
`Thermal Storage System. These include the following:
`
`*
`
`*
`
`*
`
`*
`
`*
`
`*
`
`Utilization of off-peak electrical energy to provide a significant portion of their
`cooling requirements.
`
`Capacity enhancement of turbine-generator by up to 35% during wann weather
`conditions.
`
`Provide a reliable source of cooling during the day, if a chiller is taken out of
`service during peak load conditions.
`
`Provide for 20,000 ton-hrs of cooling (35 percent of the tank capacity) for the
`turbine-inlet-cooling system by adding only 25 percent additional storage volume
`to the thermal storage tank (due to the increased h.. T between the supply and return
`to the tank).
`
`Eliminated the need to replace 3,325 tons of chiller capacity. Also, reduced
`reliance on CFC refrigerants for real time cooling.
`
`With the addition of the new chilled water cross-tie between the east and west
`loops: realized an increase in system reliability in the distribution system.
`
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`The entire project provided adequate savings in energy usage to provide a good rate of return on
`RCID's investment. The exact economic impact is not available for publication.
`
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