THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St., New York, N.Y

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1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St., New York, N.Y The Society shall not be responsible for statements or opinions advanced in papers or discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is published in en ASME Journal. Papers are available from ASME for 15 months after the meeting. Printed in U.S.A. Copyright 1994 by ASME 94-GT-310 THERMAL ENERGY STORAGE AND INLET AIR COOLING FOR COMBINED CYCLE Jerry Ebeling and Robert Balsbaugh Burns & McDonnell Engineering Co. Kansas City, Missouri Steven Blanchard Public Works Commission Fayetteville, North Carolina Lawrence Beaty Bearden Beaty & Associates Marietta, Georgia, BREAK ABSTRACT The paper will discuss the application of Thermal Energy Storage (TES) using ice and inlet air cooling at the Fayetteville (North Carolina, USA) Public Works Commission (PWC) Butler-Warner Generation Plant. The Butler-Warner Generating Plant consists of eight General Electric Frame 5 combustion turbines and a single steam turbine. Six of the combustion turbines exhaust through three Heat Recovery Steam Generators (FIRSG). The project consisted of modifying the inlets of all eight combustion turbines to accommodate plate fm cooling coils and new air filters; and the design and construction of the TES ice production and storage facilities. A feasibility study was completed in June Detail designed began in August Initial operation was June The modifications have been completed and the plant has experienced a 29% capacity increase as a result of the project. INTRODUCTION Combustion turbines have traditionally been used by the utility industry for peaking electric generation operation. Combustion turbines lend themselves to this type of service for several reasons. They are relatively inexpensive and can be procured, designed, Stalled and operational in less time than large conventional steam power plants. However, combustion turbines do have drawbacks. When compared to fossil fuel steam power plants, they do not provide lowest-cost energy due to their relatively high heat rates and use of premium fuels (natural gas or fuel oil). But worst of all, combustion turbines have a severe sensitivity to ambient weather conditions. Combustion turbines are mass flow machines with generating capacities dependent on combustion air density. As inlet air ambient temperatures increase (low air density), generating capacity and efficiency decrease and heat rate and exhaust temperature increase. Figure 1 illustrates the relationship between ambient temperature, heat rate and capacity for a Frame 5 combustion turbine. Because peak electrical demand for many utilities occurs in the hot summer months, a traditional combustion turbine installation is at a disadvantage for summer peaking operation. When a utility needs electric generating capacity most, the combustion turbine's capacity is at its lowest levels. Inlet air cooling is a way to avoid derating a combustion turbine's summer capacity. A combustion turbine's capacity can be increased by reducing the inlet combustion air temperature. The most traditional way to provide inlet air cooling is the use of evaporative coolers. These coolers function similar to a cooling tower and are limited in their effectiveness by the ambient air relative humidity. A second technique which has been used for inlet air cooling is on-line mechanical chillers. These chillers can deliver 4.4 C water which could be used in a direct contact cooler (such as an evaporative) or indirect by using a cooling coil heat exchanger. The coolant temperature can be lowered below 0 C if brine solutions are used, but this technique would eliminate the use of direct contact coolers. These Presented at the International Gas Turbine and Aeroengine Congress and Exposition The Hague, Netherlands June 13-16, 1994

2 systems require a significant amount of auxiliary power which reduces the peaking capacity of the unit. A third technique is the use of on-line absorption chillers. These chillers are driven by waste heat from the combustion turbine which reduces the auxiliary power consumption. They are limited to a coolant temperature of 7.2 C, but could be used with both direct and indirect cooling. This paper will describe the application of TES using ice, 0 C water and indirect cooling to lower the combustion air inlet temperature to 4.4 C at the Public Works Commission Butler-Warner Generation Plant. A simplified piping schematic is shown in Figure 2 for both the refrigeration and chilled water systems. PLANT DESCRIPTION The Butler-Warner Generating Plant is an intermediate and peak load facility operated by the Public Works Commission of the City of Fayetteville, North Carolina. The plant has eight General Electric Frame 5 Model P N/T combustion turbines (CI), one General Electric steam turbine and three Vogt HRSGs. Six of the combustion turbines exhaust to the three HRSGs. The as were installed in three phasts starting in 1975 and finishing in 1981 and were all simple cycle. The units were originally equipped to burn only No. 2 oil, but were converted to burn natural gas in The combined cycle modification was completed in As a result of the combined cycle modification, the station equipment layout is very congested. The as are installed meters from centerline to centerline. The HRSGs occupy most of the space available between those units from which they receive exhaust gas. The majority of piping and electrical services to the as is underground. The manufacturing and installation of the CTs over a six year time period resulted in three different air intake/filter systems. The combination of these factors led to many challenges in engineering and constructing the new air inlet/filter/coil structures and the pipe trestle for the chilled water supply and return lines. The plant layout with the TES equipment is shown in Figure 3. The ice tanks, equipment enclosure and piping were laid out 45 degrees off of plant north to accommodate the existing buried service lines to the cooling tower. SCHEDULE The Public Works Commission authorized Burns & McDonnell to complete a Feasibility Study, Conceptual Design, and Cost Estimate in spring These were completed in June The commercial operation date for the project was identified in the study as June 1, 1993 to be ready for the summer cooling season. The best way to meet such an ambitious schedule was to use multiple procurement and construction contracts. The equipment and tank construction contracts were awarded first, followed by foundations, inlet modifications, mechanical and electrical. The first meeting and site visit by the design team occurred on August 26 and 27, The detail design began in early September, The site earthwork preparation was started on December 11, Pile driving for the ice tanks began on December 17, Despite the use of multiple contracts, a significant amount of detail work was performed based on catalog and bid information during December 1992 and January 1993 to support issuing construction contracts for bid in late January. These estimates had to be checked against actual equipment submittals as they became available in January, February and March. These checks and refinements were completed on an as needed basis, i.e., the construction sequence dictated which item needed to be checked next The other construction contractors wore all mobilized and tank construction completed by late March The first ice was produced on June 1, 1993 and chilled water was delivered to 4 CTs on June 4, The system was fully operational on July 7, 1993 when the second ice production system was started. INLET MODIFICATIONS The adaptation of the CTs to inlet air cooling posed some of the most difficult challenges of the project. The as at Butler-Warner were supplied with three different air inlet systems. This was the result of being purchased over a six year period. Units 3 and 4 had inertial separators only, while the remainder had separators and filters. The conceptual design included installing only a coil support structure around the existing air intake systems. The expected size of the air coil support structure combined with the proximity of the as, HMG and in some cases, the boiler feed pump house resulted in a decision to demolish the existing air intake systems and install new structures to house the cooling coils, filters and support the chilled water piping. There were some additional benefits to this decision such as: Filter elements could be standardized which would reduce spare inventory costs. Filters could be added to Units 3 and 4. Filters could be placed upstream of the air cooling coils which would eliminate air side fouling and cleaning. Eliminate parasitic losses due to inertial separator blower motors. 2

3 A standard design could be developed which would shorten the design, fabrication and erection times to support the schedule. The decision to demolish the existing inlets did have a major drawback. PWC needed to keep enough CTs available for operation to meet their system demands. Therefore, the contractor was only allowed to demolish the inlets on two CTs at any one time and all the units had to be modified during a 3 month construction period. If the existing filters had been left in place, then the coil support structures could have been built while the CT remained available for operation. The following design parameters were used for the air cooling coils: Inlet Air Temperatures 33.3 C dry bulb 26.6 C wet bulb Outlet Air Temperature - Turbine 4.4 C Outlet Air Temperature - Generator 10 C Outlet Air Temperature - Auxiliary C Air Velocity - Turbine 122 m/s Air Velocity - Generator 137 m/s Air Velocity - Auxiliary 274 m/s Air Flow - Turbine 5,965 cu m/s Air Flow - Generator 736 cu nds Air Flow - Auxiliary 2,039 cu m/s Inlet Water Temperature 0.6 C Maximum Air Side Pressure Drop mm water Maximum Water Side Pressure Drop 1.31 bar The cooling coil tube material was specified as copper with a stainless steel option. The stainless option, although approximately 20 percent higher in price, was chosen for two reasons. First, copper is incompatible with the ammonia refrigerant. While the probability of an ammonia leak large enough to cause significant corrosion problems is remote, the consequences would be significant. Second, there have been some problems in the HVAC industry with borate-nitrate corrosion inhibitors when used in systems with traditional copper or aluminum coils. Many of these systems have switched to a more expensive molybdate based program. The increased reliability and lower maintenance costs of the stainless steel tubing led to the decision to choose stainless steel. The fm material is mm thick aluminum.. The fin design is a plate or sheet type rather than a spiral wrap type. The fins are stamped out of sheet stock with a wave pattern to promote air mixing. The tube holes are punched. The tube is then inserted in the sheets and a mandrel is pulled through to expand the tube into the hole. This assures good contact between the tube and sheet. The face velocity was kept below 122 m/s to insure no condensation carryover. The generators at Butler-Warner are once-through air cooled. These generators also required cooling in order to match the output of the CT. The maximum Cl' output required 10 degrees C inlet air to the generator. The air velocity is 137 m/s to minimize water carryover. The air filters are downstream of these coils and, therefore, these coils will have to be cleaned. The existing auxiliary cooling system for each CT is an air to ethylene-glycol exchanger. The Public Works Commission requested that additional cooling capacity be included in the project for this service. An additional water coil was placed in front of the existing coils. This is only a two row coil with a very low air side pressure drop, however, an airflow reduction of approximately 10 percent does occur. The air velocity is high enough to carry condensation into the existing coil which will revaporize it. This cooling system was not provided With any automatic controls and is intended to be used on an as-needed basis. Another significant factor which affected the inlet design was the existing underground services to the C1's and HRSGs. Due to the close proximity and large number of these obstacles, it was determined that all the air inlet structures would be supported on piling. The exposure of these utilities was not done prior to structural design. This resulted in design modifications as these areas were exposed during construction. Numerous pile locations had to be adjusted or pile caps enlarged or both. All chilled water distribution piping is installed above ground. The pipe trestles used to support the chilled water piping to the inlets was designed to go over the top of the existing east-west trestle. This was done in order to preserve maintenance access to the CI's. In the area north of the steam turbine enclosure, this was a significant challenge due to both below and above ground interferences. This portion of the trestle is also supported on piling due to the lack of space for spread footings. ICE PRODUCTION The ice production systems are dedicated to each ice storage tank and use ammonia for the refrigerant. The system includes compressors, evaporators, evaporative condensers, recirculation systems, ammonia tanks and controls. Each refrigeration system is dedicated to an ice storage tank and is not cross connected. The equipment quantities listed are for both systems. The refrigeration equipment ratings are: 4 Compressors 14.1 GJ/hr (1,113 tons) 4 Evaporative Condensers 14.0 CU/hr (1,106 tons) 8 Evaporators 4.1 GJ/hr (325 tons) 2 Recirculation Systems /s (520 gpm) The systems will produce 2,333,203 kg of ice in a 24-hour period. They are designed to operate 148 hours per week. 3

4 The system is a "liquid overfeed" type, i.e., only 20 (0 25 percent of the liquid ammonia is evaporated to remove heat from the water. This excess liquid is returned to the recirculation system. The following are the operating conditions for the system: Suction Pressure Suction Temperature Compressor Discharge/Condensing Pressure 1.83 bar C bar Condensing Temperature 35.0 C Ambient Dry Bulb 33.3 C Ambient Wet Bulb 25.6 C The evaporators are a plate type. Ice is formed in sheets which are 1.5 in by 2.4 m by approximately 9.5 mm thick. Each evaporator consists of 64 plates divided into two sections. Ice forms on both sides of the plate. It takes approximately 15 minutes for the ice to form and 45 seconds to be harvested. The harvesting process allows hot compressor discharge gas to displace the liquid in the plate. The temperature of the plate is raised, the ice in contract with the plate melts and the sheet falls into the tank below. The off-peak auxiliary power required by the ice production system is approximately 4.3 MWe. ICE STORAGE The ice storage capacity is based on a weekly design cycle. The ice production system will operate 148 hours per week and ice is melted the other 20 hours per week. The Cl's can be operated with inlet cooling for 4 hours per day, 5 days per week at the ambient conditions listed for the air inlet coils. A portion of the ice consumed each day is replaced during the remaining 20 hours of the day. Ice is produced 24 hours per day over the weekend. The combined volume of the two tanks is 16.9 million litres. The maximum ice inventory is 2,194 GJ or 6,549,371 kg of ice. This includes a reserve amount of 710,640 kg to insure that the supply water is always 0 degrees C. The ice storage tanks were designed and bid as concrete cast in place. The successful bidder offered a prestressed concrete composite design as a voluntary alternate. Prestressed composite tanks are widely accepted for municipal water storage, but had not been used for ice storage previously. This alternate was accepted based on no significant increase in price and a 25 percent decrease in construction time. The prestressed design also eliminated internal columns and beams for roof, piping and evaporator support and interior coatings for water tightness. The deletion of roof beams and change to a dome roof instead of flat roof required the revision of the evaporator and piping layouts. The piping to the evaporators was simplified and the distances made more equal in length. This is important for the refrigerant piping to develop balanced operation. The tank design included internal lighting and an observation door near the top of the tank shell. This was provided to allow the operator to observe ice production, ice inventory and "warm" water return from the as. The ice tank overflow pipe is 7.3 m above the base. This location was chosen to reduce the piling and base slab requirements. It was estimated that approximately $150,000 was saved on piling alone as a result of not designing for full water storage. The tank is normally filled to approximately 6.1 m of water prior to ice production. Once the ice inventory is sufficient to form a structural pile, the water level drops as the ice pile continues to build. CHII.IFD WATER The chilled water system consists of five 25 percent capacity circulating pumps, six 50. percent capacity evaporator supply pumps (three for each tank), air cooling coils, piping, valves and controls. The system has been designed to circulate 136,260 I/min of 0 degrees C water to the turbines or 27,252 l/min to the evaporators. The suction header for both sets of pumps is a steel pipe installed near the wall of the tanks. The chilled water return spray header system is similar to those used in cooling towers. The intention was to achieve the best distribution of "warm" water over the ice pile to promote even meltdown. The ice is treated the same as cooling tower fill. During CT operation with inlet cooling and when the water flow is less than 22,710 1/min. per tank, the water is directed to the evaporators. This was done for two reasons; first, the spray nozzles are not as effective below this flow, and second, melting a low spot directly below the evaporators would be the easiest to fill with ice during the next regeneration cycle. COSTS The dollar amounts shown in Table 1 are the results of competitive bidding for the project. The direct capital cost of $14.6 million represents an installed cost of $275/kw of incremental capacity. There are some additional considerations which significantly affected the costs such as: Project Schedule - Burns & McDonnell estimates that a 25% premium was paid on construction contracts to meet the June 1, 1993 commercial operation goal. Minimal clearance for construction of CT inlet structures and pipe trestle due to previous combined cycle retrofit. Redundancy/Flexibility - The use of five chilled water pumps for eight CTs added complexity and cost, but was required to meet PWC's specific needs. 4

5 New Substation - A new 12 kv to 4.16 kv substation was added to supply power to the ice production equipment. The substation was sized to also allow it to also serve as a backup source of power for the steam turbine auxiliaries. Table 1 Capital Costs Refrigeration Equipment $2,100,00 Refrigeration Installation $1,200,00 Cooling Coils $1,000,000 Electrical Equipment $900,000 Electrical Equipment Installation $1,000,000 Structural Work $3,100,000 Civil Work $1,760,000 Pumps $260,000 Piping Installation $3,100,000 Balance of Plant Controls $176,000 Total $14 5% 000 RESULTS The TES system has proven to be an excellent choice for PWC. The project has allowed them to lower the expected increase in capacity purchases from their wholesale supplier. The gross generating capacity has increased 28.8% over its previous summer rating when the as are operated in peak mode. The net plant heat rate of both the simple cycle units and combined cycle units has improved. While complete performance testing will not be accomplished until late spring 1994; some data has been collected and reviewed. This data was recorded using station instrumentation. A comparison was made between operation with chilled water and without. In this comparison the net heat rate of the simple cycle combustion turbines improved by 6.3 % decreasing from an average of 13,713 kj/kwh to 12,849 kj/kwh. The combined cycle operation improvement was lower. This is due primarily to the reduction of the temperature of steam entering the turbine throttle block. This reduction in steam temperature is a result of cooler combustion turbine exhaust temperatures caused by lower compressor inlet temperatures. This reduction in steam temperature is partially offset by the increase in mass flow through the Heat Recovery Steam Generator. Combined cycle heat rates improved from 9241 kj/kwh to 9086 kj/kwh or a decrease of 1.7%. The performance reported does not include the off peak generation required for ice production but does include on-peak loads associated with chilled water circulation. The reduction in steam temperature has aided the summer peak mode operation of the plant by reducing steam turbine inlet temperatures below the maximum allowable temperature for continuous operation. The higher exhaust temperatures of peak mode combustion turbine operation caused increases in steam temperature. The plant does not have a main steam attemperator. The inlet cooling system has been used in a load following mode. The operator has the ability to adjust the inlet temperature setpoint and effectively dispatch the ice inventory. This has allowed PWC to avoid starting additional units until the load is sufficient to allow efficient operation. CONCLUSIONS The application on TIP2 Capacity for the Fayetteville Public Works Commission has been successful. The design and construction of this project was completed on a very fast track schedule. Some of the design decisions were driven by the schedule while others were based on reliability and economic considerations. BIBLIOGRAPHY Ebeling, J., The Concept and Options for Combustion Turbine Inlet Air Cooling Capacity Enhancement. Presented at Combustion Turbine Inlet Air Cooling Conference, August

6 120 EFFECT OF INLET TEMPERATURE ON CT PERFORMANCE ITS EL 95 so 15 0 TO 20 _ 30 INLET TEMPERATURE (DEGREES C) DESIGN OUTPUT 'HEAT RATE FIGURE 1 40 so RECIRCULATION YORK EVAPORATOR ace FIAI0ER1 ICE STORAGE TN( MET AIR FILTER AIR. COOLING COIL CONNISTION AIR HEAT (31 REFRIGERANT Afl EVAPCRATIVE CONDENSER HIGH PRESSUFTE RECEIVER EVAPORATOR SUPPLY PUFF CHILLED WATER CIRCUATING PLAY CCWRESSOR FUEL EXHAUST GAS TO HPSG OR STACK COFTBUSTICH TURBINE 6 FIGURE 2 PUBLIC WORKS COMMISSION OF THE CITY OF FAYETTEVILLE SYSTEM SCHEMATIC

7 COMBUSTION AIR INLET AIR COILS (TYP) I =I, HRSG! CONDENSATE TANK ebt 4 "N. COOLING' TOWER UNIT SUB STATION CHILLED WATER PUMPS & PIPING TRANSFORMER PIPE TRESSEL COMPRESSORS HR SC ri [ I STEAM TURBINE GENERATOR,) I EXIST. UNDERGROUND SERVICES TO I COOLING TOWER FUEL OIL TANKS TRANSFORMERS EVAPORATIVE CONDENSERS FIGURE 3 PUBLIC WORKS COMMISSION OF THE CITY OF FAYETTEVILLE SITE LAYOUT