U.S. Department of Energy award-ed Burns & McDonnell Engi-

neering Co. a development contractabout two years ago to design andbuild a ‘small’ integrated gas turbineenergy system based on improvedcombined heat and power technology.

Proposed plant, which recently be-gan commercial service, is poweredby a 4.6 MW Solar Turbines Centaur50 gas turbine and two-stage indirect-fired Broad Co. absorption chiller:

■■ Electric Power. For full-loadcontinuous duty the plant is nominallyrated at 4.3 MW net output with a 15ppm NOx guarantee on site emissionsfor the gas turbine alone.

■■ Chilled Water. Absorption chilleroperates off gas turbine exhaust todisplace up to 2600 tons of chilledwater previously produced by electri-cal centrifugal chillers.

■■ Capital Cost. Modular packagingdesign is expected to cut equipmentand installation costs by 15-30% de-pending on amount of infrastructureavailable at the site.

Burns & McDonnell partnered witha municipal utility, Austin Energy, to

build the CHP plant at the city’s Do-main Energy Park in an existingbuilding that was the right size tohouse the modular package layout.

Austin Energy owns and operatesthe plant as part of an existing centralutility station that generates power forthe grid and sells chilled water to in-dustrial tenants and a downtown dis-

trict cooling system.The Centaur 50 gas turbine is ISO

rated at 4600 kW continuous and11,950 Btu/kWh heat rate (28.6% ef-ficiency) simple cycle with a 42.1lb/sec mass flow (19.1 kg/sec) and950°F gas turbine exhaust tempera-ture (510°C).

For the Austin installation, the

14 GAS TURBINE WORLD: August-September 2004

4.6 MW plant with an indirect fired2600 ton chiller at 76.8% efficiencyBy Irwin Stambler

City of Austin recently commissioned a CHP plant generat-ing 5.5 to 6 cents per kWh peaking power plus 2600 tonsof chilled water, at 76.8% efficiency (LHV) with less than 15 ppm NOx and no catalyst exhaust treatment.

Centaur 50 gas turbine. Austin Energy CHP plant is powered by a packaged Solargas turbine genset in an enclosure (foreground). Gas turbine exhaust is ducted toin-line absorption chiller via a diverter valve.

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CHP plant is site rated 4330 kW andnominal 2600 tons chilled water out-put at 76.8% overall net plant effi-ciency and 5 ppm NOx emissions.

Under new State of Texas output-based emission regulations, there is anemissions offset credit for the gas tur-bine by virtue of its waste heat recov-ery to produce chilled water (withoutburning more fuel).

Emissions performance is calculat-ed on the basis of overall system effi-ciency which, for the Austin CHPplant, works out to 15 ppm NOx forthe gas turbine and 5 ppm for theCHP system.

Newer gas turbine designs like therecuperated Mercury 50, which has a38.5% simple cycle efficiency, will-guarantee 5 ppm NOx and 10 ppmCO and UHC emmissions withoutCHP credit

Project developmentThe original DOE development con-tract was awarded through the OakRidge National Laboratory to a teammade up of Burns & McDonnell (engi-neering project manager), Solar Tur-bines (prime mover), and Broad AirConditioning (exhaust-fired chiller).

Once the contract was awarded,Burns & McDonnell conducted “anextensive site selection process” tofind a site and compatible owner-op-erator partner (Austin Energy) toshare in the cost of plant equipmentand construction.

Under the project’s cost-share de-velopment arrangement, DOE put up$3 million for research and develop-ment work and some equipment (gasturbine and absorption chiller).

Austin Energy put up another $5.3million for balance of plant equip-ment, installation, start-up and inter-connection to the grid.

DOE studies show that the stan-dardized packaged system designshould cut CHP plant capital costs by15-30%, says Jan Berry, research anddevelopment program manager forOak Ridge National Lab.

In addition, the modular packagingshoud reduce installation time by asmuch as two-thirds, with a correspond-ing savings in construction costs.

Indirect fired chillerEd Mardiat, director of CHP develop-ment for Burns & McDonnell toldGTW, “This is a first of a kind projectin the sense we are using gas turbine

exhaust to operate an absorptionchiller to produce 2600 tons of chilledwater.

“Ours is unique because of its in-direct-fired cooling. We use the gas

GAS TURBINE WORLD: August-September 2004 15

Austin CHP plant operationIndirect-fired absorption chiller operates off Centaur 50 gas turbine exhaust toproduce around 2600 tons of chilled water for industrial cooling and turbine inletchilling. CHP plant is rated at 76.8% efficiency at 4.3 MW net output.

Inlet Chiller

AbsorptionChiller

4.6 MW GasTurbine

Cooling Tower

950°F exhaust

up to 225 tons

Generatorambient air fuel

4.3 MWnet power

up to 2600tons chilledwater

Broad absorption chiller. Two-stage indirect fired absorption chiller producesaround 2600 tons of chilled water, some of which is used for gas turbine air inletcooling to enhance hot day power output and efficiency.

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turbine exhaust to runs the chiller di-rectly rather than interposing anHRSG or gas firing for the asborptionchiller.” Indirect firing has been at-tempted before, he says, but on muchsmaller chillers well below the 2600ton size of the Austin unit.

There is another Centaur 50-pow-ered CHP plant developed by Honey-well, which has both an HRSG andindirect-fired chiller, that is being in-stalled at Ft. Bragg, North Carolina.For this application, some of the ex-haust can be ducted to a chiller andsome to an HRSG.

With this arrangement, splittingthe exhaust with a modulating divert-er valve, they will be able to makesteam during the winter and chilledwater during the summer.

Reportedly, Honeywell has creat-ed some innovative software thatwill allow the gas turbine to be oper-ated based upon time-of-day electri-cal rates and variable cost of naturalgas.

Control technologyAustin’s 2600 ton absorption chiller isstate-of-the-art, Mardiat notes, withadvanced control technology notavailable on earlier designs.

The programmable logic controlsystem used, both electronics andsoftware, was developed in China. Itoperates to continually match chilleroutput to demand.

Say, for example, that only 1200instead of 2600 tons of chilled wateris required. The controller will auto-matically bypass some turbine ex-haust flow into the atmosphere andthereby reduce output to maintain aconstant delivery temperature for thechilled water supply.

Overall plant integration, howev-er, is controlled by Allen Bradleysoftware which also provides pro-grammable logic remote monitoringcapability for the complete system.The system is intended to run in con-tinuous duty operation at full baseload output 24 hours a day.

In addition to integrating opera-tion of the gas turbine and chiller, thesystem control system can activateback-up electric centrifugal chillers tosupplement output whenever require-ments exceed the plant’s 2600-ton ca-pability.

As mentioned earlier, the conceptof using gas turbine exhaust to pro-duce chilled water is not new. How-ever, early attempts were not all thatsuccessful. At the time, absorptionchillers did not have digital controlsneeded to control the chiller’s lithiumbromide heat exchange process.

Gas turbine exhaust Generally, absorption chillers operateon steam, hot water or natural gasfuel. An indirect fired design uses awater and lithium bromide solutioninstead.

Gas turbine exhaust heat is addedto the solution to boil off some of thewater. In this thermal-chemical pro-cess, water under a vacuum is used asthe refrigerant for producing chilledwater.

Essentially, gas turbine exhaust re-places gas firing to achieve the sameeffect. The only design modificationsare in the high-stage generator, not inthe chiller itself. Nothing else changes– except for the control system.

Not all of the chilled water pro-duced by the Austin plant is sold,Mardiat points out. During hot dayoperation, up to 225 tons chilled wa-ter will be used for air inlet cooling,to improve gas turbine power outputand efficiency.

On a 95°F day in Austin, he ex-plains, that amount of coolant willlower the temperature of the Centaur50 gas turbine inlet air to 54°F whichproduces measurably better perfor-mance than at 95°F – and slightly bet-ter than its 59°F ISO rating.

Modular designIn keeping with one of DOE’s majordevelopment goals, the Austin CHPsystem design stresses flexibility andmodularity. The basic equipment lay-out is readily adaptable to various ca-pacity, space limitation and grid inter-connection requirements.

16 GAS TURBINE WORLD: August-September 2004

Modular package design. Turbine exhaust diverter valve and bypass damperstack (center) controls the hot gas flow supplied to the absorption chiller (on the left)by the gas turbine (far right).

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Project design engineers separatedthe equipment into seven packages:inlet air filter and cooling skid, gascompressor skid, gas turbine skid, ab-sorption chiller skid, chiller exhauststack, chilled water pump and controlskid, and bypass damper stack

If necessary, the modules can bedisconnected, mounted on trucks, andmoved to other locations. Basicallywhat this means, says Mardiat, is thatintegrated energy system designs ofthis type can be erected at a cus-tomer’s facility nearly anywhere inthe world.

“We are currently evaluating a va-riety of facilities at hospitals, univer-sity and college campuses, airportsand large corporate or government of-fice complexes where it makes senseto install similarly packaged CHPsystems.”

Future projectsDepending on location and other fac-tors, his firm is considering designsbased on either the 4.6-MW Centaur50 or newly introduced Mercury 50gas turbine with the same output rat-ing but which is recuperated to oper-ate at 38.5% efficiency – with a 5ppm NOx guarantee for the gas tur-bine alone.

As Solar emphasized in announc-ing the commercialization of the Mer-cury 50 last December at Power GenInternational, the design is ideallysuited for commercial CHP applica-tions where steam loads are less than25,000 lb/hr and there is a require-ment for chilled or hot water.

(See December-January 2004 is-sue of GTW for information on recu-perated Mercury 50 design featuresand performance.)

Reportedly, Solar and San DiegoGas & Electric plan to install a Mer-cury 50 based CHP plant at a hospitalcomplex in the San Diego area.(Some of the funding may come fromthe California Energy Commission).

Mardiat says his company is cur-rently discussing nominal 4.3-MWMercury 50 gas turbine powered CHPplant designs, each containing an un-fired HRSG and chiller, for hospitalsites in Austin and in Chicago.

The unfired HRSG for Austin willsimultaneously provide 120 psigsteam for a nominal 1000-ton twostage absorption chiller and 7,000 lbsof 120 psig steam for hospital heatingand process loads.

Recuperated Mercury 50 gas tur-bine has guaranteed limits of 5 ppmNOx and 10 ppm CO even withoutthermal credits. Projected CHP plant

efficiency is calculated at 74% in-cluding allowances for auxiliaryequipment.

For the Chicago hospital the un-fired HRSG will provide simultane-ous steam for a nominal 1200-ton twostage absorption chiller and for heat-ing and process load. Projected CHPefficiency for this project is calculat-ed at 72% including auxiliaries. ■

GAS TURBINE WORLD: August-September 2004 17

Chilled water pump. Pumping module delivers the main supply of 2600 tons pro-duced to the city’s district cooling system and a small percentage (up to 225 tons) tothe gas turbine for air inlet chilling.

Austin Energy CHP Plant Design PerformanceCentaur 50 genset at Austin, with losses, is nominally site rated at 4330 kW netoutput and 11,630 Btu/kWh heat rate (29.3% efficiency) simple cycle. In theCHP mode of operation, producing 2600 tons of chilled water, the CHP plant israted at 88.8% efficiency (LHV).

Parameters Genset CHP Plant

Net site ouput . . . . . . . . . . . . . . . 4330 kW 4330 kW

Chilled water . . . . . . . . . . . . . . . . None 2600 tons

Heat rate (LHV) . . . . . . . . . . . . . . 11,630 Btu/kWh 11,630 BtukWh

NG fuel compressor . . . . . . . . . . 265 kW 265 kW

GT parasitic load . . . . . . . . . . . . 5 kW 5 kW

Chiller parasitic load . . . . . . . . . . 35 kW 35 kW

Max efficiency . . . . . . . . . . . . . . . 28.6 % 76.8 %

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