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Distributed power generation using

microturbinesS SZEWCZUK

CSIR Built Environment, PO Box 395, Pretoria, 0001, South Africa

Email: s szewczuk@csir.co.za www.csir.co.za

INTRODUCTIONAt present, the bulk of the worlds electricity is generated in central power stations.This approach, one of economy of size, generates electricity in large powerstations and delivers it to load centres via an extensive network of transmissionand distribution lines. An alternative approach, that of distributed generation,which can be described as economy of mass production, generates electricityby many, smaller power stations located near the load centres. One such form ofsmall power generation system is that based on microturbines. Microturbines, asthe name implies, are much smaller versions of the conventional gas turbine. Amajor advantage of a microturbine is its ability to provide firm power, providedthat it is kept supplied with fuel. The primary source of fuel is currently based onfossil fuels. However, gas turbines have the ability to accept various fuels, such

as those based on liquid or gas. Being able to accept a diverse range of fuels,this opens possibilities of non-fossil based fuels to be used. Microturbines aresmall combustion turbines that produce between 25 kW and 500 kW of power.Microturbines were derived from turbocharger technologies found in largetrucks or the turbines in aircraft auxiliary power units (APUs). Most microturbinesare single-stage, radial flow devices with high rotating speeds of 90 000 to120 000 revolutions per minute (rpm). This poster describes the research workundertaken by the CSIR that led to the demonstration of a microturbine to generateelectrical power. The CSIR is currently undertaking research into the productionof biogas from wet organic waste sources as an alternative renewable fuel formicroturbines.

EXPERIMENTAL AND NUMERICAL INVESTIGATIONSTo complement research onaxial turbines for aerospaceapplications, research wasundertaken to develop an under-standing of the characteristicsand performance of radial inflowturbines. Initially this was doneon a diesel engine dynamometerwhere a turbocharger, whichuses a radial compressor and aradial turbine, was instrumentedto be able to measure the

various characteristics of theturbocharger. Figure 1 shows an

instrumented turbocharger on a diesel engine where experiments were done ina CSIR engine dynamometer test cell.

A dedicated test rig was designed and built in a test cell in the CSIRs highspeed wind-tunnel building to evaluate the performance of a 9,6 cm radial inflowturbine of a turbocharger1. This was done so that radial inflow turbine designtools could be validated experimentally. Testing was done under cold steadyflow conditions and the tests were automated and controlled by a computer-based data acquisition system. This test rig allowed for a comparison betweenan experimental investigation and analytical prediction of the performance of theradial inflow turbine. A novel feature of this test rig was the design and construction

of an air bearing dynamometerto measure the power generatedby the turbine directly. Here thecompressor housing was isolatedfrom the rest of the turbocharger,mounted on air bearings andtorque measured on a loadcell.Evaluation of the aerodynamicperformance of the turbinewas conducted over a range ofpressure ratios and shaft speeds.Figure 2 shows the instrumentedradial inflow turbine with thecompressor connected to the airbearing dynamometer.

MICROTURBINE TECHNOLOGY DEMONSTRATORHaving acquired understanding of and expertise in the characteristics of radialinflow turbines, resources were allocated towards the understanding of thevarious components that would convert a turbocharger into a microturbine.Barnard and Caroline2 discussed the radial flow turbomachines as a powergenerator and describe in detail the components of the microturbine technologydemonstrator. Figure 3 shows the core of the demonstrator, which is based onthe Holset Model 3LD turbocharger, with a reserve flow combustor designed byStellenbosch University. A digitally-controlled single-point injection system wasemployed and a Bosch L-Jetronic injector was combined with A Hago 80o swirlatomiser to deliver the fuel, diesel, into the combustor. The control system, forwhich software was developed, made use of an Intel8052AH central processingunit that controlled the various parameters of the microturbine. The Holsetturbocharger has an operational range of between 40 000 and 120 000 rpm.A high speed alternator, with a maximum shaft speed of 30 000 rpm, had beendesigned by the then University of Natal and to integrate the alternator withthe microturbine, a power turbine was added to accept the hot exhaust gasesfrom the Holset turbocharger-based free turbine. The power turbine was basedon a standard KKK turbocharger that was directly coupled to the high speedalternator. The reason why direct coupling of t he alternator to the microturbine

was investigated was to demonstrate that the overall system could be simplifiedby eliminating the need for gearboxes.

Killey3 described the testing of the microturbine technology demonstrator wherethe concept of a turbocharger-based gas turbine was coupled to a high speedalternator. The electricity that was produced by the demonstrator proved thatthe concept was feasible. However, it was recognised that further optimisationof the overall concept was necessary.

The purpose of t he microturbine/alternator demonstrator had been to illustratethe feasibility of such a concept by using as many off-the-shelf components as

possible. To illustrate a moreoptimal microturbine/alternatorconfiguration, a mock-up wasmade of a high-speed alternatordirectly coupled to the freeturbine, as shown in Figure 4.

RADIAL TURBINE CASTINGTECHNOLOGIESDue to the high temperaturesand stresses that turbinesneed to endure, nickel-based

super alloys are employed toensure that turbines achieve thedesigned operational life. Aspart of the overall programme todemonstrate a microturbine theCSIR developed technologiesto cast the complex three-dimensional (3D) shapes ofradial inflow turbines out ofthese superalloys. This was donewith the view to gain experienceto cast turbine components aspart of the concept of economyof mass production. Figure 5shows the iris type dye that wasmade to cast the component outof wax. A wax casting of theradial turbine is included on the photo. Figure 6 shows the superalloy casting ofa radial inflow turbine with its complex 3D flow passages.

MICROTURBINE SPECIFICATION DEVELOPMENTTo develop an optimised microturbine, investigations were undertaken by the CSIRinto various gas turbine thermodynamic cycles using turbocharger components.Szewczuk4,5 investigated a 10 kW and a 25 kW microturbine with a simplecycle and a simple cycle with heat exchanger. Roos and Mtyobile6 undertooka comprehensive investigation into further thermodynamic cycles that includedintercooling, reheating and heat exchange using a TSA turbocharger as the coreof the machine. Further work on the development of a microturbine was stoppedat that stage. However, internationally, further developments on microturbinescontinued and these are now becoming readily available on t he market with arange of available power outputs.

ALTERNATIVE FUELS - BIOGASThe most common types of fuelsused to power microturbinesare based on fossil fuels,namely diesel and naturalgas. Pointon and Langan7investigated the feasibilityof using microturbines fordistributed generation andbeing fuelled with biogasderived from organic waste.The investigations indicatedthat the microturbine willfind application on biogasinstallations that will benefitfrom its fixed output, high-part load efficiency andreliability. When the wastemanagement and energy-efficiency elements are linked,analysis indicated that theeconomic performance makesthe concept of distributedpower generation usingbiogas-fuelled microturbinesattractive under current

market conditions. Anexample of a microturbinebeing fuelled by biogascan be found in Hagawikin Sweden and the overallinstallation is shown in Figure7. The blue container housesthe microturbine and the large reservoir is the biogas anaerobic digestor.

The CSIR is currently investigating the use of biogas to fuel a microturbine. Researchis being done into the quantification and c haracterisation of various wet organic

waste sources (sewage, animal slurries, food waste) and the quantification ofthe significant parameters to maximise biogas production. These parametersinclude loading rate, gas production rate and temperature of the digestor.Temperature directly influences the gas production rate; at 35C a mesophylicprocess takes place and at 65C a thermophylic process takes place. Figure 8shows a laboratory-scale investigation into the mesophylic and thermophylicanaerobic digestion of food waste as part of the research into maximising biogasproduction from wet organic waste.

CONCLUSIONThe CSIR has developed capacity and expertise in the field of microturbines,which have the potential to be used for distributed power generation. To reducethe use of fossil fuels, investigations are underway to maximise the productionof biogas from various sources of wet organic waste, for example sewage,animal slurries and food waste. Biogas can be used as an alternative fuel formicroturbines.

REFERENCES1. Szewczuk S, An Experimental Investigation and Analytical Prediction of the Aerodynamic Per-

formance of a 9,6cm Radial Inflow Turbine, MSc dissertation, University of the Witwatersrand,1988.

2. Barnard JP and Caroline GM, Radial Flow Turbomachine as a Power Generator, CSIR ReportNo. DASCT 90/240, August 1990.

3. Killey K, Demonstration of a Radial Gas Turbine Power Generation Concept, CSIR Report No.AERO91/225, September 1991.

4. Szewczuk S, Development of a Specification for a 10 kW Turboalternator, CSIR Report No.AERO 92/081, February 1992.

5. Szewczuk S, Development of a Specification for a 25 kW Alternator, CSIR Report No. AERO92/082, February 1992.

6. Roos TH and Mtyobile V, Investigation into Gas Turbine Cycles using Turbocharger Componentsfor Rural Power Generation, CSIR Report No. DEF 2000/043, May 2000.

7. Pointon K and Langan M, Distributed Power Generation using Biogas Fuelled Microturbines,ETSU Contract B/U1/00670/00/Rep DTIti/PubURN 02/1345, 2002.

CPO-0021

Figure 1: Instrumented turbocharger ondiesel engine

Figure 2: Instrumented turbocharger intest cell

Figure 3: Core of microturbine technologydemonstrator

Figure 5: Iris type dye with waxcasting

Figure 6: Superalloy casting of radialturbine

Figure 7: Example of a facility producing biogasto fuel a microturbine.

Figure 4: Mock-up of directly coupledmicroturbine/alternator

Figure 8: Thermophylic and mesophylicanaerobic digestion of food waste.

To helpsecureSouthAfricasenergyfuture,CSIR re-searchersare usingaerospaceknow-how andexperienceto

integratenovelenergysystemsandoptimisethe use ofresourcesforsustainable

economicdevelop-ment.