Energy | MTU REPORT 2/2008 | 29

E N E R G Y F A R M I N G

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Pig power“I produce nearly 2.3 million kilowatt-hours of electricity ayear,” proudly states farmer Martin Schmid. A few years ago, heconverted his farm business from specializing in pig rearing toproducing energy from biogas and now not only supplies elec-tricity to the local power grid but also heats the village schoolwith the recovered heat. In so doing he is using the modernheat and power plants supplied by MTU Onsite Energy to theirfull potential.

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1, 2, 3 Energy from corn and slurry: theSchmid family use maize grown in theirown fields and slurry from their pig farmto supply their biogas plant.

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Down on the farm, somethingis brewing. Hidden underconcrete covers, a gluey,

yellow broth is bubbling away tothe great delight of those who haveconjured it up. But there are noweird sisters at work here, just aspecial breed of farmers. What theyare producing from the giganticfermentation tanks sunk deep intothe ground and filled with a seethingmixture of decomposing corn andslurry is biogas.

Biogas is roughly 50 percentcombustible methane, familiar tomany as the main constituent ofnatural gas. The methane is whatmakes the bio-fuel so valuable,because it is easily converted intoenergy in the lean-burning enginesused in MTU Onsite Energycombined heat and power plants.The plants produce electricity from agenerator while also providing heat,which is recovered from the enginecoolant and exhaust. Due to the factthat they recover and utilize the heatfrom the process as well as gener-ating electricity, the modular CHPplants convert virtually all of theenergy in the fuel into useful energy.

CARBON-NEUTRAL BIO-FUEL.The principle of combined heat andpower generation is in any casemore climate-friendly than thetypical separate production of heatand electricity because it creates lesscarbon dioxide. Using biogas madefrom maize, for example, this type of small-scale power plant is actuallyentirely CO2-neutral because thecombustion process only releases theamount of carbon dioxide that wasabsorbed by the plants while theywere growing. Thus fossil fuels suchas natural gas, oil and coal, whichcontribute to the greenhouse effect,can be replaced with climate-friendlyrenewable energies. And, of course,the same applies to gas made fromorganic waste or slurry. So it is nosurprise that many farmers aregiving over some of their land togrowing fuel crops such as feedmaize instead of wheat or barley.Farmers like the Schmid family fromUnlingen in the Swabian region ofSouth Germany.

MUCK AND BRASS. The biogasfor generating heat and power isproduced right next to the piggeryfrom corn and slurry. “Once thefermentation process is running,the digester is a good source ofincome,” Martin observes, “becausethe material we feed into thefermentation process turns intobiogas all on its own.” The mixturein this case consists mostly of maize,sometimes a small amount of grass,and about one sixth pig manuresupplied by the thousand or soanimals in the pig pens next door.The concoction of biomass andslurry is mixed together in thedigester tank. An electric mixer thatstirs the gooey liquid around atregular intervals ensures a homoge-neous mixture and even distributionof heat. But the surface of the brewis by no means still when the mixerstops. The fermentation processcauses millions of bubbles to rise tothe surface – everyone of them filledwith biogas.

CHP PLANT HEATS THE LOCALSCHOOL. The Schmid family werecaught up by the biogas idea a fewyears ago. They built their firstdigester next to the piggery and asmall 100-kilowatt modular CHPplant with which they generatedpower for the local electricitycompany and heat for the pig pens.It went so well that before long thefamily took the decision to expandproduction. But first they neededto identify potential consumers. Acombined heat and power plant isonly viable if it there is a need forheat in the immediate vicinity. TheSchmids’ biogas plant is about akilometer from the edge of thevillage and their own needs werealready catered for. A heat pipelineover 1,000 meters long would nothave been worthwhile because thehot water would have cooled down

The used slurry-and-maize mixture is stored in a secondary digester and then used as fertilizer on the corn field or sold.

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1 Pig rearing and biogas production:right next to the piggery housing1,000 animals are the main digester tanksand the secondary digester (foreground). 2 Raw material: shredded maize is storedin large quantities near the digesters.3 Filling the digesters: maize is fed in tothe digester by a hopper on top of theconcrete cover.

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too much by the time it reached theconsumers. So the Schmids decidedto look for a suitable customer inthe village of 2,500 people wherethey could install a modular CHPplant directly on site. It didn’t takelong. The obvious candidate was thelocal school where the family’s sonshad also been pupils.

The Donau-Bussen-Schule is aprimary and secondary school with280 pupils. The classrooms werepreviously heated by an oil boiler.But the price of oil was rising andthe heating costs were increasingenormously. An economical alterna-tive such as an energy-efficient CHPmodule was an attractive option andso a CO2-neutral modular heat andpower plant was installed in theboiler room under the sports hall.Fueled by biogas, it is able to supplyheat substantially more cheaply thanthe oil boiler because of thediscount for the electricity it suppliesto the grid.

EXCLUSIVE HEAT SUPPLIER TOLOCAL SCHOOL. To supply the fuelto the school 1,700 meters away,the energy farmers built their ownpipeline. They have now been exclu-sive heat suppliers to the schoolsince 2006. A 20-year energy-supplycontract safeguards the business sothat investment in infrastructure andtechnology is economically viable.And the school benefits too becauseits heating costs are now lower thanbefore. Among the reasons why theTognum business unit Onsite Energy& Components was chosen tosupply the CHP plant were theadvanced engine technology and theperfectly matched peripherals. MTUOnsite Energy GmbH Gas PowerSystems – until recently known asMDE – has been making natural-gas,biogas or sewage-gas fueledmodular CHP plants with electricaloutputs from 125 to 400 kilowattsfor over 30 years. For a good twoyears now, the company has alsobeen the Tognum Group’s center ofexcellence for gas engines, creatingsynergetic effects between itself andMTU. The Augsburg factory nowproduces more powerful gensets(electricity generator modules) and

natural-gas fueled CHP plantscapable of supplying between 775and 2,000 kilowatts of energy.

SMOOTH-RUNNING ENGINE. Themodular combined heat and powerplant in the Donau-Bussen-Schuleis driven by a six-cylinder Series 400engine. It is compact, housed in anenclosure about the size of a largewardrobe, and extremely quiet.“We couldn’t have a noisy, vibratingmachine here as we are right under-neath the sports hall,” MartinSchmid explains. Thanks to its effi-cient sound insulation and thesmoothness of the gas engine, theCHP plant disturbs nobody. Its200 kilowatts of heat output coverthe demand for hot water andheating during the summer and thespring and autumn transition periods– in other words it supplies the base-load requirement. In the winter, it issupplemented by the oil heating.Dimensioning the cogenerationmodule to cover the base loadensures that it is continuously inoperation. That is important for tworeasons. Firstly, a CHP module paysfor itself more quickly the morefrequently it is in use. And secondly,the biogas produced has to be usedcontinuously, so that ideally the CHPplant should never shut down.

Engine output and biogas productionperfectly matched. “On average, wehave been able to utilize 95 percentof the overall plant output,” MartinSchmid explains, revealing his satis-faction with the heat and powerrevenue. The biogas plant enabledthe family to sell 2.3 million kilo-watt-hours of “green electricity” lastyear, amounting to an income of300,000 euro. “The plant has alwaysbeen reliable. The anti-knock controlsystem adjusts the engine perfectly tothe biogas quality and it has workedlike a dream.” Almost incredibly, itwas only idle for 25 hours last year –while it was serviced. That adds up toan availability rate of 99.7 percent.And after 11,000 hours of service todate, the unit hasn’t even needed anoil change because the 1,000-liter oilcirculation system showed virtually nosigns of contamination.

Ralf Dunker

Ô Benno Müller

benno.mueller@mtu-online.comTel. +49 821 7480 263

Production process of the biogas farm: the Schmid family makes biogas usingshredded maize, some of which they grow themselves, and slurry from 1,000 pigsplus small quantities of grass. The two fermentation tanks (digesters) are locateddirectly adjacent to the piggery. The maize is also stored near the digesters. Thebiogas is stored in a holding tank and supplied by a 1.7 km pipeline to the MTU Onsite Energy CHP plant located directly in the client’s basement.

1, 2 Martin Schmid monitors theoperation and the heat & energy generation process of the cogenera-tion power plant and analyzes the acidcontent of the ferments in his mini-laboratory. 3 The local school is the appreciativeheat consumer.

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MORE EFFICIENT THAN POWER FROM THE GRID

Combined heat and power modules simultaneously provide electri-cal and thermal energy from a single process. Their main compo-nents are usually an engine, a generator, a coolant heat exchangerand an exhaust heat exchanger. The engine, fueled for instance bybiogas, drives the electricity generator attached to its output shaft.The engine heat inevitably produced is recovered from the enginecoolant or the exhaust by a heat exchanger and transferred to a heatconsumer. That could be an apartment building that requires hot wa-ter and heating, a public swimming baths or a commercial green-house.

Because the cogeneration plant converts around 90 percent of thegas energy content into usable energy, it is significantly more effi-cient and environmentally friendly than generating electricity inlarge-scale power stations and using heat from a separate sourcesuch as a central heating system. Why? Because the heat byproductfrom large-scale power plants (such as coal-fired power stations) isalmost entirely wasted. And that means that combined heat andpower generation saves around a third the amount of carbon diox-ide compared to power from the grid.

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