A gas-powered cogeneration module operates at an exceptionally high operating efficiency, thanks to a heat extraction system and use of a heat pump, at a district energy station in Germany. A comprehensive long-term maintenance agreement completes the benefits package, as Peter Grüner reports.

The Freiburg suburb of Vauban has undergone massive changes in recent years. The area, which covers 0.4 km², was used as barracks by the French army up until 1992. Now it is a residential suburb with living space for some 5000 citizens, most of whom live in low-energy, ‘passive’ houses. Many of the houses use photovoltaic systems to generate electricity, thus helping to protect the climate. Heat is also provided in an ecologically sound way, thanks largely to a 14 km local heating grid operated by Badenova Wärmeplus, Freiburg.

GAS-POWERED COGENERATION

Badenova Wärmeplus, the subsidiary of the regional energy and environmental services provider Badenova, specializes in eco-friendly, efficient heat and power generation. From 2001 to 2002, the company built a CHP station in Vauban to feed the local heat network. It went into service in April 2002, initially with a wood-fired cogeneration plant, supported by two peak load boilers fuelled with natural gas or oil. The wood chip heating system realized a thermal output of 2.5 MW, and the gas and oil boilers 2.5 MW each.

Also in place was a steam engine/generator combination with an electrical output of 240 kW. While the wood chip heating system functioned impeccably, the steam engine/generator combination was prone to frequent failure. The reciprocating engine, using hot steam at 250°C and 26 bar pressure, often broke down, with only a fraction of the targeted electrical output being achieved by the engine over the years.

Instead of continuing to refurbish the repair prone steam engine, Badenova Wärmeplus replaced it in spring 2011 with a gas-powered cogeneration module based on an MTU Series 4000 engine for the main supply. The existing wood chip heating system, originally the main generator, has now been re-configured to support the CHP module when the demand for heating is high. During peak demand times, the gas and oil boilers also step in to accept load. The electrical power generated is fed into Badenova’s 20 kV network via a new transformer station.

Being extremely well insulated, the houses in Vauban have relatively low heating requirements. Hot water consumption accounts for a high share of the total heating demand of around 14,000 MWh/year. With its thermal output and a scheduled runtime of 7,200 operating hours per year, the CHP unit is able to cover over 50% of that requirement. The rest is covered by the wood chip heating system and peak load boilers.

To make optimum use of the energy used, the heating experts at Badenova Wärmeplus worked with the consulting engineers Eser, Dittmann, Nehring & Partner to elaborate a plant concept offering a level of efficiency several percentage points higher than that of a conventional cogeneration power plant. At the heart of the solution is the engine from MTU Onsite Energy. The operator was impressed not only by the sound technical basis of the concept, but also by its good cost/benefit ratio and plant efficiency.

An effective replacement for the steam engine: the new cogeneration module is based on the Series 4000 gas engine – here in the 8-cylinder version with an 850 kW electrical output

Combined with long-term maintenance agreements, a high degree of planning certainty has been created. ‘We have long-standing experience of operating various MTU installations and we rate the company highly as a reliable and competent partner,’ said Klaus Schipek, project leader for CHP and heating systems at Badenova Wärmeplus. ‘As far as electrical efficiency is concerned, the 8-cylinder Series 4000 engine in the cogeneration module delivers a convincing performance,’ he said.

The engine achieves such a high output that the generator at the shaft end supplies around 850 kW power, meaning that some 40% of the energy contained in the fuel is utilized. The power generated by the cogeneration module is enough to cover the electrical needs of the Vauban district all year round.

ACHIEVING MAXIMUM HEAT EXTRACTION

‘Tapping’ of the coolant, oil circuit and exhaust gas is optimized in order to extract around 1150 kW of heat. In Vauban, two exhaust gas heat exchangers are installed for this purpose. The second exhaust gas heat exchanger cools the exhaust gas a second time from around 120°C to below 66°C using the additional thermal energy contained in the exhaust gas. The heating water arriving at the grid return at a temperature of around 62°C is initially pre-heated by a moderate 1.3 K by the low-temperature exhaust gas heat exchanger. The heat from the mixture cooler, engine oil and engine coolant then increase the temperature to around 67.5°C. Finally, the heating water flows through the first exhaust gas heat exchanger (as viewed in the exhaust gas flow direction), which heats it to the target temperature of about 90°C.

Use of an additional low-temperature exhaust gas heat exchanger raises the overall efficiency of the cogeneration module to 93%. To raise the efficiency level even higher, the waste heat from the cogeneration module – especially from the generator and the engine block – does not simply flow into the atmosphere via a ventilation system. Instead, an electric heat pump cools the area where the cogeneration module is erected and the second-stage section of the mixture cooler, and forwards the heat extracted to the district heating grid. In that way, up to 140 kW of additional output is mobilized, which is enough to heat a further 160 households. The cooling of the area by the heat pump also benefits the engine, particularly in the summer, when outside temperatures are high. Keeping down the temperature of the intake air to the engine enhances its performance.

If the energy available in the form of power and heat is added together and then divided by the sum of the power and fuel used to generate it, the plant achieves a possibly record-breaking efficiency level of over 96%. That value puts the Vauban CHP module well ahead of its conventional counterparts, which achieve on average 90% efficiency.

CARBON REDUCED BY OVER 4000 TONNES

For heat demand-based operation, Badenova Wärmeplus installed a 100,000 litre stratified heat accumulator next to the cogeneration module. Hence the cogeneration plant can operate at full load for one to three hours and charge the accumulator.

As part of the modernization programme, a new exhaust stack also had to be built. Although the output of the cogeneration plant is now significantly higher, the height of the new stack is relatively low at 24 metres – just 2 metres taller than the previous stack. That is thanks to the fact that emissions from the Series 4000 engine are 50% lower than the limits prescribed by TA Luft (Technical Instructions on Air Quality Control).

Converting the power station has not only brought benefits in terms of higher energy utilization, but has also made a contribution to environmental protection. The higher overall level of efficiency allows primary energy consumption to be reduced by about 40% in comparison to conventional power plants. Furthermore, the cogeneration module brings a saving of 1.8 million litres of heating oil and 4200 tonnes of carbon dioxide per year. Vauban’s cogeneration power plant is a near perfect example of sustainable power generation.

LONG-TERM MAINTENANCE AGREEMENTS

On installation of the new cogeneration module, Badenova Wärmeplus signed a maintenance agreement with MTU Onsite Energy initially covering a period of 10 years. The ‘peace-of-mind’ package includes the supply and disposal of engine oil, as well as all preventive maintenance work and repairs. That means that all possible damage to the plant is covered. Even the first major overhaul after 64,000 operating hours is included. The operator therefore benefits from the highest degree of service quality, with simultaneous cost cover.

CONCLUSION

The power plant team is satisfied: the MTU cogeneration module operates reliably and with low emissions. Designed for using natural gas, it can also be powered by bio-fuels. The Badenova company has been operating biogas plants since 2010, which turn biogas into bio-natural gas for supply to the gas grid. All in all, the Vauban combined heat and power station is an example of an ecologically sound solution that meets the environmental requirements of local citizens, at the same time saving them money for heating thanks to its outstanding efficiency.

Peter Grüner is sales manager at MTU Onsite Energy, Gas Power Systems, Germany. Email: peter.gruener@mtu-online.com

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