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A showcase for cogeneration

A combined heat and power unit at Stapelfeld in the Hamburg metropolitan area is a benchmark example for the many that will soon follow in Germany, writes Dr Jacob Klimstra

The Stapelfeld CHP plant with its 40 metre-high chimney

Credit: GE

On Saint Martin’s day, 11 November, 2015, Hamburg’s Environmental Senator Jens Kerstan inaugurated a showcase cogeneration plant at Stapelfeld in the Hamburg metropolitan area.

The plant is owned and operated by HanseWerk Natur GmbH, a wholly-owned subsidiary of HanseWerk AG.

Cogeneration of electricity and heat is nothing new to Hansewerk Natur, since they already operate some 200 combined heat and power (CHP) units in the north of Germany. The Stapelfeld plant, however, is equipped with the latest reciprocating engine-based generator set developed by GE, the J920 FleXtra unit.

This unit has a power capacity of 9.5 MW and achieves a combined fuel efficiency of 95% to 97%. Due to an innovative heat recovery system, the heat utilization results in 47% to 49% of the fuel input, and the electrical efficiency was optimized to around 48%.

A combined fuel efficiency of more than 95% is a great achievement for a CHP unit operating in a 60à‚°C in/105à‚°C out district heating system.

HanseWerk Natur

HanseWerk Natur GmbH operates 150 district heating systems in the metropolitan area of Hamburg and the states of Schleswig-Holstein, Mecklenburg-Vorpommern and Nordniedersachsen. The pipelines of their district heating systems cover a total length of 900 km. Next to natural gas, they convert biogas, landfill gas and sewage gas into electricity and heat.

In addition to using their own 200 CHP units, HanseWerk Natur integrates the heat output of a number of cogeneration units owned by their customers into the district heating systems. Some 40,000 customers are provided with a total of 1.3 TWh of heat per year, equivalent to 4.7 petajoule. Heat storage is an important part of the system, since it enables a decoupling of electricity and heat use.

Thomas Baade, director of operations and technology for HanseWerk Natur, informed Decentralized Energy that his company operates a combination of 65 CHP installations as a virtual power plant. This enables them to participate in the German power market by not only offering kilowatt-hours, but also operating reserves and control power. Such a virtual power plant is very convenient in the German power system, which is characterized by the fluctuating output of a large installed base of solar panels and wind turbines. Heat storage is, of course, an essential element in this.

Baade is convinced that proper integration of electricity and heat is the basic precondition for a further expansion of renewable energy. Having so many individual CHP generating units combined offers an unsurpassably high probability of complying with the capacity offered on the market. It also provides super flexibility.

The new Stapelfeld CHP plant has been integrated into the virtual power plant. The fast response of this engine to required load changes and its rather flat electrical efficiency in the upper load range offer excellent economic control opportunities with respect to the output.

GE’s Jenbacher J920 FleXtra engine

The J920 FleXtra gas engine-based CHP unit is a relative newcomer in GE’s Jenbacher portfolio. A first unit has been tested and optimized at the Stadtwerke Rosenheim in the south of Germany over a timespan of about two years.

The nominal electrical output of the unit is 9.5 MW in a 50 Hz grid system and 8.6 MW in a 60 Hz system. The background of this is that, in a 50 Hz system, the unit is running at 1000 rpm, while in a 60 Hz system the running speed is only 900 rpm. In both cases, the brake mean effective pressure equals 22 bar.

At nominal output, the electrical efficiency is 48.7% for the 50 Hz version and 49% for the 60 Hz version at a power factor of 1, a specific NOx production of 500 mg/m3 at 5% O2 and a methane number (MN) higher than 80. The tolerances of ISO 3046 apply for these performance data.

The high performance of this unit is only possible thanks to the efficient turbocharger system. The two-stage turbocharging with two intercoolers more closely approaches isothermal compression than a single-stage turbocharger. This results in high efficiency of the turbocharging process, meaning that less back pressure is required to create the high charge air gauge pressure of about 6 bar.

The engine is equipped with electronically controlled fuel gas injection per cylinder. The fuel gas pressure has to overcome the charge air pressure in the intake section and to provide control range for the gas injection valves. Therefore, the required gas-supply gauge pressure for this engine is 8 bar. Such a gas pressure is common practice in regional gas supply lines and in industrial areas.

From standstill to full output of the plant takes only five minutes. This makes it possible to sell secondary reserves with a non-spinning machine. When running, the unit has a ramp-up rate of 4.8 MW per minute and a ramp-down rate of 6 MW per minute. Therefore, ramping up equals 50% of nominal output per minute and ramping down equals 63% of nominal output per minute. Such high ramping rates are impossible to achieve with large central power plants.

The engines have also been equipped with a diagnostic system that predicts the remaining useful life of typical components. As an example, the breakdown voltage of the spark plugs is constantly monitored. This breakdown voltage is directly proportional to the electrode gap of the spark plugs.

Maintenance intervals

The J920 FleXtra is designed for a maintenance schedule of up to a 40,000/80,000-hour interval. With GE’s remote monitoring and diagnostic system, all critical parts are monitored. The system uses fleet comparison, which helps to increase availability for the customers.

The unique power unit concept allows GE to exchange a whole unit consisting of cylinder head, water jacket, piston, liner and con rod. This allows for easier maintenance and reduces the downtime of the engine significantly.

The Stapelfeld CHP installation

At first sight, it seems impossible to reach a combined fuel efficiency of more than 95% with a cogeneration plant in a district heating system having a return temperature of 60à‚°C. The coolant temperature of the low-temperature intercooler is much lower than 60à‚°C and exhaust gas condensation is normally not possible for such a return temperature.

Also, the low temperature heat released by the generator is generally lost. In many cogeneration installations, the convection heat (‘radiation heat’) from the engine block is also at least partly lost to the environment.

However, this is the really innovative part of the installation. Here, the combustion air and ventilation air are drawn from the side of the room where the turbocharger block is positioned. At the exit of the ventilation air, at the generator side, a heat exchanger captures the heat from the ventilation air.

The heat from all four sources of low temperature is subsequently offered to an electrically driven heat pump with a coefficient of performance of more than 5. This heat pump uses ammonia as the medium. The power consumption of the heat pump is only 150 kW. Such a solution also enables low-temperature condensation of the exhaust gases.

Since the condensate has to be released to the sewage system, it is treated with active carbon to capture undesirable components originating, e.g., from the lubricating oil additives. Further, the relatively acid condensate is treated so that the maximum pH (= acidity) is well below the value acceptable for the sewage system.

The installation is also equipped with an oxidation catalyst to comply with the German TA-Luft emission limits for carbon monoxide and aldehydes. Further, space has been foreseen in the exhaust system for inserting a selective catalyst in case the NOx emission rules are further sharpened.

Maximum attention has been given to safety. Rupture disks have been installed at critical locations before and after the first noise damper, as well as close to the catalyst. The probability that exhaust explosions will occur in CHP units is very low, but if it happens, no damage will occur to the installation and no unsafe conditions will occur for the operators. After each stop, the exhaust system is purged with air to exclude the presence of ignitable mixtures when starting.

The housing of the installation is made from concrete. Plenty of space is available for maintenance. The exhaust gas heat exchanger is positioned together with the catalyst and noise damper on a separate floor, some 10 metres higher than the engine base. This further improves access in case maintenance has to be carried out.

The total impression is an optimum installation, not only from the point of view of fuel efficiency but also considering the total layout.

Economic aspects

The building time of the CHP plant has been exceptionally short. Groundbreaking was done in March 2014 and the whole installation was ready to run at the end of September 2014. The official inauguration was originally scheduled for the beginning of 2015, but organizational circumstances required a postponement to early November 2015.

A short building time drastically reduces the capital costs of an installation. Moreover, a rapid response to expected increases in power and heat demand can be made.

One might expect that the spacious layout of the installation and the additional investments in the heat pump, rupture disks and diagnostic system would lead to high investment per kW. However, HanseWerk Natur revealed that the investment was just €715 per kW. This is certainly a benchmark value for such an elaborate installation.

Low investment costs and high fuel efficiency are not only attractive for the owner of the installations. The customers will also benefit from the lower costs. The environmental performance of this CHP plant is so good that the customers connected to the system can get a raised environmental rating for their houses without having to invest in additional measures.

The J920 FleXtra generating set consists of three modules: the generator (left), the engine and the turbocharging auxiliary block

Credit: GE

A benchmark

This CHP installation is a good example for the many that will soon follow in Germany. Existing German CHP installations produce some 96 TWh per year, or roughly 16% of the country’s electricity needs. The installed capacity is estimated at 19 GW, or a capacity factor of 58%, which is reasonable for CHP in the typical German climate. Since the German policy goal is to generate 25% of electricity from CHP by 2020, at least an additional 10 GW must be installed. That compares with more than 1000 installations of the size of the Stapelfeld unit.

This means that there are great opportunities for CHP in Germany. The Stapelfeld unit can act as a benchmark.

The floor with the noise damper and the box for the catalyst, together with the three overpressure relief channels

Credit: GE

Dr Jacob Klimstra is Managing Editor of Decentralized Energy

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