By James Varley

Gas engines, cogeneration and thermal storage. This is a combination of technologies that could play an increasingly important role in making Germany’s Energiewende (energy turnaround) work. And there is already a good body of practical experience, notably in the southern German town of Rosenheim (pop about 61000), and its municipal energy company, Stadtwerke Rosenheim.
Stadtwerke Rosenheim has for some years has been actively engaged in an Energiewende of its own, demonstrating the advantages of being a relatively small but innovative municipality-based player – with a power plant in the centre of the town, and therefore well placed to provide heat not just power.
With the official start-up of a 9.5 MWe GE Jenbacher J920 Flextra unit on 10 April – the "field validation" machine for this engine type – Stadtwerke Rosenheim now has five gas engines in operation at its CHP facility in the town, together with a waste incineration plant. The expanded cogen facility, with a total installed capacity of 36.1 MWe and 43.8 MWt, meets about 40% of Rosenheim’s electricity needs and some 20% of the heating load, including a hospital and a dairy factory.
The other four gas-engines at the site, three 3.35 MW J620s plus a 4.4 MW J624 (also two-stage turbocharged), were all supplied by GE Jenbacher, which is based in nearby Austria and has been working with Stadtwerke Rosenheim for about ten years on engine projects.

Carbon neutral by 2025
These engines plus a planned additional J920, coupled with biomass gasification (which has been under development since 2006 as part of the municipal utility’s own R&D programme) are key to Stadtwerke Rosenheim’s future energy concept, which was outlined by CEO Dr Götz Brühl at the J920 inauguration event – see generation mix projection and Sankey diagrams on p 22 – with the basic goal that the town will be "carbon neutral" by 2025 in terms of power and heat (excluding transportation).
Dr Brühl noted that the importance of heat in the German energy system had tended to be underestimated, with some politicians thinking that improved insulation would eliminate it as an issue. But he pointed out that provision of "heat is and will remain important", being the largest single end-use of energy in Germany, accounting for more than 50% of the country’s energy consumption, and for around 85% of domestic consumption, a figure not projected to change much by 2050.
Another consideration is the need for flexible generation to accommodate a growing share of intermittent renewables, in particular to provide the sharply fluctuating "residual load" (see graph above, left), which is what Germans call the gap between the load provided by the must-run renewables and total power demand.
"CHP plant with district heating and thermal storage fit perfectly with wind and solar power," says Dr Brühl, while "huge gas plants waste energy (heat)" and their "start-up and shut-down is too slow." It is also worth noting that Germany has a national goal of increasing the power from CHP to 25% of the country’s total supply by 2020.
A particular attraction of distributed gas engine based CHP is very fast start-up and shut-down, quick load change and extremely high efficiency.
When coupled with thermal storage, which Stadtwerke Rosenheim plans to expand significantly (by a factor of about 20) in the coming years – allowing the decoupling of heat provision from power generation – an operating regime of the type shown left becomes feasible, and profitable, in a market with a large proportion of intermittent renewables.

Fast start
Stadtwerke Rosenheim’s new J920 FleXtra can be "on the grid" in about five minutes (from hot), while it has an electrical efficiency of 48.7% (heat rate 7392 kJ/kWh) – the highest for a machine in the 10 MW class, with the use of two-stage turbocharging adding about 2.5 percentage points of efficiency. The thermal output is 8.1 MWt, and in cogeneration mode the total fuel efficiency is around 90%. The engine speed is 1000 rpm.
GE says it expects the J920 FleXtra to be available in 60 Hz regions in 2014. Further information about the new engine will be provided in a future issue.