Power2 proves its worth

As well as over two years of fault-free operation, its two turbocharged gas engines have brought about a 10% improvement in power efficiency at a commercial market garden in France, from 4.0 MW to 4.4, since the ugrade in 2011. The power density improvement has been achieved by incorporating, in conjunction with GE, ABB's two-stage turbocharging solution, Power2, which the company reports has been 'setting new standards for engine efficiency and performance while minimising environmental impact'. At the beginning of 2011, in Machecoul, Power2 was installed on two GE Jenbacher J624 lean-burn, spark-ignited gas engines at the Serres Vinet market garden - a first in France. This solution is still at the heart of two cogeneration plants powering Serres Vinet's original greenhouses and a 17-hectare expansion. Each Jenbacher J624 engine offers about 4.4 MW of electrical output and 4014 kW of thermal output at 44.4 % electrical efficiency and 47 % thermal efficiency. The units operate on natural gas and are the key that enables Serres Vinet's cogeneration plants to achieve an overall energy efficiency of more than 90 %, with NOx emissions continuously below 350mg/Nm³.

These engines, set up for CHP, generate all the hot water and electricity required for the facility's extensive tomato, cucumber and lettuce greenhouse cultivation.

As well as improved technical operation, the installation has helped minimise the environmental impact of Serres Vinet's operations. Heat from the engines is used to warm the greenhouses when it is cool, or is stored for later use, and the CO2 from the exhaust gas, having passed through special catalytic converters for cleaning (SCR and oxidation), is cooled to about 50°C and fed to plants in the greenhouse. This process increases the photosynthetic conversion of CO2 and water to sugar and oxygen in plants. So apart from providing heat and electricity, the CHP system also helps increase crop yields.

Results

Adding Power2-enhanced turbocharging modules to the system guaranteed very high reliability and turbocharging efficiency. In more than two years of operation, there was no unplanned downtime, while power output was reliably improved by 10%. And when ABB engineers inspected the installation for the first time last year, they found that all the components in the Power2 340-H turbocharging system were fully operational and that the installation could have continued running reliably without alteration.

In fact, the installation's performance has been impressive enough to prompt the installer Forclum, which specialises in the installation of electricity systems, two purchase two more gas engines with Power2 for another greenhouse.

‘The efficiency, flexibility and reliability of these two-stage turbocharged gas engines have been outstanding,' commented Gilles Marquerat, director of Forclum's power production department.

Two stage turbocharging

Power2 two stage turbocharging technology is capable of producing pressure ratios as high as 8 and above and it is already widely accepted as a major enabling technology for the strong Miller Cycle which can substantially reduce NOx emissions on diesel engines while realising the potential for enhanced power densities and fuel efficiencies. On gas engines Power2 is also seen as a Miller enabler, but in these applications the major focus is to employ the high order of charge air pressures that can be produced to increase power density and efficiency.

The first commercial application was with a GE J624 gas engine in a horticultural application in the Netherlands. The 24 cylinder J624 with two stage turbocharging achieves a rated output of 4.4 MW compared to the 4 MW of its predecessor with conventional turbocharging and offers an efficiency increase of about 2% while, like modern lean burn gas engines, undercutting Tier III limits on NOx by a wide margin. Moreover, the higher level of charge air pressure from the ABB two stage turbocharging system helps the engine maintain rated outputs in climates with high ambient temperatures and humidity and on gases of variable calorific value.

At the present stage of development, these enhancements are based on pressure ratios well over 6 bar, enabling the gas engine to run at mean effective pressures above 22 bar and giving scope for the future application of advanced Miller Cycles.

The Power2 system consists of two turbochargers of different, tuned frame sizes connected in tandem on the compressor side via an intermediate air cooler - cooling the compressed air issuing from the first turbocharger means the second turbocharger needs to do less work and can be more compact.

By assisting engine builders to attain a strong Miller Cycle, Power2 is instrumental in alleviating one of the hitherto most intractable constraints in the search for lower NOx emissions without incurring penalties on the fuel efficiency side - the compromise between NOx emissions and specific fuel consumption (SFC), which reflects the fact that NOx formation reduces with lower combustion temperatures while engine fuel efficiency increases with higher combustion temperatures. So in the early days of emissions reduction on diesel engines, a widely used measure was to reduce combustion temperatures by retarding fuel injection to reduce the rate of heat released from the fuel, incurring a fuel consumption penalty. ABB and its development partners found that using a combination of much higher turbocharging pressures, variable valve timing, advanced fuel injection technology (eg common rail), and electronic control, the Miller Cycle is capable of shifting this compromise between NOx emissions and specific fuel consumption values into a new, far lower range. The effect of the higher turbocharging pressures used is a useful increase in engine power density, which is clearly demonstrated in gas engines.