SIEMENS

Stephen Kiprotich, who won the marathon event at the 2012 Olympics, weighs only 56 kilos, but his lungs have a volume of around eight liters, double that of the average person. This allows him to convert his body’s energy into movement very efficiently. Usain Bolt, who holds the world record for the 100-meter race, weighs 38 kilos more than Kiprotich—additional muscle mass he uses to achieve faster acceleration.

Like runners, state-of-the-art coal and gas-fired power plants have to be both efficient and dynamic. That’s because every efficiency gain of one tenth of a percent at a major plant reduces CO₂ emissions by up to 7,000 tons per year – or it increases output by seven gigawatt-hours while maintaining the same level of resource consumption. Meanwhile, the rising share of electricity produced from renewable energy sources requires fossil-fuel power plants to operate more flexibly. For example, if the sky is cloudy and there’s little or no wind, these power plants need to ramp up to full capacity as quickly as possible.

Efficiency and dynamic operation are built into new power plants from the start. Today’s coal-fired plants have an electrical efficiency of 46 percent – 15 percent higher than the global average for generation facilities. Combined cycle plants have efficiencies of over 60 percent. Although new coal-fired power plants with a collective output of over 350 gigawatts (GW) have been built in the last five years, the existing total global capacity in this area is over 1,600 GW. The ratio between old and new is even less favorable in certain countries. For instance, more than 80 percent of the coal-fired plants in Russia are over 20 years old, and the efficiency of some facilities is only 23 percent – meaning that they emit twice as much CO₂ per kWh as necessary.

Old and new facilities differ mainly in two respects. First, the maximum steam temperature can exceed 600 °C in modern facilities, but tops out at only around 500 °C in most older plants. Old plants would have to be completely rebuilt to raise this maximum by 100 °C. That’s not the case with the second difference, the turbines, because they are completely overhauled every 20 to 25 years. The modernization of more than 100 steam turbines in the U.S. alone since 2000 has cut CO₂ emissions by over 20 million tons a year, says Steve Welhoelter, who coordinates Siemens Energy’s global service activities out of Orlando, Florida. The upgrades have enabled the plants to raise their output by four percent on average without increasing the amount of fuel they consume.

The potential for further improvements is great, as 2,100 Siemens steam turbines are now operating in power plants worldwide. For example, Siemens’ overhaul of the Ibbenbüren coal-fired power plant near Münster, Germany, increased its maximum output by over ten percent. The plant, which entered service in 1985, has a steam temperature of 530 °C, a typical value for plants built at that time. In 2006 Siemens maintenance specialists were commissioned to carry out a study of the facility’s improvement potential together with the operating company RWE. A scheduled maintenance shutdown in 2009 was used to overhaul the steam turbines and heat exchangers. All the internal parts in the five turbines for the high, medium, and low-pressure cycles were replaced. This included asymmetrical turbine blades, which have substantially reduced flow losses, and a new sealing concept for the high-pressure turbine that helped reduce the gap between the turbine rotor and the housing, thus slashing the amount of steam that flows unused past the rotor by 30 percent.

Following a 72-day shutdown, the plant now generates up to 86 MW more power. “A general overhaul is a good way to increase output capacity, especially in countries where construction requires lengthy approval procedures,” says Dr. Norbert Henkel, who is responsible for global marketing of such projects at Siemens Energy. “Our measures also improve the reliability of power plants and extend their service life by 20 to 25 years.”

However, modernization isn’t just about increasing efficiency at full load. That’s because the growing use of energy from renewable sources means that large coal-fired plants now often operate under partial load. This lowers a plant’s efficiency, because each turbine is designed to operate at an ideal level, which is usually 95 percent of its rated output. “If you adjust that point a little, you not only can improve efficiency but also halve the time it takes to ramp up a coal plant to full capacity,” Henkel explains. State-of-the-art facilities can reach start-up ramp gradients of up to six percent of their rated output per minute; the figure for combined cycle plants is eight percent.

Gas around the Clock. Because of their high level of flexibility, in Germany, gas-fired power plants are considered an ideal complement to facilities that generate electricity from renewable energy sources. Such plants are also enjoying success in the U.S. where the declining price of natural gas is making them increasingly attractive compared to coal-fired power stations. The drop in price is due to the increased domestic extraction of natural gas from unconventional sources, especially shale gas. In the past, gas-fired power plants were used mainly to meet the higher demand for electricity during the day, but today many of these plants operate around the clock. The longer operating hours motivate plant owners to invest in modernizing their facilities. “We expect to see a big upswing in this market segment beginning in 2015,” says Welhoelter.

The Klamath power plant in Oregon is another good example of what modernization can achieve. Its output has risen by 30 MW, to 563 MW since its two turbines were overhauled in 2009 and 2010. It has also become more flexible, and the amount of extra fuel it uses during load adjustments has been cut in half. In addition, a new air inflow system was installed upstream of the turbine to ensure that operation at low loads does not lead to more pollutant emissions. The system uses waste heat from the turbine to preheat live air before it enters the gas turbine, which means that combustion occurs at a higher temperature and is therefore more complete. Other measures differ little from the procedures used to optimize steam turbines. The idea is always to minimize flow losses. Here as well, the shape of the turbine blades and the seals were of the utmost importance.

Combined cycle power plants can also be made more flexible if their steam cycles, which are sluggish compared with gas combustion facilities, can be brought up to speed more quickly. Siemens engineer Erich Schmid has worked in this area for over a decade. His 138 patents earned him the Siemens Inventor of the Year 2012 award. His idea involves transporting heat faster when necessary. One way to do this is to briefly open an overload valve on a turbine, which leads to a drop in pressure that causes more steam to flow from the boiler. Another option is to not feed steam into the high-pressure turbine when the facility starts up but instead to reroute the steam into the medium-pressure turbine and bring it up to power first. In the future, such measures are expected to increase combined cycle steam turbine output by more than 30 MW per minute. “Basically, you only need a few valves and pipes and a lot of software. It’s easy to retrofit a power plant with such systems,” Schmid explains.

Power facility modernization is also important in high-growth emerging markets such as China, India, and Russia, where first-generation plants are reaching the end of their lifecycles. “This is a great opportunity for us,” says Henkel. Companies are increasingly modernizing not only the turbines they have supplied but also those of other firms. Sometimes the latter are replaced completely. That’s what Siemens did at the Kirishi power plant near St. Petersburg, Russia. By installing two new 279 MW turbines and a new control system and upgrading the steam cycle, Siemens increased the facility’s overall efficiency from 38 to 55 percent. Clearly, marathon power generation facilities with sprinter qualities are in great demand all over the globe.