Pictures of the Future    Fall 2007

Power from Heaven and Earth

Kramer Junction is 160 km east of Los Angeles. As its name suggests, it’s not much more than an intersection. It is, however, the site of the world’s largest solar thermal power plant, which has an output of 354 MW. Located in the middle of the Mojave Desert, the facility, which employs parabolic mirrors to focus solar rays and vaporize water using captured heat, has generated more than 12 billion kWh of energy since 1991. It now has some competition, however—in Boulder City, Nevada, where Spanish company Acciona put a 320-acre solar thermal power plant into operation in June 2007. And two similar facilities are being built simultaneously in Spain. Siemens has been involved in all of these projects, with Power Generation (PG) supplying their steam turbines.

Nevada Solar One has 219,000 individual parabolic mirrors with a total length of 76 kilometers. These mirrors reflect solar rays onto a receiver containing a special thermal oil that is heated to a temperature of around 400 °C. The oil is used to create steam in a heat exchanger located in the plant’s central block, and the steam powers a turbine, which generates electricity. With a rated output of 64 MW, the facility generates 134 million kWh per year, enough to power 14,000 households. The Schott company estimates that solar thermal electricity costs approximately 0.12 €/kWh. That’s much less than electricity produced by solar cells. What’s more, the International Energy Agency forecasts that the cost of solar thermal electricity will fall to only 0.06 €/kWh by 2020, which would put it around the same price as power generated from fossil sources. Solar thermal power is more environmentally friendly in any case, as 64 MW of rated output reduce annual carbon dioxide emissions by around 80,000 t in the global energy mix (600 g of CO₂ per kWh).

Siemens supplied the steam turbines for Nevada Solar One, and for the two 50-MW facilities that will go online in Andalusia in 2008 and 2009. The turbines must meet special requirements because solar thermal power plants produce electricity only during the day and therefore have to be shut down every evening and then started up quickly again the next morning. To ensure that the oil in the heat exchanger does not decompose, the steam isn’t heated as much as in a conventional power plant. The turbines therefore operate with two sections: one for low pressure and one for high pressure. "This enables more flexible operation of the turbine," says Samuel Fällman from Siemens PG in Sweden. Siemens' first such turbine—which was built in 2005—was a great success. The company has since sold an additional six turbines for new solar thermal plants planned for Spain, and is now the market leader in this segment. Another pioneering development is direct steam generation (DSG), in which water is heated to more than 500 °C and turned to steam in the pipes. This eliminates the need for both the heat exchanger and the toxic thermal oil. DSG technology is being developed and tested by the German Aerospace Center (DLR). Despite its complex flow relationships, DSG functions perfectly and is ready for practical use. Engineers from Siemens PG in Erlangen, Germany, are also involved in the technology’s development, as PG’s Innovative Power Plant Concepts department is now determining the optimal setup for linking a DSG solar field with a conventional power plant block. Together with other measures, such a concept can lower the cost of generating power over the long term. Plans call for a small solar thermal test facility that will use water instead of oil to be built and put into operation in a few years.

Producing energy with heat from the Earth harbors just as much potential as solar power generation. The rule of thumb here is that the temperature increases by 3 °C for a depth increase of 100 m. Temperatures at a depth of 3 km range from 80 to 120 °C; at 5 km the mercury climbs to 130 – 160 °C. The energy stored at such depths is available around the clock and can be harnessed in two ways. "The hot dry rock" process involves pumping water at high pressure into the ground, thereby turning the area there into a continuous-flow heater. Hydrothermal techniques directly utilize hot water already present at such depths.

In Unterhaching, a town southeast of Munich, for instance, the power of naturally-occurring hot water is being tapped. Unlike some other mayors in the area, Unterhaching’s Dr. Erwin Knapek, didn’t want to use the water to open a spa. A physicist, Knapek instead arranged to have a district heating network and a geothermal power plant built. With the help of Siemens Industrial Solutions and Services (I&S), the facility will soon feed its first kilowatt-hours into the grid, and cover around 70 % of Unterhaching’s electricity and heating needs. The town, which has slightly more than 22,000 people, will thus become the site of the world’s most modern geo-thermal power plant.^

An unobtrusive stainless steel pipe protrudes from the ground at the site. Not too far away is a 20 × 12 m² hall. This facility—the heart of the power plant—contains a compact green generator and a pink condensate tank. Various pipes run through the building. One set, for the district heating system, extracts 25 of the 150 l of thermal water that pass through the facility per second. A second set leads to the turbine that produces the electricity, while a third single pipe pumps the water, now cooled to 60 °C, into a borehole three kilometers away, where it is returned to the depths in order to retain the underground water balance.

The pink color of the condensate tank stands for ammonia, the true secret behind the facility. The problem is that the thermal water source in Unterhaching is not hot enough for a conventional water-steam power-generation cycle. Siemens engineers therefore employ the Kalina Technique, named after its Russian inventor. Here, hot water heats a mixture of around 89 % ammonia and 11 % water that is already simmering at 50 °C. That’s enough for the turbine—and to generate 3.4 MW of electricity in Unterhaching. This output decreases slightly in the summer due to higher outside temperatures, and then rises in the winter. Because of its relatively low temperature and pressure, the facility has an efficiency rating of only 12 % (a coal power plant has a rating of 40 % or more). Nevertheless, the plant operates at a profit because Germany’s Renewable Energy Act fixes a 0.15 € price for every kilowatt-hour produced in such a manner. That makes good sense given that the plant will reduce annual carbon dioxide emissions by 30,000 t, or half of what Unterhaching was producing to meet its electricity and heating requirements.

The next few months will be very exciting for the town—which has invested €60 million in the geothermal project—and for the Siemens engineers who worked on it. That’s because the Unterhaching facility is a prototype. "A successful conclusion to this project will spur development of other geothermal power plants," says Sameer Joshi, who is responsible for geothermal activities at I&S. A borehole a few kilometers away in Sauerlach has made available an ample source of hot water, and Siemens will complete another geo-thermal power facility in neighboring Oberrheingraben in the summer of 2008. There is thus huge potential under the surface in Germany for a form of power previously neglected. In fact, a study conducted by Germany’s Ministry of the Environment estimates that geothermal power sources could be providing as much as ten percent of the country’s energy requirement by 2050.
                                                                                                            Jeanne Rubner