Siemens turbine axles are forged in a single piece, and then given their final form in a high-precision lathe.

Siemens turbine axles are forged in a single piece, and then given their final form in a high-precision lathe.

The most effective way to generate electricity on a large scale from solar energy is to somehow convert the sun’s heat to kinetic energy, or motion, and use it to drive a generator. A variety of approaches have been developed, but the most technically and economically feasible solution is the solar turbine.

A solar turbine works on the same principle as any steam-driven generator, differing only in way steam is produced to power the turbine. Rather than burning fossil fuels, the solar turbine captures sunlight in a number of parabolic mirrors to heat a transmission fluid which in turn boils water. Just as in a fossil-fuel generator, the steam is used to drive a turbine which generates electricity.

The technology is based on nearly one hundred years of turbine development. Siemens Industrial Turbomachinery AB is the modern incarnation of STAL (Svenska Turbinfabriks AB Ljungström), founded in 1913 by the Swedish brothers Birger and Fredrik Ljungström.

Major investment, low operating costs

The economic and environmental advantages of solar turbines are straightforward: they do not consume fossil fuel.

But a solar electric installation is a substantial investment. A 50 MW plant costs some €300 million to build, with the turbine accounting for about €15 million of that amount. The bulk of the expense covers permitting, construction and land—the mirror park requires about 2 sq km for a 50 MW plant.

Infrastructure also boosts the price substantially. The plant must be connected to the electric grid, and that often involves construction of access roads. But on the upside, operational costs are very low. Solar turbines are built for seven to eight years of operation before service is required, and because the materials are actually designed for higher temperatures than a solar plant achieves, reliability is increased.

While the solar electricity industry is still in its infancy, the potential market is enormous. Almost all of northern Africa is suitable territory, as well as the Middle East, northern Australia and the southwestern United States.

Spain is the current world leader in electrical energy produced by solar turbines. More than 40 plants have been built or approved, and most of these will feature turbines manufactured by Siemens in the Swedish city of Finspång.

Through the end of 2008 the factory had sold 43 turbines—of which 37 will be used in solar plants—and those existing orders represent full capacity through the summer of 2011.

Mirrors in the shape of parabolic troughs track the sun throughout the day.

Mirrors in the shape of parabolic troughs track the sun throughout the day.

Technological challenges

Solar turbines are the result of thousands of hours’ work by engineers. As with any industrial technology, factors such as conversion efficiency, installation costs and operating expenses are all essential considerations in determining the economic viability of a solar power plant. Technical quality is the primary competitive tool for any company in power engineering.

The Finspång plant gives Siemens a solid foundation in turbine engineering, but in fact the solar turbines produced today were originally designed for a different application altogether. When the global shipbuilding industry was hit by the oil crisis of the early 1970s, sales stagnated for steam turbines, the company’s most important product line. As diesel engines became the most common power source for large ships, Siemens saw that the Finspång factory had the right expertise and equipment to re-tool its ship turbines for land-based applications.

The most important difference between powering steam turbines by fossil fuels and solar energy is the operation cycle. Solar turbines need to work effectively during repeated starts and stops throughout the day as exposure to solar radiation shifts with the sun’s trajectory and changes in weather. This requirement is similar to the demands placed on ship’s turbines.

Steam turbines are comprised of two main units, a high-pressure turbine rotating at high speed and connected to the generator through a gearbox, and a low-pressure turbine rotating at the same rate as the generator. Proper disposition of these two units increases electrical output since the separate turbine sections are allowed to operate at their most efficient rotational speed.

The future of thermal solar energy

Lars-Göran Sjöberg shows how the turbine blades are attached to the axle.

Lars-Göran Sjöberg shows how the turbine blades are attached to the axle.

For the foreseeable future, turbine technology is expected to dominate the market for solar power plants between 50 and 150 MW, and several interesting developments are in the works. The challenge is to achieve the highest possible efficiency rate from the turbines as this reduces the size of the mirror park and lowers initial investment.

One shortcoming is that the transmission fluids available today cannot be heated above about 400° C. Most solar plants currently operate at about 380° C to maintain a safety margin, but the turbines are capable of operating with steam heated to as much as 540° C, which would generate more power.

A solution to this problem could be in placing the turbine on a high tower with the mirrors aimed to focus the sun’s rays directly on the steam boiler rather than using transmission fluid in a system of pipes.

Exploiting the enormous solar potential in the deserts of northern Africa will require interconnections between electrical networks. Solar enthusiasts dream of a Mediterranean network joining African and Middle Eastern solar plants to the European electrical grid. Some have called for the European Union to study a possible investment.

The benefits would be many. Energy security is a priority policy area, and a Mediterranean network would reduce reliance on petroleum, gas and oil. At the same time, greater economic integration between North Africa and Europe could reduce the risk of conflicts while offering helping to encourage the spread of environmental technology. The importation of solar energy to Europe would be a step toward meeting EU climate goals that call for reductions in carbon dioxide releases.

Article published in June 2009