IN TODAY'S green world, hydroelectric dams are often unwelcome. Though their power is renewable and, on the face of it, carbon-free, there are lots of bad things about them, too. Blocking a river with a dam also blocks the movement of fish upstream to spawn and the movement of silt downstream to fertilise fields. The vegetation overwhelmed by the rising waters decays to form methane—a far worse greenhouse gas than carbon dioxide. The capital cost is huge. And, not least, people are often displaced to make way for the new lake. The question, therefore, is whether there is a way to get the advantages without suffering the disadvantages. And the answer is that there may be.

The purpose of a dam is twofold. To house the turbines that create the electricity and to provide a sufficient head of water pressure to drive them efficiently. If it were possible to develop a turbine that did not need such a water-head to operate, and that could sit in the riverbed, then a dam would be unnecessary. Such turbines could also be put in places that could not be dammed—the bottom of the sea, for example. And that is just what is starting to happen, with the deployment of free-standing underwater turbines.

The big disadvantage of free-standing turbines is that they are less efficient in transforming the mechanical energy of water into electrical energy than turbines in dams are. They are also subject to more wear and tear than turbines protected by huge amounts of concrete. They can be hard to get at to repair and maintain. And the generators they run, being electrical machines, need to be protected from the water that surrounds the rest of the turbine.

A discouraging list. But in the past three decades computing power has become cheaper, helping developers to simulate the behaviour of water and turbine blades—something that is hard to do with paper, pen and formulas. Moreover, prototypes can be built directly from the computer model. All this has helped scientists and industry to solve the weaknesses inherent in free-standing turbines.

The first new design was by Alexander Gorlov, a Russian civil engineer who worked on the Aswan High Dam in Egypt. He later moved to America where, with the financial assistance of the Department of Energy, he produced the first prototype of a turbine that could extract power from free-flowing currents “without building any dam”. The Gorlov Helical Turbine as it is known, allows you to use any stream, whatever the direction of its flow. The vertical helical structure, which gives the device its name, provides a stability that previous designs lacked. It increases the amount of energy extracted from a stream from 20% to 35%. In addition, as the shaft is vertical the electric generator can be installed on one end above the water—without any need of waterproof boxes.

In 2001 Mr Gorlov won the Edison patent award for his invention, and the turbines have now been commercialised by Lucid Energy Technologies, an American company, and are being tested in pilot projects in South Korea and North America.

A second design is by Philippe Vauthier, another immigrant to America (he was originally a Swiss jeweller). The turbines made by his company, UEK, are anchored to a submerged platform. They are able to align themselves in the current like windsocks at an aerodrome so that they find the best position for power generation. As they are easy to install and maintain, they are being used in remote areas of developing countries, as well.

Finally, a design by OpenHydro, an Irish company, is not just a new kind of turbine but also a new design of underwater electric generator. Generators (roughly speaking) consist of magnets moving relative to coils. Why not have the magnets encapsulated in the external, fast moving part of a turbine? The turbine is then installed in an external housing, containing the coils. The result looks like an open-centre turbine contained within a tube. OpenHydro's generators do not need lubricant, which considerably reduces the need for maintenance, and are said to be safer for marine life.

These new designs, combined with the fashion for extracting energy from the environment by windmills and solar cells, means money that previously shied away from the field is now becoming available. According to New Energy Finance, a specialist consultancy, investments in companies proposing to make or deploy free-standing turbines have risen from $13m in 2004 to $156m in 2007. Projects already underway include the installation by American Verdant Power of a tidal-turbine in New York's East River and pilot projects in Nova Scotia with UEK, OpenHydro and Canadian Clean Current.

And that, optimists hope, is just the beginning. Soon, many more investors will be searching for treasures buried in the ocean sea beds—or, rather, flowing above them.