StreamDiver utilises new hydropower potential

Even though hydropower accounts for the largest share of renewable energies worldwide there is still sufficient potential for further development. Until recently, run-of-river plants with low heads were regarded as uneconomical and therefore often remained unused. In order to take advantage of this unused potential Voith has developed the StreamDiver, a new compact propeller turbine particularly suited to taking over where conventional plants may not be viable.

In an effort to balance economy and ecology, several new technologies have emerged which place the entire generating unit directly in the waterway. However with this new placement challenges appear, such as installation and access to the hydropower equipment for maintenance. In combination with the StreamDiver technology this innovative power plant concept can be the beginning of a new run-of-river power plant generation.

StreamDiver's main goal was the development of a compact, robust turbine design that reduces civil interfaces and has minimal maintenance requirements. The design innovations include a water-filled bulb turbine which houses a floating-type, permanent magnet generator (PMG) and rotates on river water lubricated bearings. This arrangement allows installation of the unit directly in the waterway thus eliminating the need for a dry power house floor. Furthermore, the compact design is nearly maintenance free. Both of these make the unit more economically attractive while offering an oil-free design and unit arrangement for better ecological performance.

Technology

The StreamDiver is of the propeller bulb type construction wherein the runner is directly mounted on the shaft of the synchronous PMG. The target of the StreamDiver project was to develop a standardised turbine concept in a compact and submersible design without the need for peripheral systems (eg hydraulic power unit, cooling system and dewatering system). Therefore the wicket gates and runner blades are not regulated. The bearings are of a self-lubricated type operated with water. No peripheral system is needed to supply the bearings with a lubrication medium. The bulb of the StreamDiver is filled with river water to avoid leakage from the surrounding water.

In addition the bulb of the StreamDiver is not pressurised. In case of applications where shaft sealing is required, various filters are integrated in the turbine housing and the bulb hub to avoid over pressure. The power unit is equipped with two guide bearings: one downstream close to the runner integrated in the turbine housing and another upstream connected to the generator housing. The thrust bearing, for carrying the hydraulic thrust, is combined with the downstream guide bearing. A counter thrust bearing has been incorporated. To avoid ingress of the water inside the stator body and to avoid damage to the winding, the stator of the generator is equipped with a static sealing.

Operational criteria for the StreamDiver-equipped power plant are guided by the following important considerations:

• The discharge through turbine for single unit is limited in a range of 2 to 16m³/sec. Since the StreamDiver is a non-regulated machine, multiple units can be installed to completely utilise the potential of any site.
• The typical head range for StreamDiver is 2 to 8m.
• The civil structure facilitates the minimum submergence of the machine for cavitation-free operation of the StreamDiver.
• StreamDiver uses a modular concept with five different runner diameters. Runner diameters vary between 0.8 to 1.3m.

Typical application for the StreamDiver include:

• Alternative for a bay type bulb turbine power plant: The StreamDiver allows a significant simplification of the civil design in comparison to a conventional bulb turbine. Despite the fact that the StreamDiver does not require a power house building also the total plant length and the excavation depth can be reduced.
• Overflooded power plant for navigation dams: Large lock and dams built across the rivers for the purpose of navigation are potentially ideal candidate sites for power generation with StreamDiver units. This assumes that the existing structure allows the implementation of proposed configuration.
• Refurbishment project: Due to its compact design, the StreamDiver can be integrated in old shut-down power plants without extensive construction measures.

Further applications include:

• Residual Flow applications
• Penstock applications
• Energy recovery application
• New greenfield projects with head below 8m

Assembly and service

The typical StreamDiver power plant foresees limited access to the turbine parts during operation. To service the turbine, the complete power unit needs to be removed out of the power plant. The shutdown of the turbine occurs via an automatic gate valve and the power unit can be removed from the water via a mobile crane. To avoid the requirement of divers and time consuming mounting procedures, the turbine is supported with a steel structure, and the supporting structure allows an assembly similar to a stop log system. The maximum weight of the StreamDiver will be kept on a weight level to allow the handling via a mobile crane.

Turbine servicing can be undertaken after disassembly in the factory workshop. The assembly time and tools required for the StreamDiver are very small compared with a conventional bulb type turbine. The alignment of shaft and bearing is very easy and less time consuming. The focus in the development was to avoid special alignment procedures. The disassembly of the StreamDiver in its main parts is a four-step process. (Refer to Fig. 8).

Development and prototype results

Since August 2012, a full scale prototype of the StreamDiver has been in operation in a power plant close to Vienna. The over flooded power plant is equipped with 12 small turbines. To test the new Voith technology, the owner of the power plant offered to remove one of the turbines and installed a 450kW StreamDiver. Up to now, the prototype has been running for more than two years without any breakdown or maintenance required.

Subsequently, besides the test of the new bearing and generator design, different grid connection types have been investigated. As a standard solution, the PMG generator of the StreamDiver will be connected directly to the grid. In case turbine flow control is required, the generator can be connected via a frequency converter to the grid. In this case the turbine can be operated with variable speed. A further advantage is that the reactive power can be controlled via the frequency converter.

Beside the full scale prototype installation, further tests have been carried out during the development. The StreamDiver technology has been developed based on the proven hydraulic profiles of existing Voith Hydro axial flow turbines which are model tested in a hydraulic laboratory. The PMG technology is developed along with the supplier of the generator and the water floated PMG is tested in the laboratory for satisfactory operation. The magnets used for the generator are of high performance type which does not require the active cooling of the rotor.

The water lubricated bearings are designed for rough operating conditions. The synthetic bearing pads and shells with high-strength abrasion surface offer excellent wear resistance. The bearing concept is a proven design derived from other Voith products, e. g. tidal flow turbines, HyTide and Voith inline Thruster.

Conclusion

Technological progress and innovation is an important contributor to the continued development of sustainable energy portfolios. It is also very important to consider the impact of these innovations on the environment and ecology. The StreamDiver offers the new possibility to design run-of-river power plants or utilise existing dam structures where conventional solutions are not feasible for environmental or economic reasons. Some of the advantages of Stream Diver are:

• Oil free Technology
• Minimum maintenance effort and costs due to the reduced technical complexity and the non-requirement of auxiliary systems in the power plant. The compact design allows flexible installation possibility and a simplification of the civil structure.
• Due to the standardised and modular construction of StreamDiver, the delivery and installation timelines are short.
• Due to the limited complexity the service and spare part administration is very cost effective.

The first commercial StreamDiver project is currently being executed.

The author is Jörg Lochschmidt, Product Manager Small Hydro, Voith Hydro Holding GmbH& Co. KG, Alexanderstraße 11, 89522 Heidenheim, Germany
Email: joerg.lochschmidt@voith.com