Thinking inside the box: marine propulsion technology

An advanced new hydroelectric turbine/generator now in development has been designed to enable more low-head projects in the US and worldwide. US firm Amjet Turbine Systems (ATS) is developing its ATS-63 unit for customer delivery starting in the Fall of 2015.

The ATS-63 (named for the 63-inch/1.6m internal diameter of its turbine housing) is intended for sites with 1.5-28m of head, and 8.8-26.2m3/sec of flow. This range will enable electric generation output from 100-2500kW. Sites with higher heads and larger flows can be accomplished using multiple ATS-63 units in series (for higher heads) and in parallel (for higher flow rates).

ATS claims its latest development represents a dramatic departure from current technologies. "The ATS-63 has been designed from the tailrace-up to negate the disadvantages of low-head hydro", said David Jansson, ATS CEO. "Those disadvantages include high initial unit cost, heavy weight, requirements for extensive civil work including power house structures, long lead times from order-to-delivery, complicated operating machinery, and lengthy time and labour requirements for maintenance, repair or replacement."

Genesis in Marine Propulsion

The multi-patented technology comes from the marine propulsion industry. American Hydro Jet Company (AHJ), a sister company to ATS, is the developer of hydro turbine engines for large boats. Think jet-skis, but a lot bigger. ATS has reversed this technology to create applications for low-head hydroelectric sites. The water drives the turbine, and the turbine drives an electric generator.

Most current low-head hydro technologies use multiple separate and site-customised components brought together to achieve a complete generating plant. These individual "boxes" as shown in Figure 2 are assembled to work together to go from water-to-wire. Water flows through a turbine "box". The turbine box typically has mechanical wicket gates to control the water flow in such a way to enable the turbine to operate at constant speed. The pitch of the turbine blades is typically also varied by a hydraulics box as water conditions vary. The turbine is connected to a gear box to change the turbine rotational speed into the required constant generator speed. Finally, the generator box converts the rotation of the shaft into electricity and feeds it to the switchgear box for connection to the electric grid.

Some conventional technologies have up to 70 mechanical moving parts in all of the boxes. In contrast, the ATS-63 incorporates all of these functions into a single, integrated turbine/generator unit. It has only one moving part - the turbine rotor. The result is reduced mechanical complexity, weight and maintenance. "We smile when we hear organisational theorists encouraging everyone to 'think outside the box'", said Paul Roos, ATS Vice President of Technology and creator of the technology. "With the ATS-63, we emphasise the advantages of thinking inside the box."
Integrated Turbine/Generator Unit

Within the integrated turbine/generator, permanent magnets are located on the tips of the turbine rotor blades. As the turbine turns, these magnets rotate inside the generator stator ring that encircles the turbine tunnel. The generator stator features multiple individual power segments connected to work together. If one segment fails the others keep operating, albeit at lower total electric output. This provides a 'fail-soft' mode of generator operation.

Power electronics convert the alternating current output of the segments to direct current, and control the speed of the turbine. The direct current is then inverted into 480 VAC, three- phase output for step-up to the voltage of the local electric grid using standard distribution transformers.

Other low-head hydro technologies are typically manufactured from metal in a one-off, site-specific basis using time- and labour-intensive, subtractive approaches such as grinding and milling to remove excess metal from poured castings. In contrast, the ATS approach uses lightweight but strong composite materials like those now appearing in extensive use in advanced jet aircraft. Such materials can be moulded or printed using additive (that is, non-subtractive) manufacturing techniques. And they are corrosion-resistant.

“The turbine and generator are integrated into a single unit", Roos said. "Each unit will be the same physical size across the range of electric outputs, so they will share common parts. Together with the composite materials, this will result in light-weight turbine/generators that can be mass-produced, lowering manufacturing time, labour and cost".

The ATS-63 has been in development since 2009, with investor funding and assistance from the State of Iowa, Lee County, and the City of Keokuk. A 1/8-scale model unit was successfully tested for hydraulic and electrical performance by the University of Iowa's Hydraulics Lab in 2012, confirming computer performance designs and computational fluid dynamics (CFD) performed earlier. With funding from the DOE Energy Efficiency and Renewable Energy (EERE) programme in 2013, ATS completed the full-scale unit design and is now ready to manufacture them. In April 2014, the company received an additional US$1M DOE/EERE Small Business grant for continued efforts. A full-scale demonstration test at an actual operating site is planned for early 2015.

With the exception of the generator magnetic and electric conductive elements, the demonstration unit will be made entirely of composite materials. Portions of the unit will be made using 3D printing techniques, within the current state-of-the-art. The demonstration project includes additional research to enable additional portions of the unit to be built using 3D printing materials and techniques as that technology continues to mature.

Advantages of Variable Speed

The ATS unit offers further advantages in terms of operating flexibility. Traditional technologies operate at constant speed to enable their generators to synchronise with the electric grid. Instead, the ATS turbine/generator will operate at variable turbine rotor speeds in accordance with changes in the available hydro resource. This is made possible using modern power electronics.

Variable speed means the unit does not need the mechanical wicket gates, hydraulic blade pitch controls or gearboxes to change turbine speed to generator speed typically found in traditional technologies. This saves operating complexity, maintenance costs and weight. "Instead of managing the variable water flow to work around the limitations of the turbine and generator technologies, we use our advanced technology to accommodate the natural variations in the water resource," said Roos.

The integrated turbine/generator units are designed for a wide variety of applications. They can be mounted with the turbine shaft vertical, horizontal, or at any angle in between. Their relatively light weight (3600 pounds for a 100kW unit, to 12,000 pounds for 2500 kW) and the fact they can be installed either above water or completely submerged means less civil work. A power house building is not necessary.

Figure 4B is an illustration of the ATS and an example mounting rack for installation. The unit can be mounted on rollers so it can be easily moved from its operating location to a nearby maintenance station (Figure 4A) where it can be tilted and maintained. The unit is designed for removal/repair/replacement in only one day. This contrasts with weeks or months required for other technologies.

The units can be installed in dam bulkheads in a weir, in canals or conduits, submerged in a bypass dam or canal, or in any variety of retrofit applications to replace existing older units. For example, if an existing plant has its hydro turbines in a pit outside the generator hall and connected through a bulkhead to the generators by power shafts, the ATS-63 turbine/generator can replace the turbines submerged in the turbine pits. The generator hall can be emptied of equipment because it would no longer be required. In multi-unit applications, the mass-produced ATS units can be installed in standardised, "cookie-cutter" fashion, minimising engineering design requirements.

More practical

Transport of the modular ATS-63 from the factory to the site is simple. With maximum outside dimensions of only 2.3m in diameter by 1.6m long, it is designed to fit in a standard truck or shipping container. "It is designed for applications across the US or around the world," ATS CEO David Jansson says.

The advantages of the ATS-63 technology will make more new and retrofit low-head hydro projects technically feasible and cost-effective. It is a technology not just for developed countries, but developing ones as well.

“We can light more homes and businesses in the US and other developed countries using clean, renewable energy because it costs less. We can also enable the advent of electricity in communities in developing countries that simply could not have it before. The possibilities are endless," said Bob Schulte, ATS' vice president of marketing.

For more information about the ATS-63, refer to the company's website at www.amjetturbines.com, or contact Bob Schulte via e-mail: rhschulte@amjetturbines.com