Efficiency gains for wind turbines

An alternative transmission system may improve the efficiency and reliability of wind turbines. David Wilson reports.

A new type of hydraulic technology could be used to build more reliable and less expensive transmission systems for wind turbines, its developers have claimed.

The majority of large modern wind turbines use mechanical gearboxes to couple the slow rotating blades to high-speed electrical generators. However, gearboxes are heavy and prone to failure and their replacement can be expensive.

Alternative wind-turbine transmissions use low-speed electrical generators coupled directly to the shaft of the wind rotor. Although reliable, these machines are costly, heavy and require electronic frequency and voltage converters to condition the power from them before it can be supplied to the network.

Now, by using hydraulic pumps and drive motors based on what it calls Digital Displacement technology, Edinburgh-based Artemis has developed a prototype hydraulic transmission system for a 1.5MW wind turbine that it claims could be a more reliable and less expensive alternative to the mechanical or electrical solutions that have been deployed in the past.

The Digital Displacement concept originated from research by Prof Stephen Salter at Edinburgh University. While developing devices to generate power from the ocean, one of which was the well-known Salter Duck, he recognised that a hydraulic transmission system could provide a means to transfer wave energy into a source of electricity.

However, he also realised that conventional hydraulic systems would not be efficient enough to do so, so he set about developing a technology that was. The result is the system now being commercialised by Artemis.

Digital Displacement, which uses electronically actuated valves to control the flow of hydraulic fluid in hydraulic pumps and motors, provides some important technical advantages over conventional designs. Yet a superficial look at the design of a hydraulic pump or motor based on it reveals mechanical structures not entirely dissimilar to those found in radial piston pumps and ring-cam motors.

In a conventional pump, the ends of a set of piston rods are connected to an eccentric on a camshaft that rotates to draw hydraulic fluid into the piston’s cylinder chambers. The actual flow of the fluid into the chambers is controlled by two valves — one to allow low-pressure fluid in and one to allow high-pressure fluid out.

However, in a conventional variable displacement machine, such as an axial swash plate pump, no means is provided to regulate the flow of the fluid that leaves each of the cylinders. It is left to a hydraulically or mechanically driven swash plate to adjust the stroke of all the pistons together and hence control the amount of displacement that the pump provides.

That is not the case with a pump based on the Digital Displacement technology. Here, the high and low-pressure valves associated with each of the cylinders are controlled by a small electro-magnetic latch so that they can be opened and closed individually on a stroke-by-stroke basis. The solenoid coil in each latch is activated by a power FET, which is connected directly to the digital output of an embedded controller.

In the Digital Displacement pump, it is only necessary to control the actuation of the low-pressure valves. If a valve remains open, the pump does no work as hydraulic fluid flows from the piston cylinder to and from a reservoir in an almost lossless cycle. But if the low-pressure valve is then closed, any fluid in the cylinder then becomes pressurised, causing it to passively open the high-pressure valve through which the fluid is then pumped.

The Digital Displacement motor, on the other hand, uses an electronically driven high-pressure valve in place of the passive high-pressure valve deployed in the pump. High-pressure fluid entering the motor through the open valve then causes the pistons to be actuated and the crankshaft to rotate.

Hence, the displacement of a pump, or the speed of a motor, can be accurately controlled by opening or closing the valves associated with each cylinder. The effect of controlling the valve timing, and hence the fluid flow, overcomes the loss of energy caused by leakage of pressurised fluid out of the cylinders, which is inherent in the axial swash plate pumps or motors when they operate at less than full displacement. Controlling the valve timing electronically also eliminates the high-frequency noise associated with existing hydraulic pumps and motors.

‘Pumps and motors based on Digital Displacement technology are more than 90 per cent efficient, even at partial flow rates. In fact, they are close to uniformly efficient over the whole displacement range. In applications such as wind energy, where the average load on a system is 30 per cent of the peak power, that part-load efficiency is critical,’ said Dr Michael Fielding of Artemis.

It is unsurprising then that such a controllable motor/pump combination is now being seriously considered as a replacement for the traditional transmissions used in wind turbines.

In a typical wind turbine, blades attached to a turbine hub turn a shaft supported by a large bearing. The main shaft is mechanically coupled to a gearbox whose output is coupled to an electric generator. Although such systems are proven, they have many limitations.

Even though the mechanical couplings between them allow for some slight variations, it is still important to accurately mechanically align the rotor, the gearbox and the generator, so the underlying mechanical structure must be inherently rigid. This adds a lot of weight to the transmission. Any misalignment that is inevitably created concentrates mechanical loads within the gearbox that leads to wear and eventually more serious problems, including failure.

The prototype transmission under development at Artemis has resolved those issues. Here, the wind turbine’s rotor is connected to a 15-20rpm Digital Displacement hydraulic pump, which then produces enough high-pressure fluid to power a hydraulically coupled 1,500rpm hydraulic Digital Displacement motor that drives a generator.

This continuously variable transmission allows the rotor in the wind turbine to be operated at its optimal speed to capture as much power as possible, while the synchronous generator used to produce electric power can be run at a much higher constant speed.

Through the use of such a hydraulic transmission, it is no longer necessary to mechanically couple the rotor to the generator through a gearbox, hence the rigidity of the entire system can be reduced. What is more, the transmission weight is between 30-40 per cent lower than that of a comparable gearbox, according to Fielding.

Using a hydraulic system also overcomes the issue of dealing with unwanted output fluctuations from a wind turbine caused by gusts of wind. Where these would only produce additional stress on a gearbox, in a hydraulic system, the use of an accumulator can be used to buffer the energy for a number of seconds, acting as the equivalent of a smoothing capacitor in an electrical circuit.

Fielding added that the Artemis transmission, unlike a gearbox, is inherently scaleable. While he claims that it will be difficult to maintain the reliability of gearboxes as they are scaled up, he said that there are no such fundamental issues involved in producing larger hydraulic transmissions for wind turbines with up to 10MW capacities.

Although it was born out of the need to make more efficient systems for wave power systems, Digital Displacement technology has already proved its merits in a hybrid transmission for a car. Here, a transmission comprising a Digital Displacement pump mounted to an internal combustion engine that drove hydraulic motors coupled to the wheels of a BMW 530i increased the miles per gallon by 50 per cent compared to a six-speed manual transmission in city driving.

Ironically, it is only now that the technology is once again being considered for the application for which it was originally intended — to convert green energy from wind, and perhaps soon tidal, power into electricity.