Hydraulic system

2.4.1.1 Over view

Hydraulic systems are frequently proposed for power take off in wave energy devices and the current version of the Stingray device. Typically a hydraulic ram is used to convert the motion of the device into high pressure oil, which is then fed into a hydraulic motor driving an electrical machine. Slow speed high forces are hence converted into rotary high speed motion with a minimum onboard weight. Furthermore, the reactive force of the rams can be controlled by the working pressure of the system. The addition of one, or possibly two, accumulators into the system allows some provision for energy storage. The system of Figure 2.2 shows a double action piston feeding into a high and low pressure accumulator. A throttling valve controls oil flow which then drives a hydraulic motor and electrical generator.

Figure 2.2: Hydraulic power take off system

Any configuration of a hydraulic power system would likely have the electrical plant close to the device, the advantage of remote shore based conversion being offset by the disadvantage of long, costly and inefficient pipe work.

Realistically, a hydraulic system is the most likely competitor to a direct drive system, because gearboxes are accepted as cumbersome and rarely proposed in WECs and the pneumatic system of an OWC is inherent in its design. As such, a typical hydraulic system is examined in further detail, and its specific parts are outlined below. Rams

Typically used as actuators in hydraulic the industry, in this application they are used to displace the hydraulic fluid at high pressure. The dynamic seals enforce a restriction on the extraction and contraction velocity of the piston. Pressure inside the ram can be of the order of 400 Bar, allowing it to exert a very large thrust.

Accumulator

Gas-pressurised accumulators are the preferred method in the hydraulics industry for energy storage. They consist of a pressurised vessel containing hydraulic fluid and an inert pressurised gas, possibly separated by a variable membrane to avoid contamination. As more fluid is forced into the accumulator, the gas is compressed and hence the pressure of the vessel increases. If the variation in pressure due to the WEC is assumed to be sufficiently fast, and the accumulator is assumed to be a thermal insulator, the process is adiabatic and dictated by (2.1).

c = constant Non return Valves

These are necessary to enforce the correct flow of oil around the hydraulic circuit. They will have some pressure drop, Ap, and hence power loss associated with them, the value of which may be calculated using (2.2), which assumes they may be modelled as a simple orifice of area A.

|j, = discharge coefficient = 0.61 [13] po = density of oil (kgm-3)

Motor

The ideal motor would have variable displacement volume and be able to work at the desired pressures and flow rates. Axial piston or wing motor appear to be suitable for this application, although the efficiency of these drops off at part load. A specific hydraulic motor suitable for this type of application has also been proposed, [50], which claims to be able to deliver varying input powers and pressures to a constant output load at high efficiencies.

2.4.1.2 Limitations of hydraulics

Hydraulic systems are expensive and designed to operate at speeds even lower than those of a typical WEC. Although the technology of hydraulics is well established, this application is using the components in a role reversal, driven by linear motion (acting as rams not actuators) and driving rotary motion (motor not pump). As such a degree of research into their mass utilisation is still required. If oil is to be the working medium then the marine environment strictly enforces its containment to prevent contamination. This places a stringent limit on the maintenance interval in terms of moving seal life. There is a theoretical advantage in using fresh water as the medium, but leakage could then be expected to increase one thousand fold when compared to hydraulic oils, in line with the three order of magnitude decrease in viscosity.

The need for moving seals in the hydraulic rams limits the speed with which they can be operated. The maximum speed would be typically around 0.5 m/s. With the life of the seal being inversely related to the speed, distance and length of its application, it may prove desirable for the speed to be kept even lower, in the region of 0.1 m/s. An example of how hydraulic systems are the bench mark for WECs is demonstrated by a device known as the Pelamis [19] in which the designer deliberately reduces the speed of movement to those utilisable by hydraulic systems. A further example is highlighted in Chapter 7 for the design of the Stingray tidal stream device.

The requirement for non return valves, crucial to the use of accumulators, will detract slightly the efficiency of the system due to the inherent pressure drop across them.

The use of flexible hose potentially offers an attractive solution to the problem of locating the hydraulic motor and accumulators remote from the rest of the device. However, flexible hydraulic hoses do not perform well in the marine environment, especially not in comparison to electrical cable [51]. Using rigid steel pipe work clearly avoids this problem but enforces all parts of the hydraulic system to be mounted on the same platform.

Hydraulic technology is a mature technology, with a known and proven reliability. Similarly, however, the disadvantages of slow speed and low efficiency at part loads are equally proven.

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Responses

  • ruaridh
    Is hydraulic energy renewable?
    7 years ago

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