Irish company OpenHydro is a technology business that designs and manufactures marine turbines that generate renewable energy from tidal streams. The company has achieved a number of industry firsts including being the first to deploy a tidal turbine at the European Marine Energy Centre (EMEC), the first to connect to and generate electricity from tidal streams onto the UK National Grid and the first to successfully demonstrate a method of safely and economically deploying and recovering turbines directly on the seabed.

Many renewable energy technologies, such as solar, wind and wave, suffer from the difficulty of predicting when supply will be available, and what volume of electricity will be generated, due to the difficulty of accurately forecasting weather in the mid- to long-term. As it is dependent on gravitational forces rather than the weather, the tidal energy resource is reliable and easy to predict. Once tidal currents have been properly studied at a particular location, variations during the tidal cycle can be predicted with a high level of accuracy far into the future, making it possible to reliably forecast the energy supply.

OpenHydro has developed its open-centre turbine technology to take advantage of this resource and to extract energy from the oceans in an economically viable and environmentally sensitive manner. The open-centre turbine is designed to be deployed directly on the seabed. Installations are silent and invisible from the surface and, as the turbines are located at depth, they present no navigational hazard. It is the company’s vision to deploy arrays of tidal turbines under the world’s oceans, silently and invisibly generating electricity at no cost to the environment, allowing communities to benefit from power supplied by the technology without ever being conscious of the turbines' existence.

The Irish company believes that the open-centre turbine is an example of a simple idea proving to be the most effective solution. The functionality and survivability of equipment in an underwater environment demands simplicity and robustness. The open-centre turbine meets these demands, with its slow-moving rotor and lubricant-free operation, minimising risk to marine life. The turbine features a horizontal axis rotor with power take-off through a direct drive, permanent magnet generator. The turbine’s large open centre provides a safe passage for fish and its clean hydrodynamic lines ensure that marine life will not become entangled. The blade tips are retained within an outer housing which clearly defines the moving component, and the turbine is designed to generate energy at a slow rotational speed. The design avoids the use of oils or other lubricating fluids that could present a pollution risk. Tests have also confirmed that the unit produces very low levels of mechanical noise.

A number of factors need to be taken into consideration when selecting a site for a facility to harness tidal energy, these include tidal velocity (the speed and volume of water passing through the site) and bathymetry (the depth of the water and the geology of the seabed); and it is these that determine the position and number of turbines deployed. In addition, the distance from the proposed site to a grid access point will help determine the viability of an installation, as distance to shore can significantly influence costs. For an installation located at 100 km from the shore, the grid component can represent up to one third of the cost.

©Image: OpenHydro

OpenHydro's open-centre turbine is one of the world's first tidal energy technologies to reach the development stage of permanent deployment at sea. The company’s new deployment methodology, which was developed in-house, has eliminated dependency on expensive marine equipment and uses a custom-built tidal turbine deployment barge, delivering what the company believes is a step change in the economics of tidal energy. The open-centre turbine design will increase in size as development progresses. The first 6m test unit produced enough energy to supply 150 average European homes, mitigating over 450 tonnes of CO₂ greenhouse gas each year. OpenHydro has demonstrated that its technology is scalable by increasing the diameter of the turbine from a 10m, 1MW rated machine to a 16m, 2.2MW rated machine, generating up to 500kW. Larger diameter turbines result in higher outputs and improved economics.

However, proponents of the competing horizontal axis turbine technology have expressed some doubts about the commercial viability of open-centre turbines, arguing that open-centre turbines increase their output by increasing their size and that, in order to be highly efficient, a turbine must be smaller and cheaper to build, transport, install and operate. Rather than increasing turbine size, increases in efficiency are better achieved by optimising the blade, which can only be done if the blade operates in a single flow direction. An open-centre turbine uses fixed pitch blades, allowing them to operate in both tidal ebb and flow. This means that there is no pitch control, which supporters of other tidal technologies see as crucial for a turbine to perform to its best advantage over the complete tidal cycle. Pitch control is also used to limit the forces that are applied to the rotor under extreme weather conditions. Yawing is another issue – this is what allows a turbine to turn directly into the flow for maximum efficiency, and this is seen as another advantage of competing technologies, as on most sites the tidal reversal is not a simple 180 degree switch. There is often an offset caused by the shape of the seabed resulting in the reversal deviating from 180 degrees, which reduces yield and increases loads if it is not possible to adjust the turbine. The counter argument is that an open-centre turbine, with its direct drive and fixed pitch, is more reliable and therefore operating costs are lower.

While no conclusive argument has yet been made in favour of one technology or another, independent industry experts believe that it is important to maintain technological diversity at the early stages of the industry’s development, so as not to exclude concepts that may have the potential to deliver significant benefits in the longer term. The market will ensure that the strongest concepts succeed. Whatever the future brings, right now OpenHydro is in a strong position. It has a commercial-scale tidal turbine with proven ability to generate electricity, the technical ability to connect successfully to a national grid and a method to deploy turbines quickly, safely and economically on the seabed. These are fundamental elements to support the commercialisation of the technology. Currently, the cost of generating electricity using an open-centre turbine is comparable to that of offshore wind. In the longer term, economics of large-scale deployment will trend costs toward those for onshore wind. As fossil fuel prices continue to increase, the case for tidal energy strengthens further. According to OpenHydro estimates, the value of energy produced from the world's identified tidal resources should exceed €16 billion per year.

OpenHydro’s Open-Centre Turbines are only one of a range of tidal technologies that includes:

Horizontal axis turbines

Horizontal axis turbines operate on the same principle as wind turbines, extracting energy from moving water in the same way as wind turbines extract energy from the air. The tidal stream causes the rotors to rotate around the horizontal axis, thereby generating power.

Vertical axis turbines

Vertical axis turbines extract energy from the tides in a similar manner to their horizontal equivalents, except that the turbine is mounted on a vertical axis, around which the tidal stream causes the rotors to rotate.

Oscillating Hydrofoils

A hydrofoil is attached to an oscillating arm. The oscillating hydrofoil induces hydrodynamic lift and drag forces due to a pressure difference on the foil section caused by the relative motion of the tidal current over the foil section. This motion then drives fluid in a hydraulic system that generates electricity.

Enclosed Tips (Venturi)

Venturi Effect devices are housed in a duct which has the effect of concentrating the flow of water past the turbine. The funnel-like collecting device sits submerged in the tidal current. The flow of water can drive a turbine directly or the induced pressure differential in the system can drive an air-turbine.

Archimedes Screws

The Archimedes Screw is a helical corkscrew-shaped device (a helical surface surrounding a central cylindrical shaft). The device draws power from the tidal stream as the water moves up/through the spiral, turning the turbines.

Tidal Kites

A tidal kite is secured to the sea bed and carries a turbine below its wing. The kite flutters in the tidal stream, describing a figure-of-eight shape, which increases the speed of the water flowing through the turbine.