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Hybrid converters: bringing together the best of each technology to harness the energy of ocean waves

13 January 2021
Article by Henrique Cestaro, development engineer at INEGI in the area of technologies for the sea.


The worldwide consumption of electricity continues its growth trajectory. And if, in the very near past, energy sources were selected according to criteria of price and local availability, today we are witnessing a greater demand for renewable energy sources, with lesser environmental impacts.

The term "renewable energy" evokes images of solar panels and wind turbines for most people. However, it is increasingly clear that wave energy will play an important role in the global energy mix. A future that is shaped thanks to advancing technology, especially hybrid solutions.

As such, we at INEGI also recognise the clear need to invest in the development of these hybrid solutions, explore new possibilities, and validate their real-world use.

The energy mix of the future must be diversified

Of the energy we currently consume, about 20% is in the form of electricity. However, it is expected that this slice will increase to around 50% by 2050[1]. Until then, it is projected that two thirds of the energy consumed will come from renewable sources. This increase in the production of renewable electricity, in line with the increase in electrification of means of transport, from cars to ships, can result in a reduction in carbon dioxide emissions of up to 60%.

Greater diversification of energy sources, in particular by prioritising local resources, can thus contribute to greater energy independence[2], so that consumers are less exposed to external factors, such as oil prices or political issues.

In this context, the sea is seen as a renewable energy source with great potential. Not only is it available in all coastal locations, but it has an estimated electricity production capacity of 20 to 80 thousand terawatt-hours (TWh) per year[2], which represents between 100% to 400% of global energy demand electrical.

Technologies for converting wave energy are evolving

In order to transform this potential production capacity into real production capacity, several technological solutions based on Wave Energy Conversion(WEC) systems have emerged.

One of these most studied solutions is the Oscillating Water Column (OWC) system[3].

This system of harnessing wave energy consists of a concrete or steel structure partially submerged, with an opening to the sea, below the water line, forming a pneumatic chamber. As the sea surface oscillates with the waves, the sea compresses the air inside the chamber, moving a turbine and generating electricity.

These OWCs have already been built for testing and production of electrical energy, and one can highlight the station that, until recently, was located at the Pico Ondas Power Station, in the island of Pico, Azores[4]. However, the high costs of construction and maintenance of these structures, combined with the complexity in controlling the turbine-generator set, still hinder the expansion of the use of this technology.

Another technology under development that deserves to be highlighted is the capture of wave energy by a wave energy converter by Spreading or Overtopping (OWEC, for the Overtopping Wave Energy Converter).

It is a system based on the capture of water that spreads in the structure of reservoirs, where it is stored at a higher level than the average level of the surrounding water surface. The increase in potential energy obtained through spreading / overtopping is transformed into electrical energy through low-fall hydraulic turbines. The productivity of this type of device, however, is highly dependent on the marine agitation present on site, the bathymetry of the bottoms around the device, the design of the components of the structures where the spread occurs, and the performance of the turbine)[5].

Hybrid solutions are at the forefront of development

Hybrid solutions are at the forefront of development in this context, with new solution that propose to combine different WECs in one device, called Hybrid Wave Energy Converter (HWEC), so that the strengths and weaknesses of each technology are offset, thus increasing the efficiency of the installed equipment.

This work is also in the hands of INEGI's engineers, with an approach centered on ocean engineering, using advanced computational tools and experiments in a laboratory and real environment.

One can highlight the SE@Ports project, during which different geometries of the HWEC system were explored, to create an energy conversion system combining the concept of an OWC with an OWEC, to be installed on the North breakwater of Porto of Leixões. Today, INEGI continues to play an active role in the technological evolution of these solutions, namely in the scope of the PORTOS and WEC4Ports projects, both focused on the sustainable production of energy in ports with HWEC solutions.

The challenges and opportunities driven by these technologies are highly motivating to position ocean waves as a renewable energy source with an impact on the production of electricity on a global scale. Now it's up to investment in the sector, to drive the creation of technical-scientific knowledge, and propel us towards a viable technological state.



References

[1] IRENA. Global Energy Transformation: A Roadmap to 2050 (2019 Edition). International Renewable Energy Agency, Abu Dhabi, 2019. ISBN 9789292601218.

[2] Linus Mofor, Jarett Goldsmith, and Fliss Jones. Ocean Energy -Technology Readiness, Patents, Deployment Status and Outlook. Technical Report August, International Renewable Energy Agency, 2014.

[3] Aquatic Renewable Energy Technologies (Aqua-RET)

[4] História da Central de Energia das Ondas do Cachorro, Programa de recuperação da Central CAO no Pico, WavEC Offshore Renewables

[5] Wave Devices, The European Marine Energy Centre (EMEC)