Author: Camila Santos Oliveira and Luciano Hocevar
Energy Review, Vol 4. Issue 5. 2022
A country of considerable geographic diversity, Brazil has the potential to generate electricity from the most different sources. It, however, focuses on a few forms of power generation, primarily hydroelectricity. Seasonal or unforeseen climatic factors can cause instability in the generation capacity of hydroelectric power plants, which come to be guaranteed by thermoelectric plants, with more expensive and polluting production. Thus, it is necessary to develop and improve technologies related to electricity generation through renewable and clean sources. One of these sources is the ocean, and Brazil has more than eight thousand kilometres of coastline capable of generating electricity.
To convert existing energy in the oceans into electrical energy, there are a variety of devices that can be used to harness this potential in different ways, such as the mechanical energy of waves, currents and tides, thermal energy in the temperature gradient and the osmotic pressure generated using a gradient of water salinity. While most of this equipment is still in the development and testing process, a few are in operation and are participating in the local electricity matrix, as in the case of the La Rance plant in France. Existing models of devices that directly or indirectly use wave motion to generate electrical energy can be classified concerning their distance from shore or by device operation. We have offshore, nearshore and onshore models of devices, assessing the distance from the coast. This classification is important, as it guarantees attention to the wave regime to which the device will be subjected, since the greater the distance from the coast, the greater the depth and the smaller the effects of energy dispersion by friction with the bed, which culminates in a regime of waves that needs a stronger construction to support it.
We can classify wave generation devices as OWC – Oscillating Water Column, Floating bodies, Point Absorbers or progressive Surging Devices and Overtopping Devices. Devices of the OWC type work to convert kinetic energy using a self-rectifying turbine, which maintains the direction of rotation independent of the flow direction to capture the airflow generated by the back and forth movement of a wave in space, closed on top and open to the sea.
Floating bodies, on the other hand, convert the kinetic energy of the wave passing through a certain point (point absorbers) or through a delimited extension (Suring Devices). Other scholars divide the classification of this type of device into three categories, Terminators - devices with large horizontal extensions parallel to the direction of wave propagation; Attenuators - which have large horizontal proportions orthogonal to the propagation of waves; and Point Absorbers - devices of small proportions compared to the wavelength. They use different mechanisms to focus different motions that capture the wave motion to convert it.
Another category of devices is made up of overtopping devices, in which the water from the waves uses the ramp to pass over a barrier and reach a reservoir, from which the water leaves, passing through a series of low head turbines and generating electricity.
There is also the classification system by the IEA, termed the Ocean Energy Systems, which uses three criteria: 1) Oscillating water column with fixed or floating structure; 2) Floating or Submerged Wave Activated Bodies 3) Overtopping with Fixed or Floating Structure. This classification is also based on the functioning of the device. However, instead of “floating bodies” as in the previous classification, there are “wave-activated bodies”, and within this classification, floating bodies, accompanied by submerged bodies generate electricity with the movement of water layers below the surface.
Another possibility of converting ocean energy into electrical energy uses the temperature difference between the surface and the bottom of tropical waters, similar to a set of condenser and evaporator of a thermoelectric plant operating in the Rankine cycle. This uses a fluid with a low point boiling point to drive a turbine. It is also possible to use the salt concentration gradient between seawater and water from the mouth of a nearby river to generate electricity. In this case, two tanks with constant volume are filled, separated by a semipermeable membrane, one with seawater and the other with water from the river mouth. The concentration of salt (solute) is higher in seawater, so naturally, the water (solvent) will pass from the tank with water at the mouth to the tank with seawater, through osmotic pressure. As a consequence, the tank with the most concentrated medium, having a pre-defined volume when receiving a greater amount of material, will have greater pressure. The pressurized salt water is directed to a turbine connected to a generator.
There are many ways to convert the energy present in the oceans into electrical energy that will benefit hospitals, banks, industries, companies from the most diverse sectors and homes contributing to the elimination of blackouts, stability in the electricity grid, leading to national independence and low energy prices. Therefore, investment in science and technology is necessary to develop devices that take advantage of the energy potential of the oceans in Brazil and other countries with coastlines. In addition, the devices are beneficial to the environment by reducing the need to use fossil fuels, which culminates in a reduction of gases from their combustion, mainly carbon dioxide.
(Ms. Camila Santos Oliveira is a researcher on Ocean Energies at the Universidade Federal do Reconcavo da Bahia-UFRB, Brazil. Dr.Luciano Hocevaris a Professor at the Energy Engineering department, UFRB / CETENS - Center for Science and Technology in Energy and Sustainability, in Bahia, Brazil.)