Turbine speed control could bolster Mutriku wave plant output

Researchers from the University of the Basque Country (UPV/EHU) have found that wave power plant located off the coast of Spanish town of Mutriku could improve its efficiency.

The research focused on the study and analysis of the operational data provided by the Basque Energy Agency (EVE), which manages the plant.

Mutriku power plant, operating since 2011, comprises oscillating water column (OWC) wave energy devices with the capacity of 296kW.

The team of researchers from UPV/EHU’s EOLO group analyzed the behavior of the plant during the 2014-2016 period.

After the analysis of the data, the researchers have determined that there is a way of increasing the efficiency of the Mutriku power plant.

Gabriel Ibarra-Berastegi, the lead author of the study, said: “We saw that one output indicator is the Capacity Factor (CF), which allows different technologies for producing electricity to be compared. In this case we calculated the CF of the Mutriku farm and its value is 0.11, while wind energy facilities have a CF in the region of 0.2-0.3, and solar plants 0.4.

“That indicates that the OWC technology at Mutriku needs to improve its CF to put it on a par with the values of the remaining renewable energy sources.

“We believe that the way to achieve this is to improve the regulation and control of the speed at which the turbines rotate, in other words, to properly manage the speed at which the turbine rotates in relation to the advancing waves.”

In OWC technology the turbines are not driven by the waves directly but by a mass of compressed air. It is a structure in which the upper part forms an air chamber and the lower part is submerged in the water.

That way, the turbine takes advantage of the movement produced by the wave both when it advances and recedes, and the generator to which it is coupled feeds the power into the grid.

In its first five years of operation, the Mutriku wave power plant produced and exported over 1.3GWh of power to the Basque grid (Photo: tethys.pnnl.gov)

According to Gabriel Ibarra, the conclusions drawn from the data on a real wave farm like that of Mutriku constitute a step forward whereby for further advancement of OWC technology so that it can reach a level of maturity to facilitate the introduction and deployment of such farms.

“The turbines generate electricity which is regularly sold to the electricity grid. In the case of Mutriku this happens 75% of the time. The plant shuts down from time to time when the waves are very calm or even when they are too rough,” added Ibarra.

The offshore power plant or wave farm at Mutriku is said to be the only commercial facility in the world that operates by regularly feeding the grid with electrical power produced by waves.

The cost of constructing the plant was €2.3 million, and the energy produced is used to cover the needs of 100 households.

Aside from producing the energy from waves, the plant also acts as a test center for new turbine technologies and control systems.