Feasibility of ocean energy and offshore wind hybrid solutions

Abstract

In order to meet future energy demand, a more sustainable energy society is essential because fossil fuel reserves are depleting while the total energy consumption worldwide increases. One type of renewable energy which can play a role is ocean energy having the theoretical potential to exceed both current and future human energy needs. This study researches the feasibility of a hybrid system using two sources of ocean energy, namely offshore wind and wave energy. It is concluded that a single horizontal floating wind turbine sharing a foundation with a wave energy converter is most interesting to research. The main reason for this is that offshore wind energy is still limited to relative shallow depths and thus only limited locations are suitable. At further depths, floating foundations become more important. By sharing the mooring system, electrical infrastructure and other structure components with a wave energy converter, costs can be reduced while increasing energy yield in comparison to two separate systems. A classification of existing concepts within the same category is made. By comparing these different concepts based on platform motions and energy production, it is chosen to look into the combination of a WindFloat with a buoy point absorber. Motivation for this combination is the technical stage at which the WindFloat currently is. In addition, it has good overall stability and minimal heave motions which were found through numerical simulations in the time domain when comparing different floating wind turbines under similar wind and wave conditions using state of art software. Furthermore, a point absorber is used since minimal changes would be necessary to the WindFloat platform. In addition, the power take off system can be placed outside the water making contingent operation and maintenance easier to fulfill. The point absorber is modeled as a mass spring damper system with a power take off system relative to both a fixed and a floating platform. For both cases it is concluded that the relative heave oscillation between the WEC and platform should be maximized and thus the natural frequency of the system should match the peak frequency of the sea state. The steady state behavior of the hybrid system is investigated in eight sea states, which is modeled using the JONSWAP wave spectrum. Different buoy shapes are investigated which eventually leads to an optimal conical shaped buoy with a diameter of 5 meter. Hydrodynamic parameters of this buoy are found in literature and validated using state of art software. The power production of the WindFloat is found by introducing aerodynamic theory coupled to a wind wave relationship based on the Sverdrup-Munk-Bretschneider nomogram. Now that both response and power production of both subsystems are known they are coupled to see its effect. The wave energy converter will have no effect on the WindFloat motions due to the relatively low forces interacting between the power take off system and the platform. It can be concluded that the coupling results in a very low impact of wave energy on the total increase in energy production. Absorbed wave energy is between 0.75 and 73.07 [kW] which is in terms of contribution is respectively 0.07 and 1.44 [%]. Increasing the buoy size up to 25 meter shows the potential of wave energy contribution which can go up to 10.7 [%] for strong wave environment.