Wave To Energy

(a) Backward Bent Duct Buoy (BBDB) component of the device has high efficiency in a certain wave frequency bandwidth according to the natural frequency of the water confined inside it (mass m_3 in Figure 1). The BBDB system acts similarly to a single degree of freedom oscillatory system (as shown in Figure 1, PTO is modeled like a dashpot with the damping c_3). It is analytically and numerically proved by Rezanejad and Guedes Soares (2018) that the single degree of freedom oscillatory systems can absorb most of the energy of the excitation resources (wave energy in the present case that causes imposing force F(t) on the mass m_3) in the frequency range close to the natural frequency of the system. Hence, high efficiency is attained in the wave frequencies close to the natural frequency of the mass m_3 in the BBDB system that has the dominant role to capture the energy in this condition.

(b) In addition, it is also proven in the literature that the tendency of the BBDB systems to capture higher energy is increased in the pitch resonance frequency of the FOWC rigid body. Hence, the efficiency increases in the pitch resonance frequency in which the BBDB system has the dominant role to capture the energy from waves.

(c) Similarly to the mass m_3 in the BBDB part of the system (Figure 1), the efficiency is increased in the frequency ranges close to the natural frequency of the mass m_1 (confined inside the OWC part of the system). Hence, the role of the OWC part of the system becomes dominant to capture most of the incident wave energy in a specific range of frequencies.

(d) The underneath plate in the OWC part of the system imposes an additional resonance mechanism to capture the energy from waves. In this case, the mass of water located outside of the OWC chamber and above the plate (mass m_2in Figure 1) will contribute to the power capture mechanism (similar to the coastal OWC devices installed in the stepped sea bottom condition, see Rezanejad et al., 2013 for more details). In this condition, the OWC part of the device acts similar to the dual mass oscillatory system (as shown in Figure 1) that significant improvement in the efficiency can be reached consequently (which is proved analytically and experimentally for the fixed OWCs in the stepped sea bottom by Rezanejad and Guedes Soares, 2018). In conclusion, substantial efficiency improvement is expected to occur in the specific frequency ranges related to the natural frequency of the two masses of m_1 and m_2.

(e) The mutual interaction between the two chambers enhances the energy capture of the whole system as was proved by Rezanejad et al. (2015) (for the case of dual chamber fixed OWC devices). In addition, application of dual chambers imposes a kind of natural phase control to the system which eliminates the need to use complex control strategies (for instance re-active control). In the re-active control strategy, a certain portion of energy is applied to the PTO system in some circumstances to increase the overall performance. Re-active control tools are sophisticated and expensive to implement. Moreover, some portion of the produced energy should be imposed (consumed again) to the system as well. In the dual chamber floating OWC system, the equivalent effect of re-active control is naturally applied to the system without the need to apply some portion of the produced energy to the PTO and without the need for using complex controlling strategy (see Rezanejad et al. 2015 for more details).

In conclusion, the consecutive occurrence of the above mentioned mechanisms will cause the primary efficiency (efficiency of converting the wave power to the pneumatic power) of the system to remain in the high level in a broad range of wave frequencies. Consequently, the efficiency of the system in random waves that include various wave frequency components increases significantly. Therefore, it is expected that the average primary efficiency of the FOWC in random waves would be around 60-80%. The efficiency of the high-tech impulse air turbines (which have been developed recently) is up to 75% (efficiency of converting pneumatic power to electric power). Hence, it is estimated that the wave to wire efficiency (overall efficiency) of the optimized design of FOWC device can reach the average level of 50% in random waves (that can convert the 50% of the available wave power to the electric power in the installation site), which (from the knowledge of the researchers developing this system) have not been observed yet in other types of WEC systems.

In addition to the high performance (that leads to energy generation with the cost less than 0.12 €/kWh in the commercial phase), the dual chamber floating OWC system has the following innovative capabilities/characteristics/beneficial aspects that may cause extending its applicability and further reductions in the energy production costs:

(1) As was explained earlier, two buoyancy modulus are deployed in the fore and rear part of the device (Figure 1) providing the required floatation for the WEC device. The vacant space inside these modules can also be used to store the produced energy by using hydrogen (or other possible forms of clean fuels like ammonia). In this case, the need for transmitting the energy in terms of electricity using underwater cables would be eliminated or reduced, which can further reduce the energy production costs of the units installed in offshore zones. The produced energy in the form of hydrogen can be used to support the energy demand of future generation of marine vehicles and offshore platforms. In this context, the future generation of marine vehicles is the main target market of this WEC technology. It should be noted that the marine transportation sector produces 3% of the carbon dioxide in the globe. The entire maritime sector (transporting 90% of goods in the globe) should seriously look into renewable fuels such as hydrogen and ammonia, because the International Maritime Organization (IMO) obliged them to reduce their total GHG (global greenhouse gas) emissions by at least 50% by 2050. Hence, the proposed FOWC technology would be an appropriate solution to support the indicated demand and combat against the climate change effects.

(2) The FOWC technology can be implemented in energy parks with other renewable energy generation systems (floating wind turbines, tidal turbines). As the device has two relatively large floating modules, (in addition to renewable energy generation,) it can simultaneously be used as the floating energy storage platform (to store the energy in the form of Hydrogen) for all the renewable energy production units in the energy park. The produced hydrogen in the energy park can be carried out to the shoreline or to a floating storage unit by means of marine vessels.

(3) The introduced FOWC concept can also be implemented as a combined system to simultaneously produce energy and protect the shorelines from harsh waves (Figure 2). In other words, it can also act as an efficient floating breakwater (FBW) system as it absorbs a significant portion of the energy in addition to reflecting a part of the energy emitted to the device (and does not permit the waves to pass toward the downstream of the device). Hence, it can provide an appropriate sheltered area downstream of the device. The energy production costs will therefore be reduced as construction and maintenance expenses are shared with the shore protection application. In this context, the variation of the wave transmission coefficient has been calculated numerically for the case study model (with the characteristics of the 1/50 scale model in experimental study). The wave transmission coefficient (C_t) is defined as the ratio of the wave energy in downstream of the device to the incident wave energy. In this regard, the variation of the wave transmission coefficient for the 1/50 scale model as well as for the equivalent box-type (Pontoon) floating breakwater (with the same mass properties, draft and width with the case study model) is shown in Figure 3 with respect to the wave period for various PTO damping coefficients applied to the system (Rezanejad and Guedes Soares, 2021). It is observed that the wave transmission coefficient of the floating OWC device is in general, smaller than the box-type FBW. Hence, the dual-chamber floating OWC device substantially has better functionality compared to the conventional box-type FBW to create sheltered areas protected from waves. It is noteworthy to remark that the main drawback of conventional floating breakwater systems is related to their limited capability to protect sheltered areas from waves in a wide range of frequencies (only they have efficient behavior in a limited wave frequency bandwidth). However, the indicated drawback can be compensated significantly by application of FOWC systems as they have high capability to capture the energy of waves in addition to other wave dissipation mechanisms (reflecting the energy upon interacting with device, viscous damping).

(4) The WEC technology is designed in such a way that can be towed with common tugboats to the installation site and its anchor and single mooring can be carried out and installed by using Anchor Handling Tug Supply vessels. Hence, the installation process is less complex and requires minimum time and financial resources. Moreover, in the case of intending to store the energy on-board the device (no underwater cables are planned to be used), the commissioning process of all the sub-components of the device can be undertaken in the harbour area of the construction shipyard. In this case, the commissioning costs might be reduced significantly as the need to undertake technical actions at the installation site is minimized.