Optimising the number of turbines in Tidal Range Schemes

An energy transition from fossil fuel energy to renewable energy is needed to alleviate global climate change and achieve the Net Zero emission target. While focus has been put on wind and solar energies, they are intermittent and non-dispatchable. Consequently, there will be a significant need for filling the gap between renewable energy supply and energy demand. Tidal energy is potentially capable of contributing to balancing this gap because of its predictability and dispatchability. Tidal range schemes (TRSs) utilise the tidal range to create artificial head differences across the structures. The head differences then drive the turbines to generate electricity. TRSs can also be used as energy storage by controlling the turbine operation and even pumping water into or out of the impoundment. The UK has vast tidal energy resources which are mainly unutilized. While several TRSs have been proposed, there are currently no TRSs developed in the country because of their large initial investment cost and significant impacts on their adjacent coastal areas. Approaches to determine the optimal design and operation of TRSs are needed to improve their cost-effectiveness.

            This research studies the optimal number of turbines for TRSs. There have been discrepancies concerning whether an optimal number of turbines exists for any given TRS by merely optimising its annual energy production (AEP).  Several published works (Aggidis and Feather, 2012; Petley and Aggidis, 2016; Vandercruyssen et al., 2022) showed that the AEP increases as more turbines are used until the turbines fully utilise the available tidal energy. Once the tidal energy is fully utilised, adding more turbines does not increase or decrease the AEP. However, other published works (Xue et al., 2021; Hanousek et al., 2023) showed that there is an optimal number of turbines for a given TRS that produces the maximum AEP and using turbines more than the optimal number reduces AEP. This research reconciles the two aforementioned statements by optimising the AEP of a proposed TRS at an unused dock with a 0D model with and without constraining the maximum starting head to 7.5 m, which is lower than the tidal range at the TRS site. Such a constraint would be necessary if the TRS is used as a flood protection measure as well as an energy source. Results showed that AEP increased with an increase in the number of turbines until the entire tidal range was utilised if the constraint was not applied. With the starting head constraint applied, there was an optimal number of turbines beyond which the AEP decreased. The absence of the optimal number of turbines in the unconstrained condition demonstrates the importance of cost models in selecting the optimal number of turbines. Reference: (i) Aggidis and Feather (2012). 10.1016/j.renene.2011.11.045; (ii) Hanousek et al. (2023). 10.1016/j.renene.2023.119149; (iii) Petley and Aggidis (2016).  10.1016/j.oceaneng.2015.11.022; (iv) Vandercruyssen et al. (2022). 10.1016/j.heliyon.2022.e11381; (v) Xue et al. (2021). 10.1016/j.apenergy.2021.116506

Figure 1. The annual energy production for Barry Dock Lagoon with an increasing number of turbines with and without the maximum starting head constraint (≤ 7.5m).