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Grid connection of Wave Energy Converters: investigation on storage requirements and solutions

Final Report Summary - SEA2GRID (Grid connection of wave energy converters: investigation on storage requirements and solutions)

The two-year research activities carried out under the SEA2GRID project led to a series of relevant conclusions about the approach, results, and still open issues in the wave energy sector.

At first, the research activities showed the paramount importance of keeping a multidisciplinary perspective when dealing with wave energy converters (WECs). To reach a deep understanding of the operation and performance of a WEC, an insight into different field of investigation is required:

(a) from hydrodynamics - for studying the interaction between the waves and the prime mover - to mechanics for the construction and survivability of the prime mover;
(b) from electrical machines and power electronics which are the basis for the power take-off (PTO) study to the field of power systems, to deal with the grid connection of a WEC device.

To correctly deal with a wave energy project, and correspondingly to simulate it by a 'wave-to-wire model' it is fundamental to identify the main characteristics of all these subsystems and especially their mutual interactions. It has been also underlined that it is important to clearly define the purpose of any specific analysis, to consequently identify the level of detail needed in the corresponding simulation model. This is relevant since phenomena occurring in different time scale are involved in the wave energy conversion (from the millisecond to the minute or hour timescale) and it would be impossible or unpractical to treat them with equal precision in the same model.

During the SEA2GRID project it has been specifically underlined, through several test-cases and examples, the importance of control both to improve the power extraction from sea waves and to allow the grid connection of the WECs, while ensuring proper power quality and grid code compliance at the point of connection. The role of the PTO has been also highlighted as enabling technology of the control action and the mutual relationship between selected control strategy and PTO topology and operation. Finally, the importance of an optimised design of the PTO itself has been stressed, especially as regards its efficiency, since this parameter can severely affect the overall power performance of the WEC. The specific analyses performed during the SEA2GRID confirmed that direct-driven point absorbers are an especially challenging application due to the extremely high peak to average extracted power ratio (possibly higher than 20), which calls for a consistent oversizing of the PTO system. It is worth noting that when multiple WECs are deployed in a wave farm, a consistent reduction (more than 40 %) can be obtained in the peak-to-average power ratio due to the natural special displacement among the WECs in the farm. This results in a natural power smoothing, but possibly also in a small reduction (some per centage points) in the average power capture, due to possible masking effects of point absorber in the front rear on those on the back rear.

The specific test case of points absorbers of rated power of few hundred kW connected to the Biscay Marine Energy Platform (Bimep) infrastructure was considered. Bimep is an infrastructure for research and testing of offshore WEC, which is located in the Basque Country. Its point of connection to the main power system constitutes a strong electric grid. It has been shown that the connection of a single point absorber to such strong grid has a negligible effect on the voltage drop and frequency level. It is however worth noticing that when a single point absorber is connected to an infrastructure having a rated power of several MW (20 MW in the case of Bimep) it is recommended to de-energise the unused berths, to avoid the circulation or reactive power that represents a consistent fraction of the active power delivered at the connection point by the single WEC. It has been shown that also in the case of a 20-MW wave farm of point absorber, the voltage and frequency drops are very small and the low voltage ride-through behaviour of the farm is fully satisfactory, at least in the most widespread case of WECs equipped with a fully controlled power electronics interface (e.g. back-to-back solution). The case of WECs equipped with an asynchronous machine directly connected to the main grid (with no power electronics) is potentially more critical, anyway this solution is strongly discouraged since the desired controllability of the WEC is totally lost.

It is worth noting, however, that in all the considered case of grid connection to the Bimep strong grid, grid code compliance is ensured, thus showing that grid integration of single WEC / wave farm to a strong grid does not present special concerns and specific energy storage provisions are not required. It should be mentioned however, that any considerations about grid integration is specifically dependent on the specific grid code and on additional requirements from local grid operators, thus an absolute generalisation is not possible.

On the other hand, it has been shown that the grid integration into a weak grid is much more critical, especially in the case of a multi-MW wave farm. Considering in this case the real test case offered by Atlantic Marine Energy Test Site (AMETS), it has been shown that the grid integration of a 20-MW wave farm would provoke high voltage drops both in steady state conditions and under transient conditions, thus violating the limits imposed by the local grid codes. In this weak grid scenario, energy storage deployment becomes fundamental to solve power quality issues and ensure grid code compliance. Performed analyses on the required short-term energy storage required to solve such issues showed that a power rating of the storage in the range of the wave farm installation and an energy rating in the order of magnitude of few hundred KWh would be required. Based on such power and energy rating, suitable storage technology technologies could be lead-acid batteries, flywheels and potentially nickel-cadmium (Ni-Cd) batteries.

It is worth noting, however, that this should be considered as a preliminary indication, since final technology selection should be based on the analysis of multiple wave profiles it must also consider other parameters as storage life-time and efficiency. Thus, further investigation is encouraged to evaluate in detail these aspects before final decision. Moreover, the analyses showed the importance of a smart selection of the energy storage control strategy depending on the reference set and potentially affected by the availability of good weather forecast. This is another interesting field of investigation for the future. Studies focused on the application of long-term energy storage for energy management, showed potential for the combination of wind and wave energy farm, with a potential improvement in further stabilising the output power if a centralised energy storage device is jointly deployed.

Finally an additional investigation focused on the grid integration of a wave farm to cover the energy need of an isolated community (real test case of the La Palma island), showed that in isolated networks matching power generation and consumption is a clear priority and that their correlation is fundamental to define the energy storage requirements. In this case the convenience to deploy an energy storage device and the corresponding decision on the optimal rating was based on economical parameters, in order to minimise the final cost of energy. It was shown that this approach may significantly differ from the one based on technical requirements only. Moreover, in this case a stochastic approach to energy storage sizing, which takes into account all the different operating conditions of the storage along the years was adopted. This is considered the most advisable approach to energy storage application and further investigation is encouraged in order to consider additional parameters (extended data-base for input data, efficiency of different storage technologies, storage lifetime, etc.) to refine the storage sizing procedure.

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Contact details: Jose Luis Villate
Marine Energy Area Director
Parque Tecnológico de Bizkaia, C/Geldo, Edificio 700, 48160 Derio - Bizkaia (Spain)