# Wave Power Output Smoothing through the Use of a High-Speed Kinetic Buffer

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## Abstract

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## 1. Introduction

- The development of control methods for power output regulation at hybrid wave energy installations combined with a flywheel is presented. The proposed control strategy can be used for wave power output regulation in order to satisfy future grid codes for marine energy sources.
- The proposed control considers the application of flywheel ESSs for wave energy plants to smooth the power output delivered to the grid.
- It is worth highlighting that the developed control strategies were applied using data from an existing wave emplacement, namely, the Lysekil Wave Energy Site located in Sweden.

## 2. Wave Energy Characteristics

- Wave energy is characterized by the height and frequency of the waves, which experience constant variation; as a result, a marine installation’s power output does not comply with the quality criteria required for electric grids. To improve the power quality of the delivered energy, it is possible to install storage systems in marine emplacements or even group wave generators in a way that allows oscillations of the power output of some units to be compensated by the power output of other units in the same cluster.
- Wave direction is very changeable, and for this reason, it is necessary to carry out exhaustive studies to determine the optimal location of the marine generation units’ mooring systems.
- With respect to grid connection requirements, there are no grid codes regulating the coupling of marine generation emplacements; therefore, the codes that have been used for wind installations are applied instead. However, the variability in the power output that marine installations inject into the grid must be considered because their behavior differs greatly from other installations.

## 3. Wave Power Output Signal Improvement

- The first approximation consists of clustering a set of WECs at the same connection point (PCC) in order to generate power that can be injected into the grid with reduced oscillations. This power output aggregation also improves the on-shore connection system [22].
- Another option is the control of the wave power converter. The active control of the power take-off (PTO) system can improve the power output of a grid-connected marine emplacement [23].
- In those cases in which hydraulic PTOs are used, it is possible to reduce the power output oscillations by means of water accumulators at sea level [24]. The system utilized by overtopping reservoirs allows for a small power output regulation. Nevertheless, storing wave energy in its hydraulic form in other types of installations could lead to extra installation costs [25].

- Capacitors were used in [26], in which a power converter controlled the charge/discharge operation to smooth the power output oscillations of the marine installation. It was demonstrated that the size of the capacitor depends on the size of the installed WEC power. Consequently, its use is restricted to small wave energy plants.

## 4. Flywheel Energy Storage Systems

- The primary component is a rotating mass that is commonly known as a flywheel. The rotating mass is able to store kinetic energy, so the bigger the size of the flywheel, the greater the stored energy. The stored kinetic energy is strongly dependent on the mass rotational speed.
- An electrical machine is in charge of transforming the mechanical energy to electrical energy or vice versa (generator/motor).
- Power electronics are used for controlling the performance of the electrical machine.
- Magnetic bearings hold the flywheel’s weight.
- An external inductor is used for improving the Total Harmonic Distortion (THD) that is created by the generators of the permanent magnets in the electrical machine.

- Moreover, flywheels have a higher power and energy density [27].
- They also have a shorter charging time. This translates to a power output that can come from primary sources with high variability in a short period of time, such as wind and wave power [27].
- Flywheel ESSs are cheaper compared with other batteries storage systems, such as Li-ion and flow batteries [45], and because their performance does not degrade over time, they do not need to be replaced, contrary to the requirements of battery devices. In general, the economics of flywheel units is superior compared with battery technologies; for example, for one cycle per day, the battery cost of a Li-ion and flow battery can be up to 14.2% and 24% higher than the flywheel cost, respectively. This difference increases for three or more cycles per day, as shown in [45].

#### 4.1. Flywheel Components

#### 4.1.1. Flywheel Rotating-Mass

#### 4.1.2. Magnetics Bearings

- Passives: The main advantages of these elements are their low friction losses and low cost [48]. However, they are used as backup bearings because their performance presents instabilities.
- Actives: These bearings are used as additional units to reduce rotor vibrations. Their main advantage is that they present higher controllability and longer life cycles than passive ones, but active magnetic bearings do present higher losses due to the existence of bias currents. Combining active and mechanical bearings is a good choice in terms of controllability, stability, viability, and costs [41].
- Lastly, superconducting magnetic bearings are the most widely utilized type in applications that require high speed since they have more stability, longer life cycles, and lower losses. The main inconvenience is their very low operating temperature, which requires the addition of a cryogenic system that can provide these low temperatures. Consequently, the cost of the equipment increases [41].

#### 4.1.3. Electrical Machine

#### 4.1.4. Power Converters

## 5. Lysekil Research Site

#### 5.1. Location

#### 5.2. Technology Used

#### 5.3. Marine Substation

## 6. Lysekil Research Site Modelling for Power Output Smoothen

#### 6.1. LRS Input Data

#### 6.2. LRS Modelling

#### 6.2.1. General Description

- Filter: the purpose of the filter is to create a reference signal for the FESS in order to store wave energy in this device. To accomplish that, the filter decomposes the input signal (power from WECs) into two components: one that corresponds to the smoothed power output and another that corresponds to the fluctuating part of the signal and that must be stored in the FESS (Figure 14, Reference signal Block).
- The KESS is formed by a high-speed flywheel that is connected to the DC link (see Figure 14, FESS Block).
- The DC/AC inverter is an IGBT inverter that improves the wave farm power output by filling the power valleys with the energy stored in the KESS (see Figure 14, Grid-Side Converter Block).
- Marine transformers are used for integrating the wave power output to the on-shore power grid. The transmission network at the Lysekil emplacement consists of two power transformers with five winding taps connected at each extreme of a submarine transmission line. In this study, only the first transformer was considered [63].

#### 6.2.2. WEC Units

#### 6.2.3. Flywheel Model

- Drag losses occur when the FESS model is operated between 15 and 30 krpm. In this speed range, the air in the flywheel chamber develops a turbulent velocity regime. Under this condition, the power that dissipates because of the drag losses from a rotating cylinder in a concentrically cylindrical case can be expressed as (6) [67]:$${P}_{d}=\omega R{F}_{d}$$$${F}_{d}=\lambda \rho \pi {R}^{3}L{\omega}^{2}$$
- For electrical machine losses in the FESS model, an electrical machine whose stator is coreless is considered; thus, the main source of losses in the machine is Joule losses, as expressed in (8):$${P}_{i}=R{I}^{2}$$
- Lastly, the magnetic bearing losses associated with the magnetic bearing that is responsible for sustaining the flywheel rotor can be classified as hysteresis losses or eddy losses. At high flywheel speeds, the eddy losses are those that surpass the hysteresis losses [64]; thus, for the developed FESS model, magnetic bearing losses are calculated according to (9), and these losses are directly proportional to the square of the flywheel rotational speed.$${P}_{eddy}={K}_{eddy}{\widehat{B}}^{2}{f}^{2}$$

#### 6.2.4. Grid Connection

## 7. LRS Power Output Smoothening

#### 7.1. First Stage: Generators Clustering

- Peak Power, Average Power, and Peak/Average Ratio.As the number of generators clustered at the grid connection point increases, both the peak and average power values of the power output from the wave energy plant increase, leading to a simultaneous decrease in the peak/average power ratio.
- Normalized Standard Deviation (nSD).The normalized standard deviation is measured in p.u. and gives a measure of the relative power output fluctuations. This value is more intuitive than the value of absolute fluctuations.
- Point of delivery KPIs.Table 1 shows the power delivered to the grid by the wave energy farm at the grid connection point. From the results, it can be concluded that clustering the WEC units improves the power output in terms of the peak/average ratio and standard deviation. Nevertheless, the power output variability is not completely improved. Consequently, additional techniques need to be carried out in order to achieve this goal; these techniques include filtering and valley filling, and their procedures are fully shown in Table 1.

#### 7.2. Second Stage: Filtering and KESS Stage

#### 7.3. Active Power Control

## 8. Discussion

- First, aggregating a group of generators into one cluster results in a decreased Peak/Average power ratio and an improved normalized standard deviation (nSD).
- In addition, two clustered wave generators share the same magnitude order when referencing the peak values; this results in a notable increase in both the energy losses and the energy delivered to the grid (to the shore). This can be seen in Table 1, in which the results of aggregating Generators 2 and 3 are reported.
- Furthermore, the cut-off frequency of the low-pass filter is critical for the smoothing process. The study found an optimal cut-off frequency of 256 Hz; it not only filters the output power but also reduces the energy curtailment. Increasing the cut-off frequency of the filter above this optimal value does not decrease the losses in the substation or flywheel, but it increases the energy curtailment, which is not efficient.
- Moreover, increasing the moment of inertia of the flywheel (that is, increasing the cylinder height) results in a considerable decrease in the energy curtailment. Nevertheless, an appropriate criterion for choosing the cylinder size could be based on the related improvement in (1) the energy that is delivered to the shore and (2) the curtailed energy. This optimal value is achieved for a moment of inertia that is equal to 3.18 kg m${}^{2}$, which corresponds to a flywheel size that is roughly 10% of the inverter’s size.
- In conclusion, almost all of the analyzed KPIs are considerably improved if the aforementioned smoothing techniques are carried out, such as the peak power to the shore, peak/mean power to the shore ratio, normalized standard deviation, and grid connection losses, all of which are decreased. These findings are a clear indicator of the effectiveness of the proposed simulated approach.

## 9. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## Appendix A

Nominal power at 0.7 m/s | 10 kW |

Voltage (line to line, rms at 0.7 m/s), ${V}_{d}$ | 200 V |

Generator resistance, ${R}_{G}$ | 0.44 ± 1% $Omega$ |

Generator inductance, ${L}_{S}$ | 11.7 mH |

Air gap | 3 mm |

Size of the magnet block | 6.5 × 35 × 100 mm${}^{3}$ |

Pole width, ${w}_{p}$ | 50 mm |

Number of stator sides | 4 |

Vertical stator length | 1264 mm |

Vertical translator length | 1867 mm |

Transator resp. stator width | 400 mm |

Translator weight | 1000 kg |

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**Figure 4.**Structure and components of a flywheel. (Reproduced with permission from the authors [33]).

**Figure 6.**Electric schemes of a grid-connected flywheel. (

**a**) back-to-back topology (

**b**) back-to-back wind-FESS combined topology [41].

**Figure 8.**Wave climate matrix at Lysekil research site [54].

**Figure 9.**Outline of the basic components of the LRS [57].

**Figure 11.**Electrical diagram of the LRS on-shore connection [58].

**Figure 23.**Comparison of energy curtailed, energy to shore, and mean power to shore between different filtered frequencies and flywheel moments of inertia.

Peak ${\mathit{P}}_{\mathit{shore}}$ (kW) | Mean ${\mathit{P}}_{\mathit{shore}}$ (kW) | Peak/Ave. ${\mathit{P}}_{\mathit{shore}}$ | Normalized Standard Deviation | Energy to Shore (kWh) | Grid Losses (Wh) | |
---|---|---|---|---|---|---|

WEC1 | 9.94 | 0.81 | 12.28 | 1.54 | 0.81 | 0.85 |

WEC2 | 40.82 | 1.29 | 32.71 | 1.75 | 1.25 | 2.42 |

WEC3 | 30.09 | 1.35 | 22.32 | 1.52 | 1.35 | 2.48 |

WEC (1 + 2) | 42.83 | 2.06 | 20.82 | 1.23 | 2.05 | 4.12 |

WEC (1 + 3) | 33.98 | 2.16 | 15.75 | 1.12 | 2.15 | 4.26 |

WEC (2 + 3) | 45.30 | 2.60 | 17.45 | 1.16 | 2.59 | 6.20 |

All WECs | 45.58 | 3.41 | 13.39 | 0.96 | 3.4 | 8.840 |

Peak ${\mathit{P}}_{\mathit{shore}}$ (kW) | Mean ${\mathit{P}}_{\mathit{shore}}$ (kW) | Peak/Ave. ${\mathit{P}}_{\mathit{shore}}$ | Grid Losses (Wh) | Curtailed Energy (kWh) | Energy to Shore (kWh) | Normalized Standard Deviation | |
---|---|---|---|---|---|---|---|

WEC clustering | 45.58 | 3.41 | 13.39 | 8.84 | − | 3.40 | 0.96 |

LPF at 256 Hz | 9.23 | 2.52 | 3.66 | 12.72 | − | 2.51 | 0.58 |

KESS (3.18 kg.m2) | 9.23 | 2.91 | 3.17 | 4.37 | 0.36 | 2.92 | 0.49 |

Active power control | 4.56 | 2.68 | 1.71 | 3.47 | 0.70 | 2.68 | 0.44 |

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## Share and Cite

**MDPI and ACS Style**

Rojas-Delgado, B.; Alonso, M.; Amaris, H.; de Santiago, J.
Wave Power Output Smoothing through the Use of a High-Speed Kinetic Buffer. *Energies* **2019**, *12*, 2196.
https://doi.org/10.3390/en12112196

**AMA Style**

Rojas-Delgado B, Alonso M, Amaris H, de Santiago J.
Wave Power Output Smoothing through the Use of a High-Speed Kinetic Buffer. *Energies*. 2019; 12(11):2196.
https://doi.org/10.3390/en12112196

**Chicago/Turabian Style**

Rojas-Delgado, Brenda, Monica Alonso, Hortensia Amaris, and Juan de Santiago.
2019. "Wave Power Output Smoothing through the Use of a High-Speed Kinetic Buffer" *Energies* 12, no. 11: 2196.
https://doi.org/10.3390/en12112196