1 Introduction

In the early days, a collection of wind turbines, or a wind farm, was seen as an interesting but insignificant component in the power generation system. Terms such as intermittent generation and negative load were used to describe them [1–6]. If there was a disturbance on the network, the normal thing to do was to disconnect the wind turbines, wait until the network settled down, and then reconnect [5, 6].

The wind and tide are simulated by servo-motors that drive the wind-generator and tidal generator, respectively. The offshore-wind turbine load is complicated because the load of the offshore-wind turbine fluctuates during the day. These fluctuations create imbalances in the power distribution that can affect the frequency and voltage of the power system. Offshore-wind and tidal energy systems are omnipresent, freely available, environmentally friendly, and considered to be promising power generating sources because of their availability and topological advantages for local power generation. Prototype of HOTT Generation System (PHGS) energy systems, using two renewable energy sources, allow improvements in system efficiency and power reliability, and reduce the energy storage requirements for autonomous applications. Offshore-wind systems are becoming popular around the world for power generation applications because of the advances in renewable energy technologies and substantial rise in the prices of petroleum products. This experiment was conducted to analyze the current state of the prototype, as well as the optimization and control technologies for autonomous hybrid wind-tidal energy systems without battery storage.

2 Proposed PHGS Model System

2.1 Model Setup

Figure 1a shows how the HOTT system will be set up offshore, and how it will function. A wind and tidal turbine experimental model plays an important task in the hybrid turbine modeling, particularly for analyzing the interaction between the tidal and offshore-wind power systems, which are connected on the DC side [7–10].

Fig. 1
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a HOTT conceptual image ([10], ©IEEE 2010). b Photo of laboratory scale prototype model of hybrid offshore-wind and tidal turbine system with flywheel ([10], ©IEEE 2010). c Schematic of prototype model of hybrid offshore-wind and tidal turbine system with flywheel ([10], ©IEEE 2010). d Schematic of Hybrid Offshore-wind and Tidal Turbine (HOTT) ([10], ©IEEE 2010)

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Figure 1a–c show a photo and schematic view of the small laboratory-based hybrid power system model that designed and fabricated. The system has two types of generation, the tidal motor/generator and the offshore wind turbine generator. The tidal turbine (induction machine) can act as either a motor or generator, depending on the need. The tidal generator provides smooth output power, whereas the output power of a wind turbine depends on the wind velocity.

Figure 1a–d show how the HOTT system is set up in the laboratory and how it will function. These figures also show the conceptual schematic of the proposed HOTT system connected to the power system, and the detailed circuit configuration. The AC power generated by the wind and tidal turbine generators is converted into DC power. It is converted again into AC power through the maximum power point tracking (MPPT) inverter.

2.2 Offshore Wind Turbine

Figure 2 shows an experimental model of the offshore-wind turbine generator system. It consists of a coreless synchronous generator and a servo-motor. The offshore-wind turbine is simulated by the servo-motor. In this model system with the small servo-motor, the rated rotating speed is 2,500 rpm and the gear ratio is 10.5:1. In the real system, the wind turbine would have a slower rotating speed without the step-down gear. The rotating speed or the torque of the servo-motor is controlled by a computer. The electrical energy depends on the rpm (rotations per minute) of the servo-motor that rotates the coreless generator. Wind turbine generated AC power is converted to DC power with the 6-pulse diode rectifier [7–10].

Fig. 2
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Offshore-wind turbine generator experimental model ([10], ©IEEE 2010)

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The parameters of the servo-motor and the coreless synchronous generator are listed in Tables 1 and 2, respectively.

Table 1 Rating of main components (offshore-wind servo-motor) ([10 ], ©IEEE 2010)
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Table 2 Rating of main components (coreless synchronous generator) ([10], ©IEEE 2010)
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2.3 Tidal Turbine (Flywheel)

An induction generator produces electrical power when its shaft is rotated faster than the synchronous frequency of the equivalent induction motor. Induction machines are used in tidal system installations because of their ability to produce useful power at various rotor speeds. Induction machines are mechanically and electrically simpler than other generator types.

The energy scenario in the world is calling for efforts toward more efficient use of electrical energy, as well as improvements in the quality of its delivery. This issue involves the use of energy storage devices, such as a tidal turbine used as a flywheel. The demand for and use of such equipment are increasing. One type of energy storage system is a Tidal Turbine Flywheel Energy System (TTFES). Due to the advancements in machines and power electronics, the flywheel is becoming more popular. Many feasible projects employing flywheel storage systems have been implemented all over the world [7–10].

Figure 3 shows the experimental model of a tidal turbine induction generator/motor and a servo-motor. The main concept in this project is to apply and control a bi-directional (two way) energy flow scheme, so that energy is injected into the offshore wind turbine or stored as kinetic energy from/to the tidal system (induction machine).

Fig. 3
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Tidal turbine generator/motor experimental model. ([10], ©IEEE 2010)

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Flywheels are one of the oldest forms of energy storage, having been used for thousands of years. The potter’s wheel is one of the earliest applications of a flywheel. The kinetic energy stored in the flywheel results from spinning a disk or cylinder coupled to a machine’s rotor. This energy is proportional to the flywheel mass and the square of its rotational speed:

$$ {\text{E}} = \frac{1}{2}{\text{I}}{\upomega }^{2} $$

where I is the moment of inertia in Kg m2 and ω is the rotational speed in rad/s. The induction machine (tidal system) works as a motor with almost no load, and the rotational kinetic energy is stored as a function of the square of the rotational speed. The stored energy is extracted by decelerating the induction machine.

TTFES systems, in comparison with conventional batteries, present some interesting characteristics when used as an energy source to compensate for voltage sags and momentary power interruptions. The induction machine is used for bi-directional energy conversion from/to the tidal turbine. The servo-motor is used as an input model of tidal energy to the induction generator, which converts the mechanical energy into electrical energy. The induction machine can work as a motor by using the bi-directional IGBT converter and a one-way clutch. When the induction machine’s rotational speed is larger than that of the servo-motor, the servo-motor clutch turns to the off-state.

The design parameters of the servo-motor and the induction machine are listed in Tables 3 and 4, respectively. A speed of 1,110 rpm is selected for the induction machine to store the rotational kinetic energy. In a real system, the tidal turbine rotational speed should be much lower than that of the servo-motor, and a step-up gear will be necessary.

Table 3 Rating of main components (tidal servo-motor) ([10], ©IEEE 2010)
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Table 4 Rating of main components (Induction machine) ([10], ©IEEE 2010)
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In this application, the TTFES supports the offshore-wind turbine, supplying power to the DC load in the case of overloads or dips. This occurs when there is a low wind speed, causing an offshore-wind turbine voltage or frequency dip or overload in the hybrid side. Thus, an offshore wind system is not capable of supply all the power needed by the DC load. The system gets help from the TTFES, which has stored kinetic energy. Therefore, the main purpose of the flywheel is to accumulate rotational kinetic energy, which can be injected into or extracted from the DC side whenever it is required.

2.4 Maximum Power Flow Control

The grid connected inverter is a current control type because the AC voltage is fixed by the grid. Therefore the DC link voltage in the HOTT is kept within a certain range for stable operation by controlling the AC output current of the grid inverter. The AC output current of the grid inverter is controlled so as to give the maximum output power with a certain DC voltage. This control is based on the Maximum Power Point Tracking (MPPT) algorithm.

In the MPPT control (Fig. 4), in order to search the DC link voltage which gives the maximum DC output power, small perturbation, ΔV (±4 V), is given to the DC reference voltage and check the DC output change. If the DC power increases, the perturbation is approved make a new DC reference voltage. Conversely, if it decreases, the reference voltage is changed in the opposite direction. This algorithm makes it possible to find the maximum electric power point when the characteristic of the DC side changes by wind and tide speed changes, etc. [11].

Fig. 4
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MPPT electric power follow control algorithm

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MPPT algorithm

(1)–(8). A series of processes for MPPT

  • (1), (4) and (7) : Period for calculation of average value of DC voltage, current and power.

  • (3) and (6): Period for DC voltage change [perturbation ΔV (±4 V)]

  • (2) and (5): Check and memorize the deviation of DC output and voltage by corresponding perturbation.

  • (8): New DC voltage reference is determined by the results at (2), (5) and (8).

    • When the DC output power increases with DC voltage change +ΔV, the new DC voltage reference moves to (9) and start again from (1).

    • Otherwise, it moves to (10) and restart from (1).

2.5 Inverter Circuit Configuration

Figure 5 shows the HOTT inverter circuit for this method. The output of the inverter is a single-phase three-wire system. A general domestic power supply is often a single-phase three-wire system. The inverter has a circuit configuration that combines two half bridges. The input can share one line with the output by using the half bridge type inverter [12]. In order to meet the grid voltage (200 V), the boost up chopper circuit is adopted in Fig. 5 to increase the voltage of the DC side.

Fig. 5
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Boost up chopper with a half-bridge inverter circuit

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2.6 Hybrid System (Circuit Configuration)

This section describes the system and circuit configuration of the proposed HOTT with TTFES, which was designed and constructed based on the reviews of the alternatives and the components in the successive sections [7–10, 13].

The following is a block diagram of TTFES with an offshore wind system (also shown in Fig. 6). The offshore-wind coreless synchronous generator output is simply rectified by a 6-pulse diode bridge to charge a DC capacitor. The tidal turbine induction generator/motor output is connected to the DC capacitor through a 6-pulse IGBT dual converter. The DC link capacitor is connected to the commercial grid through a grid-connected, single-phase, 3-wire inverter. The grid-connected inverter is of a transformer-less half-bridge type with a boost-up chopper circuit. The voltage-source inverter output current is controlled by a PWM controller under maximum power point tracking (MPPT) control. The MPPT control monitors and maintains the DC link capacitor voltage, providing the maximum output power by controlling the output AC current. It monitors the DC voltage perturbations of 4 V up and down (2 V/s) every 4 s, calculates how to change the output power caused by them, and then determines the DC-voltage reference at the next stage to give more power. Several small controllers are implemented at both ends to provide the required performance for the system.

Fig. 6
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PHGS system configuration ([10], ©IEEE 2010)

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3 Changing Voltage Frequency 50–46–50 Hz

In order to simplify the description, an autonomous power supply with a limited capacity is substituted for the HOTT system (Fig. 7). A flywheel induction machine is connected to the offshore-wind system.

Fig. 7
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DC Power output

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In order to compensate for wind power fluctuation, the output of the induction generator was controlled so that the total output was steady. The reference frequency for the PWM inverter signal of the bi-directional inverter was manually changed from 50 to 46 to 50 Hz; it is assumed that the servo-motor (tidal) was controlled to maintain a stable rotating speed (1,000 rpm), because tidal flow is more stable than wind flow. This experimental model system was able to keep the total output steady by controlling the induction generator by changing the AC voltage frequency input from the bi-directional converter.

4 Experimental Results

Figures 8, 9, 10, 11 and 12 shows some of the experimental results. From top to bottom, these figures show the generator voltages, currents, and instantaneous active and reactive powers of the tidal generation system; those of the offshore-wind generation system; the voltages, currents, and powers of the DC link circuit; those of the load (AC grid) side; and the rotating speed of the induction generator, servo-motors, and coreless synchronous generator.

Fig. 8
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Experimental results of tidal turbine generator voltages (Vuv1 and Vwv1), currents (Iu1 and Iw1), active power (P1), and reactive power (Q1) under the generator condition

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Fig. 9
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Experimental results of offshore-wind turbine generator voltages (Vuv2 and Vwv2), currents (Iu2 and Iw2), active power (P2), and reactive power (Q2) under the generator condition

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Fig. 10
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Experimental results of DC (Hybrid) side voltage (Vdc), currents (Idc1, Idc2, and Idc3), and powers (Pdc1, Pdc2 and Pdc3)

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Fig. 11
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Experimental results of load (AC grid) side voltages (Vuv3 and Vwv3), currents (Iu3 and Iw3), and active power (P3)

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Fig. 12
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Experimental results of rotating speeds (rpm) of coreless generator (offshore wind generator), induction machine (tidal generator), and servo-motor (for tidal generator)

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4.1 Tidal Turbine (Induction Machine as Generator Mode)

Figure 8 shows that tidal turbine servo-motor working at 1‚371 rpm (shown in Fig. 12). The induction machine was driven in generator mode at 1‚371 rpm (more than 1‚200 rpm 60 Hz). The tidal turbine voltages (Vuv1 and Vwv1), currents (Iu1 and Iw1), and powers (P1 and Q1) were almost steady state.

4.2 Offshore Wind Turbine (Coreless Generator)

Figure 9 shows that the offshore wind turbine servo-motor and generator were operated at a constant speed of 82 rpm (shown in Fig. 12) throughout the test. The voltages (Vuv2 and Vwv2), currents (Iu2 and Iw2), and powers (P2 and Q2) were steady state, with a small fluctuation in the active power caused by Iu2 and Iw2.

4.3 DC Side

As shown in Fig. 10 of the DC link voltage, Vdc, the MPPT controller gave DC voltage perturbations of 4 V up and down (2 V/s) every 4 s. The DC side power, the offshore-wind generated power Pdc2 (offshore-wind), was almost a constant 190 W, and the tidal generated power, Pdc1 (tidal), stepped up from 230 W, with small fluctuations at 4–7 s caused by the MPPT and inverter system. The hybrid power (Pdc3) was 420 W for the PHGS steady state, although there were some fluctuations in Vdc.

4.4 Load Side

As shown in Fig. 11, the load (AC grid) side AC current and voltage were in a steady state condition, while the active power was 420 W.

4.5 Rotating Speed (rpm)

As shown in Fig. 12, the induction machine and servo-motor rotating speeds were the same (1,371 > 1,200 rpm: generator mode), and the rotating speed of the off-shore wind generator was 82 rpm. Both systems were in the generator mode and each output was summed up to the hybrid output flowing to the grid.

5 Discussion

The specific task of this research was using the prototype to evaluate the offshore wind energy and tidal technologies with respect to hybrid integration, and conveying this know-how into the ocean energy domain. The following tasks were included:

  • Analyses of offshore wind and tidal energy conversion technologies and their conversion principles, control devices, and operational behaviors.

  • Analyses of power quality and hybrid impact issues, as well as other peripheral technical barriers.

  • Identifying possible similarities associated with hybrid integration (mixing) of offshore wind energy and tidal (induction machine) power technologies.

The PHGS conceptual demonstration prototype was constructed and successfully operated [9]. The PHGS hybrid performance was studied experimentally in stable generation ranges of the wind/tidal conditions. The proposed PHGS is more flexible than a single system, allowing the stable generation ranges of the wind/tidal conditions to be extended using an adequate system control strategy. The performance of modular hybrid energy systems can be improved through the implementation of advanced control methods in bi-directional and MPPT system controllers [11, 12, 14–16].

Optimum resource allocation, based on load demand and renewable resource forecasting, promises to significantly reduce the total operating cost of the system. The application of modern control techniques to supervise the operation of modular hybrid energy systems allows the utilization of the renewable resource to be optimized.

6 Conclusion

Executing the experimental design and analysis is a critical component in the HOTT system. The focus of this chapter is to describe the methodology and performance of a prototype experimental design hybrid system.

First, the basic performances of the conceptual demonstration prototype PHGS hybrid were studied while electric power was generated from both the offshore wind and tidal systems, and distributed to a load system. The proposed PHGS is more flexible than a single system, allowing the stable generation ranges of the wind/tidal conditions to be extended by an adequate system control strategy. The output of the tidal generator (induction machine) is controlled using an IGBT bi-directional converter system to compensate for the power fluctuation in the offshore-wind turbine generator. Additionally, the tidal induction machine rotor can be mechanically isolated from the tidal turbine shaft by a one-way clutch, allowing the induction machine to work, not only as a generator, but also as a motor (flywheel energy storage [15, 16]) using the IGBT converter control. The wind and tidal hybrid generation system circuit models linked using a DC link capacitor and MPPT grid inverter showed stable operation [13]. A boost up chopper was used to improve the power factor of the system. The grid inverter was controlled using the maximum power point tracking (MPPT) control, which kept the DC capacitor voltage at the maximum power output.

Second, two kinds of control strategies were proposed and experimentally demonstrated using the prototype test setup.