Today, the whole world faces a great challenge to overcome the environmental problems related to global energy production. Most of the islands throughout the world depend on fossil fuel importation with respect to energy production. Recent development and research on green energy sources can assure sustainable power supply for the islands. But unpredictable nature and high dependency on weather conditions are the main limitations of renewable energy sources. To overcome this drawback, different renewable sources and converters need to be integrated with each other. This paper proposes a standalone hybrid photovoltaic- (PV-) wave energy conversion system with energy storage. In the proposed hybrid system, control of the bidirectional buck-boost DC-DC converter (BBDC) is used to maintain the constant dc-link voltage. It also accumulates the excess hybrid power in the battery bank and supplies this power to the system load during the shortage of hybrid power. A three-phase complex vector control scheme voltage source inverter (VSI) is used to control the load side voltage in terms of the frequency and voltage amplitude. Based on the simulation results obtained from Matlab/Simulink, it has been found that the overall hybrid framework is capable of working under the variable weather and load conditions.
In developing countries like Malaysia, the development of islands is mostly related to the electric power availability, because there are many islands all over Malaysia where electric power grid is not available. Among these island communities electricity is supplied by traditional energy sources, but the fuel cost increases significantly with remoteness. Furthermore, the energy produced by the conventional sources raises the greenhouse gas emissions, which may be the key source of global warming. It is projected that, by 2020, Malaysia will release 285.73 million tons of CO2 which is an increase of 68.86% compared to the amount of CO2 emitted in the year 2000. In Malaysia, electricity generation alone contributes 43.40% of the total CO2 emission, which is the largest among all sectors [
Among the renewable energy sources, solar energy is an environmentally friendly and the fastest growing green energy source. But the main drawback of the PV system is that the power produced by this system is highly dependent on climatic conditions. For example, a PV system could not able to produce any power at night and during cloudy periods. So the PV system intermittently produces power, which means that PV system may not totally satisfy the load demand at each instant. This problem can be solved by combining PV system with other renewable energy sources and/or energy storage systems (such wind, wave, fuel cell, battery bank, ultracapacitor bank, and hydrogen storage tank) in a suitable hybrid framework [
Wave energy is a renewable energy generated by the force of surface waves from the ocean. Although many wave energy conversion techniques have been patented and new patents are granted each month [
An extensive review based on the solar and other relevant areas has been reported in the literature to model hybrid renewable energy system. Among them, Onar et al. [
In this paper, detailed modeling, control, and simulation of a PV-wave hybrid renewable power generation system are developed for island communities. OWC wave energy device is used to generate the electrical power from the sea waves and PV model is used to generate power from solar radiation. A control algorithm is developed using a BBDC between the battery bank and dc-link, and a switch mode inverter is placed at the load side end. A simple passive L-C filter is placed after the inverter at load side end to eliminate the unwanted high frequency harmonics, which are generated by the load side VSI based on the inverter switching frequency.
The simulation model can be used not only for analyzing the battery storage based PV-wave hybrid system performance, but also for designing and sizing the system HRES to meet the consumer load demands for any available meteorological condition. The proposed standalone PV-wave hybrid system model in this paper has been modeled, designed, and simulated using Matlab, Simulink, and SimPowerSystems software packages. In addition, simulation results are presented to verify the effectiveness of the proposed system under variable weather conditions.
The sequential workflow hints of this paper are as follows. In Section
In this section, the detailed simulation model of PV-wave hybrid renewable power generation system is briefly described. Figure
Block diagram of the proposed standalone PV-wave hybrid system.
In the HRES, the renewable PV and wave energy system is considered as a main power generation source to meet the system load demand and battery bank is used as a backup energy storage system. The HRES is proposed to implement in island areas in Malaysia; hence, if generated power from HRES is not enough to meet the system load demands, then battery bank will deliver power to balance the system power demand. To interface PV, wave, and battery bank in hybrid framework, the dc-link voltage must be constant. Hence, a BBDC with PI controller is used in the HRES to maintain the constant dc-link voltage. A three-phase VSI with relatively complex vector control scheme is used at load side to control load side voltage in terms of the amplitude and frequency. The detailed description of each component of the overall HRES and controller is given in the following parts.
Solar PV systems generate electric power by converting solar photon energy into electrical energy in the form of direct current using solar cell or PV cell. Crystalline or polycrystalline materials are commonly used for solar cell [
Circuit diagram of single diode PV model.
The output power from solar PV array is given by
KOYCERA KC85T-87W PV model I-V characteristics curve with varying irradiation.
KOYCERA KC85T-87W PV model P-V characteristics curve with varying irradiation.
According to solar irradiation or load current, the maximum output power of the PV module varies. Therefore, a proper control system is needed to use the PV model more efficiently as an electric power source by building a MPPT. There are many different MPPT methods discussed in [
The Simulink diagram of the PV model with MPPT. (a) PV Simulink model; (b) MPPT model; (c) complete Simulink PV model with MPPT.
It should be noted that this paper focuses on the designing of a battery storage standalone PV-wave hybrid supply system for island communities and, therefore, the mathematical modeling for individual elements such as OWC wave chambers is simplified. The detailed design and complete mathematical modeling of OWC wave energy system can be found in [
The operating principle of the OWC as shown in Figure
OWC chamber parameters.
In this section, a set of equations is present to describe the power generated by OWC system. As mentioned earlier, OWC wave energy operating principle is much like wind turbine system, so the power available at the wave turbine consists of two terms: air velocity term
Figures
OWC full chamber arrangements [
The Simulink diagram of the OWC model.
A standard battery model presented in [
The battery is modeled using a simple series connected controlled voltage source with a constant resistive value, as shown in Figure
Nonlinear standard battery model [
The battery model based on (
The Simulink diagram of the battery model.
The circuit topology of the proposed PV-wave hybrid standalone system is shown in Figure
Circuit topology of the proposed PV-wave hybrid standalone system with emergency backup and dump load.
In this paper, through BBDC the dc link side is connected to batteries bank; the primary objective of the control of this BBDC is to maintain constant dc-link voltage as a reference value in addition to discharge/charge current from/to batteries bank according to the required load power. The schematic diagram of the battery bank BBDC controller is depicted in Figure
The schematic diagram of DC-DC converter controller.
The value of the inductor used in BBDC is crucial for the conduction mode operation of it. And also the inductor existence in the batteries bank side is shown lower ripple current results which gives long lifetime and higher efficiency. Conduction mode operation also depends on input and output current, capacitors value, and switching frequency. The value of the inductor and capacitors is as follows [
In this paper, the battery bank can act either as a power supply or as a sink. As a result, it should discharge/charge within specified limits when there is lack/surplus of hybrid power due to the weather condition. In this work, due to high wave and solar power condition, the surplus power at first is supplied to the battery bank until it reach its upper limit of charge carrying capacity and then additional power is absorbed by the dump load and is regulated via the chopper control shown in Figure
The schematic diagram of dump load controller.
In case of long term when there is no PV and (/or) wave or lower PV and (/or) wave power the battery bank may not able to meet the load demand. In this case, an emergency backup is integrated with hybrid system. Control action algorithm of emergency backup is depicted in Figure
The schematic diagram of emergency backup controller.
Figure
Flowchart for dc-link voltage control.
At the load end, a three-phase vector control VSI is used as interface element between the consumer load and DC link voltage. The load side VSI control is responsible to control the frequency and voltage at the consumer load end. In the proposed HRES system, the output load voltages should be controlled in terms of frequency and voltage amplitude because there is no electric power grid connection. The space vector control technique is used to regulate the output voltage during the variation of required hybrid power or load power.
In this paper, the vector control technique is used based on the synchronously rotating frame described in [
The equations of voltage using reference rotating
The load side three-phase VSI controller.
Based on the inverter switching frequency, the high frequency unwanted harmonics will generate in output ac voltage by the load side VSI which ultimately creates power quality problem in the customer end. In this controller, space vector PWM (SV-PWM) method is used because it slightly reduces the system harmonics contents in the output voltage. In addition, it raises the fundamental load output voltage. A simple passive
Malaysia is situated between 1° and 7° in the North Latitude and 100° and 120° in the East Longitude. But Malaysia is vastly surrounded by water and it has the 29th longest coastline in the world. It has a total coastline of 4,675 kilometers and 878 islands [
Location of target site (Perhentian Island) (modified from [
Perhentian Island has uniform climate characteristics and it has abundant rainfall and temperature with high humidity. Since it is located in the equatorial doldrums area, it has naturally great sunshine and solar radiation. But it is quite difficult to have a fully sunny day with completely clear sky. Perhentian Island receives daily average 5.5 hours of the sunshine. The solar data of Perhentian Island is as shown in Figure
Annually solar irradiation.
Solar energy cannot meet the consumer load demand at each instant because of its daily limited sunshine hours. So to build HRES in this island, wave energy can be considered one of the efficient power generation sources. The wave data of Perhentian Island is illustrated in Figure
Monthly average WH/WP data of a year for Perhentian Island.
At this site, the maximum average WH measured in the month of November was 2.1 m and the minimum in the month of June was 0.88 m. On the other hand, the maximum average WP measured in the month of December was 6.1 s and the minimum in the month of July was 4.64 s. Based on the wave theory and equations in [
Average wave power level of a year for Perhentian Island.
It could be found from [
The simulation model of the proposed standalone PV-wave hybrid system with energy storage is built in Matlab Simulink environment under different operating conditions. PMSG is modeled in Matlab Simulink from the literature [
The performance of buck-boost DC-DC bidirectional converter controller is presented in Figures
PV-wave hybrid system powers distribution.
Dc-link voltage.
The output load current response during the long time simulation is shown in Figure
Output line current response with change in required load power. (a) Output line currents throughout the full simulation time; (b) output line current when the load increases at simulation time from 3.96 s to 4.06 s; (c) output line current when the load decreases at simulation time from 11.97 s to 12.07 s.
Output line voltage response with change in required load power. (a) Output line voltages throughout the full simulation time; (b) output line voltage when the load increases at simulation time from 3.96 s to 4.06 s; (c) output line voltage when the load decreases at simulation time from 11.97 s to 12.07 s.
From Figures
The output RMS phase voltage.
The total harmonic distortion throughout the whole simulation time. (a) THD of line voltage; (b) THD of line current.
Modulation indexes for all three phases.
In this paper, a novel standalone PV-wave hybrid system with appropriate energy flow controllers is designed and modeled for island users where the electric power grid is not available. The power generated by PV sources is highly dependent on environmental conditions. To overcome this intermittent power generation nature of PV system, in this paper, PV system integrated with the wave energy converter device and battery bank, because wave energy is easily predictable and consistent than other green energy sources such as wind. The hybrid PV-wave topology with broad analysis and Matlab Simulink simulation results is presented in this paper. It has been seen from the simulation results that the controller can maintain the dc-link voltage at constant value in spite of variation in generated hybrid power and required load power. Furthermore, the controller is developed in such a way that the battery bank has been able to accumulate the excess power generated by hybrid system and supply it to the system load during the hybrid power shortage by controlling BBDC. This controller not only maintains the constant dc-link voltage, but also acts as a dc-link side active filter and reduces the generator toque oscillation of OWC PMSG during the variation in load. Finally, it has been explained how the space vector controls three-phase VSI controller to control the load side output voltage in terms of frequency and voltage amplitude to the resistive load. The THD in voltage and current at load side is about 1.7% and 1.55%, respectively, which illustrates the good quality of voltage and current generated at the consumer side end. The simulation results show that the performance of the proposed hybrid system is satisfactory under the steady-state power as well as transient solar, wave, and load power conditions. This study can be considered as the initial part of building prototype standalone PV-wave hybrid system. The future work will aim to set up a standalone PV-wave hybrid system in the University of Malaya laboratory to verify the simulations results with experiment.
The battery bank rating calculation is
The equations [
See Tables
Parameters of PMSG.
Number of poles | 4 |
Rated power | 3 kW |
Rated speed | 241 rad/s |
Per phase stator resistance ( |
0.4578 |
|
0.00334 H |
Magnetic flux induced in the stator windings ( |
0.171 Wb |
Rated torque | 14.2 Nm |
Parameters of OWC.
OWC chamber length ( |
1.5 m | ||
Water surface area inside the chamber ( |
1.4 m2 | ||
Turbine inlet area ( |
0.012 m2 | ||
Water depth ( |
|||
WH (m) | 0.98 | 0.9 | 0.88 |
WP (s) | 4.9 | 4.79 | 4.79 |
|
16.47 | 15.75 | 15.73 |
Parameters of Darrieus turbine.
Swept area by balde ( |
0.012 m2 |
Air density | 1.22 Kg/m3 |
Height of the rotor ( |
240 mm |
Diameter of the rotor ( |
100 mm |
Parameters of PV array.
Maximum rated power ( |
87 W |
Maximum voltage ( |
17.4 V |
Maximum current ( |
5.02 A |
Open circuit voltage ( |
21.7 V |
Short circuit voltage ( |
5.34 A |
Number of modules required | 5 |
Using the literature [
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank KeTTHA, Ministry of Energy, Green Technology and Water, and Postgraduate Research Grant, University of Malaya, for providing financial support under the Research Grant “Active Control and Efficiency Optimization of Turbine Blades for Oscillating Water Column Device.”