해양시설물용 복합전력 생산시스템의 설계에 관한 연구

Abstract

There is worldwide competition in the field of energy security. As gasoline prices remain high, South Korea, which is highly dependent on the Middle East for oil, will need to develop alternative technologies and energy sources to achieve energy independence. Further, South Korea needs to develop renewable energy technologies in view of environmental issues and new laws on carbon emissions.

The most widely used new renewable energy technologies are solar power, wind power, wave power, sea power, and tidal power. Land-based renewable power generation has problems such as large land requirements (generating power with solar panels requires a great deal of space) and noise (generating electricity with wind power systems creates noise), which have led to a transition to offshore power generation systems. In the case of South Korea, which is surrounded on three sides by water, the western and southern shores have long coastlines and deep waters. Consequently, these offshore regions are suitable for solar power, wind power, and wave power generation. To develop renewable energy offshore, large-scale power generation systems as well as an offshore base, offshore housing, large-scale seacraft, leisure seacraft, and other offshore facilities are required.

In the development of even a standalone solar power generation system, the initial stages involve significant expenditures of capital, and maintenance and management are difficult. Consequently, an effective approach is to apply a strategy that utilizes a cascaded multi-level inverter in which multiple small-scale power generation systems are installed and then combined to create a larger-scale power source. A cascaded multi-level inverter employs a structure in which one power source is used to charge a separate power source. To develop power systems, it is common to use a battery as an energy storage device. This type of small-scale system is generally based on DC power. A standalone power generation system (powered by just one type of renewable energy) represents major changes for the environment in terms of the amount of energy generated. To resolve such a problem, a hybrid power generation system, in which two or more types of power generation are combined, is used. Together, the various electric power sources constitute an electric power source having mutually complementary properties in the environment where they are deployed, and have the ability to maintain a constant power output level even when the environmental conditions vary.

A small-scale hybrid power generation system is a structure in which power diodes are used to combine electricity from two different power sources. When the different output voltages and electrical currents from the two power generation sources are combined into a single power source, it is difficult to optimize the output of each of the power generation sources. Moreover, because the two individual power sources store energy in a single storage battery, if the storage battery’s voltage changes owing to an increase in the power generated by one of the electric power sources, the output of the other electric power source must also change. To effectively control these problems, a power controller for each power generation system must be installed and used for controlling both output voltages. Further, to reflect the properties of the two power generation sources, an overall power controller design and control algorithm must be configured. In this paper, we propose a power control system and algorithm that reflect the respective properties of a solar power system and a wave power system.

Solar panels are installed at offshore facilities, oriented in different directions, for reasons including the lack of space and the movement of the offshore facility. In so doing, because of an inflection point in the output of the solar panels, it is impossible to track the maximum power point using an MPPT(Maximum Power Point Tracking) algorithm typically used on land. This paper proposes an MPPST(Maximum Power Point Searching & Tracking) algorithm for solar power generation systems. To optimize the search range and interval of the MPPST algorithm, experiments were conducted on land and in simulations.

In this paper, an oscillating water column scheme is used for the wave power generation structure. A Wells turbine and a permanent magnet power generator generate power from the oscillating water column of the wave power generator. This paper provides a mathematical model of an entire system and proposes a wave power generation control algorithm that reflects the properties of the wave power generation system. The wave power generation control algorithm we propose incorporates properties that have been identified through experiments carried out using a wave power generation simulator.

For actual experimentation at sea, a combined solar power and wind power generation system was tested using a buoy, which is a type of offshore floating device. The results of the experiment confirmed that when the solar and wind power levels change relative to each other because of fluctuations in environmental conditions or the voltage of the storage battery, the power sources simultaneously store charge in the storage battery. Moreover, on clear days, the MPPST algorithm resulted in an 18% increase in solar power output during offshore testing compared to the MPPT perturb and observe (P&O) control algorithm. Even when wave power generation system had a lower output voltage, current was generated, and output was increased by about 5%. Further, we also found that the use of a hybrid power generation system provided about 19% improvement in the stability of the power produced compared to a standalone power generation system. And also further study will be required to extend whole system using multi-level serial inverter.