DC Microgrid System

DC microgrids
 

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A DC micro grid system has been proposed as a power network that enables the introduction of a large amount of solar energy using distributed photovoltaic generation units. To test the feasibility of the system, we have developed a demonstration facility consisting of silicon photovoltaic (Si-PV) units, copper indium gallium (di)selenide photovoltaic (CIGS-PV) units, concentrator photovoltaic (CPV) units, an aerogenerator, and a redox flow battery. The redox flow battery, a key component for supply-demand adjustment in the micro grid system, successfully balanced supply and demand in the grid by its rapid charge-discharge ability even under the fluctuating condition of power generation and consumption.

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How can we increase the amount of photovoltaic (PV) generation? From this viewpoint, we are overviewing elec- tric facilities from power plants to electric appliances in demand sites. PV modules generate DC electric power. The power should be converted to AC that is synchronized with commercial grids to be transmitted and distributed to demand sites. To reduce energy dissipation through the transmission, the power is sent near the demand site after being raised the electric voltage to 66 kV or higher. The power is transformed to 100 V and provided to residential outlets after multi-processed reduction in voltage at substations and polemounted transformers. Therefore, we should consider how we can establish efficient transmission and distribution systems for PV generation in addition to cost, efficiency and lifetime for generation facilities, if we utilize the power source as infrastructure.

Transmission facilities for PV generation often stay idle as well as generation facilities themselves, because they do not yield electricity during night and poor weather. If contribution from solar power were much smaller than transfer capability, existing facilities could take care of it. To understand this problem easily, we assume a huge PV farm comparable to a nuclear power plant with a giga wattage class output. PV generation, which has poor yield for its footprint, needs vast ground to generate such a big power. Consequently, the generation facilities must be set up in sites far from consuming regions. Transmission facilities must have enough large capacity for maximum cur- rent which can be generated under the best weather condition. They do not work during off-generating time such as at night and under poor sunshine. If PV plants sup- plied constant huge power as dam type hydraulic or nu- clear plants, we would make choice of a far-reaching transmission system that connects distant sources and a consuming center.

As mentioned above, large scale PV plants in remote sites have a serious problem on economic efficiency. We need a new power system that enables the introduction of a massive amount of distributed PV units in demand sites. This article proposes DC microgrid systems as an option for such a purpose.

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