1. Technical Field
The technical field relates to a method for controlling an alternating current (AC) output of a photovoltaic (PV) device, and an AC PV device.
2. Background
A basic structure of a solar cell is a semiconductor device with a P-N junction, and a direct current (DC) output is generated at two terminals of the P-N junction under sunlight irradiation. However, since the AC current predominates in current grid power system, the DC output of the solar cell cannot be directly integrated with the grid power for application. Therefore, the DC output can be merely supplied to drive a DC load, and needs a good load matching in order to achieve an optimum efficiency performance. Such a system is frequently found to be used in an apparatus in urgent need of power or in an outdoor area without power supply.
For the purpose of feeding the DC output of the solar cell into the grid power system, a PV inverter is a main device currently used for converting electric energy types. The DC output of the solar cell can be converted into an AC output through boost and frequency modulation of the PV inverter, which can be used as a private power source by a user, and also used as a public power source.
However, since the PV inverter involves electrical safety, the fabrication is difficult and a certification procedure is severe. Moreover, in order to effectively achieve high efficiency performance of photoelectric conversion, some PV cell systems are installed at positions at which it is hard to frequently maintain and displace the systems, for example, a sparsely populated desert district. Therefore, besides the limitations on the cost, size and maintenance of the conventional inverter depending on power rating per unit PV cell, complicated circuit design and other factors (e.g. a severe operating environment) also result in undesirable reliability and inconvenience of the inverter.
U.S. Pat. No. 4,217,633 discloses a technology in which a combination of three circuits is used to replace the conventional inverter.
U.S. Pat. No. 4,533,783 discloses that two symmetrical areas are made on a PV cell structure, and selection of an operating area in a single timing is determined by using an AC input, so as to control an electric energy output direction and obtain an AC output.
U.S. Pat. No. 4,577,052 discloses that two PV cells with contrary polarities are connected in parallel, incident solar radiation is controlled to be projected on a single PV cell unit by using an optical element, and timing swap is controlled by an AC signal to obtain an AC output.