|Solar power system|
|Sunday, 19 November 2006|
Solar energy can be collected and concentrated, and then converted into electrical energy. Various solar power systems have been developed for collecting the heat energy of the sun or converting the radiant energy of sunlight into electric power. One type of solar power system includes a solar panel, which is a photovoltaic device that converts the incident light energy into electrical power. Such solar power systems are used on many types of spacecraft to provide a replenishable source of power. Solar power systems that employ photovoltaic cells to convert the radiant energy of sunlight directly into electrical energy find particular use in remote locations and during emergency situations in which conventional electrical sources have been disrupted. The systems are typically employed to power communications equipment, to supply field hospitals and aid stations, to provide emergency lighting, and for other similar purposes. Effective utilization of the power generated by a solar cell array requires that the solar array be controlled to operate at its most efficient point. The typical solar cell is generally a semiconductor solar cell that converts light energy from the sun into electrical energy. Photons interact with a depleted region of a p-n junction and produce electron-hole pairs therein. The electrons freed on the n side of the p-n junction recombine with the holes created on the p side, thus, resulting in an increased electric potential across the p-n junction. A solar cell has such a property that because of its characteristics, the voltage is varied depending on a given load factor. For instance, the open-circuit voltage of a solar cell in a non-loaded state is always greater than the optimum operating voltage at which the output of the solar cell becomes maximum. The most efficient operating point of a solar cell or solar cell array may vary dependent upon a variety of factors including temperature, illumination level, the type of cell, radiation damage to the cell, the number of cells in series and other cell properties. The output power of the solar cell increases with the intensity of the incident light, and it is therefore desirable to increase the incident intensity above that associated with normal incident sunlight. In general, the solar cell array will operate at its most efficient point and output the greatest amount of power at a specific power maximizing voltage which is determined by the operating conditions. Flat solar panels normally are formed in rectangular boxes with one or two layers of glazing above the absorbing surface, and the sidewalls that support the glazing cast shadows on the absorbing surface in early morning and late afternoon. Another approach to increasing the intensity of the incident light is to use a solar concentrator, which is a reflective panel positioned adjacent to the solar panel to reflect incident light toward the solar panel. The intensity of the light incident upon the solar panel is therefore up to several times that of the normal intensity of sunlight. High concentration solar power systems typically use reflectors, drive systems, and receivers to provide thermal energy for various industrial and commercial processes. Each reflector transfers solar radiation to a receiver and is typically a parabolic trough, a dish concentrator, or a field of heliostats. In a solar concentrator system, one or more parabolic dishes or concentrators, each having a reflective surface, are driven in azimuth and elevation so as to track the diurnal and seasonal movement of the sun in order to collect and concentrate solar radiation in or on a suitable receiver. At the receiver, the thermal energy produced by the concentration of solar radiation is usually conducted away from the receiver to a heat engine, generator or the like for the production of electrical power. The concentrator is much lighter in weight than the solar panel, so that a greater incident energy of sunlight is effectively utilized for generating electrical power without a comparable increase in weight. The reflected radiation heats a working fluid and drives a power conversion system to produce electricity.
In solar power systems, solar energy is collected and concentrated to power various types of heat engines based on power conversion cycles to produce electricity. The absorbed optical energy provides a source of thermal energy to operate a power conversion cycle or heat engine, such as the Stirling cycle engines, Brayton cycle engines, or Rankine cycle engines. The temperature of the thermal energy at the absorber depends on the concentration ratio, the optical/absorber configuration, and the rate of heat removal to the heat engine and to the environment through losses. Advanced power systems integrate the solar power charging system which includes the solar battery and the electric double layer capacitor, so that a power is generated during the daytime and the generated power is consumed in the night in order to respond to environmental and energy requirements. Solar power generation systems used as emergency power supplies comprise an inverter for converting DC electric power supplied from a solar cell into AC electric power and supplying it to consumer loads, and a switch for connecting the output from the inverter to a main AC power system such as a commercial AC power system or the like. When electric power supplied from the solar cell is insufficient for customer consumption power, the short electric power is received from the main AC power system; when electric power supplied from the solar cell exceeds customer consumption power, excess electric power is supplied to the main AC power system A power conditioner for a solar power generation system includes an inverter unit as a main circuit, an inverter driver unit, a controller unit, a display unit, an operation unit, and a power supply circuit unit which provides required power to each of these units. The display unit provides respective indications of instantaneous output power and integral power of the power conditioner that are detected and calculated by the controller unit, or an indication of a period power amount which can arbitrarily be initialized by a user. Since the power generated by a solar cell array fluctuates significantly depending upon the amount of solar radiation and the temperature as well as the operating-point voltage and current of the solar cells, the power control apparatus performs maximum power point tracking control (MPPT control) for adjusting the load of the solar cell array to extract the maximum power from the solar cell array at all times. Such peak power control systems generally utilize a variety of techniques to ascertain the maximum power available from the solar array. Some peak power control systems determine the maximum power available from the solar array to constantly apply that peak power to a load. One system for determining the maximum power available from the solar array detects the current and voltage at either the load or the solar array to calculate the power value. To realize MPPT control, the operating point of a solar cell array is made to fluctuate by varying the output voltage and current of the solar cell array, which is constituted by a plurality of solar cells, and the output power characteristic of the solar cell array is examined by checking the output power of each operating point. On the basis of the voltage-power characteristic obtained, operating point at or in the vicinity of maximum power of the solar cell array is tracked.