外文資料翻譯--太陽能光伏發(fā)電(文件)
【正文】 nt to understand what these figures mean and how they relate to the power available from installed photovoltaic generating systems. Power Ratings Photovoltaic power generation systems are rated in peak kilowatts (kWp). This is the amount of electrical power that a new, clean system is expected to deliver when the sun is directly overhead on a clear day. We can safely assume that the actual output will never quite reach this value. System output will be promised by the angle of the sun, atmospheric conditions, dust on the collectors, and deterioration of the ponents. When paring photovoltaic systems to conventional power generation systems, one should bear in mind that the PV systems are only productive during the daytime. Therefore, a 100 kW photovoltaic system can produce only a fraction of the daily output of a conventional 100 kW generator. Efficiency Ratings The efficiency of a photovoltaic system is the percentage of sunlight energy converted to electrical energy. The efficiency figures most often reported are laboratory results using small cells. A small cell has a lower internal resistance and will yield a higher efficiency than the larger cells used in practical applications. Additionally, photovoltaic modules are made up of numerous cells connected in series to deliver a usable voltage. Due to the internal resistance of each cell, the total resistance increases and the efficiency drops to about 70% of the singlecell value. Efficiency is higher at lower temperatures. Temperatures used in laboratory measurements may be lower than those in a practical installation. CONVERTING SUNLIGHT TO ELECTRICITY A typical photovoltaic cell consists of semiconductor material (usually silicon) having a pn junction as shown in Figure 1. Sunlight striking the cell raises the energy level of electrons and frees them from their atomic shells. The electric field at the pn junction drives the electrons into the n region while positive charges are driven to the p region. A metal grid on the surface of the cell collects the electrons while a metal backplate collects the positive charges. Figure solar cells work Thin Film Technology Thinfilm solar cells are manufactured by applying thin layers of semiconductor materials to a solid backing material. The position of a typical thinfilm cell is shown in Figure 2. Sunlight entering the intrinsic layer generates free electrons. The ptype and ntype layers create an electric field across the intrinsic layer. The electric field drives the free electrons into the ntype layer while positive charges collect in the ptype layer. The total thickness of the ptype, intrinsic, and ntype layers is about one micron. Although less efficient than single and polycrystal silicon, thinfilm solar cells offer greater promise for largescale power generation because of ease of masspro
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