Vật liệu Thiết kế nội thất Chúng tôi đã xem xét những cách IAQ có thể trở thành bị ô nhiễm, làm thế nào ô nhiễm ảnh hưởng đến người cư ngụ xây dựng, và làm thế nào thiết kế của tòa nhà có thể ảnh hưởng IAQ. Bây giờ chúng ta hãy kiểm tra nội thất xây dựng và vật liệu trang trí nội thất liên quan đến các vấn đề về chất lượng không khí trong nhà. | How Electrical Systems Work 219 them. Photovoltaic cells are made from a very pure form of silicon an abundant element in the earth s crust that is not very difficult to mine. Photovoltaic cells provide direct electrical current. When enough heat or light strikes a cell connected to a circuit the difference in voltage causes current to flow. No voltage difference is produced in the dark so the cell only provides energy when exposed to light. A cell can be connected to a battery to provide continuous power. Even the best PV cells turn less than a quarter of the solar energy that strikes them into electricity with the rest given off as heat. Commercially available cells are currently only about 10 to 12 percent efficient. New designs currently being researched are up to 18 percent efficient. Individual PV cells are wired together to produce a PV module the smallest PV component sold commercially and these modules range in power output from about 10 to 300 W. Usually individual modules are mounted onto an existing roof. Some modules can be designed directly into the roof acting as both a roofing material and an electricity generator. To connect a PV system to a utility grid one or more PV modules is connected to an inverter that converts the modules DC electricity to AC electricity. The AC power is compatible with the electric grid and can be used by lights appliances computers televisions and many other devices. Some systems include batteries to provide backup power in case the utility suffers a power outage. Small commercial and industrial PV applications include lighting traffic counters signaling and fence charging. Larger systems provide electricity for residential office educational and mobile electrical needs. Systems are not limited to sunny tropical areas. A solar electric system in Boston Massachusetts will produce over 90 percent of the energy generated by the same system in Miami Florida. In areas with low-sun winter seasons like New England these systems
