برچسب‌گذاری نمودار، فلوچارت و پاسخ کوتاه

A solar water heater uses energy from the sun to provide hot water for a home. In sunny climates it can supply more than half of the hot water a family uses in a year. A typical system has four main parts: a collector, two pipes and a storage tank. The collector, often called a solar panel, is a flat, dark box fixed to the roof, usually on the side that faces the sun for most of the day. Inside it, a network of narrow copper tubes lies beneath a sheet of glass. The dark surface absorbs sunlight and becomes hot, and this heat passes to the water in the tubes. The glass stops much of the heat from escaping, in the same way that a greenhouse stays warm on a cold but sunny day. Once the water has been heated, it leaves the top of the collector through the flow pipe and travels to a storage tank in the loft. The tank is wrapped in a thick layer of insulation, so the water can stay hot for many hours, even overnight. When someone turns on a hot tap, hot water is drawn off from the top of the tank and runs down to the kitchen or bathroom. Meanwhile, the cooler water at the bottom of the tank goes back to the lower edge of the collector through the return pipe, where it is heated again. A small electric pump keeps the water moving round this circuit, and a sensor switches the pump off whenever the collector is colder than the water in the tank, for example at night. In winter, or after several cloudy days, the sun alone may not heat the water enough. For this reason, most systems also have a backup heater inside the tank, which runs on gas or electricity and is only used when it is needed.