سؤال قراءة ستيب محلول — الفكرة الرئيسية (تدريب 7915)

Passage A Solar Energy 1) The energy falling on the Earth from the Sun is mostly in the form of light and the light that falls in one hour equals the total energy used by the world in a year. Unfortunately, its low energy density requires large collecting devices and its availability varies. Its greatest potential use is as an energy source for low-temperature water heating. This uses solar panels as the energy transfer devices, which convert light into heat energy. They are used increasingly to produce domestic hot water at about 70°C and to heat swimming pools. 2) Solar energy can also be used to produce high temperature heating, up to 3000°C or so, if a large curved mirror (a solar furnace) focuses the Sun's rays on to a small area. The energy can then be used to turn water to steam for driving the turbine of an electric generator in a power station. 3) Solar cells, made from semiconducting materials, convert sunlight into electricity directly. A number of cells connected together can be used to supply electricity to homes and to the electronic equipment in communication and other satellites. They are also used for small-scale power generation in remote areas of developing countries where there is no electricity supply. Recent developments have made large-scale generation more cost effective and there is now a large solar power plant in California. There are many designs for prototype light vehicles run on solar power. Wind energy 4) Giant windmills called wind turbines with two or three blades each up to 30m long drive electrical generators. "Wind farms" of 20 to 100 turbines spaced about 400m apart, supply about 400MW (enough electricity for 250,000 homes) in the UK and provide a useful "top up" to the National Grid. Wind turbines can be noisy and may be considered unsightly so there is some environmental objection to wind farms, especially as the best sites are often in coastal or upland areas of great natural beauty. Passage B Wave energy 5) The rise and fall of sea waves have to be transferred by some kind of wave-energy converter into the rotary motion required to drive a generator. It is a difficult problem and the large-scale production of electricity by this means is unlikely in the near future, but small systems are being developed to supply island communities with power. Tidal and hydroelectric energy 6) The flow of water from a higher to a lower level from behind a tidal barrage (barrier) or the dam of a hydroelectric scheme is used to drive a water turbine (water wheel) connected to a generator. 7) One of the largest working tidal schemes is the La Grande I project in Canada. Feasibility studies have shown that a 10-mile-long barrage across the River Severn Estuary could produce about 7% of today's electrical energy consumption in England and Wales. Such schemes have significant implications for the environment, as they may destroy wildlife habitats of water birds for example, and also for shipping routes. 8) In the UK, hydroelectric power stations generate about 2% of the electricity supply. Most are located in Scotland and Wales where the average rainfall is higher than in other areas. With good management hydroelectric energy is a reliable energy source, but there are risks connected with the construction of dams, and a variety of problems may result from the impact of a dam on the environment. Land previously used for forestry or farming may have to be flooded.