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【译林湖南版】2011高考英语一轮复习提能训练 Module 5 Unit 2 The environmentⅠ.填词 For those who are concerned about pollution，clean and renewable energy sources that help to protect the environment such as solar energy are becoming possible alternatives to oil and coal. Increasing demand for clean，renewable energy drives a growing market for solar energy products.Two of the main types of heating products that take advantage of solar energy are passive and active solar products.These two types of solar heating systems differ in how they are constructed and can also differ greatly in cost.Passive solar technology can be used to heat air or water directly using the sun’s energy.Unlike an active solar application，passive solar set-ups accomplish their task without the use of additional electrical components，such as fans that need an external electricity supply.Some solar space heating or water heating systems are active solar heating systems that have various components that require external power sources.However，they don’t use the solar energy directly to heat air or water.Because of their simplicity compared with active systems，passive solar heating systems generally have the lower cost of the two. In addition to these solar heating systems，there are also solar power systems available.Rather than use the sun’s energy for heating applications，these systems change solar energy into electricity.These products are often constructed from hundreds of solar energy collection cells called photovoltaic cells，which generate electricity from light.As sunlight falls on a photovoltaic cell，the light’s energy is changed into electricity that can then be used to power many kinds of electronic devices.Because of the complicated technology involved，these kinds of solar power systems can be very expensive.However，the cost savings may eventually more than pay off your initial investment in the solar cells. | 
