Scientists are now looking for cheaper way to use solar energy to heat air and water. Many buildings are using solar energy to heat water now. Solar units on the (1) _____ of buildings collect the energy and use it to heat water. (2) _____ these units are still very expensive, more and more people are buying solar hot water heaters. Universities and colleges give solar energy courses where students learn to build their own solar collectors. There are also many books (3) _____ explain the construcstion of solar collectors. (4) _____ the future, there will be more and more solar collectors, especially in the sunny desert areas of the world.

Solar units on the (1) _____ of buildings collect the energy and use it to heat water. (