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Figure 21.11 Solar radiation varies from
                                                                                        place to place. Harnessing solar energy is
                                                                                        more profitable in sunny regions such as the
                                                                                        southwestern United States than in cloudier
                                                                         GERMANY
                                                                                        regions such as Alaska and the Pacific North-
                                                                                        west. However, compare solar power leaders
                                                                                        Germany and Spain with the United States.
                                       UNITED STATES                                    Spain is similar to Kansas in the amount of
                                                                                        sunlight it receives, and Germany is cloudier
                                                                                        than Alaska! This suggests that solar power
                                                                          SPAIN         can be used with success just about anywhere.
                                                                                        Data from National Renewable Energy Laboratory, U.S.
                      Hawaii                                                            Department of Energy.
                                                                                              Roughly how much more solar
                                Alaska       Solar radiation                                  radiation does southern Arizona receive
                                             (kWh/m 2 /year)                            than Germany?
                                                 > 2600       2000–2200     1400–1600
                                                 2400–2600    1800–2000     1200–1400
                                                 2200–2400    1600–1800     < 1200



                     electricity (see Figure 21.5). It may take 20 years or more   Wind Power
                     for most homeowners to break even on an investment in PV
                     arrays or solar collectors. The high costs are due to the fact that   As the sun heats air in the atmosphere, the movement of dif-
                     the technologies are young and developing. Moreover, they   ferentially heated air masses produces wind. We can harness
                      are competing against energy sources (fossil fuels and nuclear   wind  power from air’s  movement by using  wind  turbines,
                     power) that have remained relatively cheap as a result of dec-  mechanical  assemblies  that convert  wind’s  kinetic  energy
                     ades of taxpayer support and whose external costs (pp. 164,   (p. 48), or energy of motion, into electrical energy.
                     183) are not included in market prices. As a result, market
                     prices have given governments, businesses, and consumers
                       little economic incentive to switch to solar energy thus far.  Wind turbines convert kinetic energy
                        However,  declines  in  price  and  improvements  in  effi-  to electrical energy
                     ciency of solar technologies so far are encouraging, even in the
                     absence of significant funding from government and industry.   Today’s wind turbines have their roots in Europe, where
                     At their advent in the 1950s, solar technologies had efficien-  wooden windmills were  used for 800 years  to grind grain
                     cies of around 6% while costing $600 per watt. Today, PV   and pump water. The first wind turbine built to generate elec-
                     cells are showing up to 20% efficiency commercially and 40%   tricity was constructed in the late 1800s in Cleveland, Ohio.
                     efficiency in lab research, suggesting that future solar cells   However, it was not until after the 1973 oil embargo that gov-
                     could be more efficient than any energy technologies we have   ernments and industry in North America and Europe began
                     today. Solar systems are becoming less expensive and now can   funding research and development for wind power.
                     sometimes pay for themselves in less than 10–20 years. After   In a modern wind turbine, wind turns the blades of the
                     that time, they provide energy virtually for free as long as the   rotor, which rotate machinery inside a compartment called a
                     equipment lasts.                                     nacelle, which sits atop a tower (Figure 21.12). Inside the nacelle




                                                                    Gearbox     Generator
                                                                (increases rotational  (produces
                                                                 speed of blades)  electricity)
                                                                                               Figure 21.12 A wind turbine
                                           Blades                                              converts wind’s energy of motion
                                                                                               into electrical energy. Wind
                                                                                               causes a turbine’s blades to spin,
                                                                                               turning a shaft that extends into the
                                                                                               nacelle. Inside the nacelle, a gearbox
                                                                                               converts the rotational speed of the
                                         Tower
                                                     Nacelle                                   blades, which can be up to 20 revo-
                                                                                               lutions per minute (rpm) or more, into
                                                                                               much higher speeds (over 1500 rpm).
                                                                                               This provides adequate motion for
             612                                                                               the generator to produce electricity.







           M21_WITH7428_05_SE_C21.indd   612                                                                                    12/12/14   4:29 PM
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