1. Copyright © 2011 by Oxford University Press, Inc. Energy and the Environment James A. Fay / Dan S. Golomb FIGURE 8.11 A parabolic mirror system focuses sunlight on a Sterling cycle heat engine that produces electrical power. (By permission of DOE/NREL-PIX.)

2. Copyright © 2011 by Oxford University Press, Inc. Energy and the Environment James A. Fay / Dan S. Golomb FIGURE 8.12 A spherical mirror system consisting of individual focusing mirrors arrayed around a central tower that collects the focused light. (By permission of DOE/NREL-PIX.)

3. Copyright © 2011 by Oxford University Press, Inc. Energy and the Environment James A. Fay / Dan S. Golomb FIGURE 8.13 A cylindrical collector mirror focuses sunlight on a glass pipe containing fluid. (By permission of DOE/NREL-PIX.)

4. Copyright © 2011 by Oxford University Press, Inc. Energy and the Environment James A. Fay / Dan S. Golomb FIGURE 8.14 The elements of a photovoltaic cell of area A exposed to a solar irradiance I s and supplying a total current jA to an external load at an electric potential difference V. The current density, electric current per unit area, in the cell is j.

5. Copyright © 2011 by Oxford University Press, Inc. Energy and the Environment James A. Fay / Dan S. Golomb FIGURE 8.15 The maximum fraction f of the transmitted solar irradiance that can be converted to electric power in the external circuit as a function of the dimensionless photovoltaic cell energy gap x g of Equation (8.10).

6. Copyright © 2011 by Oxford University Press, Inc. Energy and the Environment James A. Fay / Dan S. Golomb FIGURE 8.16 (a) The open circuit voltage V oc and short circuit current density j ss of a photovoltaic cell as functions of solar irradiance. (b) The voltage versus current density characteristics of a photovoltaic cell for a fixed value of solar irradiance, showing the point of maximum power.

7. Copyright © 2011 by Oxford University Press, Inc. Energy and the Environment James A. Fay / Dan S. Golomb FIGURE 8.17 The flow of wind having a speed V and passing through a wind turbine whose blade radius is R is slowed to a speed V t at the turbine disk and even further to a speed V w in the wake region downstream of the turbine.

8. Copyright © 2011 by Oxford University Press, Inc. Energy and the Environment James A. Fay / Dan S. Golomb FIGURE 8.18 The motion of the wind relative to a turbine blade consists of an axial speed V t and a tangential speed 2πrf that generates a lift force L and drag force D.

9. Copyright © 2011 by Oxford University Press, Inc. Energy and the Environment James A. Fay / Dan S. Golomb FIGURE 8.19 The power coefficient of a wind turbine of fixed pitch is a function of the ratio of the tip speed to wind speed. Its maximum value is less than the theoretical value of 16/27 = 0.593.