The graph shows the variation with the incident solar intensity $\mathrm{I}$ of the temperature of a solar panel used to heat water. Thermal energy is extracted at a rate of $320 \mathrm{W}$ . The area of the panel is  $2.0 {\mathrm{m}}^2$. Calculate, for a solar intensity of $400 \mathrm{W}/{\mathrm{m}}^2$, the efficiency of the panel.

 

The graph shows the power curve of a wind turbine as a function of wind speed. For a wind speed of $10 {\mathrm{ms}}^{-1}$, calculate the energy produced in a course of one year, assuming that the wind blows with this speed for $1000 \mathrm{hrs}$ in a year.

Wind of speed $\mathrm{v}$ is incident on the blades of a wind turbine. The blades present the wind with an area $\mathrm{A}$. The maximum theoretical power that can be extracted is given by,

$\mathrm{P}=\frac{1}{2}{\mathrm{\rho Av}}^3$.

State the assumption made in deriving this equation.