Chapter 1, Problem 1.42P

One method for growing thin silicon sheets for photovoltaic solar panels is to pass two thin strings of high melting temperature material upward through a bath of molten silicon. The silicon solidifies on the strings near the surface of the molten pool, and the solid silicon sheet is pulled slowly upward out of the pool. The silicon is replenished by supplying the molten pool with solid silicon powder. Consider a silicon sheet that is $W_{\text{si}}=85\text{mm}$ wide and $t_{\text{si}}=150\mu \text{m}$ thick that is pulled at a velocity of $V_{si}=20\text{mm/min}\text{.}$ The silicon is melted by supplying electric power to the cylindrical growth chamber of height $H=350\text{mm}$ and diameter $D=300\text{mm}\text{.}$ The exposed surfaces of the growth chamber are at $T_s=320\text{K,}$ the corresponding convection coefficient at the exposed surface is $h=8{\text{W/m}}^{\text{2}}\cdot \text{K,}$ and the surface is characterized by an emissivity of ${\epsilon }_s=\mathrm{0.9.}$ The solid silicon powder is at $T_{si}{}_{,i}=298\text{K,}$ and the solid silicon sheet exits the chamber at $T_{si,o}=420\text{K}\text{.}$ Both the surroundings and ambient temperatures are $T_{\infty }=T_{\text{sur}}=298\text{K}\text{.}$

1. Determine the electric power, $P_{\text{elec}},$ needed to operate the system at steady state.
2. If the photovoltaic panel absorbs a time-averaged solar flux of $q_{\text{sol}}^{"}=180{\text{W/m}}^{\text{2}}$ and the panel has a conversion efficiency (the ratio of solar power absorbed to electric power produced) of $\eta =0.20,$ how long must the solar panel be operated to produce enough electric energy to offset the electric energy that was consumed in its manufacture?