Using a Zener to protect devices from solar overvoltage
The zener doesn't "short" the power source (panel), it simply limits the terminal voltage to a fixed value. The zener will be dissipating power equal to the zener voltage multiplied by whatever current the panel can deliver at that voltage (minus whatever current the load is drawing, of course), so it needs to have a suitable power rating.

You can "amplify" the power handling capability of a zener diode with a power transistor, which you can then bolt to a heatsink. Such a setup would be able to absorb the full 100W of the panel if necessary.







simulate this circuit &ndash; Schematic created using CircuitLab

This circuit doesn't do anything at all as long as the input voltage is less than the zener voltage. However, once the zener starts conducting, the transistor will also conduct about 50&times; the zener current, and will dissipate about 50&times; the power. If the panel can deliver, say, 3A @ 30V, the transistor will dissipate about 88W and the zener will dissplate about 2W.