A Shunt Regulator is a small electronic component that clamps a power supply voltage at a fixed level. Hundreds of times per second it checks the power supply voltage, and if the voltage is over the limit (set by adding a couple of resistors to the regulator in a small circuit) then the extra voltage is bled off through a resistor. If the voltage is below the fixed maximum, then no current passes through the resistor.
How can shunt regulators be useful to alternative energy generators. Well, for example a solar panel rated at 12V will actually generate up to 20V. If this panel is to be connected directly to equipment requiring an input of 12V, that equipment could be damaged if the voltage is too high. Therefore, by passing the solar panel's generated current through a shunt regulator circuit, the voltage reaching the equipment can be clamped at 12V very accurately.
Rather than dumping the excess power into a resistor and losing it in the form of dissipated heat, it is possible to put it to better use, for example to heat or pump water.
A common shunt regulator used in solar applications is the LM431. This is a 3-terminal adjustable shunt regulator which can have its output voltage set to any value between 2.5 and 36V by the correct selection of two resistors. LM431's are available for example from eBay at just $4.50 for 25.
http://www.reuk.co.uk/Shunt-Regulator.htm
