Use a diversion load (or "dump load") to convert excess electrical energy generation from a solar, wind, or hydro power system into a more useful form of energy. The most common kind of diversion load is a water heating element.
In most battery-based wind and hydro power systems, the charging source (turbine) cannot be disconnected from the batteries while running without damaging them from excess voltage. So a diversion load or dump load - in combination with a charge controller that can be configured for diversion-type control (like the Morningstar TS-45 or Morningstar TS-60 or OutBack FLEXmax 60) - is used for regulating battery charging voltage. In addition to making your renewable energy harvest more well-rounded and efficient and protecting your batteries, a diversion load / dump load can also protect the wind or hydroelectric turbine itself; keeping the turbine on a load prevents it from spinning too fast.
Diversion loads are also sometimes used in solar power systems where the PV array is much larger than necessary to charge the battery bank.
Dump loads can be used in series or in parallel, and need to be matched to the maximum power output your turbine is capable of. Use this formula to figure out the total resistance you'll need when sizing the diversion load for your turbine:
Resistance Needed = (DC Voltage of your system)2 / Peak Power of Your Turbine