Controllers

One of the great things about microhydro turbine generation is that it tends to generate a very steady current of electricity once the system is up and operating.  The downside of this is that if you are storing your energy in battery banks as part of an off-the-grid or hybrid system, there is the risk that you could easily overcharge your batteries. If a battery is routinely allowed to overcharge its life expectancy will be greatly reduced. To avoid this problem you will need to use an electronic component called a charge controller to bleed off the excess energy once your battery bank is fully charged. The excess energy you are getting rid of is often referred to as a dump load. 

Unlike a solar-electric controller, a microhydro system controller does not disconnect the turbine from the batteries when it senses that overcharging could occur.  This could end up creating voltages that are higher than the components might be able to take.  Usually it shunts the excess electricity to a secondary artificially created low, often created by using electronic components called resistors.  Another approach is to transfer the excess electricity to water heating elements so they provide the additional advantage of creating hot water for the house.  If the system is a hybrid system that uses both batteries and a grid connection, the excess electricity can be sent on to the grid once the batteries are fully charged.  This is one of the major advantages of hybrid systems.

Some charge control systems are designed so that they will run the system at whatever voltage produces the most wattage. A good controller can sometimes increase the amount of usable energy you get out of your system by as much as 15%.

Governors

If you are using a water turbine that generates AC current and are not using batteries you will still probably need a load-control governor.  This type of device monitors the voltage or frequency of the current being created by your water turbine and ensures that the generator is producing the appropriate current for a household system.  It can do this in a couple of different ways.  One approach is to use a system that deflects water away from the runner if the voltage gets too high.  This slows down the generator and correspondingly reduces the current.  Another approach is for the system to periodically adjust the current by dumping excess load, much in the way a charge controller would work.