Although most microhydro systems produce DC electricity, there are some times when AC generators are advisable, notably for larger commercial applications or for grid-connected systems. These systems can produce more than 3,000 watts of continuous power or 72 kWh per day — far more than most homes ever consume* As Dan New pointed out in an e-mail to me, these systems Tan often supply all of the energy needs of a home.” DC systems typically only provide electricity for
lighting, appliances, and electronic heaters, but not space heat, hot water, or dryers* Those demands are typically satisfied by propane or natural gas* An AC microhydro system, however, could supply heavy-duty machines, compressors, and welding and woodworking equipment* "Most DC systems cannot cover most of this list,” concludes New, so homeowners must rely "on nonrenewable sources of energy to accomplish these tasks.”
According to Schaeffer, AC systems often involve considerable engineering, custom metalwork, formed concrete, permits, and a fairly high initial investment* AC systems also require governors to ensure that the AC generator spins at the correct speed to produce electricity of the proper frequency to match household needs* In grid-connected systems, additional controls are required to ensure that power going onto the electric grid matches line current with respect to voltage, frequency, and phase. These controls will automatically disconnect the system from the grid if major fluctuations occur on either end* Although AC systems cost more upfront, "AC power is almost always cheaper per kilowatt-hour,” says New. AC hydro systems also require no batteries and do not require inverters* AC power is used directly as it is produced* Surplus can be diverted to dump loads, usually resistance heaters, or can be diverted to the electrical grid* "So,” says New, "if a homeowner can find a use for most of the power most of the time, a moderately sized AC system, say 4 kW to 15 kW, will provide the energy needed to power a home renewably for less money.” (For details on grid-connected renewable energy systems, see Chapter 8.)
For those who want battery backup, in case the system must be shut down, remember that AC power can be converted to DC power by a bank of diodes and then stored in a battery bank for later use* However, also remember that to be useful, DC electricity from the battery bank must be converted back to AC power* Thats a function of the inverter* As Dan New pointed out to me, systems that convert AC to DC power are rare*
AC systems can be quite costly, warns Schaeffer, so this "isn’t an undertaking for the faint-of-heart or thin-of-wallet.”This is not to say these systems are uneconomical* Quite the opposite: they often provide a very good return on investment*"Far more streams can support a small DC-output system,” says New, "than a higher output AC system.” Those who think they have a site that is suitable for low-head AC power production, can download information on these systems from the following website: eere. energy. gov/RE/hydropower. html* For more on the cost issue, see the sidebar, "AC vs DC Costs.”
As noted earlier, turbines and generators must be protected from the elements to ensure long service. Protection is provided by a powerhouse. Powerhouses range in complexity from simple wooden or cement block boxes (designed so you can work on the system) to small sheds with room for battery banks.
Be sure when connecting pipes to the turbine in a powerhouse to use as few elbows as possible. Bends in pipe dramatically reduce head by creating turbulence. Reduced head, in turn, reduces system output. “Likewise," notes Dan New, “any restrictions on water exiting the turbine may increase resistance against the turbines moving parts," This, too, reduces power output.