When people think of grid-connected hydro electricity, images of something really big—like Hoover Dam—come to mind. But on a residential scale, high-power AC-direct hydro turbines in the 10 to 25 KW range have been feeding electricity to the grid for decades. What is new in the hydro world is coupling batteryless inverters to low-power microhydro turbines generating anywhere from 500 to 4,000 W for grid-tied applications.
Like PV and wind systems, microhydro systems can be battery-based or batteryless, depending on your needs and specific site conditions. In the past, home-scale batteryless hydro systems were high-power units where the hydro plant and controller interfaced directly with the utility grid without an inverter. With the advent of modern batteryless inverters, hydro system designers have been pushing the envelope and installing systems using low-power hydro turbines that can feed the grid without including batteries in the system. (For more information on AC hydro turbines, see page 68 of this issue.)
The success of a low-power grid-direct microhydro system relies on matching a batteryless inverter to the hydro site’s characteristics (head and flow), the output of the specific hydro turbine, and its voltage and power compatibility. Additional design aspects come into play. To control voltage and rpm, hydro turbines must be electrically loaded at all times. In grid-tied applications, diversion control is needed to shunt the turbine’s output to a dump load in the event of a utility outage. A voltage clamp to eliminate a voltage spike in the case of grid failure is required in many systems to protect inverters. Currently, the only controllers available for batteryless hydro applications are ones originally designed for batteryless wind systems.
One of the earliest low-power batteryless hydro systems installed in the United States was covered in HP80 in 2001. Installer Kurt Johnson designed an innovative system in North Carolina comprised of a Harris Hydro turbine that fed 57 VDC to an Advanced Energy Systems GC-1000 inverter. A Trace C-40 charge controller and an air-heater diversion load kept the turbine electrically loaded and its rpm regulated when the grid went down. System output ranged between 200 and 400 W depending on the seasonal creek’s flow rate. (See the original article at www.homepower.com/johnsonhydro.)
There wasn’t much further movement in the low-power batteryless microhydro world until 2006, when Jeff Clearwater from Village Power Design teamed up with Derik Veenhuis from Hi Power Hydro to design and install a batteryless system in Colorado. The water source delivers approximately 70 gpm at 110 psi to two 2.3 KW Hi Power turbines. Each turbine is paired with an SMA America 2.5 KW Sunny Boy inverter. Turbine output is three-phase 240 VAC that is rectified to 300 VDC in the power room and fed to the Sunny Boys. The inverter manufacturer tweaked the inverters’ software, lowering the start-up voltage from 300 to 240 VDC to ensure smooth operation with the turbines. Because the inverters have a maximum open- circuit design voltage of 600 volts, controllers manufactured originally for wind turbines by Abundant Renewable Energy were installed to limit output voltage and divert hydro electricity to air-heating loads when utility power fails. Combined, the two turbines generate 4.4 KW continuous and feed 106 KWH a day directly to the grid.
There are no special hydro inverters on the market, but some off-the-shelf PV inverters can be used. The critical design component of low-power batteryless hydro systems is matching the turbine output voltage with a specific batteryless inverter. Low-power turbines come in two basic designs—lower-voltage DC alternators and AC alternators that produce unregulated three-phase AC power. The turbine output voltage is the main consideration when investigating a compatible inverter.
DC turbines are manufactured with nominal outputs of 12 to 120 VDC. The trick is selecting a turbine with a high enough voltage to meet the inverter’s input voltage requirements. For example, PV Powered manufactures two batteryless inverter models (PVP1100 and PVP2000) that have operating voltage ranges between 115 and 450 VDC, with a maximum voltage limit of 500 VDC. A 96-volt nominal DC turbine will output approximately 120 VDC—enough voltage to integrate with the PV Powered inverters. If the turbine happened to become unloaded electrically during a grid failure and the rpm and output voltage increased, the inverter would not be damaged due to its 500 VDC maximum input voltage. With this design approach, a voltage clamp may not be necessary, but a diversion load should be included so the turbine does not run in an unloaded state for long periods of time. Batteryless inverters with lower input voltages are not currently being manufactured, but discontinued inverter models like the Xantrex SunTie (48 VDC nominal input voltage) can still be found online and used with 48 VDC nominal hydro turbines.
Alternatively, turbines with three-phase AC alternators can be configured for up to 480 VAC nominal output. Compared to lower-voltage DC turbines, the higher voltage makes it easier to integrate them with a variety of batteryless inverters designed for PV arrays operating at up to 600 VDC. The three-phase AC output is rectified to high-voltage DC, and then routed to the inverter. A controller with both diversion load and voltage clamping capability is required to eliminate the possibility of excessive system voltage if utility power fails.
Compared to grid-direct PV or wind systems, low-power batteryless microhydro is still an underdeveloped technology. That’s not to say that the equipment and system designs are unreliable, but the limited number of installations to date reflects a general unfamiliarity with these efficient and durable systems. If you’re considering a low-power batteryless hydro project, work with the hydro manufacturer to make sure that the turbine and the inverter will couple well. Matching turbines to inverters can be challenging, so experience is crucial.
Over the last decade there has been continual growth of batteryless renewable energy systems, related equipment, and design and installation expertise. Batteryless PV systems now dominate that market, batteryless wind systems are becoming very popular, and soon batteryless low-power hydro systems will likely become a common feature in the microhydro landscape.
Jay Peltz is owner of Peltz Power, an RE design and installation company based in Redway, California. Jay specializes in off-grid PV, wind, and hydro-electric systems. He is currently working on one of the first residential-scale, grid-direct, low-power hydro systems in California.
Low-Power Hydro Turbine Manufacturers:
Alternative Power & Machine • www.apmhydro.com
Energy Systems & Design • www.microhydropower.com
Harris Hydro • 707-986-7771
Hi Power Hydro • www.hipowerhydro.com
Voltage Clamp Controls:
Abundant Renewable Energy • www.abundantre.com