Impulse turbines are generally used in higher-head, lower-flow applications. The runners are not immersed in the flow, and are driven by high-speed jets of water. They are controlled by adjusting the number and size of nozzles that produce these jets.
Reaction turbines are used in higher-flow, lower-head (LH) situations. The runners are fully submerged. Water pressure acts directly on the runner; flow is harder to control, but will depend on the turbine rpm and can be tuned by MPPT devices.
Some low-head turbines stand at the top of a “draft tube” that sucks water through the blades. Some, therefore, do not even need a pressurized penstock. One advantage is that the turbine can be kept clear of floodwaters. While the difference in turbine details is fascinating, the end result makes little difference to the buyer.
Connecting directly to all home loads via the main distribution panel is an option more common in large systems (beyond residential scale). Direct AC hydro turbines less than 5 kW do not offer good enough power quality for modern appliances. Also, the available peak power is limited by the turbine output. When you use less than peak, the rest is often dumped into waste heat by the onboard voltage regulator.
Connecting via a battery is an obvious choice if your home is off-grid. You may already have a battery-based solar-electric system in place, and the extra charging input from the microhydro system can help balance out the system’s production, especially in the wintertime.
The wonderful thing about a microhydro turbine is that (unlike PV) it produces electricity continuously. Because it works over many hours, a small amount of power (W) can add up to a large amount of energy (Wh) put into a battery. Used with an inverter, it can deliver both high peak surges and high-quality power, with excellent reliability.
Connecting directly to a heater is doable with simple, low-cost parts if all you want from your turbine is some heat, and you do not need other electrical power. A special heater or generator winding may be needed to optimize the turbine speed. Complications can arise if a thermostat is used, because the load will need to be transferred to another heater to prevent turbine overspeed.
Controller Options for Battery Charging
Direct connection to the battery, using a pulse-width modulated (PWM) charge controller in diversion mode (independently connected to the battery). The controller manages any excess energy by sending it to a diversion load. There is a danger of overcharging the battery if the controller fails, so you should follow the Code requirement to have two independent diversion-load controllers.
Battery voltage may be unsuitable for long wire runs. But many hydro manufacturers offer high-voltage alternators designed for AC transmission and a transformer with rectifier to step down the voltage at the battery.
Connecting to the battery via an MPPT controller is recommended by some manufacturers instead of direct connection. MPPT controllers combine three functions:
Just as with a PV array, you need to take care that the Voc of your turbine cannot damage the controller. The maximum safe voltage for many MPPT controllers is 150 V. The OutBack FM is ideal for hydro below this limit. A turbine’s “runaway” voltage must be safely below this. But a turbine’s operating voltage or Vmp may only be one-third of its Voc, which is therefore 50 V. This is better than 12 or 24 V for wire sizing, but is too low to charge a 48 V battery.
MidNite Solar serves the microhydro market with its Classic controllers, which can survive voltages above their nominal limit. The Classic has 150, 200, and 250 V versions. For a 48 V battery, or for saving on cable cost, use a higher-voltage Classic that allows a higher-voltage turbine. MidNite’s smaller Kid controller is worth considering for lower-power systems. It’s affordable and contains a built-in solid-state relay that can send energy to a diversion load.
Morningstar and Schneider make 600 V controllers that can handle a turbine at 200 V without fear of turbine open-circuit voltage damaging the controller. But always check with your turbine supplier before using it with any MPPT controller. A controller designed for solar MPPT may not always track a turbine’s maximum power. And very few controller manufacturers offer warranty or support for microhydro applications.
AC coupling is another option for using MPPT with battery-charging hydro.
Batteryless grid-tied inverters (GTIs) made for PV systems are an attractively simple way to connect a turbine to the utility grid because they are already approved by the utilities. Another popular arrangement is to install a permanent-magnet alternator or stand-alone induction generator producing three-phase AC that is rectified to DC and fed to a GTI. The alternative is to connect an IG directly via a code-compliant relay, which is harder to find.
Not all GTIs will work properly with a microhydro turbine, since their MPPT often tracks too fast. It is hard to find suitable products, and the market changes rapidly. Ask your supplier for advice before buying a GTI.
You can also use some battery-inverter systems on-grid if you want to be independent during outages; they can also export surplus energy to the grid when connected. Use the hydro to charge the battery directly, or connect the hydro power (via a PV GTI) to the inverter output and feed your system by “AC coupling.”
Clippers and voltage clamps are products that protect your MPPT controller or inverter against excessive voltage by diverting turbine power to a heater or by short-circuiting the alternator. MidNite Solar and some turbine manufacturers offer suitable products. The advantages are that you can run your turbine at a higher operating voltage and that it will not overspeed when unloaded. Such products are expensive and rare.
Whatever road you follow, we hope that you will persevere and enjoy the process of creating your own hydro system. Use the information here to connect with the right manufacturers, and work your way up the hydro learning curve with their help. It is hard to beat the satisfaction of getting all the energy you need from a local source using a hydro system that generates energy night and day.