Before you get to work installing your hydro system, make sure you’re up to speed on all of the available turbine and transmission options. The most common approaches are listed below, and the specific characteristics of your site—primarily how far you need to move the energy generated—will be the main driver in your transmission design.
Low-voltage DC. If your turbine will be located within a few hundred feet of your home, choosing a hydro plant with output at the nominal voltage of the battery bank (12 to 48 VDC) is the simplest approach. Some home-scale hydro plants use generators that produce low-voltage DC directly. But most modern turbines have brushless permanent-magnet alternators that produce wild three-phase AC, which is rectified to DC at the turbine. The output of both turbine types can be routed right to the battery bank without further processing or conversion.
High-voltage DC. Some manufacturers offer turbines with DC output voltages above the standard battery voltage in most modern residential systems (48 V). The higher the transmission voltage, the smaller the wire size required. At the battery bank, a maximum power point tracking (MPPT) controller with voltage step-down functionality can convert the higher transmission voltage down to the nominal voltage at the battery, just like it’s done in many battery-based PV systems. UL-listed controllers manufactured by Apollo, OutBack Power Systems, and Xantrex all have this functionality.
While high-voltage DC transmission is an option to consider, the controllers needed to step down the voltage are sophisticated and relatively expensive electronic devices. Most importantly, they will be damaged if they’re subjected to voltages that are higher than they are designed to handle. Most of these controllers max out at 140 to 150 VDC open circuit. If the hydro turbine ever generated a greater voltage, or a breaker was inadvertently shut off, allowing the turbine to overspeed, the controllers will be damaged. Because of this, the simplicity of using a high-voltage alternator with a step-down transformer at the battery may be preferable—reliability is better, net efficiency is about the same, and the cost is less.
Wild three-phase. If you’re facing a long wire run between the turbine and the batteries, one option is to purchase a turbine with a high-voltage alternator and transmit the three-phase AC output over the wire run. At the battery bank, a transformer drops the AC voltage down to the battery voltage, and it is rectified to DC for battery charging. A transformer/rectifier used in this application will typically have a conversion efficiency of about 90%. This approach will require a third power conductor for the third-phase, but is almost always cost-effective compared to DC transmission due to the smaller wire size required. Turbines with AC output voltages between 120 and 480 V are available.
AC-coupled systems. The vast majority of off-grid systems are DC coupled—PV arrays, and wind and hydro turbines ultimately feed DC to the battery bank. In contrast, AC coupling uses one or more batteryless inverters to parallel charging sources on the AC side of the system. An additional battery-based inverter sets a baseline voltage and frequency to which the batteryless inverters synchronize. In an AC coupled hydro application, a dedicated batteryless inverter can be located at the turbine. The inverter’s 120 or 240 VAC output will in turn be synchronized with the AC waveform of the battery-based inverter. While this approach is still uncommon in single-household off-grid applications, it does offer some potential advantages in village-scale applications, or when a single battery-based system is charged by multiple power sources that are not located close to one another. Because multiple inverters are required, AC-coupled systems are usually more expensive. The main advantage is that, in some cases, this type of system can overcome design hurdles that a DC-coupled system can’t.
Because hydro turbines run 24 hours a day, most systems produce a lot of energy. As a result, the battery bank spends much of the time completely charged and in float mode. To keep the batteries from being overcharged, voltage control is required. Unlike PV arrays, hydro plants must remain electrically loaded at all times to keep both the turbine’s rpm and the peak output voltage in check. That means that series-type voltage regulators, like the one in your car, cannot be used because they simply open the circuit when they hit their voltage set point. In a car, this isn’t an issue because the alternator speed is limited by engine speed—but hydro plants have no such limitation.
Electrically unloaded, a hydro turbine’s rpm will almost double, and output voltage may triple. Most turbines can handle the increased rpm, but the extremely high output voltage will destroy system controls. To avoid this scenario, hydro systems rely on diversion controllers that shunt (route) the turbine’s output to a diversion load when the battery bank is fully charged. The TriStar controller manufactured by Morningstar is a very popular diversion controller, as are the C-series controllers manufactured by Xantrex. MPPT controllers manufactured by Apollo, OutBack, and Xantrex all have an auxiliary output-control feature that’s capable of driving a separate high-current relay to shunt excess power to the diversion loads. In addition, several battery-based inverter models feature auxiliary control functionality.
In off-grid hydro systems, resistive loads like water- or air-heating elements are used to dissipate excess energy. These diversion loads are usually sized to handle the turbine’s full power output. The National Electrical Code (NEC) requires a second independent diversion setup to protect the battery bank from overcharging if one controller fails. One common approach is to use a dedicated controller and water- or air-heating element as the primary diversion system. The secondary diversion setup can utilize the inverter’s auxiliary output and a relay to dump AC power to a standard 120-volt space heater.
In battery-based grid-tie applications, the grid functions as the primary diversion load, with excess hydro-generated energy fed back via a utility-interactive inverter. But even grid-tied systems require a separate, dedicated diversion controller and load as a backup. Without it, in the event of a utility outage, the system’s battery bank would still be vulnerable to overcharging since the primary diversion load (the grid) is no longer available.