The Electric Side of Hydro Power

When people dive into developing hydro-electric systems, a lot of thought goes into diverting debris-free water from the creek and sizing the pipeline to carry it from the intake to the turbine. Both of these topics have been covered in recent issues of Home Power (HP124 & HP125).

Designing the electrical side of your hydro system deserves equal attention. When it comes to transmitting and regulating the energy generated by your hydro turbine, making the right design choices for your site will result in maximum energy production and minimize up-front costs.

Choosing a hydro turbine with the optimal mechanical characteristics (runner type and flow capacity) for your particular application is an important step in system design. So is designing the ideal intake and penstock setup. But all these will be for naught if the electrical side of your system is poorly thought-out.

Turbine Output Options

There are two basic types of hydroelectric turbines—high-power AC-direct plants and low-power DC or AC/DC turbines. Generally, high-power AC direct turbines are suitable for sites that have enough hydro potential to meet the peak demand of the combined electrical load on the system—most or all of the appliances running at once. Appropriate sites often have high flow rates (think cubic feet per second rather than gallons per minute). In a residential application, and depending on the water source and site characteristics, these turbines may generate from 4 to 15 KW (or more) continuously. Batteries are not necessary in these systems. Instead, the turbine produces 120 or 240 VAC at a controlled frequency (60 hertz in the United States) and output is fed directly to the electrical loads. Any excess power output beyond what the appliances require is routed to an electrical diversion load, typically either a water- or air-heating element.

Since most people don’t live on a property with enough hydro potential for a high-power AC-direct turbine, the majority of residential hydro systems utilize low-power DC or AC/DC units. The output of this range of turbines may be from 100 to 1,500 W (and up) depending on the hydro resource. In most cases, the turbine charges a battery bank, and an inverter (instead of the turbine itself) is sized to meet the peak electrical demand. Another relatively new design approach using low-power turbines in grid-tied applications involves coupling the turbine with a batteryless inverter that, in turn, synchronizes system output with the utility grid (see page 48 of this issue for more information).

Low-power turbines are available in an array of output voltages, 12 to 120 VDC, and 120 to 480 VAC. The ideal turbine voltage for a given site will depend on the transmission distance between the turbine and the batteries. It’s important to note that the output of low-power turbines at higher voltages (120 to 480 VDC) is almost always three-phase and the frequency will vary with the rotational speed of the turbine. Unlike high-power AC-direct turbines, the electricity generated is not compatible with your electrical loads without some additional power processing—the variable frequency three-phase AC output is rectified (converted) to DC and used to charge batteries.

Transmission Basics

If your turbine will be located within a couple of hundred feet of your home, you are fortunate—your wire routing, wire sizing, and associated costs will be relatively easy to deal with. But in many situations, geographic circumstances necessitate locating the turbine far from where the electrical energy needs to end up. Transmission distances of 1,000 feet are common, and distances up to 1 mile or more are surmountable with today’s hydro and control technology.

Similar to penstock design, longer wire runs require larger diameter wiring—the pipeline for electrical energy. The power equation (watts = volts x amps) shows us that voltage and current share an inverse relationship to each other. Higher turbine/transmission voltage results in lower amperage for the same amount of power (watts). Lower amperage means less energy loss in transmission and smaller-diameter (and therefore, less costly) wire can be used. To drive the point home, take a look at the Conductor Sizing table. The transmission distances, wire sizes, and conductor costs are based on a turbine output of 500 W.

Higher-voltage transmission strategies will keep wire costs down, and allow you to site the turbine farther from the electrical loads, which may also give you access to additional vertical drop (and power output) along the stream course. In the field, most hydro system installers end up making a lot of trade-off decisions based on cost, topography, the amount of energy that’s required, and the distance between the turbine and the batteries. For example, siting the hydro plant a little closer to the house may shorten the wiring run enough to make it affordable, and not result in a significant reduction in turbine output. Or maybe an extra 50 feet of cable will make it possible to move the whole penstock upstream far enough to use a natural spillway for an intake. The bottom line is that there is a lot of give-and-take during the design process. This is where experience-based advice will result in optimal system production.