If your stream or pond has sufficient head (vertical drop) and flow, a microhydro-electric system can be a cost-effective and reliable choice to provide renewable electricity for your home. To tap the power in falling water effectively, you need to understand basic physics, how each component works, and how to select and install the appropriate turbine and balance-of-system components for your site.
Hydropower results from the marriage of two forces—gravity and the flow of water—both used to determine how much power and energy can be had. Gravity is what creates the pressure between the inlet and outlet of the turbine. For every 2.31 feet of vertical drop in the pipe, 1 pound of pressure per square inch (psi) is gained. This vertical drop is also called “head”—the vertical distance between where water is taken out of a stream and where it leaves your turbine. The horizontal distance between the source and turbine is also important because of pipe cost and friction losses in the pipe—but it does not affect the basic head measurement.
Flowing water, whether measured in gallons per minute, cubic feet per second, or some other measure, is the other key factor in the hydropower equation. A continuous flow of falling water is needed to make electricity. Measuring this flow accurately is crucial to hydro site assessment and system design.
Once you have these two measurements, you can make at least a rough estimation of the power available. Multiplying the gross head (in feet) by the flow (in gallons per minute) and dividing by a specific factor will give you the potential output wattage. The factor, which is derived from real-world experience with hydro systems, will vary from 9 for larger AC systems to 13 or more for smaller battery-based systems.
Once you have figured power (watts), it’s easy to calculate energy (watt-hours): Just multiply by 24 hours in a day to arrive at daily watt-hours, since hydro turbines run around the clock. The relationship of power production with water flow and head is linear, meaning that a site with 1 unit of water flow times 2 units of elevation difference will give roughly the same power production as a site that experiences 2 units of water flow times 1 unit of elevation difference, if all other things are equal. For example: If your stream has 120 feet of head and 45 gallons per minute of flow, you might expect to generate about 11 kilowatt-hours per day.
120 ft. head x 45 gpm ÷ 12 factor x 24 hrs./day =
A hydro-electric system, like any renewable electricity system, is a collection of components. Buying only the turbine will get you nowhere. Hydro systems typically contain these basic components, listed here with their basic purpose:
Hydro system design is not simple, nor is it recommended for those with little experience with electrical, mechanical, and hydraulic systems. Because good hydro sites are few and far between, it is sometimes difficult to find expertise. Many systems are also deep in the back woods and not on public display, so you may need to do some research and networking to find the right people to help you.
Hydro systems come in four primary configurations, with other variations and permutations. Which type you choose depends on your site, goals, budget, and energy needs.
Battery-based off-grid systems are appropriate for smaller systems far from the utility lines, where the peak load exceeds the peak generation on a regular basis. If your hydro system produces 800 W, you’ll generate about 19 kWh per day, which is substantial. But without a battery bank and higher-powered inverter, you could not run many appliances or electronics simultaneously, and many loads, such as an 1,100 W microwave, would be impossible to power.
Batteryless off-grid systems are appropriate when the generating capacity is 2 kW or more. As household loads decrease and increase, load-control governors constantly adjust the amount of energy to the diversion load to maintain a constant voltage and frequency. Because the system cannot store energy, considerable amounts of power are typically diverted to the diversion load. For this reason, it’s worth considering how to use it most effectively. One of the most common ways to use the excess energy is for heating water for domestic use.
Battery-based on-grid systems are very similar to their off-grid counterparts. The first of two primary differences is that excess energy can be sold to the grid for payment or credit. The other is that the grid can be used for backup if the hydro system doesn’t provide enough energy.
Batteryless on-grid systems use the grid as the “dump load,” sending excess energy back to the utility’s grid for their customers to use. These systems still may require a controller and dump load which only come into play in the event of a utility outage. Batteryless grid-tied systems are perhaps the simplest and most reliable systems because they incorporate no batteries but have the grid available. Their drawback is the lack of backup for any utility outages.
All hydro-electric turbine generators, like electric motors, work on the principle of electrons moving through wire as a result of wires passing through magnetic fields (the electromagnetic effect). Hydro-electric turbines use the moving water to turn a wheel and provide the rotational movement necessary to cause the electromagnetic effect in their generators.
Microhydro turbines are generally classified in the range of 100 W to 100 kW, though most turbines used by homeowners are less than 25 kW. Another classification is based on the “head” (water pressure) that drives the turbine.
Low-head turbines are used in systems with 3 to 20 feet of head. Medium-head turbines are for 20 to 60 feet of head, and high-head turbines can use 60 to 1,000 feet (or more) of head.
Low-head turbines are typically “reaction” turbines, in which the turbine blades are submerged and produce electricity as an integral reaction with the water pressure. Because they work with low head, these turbines normally require a significant amount of water to produce useful power. For instance, the Energy Systems and Design LH-1000 low-head propeller turbine requires 1,000 gpm of water operating at 10 feet of head to produce 1,000 W.
Medium-head turbines are often reaction turbines. A Francis turbine is a common type. Medium-head turbines often have adjustable flow-control devices to deal with variable water flow under the same head conditions.
Another type of reaction turbine is a pump that runs in reverse as water flows through its centrifugal works (see the “Pumps as Turbines” sidebar). These can be a simple and cost-effective solution in the right situations.
High-head turbines are the most common microhydro turbines installed in residential systems and are known as impulse or impact turbines. Water is passed through nozzles, converting pressure into velocity and sending a jet of water that “impacts” buckets or vanes attached to a rotating wheel, making it turn.
Electricity produced by most micro-hydro turbines is unregulated and is normally converted from “wild” (unregulated voltage and frequency) AC to DC using a rectifier. DC is then used to charge batteries from which an inverter can provide true 60 Hz AC electricity.
Larger (2 to 100 kW) microhydro turbines can produce 60 Hz electricity directly through regulation using an electronic load governor, which maintains a constant load on the generator through dump loads when electricity is not needed.
Off-grid microhydro turbines require the means to “dump” excess energy when batteries are full or AC loads are reduced. Generally it is best to have redundant (duplicate) diversion loads and/or an overvoltage trip device for protection in the event that a dump load or charge controller fails.
Model is dependent on each manufacturer. Each manufacturer should be contacted to verify a turbine will suit a particular site.
The generator type associated with microhydro power is normally either a permanent magnet, a wound-field, or induction. Most smaller turbines use permanent magnet generators, some of which have adjustable gaps between the magnets and the windings for tuning the output. Stand-alone synchronous generators have a wound-field that produces its own magnetic excitation, and induction generators receive their magnetic excitation from the stator, either via capacitors or the grid.
Maximum power is determined by the watts produced by the turbine at maximum water flow and net head. This number is used to calculate the size of charge controllers and dump loads necessary to protect turbines and battery banks, adding a safety factor.
Voltage of the type of generator used. Alternating current (AC) generators are used for either standard 60 Hz electricity or to produce “wild” unregulated voltage and frequency electricity, which is rectified to DC to charge batteries. “Wild” indicates that the turbine is not producing steady 60 Hz AC, and the frequency and voltage may vary. High-voltage generation (hundreds of volts instead of dozens of volts) can be useful in overcoming line losses.
AC/DC stands for alternating current and direct current. Most smaller (100 to 1,000 W; less than 2 kW; 48 kWh/day) hydro-electric turbines use permanent-magnet, “wild” AC generators. Most larger microhydro systems (2 to 100 kW) use either an induction or synchronous AC generators. Virtually all spinning generators make AC natively, and how it is transferred and conditioned is based on the application. Battery charging turbines end up producing DC. The grid and your home loads are AC systems, so turbines designed to directly interface with them produce AC in the end.
Grid connection is possible with certain makes and models. The grid connection for a smaller (less than 2 kW) hydro system commonly uses a grid-tied inverter, as for PV systems. Larger systems (2 to 100 kW) are connected through switchgear and inductive generators or synchronous generators and governors.
Runner type identifies the turbine wheel used to convert water power to rotational power, and is determined by the head and flow available. Through testing, manufacturers have determined the best runner types for various head and flow conditions. Common types are the Pelton wheel, the turgo, the crossflow, and the propeller. Your turbine supplier and contractor can give good advice about the choices.
Runner Material. Runners for microhydro applications are commonly made of an alloy, since these materials resist corrosion and are easily cast and machined into shape. Stainless is most common in larger systems. Stainless steel and various bronze alloys are common, long-lasting materials. Plastics are used for smaller, less expensive runners.
Runner diameter selection is associated with the velocity of water impacting the runner, which is directly related to available head. The higher the head, the smaller the runner diameter for a given/constant shaft speed. Under ideal conditions, the runner velocity is approximately half the water jet velocity. For practicality, runners for smaller turbines are usually limited to just a few. The runner’s speed is adjusted by means of the generator field in relation to battery voltage, or using belt pulley ratios in relation to the output frequency of direct AC systems Again, your suppliers are your best resources for helping make this choice.
Number of nozzles is a choice dependent on the range of water flow available to the turbine. Nozzles are opened or closed (manually for most small turbines, and occasionally automatically for larger turbines) to maintain maximum pressure in the turbine pipeline while taking advantage of available flow. Having multiple nozzles is especially important where stream flow varies widely over the year, so you have the option of using more or less water.
Nozzle size options are associated with available water flow. Smaller-diameter nozzle sizes are used for lower-flow situations. Nozzles are sized by manufacturers based on potential range of flow. Generally, these parts are removable and replaceable. Larger systems sometimes have adjustable “needle nozzles” or “spear valves.”
Head range is associated with types of turbine runners that can be used. Higher-head turbines use impact runners, which are generally Pelton or turgo designs. Mid-range turbines (suitable for 20 to 60 feet of head) use reaction runners, which are submerged fully or partially, and include Francis and propeller runners. Low-head turbines (3 to 20 feet) may also use propeller reaction turbines.
Flow range will vary for every project site. The table shows the actual flow used in the turbine, which may be 10% to 50% of the stream flow.
Controls and over-speed control are necessary for stand-alone AC turbines to maintain 60 cycles per second output under varying load conditions. Electronic load governors usually provide this control for AC units, shunting energy to resistive loads. Control is also necessary for grid-tied systems when utility outages occur. Without the load of the utility grid, a hydro turbine will over-speed, possibly resulting in mechanical and electrical failure.
Controls, dump load, and metering included describes what comes with a turbine and what must be purchased separately.
Turbine selection usually begins with determining the site’s available head and flow, including variation in seasonal flows. Selection is also based on whether your system will be grid-tied or off-grid, and with or without batteries. Most turbine manufactures provide online questionnaires to assist in turbine selection.
Most turbine manufacturers publish test results for their turbines at various heads and flows, and with various turbine runners. Charts are prepared and compiled for various nozzle sizes and will be used by the manufacturer to recommend a specific turbine.
The intended use of energy will further determine turbine sizing. There is no point in generating more energy than can be used. Unlike PV systems, hydro-electric turbines generate electricity 24 hours a day, seven days a week. Unused energy must be shunted through to the grid, or to diversion loads—typically water or air heaters—to protect the generator. Inverters and battery banks for hydro-electric systems are normally sized to meet peak load, and store excess energy for these loads and motor-starting surges.
The location of a turbine relative to its interconnected battery bank or loads normally dictates turbine generation voltage. A distance of 100 feet or less may permit use of a low-voltage DC generation turbine. Transmission wire size and voltage drop beyond 100 feet may be excessive at low voltage and will often dictate the selection of an unregulated high-voltage AC turbine (400 to 500 V wild AC) feeding transformers at the battery shed, depending on the wattage.
Careful planning is called for, even beyond the selection of the turbine. Each system component must be selected and integrated into the whole. For instance, the design and installation of valves, and connection and discharge pipes, are critical to the proper operation of hydro-electric turbines, because water discharging from a turbine will mix with air and commonly double in volume as it leaves the turbine. Advice from turbine manufacturers is also helpful when considering inlet and outlet piping size.
Ken Gardner is a consulting civil engineer and master electrician. He owns an RE company in Ogden, Utah, and teaches classes for Solar Energy International.
Ian Woofenden teaches and writes about hydro electricity and other renewable sources in North and Central America. He dreams of head and flow at his solar- and wind-powered homestead on his flat island property.
Chris Greacen builds picohydro-power projects in rural Thailand and Vanuatu when he’s not living on another flat island near Ian Woofenden.