If you have a suitable site, harnessing the energy in a stream or creek can be the most cost-effective way to make renewable electricity. Compared to the sun and wind’s variability, a stream’s flow is relatively consistent, making microhydro-electric system output the most predictable of all the renewable energy (RE) electrical systems. Hydro resources are also the most site specific, since your property must have a usable water source. If you are one of the lucky few with a stream running down your hillside, it’s the resource to assess first.
The first step in designing a microhydro system is to evaluate your water resource by measuring the head (vertical drop) and flow of your stream. (For detailed instructions, see Dan New’s article in HP104.) These two measurements are necessary to calculate the energy potential of your stream. The next step is to design a system that will effectively harness that potential.
A microhydro-electric system is made up of a number of components, not just the turbine. Hydro sites and end users’ needs vary, and a wide range of equipment and system configurations are available to properly match the conditions. This article will give you an overview of the components, and help you understand the different ways they can work together to make electricity from falling water.
Intakes can be as simple as a screened box submerged in the watercourse, or they can involve a complete damming of the stream. The goal is to divert debris- and air-free water into a pipeline. Effectively getting the water into the system’s pipeline is a critical issue that often does not get enough attention. Poorly designed intakes often become the focus of maintenance and repair efforts for hydro-electric systems.
A large pool of water at the intake will not increase the output of the turbine, nor will it likely provide useful storage, but it will allow the water to calm so debris can sink or float. An intake that is above the bottom of the pool, but below the surface, will avoid the grit on the stream bottom and most of the floating debris on top. Another way to remove debris is to direct the water over a sloped screen. The turbine’s water falls through, and debris passes with the overflow water.
Most hydro turbines require at least a short run of pipe to bring the water to the machine, and some turbines require piping to move water away from it. The length can vary widely depending on the distance between the source and the turbine. The pipeline’s diameter may range from 1 inch to 1 foot or more, and must be large enough to handle the design flow. Losses due to friction need to be minimized to maximize the energy available for conversion into electricity. Plastic in the form of polyethylene or PVC is the usual choice for home-scale systems. Burying the pipeline is desirable to prevent freezing in extremely cold climates, to keep the pipe from shifting, and to protect it from damage (cows, bears, etc.) and ultraviolet (UV) light degradation.
The turbine converts the energy in the water into electricity. Many types of turbines are available, so it is important to match the machine to the site’s conditions of head and flow.
In impulse turbines, the water is routed through nozzles that direct the water at some type of runner or wheel (Pelton and Turgo are two common types). Reaction turbines are propeller machines and centrifugal pumps used as turbines, where the runner is submerged within a closed housing. With either turbine type, the energy of the falling water is converted into rotary motion in the runner’s shaft. This shaft is coupled directly or belted to either a permanent magnet alternator, or a “synchronous” or induction AC generator.
The function of a charge controller in a hydro system is equivalent to turning on a load to absorb excess energy. Battery-based microhydro systems require charge controllers to prevent overcharging the batteries. Controllers generally send excess energy to a secondary (dump) load, such as an air or water heater. Unlike a solar-electric controller, a microhydro system controller does not disconnect the turbine from the batteries. This could create voltages that are higher than some components can withstand, or cause the turbine to overspeed, which could result in dangerous and damaging overvoltages.
Off-grid, batteryless AC-direct microhydro systems need controls too. A load–control governor monitors the voltage or frequency of the system, and keeps the generator correctly loaded, turning dump-load capacity on and off as the load pattern changes, or mechanically deflects water away from the runner. Grid-tied batteryless AC and DC systems also need controls to protect the system if the utility grid fails.
A dump load is an electrical resistance heater that must be sized to handle the full generating capacity of the microhydro turbine. Dump loads can be air or water heaters, and are activated by the charge controller whenever the batteries or the grid cannot accept the energy being produced, to prevent damage to the system. Excess energy is “shunted” to the dump load when necessary.
By using reversible chemical reactions, a battery bank provides a way to store surplus energy when more is being produced than consumed. When demand increases beyond what is generated, the batteries can be called on to release energy to keep your household loads operating.
A microhydro system is typically the most gentle of the RE systems on the batteries, since they do not often remain in a discharged state. The bank can also be smaller than for a wind or PV system. One or two days of storage is usually sufficient. Deep-cycle lead-acid batteries are typically used in these systems. They are cost effective and do not usually account for a large percentage of the system cost.
System meters measure and display several different aspects of your microhydro-electric system’s performance and status—tracking how full your battery bank is, how much electricity your turbine is producing or has produced, and how much electricity is being used. Operating your system without metering is like running your car without any gauges—although possible to do, it’s always better to know how well the car is operating and how much fuel is in the tank.
In battery-based systems, a disconnect between the batteries and inverter is required. This disconnect is typically a large, DC-rated breaker mounted in a sheet metal enclosure. It allows the inverter to be disconnected from the batteries for service, and protects the inverter-to-battery wiring against electrical faults.
Inverters transform the DC electricity stored in your battery bank into AC electricity for powering household appliances. Grid-tied inverters synchronize the system’s output with the utility’s AC electricity, allowing the system to feed hydro-electricity to the utility grid. Battery-based inverters for off-grid or grid-tied systems often include a battery charger, which is capable of charging a battery bank from either the grid or a backup generator if your creek isn’t flowing or your system is down for maintenance.
In rare cases, an inverter and battery bank are used with larger, off-grid AC-direct systems to increase power availability. The inverter uses the AC to charge the batteries, and synchronizes with the hydro-electric AC supply to supplement it when demand is greater than the output of the hydro generator.
The AC breaker panel, or mains panel, is the point at which all of a home’s electrical wiring meets with the provider of the electricity, whether that’s the grid or a microhydro-electric system. This wall-mounted panel or box is usually installed in a utility room, basement, garage, or on the exterior of a building. It contains a number of labeled circuit breakers that route electricity to the various rooms throughout a house. These breakers allow electricity to be disconnected for servicing, and also protect the building’s wiring against electrical fires.
Just like the electrical circuits in your home or office, a grid-tied inverter’s electrical output needs to be routed through an AC circuit breaker. This breaker is usually mounted inside the building’s mains panel. It enables the inverter to be disconnected from either the grid or from electrical loads if servicing is necessary. The breaker also safeguards the circuit’s electrical wiring.
Most homes with grid-tied microhydro-electric systems will have AC electricity both coming from and going to the utility grid. A multichannel KWH meter keeps track of how much grid electricity you’re using and how much your RE system is producing. The utility company often provides intertie-capable meters at no cost.
Hydro-electric systems have great potential, but several things can make using this technology difficult. Diverting the water in a stream or creek is likely subject to regulation by local authorities and may require seeking approval. You also may need to contend with droughts or floods. All hydro turbines have moving parts that require maintenance and periodic replacement. The most common maintenance chore is keeping debris out of the intake.
Despite the various challenges, most of the problems can be easily overcome. If installed correctly and properly maintained, a microhydro system can provide many years of service. The predictable and often ample output is the envy of those restricted to using only wind or solar electricity. As an owner of a microhydro system, you’ll go to bed at night with the knowledge that while you are sleeping, your system is charging and will be ready for another day of energy use when you rise.
Paul Cunningham, Energy Systems & Design, PO Box 4557, Sussex, NB, E4E 5L7, Canada • 506-433-3151 • Fax: 506-433-6151 • www.microhydropower.com
Ian Woofenden, PO Box 1001, Anacortes, WA 98221