Microhydro Power Basics

What is Micro-hydro Power?

Hydro-electricity is fundamentally the combination of water flow and vertical drop (commonly called “head”). Vertical drop creates pressure, and the continuous flow of water in a hydro system gives us an ongoing source of pressurized liquid energy. Pressurized, flowing water is a very dense resource, and hydro-electric systems convert a very large percentage of the available energy into electricity because the resource is captive in a pipe or flume.

People have been tapping the energy in flowing water for centuries, first for mechanical power, and, in the last hundred years, for electricity. Early applications included milling, pumping, and driving machinery. Unlike wind and sun, the right hydro resource can be available 24 hours a day, 365 days a year. This allowed pioneers to run irrigation pumps and grain mills, and allows people today to make clean, renewable electricity at a reasonable cost.

A simple formula can give you a rough idea of how much capacity your stream might have. Take the head in feet, multiply it by the flow in gallons per minute (gpm), and divide by a factor of about 12. This will give you the potential wattage of a reasonably efficient, small system. For example, if you have 30 gpm available and 40 feet of head, you will be able to generate something in the range of 100 watts [(30 ´ 40) ¸ 12 = 100). Over the course of an entire day, the generation would be 2,400 watt-hours or 2.4 kWh (24 hours/day x 100 W).

Within this formula is the understanding that systems with low vertical drop (head) need more flow to generate the same amount of energy. Typically, low-head systems will have high flow, and high-head systems will have lower flow. Adapting the example above, if we have 400 hundred feet of head, we only need 3 gpm to generate the same 100 W.

There are a wide range of small hydro turbine types to suit the head and flow of the site. Large wooden overshot and undershot wheels tend to be less efficient for generating electricity, though they may be appropriate for mechanical work. For electricity generation, systems can be divided into “low head” and “high head.”

Low-head systems may have less than 5 feet of vertical drop - sometimes they may have only 10 or 20 inches. In this case, most or all of the water in a small stream will run through the turbine to maximize output. The runner (the part of the turbine that receives the water and turns its energy into rotation in a shaft) for low-head turbines may be a Turgo or Francis type. These systems typically have short pipelines or sluice ways that then allow the water to drop through the runner.

High-head systems may be defined as any site with more than 10 feet of head. Common runners are Turgos on the low end, and the most common, Pelton, for medium to high heads. These systems may have hundreds of feet of pipeline to develop the head (pressure), with the water delivered to the runner via multiple nozzles.

The basic components of a small hydro-electric system, powered of “water to wire” are:

Diversion and intake screen - Directs water from the stream or river into the pipe or channel
Penstock (pipeline) - Carries the water to the turbine
Turbine - Generates electricity (includes nozzles, runner, and generator)
Electronics and batteries (if used) - Regulates turbine and stores energy
Dump load - Absorbs surplus energy
Transmission and distribution - Delivers the energy to its end use

Why Use Micro-hydro Power?

People choose micro-hydro-electric systems for a variety of reasons. Environmental motivations are very common. Micro-hydro-electric systems can eke a large amount of energy out of a small water flow with minimal impact. Because they run 24 hours a day, these systems can be low-wattage while generating enough energy to make a big dent in a typical home’s energy use, and, in off-grid systems, even minimize or eliminate the need for having batteries. While care needs to be taken not to impact wildlife, micro-hydro-electric systems can be unobtrusive, use only a portion of stream flow, and quietly produce clean electricity with almost no ongoing impact. While all energy generation and use has some impact, it’s instructive to compare the impact of grid sources such as coal, oil, and nuclear with renewables. If we had a truly level playing field, where all players had to take their cost, embodied energy, and environmental impact into account, small hydro systems would likely be shown to have the least impact.

In situations where the resource exists reasonably close to the end use, and pipeline and transmission distances can be moderate, hydro-electricity may be more economical than tapping other renewable resources. A combination of a solid resource, a well-designed system, good maintenance, available incentives, and utility costs may even make a compelling economic argument to tap that stream on your property. Micro-hydro systems can, at a minimum, save you dollars, while providing clean, reliable electricity.

These systems can make you entirely independent of the grid, or they can be connected to the grid, allowing you to “sell back” surplus electricity for a credit and providing backup when the utility fails, giving you the best of both worlds. Additional benefits include that maintenance is done at ground level (unlike wind) and the system’s production is around the clock (unIike wind and solar). If you want a reliable electricity supply, it’s hard to beat a micro-hydro system.

Types of Micro-hydro-Electric Systems

As with wind- and solar-electric systems, hydro-electric systems can be divided into four configurations:

On-grid without batteries. This a simple and efficient system that sends any surplus energy back into the grid to be credited to you for use at other times. These systems typically do not provide backup for utility outages.
On-grid with batteries. This system type also sells back surplus electricity, but also provides backup during utility outages. The amount of backup will be determined by the system’s capacity and the battery size.
Off-grid without batteries. This configuration is generally for larger, AC-generating systems. The peak load capacity (how many things you can operate at once) is determined by the hydro system’s peak generating capacity. This configuration is generally not used for systems that generate at less than about 2 kW.
Off-grid with batteries. This is the most common off-grid option, and is similar to off-grid solar- or wind-electric systems. The charging source puts energy into a battery bank, while loads are run from the batteries—directly, if DC; via an inverter, if AC.

Micro-hydro-electric systems can power most, if not all, electrical loads, depending on the size of the resource. The smallest systems may only provide for lighting, electronics, and basic refrigeration. But with sufficient head and flow, these systems can run heating and cooling systems, tools, and even commercial equipment in a modern, on-grid home, ranch, or business. The head and flow are the limitation, and it all comes down to how much power (wattage) and energy (kilowatt-hours) you have at your disposal.

While people with solar-electric and small wind-electric systems may be pushed toward serious efficiency and conservation measures because of the cost of these renewable kilowatt-hours, small hydro systems frequently have ample resources behind them, and can be more generous with their output. However, using your energy wisely can mean using less water and smaller equipment, which means lower environmental impact and lower cost.

Get Started with Micro-hydro Power

After you’ve done your load analysis and know how many kilowatt-hours you want to generate, a micro-hydro system site survey primarily focuses on four measurements:

The flow - how many gallons per minute (or in larger systems, cubic feet per second) are available, and how much water you want to divert from the stream
The head, or vertical drop, between where the water is removed from the stream and where it leaves the turbine runner. Exactly where or how the pipeline runs is not vital for this measurement, though calculations of losses for pipe and fitting friction will need to be made
The pipeline length, which, combined with its diameter, will allow you to price what may be one of the most expensive parts of the system
The length of the transmission wiring, which may also be a significant cost, and must be sized to minimize energy losses, and be well within safety parameters.

For most people, a combination of motives—environmental, independence, reliability, and cost—make hydro-electric systems attractive. The “bottom line” may end up being what the actual cost per kWh is. To arrive at this, you’ll need a complete design along with construction bids or estimates. If it’s a grid-connected system you’re after, you’ll also need to know what your local utility policies are for renewable energy systems, and at what amount you will be credited or paid. You’ll also need to know if any incentives (utility or government) exist. Often, micro-hydro system incentives are less than those for solar energy systems, and sometimes non-existent. Available incentives, though, may be generous because of hydro’s 24-hour generation capability. Once you have these figures, you’ll need to predict how many years your system will operate and the annual maintenance costs, and then you’ll be able to calculate the cost per kWh.

Throughout your design, consider strategies to get the most out of your precious flowing water resource. Properly sizing the pipe will get the most energy to your turbine, minimizing friction loss. Choosing the right turbine and runner for the job will maximize production for your stream’s specific head and flow. And sizing the wire correctly will keep the system safe, and keep you from losing energy in the transmission of your hydro-electricity.

Get an education about common hydro myths, and avoid scams or schemes that promise more than they deliver. Lean on professionals and others with experience in the field to discover what has worked well to produce hydro-electricity. If you do your homework, and apply what you learn with care, hydro-electricity can provide low-cost, clean energy for many years.

ELECTROLUX
WIND & WATER