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Microhydro Power in Your Backyard? How to Assess Your Site
Posted By dpacheco On May 6, 2009 @ 11:19 pm In Renewable Energy | No Comments
One of my coworkers dreams of owning a home with a little stream running through the yard helping to power his hybrid microhydro/photovoltaic and carbon-free home.
If you’re lucky enough to have access to your own stream, complete with a small waterfall, just think: you could be harnessing that energy for your own renewable electric system—via microhydro turbine. Here’s the first step to get you started on the road to clean, off-grid power.
Where can a microhydro turbine be installed? Good sites have either a lot of water flow, a lot of elevational drop (head), or a workable balance of the two. To be practical, microhydro systems need a combination of head and flow somewhere between these extremes: 2 feet of drop (the head) and 500 gallons per minute, or 2 gallons of water per minute and 500 feet of drop.
How do you assess your site? You must try to estimate the available flow (gallons per minute) and head separately. Measuring flow and head can be a tricky business, involving buckets, gauges, levels, water lines, and measuring tapes. If you have access to a stream that you think might work, then a more thorough analysis than we detail here could be worthwhile. For a quick easy guess at flow in gallons per minute, if you have an accessible spot such as a small waterfall you can stick a 5-gallon bucket in the flow and measure how fast it fills—for example, filling in 30 seconds means 10 gallons per minute. If there isn’t an accessible spot, the other way to measure flow is to build a weir, a labor-intensive project that involves basically building a small dam with a hole in the middle that the water has to flow through, enabling you to catch the flow and measure it. If you have a watercourse where flow is obviously huge, then you probably need to worry only about the second part of the equation—head.
Approximating head is tricky. Head is measured as the vertical distance between the start of the captured water line and the input of the turbine. You can make a guesstimate, perhaps with an altimeter watch read at the intake spot and the turbine location, but before investing in a system you should get an accurate measurement.
To measure head you need to start at the bottom of the potential penstock route (where the turbine would sit) and, working with a friend, try to estimate the rise to a possible point of intake (where the water enters the route to the turbine). This can be done with a level and a stick of known height (5 feet in this example). Start at the lowest elevation and work your way toward the highest spot—the intake. Using your 5-foot-tall measuring stick, sight along the top of the stick with your level to a reference point at ground level, such as the base of a tree or a rock or your friend’s feet. Instead of sighting along the stick, you could instead use a string level or water level from the top of the stick to the reference point for more accuracy. The height (head) between you and the reference point will be 5 feet. Your friend can mark the reference spot, and you can start over again at the reference spot and work your way to the intake point. Keep track of the number of times it takes you to make it from the lowest spot—the turbine’s future home—to the highest spot, the intake, and multiply that times five. If it takes three different markings and movements to get to the top, you have a 15-foot head. Voilà: a slightly inaccurate but very good ballpark guess at your head!
Another good measurement to make at this point is the horizontal distance from turbine to intake. This will tell you how much pipe you would need to build the system. If you are looking at 50 feet of drop over 600 feet of horizontal distance, for example, that’s a lot of distance; it’s certainly doable, but it will add expense to the system. Friction losses vary by pipe diameter and distance. Longer distances mean more friction losses, which can be offset with larger pipe. Larger pipe entails lower friction losses but adds expense and trouble to a system design.
Once you have a ballpark notion of your head and flow, take some time to peruse different turbine manufacturers’ Web sites. Microhydro turbine manufacturers print data charts, such as in table 2.4, that show power output at different combinations of flow and head. Examining these charts can give you a good idea of which turbine might fit your site and how much power you could expect it to produce.
You not only need a good amount of water and head for microhydro but also must provide a screened intake, a route (usually piped) for the captured water to flow out of the source and into the turbine and back to the stream, a safe (often enclosed) location for the turbine, a transmission route for the electricity, and a protected space for the electrical components. That’s a lot of infrastructure, which can be exposed to freezing temperatures depending on location. Freezing temperatures aren’t as much of a concern for wind or PV (batteries excepted, which can be sensitive to temperature swings) but make microhydro that much harder to install, as frozen pipes mean zero power and reconstruction expenses.
You must also make sure there aren’t restrictions on water access: remember these systems almost always remove some percentage of the water from a stream or creek to get it to the turbine, sometimes for hundreds of feet. Questions to ask yourself include: How will the removal of water affect wildlife in the creek? Do you own the entire watercourse? Will you need county or jurisdictional permission?
What if you live in an off-grid home with a seasonally dependent water supply, so that your microhydro site provides power for only certain times of the year? This can be overcome as long as you plan a hybrid system that relies on other power sources to provide electricity in the downtime when little water is available.
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