Personal Hydropower

When exploring small-scale renewable energy sources, the most commonly considered techniques are wind and solar generators. Depending on several factors, a far better option may be small hydroelectric systems, or “microhydro”.

We learned in our discussion on ram pumps that we can use the energy of water falling from a height to pressurize tap water. Of course, that same energy can be used to drive a turbine and produce electricity, just as large projects like the Hoover Dam.

Energy Alternatives claims you can get 10 to 100 times more power per dollar with microhydro than with wind or solar. Hydropower has a long history, and was most commonly used to power grain or saw mills.

Water turbine with a PVC sluice (via OtherPower.com)

Large commercial hydroelectric operations like Hoover, Aswan, and Three Gorges rely on massive dams in order to establish strong head (height) pressure and secure a reservoir in order to smooth out variations in water flow. These techniques are typically very environmentally destructive – they flood areas upstream and dehydrate the rivers downstream.

Using a dam for small-scale production is much lower-impact, but could disrupt the local environment in similar ways. Fortunately, there are other options including using a sluice or simply dipping a wheel directly in the flow. I even saw a mill wheel in St. Augustine, FL that had used water from an artesian well for over a century.

Using water turbines for energy production is more common nowadays than you might suspect, as seen at Otherpower. Water turbines have a number of advantages over solar and wind, including lower cost, better maintainability, improved reliability, easier construction, and easier-to-obtain parts. All of these factors are particularly important in the context of Peak Oil – highly-sophisticated devices (such as most solar panels) could be very difficult to repair or replace on your own.

The important parameters when working with hydropower are the flow rate of the available water source (volume/time) and the head (height) from source to target. There are three main types of water turbine: gravity wheels, impulse turbines, and reaction turbines.

Gravity wheels are what you might find attached to an old mill – typically large wooden wheels with many small buckets fed from an overhead supply of water. They are very heavy and spin slowly, but produce a high torque ideal for their traditional milling applications. While it is possible to use a gravity wheel for producing electricity, they are the least suited for the purpose.

Impulse turbines are comprised of curved paddles acted upon by a jet of water. The force of the water striking the paddles turns the turbine at a high speed, making impulse turbines a good choice for electricity production.

A reaction turbine is designed to capture water energy by changing the fluid’s pressure from high to low. This type of turbine requires a sturdy housing to contain the pressure and is usually the most difficult type to construct.

Before you can decide whether microhydro is the right choice for your energy needs, you must take some measurements. The flow rate of a stream or river can be calculated as follows: First, estimate the cross-sectional area. Pick a relatively straight stretch, then measure the distance from bank to bank with a tape measure or rope. Use a graduated stick to probe the depth at various points in the cross section. Average your results.

Now you need to measure the speed of the water. Mark off several points along the bank an equal distance apart. Find an easy-to-see object that will float and send it down the river several times, starting at different points along the cross-section. Record the time it passes each of your markers, and calculate the speed. Average your results.

Multiplying the cross-sectional area by the speed will give you a rough estimate of your flow rate (volume/time). For best results, you should perform this measurement at several different times during the year so you get an idea of the highest and lowest flow rates.

The second measurement you need is the head height from stream source to target wheel. You may need to search around your property to find the best locations for both the diversion point (where you redirect the water) and the turbine position (remember these can be some lateral distance apart). If you plan on using a dam or weir, remember to measure not from the water surface, but from the top of the spill location. Similarly for a sluice, you should measure from the bottom of the sluice surface, not the top of the water.

The bottom of the head measurement is where the water strikes the paddle (for impulse turbines) or enters the housing (for reaction turbines), not the center of the wheel.

In the next installment on microhydro, I’ll talk about calculating performance.