If a hydro turbine is the “engine,” the penstock is the fuel line, delivering the power of water to the turbine. For optimum performance, it’s critical that this pipe be sized properly. Using too small a pipe could mean losing much of the potential energy to friction. Too big of a pipe, and you’ll be spending more money than needed for no significant increase in energy.
To balance cost with efficiency, hydro designers typically aim for 10% to 15% pipe friction loss (also called “head loss”). Measuring the pressure (psi) at the bottom of the full penstock with no water flowing gives the “static pressure.” When the valve is opened and water flows at the rate the turbine requires, you can measure the “dynamic pressure,” or net head. The difference between these two numbers is the friction loss of the penstock, which can be identified in psi or feet of head.
To size pipe correctly, you need to know the total static head, the design flow—the amount of water you will take out of the stream (which will generally be some fraction of the stream flow)—and the penstock length. Friction- or head-loss tables give losses in feet per hundred feet of pipe for various pipe types and sizes, so you can do the math to figure out what pipe to buy. If you’re on the fence between two pipe sizes, round up!
Here’s an example: Let’s say you have a fairly steep site with 120 feet of head in 500 feet of run, and a design flow of 100 gpm. Tables for PVC pipe show head losses of 14.53 feet per hundred feet for 2-inch pipe, 2.125 for 3-inch pipe, and 0.578 for 4-inch pipe. Multiplying the head loss by 5 (for 500 feet of pipe) gives 72.65 feet for 2-inch, 10.625 for 3-inch, and 2.89 for 4-inch. Dividing these by the 120 feet of static head, we get about 61% loss for 2-inch, about 9% for 3-inch, and less than 3% for 4-inch. In this example, the right pipe to choose is 3-inch, as head loss is significantly lower than the 2-inch pipe option, and the higher cost of the 4-inch pipe will probably outweigh the minimal 6% decrease in head loss.
You’ll also need to choose the type of pipe to use. Hydro penstocks can be made out of steel, PVC, or polyethylene (usually high-density polyethylene—HDPE). Pipes of the same material can come in different wall thicknesses to handle different amounts of pressure. Your choices will depend on the pressure rating required, the size of your pipeline, what is available locally, friction losses, and your budget.
Penstocks must have appropriate pressure rating! When you map out the head, you can also convert to psi at various stages along the pipe run, adding a 40% safety factor. Most often, you’ll buy one type of pipe that can handle the highest pressure in the system, which is at the turbine. But if the pipeline is long or the pipe is costly, you may opt to use pipe with a lower pressure rating at the top of the penstock, and switch to an appropriately higher pressure-rated pipe as it gets closer to the turbine.
Pipe availability depends on local demand. If you live in an agricultural area, you’ll find a wide variety of irrigation and other pipe available. PVC sewer pipe in the 3- to 8-inch range can be a low-budget option for lower-head systems. However, it is not pressure-rated, so you’ll have to get some inside advice on what it will actually handle. Standard schedule 40 PVC pipe is often a readily available option, and bell-and-socket gasketed versions are commonly used. PVC is subject to ultraviolet light degradation and physical damage, so it is normally buried, covered, or sometimes painted. HDPE is the toughest of the plastic options. This material can be fused together with a “welder,” and is tough enough to drag behind a tractor or mule to get it into place.
Aluminum and steel pipe are used less often because of cost and higher friction losses. They can sometimes be found surplus and are very durable, though there are concerns with rust and corrosion. I’ve seen steel used for the lower ends of penstocks, where the pressure is higher, and at high-wear points like road crossings and places where the pipeline comes to the surface. Common aluminum irrigation pipe cannot handle much pressure, nor can it be buried, since it will corrode.
Every hydro designer has their own pipe preferences, based on the site, experiences, and values. For exposed penstocks 3 inches and larger, I lean toward HDPE. For buried pipelines, the additional expense may not be warranted. Evaluate your situation, educate yourself about the benefits and costs, and make a choice that will serve your needs.
Most home-scale hydro sites in North America are “high-head” systems, with drops from 10 feet to hundreds of feet. While there are many potential “low-head” (18 inches to several feet) sites, they require more flow and can be subject to more political challenges, since the complete stream or river flow is often diverted, requiring a dam across the whole water course. We’ll primarily focus on high-head systems.
A hydro turbine includes a “runner”—the wheel that receives the water’s kinetic energy to drive a shaft. Two types of runners—the Pelton and the Turgo—are used in most of the small, high-head turbines. In general, the Turgo tends to work more effectively at lower heads and higher flows than the Pelton. But there is a broad range of overlap between the two runners, and each turbine manufacturer will have a preference and point of view. Other runner types, such as the Francis, propeller, and cross-flow, are generally used in low-head, high-flow conditions, and may be appropriate for your site. Pumps used as turbines are another option that some designers use in a number of different situations.
When deciding on turbine type, ask the manufacturers for information and get more than one opinion. Most of us will never be experts at selecting the perfect runner for a specific site, and most hydro homeowners only make this decision once or twice. Most manufacturers will fully disclose their products’ limitations, and in this small industry, a few phone calls can net you much of the available information. Ask the suppliers what they recommend for your site. Look at what they offer, and whether they have choices of different runners in their lines. And realize that different runners can do a reasonable job on the same site.