The Two Parts Nobody Photographs
Everyone looks at the turbine. But before any water reaches it, two quiet components have to do their job. The intake catches the water, and the penstock carries it down the hill. Get either one wrong and your turbine just sits there, dry.
What Does A Penstock Do?
A penstock is a closed pipe that delivers water under pressure to a turbine. That's the whole job.
Think of a water slide sealed all the way round. The water can't splash out, so the drop from the top of the hill turns into pressure at the bottom, right where the turbine needs it.
At a big hydroelectric dam, the penstock is a huge tube that drops water through the dam structure to the turbines. In home systems it is usually a plastic pipe in a trench.
Why Is It Called A Penstock?
The word is Scots, and it comes from old mill ponds and watermills, long before anyone made electricity with water. Back then a penstock meant the sluice or gate that controlled the water from a mill pond.
Today the name covers the whole pipe.
The Intake And Its Channel
The intake is a small structure that scoops part of the flow of water out of the stream and into the pipe. It can sit right in the stream or at the end of a short channel dug off to the side.
Its height decides your power. Head is the vertical drop from the intake down to the turbine, so a higher intake means more head. Measure head and flow before you pick the spot, or ask a surveyor or engineer, then check if you have enough water.
But a longer pipe also loses more energy to friction, so moving the intake upstream only pays while the extra drop beats the extra pipe. The best spot depends on your stream.
Every intake should have a screen or trash rack across the opening. Its job is to prevent leaves, sticks and fish from getting into the pipe, and a clogged one starves the turbine. Many systems also have a gate or valve installed here, so you can control the flow and shut it off for repairs.
The Penstock
Most small penstocks are built from rigid PVC, HDPE (a tough black plastic pipe) or steel. The plastic materials are common because they are light and cheap, while steel is the heavy choice for bigger jobs.
Penstocks are often installed underground. That keeps the sun off, because sun-heated water wears out PVC, and it shields the pipe from animals and falling trees. A pipe on the surface needs that shield added, and supports may be required at bends.
Size is where people go wrong. Too narrow, and the water rubs along the inside so hard that friction eats your head. US farm guides say friction often takes about 20 percent of it. Too wide, and the pipe cost climbs fast. Friction loss tables, the same ones used for irrigation, show the pipe size required for your flow.
Elbows add loss too, so a good penstock design runs as straight as the ground allows.
Air And Water Hammer: Damage Prevention
Bubbles drift to the high points of a pipe and collect there, and a growing air pocket squeezes the space left for water until the flow can stop.
The fix is an air release valve installed at each high point, a little valve on top of the pipe that lets trapped air out.
Air has to get back in too. If a full penstock is emptied fast, the pipe can collapse from the vacuum inside. A combination air valve does both jobs.
Water hammer is the other danger. When moving water is forced to stop suddenly, a shock wave slams back up the pipe. At home you hear it as a bang in the plumbing when a tap shuts fast, and in a penstock it can break the pipe.
Slowly closing valves are the main fix, and big hydroelectric plants add a surge tank that soaks up the jump. For big pipes, that part is a job for a hydro engineer.
See what the water does inside a water turbine, keep the screen and valves on your yearly maintenance list, or go back to hydro power for the big picture.
Sources
- Wikipedia, Penstock
- Wikipedia, Water hammer
- University of Missouri Extension, Micro-hydropower site assessment (EM0802)
- University of Wyoming Extension, Micro-hydropower for the home, farm, or ranch (B-1285)
- Alternative Energy Tutorials, Penstock design and penstocks for small hydro schemes
- CED Engineering, Selection and sizing of air release valves
Hard Words
| Term | What it means |
|---|---|
| Intake | The structure where you catch some water out of the stream and send it into the pipe. |
| Penstock | The closed pipe that carries water under pressure from the intake down to the turbine. |
| Screen | The bars or mesh across the intake opening, also called a trash rack. |
| HDPE | High-density polyethylene. A tough black plastic pipe used for many small penstocks. |
| PVC | Rigid plastic pipe. Sun-heated water wears it out, so it is often buried. |
| Air release valve | A small valve at a high point of the pipe that lets trapped air escape. |
| Water hammer | The shock wave when moving water in a pipe is forced to stop suddenly. |
| Surge tank | A chamber on the pipe that soaks up sudden pressure jumps. |
Common Questions
What is a dam penstock?
It is the pipe that carries water from behind a dam down to the turbines of a hydroelectric plant. At home a buried plastic pipe does the same job.
What are the different types of penstocks?
Most are sorted by material: rigid PVC, HDPE plastic or steel. They are also either buried or laid on the surface with supports.
Why are they called penstocks?
The word is Scots and comes from old mill ponds and watermills, where the penstock was the sluice or gate for the pond water.
Why do penstocks have small valves on top?
Those are air valves. Air collects at high points and blocks the pipe, so the valves let it escape.