The Two Parts You Never See On A Brochure
When people look at a hydro setup, they usually look at the turbine. The shiny spinning wheel doing the cool job.
But the turbine is only the tip of the system. Before any water reaches it, two other parts have to work perfectly. The intake catches the water. The penstock carries it.
These are the boring parts. Buried in the ground, surrounded by concrete, easy to ignore. But mess either one up and your turbine just sits there, dry and useless.
The Intake
The intake is the structure where you scoop a bit of the stream out and send it into your pipe.
It usually sits at the highest point you can reach upstream. A small concrete or stone box, partially submerged in the stream, with an opening on one side that lets water in.
The opening always has a screen across it. Without one, every leaf, twig, fish, and frog ends up in your turbine. Some pretty quickly grind the blades to nothing.
A good screen has bars or wire mesh small enough to keep debris out but big enough that water flows through easily. The standard rule is the spaces between bars should be no wider than half the diameter of the smallest part inside the turbine.
The intake also usually has a small gate or valve. Close it and you can stop the flow, which is handy when you need to clean the screen, fix the penstock, or shut down for winter.
→ How a turbine actually uses the water once it arrives
Why Position Matters
The intake's position decides your head.
Take the intake higher up the stream and you get more drop down to the turbine. More drop means more power. Lower intake means less drop, less power.
So you want the intake as high as you reasonably can. There is a limit. Eventually the pipe gets so long that you lose power to friction inside it. There is a sweet spot, usually 100 to 300 meters of pipe length, where you get the most drop without too much loss.
For mountain streams, this often means trekking 200 meters up a steep slope to find a good intake spot. For a flat stream, you might only have 20 meters to work with.
→ How to measure head and flow on your stream
The Penstock
The penstock is the pipe that carries water from the intake down to the turbine.
The name comes from the old English word for a sluice gate. It is a fancy word for "pipe", but it has stuck around in hydro talk for two centuries.
A penstock looks simple but it is doing a lot of work. It is keeping the water sealed, so it builds up pressure as it falls. It is keeping the water clean. It is sometimes carrying tons of pressure when the turbine briefly closes its valves.
Pipe Material
Three materials show up most often.
HDPE (high-density polyethylene) is a tough black plastic pipe. Cheap, flexible, lasts 50 years, easy to bury. It is the most common choice for home hydro setups. The downside: it can't handle super-high pressures, so it is best for drops under 50 meters or so.
PVC is white plastic pipe. Even cheaper than HDPE. Lighter. But it gets brittle in cold weather and from UV light, so it has to be buried or shaded. Good for short runs.
Steel is the heavy-duty option. Used when the pressure is very high, like the penstock at a Pelton wheel site with a 100-meter drop. It can handle anything. It also costs a fortune and rusts if you don't paint it. Mostly for serious installations.
For a typical home setup with 5 to 20 meters of drop, HDPE is the answer 90% of the time.
Pipe Size
This is where people often go wrong.
The pipe needs to be wide enough that water flows through without getting choked. Too narrow, and the water rubs against the inside of the pipe so much that it slows down. That friction wastes your hard-won pressure.
But too wide is also bad. A pipe that is too big costs more, takes more digging, and the water spreads out and slows down on its own.
The rule of thumb: pick a pipe that gives you a water speed of about 1 to 2 meters per second inside it.
There are pipe-sizing charts and online calculators that match flow rate to pipe diameter. For a typical home setup with 20 liters per second, a 100-millimeter (4-inch) HDPE pipe is about right.
A pro will calculate exact friction losses and pick the size that gives you the best balance between cost and efficiency.
Bends And Joints
Every bend in the pipe steals a little energy from the water.
A sharp 90-degree elbow is the worst. The water has to slam into the corner and turn. Each one of those costs you maybe 1% of your power.
Smooth bends (long sweeping curves) are much better. A long radius bend costs you almost nothing.
So a good penstock is as straight as possible. When it has to turn, it turns gently. When the pipe goes through a joint, the joint is flush on the inside (no lips or ridges sticking out).
The Air Vent
Here is a thing nobody talks about. Air can sneak into a penstock and ruin everything.
A small bubble of air inside a pipe can sit at the high point and slowly grow. Eventually it blocks part of the flow, and water rushes past it, then suddenly slams into the bubble. This causes a thumping noise and can shake the pipe to pieces.
The fix is an air vent at the top of any high points. A small valve that lets trapped air escape. Looks like a tiny chimney sticking up out of the pipe.
If you ever see a hydro pipe with mysterious little valves along its length, that is what they are.
Pressure Surge: Water Hammer
The other thing that scares hydro engineers is water hammer.
When you suddenly close a valve at the end of a long pipe full of moving water, the water slams to a stop and the pressure spikes. The pressure wave can shoot back up the pipe and burst it.
It is the same banging noise you hear when you turn off a faucet too fast in an old house. In a hydro penstock, the spike can be ten times the normal pressure.
The fix is to use turbines that close their valves slowly, or to install surge tanks (small chambers off the side of the pipe that absorb the pressure jump). Most home turbines have built-in protection for this.
When It All Works
A well-built intake and penstock are the difference between a hydro setup that runs for 30 years with no problems and one that breaks every winter.
Once they are in, they are almost invisible. The pipe is buried. The intake just looks like a small concrete box next to the stream.
But every kilowatt your turbine ever makes had to travel through them first.
→ The pipe feeds the turbine: now see inside → Yearly maintenance for the whole setup → Back to the full hydro overview
