Pelton vs Turgo vs Cross-Flow Turbines
Updated 8 October 20267 min readWind & Hydropower
An impulse turbine converts the whole of the available head into the speed of a free jet, then extracts energy by turning that jet around. Nothing downstream of the nozzle is pressurised, which makes these machines simple, tolerant and easy to maintain. The three common designs differ in one thing — the geometry by which the runner meets the jet — and that single difference sets which heads and flows each can work with, and how gracefully each copes when the stream is low.
Key takeaways
- An impulse turbine turns all the head into jet velocity first, so the runner spins in air at atmospheric pressure.
- A Pelton bucket reverses the jet almost completely, which is why it extracts so much of the energy available.
- The Turgo takes the jet at an angle so water enters one side and leaves the other, letting a given runner swallow more flow.
- A cross-flow turbine passes water through the runner twice and can be split into part-width sections, which is what makes it forgiving of varying flow.
- On a real stream the flow changes all year, so efficiency at part flow usually matters more than peak efficiency.
On this page
All the pressure becomes speed first
The defining feature of an impulse machine is where the pressure disappears. In the nozzle, entirely. By the time water reaches the runner it is a free jet travelling through air, and the runner is turning in atmosphere rather than running full.
Two consequences follow. The casing does not have to contain pressure, so the machine is simpler, lighter and easier to open up. And the energy extracted is the change in momentum of the jet — so the more completely the blade reverses the flow, the more of the jet's energy is taken.
That single sentence explains the three designs. Each is a different answer to "how do I reverse as much of the jet as possible, while letting the spent water get out of the way?"
Pelton: reverse it almost completely
The Pelton answer is a ring of double bowls with a sharp ridge down the middle.
Turning the jet through close to one hundred and eighty degrees takes nearly all the momentum available, which is why a well-made Pelton is the most efficient of the three at its design point. The splitter also balances the side thrust, so the bearings only carry the intended loads.
The constraint is the spent water. It has to fall clear of the runner before the next bucket arrives, which limits how large a jet a given runner can accept. Pelton machines therefore favour high head and modest flow — a lot of speed in a small stream of water.
Turgo: take the jet at an angle
The Turgo changes one thing: the jet arrives at an angle to the runner plane, so water enters one face and leaves the other.
Because the exit path is clear of the inlet, the flow constraint that limits a Pelton is relaxed. A Turgo of a given diameter accepts a larger jet, so it takes more flow — or runs faster for the same flow, which can remove a gearbox between the runner and the generator.
The price is that the jet is not turned quite as completely, so peak efficiency is a little below a comparable Pelton. In exchange, the machine occupies the middle ground: medium head, more flow than a Pelton of the same size.
Cross-flow: through the drum, twice
The cross-flow is a different shape entirely — a wide drum with blades around its circumference, fed by a rectangular jet across its full width.
Water passes through the blades on the way in, crosses the hollow centre and passes through again on the way out, taking energy on both passes. Peak efficiency is lower than the other two, and the construction is far simpler — a drum, straight blades, a rectangular nozzle.
The feature that matters is the divided inlet. Split the nozzle into a third and two thirds, each with its own gate, and the machine has three sensible operating widths instead of one. A stream at a third of its design flow can be run through the small section at close to its proper velocity rather than through a large nozzle barely open.
Matching the machine to the site
Head and flow, established by a micro-hydro site survey, select the machine before anything else does.
| Pelton | Turgo | Cross-flow | |
|---|---|---|---|
| Head | High | Medium to high | Low to medium |
| Flow for a given size | Low | Moderate | High |
| Jet reversal | Nearly complete | Partial | Two passes, partial each |
| Peak efficiency | Highest of the three | Close behind | Lower |
| Part-flow behaviour | Good with multiple jets | Good | Very good with a divided inlet |
| Runner speed | Lower for a given head | Higher — may avoid a gearbox | Lower, wide runner |
| Construction | Precise cast buckets | Precise cast blades | Simple fabricated drum |
| Tolerance of debris and silt | Sensitive at the nozzle | Sensitive at the nozzle | More forgiving |
Design tendencies, not thresholds. The boundaries overlap considerably and a specific manufacturer's range will not match these generalisations exactly.
The last row deserves emphasis in a small scheme. A nozzle is a precision orifice and silt abrades it; a cross-flow's rectangular inlet and simple blades put up with more of what a stream carries, which is why intake design and settling matter more for the first two. It is the same survivability-against-output trade that decides wave and tidal devices, where the water is rougher still.
Part flow is what decides it
Here is the conclusion that surprises people who compare peak efficiencies.
A stream is not at its design flow for most of the year — the flow duration curve says so plainly. What a machine delivers annually is its efficiency *at each flow*, weighted by how many hours the stream spends there. A machine that holds 70 per cent of its rating across a wide range can beat one that reaches a higher peak and then collapses below half flow — and in the dry months it is that part-flow output, not the peak, that has to cover a home's off-grid energy load.
This is why multi-jet Peltons exist: shutting off jets is the Pelton's version of the cross-flow's divided inlet, keeping the remaining jets at full velocity rather than throttling all of them. And it is the same principle that governs the Betz limit's cousin in wind, where a turbine is judged on the whole distribution of wind speeds rather than on its best one.
Too much water is the opposite problem. Small schemes usually hold the runner at a steady speed by keeping it fully loaded, sending whatever the house is not using to a diversion load; if the generator loses its load altogether, a Pelton or Turgo swings a deflector into the jet so the runner cannot race towards runaway speed — the hydro counterpart of a wind turbine shedding surplus power in strong wind.
The honest summary: let head and flow narrow the field, then choose on part-flow behaviour and on how much of what the stream carries the machine will tolerate. Peak efficiency is the last thing to look at, not the first — the same reasoning that makes round-trip stability matter more than peak efficiency in storage, and for the same reason: what you keep across a year is not what a data sheet reports at one operating point.
Frequently asked questions
What makes a turbine an impulse turbine?
That the pressure drop happens entirely in the nozzle, before the water reaches the runner. The runner spins in air at atmospheric pressure and is driven by the momentum of a free jet. A reaction turbine, by contrast, runs full of water with pressure dropping across the runner itself, which is a different machine with different sealing and siting requirements.
Why does a Pelton bucket have a splitter down the middle?
So the jet is divided and turned back on both sides symmetrically. Turning the water through nearly one hundred and eighty degrees extracts close to the maximum momentum available, and splitting it keeps the axial forces balanced so the bearings are not pushed sideways.
Is a Turgo just a cheaper Pelton?
No — it solves a different problem. Because the jet enters one side of the runner and exits the other, the spent water does not have to get out of the way of the incoming jet. That lets a Turgo accept a larger jet for a given runner diameter, so it handles more flow, or the same flow at a higher speed, than a Pelton of the same size.
Why is a cross-flow turbine good at low flow?
Because it can be built with a divided inlet, typically in a one-third and two-thirds split. Running one section, the other, or both gives three sensible operating widths, so the machine can be matched to whatever the stream is doing without running one large nozzle badly throttled.
Which is most efficient?
At its design point a well-made Pelton is usually the highest of the three, with the Turgo close behind and the cross-flow lower. That ranking matters less than it appears, because a stream spends most of the year away from any single design point, and the flatter part-flow curve often delivers more energy across a year than the higher peak does.
Sources
Named organisations whose published material underpins this article. Where no link is given, the source is named rather than linked.
- U.S. Department of Energy, Water Power Technologies OfficeBackground on hydropower turbine types and small-scale hydro.
- International Renewable Energy Agency (IRENA)Hydropower technology overviews, including small and micro-scale schemes.
- Standard hydraulic machinery referencesSpecific speed, jet-to-runner speed ratios and part-flow efficiency curves are treated in the turbomachinery literature; values are design-specific.
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Last reviewed 8 October 2026. How we research and review