Deriaz Turbine: The Variable-Pitch Diagonal Pump-Turbine

The Deriaz turbine is an unusual hydraulic machine. Water flows through it neither straight along the shaft, as in a propeller or Kaplan turbine, nor inward from the rim toward the centre, as in a Francis turbine, but diagonally, a mixture of the two. Its runner blades can pivot while the machine is running, so it can keep a high efficiency across a wide range of flows and heads. And it was designed from the start to run in both directions: as a turbine that generates electricity and as a pump that lifts water back uphill.

Wikipedia describes it as having been presented by the engineer Paul Deriaz and as "the first diagonal hydraulic pump-turbine to be designed." Researchers at the Université Libre de Bruxelles, writing in the Journal of Energy Storage in 2019, put its place in the family of water turbines neatly: the Deriaz turbine is to the Francis turbine what the Kaplan turbine is to the fixed-blade propeller.

Deriaz machines are rare. Most were built around the 1960s, and only a few dozen units are known in pumped storage plants worldwide. They are interesting because of the problem they were designed to solve, which has become important again: how to store and release energy efficiently when conditions keep changing. To understand that, it helps to start with the wider family of water turbines and with pumped storage.

Background: the family of water turbines

Impulse and reaction turbines

Water turbines turn the energy of falling water into rotating shaft power. Wikipedia's article on water turbines divides them into two broad groups:

• Impulse turbines convert water pressure into a fast jet in a nozzle, so the pressure at the blades is constant. The jet strikes buckets on the runner. Impulse turbines do not need a sealed housing and are often used at heads above about 300 metres. The Pelton wheel is the best-known example.
• Reaction turbines run full of water, and the pressure of the water changes as it passes through the blades. They must be enclosed or fully submerged. Wikipedia describes them as suited to low heads (below about 30 metres) and medium heads (about 30 to 300 metres). Francis and Kaplan turbines are reaction turbines, and so is the Deriaz.

"Head" is the height through which the water effectively falls, and it is the single most important number in choosing a turbine.

Key inventions

Typical head ranges

Wikipedia gives these approximate ranges for common turbine types:

The Deriaz sits in the overlap between Kaplan and Francis. Morabito and colleagues explain why that overlap needed filling. They note that Kaplan turbines are not usable at high falls, of roughly 80 to 200 metres or more, because of mechanical and hydraulic limits, so Francis turbines are used there even though they perform poorly at part load.

Specific speed

Engineers compare turbine shapes using a quantity called specific speed. Wikipedia explains that it characterises a turbine's shape independent of its size, so a new design can be scaled from a known one, and that it is the main criterion for matching a site to a turbine type. Low specific speed machines, such as Pelton wheels, suit high heads and small flows. High specific speed machines, such as Kaplan turbines, suit low heads and large flows. Francis turbines cover the middle, and the Deriaz is a lower specific speed relative of the Kaplan with Francis-like geometry.

The basic power equation

Wikipedia gives the power available from a hydraulic turbine as:

P = η × ρ × g × h × Q

where η is efficiency, ρ is the density of water (about 1,000 kilograms per cubic metre), g is gravitational acceleration (9.81 metres per second squared), h is the head and Q is the volume of water flowing per second. Large modern turbines reach mechanical efficiencies above 90 percent. A worked example using real Deriaz figures appears later in this article.

The problem the Deriaz was designed to solve

Francis turbines at part load

A Francis turbine has fixed runner blades. Flow is controlled by adjustable guide vanes, called wicket gates, around the runner. That works well near the design point, but the angle at which water meets the blades is right only for a limited range of flows. Away from that range, efficiency falls and the flow becomes disturbed.

Morabito and colleagues describe the practical consequence. Standard Francis turbines often spend long periods at low openings, and therefore low efficiency. A Francis turbine of average specific speed reaches its maximum efficiency at about 80 percent of full load, whereas some duties call for a turbine whose best efficiency is at a much lower opening. Running off-design is not just wasteful. The same paper explains that flow separation and recirculation at part load can cause cavitation that damages the runner and vibrations that reduce bearing life.

Why not use a Kaplan?

The Kaplan turbine solved the part-load problem at low heads by making the runner blades adjustable. The controller moves both the guide vanes and the blades together, called double regulation, so the water always meets the blades at a good angle. Kaplan efficiency curves are therefore very flat.

The difficulty is head. Higher heads mean larger hydraulic forces on the blades, which need larger and stronger pivots, and more blades with longer chords to avoid overloading. Morabito and colleagues explain that in a Kaplan runner the hub geometry and the space for the blade mechanism limit the number of blades, with a maximum of about 10, and that this limits Kaplan turbines to heads of approximately 60 metres at reduced maximum efficiency.

Combining the two

The logical answer was to abandon the purely axial Kaplan arrangement and combine Francis-style blades with variable pitch. Morabito and colleagues record that the idea was first called, somewhat ambiguously, a "Francis à pas variable," a Francis with variable pitch, before it became known as the Deriaz turbine.

Paul Deriaz and the first machines

Publications and patents

The two key primary documents are both by Deriaz himself:

• A 1955 paper in the Bulletin technique de la Suisse romande, volume 81, titled "La turbine-pompe réversible axio-centrifuge à pas variable," roughly "The reversible axial-centrifugal variable-pitch pump-turbine."
• United States Patent 2801068, granted in 1957, titled "Control of feathering runner vanes of hydraulic turbines and pumps."

The French title is a good summary of the machine: reversible, mixed axial and centrifugal flow, and variable pitch.

English Electric

The British company English Electric built important early Deriaz installations. A 2009 IEEE paper by Tihomir Maricic and Don Haber of Ontario Power Generation and Stanislav Pejovic of the University of Toronto states that the Sir Adam Beck Pump Generating Station at Niagara Falls was originally constructed by the English Electric Company and that, with its Deriaz units, it is unique within Canada and one of very few in the world. Grace's Guide to British Industrial History records that in 1959 English Electric received an order for a 22,000 kilowatt vertical variable-blade water turbine and alternator for the Culligran power station in Scotland, and that the consultants and the hydro board agreed a Deriaz runner offered significant advantages over the Kaplan runner originally planned.

How a Deriaz turbine works

Diagonal flow

In a Deriaz runner, water does not move purely parallel to the shaft or purely toward it. It flows on a slant, through blades whose pivot axes are not at right angles to the shaft. Morabito and colleagues describe how reducing the specific speed relative to a Kaplan shortens the radial extent of the blade's leading edge and tips the flow from axial toward centripetal, inward toward the axis. The oblique position of the blade pivots gives the machine its mixed-flow character.

Spherical hub and skirt

The defining geometric feature, according to the same paper, is that the hub and the surrounding casing, called the skirt or runner chamber, have spherical surfaces. Spheres allow the blades to rotate on their pivots while keeping a narrow, nearly constant clearance at their tips and roots. That is a key departure from a Francis runner, whose surfaces are toroidal and whose blades are fixed.

The blade-turning mechanism

The runner blades are moved by a servomotor, normally a rotary type inside the turbine shaft or runner hub. Morabito and colleagues describe how the servomotor's rotation is transmitted through a piston rod to a "spider" inside the hub, and from the spider through slide blocks to each blade's stem. The 2009 IEEE paper lists the main parts of the Niagara machines: runner blades, blade levers, spider pivots, the spider, a rotating servomotor, a restoring tube, a return motion cam, stay vanes, flap servomotors, flaps, the draft tube and scroll casing, and guide bearings.

Changing shape with the blade angle

Because the blades pivot on slanted axes, turning them changes not only their angle to the flow but also the effective diameter of the runner. Morabito and colleagues point out that closing the blades reduces the maximum diameter, or more precisely its radial extension. For a pump running at constant speed, the discharge diameter controls the head the pump can produce, so this variation is a real advantage when the head changes. The optimum blade angle for higher heads is greater than for lower heads. This combined change of angle and effective diameter has no equivalent in a purely axial Kaplan runner.

The same paper adds that the blade pivot angle chosen in the design depends on the head: for progressively higher heads the pivot angle is smaller, moving the machine from a mainly axial configuration toward a more radial one. In that sense the Deriaz is a family of shapes spanning the gap between Kaplan and Francis.

Closing without a valve

Deriaz blades can be turned until adjacent blades touch along their whole edges, closing the runner completely. Morabito and colleagues note that, in theory, the machine can therefore be shut down without a conventional gate or inlet valve, which simplifies the plant and allows a smaller powerhouse at lower cost. The 2009 IEEE paper describes the same feature at Niagara, where the machines can be shut down without a conventional gate apparatus or inlet valve, and explains that the blade tips can be seated on the runner envelope during shutdown to reduce leakage.

Switching between pumping and generating

The reversibility is the most important practical feature. To change from pumping to generating, the 2009 IEEE paper explains, it is only necessary to feather the blades, stop and restart in the opposite direction, which takes only a few minutes, with the head gate left fully open. When starting as a pump, the closed blades reduce the torque needed to spin the runner at full speed so much that the synchronous motor can be started directly on line with the runner submerged well below the tailwater. For operation as a synchronous condenser, used to support grid voltage, no compressed air is needed to push the water down out of the runner. Starting as a turbine can be done by running up as a motor, so no governor synchronising is needed. Turbine output is then adjusted simply by controlling the blade angle.

Double regulation

Like a Kaplan, a Deriaz is double regulated. Morabito and colleagues explain that the combined governor setting of adjustable runner blades and guide vanes is what gives the Deriaz its high efficiency over a wide range of part loads. At the Naussac II plant in France, described below, the regulator uses two cams, one for pumping and one for generating, to give the best combination of blade and vane positions in each mode.

At Niagara, the guide vanes carry adjustable flaps that control the angle of flow entering the runner in turbine mode and are set according to the head. In pump mode the vanes act as a diffuser and the flaps are lined up with the vanes to avoid extra losses. Renewable Energy World's account of the station's rehabilitation noted that the Niagara units were commissioned with flaps that could take only two positions, a design feature its authors could not explain.

Pumped storage: the Deriaz turbine's natural home

What pumped storage does

Pumped storage hydroelectricity uses two reservoirs at different heights. When electricity is cheap or plentiful, water is pumped up. When it is needed, the water flows back down through turbines. Wikipedia's article on the subject gives round-trip efficiency, the share of the pumping energy recovered as electricity, as 70 to 80 percent.

According to Wikipedia:

• Pumped storage was first used in 1907 at Engeweiher, near Schaffhausen in Switzerland.
• The first pumped storage plant in the United States was built in 1930 by the Connecticut Electric and Power Company near New Milford, pumping water from the Housatonic River to a reservoir 70 metres higher.
• Reversible turbine and generator assemblies, which work as both pump and turbine, became available in the 1930s. Francis designs are the usual choice.
• In 2020, pumped storage made up about 95 percent of active grid storage installations, with more than 181 gigawatts of power capacity and more than 1.6 terawatt hours of storage.
• The International Hydropower Association's 2025 figure is nearly 200 gigawatts of power and up to 9,000 gigawatt hours of storage.

The largest plants are vast. Wikipedia lists Fengning in China at 3,600 megawatts and 40 gigawatt hours, Bath County in the United States at 3,003 megawatts and 24 gigawatt hours, and Ludington in the United States at 1,872 megawatts and 20 gigawatt hours.

Why a Deriaz suits pumped storage

Morabito and colleagues argue that the operating conditions of pumped storage are perfectly suited to a reversible machine. A Deriaz reverses by changing its direction of rotation, adapts quickly to demand, and saves powerhouse space because one machine does both jobs. In small schemes, the head changes noticeably as the upper reservoir fills and empties, and the Deriaz's adjustable blades let it stay efficient across that range at fixed speed. The 2009 IEEE paper adds that the wide operating range at synchronous speed, without extra electrical equipment, reduces the risk of failure in regulating power in either mode.

Why it remained rare

The same authors are frank about why few were built. Deriaz pump-turbines were installed mostly around the 1960s, and Francis turbines were usually preferred because the double-regulation mechanism is complex and maintenance costs are high. Wikipedia notes that fixed-blade Deriaz designs have since been developed to avoid that complexity, controlling flow instead by varying runner speed with inverters. Morabito and colleagues also note that, today, only Francis units with variable speed compete with Deriaz pump-turbines across both operating modes, and the 2009 IEEE authors suggested that the mechanically complex Deriaz machine should be compared with variable-speed machines when deciding how to improve existing units.

Known installations

List of pumped storage plants

Morabito and colleagues compiled a list of pumped storage plants using Deriaz pump-turbines. Their table gives the following. The size column appears to be the rating of each unit, since six units of 29 megawatts at Adam Beck matches the station's widely reported 174 megawatt total. The paper gives 1954 for Adam Beck, while the IEEE paper and other sources give 1957 for the pump generating station, so years should be treated as approximate.

The table contains some details that sit oddly with other sources: for example, the Naussac entry lists one unit in 1995, whereas the same paper's own description of Naussac II says three identical machines commissioned in 1983. These differences are left as published. The overall picture is clear, though: Deriaz machines were built on several continents, Japan installed more than any other country in the list, and most date from the 1960s and 1970s.

Sir Adam Beck is the example named in Wikipedia's Deriaz article.

Case study: Sir Adam Beck Pump Generating Station, Niagara Falls

Layout and purpose

The Sir Adam Beck complex at Niagara Falls in Ontario is operated by Ontario Power Generation (OPG). Wikipedia lists a total nameplate capacity of 1,962 megawatts across 26 operating units: Sir Adam Beck I, which began producing power in 1922, Sir Adam Beck II, which began in 1954, and the pump generating station, rated at 174 megawatts.

The 2009 IEEE paper describes the pump generating station as a six-unit station commissioned in 1957 with 174 megawatts of capacity. Morabito and colleagues add that it was built at the same time as Beck II with a 300 hectare reservoir, and that its six mixed-flow, variable-pitch Deriaz pump-turbines can fill the reservoir in about eight hours.

The treaty and the night shift

The station exists partly because of a treaty. Under the 1950 Niagara Treaty between Canada and the United States, a minimum flow must go over the falls for scenic reasons. Wikipedia gives these minimums as 2,832 cubic metres per second during daytime in the tourist season, 1 April to 31 October, and 1,416 cubic metres per second at night and off-season. More water can therefore be diverted for power at night than during the day. The pump generating station takes advantage of this by pumping diverted water into its reservoir at night and generating with it during the day.

The IEEE paper explains further functions. Water reaches the Beck stations through an open canal and two tunnels that meet at a "crossover." The pump generating station withdraws from or discharges to that crossover, and its ability to pump rapidly is fundamental to holding the crossover water level at the correct elevation of 165 metres. When it generates, the six units can supply up to 174 megawatts directly. Their discharge also raises the head pond for Beck I and II, which operate at a head of 89 metres, increasing those stations' output by around 90 megawatts. The authors put the overall benefit at approximately 300 megawatts of additional generation. They reported that in 2008 the station delivered 90 gigawatt hours, and that the extra water passed through Beck I and II raised their production by 1.2 terawatt hours.

The authors also explained the station's role in a grid with a large nuclear component that cannot easily change output. When demand falls below what the nuclear plants produce, pumping provides a controllable load.

Refurbishment

The station has been refurbished in stages. Renewable Energy World describes runner rehabilitation on five of the six units, including work on the flap control design. Wikipedia records that a CAD 60 million refurbishment of the reservoir, built in 1957, began in 2016. In April 2024, OPG announced a C$1 billion, 15-year programme with GE Vernova to refurbish 26 units at Sir Adam Beck I, Sir Adam Beck II and the pump generating station, starting in 2025, with each unit taking about a year. POWER magazine reported that the work could increase the complex's capacity by up to 50 megawatts, but that OPG described the investment as mainly about extending the stations' lives.

Case study: Culligran, Scotland

Culligran power station lies in Glen Strathfarrar in the Scottish Highlands and is part of the Strathfarrar hydro-electric scheme built by the North of Scotland Hydro-Electric Board. As noted above, English Electric received the order for its variable-blade turbine in 1959, and a Deriaz runner was chosen in place of the planned Kaplan.

Morabito and colleagues describe Culligran as having a 17 megawatt, 10-blade Deriaz turbine alongside a 2 megawatt Francis turbine. Culligran is used for generation rather than pumped storage, showing that the Deriaz's flat efficiency curve was valued in its own right.

By the early 2000s the Deriaz unit was 43 years old and the runner chamber was suffering severe cavitation. Scottish and Southern Energy (SSE) ran an 18-month refurbishment intended to extend the plant's life by a predicted further 30 years without further major refurbishment. Water Power magazine reported in June 2004 that VA Tech Hydro, working with Weir Engineering Services, had won a turnkey contract worth US$17 million. The scope for unit 2 included a new 17.3 megawatt Deriaz runner, main inlet valve, hydraulic and digital governor, brushless excitation and new generator auxiliaries, while unit 1 received a new 2 megawatt Francis runner. The station was expected back in operation in September 2005.

The precision involved shows why Deriaz machines are demanding. According to Morabito and colleagues, components were made at 12 manufacturing facilities across Europe, and particularly high machining accuracy was needed to keep the gap between the blades and the chamber wall within 1.5 millimetres. Too large a gap invites cavitation and leakage; contact between blades and wall would be disastrous. Modern design tools, including computer-aided design, computational fluid dynamics and finite element analysis, were used to design and test the new runner, and a scale model of the turbine was built.

Case study: Kuromatagawa II, Japan

Kuromatagawa II power station in Japan adopted a vertical-shaft Deriaz pump-turbine in 1963, according to Morabito and colleagues, to cope with a large head variation and to stay efficient at part load. The maximum head was twice the minimum operating head of 39 metres.

The plant combined the adjustable blades with a two-speed generator-motor, using pole changing. The Deriaz ran at 333 revolutions per minute when pumping at high head and at 300 revolutions per minute as a turbine, over effective heads from 78 down to 39 metres, with a maximum flow of 28 cubic metres per second. The authors note that the ratio of unit speeds at best efficiency between turbine and pump operation was about 1.1, similar to a Francis pump-turbine.

Case study: Naussac II, France

Purpose

Naussac II, in France, is an example of a Deriaz installation where generating electricity is not the main goal. According to Morabito and colleagues, it is the second phase of the Naussac reservoir development, which maintains low-water flows in the Allier river and in the Loire below their confluence. The machines were optimised for pumping, to fill the reservoir, with hydroelectric production a by-product. The reservoir has a capacity of 190 million cubic metres, and the installation was commissioned in 1983.

Why a Deriaz was chosen

The authors report that the Deriaz option was preferred over a two-speed or multi-speed Francis machine, which would have required a much larger pumping basin and intake works on the dam more than 10 metres high. With Deriaz machines, the intake works needed to be only 2.30 metres high, which they describe as much better ecologically and for the landscape. Polluting fluids such as oil from moving parts in contact with the water were partly eliminated, and the blade controls, hydrostatic seal and downstream gates have a waterproof ceramic coating.

Technical data

Each unit drives a conventional asynchronous motor-generator. Morabito and colleagues report that the machines are relatively noisy until the flow reaches about 3.5 cubic metres per second, after which they become quiet. Model tests had shown that the noise is caused by cavitation, which was considered acceptable as long as the runner blades are made of stainless material to limit damage.

Worked example: checking the turbine figures

The paper's turbine-mode data give, per unit, a flow of 5.4 to 6.0 cubic metres per second at a net head of 50 to 57 metres, with a hydraulic efficiency of 92.8 percent and a maximum power of 2.65 megawatts. Using the power equation with the larger flow and the lower head:

• ρ × g = 1,000 × 9.81 = 9,810
• × Q = 9,810 × 6.0 = 58,860
• × h = 58,860 × 50 = 2,943,000 watts of hydraulic power
• × η = 2,943,000 × 0.928 ≈ 2,731,000 watts, or about 2.73 megawatts

That is close to the stated 2.65 megawatt maximum. The small difference is expected, because the result above is shaft power from hydraulic efficiency only, before generator and other losses, and because the paper gives ranges rather than a single paired operating point.

Worked example: checking the pump figures

In pump mode the relationship is reversed: the machine must supply more power than it puts into the water. The power required is:

P = ρ × g × Q × H ÷ η

Using a flow of 4.6 cubic metres per second at the top of the paper's quoted head range of 58.2 metres, and a pump efficiency of 85.5 percent:

• 9,810 × 4.6 = 45,126
• × 58.2 ≈ 2,626,300 watts delivered to the water
• ÷ 0.855 ≈ 3,072,000 watts, or about 3.07 megawatts

That matches the nominal pumping power of 3 megawatts per unit.

Modern research: small Deriaz machines

A micro pump-turbine

The 2019 paper by Alessandro Morabito, Guilherme de Oliveira e Silva and Patrick Hendrick of the Université Libre de Bruxelles, "Deriaz pump-turbine for pumped hydro energy storage and micro applications," revisited the design with modern tools. The authors noted that Deriaz pump-turbines are largely absent from the literature and that their performance data have often been kept confidential by manufacturers.

They designed a downsized model, rated about 6 to 21 kilowatts, suitable for micro-scale pumped storage, based on the geometry of the Naussac II prototype scaled to a reference diameter of 0.3 metres. They presented what they described as a practical and easily built method for making a micro Deriaz runner with a yield of about 90 percent, supported by extensive computational fluid dynamics simulations validated against experimental data.

Results

In their conclusions, the authors reported that:

• In pump mode, hydraulic efficiency stayed at a minimum of 80 percent over a wide range of discharges, down to about 40 percent of design flow.
• In turbine mode, the peak hydraulic efficiency was modelled at 91 percent with the guide vanes open.
• Combining variable blade pitch with moving guide vanes gave a wider range of highly efficient conditions, and the downsized machine kept its versatility over a wide range of part loads.

The work was supported by the Walloon Region of Belgium under its SmartWater project. Wikipedia's Deriaz article summarises this research as showing the machine's versatility over a wide range of partial loads in both pumping and generating modes.

Why small-scale storage matters

The authors placed their work in the context of decentralised energy systems. Micro hydropower, which they define as 5 to 100 kilowatts, is renewable, long-lived and available in rivers, canals and artificial basins. Small pumped storage systems have relatively small reservoirs, so their head changes significantly as they fill and empty. A machine that stays efficient as conditions change, without expensive variable-speed electronics, could be useful there.

Fixed-blade and variable-speed alternatives

Because the adjustable blade mechanism is the most complex and costly part of a Deriaz machine, Wikipedia notes that fixed-blade Deriaz turbines have been developed, with variable flow handled by changing the runner's speed using inverter technology. The German company DIVE Turbinen markets a turbine of this kind for medium heads, and Wikipedia cites its documentation giving a head range of 20 to 120 metres.

This reflects a broader shift in pumped storage. In the 1950s and 1960s, varying the speed of a large machine connected to the grid was difficult, so a mechanical solution, pivoting blades, was attractive. Today, power electronics can vary machine speed. Wikipedia's pumped storage article notes that variable-speed operation further improves round-trip efficiency. Morabito and colleagues observe that variable-speed Francis units are now the main competitor to the Deriaz in both operating modes.

Comparison with other reaction turbines

Operation, maintenance and failure modes

Cavitation

Cavitation is a constant concern for reaction turbines. Wikipedia's water turbine article lists pitting from cavitation, fatigue cracking and abrasion from suspended solids as normal forms of wear, typically repaired by welding with stainless steel. The Culligran refurbishment was driven by severe cavitation in the runner chamber, and Naussac II's machines are noisy from cavitation at low flows. In a Deriaz, the clearance between the blade tips and the spherical chamber is critical. Culligran's specification of a gap no larger than 1.5 millimetres shows how tight it has to be.

Mechanism wear

The blade mechanism, with its servomotor, spider, slide blocks and blade trunnions, works inside the runner hub under water pressure. It is the reason Morabito and colleagues cite high maintenance costs as a reason Francis machines were usually preferred. Sealing the mechanism matters too: Naussac II's use of ceramic-coated blade controls and hydrostatic seals was partly aimed at keeping oil out of the water.

General overhauls

Wikipedia notes that water turbines run for decades with little maintenance of their main elements, with overhauls every several years. Items inspected include bearings, packing boxes and shaft sleeves, servomotors, cooling systems, seal rings and wicket gate linkages, all of which apply to Deriaz machines along with the blade mechanism.

Common misconceptions

"A Deriaz is just a Kaplan with a different hub"

Not quite. A Kaplan is purely axial. A Deriaz has diagonal flow, slanted blade pivots and spherical hub and chamber surfaces. Turning its blades changes its effective diameter as well as its blade angle, which a Kaplan cannot do.

"Pump-turbines must use Francis runners"

Francis designs are indeed the usual choice for reversible pump-turbines, as Wikipedia's pumped storage article states. But the Deriaz was specifically conceived as a reversible pump-turbine, and Morabito and colleagues note that Deriaz himself first devised it as a turbine and then as a pump.

"Deriaz turbines are obsolete"

They are rare, and few have been built recently, but the 2012 Maharashtra entry in the Morabito list, the multi-decade life extensions at Culligran and Niagara, and research into micro Deriaz machines show continuing interest.

"Adjustable blades are always better"

They give flat efficiency curves, but at the cost of mechanical complexity, demanding manufacturing tolerances and higher maintenance. Variable-speed electronics now offer another way to achieve flexibility, which is why fixed-blade, variable-speed diagonal machines have been developed.

"The Deriaz is named after its flow pattern"

It is named after its inventor, Paul Deriaz. The descriptive names are "diagonal" or "mixed-flow" turbine.

Frequently asked questions

What is a Deriaz turbine?

A reaction water turbine with diagonal flow and adjustable runner blades, designed to work both as a turbine and as a pump. It was presented by Paul Deriaz in the 1950s.

Who invented it, and when?

Paul Deriaz. His key paper appeared in the Bulletin technique de la Suisse romande in 1955, and his US patent on controlling feathering runner vanes was granted in 1957.

How is it different from a Francis turbine?

A Francis turbine has fixed blades, so its efficiency falls when flow moves away from the design point. A Deriaz has adjustable blades and double regulation, giving high efficiency across a much wider range of flows and heads.

How is it different from a Kaplan turbine?

A Kaplan is axial-flow and limited to lower heads, roughly up to about 60 metres according to Morabito and colleagues. A Deriaz uses diagonal flow and Francis-like blades on slanted pivots, allowing adjustable-blade operation at higher heads.

Where can you find Deriaz turbines?

Examples include the Sir Adam Beck Pump Generating Station at Niagara Falls in Canada, Culligran in Scotland, Kuromatagawa II and other plants in Japan, Valdecañas in Spain, Naussac II in France, Niedzica and Czorsztyn in Poland, Liptovská Mara in Slovakia and Dos Amigos in the United States.

Why are they rare?

Their blade mechanism is complex and expensive to build and maintain. Francis machines were usually preferred, and today variable-speed Francis machines offer another route to flexibility.

Can a Deriaz turbine shut off water without a valve?

In principle, yes. Its blades can close until adjacent edges touch, and the Niagara machines can be shut down without a conventional gate or inlet valve.

How quickly can it switch between pumping and generating?

At Niagara, the IEEE paper describes the changeover as feathering the blades, stopping and restarting in the opposite direction, taking only a few minutes.

How efficient is it?

Published figures for the Naussac II prototype give hydraulic efficiencies of 92.8 percent as a turbine and 85.5 percent as a pump. The ULB micro-scale model reached a modelled turbine peak of 91 percent and at least 80 percent in pump mode over a wide flow range.

What is pumped storage, and how efficient is it?

Pumped storage stores energy by pumping water uphill and recovers it by letting the water flow back through turbines. Wikipedia gives round-trip efficiency as 70 to 80 percent.

Summary

The Deriaz turbine is the hydropower world's answer to a specific gap: medium heads, variable conditions and the need to both pump and generate efficiently. Paul Deriaz combined Francis-style blades with Kaplan-style variable pitch, set them on slanted pivots in a spherical hub and chamber, and produced a diagonal-flow machine whose shape effectively changes with its blade angle. English Electric built the first major installations, including six units at the Sir Adam Beck Pump Generating Station at Niagara Falls, commissioned in 1957 and still working, and a variable-blade unit at Culligran in Scotland, refurbished after four decades. Plants in Japan, Spain, Poland, Slovakia, France, India, China, Colombia and the United States followed, mostly in the 1960s and 1970s. The machine's drawbacks are mechanical complexity, tight tolerances and maintenance cost, and variable-speed electronics now offer a competing route to flexibility. But as grids take on more variable renewable generation and pumped storage becomes more valuable, researchers have returned to the Deriaz idea, including at the micro scale, because its core strength, high efficiency across a wide range of operating conditions, is exactly what flexible storage needs.

Sources

• Deriaz turbine (Wikipedia)
• Morabito, de Oliveira e Silva and Hendrick, Deriaz pump-turbine for pumped hydro energy storage and micro applications, Journal of Energy Storage 2019 (ULB repository)
• Maricic, Haber and Pejovic, Niagara Pump Generating Station Proven Functionality Unique in Canada, IEEE EPEC 2009
• Deriaz, La turbine-pompe réversible axio-centrifuge à pas variable, Bulletin technique de la Suisse romande 1955
• Deriaz, US Patent 2801068, Control of feathering runner vanes of hydraulic turbines and pumps (Espacenet)
• Sir Adam Beck Hydroelectric Generating Stations (Wikipedia)
• POWER magazine: OPG Embarks on Major Refurbishment of Niagara Falls Hydropower Stations (2024)
• Renewable Energy World: Rehab at Sir Adam Beck
• Grace's Guide: Culligran Power Station
• Water Power magazine: Culligran refurbishment contract (2004)
• Pumped-storage hydroelectricity (Wikipedia)
• Water turbine (Wikipedia)
• DIVE Turbinen: DIVE-HAX turbine for medium head