Propeller turbine: a 16 m tidal turbine designed Tidal and wind turbine simulated in CFD Kaplan turbine runner modeled in Heliciel

Hydraulic energy capture by propeller or Kaplan turbine 1/3: components and operation

Harnessing hydraulic energy with a propeller or Kaplan turbine turns the head of a site, a pressure, into torque on a shaft. Four components work together: the spiral casing that brings the water in, the distributor that sets it in rotation and regulates the flow, the runner that takes the energy, the draft tube that recovers what is left. This first part describes these components, the types of turbines, the power of a site and the efficiencies; the next two deal with the relation between distributor and propeller and the design method.

Operation of a Kaplan propeller turbine: spiral casing, distributor, runner and draft tube

A Kaplan turbine: the water comes in through the casing, rotates in the distributor, crosses the runner and leaves through the draft tube.

Hydroelectric turbines chapter:

Contents of this page:

1: Why an accessible method

Harnessing hydraulic energy is an example of inventiveness: the principles tamed over the centuries have given the most modern power plants. In the race for clean energy, a hydraulic turbine remains a way to produce localized energy, sized in harmony with its site. But the profitability of the project requires a reasoned design of each component, and many small sites are neglected because the literature, rich as it is, remains reserved for a mathematical audience. Yet mathematics is only a reflection of physical phenomena: it must be possible to describe them in natural language.

The computations remain unavoidable. We entrust them to Heliciel, which computes the runner by blade element theory (BEM, Blade Element Momentum) and sizes the structure around it in its Hydro Turbine design workshop: casing, distributor, draft tube, head losses and head balance of the water path. We thus stay focused on the physical phenomena and the energy transformations. This article offers a complete method, tools, and an example tutorial to put the design of a small propeller power plant within everyone's reach. It remains open to your contributions (contact@heliciel.com).

The useful mathematical foundations are gathered in a reminder on the relation between power, energy, work and flow, to be consulted alongside.

Heliciel 3D perspective view of a 12 m3/s Kaplan turbine: spiral casing, distributor, runner and water trajectories coloured by velocity

The example installation in 3D perspective: the spiral casing, the distributor, the runner, and the water trajectories coloured by velocity, blue in the spiral, green and yellow at the runner.

Hydro Turbine interface of Heliciel (requires a Mecaflux Suite license)

2: Impulse and reaction turbines: Pelton, Francis, Kaplan

Harnessing the energy of water takes various forms depending on the power of the site. The two determining parameters are the head and the flow, and two families are distinguished.

Impulse turbines work only with the kinetic energy of the fluid, at ambient pressure: the Pelton turbine, made for high heads and low flows, or the Crossflow turbine.

Impulse turbines: Pelton wheel and Crossflow turbine, kinetic energy of a jet at ambient pressure

Impulse turbines: a jet, a wheel, atmospheric pressure.

Reaction turbines combine kinetic energy and pressure energy, that is the pressure difference between the inlet and the outlet of a turbine placed in a conduit: the Francis turbine suits medium heads and velocities, the Kaplan or propeller turbine suits low heads and high flows.

Reaction turbines: Francis for medium heads, Kaplan or propeller for low heads and high flows

Reaction turbines: Francis and Kaplan work in a conduit, under pressure.

The Heliciel site is dedicated to propellers: we therefore detail the design of bulb, Straflo, Kaplan or propeller turbines, which turn hydraulic power into mechanical power through a propeller. The specific speed tells, for your head and your flow, which of the three families is suitable: Heliciel computes it and locates it on the manufacturers' chart.

3: The hydraulic power of the site: gross head, net head, head losses

The hydraulic power of the site depends on the height difference H, in meters, between the surfaces of the upstream and downstream basins, and on the flow Q, in m³/s, that passes through the installation:

P (W) = Q (m³/s) × H (m) × ρ (kg/m³) × g (m/s²)

with ρ the density of water and g the acceleration of gravity. A 12 m³/s site under 4 m thus carries 12 × 4 × 999.7 × 9.81 = 470,750 W, that is 471 kW gross: this is the tutorial example.

Let us distinguish the gross head from the net head. On its way down from the upstream basin to the downstream basin, the water rubs against the walls and meets obstacles that take energy from it. This loss of energy in the conduits is called the head loss and is expressed in meters: everything happens as if the head were lower.

Gross head and net head of a turbine: the head losses of the conduit reduce the useful head

Gross head, head losses, net head.

We therefore speak of gross power or net power depending on the head put into the formula. The net power, ρ·g·Q·Hn, is the one the runner can receive: Heliciel calls it the available power.

The axial velocity of the water in the runner is derived from the flow and the swept section: V (m/s) = Q (m³/s) / S (m²), S being the annulus between the hub and the blade tips.

Annular section swept by the propeller of a turbine between the hub and the blade tips

The section swept by the propeller: an annulus between the hub and the blade tips.

As it crosses the runner, the water gives up energy to it: its pressure falls from inlet to outlet, and this pressure drop, for a given flow, is what the runner absorbs. The net head must therefore be enough for the head the runner consumes, draft tube included: this is the equilibrium balance we will see in the method.

4: Efficiency of the turbine, the generator and the installation

To obtain electricity, we turn hydraulic power into mechanical power on the shaft, then into electrical power in a generator. None of these components is perfect.

Hydro generator: turbine and electrical generator coupled

On the tutorial example, Heliciel returns 390 kW at the shaft for 402 kW available and 471 kW gross: 85% for the path, 97% for the runner, 83% in total before the generator. These values are those of the model at the design point, to be compared with an engineering study of the structure.

5: The four components: casing, distributor, runner, draft tube

The runner, or turbine propeller, is the rotating part that converts the energy of the water into mechanical energy. In a conduit, it works under pressure: its wide blades must "block" the passage so that the pressure drop can build up across it. Heliciel computes it by blade element theory in penstock mode, where the stream tube is imposed by the shroud and the blade tip losses are neutralized by the small clearance.

The distributor is a ring of profiled, adjustable vanes, the guide vanes, which direct the water and control its rotation, the tangential component of its velocity, according to their opening. It also serves as the flow regulating valve. On bulb turbines, the ring of guide vanes alone provides the rotation; on Kaplan turbines, the distributor is placed at the center of a volute that has already given the stream its tangential path, and the guide vanes complete its work. The deflection imposed by the vanes costs a head loss: it must stay minimal, and a strong deflection calls for a volute.

Guide vane distributor of a bulb turbine

A bulb turbine distributor: the ring of guide vanes alone sets the water in rotation.

Vortex created by the distributor of a Kaplan turbine upstream of the runner

The vortex the distributor gives the water before the runner.

Kaplan turbine: the distributor precisely controls the rotation and the flow upstream of the runner

On a Kaplan, the volute and the distributor share the task of setting the water in rotation.

The spiral casing wraps around the distributor and distributes the flow all around it, keeping the water velocity constant from one section to the next. Heliciel draws it in eight sections and computes its allowable velocity according to its material.

The draft tube, or diffuser, is a divergent cone placed behind the runner. It slows the water down to the downstream basin and thus turns the residual velocity into a depression at the runner outlet: the runner sees a head larger than its mere height above the downstream level. This recovery system is detailed in the third part of the tutorial, devoted to the draft tube.

Diffuser or draft tube of a Kaplan propeller turbine: divergent cone that recovers the residual velocity downstream of the runner

The draft tube: a cone that slows the water down and lowers the pressure behind the runner.

Dimensioned front vertical 3D cut of a Kaplan turbine in Heliciel: casing, runner, bulb, elbow draft tube and water particles

The dimensioned front cut: the particles swirl in the casing, dive through the runner and slow down in the elbow draft tube, below tailwater level; the runner, bulb, H2 and gross head dimensions are shown.

Heliciel 3D close-up of a Kaplan turbine distributor: gate ring, guide vanes and water particles coloured by velocity

Close-up of the distributor: the gate ring, the guide vanes and the water particles, blue in the casing, green and yellow as they accelerate towards the runner.

Elbow draft tube with rectangular outlet of a Kaplan turbine in Heliciel, water trajectories and tailwater

The elbow draft tube with its rectangular outlet, seen from downstream: the trajectories slow down and spread out to the river.

After this presentation of the components, let us see how to size them and evaluate the performance of the site: the relation between the distributor and the propeller. To go further, we can also open directly the user guide of Hydro Turbine design.