1. What the simulator shows
Tidal stream energy relies on a highly predictable resource: tidal currents. However, predictability does not mean constant production.
Current speed varies significantly during a tidal cycle, and also between neap-tide and spring-tide periods. The simulator helps visualise three essential points:
- instantaneous power depends very strongly on current speed;
- mechanical loads increase rapidly as the current accelerates;
- an installed capacity expressed in MW does not correspond to continuous production in MW.
Installing a 1 MW tidal turbine does not therefore mean producing 1 MW continuously. Actual production is measured by the energy effectively generated over time, expressed in MWh.
2. From current speed to recoverable power
The power available in a marine current mainly depends on:
- the density of seawater;
- the area swept by the rotor;
- the turbine’s power coefficient;
- and, above all, the speed of the current.
The simplified physical relationship can be written as:
P = 1/2 × ρ × Cp × A × V³
where:
- P is the recoverable power;
- ρ is the density of seawater;
- Cp is the turbine’s power coefficient;
- A is the area swept by the rotor;
- V is the current speed.
The key point is the presence of V³. A seemingly modest difference in current speed can lead to a very large difference in power.
For example, with comparable rotor area and technology, a current of 4 m/s contains eight times more kinetic power than a current of 2 m/s. This is why only the fastest sites can reasonably aim for industrial profitability, subject of course to installation, maintenance, grid-connection costs and operating conditions.
Power coefficient Cp
The Cp indicates the proportion of the current’s energy that is effectively converted by the rotor. It should not be confused with the total power available in the current, nor with the final electrical efficiency of the complete system.
The power contained in a marine current cannot be fully recovered by a tidal turbine. The rotor slows the water passing through it, but the fluid must still continue to flow downstream. A machine that completely stopped the current would no longer produce anything. This is why a power coefficient, noted Cp, is introduced.
This coefficient expresses the share of the current’s kinetic power that is converted into mechanical power on the rotor shaft, before losses in transmission, generator and power electronics. A Cp of 0.40 therefore means that approximately 40% of the power available across the rotor swept area is captured as mechanical power.
The classical Betz limit gives a theoretical maximum power coefficient of 59.3% for an ideal rotor in an unconfined flow. It provides a useful physical reference, although its interpretation must be qualified in a channel where blockage effects may occur.
3. Rated power, rated speed and rotor diameter
The rated power of a tidal turbine is the maximum power it can deliver under specified conditions. It must always be associated with a rated current speed.
A machine advertised as 1 MW does not have the same meaning depending on whether it reaches that power at 2.5 m/s, 3.5 m/s or 4.5 m/s.
To reach a given rated power at a lower current speed, it is generally necessary to increase the area swept by the rotor, and therefore the diameter or useful dimensions of the machine. This strategy can improve production during intermediate-current phases, but it also increases mechanical loads and structural dimensions.
Energy sizing cannot therefore be separated from mechanical sizing.
Rated power and rated speed: what does “a 1 MW tidal turbine” mean?
A tidal turbine rated at 1 MW does not produce 1 MW continuously. Its power output depends in particular on the speed of the water flowing through its rotor.
The rated current speed is the current speed at which the machine reaches its rated power. These two characteristics must therefore be presented together, for example: “1 MW at 3 m/s”.
In the simulator’s simplified model:
- Below rated speed, power increases strongly with current speed: it is proportional to the cube of that speed.
- At rated speed, the stated power is reached.
- Above rated speed, power output is capped at its rated value. The forces exerted by the current may nevertheless continue to increase.
Thus, in this simplified model, a 1 MW turbine rated at 3 m/s produces only about 125 kW at 1.5 m/s: halving the current speed divides power by eight.
Rated speed is not the turbine’s cut-in speed, nor the maximum current speed at the site, nor its mechanical survival limit.
Why not always choose a low rated speed?
Reaching the same rated power in a slower current requires, for the same power coefficient, a larger rotor swept area. This makes it possible to capture more energy at intermediate speeds, but it also leads to larger dimensions and greater mechanical constraints.
In the simulator, changing rated speed while keeping rated power constant therefore amounts to comparing different designs. The best compromise depends on the available current resource, the expected annual energy production, and the costs of construction, installation and maintenance.
This representation remains educational: a real machine has its own power curve, a cut-in threshold and operating limits.
4. When production becomes a structural issue
The forces exerted by the current on a turbine and its supporting structure vary approximately with the square of the current speed.
F ≈ 1/2 × ρ × Ct × A × V²
where:
- F represents the hydrodynamic force;
- Ct is the rotor thrust coefficient;
- A is the exposed area;
- V is the current speed.
In very high-current areas, the challenge is therefore not only to capture energy. It is also necessary to install, maintain and remove machines capable of withstanding significant loads over the long term.
This is where the support architecture becomes decisive. A shared, stable, ballastable and recoverable structure can help reduce heavy offshore operations, mutualise certain functions and facilitate the operation of a group of machines.
5. Using current speeds from nautical documents
Tidal stream atlases and nautical documents often provide current speeds for reference conditions: mean neap tides and mean spring tides.
For educational purposes, it is possible to estimate the current speed corresponding to an intermediate tidal coefficient by interpolation between two reference values, for example:
V45: current speed at mean neap tide;
V95: current speed at mean spring tide.
The estimated speed for a tidal coefficient C can then be approximated by:
V(C) = V45 + [(C – 45) / (95 – 45)] × (V95 – V45)
This method makes it possible to connect the simulator to nautical data that can be understood by seafarers, engineering firms and decision-makers. It must, however, be used with caution: it does not replace site measurements or detailed hydrodynamic modelling.
6. From raw resource to exploitable site
A powerful current is not enough to define a good industrial site.
Feasibility also depends on:
- water depth;
- bathymetry;
- seabed conditions;
- turbulence;
- wave and sea-state conditions;
- installation and maintenance possibilities;
- distance to electrical grid connection;
- existing uses: fishing, navigation, easements and regulated areas;
- environmental constraints.
An exploitable tidal-stream site is therefore the result of a compromise between energy resource, mechanical constraints, intervention costs, grid connection and acceptability.
The transition from raw resource to exploitable potential therefore requires a broader analysis than simply reading current-speed or power curves.
7. High-potential areas in the western English Channel and geographical phase shift
The western English Channel includes several areas where tidal currents are of particular interest:
- the Raz Blanchard / Alderney Race;
- the Barfleur / Cap Lévi area;
- parts of the waters around the Channel Islands, including the Casquets and the Great Russel and Little Russel channels;
- parts of the Norman-Breton Gulf.
These areas are not perfectly synchronised. Maximum current speeds do not necessarily occur at the same time.
Combining production from geographically phase-shifted sites could therefore help produce a smoother aggregate output than production from a single isolated site.
This perspective is important because the potential contribution of tidal energy should not necessarily be considered only at the scale of one turbine or one individual site, but also at the scale of a geographically distributed group of complementary sites.
Such an approach would nevertheless require detailed hydrodynamic, electrical and economic studies before its actual benefits could be quantified.
8. Limitations of the simulator
The HydreManche® simulator is deliberately simplified.
It does not model in detail:
- local turbulence;
- vertical variation of current velocity through the water column;
- full wake interaction between turbines;
- detailed electrical losses;
- actual turbine availability;
- maintenance downtime;
- site-specific regulatory constraints;
- actual installation, operating and decommissioning costs.
The displayed results should therefore be interpreted as orders of magnitude intended to help understand the underlying physical relationships, not as a contractual energy-yield or performance forecast.
Schematic representations and Year mode
The Tide, Day and Typical week modes are primarily educational.
They provide a rapid visual representation of how changes in current speed affect recoverable power and mechanical loads.
When the site’s maximum current speed is selected using the simulator control, the resulting curve should not be interpreted as an exact reproduction of a measured current profile.
It represents a simplified current pattern intended to illustrate orders of magnitude.
The Year mode follows a different approach.
It is intended to provide a representation closer to annual behaviour, based on values considered representative of a high-current area near Pointe de Goury and the Raz Blanchard / Alderney Race.
This annual representation nevertheless remains simplified.
It does not replace measured current time series, detailed hydrodynamic modelling or a professional energy-yield assessment.
Its main purpose is to show why annual energy production cannot be calculated simply from an average current speed, but results from integrating successive variations in current speed over time.
9. Further reading
The Reference documents page provides additional material on:
- tidal-stream resource potential;
- tidal currents;
- site constraints;
- orders of magnitude for power and energy;
- tidal turbine array modelling;
- the technical and industrial context of the sector.