How this tidal energy calculator works
There are two very different ways to turn tides into electricity, and the calculator models both.
Tidal range (barrages and lagoons) trap water behind a wall at high tide and release it through low-head turbines once a height difference has built up. In effect it is hydropower driven by the Moon. The energy available scales with the basin area and the square of the tidal range.
Tidal stream turbines sit in fast currents, like underwater wind turbines. Their power scales with rotor area and the cube of current speed. Seawater is about 840 times denser than air, so an 18 m tidal rotor in a 3 m/s current produces as much as a wind rotor of 70–80 m in a 12 m/s wind, but only where currents are strong.
Tides follow the 12.42-hour lunar semi-diurnal cycle, and their amplitude swells and fades over the 14.77-day spring–neap cycle. The calculator simulates a whole year: every tide for a barrage, and every 10 minutes for a stream turbine, with range or peak current varying between your spring and neap values.
The formulas
Tidal stream: P(t) = min(P_rated, ½ × ρ × A_rotor × Cp × |u(t)|³ × η) u(t) = U_peak(t) × sin(2π t / 12.42 h), U_peak varies between spring and neap over 14.77 days
ρ = 1,025 kg/m³ for seawater. The extraction factor (default 38% for ebb generation, 32% for flood, 50% for two-way operation) accounts for turbine efficiency, the head needed before generation starts and the time spent waiting for that head.
Worked example: a tidal stream array
The defaults model an array of four 1.5 MW turbines with 18 m rotors in a channel where depth-averaged peak currents reach 3.5 m/s at springs and 1.8 m/s at neaps, similar to the first phase of the MeyGen project in Scotland’s Pentland Firth.
- With Cp = 0.42 and 90% drivetrain efficiency, each turbine reaches rated power at about 3.1 m/s.
- Turbines generate whenever the current exceeds 1 m/s (around 6,450 hours a year) but run at full power for only about 640 hours, mostly during spring tides.
- With 92% availability the array produces about 13.6 GWh a year, a capacity factor near 26%, enough for some 3,800 average EU homes.
- At 7,500 €/kW and 3.5% O&M, the LCOE is above €400/MWh, twice the €200/MWh support price assumed. That gap is why current tidal stream projects depend on dedicated contracts-for-difference and why cost reduction is the industry’s main goal.
Choose the La Rance-type barrage preset (22 km², 11 m spring and 5.5 m neap range, 240 MW, ebb generation) and the model gives about 535 GWh a year, close to the roughly 500 GWh the real plant has delivered for decades.
How to read the results
- Annual energy and capacity factor: tidal plants typically reach 20–40%.
- Rated current speed (stream). Choosing a lower rated speed raises capacity factor but wastes energy at springs; developers optimise this against cost.
- Energy per tide (barrage). The chart shows the spring–neap swing; spring tides can deliver four times as much as neaps because energy scales with R².
- Tides limited by capacity. If this is high, more turbines would capture more energy.
Typical values
| Parameter | Tidal stream | Tidal range |
|---|---|---|
| Resource needed | spring peak current > 2.5 m/s | mean range > 5 m |
| Unit size | 0.1–2.5 MW | 10–26 MW bulb turbines |
| Capacity factor | 25–40% | 20–30% |
| Capex | 5,000–12,000 per kW | 3,000–8,000 per kW |
| Example | MeyGen (UK), 6 MW | La Rance (FR) 240 MW; Sihwa (KR) 254 MW |
Tips
- Measure at hub depth. Current speed falls towards the seabed; use ADCP measurements at the rotor height, not surface speeds.
- Spring and neap both matter. A site with strong springs but weak neaps produces less than its peak current suggests.
- Two-way lagoons generate on both flood and ebb and can pump at high water to boost head; they deliver steadier output than ebb-only barrages.
- Combine with storage or other renewables. Tidal output is predictable but pulsed four times a day, so it complements wind and solar.
Limitations
The tide is modelled as a single sinusoidal constituent with a spring–neap modulation; real tides include diurnal inequality, equinoctial highs and site-specific flood/ebb asymmetry. Stream turbines are treated independently, without array wakes or channel blockage effects, and the vertical velocity profile is ignored. The barrage model uses an empirical extraction factor rather than a full operating simulation, so expect ±20–30% accuracy. Environmental effects on sediment, fish and intertidal habitats are outside its scope, although for barrages they are often the deciding factor.
Method & assumptions
Tidal range (barrage/lagoon): water trapped over area A with range R holds potential energy E = ½ ρ g A R² per filling or emptying (ρ = 1,025 kg/m³). The range is varied each tide between neap and spring values over the 14.77-day cycle, 706 tides per year are summed, an extraction factor converts potential to delivered energy and each tide is capped at installed power × generating hours. Tidal stream: P = ½ ρ A Cp u³ η, limited to rated power and zero below cut-in, simulated every 10 minutes for a year with a sinusoidal M2 tide whose amplitude follows the spring–neap cycle. LCOE = discounted costs ÷ discounted MWh.
Frequently asked questions
How is tidal energy calculated?
How much power does a tidal stream turbine produce?
What current speed is needed for tidal stream energy?
How much energy does La Rance tidal power station produce?
Why is tidal energy predictable?
Why is tidal energy expensive?
Sources & references
- Ocean Energy Systems (IEA Technology Collaboration Programme) — annual reports
- Charlier, R.H. (2007). Forty candles for the Rance River TPP tides provide renewable and sustainable power generation. Renewable and Sustainable Energy Reviews 11(9)
- Neill, S.P. & Hashemi, M.R. — Fundamentals of Ocean Renewable Energy, Academic Press, 2018
- IRENA — Tidal Energy Technology Brief (2014)
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