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Ocean Thermal Energy

OTEC Ocean Thermal Energy Conversion Simulator

OTEC generates power from the modest 20°C temperature difference between tropical surface water and deep cold water. Switch the cycle type, surface and deep temperatures and cold-water pipe size to see Carnot efficiency, actual efficiency, net power and annual energy update in real time.

Parameters
Cycle type
Sets the working fluid and the Carnot fraction
Surface temperature T_h
°C
Deep temperature T_c
°C
Cold-water pipe diameter D
m
Cold-water pipe length L
m
Cold-water flow Q_c
m³/s
Warm-water flow Q_w
m³/s
Rated plant output
kW
Target power output for the design
Results
—
Carnot efficiency (%)
—
Actual efficiency (%)
—
Total heat transfer (MW)
—
Gross power (kW)
—
Pump power (kW)
—
Net power (kW)
—
OTEC plant schematic — surface warm / deep cold / heat engine

Surface warm water evaporates the working fluid and drives the turbine; deep cold water condenses it back to liquid in a closed loop. Colour shows temperature (red = warm, blue = cold).

Carnot and actual efficiency vs temperature difference ΔT
Efficiency and net power by cycle type
Theory & Key Formulas

$$\eta_{C}=\frac{T_{h}-T_{c}}{T_{h}},\qquad \eta_{\text{actual}}=\eta_{C}\cdot k_{\text{cycle}}$$

Carnot efficiency η_C is the theoretical upper limit from the absolute temperatures. k_cycle is the cycle-dependent fraction (Closed Rankine: 0.40 / Open: 0.30 / Hybrid: 0.40 / Kalina: 0.45).

$$\dot Q=\rho_{sw}\,c_{p}\,Q_{w}\,(T_{h}-T_{c}),\qquad P_{\text{gross}}=\dot Q\cdot \eta_{\text{actual}}$$

Total heat transferable from the warm-water flow Q_w and the corresponding gross power. ρ_sw=1025 kg/m³, c_p=4186 J/(kg·K).

$$h_{f}=f\,\frac{L}{D}\,\frac{v^{2}}{2g},\qquad P_{\text{pump}}=\frac{\rho_{sw}\,g\,Q_{c}\,h_{f}}{\eta_{p}}$$

Cold-water pipe friction loss h_f (Darcy–Weisbach, f=0.02) and pump power P_pump (η_p=0.75). v=Q_c/(πD²/4). Net power P_net = P_gross − P_pump − 5% auxiliary.

What is the OTEC Ocean Thermal Energy Conversion Simulator?

🙋
OTEC — that is new to me. Can you really make electricity from just a 20°C temperature difference in the ocean?
🎓
Yes. In tropical and subtropical waters the surface sits around 27°C and there is a cold layer of 4–6°C at about 1000 m depth. OTEC is a heat engine that uses that vertical temperature gap. The mechanism is the same as a thermal power plant: a low-boiling fluid such as ammonia is evaporated by warm surface water, drives a turbine, and is condensed back to liquid by cold deep water in a closed loop. The difference is that ΔT is only 22°C. The Carnot ceiling is just 22/300 ≈ 7%, and a real machine only captures about 40% of that. So end-to-end efficiency is 2 to 3% — extremely low.
🙋
Two percent? You put in 100 units of heat and only get 2 of electricity? How can that be commercially viable?
🎓
Good question. The answer is "the heat comes from the ocean for free, as much as you want". A thermal plant pays for fuel, but OTEC's heat source is the sun warming the surface — zero fuel cost. So if you can supply enough flow, you can still get useful power despite low efficiency. The catch is "supplying that flow". The default in this tool draws 200 m³/s of cold water — that means hanging a 9 m diameter pipe from the surface down to 1000 m. The cold-water pipe alone is 50–60% of capital cost. So OTEC is really not about "efficiency", it is about "how cheap and how durable you can make a giant cold-water pipe".
🙋
There are four cycle types. Net power goes up when I pick Kalina — why is that?
🎓
The difference is the "temperature glide" in the heat exchangers. Pure ammonia boils at a single temperature, so during evaporation and condensation it stays at one temperature. That creates regions where seawater and working fluid are very different and others where they are barely different — heat exchange is inefficient. The Kalina cycle uses an NH₃-H₂O mixture, where the boiling point changes continuously with composition, so the working fluid can follow the seawater temperature change. Heat-exchanger losses are smaller. As a result, 40% of Carnot (Rankine) becomes 45% (Kalina). A 10–15% gain in power is a big deal at plant scale.
🙋
When I drop the deep temperature, efficiency jumps. So why not just draw water from much deeper?
🎓
In principle, yes. But making the pipe longer means more friction loss and more pump power. The Darcy–Weisbach equation in the theory box: h_f grows linearly with L, and pump power is ρgQh_f/η. Going from 1000 m to 2000 m doubles the loss and the pump power, while the temperature improves only by a few degrees. The trade-off between "better efficiency" and "extra pump power" lands at 800–1200 m. And a kilometre-scale subsea structure has to survive cyclones, earthquakes and biofouling, so the design margins are enormous.
🙋
So OTEC is really about civil engineering and structural design, not just thermodynamic efficiency. Are there any commercial demonstrations going on?
🎓
Yes. The Kumejima Island demonstration in Okinawa, run by Saga University, is well known (100 kW class). It uses the pumped deep-sea water not just for electricity but also for shrimp and sea-grape aquaculture, district cooling, cosmetics raw material and agricultural irrigation — a multi-use model. OTEC alone has a high LCOE, but when you sell the by-products as well, the economics start to close. Hawaii, French Réunion and Korea are also running demonstrations. For tropical island nations seeking energy independence, OTEC is one of the few options that offers 24/7 baseload power.

Frequently Asked Questions

Carnot efficiency is set by the absolute temperatures of the hot and cold reservoirs: η_C = (T_h − T_c)/T_h. For OTEC, typical values are surface 27°C (300 K) and deep 5°C (278 K), giving a temperature difference of only 22 K. So η_C = 22/300 ≈ 7.3%. The real cycle efficiency is 30 to 45% of that, giving a net efficiency of 2 to 3%. Compared with thermal power plants (η_C above 60%), this is an order of magnitude lower, so OTEC must compensate with very large seawater flow rates.
Ocean temperature drops with depth. In tropical and sub-tropical waters, depths of 800 to 1000 m reach 4 to 6°C. Deeper intake lowers T_c and raises Carnot efficiency, but the cold-water pipe gets longer and the friction loss and pump power grow. This tool evaluates friction loss with the Darcy–Weisbach equation (f≈0.02) and pump power as ρgQh/η (η=0.75). Real OTEC plants typically use 6–10 m diameter pipes that are 800–1200 m long.
Closed Rankine uses a low-boiling pure working fluid such as ammonia (NH₃) in a closed loop. It is mechanically simple but only captures about 40% of the Carnot efficiency. The Kalina cycle (NH₃-H₂O mixture) evaporates and condenses with a sliding temperature, so the heat-exchanger irreversibility is smaller and it can capture about 45% of the Carnot efficiency. In this tool, carnotFraction is 0.45 for Kalina and 0.40 for Rankine, and you can see about a 10–15% gain in net power at the same temperature difference.
Only a handful of utility-scale plants are running, including those in Hawaii (OTEC International, 100 kW), Kumejima Island in Okinawa, Japan (Saga University, 100 kW demonstration), and Korea. The main obstacle is LCOE (levelised cost of energy), currently 30–50 ¢/kWh, much higher than solar or wind at 5–10 ¢/kWh. Still, OTEC can run as baseload 24/7, and the deep-water by-product can be used for aquaculture, district cooling and agriculture, making it attractive for tropical island nations.

Real-World Applications

Baseload power for tropical islands: Regions such as Okinawa, Hawaii, Guam and Fiji have a tropical or subtropical marine climate where solar and wind cannot cover the night or calm hours. OTEC has been studied as a baseload option there. Its output barely depends on weather, season or time of day, with a capacity factor above 90%. This tool assumes 0.9 utilisation to estimate annual energy in GWh.

Multi-use of deep-sea water: The 1000 m deep water pumped by OTEC is rich in nutrients (nitrogen, phosphorus), clean and cold — three valuable properties. On Kumejima Island the same deep water is reused after the heat exchanger for shrimp, abalone and sea-grape aquaculture, district cooling, cosmetics raw material, agricultural irrigation and bottled water. This multi-use model is how OTEC plants are made to pencil out despite their high stand-alone LCOE.

Offshore hydrogen and ammonia production: There is growing interest in converting large offshore renewable power into hydrogen or green ammonia for shipping. OTEC can supply stable power in equatorial ocean areas far from shore, making it a candidate prime mover for floating green-hydrogen and ammonia plants.

Tropical freshwater production: The Open Flash cycle (flash-evaporating warm surface water under low pressure and condensing it) yields clean freshwater as a by-product, at thousands of m³/day per MW of installed power. Combined with a seawater-desalination plant, it can address both electricity and water shortages typical of tropical island regions.

Common Misconceptions and Pitfalls

The most common misconception is to equate Carnot efficiency with real plant efficiency. This tool shows a Carnot efficiency of about 7%, but that is only the ideal-reversible upper bound. Real plants reach 30–45% of it. The difference is set by turbine, heat-exchanger and pump irreversibilities, the finite ΔT at the heat-exchanger pinch points, and auxiliary load (about 5%). When you discuss OTEC "net efficiency", always use the value after pump power has been subtracted (the Net power in this tool).

Next, do not freeze the cold-water pipe friction loss at the Darcy–Weisbach f = 0.02 value. The f = 0.02 used here is a standard average, but biofouling on the pipe wall pushes roughness up and f to 0.025–0.03 in service. Inlet and outlet local losses, bends and disturbances from support structures add another 10–30%. For detailed design, validate with CFD or pilot-pipe tests and budget 20–40% more friction than this tool predicts.

Finally, do not neglect environmental impact assessment. OTEC is a fossil-fuel-free renewable source, but it pulls large amounts of cold water from the deep and discharges it near the surface, so the local sea-surface temperature, nutrient concentration and plankton distribution must be assessed carefully. When pumped flow exceeds 100 m³/s, observable changes in temperature and salinity can extend several kilometres around the plant. Commercial sites increasingly require multi-year baseline surveys before construction and post-operational monitoring. This tool evaluates the thermodynamic and hydraulic "operating point" only — it is not a substitute for environmental impact assessment.

How to Use

  1. Set surface water temperature (typically 26–28°C in tropical regions) and deep water temperature (4–6°C from 1000 m depth) using the sliders or numeric inputs.
  2. Configure cold water intake pipe diameter (0.5–2.0 m for utility-scale OTEC) and pipe length (1000–1500 m vertical/sloped descent).
  3. Run the simulation to calculate Carnot efficiency ceiling, actual cycle efficiency (18–24% typical), total heat exchanged (MW scale), and net power output after accounting for circulation pump parasitic load.

Worked Example

Tropical OTEC plant: surface water 27°C, deep water intake 4°C, cold pipe diameter 1.2 m, pipe length 1200 m. Carnot efficiency = (27−4)/(27+273) = 7.4%. With actual cycle efficiency 21%, total heat transfer reaches 85 MW through titanium heat exchangers. Gross turbine power = 17.9 kW per MW of heat. Pump power for 1.2 m diameter pipe over 1200 m with seawater friction losses ≈ 3.2 MW. Net electrical output ≈ 14.7 MW after subtracting parasitic loads and generator losses (92% efficiency).

Practical Notes