Ocean Thermal Energy Conversion (OTEC) Lab

Explore first-principles low-delta-T Rankine cycles, organic working fluids (ammonia R717), deep cold water pipe (CWP) friction hydraulics, parasitic pumping loads, desalinated water co-production, and 30-year baseload LCOE.

Take Quiz

Archetype Presets:

Net Baseload Capacity

-- MWe

Continuous 24/7 delivery

Levelized Cost (LCOE)

$-- /MWh

With desal co-revenue credit

Net Annual Output

-- GWh/yr

93.5% capacity factor

Parasitic Pumping Load

--%

Cold & warm water pumps

Desalinated Freshwater

-- m³/day

High-purity potable water

Project 30-Yr NPV

$--

Discounted net return

Internal Rate of Return

--%

Project equity IRR

Avoided Diesel CO2

-- t/yr

Island grid diesel offset

Ocean & Cycle Parameters

Gross Nameplate Capacity 20 MWe

Ocean Thermal Gradient Resource

Surface Warm ($T_w$) 26.5 °C

Deep Cold ($T_c$) 4.5 °C

Heat Exchanger Approach Pinch ($\Delta T_{\text{pinch}}$) 2.2 °C

Cold Water Pipe (CWP) Hydraulics

Pipe Length 1000 m

Pipe Diameter Ø 3.2 m

Warm Flow 42 m³/s

Cold Flow 24 m³/s

Desalination & Freshwater Co-Gen

Enable Desalinated Potable Water Co-Gen

Freshwater Yield 18000 m³/d

Water Value $2.20/m³

Installed CapEx & PPA Economics

CapEx per kW Net $6400

Marine O&M $140/kW/yr

PPA Tariff $175/MWh

Discount Rate 7.5%

Thermodynamic & Fluid Specs

OTEC Thermodynamic Cycle Architecture

Ammonia Rankine / Desal

5×5 Sensitivity Matrix (LCOE $/MWh vs Ocean Temperatures)

30-Year Pro-Forma Cash Flow Statement

First-Principles OTEC Thermodynamics & Hydraulic Pumping

1. Low-$\Delta T$ Rankine Cycle Thermodynamics

Ocean Thermal Energy Conversion exploits solar radiation absorbed continuously across tropical surface waters, creating an immense, perpetual thermal reservoir. Because the available temperature difference between surface waters ($25^\circ\text{C}$ to $28^\circ\text{C}$) and deep abyssal waters ($4^\circ\text{C}$ to $5^\circ\text{C}$) is only around $20^\circ\text{C}$ to $24^\circ\text{C}$, the maximum theoretical Carnot limit is:

$\eta_{\text{Carnot}} = \frac{T_w - T_c}{T_w} = \frac{299.65\text{ K} - 277.65\text{ K}}{299.65\text{ K}} \approx 7.34\%$

In practical closed-cycle systems using anhydrous ammonia ($\text{NH}_3$, R717), plate heat exchangers with small approach temperatures ($\Delta T_{\text{pinch}} \approx 2^\circ\text{C}$) achieve net thermal-to-electric conversion efficiencies of $3.0\%$ to $4.2\%$. While small as a percentage, the volume of ocean water available is virtually limitless, delivering massive megawatt-hours without fuel expense.

2. Deep Cold Water Pipe (CWP) Friction & Buoyancy Head

The primary technical challenge in OTEC engineering is circulating thousands of cubic meters per minute through a 1,000-meter deep vertical pipe. The total dynamic head $h_{\text{total}}$ opposing the cold water pumps consists of Darcy-Weisbach pipe wall friction and a gravitational density penalty:

$h_f = f \frac{L}{D} \frac{v^2}{2g}, \quad \Delta h_{\text{density}} = \frac{\rho_{\text{deep}} - \rho_{\text{warm}}}{\rho_{\text{warm}}} \cdot \frac{L}{2}$

Because dense $4^\circ\text{C}$ seawater ($\rho \approx 1027\text{ kg/m}^3$) is heavier than $26^\circ\text{C}$ surface water ($\rho \approx 1022\text{ kg/m}^3$), work must be done against gravity simply to lift the water column. Combined with pipe friction, parasitic pumping loads consume $25\%$ to $35\%$ of gross turbine generator output.

3. High-Value Desalination & Deep Ocean Water (DOW) Cascades

In tropical island economies dependent on imported diesel for power and reverse osmosis, OTEC's co-production of fresh potable water ($2 to $3/m³) and nutrient-rich deep ocean water for land-based aquaculture and Sea Water Air Conditioning (SWAC) generates multi-tiered revenue, drastically accelerating project equity payback.

Knowledge Mastery Assessment: Ocean Thermal Energy Engineering

Frequently Asked Questions

Ocean Thermal Energy Conversion (OTEC) is a marine renewable energy technology that exploits the natural temperature difference between warm tropical surface ocean water (25°C to 28°C) and deep ocean water (4°C to 5°C at depths of 800 to 1,000 meters). In a closed-cycle system, warm surface seawater vaporizes a low-boiling-point working fluid (such as ammonia), driving a vapor turbine generator. Cold seawater pumped from the abyss condenses the ammonia back into liquid to repeat the thermodynamic cycle continuously.

Unlike intermittent solar photovoltaics and wind turbines, equatorial and tropical ocean thermal reservoirs store massive solar heat year-round with less than 1°C to 2°C seasonal fluctuation. As a result, OTEC operates continuously 24 hours a day, 365 days a year with capacity factors exceeding 90% to 95%, providing fully dispatchable clean baseload power that eliminates the need for massive battery storage.

Because the thermal efficiency of low-temperature Rankine cycles is modest (3% to 4%), enormous volumetric flow rates of seawater (20 to 60 m3/s per 10 MWe net) must be circulated. Cold seawater pumped up a 1,000-meter deep pipe encounters Darcy-Weisbach frictional resistance and a negative buoyancy head caused by cold, dense saline water (1,027 kg/m3) being lifted through lighter surface water (1,022 kg/m3). Consequently, seawater pumping consumes 25% to 35% of gross turbine generation.

In open-cycle OTEC or hybrid closed-cycle systems with flash evaporators, warm seawater flashed under vacuum produces pure water vapor that condenses against deep cold seawater, yielding thousands of cubic meters of fresh potable water daily. For isolated island nations relying on expensive diesel-powered reverse osmosis ($2 to $3/m3), this freshwater revenue offsets capital expenditures and lowers the net levelized cost of electricity (LCOE).

Deep cold ocean water is pure, pathogen-free, and rich in essential dissolved nitrates, phosphates, and silicates. After leaving the OTEC condenser, the cold effluent can be cascaded into Sea Water Air Conditioning (SWAC) to cool coastal resorts and datacenters (cutting chilling power by 85%), as well as cold-water marine aquaculture (salmon, spirulina, abalone) and deep-sea mineral extraction.

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