page_title: Ocean thermal energy conversion
section_title: Variation of ocean temperature with depth
hierarchical_section_title: Ocean thermal energy conversion / Thermodynamics / Variation of ocean temperature with depth
caption_attribution_description: English: Own work
context_page_description: Ocean Thermal Energy Conversion uses the ocean thermal gradient between cooler deep and warmer shallow or surface seawaters to run a heat engine and produce useful work, usually in the form of electricity. OTEC can operate with a very high capacity factor and so can operate in base load mode.
  The denser cold water masses, formed by ocean surface water interaction with cold atmosphere in quite specific areas of the North Atlantic and the Southern Ocean, sink into the deep sea basins and spread in entire deep ocean by the thermohaline circulation. Upwelling of cold water from the deep ocean is replenished by the downwelling of cold surface sea water.
  Among ocean energy sources, OTEC is one of the continuously available renewable energy resources that could contribute to base-load power supply. The resource potential for OTEC is considered to be much larger than for other ocean energy forms. Up to 88,000 TWh/yr of power could be generated from OTEC without affecting the ocean’s thermal structure.
  Systems may be either closed-cycle or open-cycle. Closed-cycle OTEC uses working fluids that are typically thought of as refrigerants such as ammonia or R-134a.
context_section_description: The total insolation received by the oceans (covering 70% of the earth's surface, with clearness index of 0.5 and average energy retention of 15%) is: 5.45×10¹⁸ MJ/yr × 0.7 × 0.5 × 0.15 = 2.87×10¹⁷ MJ/yr
  We can use Beer–Lambert–Bouguer's law to quantify the solar energy absorption by water,
  where, y is the depth of water, I is intensity and μ is the absorption coefficient.
  Solving the above differential equation,
  The absorption coefficient μ  may range from 0.05 m⁻¹ for very clear fresh water to 0.5 m⁻¹ for very salty water.
  Since the intensity falls exponentially with depth y, heat absorption is concentrated at the top layers. Typically in the tropics, surface temperature values are in excess of 25 °C (77 °F), while at 1 kilometer (0.62 mi), the temperature is about 5–10 °C (41–50 °F). The warmer (and hence lighter) waters at the surface means there are no thermal convection currents. Due to the small temperature gradients, heat transfer by conduction is too low to equalize the temperatures. The ocean is thus both a practically infinite heat source and a practically infinite heat sink.
  This temperature difference varies with latitude and season, with the maximum in tropical, subtropical and equatorial waters. Hence the tropics are generally the best OTEC locations.
caption: Graph of different thermoclines (depth vs. temperature) based on seasons and latitude
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