Editing Ocean thermal energy conversion (section)

=== Variation of ocean temperature with depth ===
[[File:ThermoclineSeasonDepth.png|thumb|200px|right|Graph of different thermoclines (depth vs. temperature) based on seasons and latitude]]
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: {{math|5.45&times;10<sup>18</sup> MJ/yr &times; 0.7 &times; 0.5 &times; 0.15 {{=}} 2.87&times;10<sup>17</sup> MJ/yr}}

We can use [[Beer–Lambert law|Beer–Lambert–Bouguer's law]] to quantify the solar energy absorption by water,

:<math>-\frac{dI(y)}{dy}=\mu I</math>

where, ''y'' is the depth of water, ''I'' is intensity and ''μ'' is the absorption coefficient.
Solving the above [[differential equation]],

:<math> I(y)=I_{0}\exp(-\mu y) \,</math>

The absorption coefficient ''μ''  may range from 0.05 m<sup>&minus;1</sup> for very clear fresh water to 0.5 m<sup>−1</sup> for very salty water.

Since the intensity [[exponential decay|falls exponentially]] with depth ''y'', heat absorption is concentrated at the top layers. Typically in the tropics, surface temperature values are in excess of {{convert|25|C}}, while at {{convert|1|km|mi|sp=us}}, the temperature is about {{convert|5|-|10|C|F}}. The warmer (and hence lighter) waters at the surface means there are no [[convection current|thermal convection currents]]. Due to the small temperature gradients, heat transfer by [[heat conduction|conduction]] is too low to equalize the temperatures. The ocean is thus both a practically infinite heat source and a practically infinite heat sink.{{Clarify|date=January 2013}}

This temperature difference varies with latitude and season, with the maximum in [[tropical]], [[subtropical]] and [[Equatorial climate|equatorial]] waters. Hence the tropics are generally the best OTEC locations.