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Ocean Thermal Energy, at first glance, is precisely what it sounds like- thermal energy stored in the ocean. All matter has a temperature, and that temperature is a representation of the amount of energy its constituent molecules contain. As “heat" moves from one object to another, what actually occurs is the transfer of that energy from one body to another. The larger and/or denser a body is, the greater the amount of thermal energy it can contain. The greater the surface area that serves as the transfer interface, the greater the amount of energy transferred. You can see this demonstrated by going outside in cold weather with your head exposed versus wearing a hood or hat. While the temperature which each exposed part of your skin feels is the same (more or less!), the total amount of heat your body loses is dramatically different, as you would attest to after an hour or so!
It is important to realize that this energy isn’t just sitting around doing nothing; that heat drives chemical reactions and allows for life to run properly. Even a few degrees difference can have a dramatic impact on ocean conditions. That heat also helps to drive air and ocean currents, hurricanes being an excellent example of this . The temperature gradient (the change in temperature as a function of latitude) also serves to drive the prevailing winds.
Ocean Thermal Energy Conversion is a proposed renewable energy source that relies on taking advantage of that energy in order to provide power. This system utilizes the temperature gradient between the surface and extreme depths of the ocean to drive a heat engine. Though the basic concept has been known for a long time, technical difficulties such as the fouling of the system with biological matter and the challenges of constructing a system with pipes that descend a thousand meters into the ocean have prevented it from becoming a reality to date. This is not necessarily a bad thing, as widespread use of such systems would change the heat balance of the oceans, which could in turn alter currents and disrupt or destroy marine life.
Given all the uncertainties, we would be wise to be careful about how we employ the planet's energy reserves- it's best not to fool with things you don't totally understand.