The Oil Drum

In a tropical climate there is unlikely to be any cold weather to create cold bottom water. So unless a cold mountain stream brings in water from a much higher elevation you wouldn't expect much thermal stratification. OTEC was much discussed thirty years ago, but it seems to have been largely abandoned. I did see one mention of someone looking at it recently, so the idea is not completely dead.

I don't think a high temperature at depth is needed to run an engine off this resource (or a low temperature at depth, either). In its current form, it is blowing a stream of water 50m above the surface of the lake. That is already mechanical power. To capture that power requires water turbines as are used in a small scale hydropower facility and some ingenuity with plumbing to separate the mixed flow of gas and liquid. It does not require the high efficiency heat exchangers that are needed for OTEC. This would be a nifty local power resource. But keep in mind, this is one energy resource that definitely should be depleted, and not developed in a sustainable way.

I've heard proposals to run engines off a salinity gradient between fresh water and salt water. This would be an engine that runs off the gradient in CO2 concentration. OTEC is a heat engine. Heat engines run off a gradient in temperature. With the development of coal resource in Europe, a lot of effort went into the study of thermodynamics. A big branch of physics was created and, as a side effect, all sorts of other engines were invented theoretically. The resource needed to run any of them never had the availability of coal-derived high temperature, so they were never really turned into practical inventions.