Bluerise

[image source - the video below]
They suppose that

Although tropical regions currently account for less than 30% of the current global energy demand, the tropics will become the most significant energy market within the next 20 years
source

This solution can work well in the tropical regions - the temperature difference of the deep sea water and surface water is rather big (near 20°C)
And it is this temperature difference what drives the Bluerise system to generate energy

One of the main advantage is that it can work day and night all the year round thus we don't need a storage of energy.
Go to http://www.bluerise.nl/aboutbluerise/ for more details

They said in the video below that

We have built and tested demonstration systems ..

which as far as I understand means that they have created a prototype already