Sustainable Ocean Living--Piece by Piece

Energy Islands are floating modular renewable energy platforms that incorporate photovoltaics, solar thermal towers, wave energy, ocean current energy turbines, wind turbines, and OTEC (ocean thermal energy conversion). Designed by architect Alex Michaelis, the concept is aimed at capturing a share of Richard Branson's Virgin Earth Prize.

Each island would be built on a floating platform and at its centre would be a plant that converts heat from the tropical sea into electricity and drinking water. Below deck would be marine turbines to harness energy from underwater currents and around the edge floating devices to provide wave power.

Vegetable farms and homes for workers will complete the colony and the power will be piped back to be used on the nearest populated land mass.

Michaelis, who is working together with his father Dominic, an engineer, estimates that each island complex could produce 250MW.

NextEnergy

Combining enough Energy Island modules to form the outside of a protected lagoon, you would be on your way to renewable power, agriculture, and aquaculture for your floating city.
Aquarius is the sea-colony concept from Marshall Savage, writer of The Millenial Project. Self-sufficient Aquarius floating cities would be the first step to colonising the galaxy. The lessons learned from building sustainable and profitable colony-cities-on-the-ocean could be transferred to floating cities in outer space.

A different group has coalesced around the concept of "Seasteads". For the seastead movement, building a sustainable floating city is an end in itself.

In the past, pioneers and malcontents would head to the frontiers, of which few now exist. The oceans, which make up 71% of the earth's surface, have always been a place for those seeking new ways of life. They are the last great unclaimed region. Ships are not well suited for permanent living, but by creating new land on the oceans we can achieve both freedom and a reasonable degree of comfort.

Freedom of movement and self-sufficiency are both intimately connected with political freedom. Fixed locations such as seamounts, islands, and atolls are much more vulnerable to the whims of nearby governments [minerva link], but a mobile seastead can always move if the political climate becomes unsuitable. While a seastead is likely to import many goods, being able to supply its own basic necessities will also add greatly to its independence. This approach to nation founding reduces - but does not eliminate - the difficulty in finding sovereignty, by operating in international waters...If the seastead is parked in area that does not get regular rain storms an alternative method of fresh water replenishment is needed. Either sea water distillation or reverse osmosis will work. Both forms of sea water reclamation require pretty hefty amounts of power. Distillation can be done with solar evaporation trays and condensers; whereas reverse osmosis runs off of electricity....

Seastead Book
Seascape One, pictured above, is a combination tourist destination and high-end condominiums designed to float around the Mediterranean Sea. It incorporates multiple renewable energy features, including wind and solar power. The tall white structure projecting above the living section is a solid sail, for clean (but slow) propulsion. Lessons learned from operating such a design should be applicable to a more rough weather seastead.
Paolo Soleri designed floating arcologies which could also be classified as "seasteads." The "Nexus" floating city project is more than a little based on a Soleri design.

This is a floating city designed to accommodate 100,000 persons. 7 kilometers long and 4 kilometers wide with the capacity to be mobile, grow its own food, produce its own electricity and, owing to it existing beyond the 12 mile governmental jurisdiction boundaries, create its own government, income system and tax base. In essence, this mobile city becomes its own independent country....The city utilizes several different types of electrical power generation. Five Ocean Thermal Energy Conversion units are positioned at strategic zones of the city to supply electricity. Banks of freestanding windmills and photovoltaic solar cells produce additional electricity. The "head" of the floating city is a small mountain range with a specially designed frontal structure that cuts Tsunami tidal waves into smaller, manageable waves with little destructive effect. It is a tidal wave barrier that requires the city to head into the on-coming wave. Nexus

The video above is a graphic portrayal of some of the aspects of the "Energy Island" concept--the UK project that wants a piece of the Virgin Prize.