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Capturing Ocean Heat

Ocean thermal energy conversion offers a timely renewable alternative.

Fortnightly Magazine - October 2009

within a nation’s Exclusive Economic Zone (EEZ); or 3) free-floating facilities in deep ocean water (either within or beyond the EEZ). In determining the site selection of OTEC facilities, there are three technical considerations—thermal gradient, sea water depth, and offshore distance, which impacts efficiency of electrical and other side-product transmissions—and the territorial sovereignty that applies to adjacent waters. In a nation’s inland and territorial waters, rights of regulatory competence and judicial oversight are unquestioned. This right assumes less prominence as the off-shore distance increases until, eventually, national sovereignty disappears completely and the law of the high seas takes hold. OTEC site selection will be governed by the political and legal realities of operating outside territorial waters. If desirable near-shore sites don’t present favorable thermal conditions, OTEC operators will be compelled to locate in international waters. In OTEC markets, this calculation is particularly acute for land-locked nations, and states bordered by colder waters in the temperate north and south.

The main advantages that land-based and near-shore facilities offer are that they don’t require sophisticated mooring or lengthy power cables and side-product piping, and they offer ease of access. However, there will be additional expenses involved in the extended warm- and cold-water piping infrastructure (which are exposed to additional stresses of the shallow water environment). This siting allows for the smallest scaleof operations (hence, less cost-effective). There is the potential for local environmental issues not seen by the other two siting choices. And in order to minimize pump head losses, the heat exchangers will need to be located below sea level, thus requiring additional site expenses.

Like the state-of-the-art construction techniques used in the present-day offshore industry in building and siting deep-water oil platforms, deep-water and open-sea OTEC plants easily can be built in a shipyard, towed to the site, and—for moored deep-water platforms—fixed to the sea bottom away from shore (either by pilings or cables), thus avoiding the negative effects of the surf zone and coming closer to cold waters. Among the other advantages of the deep-water moored OTEC plants are that they have easy access to sea-water resources and can have larger scale of operations, making them more cost-effective. However, among the challenges that they face are the need for sophisticated mooring cabling systems; increased lengths of power cables and side-product piping; and the impacts of open-ocean storm conditions.

Free-floating OTEC facilities could be preferable if the plant isn’t intended to deliver electricity to shore, but is designed for production of other side-products ( e.g., fresh water, liquid fuels and mineral extraction). The advantages the free-floating OTEC plant offers include siting in areas not subject to hurricanes; largest scale of operations; and no mooring or stabilization issues. However, free-floating facilities present the difficulty of having to ship the side-products to shore, although the shipping distances would be considerably shorter than those already accomplished by the oil industry.

Potential OTEC Products

Besides providing a source of clean, renewable baseload electrical energy, OTEC has the potential to provide many useful side-products such as desalinated fresh water for industrial, agricultural, and residential uses; liquid