An Overview Of Ocean Thermal Energy Technology, Potential Market Applications, And Technical Challenges
Authors: Cohen Robert 
Published: July 2009

The geography of the ocean thermal resource, and the physics and engineering of ocean thermal power plants are described, including the ocean thermal power cycle and plant configuration. Power-plant efficiency and cost factors in generating electricity are summarized, along with potential products that can be manufactured from that electricity, using appropriate feedstocks, aboard ocean thermal factory-ships known as " plantships??. Engineering requirements and challenges that must be surmounted in achieving commercially viable ocean thermal power plants include designing and deploying wide, kilometer-long cold water pipes; mooring in depths exceeding a kilometer; operability in storms, and survivability in severe storms and hurricanes. Early and near-term markets are described, supplied by baseload ocean thermal electricity transmitted to shore via submarine cable. Early markets are in places like Hawaii and Puerto Rico, where electricity is currently being derived from oil. Near-term markets can utilize power cabled to shore from longer distances; e.g., from the Gulf of Mexico into the U.S. electrical grid at entry points such as Key West, Tampa, New Orleans, and Brownsville. Longer-term markets are for two categories of plantship products: energy carriers, such as hydrogen or ammonia; or energy-intensive end products, such as ammonia for fertilizer.

Making ocean thermal energy available for the above markets can achieve significant savings of oil and natural gas presently used to generate electricity and can be used for manufacturing plantship products. Manufacturing viable energy carriers and energy products aboard a fleet of grazing plantships could potentially supply a significant portion of global energy needs. All of these applications can help mitigate global warming, and there is perhaps a possibility that during plant operation CO2 could be removed from the atmosphere and sequestered in the deep ocean.

Introduction

It has long been recognized, starting with d'Arsonval (1881), that the oceans are a natural collector of solar radiation, which they convert into thermal energy, and that this thermal energy, in turn, can be converted into baseload electricity. To accomplish that conversion, warm surface water would be used as the heat source, and cold water, located at about a kilometer below the surface in the major oceans, would serve as the heat sink. Specialists often refer to this generating process using the acronym " OTEC??, an abbreviation for " ocean thermal energy conversion??. In principle, electricity derived from ocean thermal energy could become the world's largest source of carbon-free renewable energy, as well as a key mitigator of global warming.

However, partaking of this bountiful but theoretical energy supply poses certain practical difficulties 'twixt cup and lip:The tropical and subtropical oceans can be a hostile environment in which to operate. Notably, the warm surface waters also fuel severe storms and hurricanes.Most of the vast ocean thermal resource is remote from human habitation, although in some geographies (such as Hawaii, Puerto Rico, and the U.S. Gulf Coast) the resulting baseload electricity can probably be conveniently and economically cabled to land. To harness a significant share of the tremendous remaining amounts of ocean thermal energy that are insufficiently proximate to shore, "plantships" (factory ships) grazing the high seas will have to produce portable energy carriers and energy-intensive products that can become economically competitive after being transported to their points of use. For example, ammonia appears to be a promising candidate plantship product for use as combustion fuel, hydrogen carrier, or fertilizer.Significant technological challenges still need to be successfully addressed with system solutions that are at once viable and acceptable technically, economically, and environmentally.