Survey of Energy Resources 2007
Types of OTEC Plant
Depending on the location of the cold and warm water supplies, OTEC plants can be land-based, floating, or - now not such a longer-term development - grazing. Floating plants have the advantage that the cold water pipe is shorter, reaching directly down to the cold resource, but the power generated has to be brought ashore, and moorings are likely to be in water depths of, typically, 2 000 metres. The development of High Voltage DC transmission offers substantial advantage to floating OTEC, and the increasing depths for offshore oil and gas production over the last decade mean that mooring can now be classed as 'current technology'- but remains a significant cost item for floating OTEC. Land-based plants have the advantage of no power transmission cable to shore, and no mooring costs. However, the cold water pipe has to cross the surf zone and then follow the seabed until the depth reaches approximately 1 000 metres - resulting in a much longer pipe which has therefore greater friction losses, and greater warming of the cold water before it reaches the heat exchanger, both resulting in lower efficiency.
The working cycle of an OTEC plant may be closed or open, the choice depending on circumstances. All these variants clearly develop their power in the tropical and sub-tropical zones (Fig. 15-1), to the benefit of countries in those parts of the world, but a grazing plant would allow OTEC energy use in highly-developed economies which lie in the world's temperate zones. In this case, the OTEC plant is free to drift in areas of ocean with a high temperature difference, the power being used to split sea water into liquid hydrogen and liquid oxygen. The hydrogen, and in cases where it is economic, also the oxygen, would be offloaded into shuttle tankers which would take the product to energy-hungry countries, where the infrastructure for liquid hydrogen distribution is now being initiated - for example in California. Also, the hydrogen may be an intermediate product, being used in turn to produce ammonia. At present, use of ammonia fertilisers is determined in part by production capacity from natural gas; the use of such fertilisers in the developing world - much of it in the tropical and sub-tropical zones where OTEC processes are available - could make a major contribution to world food production.
An especial benefit of OTEC is that, unlike most renewable energies, it is base-load - the thermal resource of the ocean ensures that the power source is available day or night, and with only modest variation from summer to winter. It is environmentally benign, and some floating OTEC plants would actually result in net CO2 absorption. And a further unique feature of OTEC is the additional products which can readily be derived - food (aquaculture and agriculture); pharmaceuticals; potable water; air conditioning; etc. Many of these arise from the pathogen-free, nutrient-rich, deep cold water. OTEC is therefore the basis for a whole family of Deep Ocean Water Applications (DOWA), which can additionally benefit the cost of generated electricity. Potable water production alone can reduce electricity generating costs by up to one-third, and is itself in very considerable demand in most areas where OTEC can operate.
In order to incorporate all these variables into an economic model it is necessary to assess:
the objectives of each application;
the state of the art;
other fields of application for the technology;
opportunities for further development.