Marine renewables, also known as ocean energy, refers to a broad range of technologies that extract energy from the ocean; this energy can be in the form of ocean waves, tidal movements or thermal gradients. Marine renewables are, in general, at a relatively early stage in their development and, as such, the methods of converting these potential energy sources into useful electrical power are still highly diversified, with many technologies competing for commercial viability.
Ocean Thermal Energy Conversion systems take advantage of the small difference in temperature between the ocean’s surface and depths that is found at certain locations. This temperature gradient can be used to generate power using specialised equipment; however at this time the technology is still at the relatively early stages of development and is not cost competitive.
Ocean thermal energy conversion (OTEC) uses the temperature difference that exists between deep and shallow ocean water to run a heat engine. Pipes are used to bring large volumes of colder water up from the depths while warmer water is also drawn into the OTEC plant from the ocean surface. Although this temperature difference generally increases with decreasing latitude, i.e. near the equator, it is still very small, roughly 20 degrees Celsius, even at good sites (Berger and Berger, 1986). This limits the theoretical maximum efficiency to between 6 and 7 percent which must be then reduced further to account for real world pumping and plant operation loads. OTEC systems face further technical hurdles in overcoming heat exchanger biofouling which reduces efficiency, dissolved gas heat exchanger erosion as cold water is drawn up and difficulties in maintaining the very low pressures required in many systems. However one of the main benefits of OTEC systems is that they could potentially be used to provide baseload as there is no intermittency associated with their resource.
There are three types of OTEC systems: closed-cycle, open-cycle, and hybrid. Closed-cycle systems use the warm water at the ocean’s surface to vaporise a working fluid with a low-boiling point such as ammonia. The expanding vapour rotates a turbine. Open-cycle systems actually boil the seawater by operating at low pressures and pass this through a turbine in a Rankine thermodynamic cycle. Hybrid systems combine closed- and open-cycle designs. The ammonia or sea water is then cooled/liquefied by the colder water that has been drawn up from depth.
The total energy available is one or two orders of magnitude higher than other ocean energy options such as wave power; but the small magnitude of the temperature difference makes energy extraction comparatively difficult and expensive, due to low thermal efficiency. The opportunities for OTEC extraction improve closer to the equator in the tropics, but it should still be understood that the technology is quite far from commercial application and relatively little research is done on it globally.
The best OTEC locations are those that have high surface temperatures and access to cold deep water such as Hawaii. Without adjacent deep water (a significant constraint for many locations due to their continental shelves) the capital cost of lengthy pipelines or offshore OTEC can be a considerable economic deterrent.
The installation of an OTEC system would require a significant component of fabrication for the necessary piping and also marine operations for the installation. Specialised low pressure generation equipment would likely need to be purchased from abroad, but at this stage there is no company is manufacturing such equipment. Much of the onshore work (assuming the system is located on land) would be similar to that required in a conventional thermal power station regarding pipework, electrical work etc.
Maintenance of OTEC systems is hard to determine at such an early stage of development. As it stands OTEC systems have not necessarily overcome the issues of biofouling, heat exchanger degradation and sealing. Broadly the maintenance requirements of an onshore OTEC plant should not be dissimilar from a conventional thermal plant.
The installation of a commercial OTEC plant in a modern setting would require a rigorous programme of consultation and environmental impact assessments. Regulatory experience could possibly be drawn from schemes such as geothermal power plants, large geothermal ground heat pumps or the deep water lake cooling systems such as the one installed in New York, US (Cornell, 2006). There is unlikely to be any kind of established regulatory framework in any country possibly with the exception of the US due to their experience with the Keahole Point project in Hawaii.
There is very little experience with public acceptance of OTEC systems due to the extremely limited number of projects completed to date. It is reasonable to expect that public perception of OTEC would be largely positive as long as the environmental issues were well understood and managed. The plant has an onshore footprint not much different to a conventional geothermal installation and its operation is invisible to the public.
There are very few OTEC installations around the world. At Keahole Point in Hawaii the US Government has had a small test facility operating since 1974, a small plant was installed and operated on the island of Naru by a Japanese firm and the southern state of Tamil Nadu in India also investigated the possibility of a floating OTEC platform; however all these developments date from a number of decades ago.
After a significant research focus in the USA in the 1980’s following the oil crisis of the mid seventies, there has been a significant decline in the level of attention given to OTEC systems. There are plans in the USA to build a 1MW OTEC facility in Hawaii (HBEDT, 2009) where much of the prior research into OTEC systems has taken place. Research is also continuing into pipeline fabrication techniques to reduce costs and maintenance (Lockheed Martin, 2008). At this point in time OTEC should be considered as a technology under development and not ready for commercial application.
The environmental impacts of OTEC systems are largely unknown. The main concerns around the technology would be the effect on the local ocean surface ecosystem due to the release of large volumes of cooler water and the possibility that marine creatures were drawn into the piping that feeds the OTEC plant. Only during the course of further development, EIA studies and larger projects will the marine energy community be able to gain a firmer idea of any potential impacts on marine life.
OTEC systems directly contribute to climate change mitigation by providing a completely renewable energy source free of GHG emissions (beyond the initial GHG gases associated with construction). However, the relative lack of development of OTEC schemes and their low efficiencies means the total installed capacity will likely remain very small in the short to medium term meaning that their overall contribution to mitigation with the next decades will be relatively small.
As marine renewables are still largely at the R&D phase, with a corresponding lack of commercial devices, it is very difficult to accurately estimate costs. The IEA (2009) put lifetime delivered energy costs at USD 150/MWh to 300/MWh across the range of technologies (minus tidal barrage), generally well outside the range of current electricity revenues even when current carbon finance incentives (available in certain countries) are considered. They estimate that costs will need to reduce to between a third and a quarter of their current levels to be feasible without significant support.
OTEC systems are limited by the high infrastructure costs involved and low net efficiencies that can be achieved along with significant maintenance costs in the pumping and piping infrastructure.
Uncertainties about the costs and technical performance of OTEC systems must be overcome before significant commercial investment can be attracted. Large-scale prototype/demonstration schemes can help in this respect to inform investors. However at this stage OTEC must be seen as a technology at the early stages of development with a relatively low research profile.
Berger, L.R. & Berger, J.A. 1986. Countermeasures to Microbiofouling in Simulated Ocean Thermal Energy Conversion Heat Exchangers with Surface and Deep Ocean Waters in Hawaii, Applied and Experimental Microbiology, vol. 51, no. 6, pp. 1186 – 1198.
Lockheed Martin, 2008. U.S. Department of Energy Awards Lockheed Martin Contract to Demonstrate Innovative Ocean Thermal Energy Conversion Subsystem , available at: www.lockheedmartin.com/news/press_releases/2008/100808_OTEC_Contract.html, last accessed 17/05/2010
Hawaiin Department of Business Economic Development and Tourist (HBEDT) 2009. Ocean Thermal Energy, available at: hawaii.gov/dbedt/info/energy/renewable/otec, last accessed 17/05/2010
Cornell, 2006. Lake Source Cooling, available at: www.utilities.cornell.edu/utl_ldlsc.html, last accessed 17/05/2010