Abstract
Process and Technology Status – Ocean Thermal Energy Conversion (OTEC) technologies use the temperature difference between warm seawater at the surface of the ocean, and cold seawater at between 800–1000 metres (m) depth to produce electricity. The warm seawater is used to produce a vapour that acts as a working fluid to drive turbines. The cold water is used to condense the vapour and ensure the vapour pressure difference drives the turbine. OTEC technologies are differentiated by the working fluids that can be used. Open Cycle OTEC uses seawater as the working fluid, Closed Cycle OTEC uses mostly ammonia. A variation of a Closed Cycle OTEC, called the Kalina Cycle, uses a mixture of water and ammonia. The use of ammonia as a working fluid reduces the size of the turbines and heat exchangers required. Other components of the OTEC plant consists of the platform (which can be land-based, moored to the sea floor, or floating), the electricity cables to transfer electricity back to shore, and the water ducting systems. There is considerable experience with all these system components in the offshore industry. The technical challenge is the size of the water ducting systems that need to be deployed in large scale OTEC plants. In particular, a 100 megawatt (MW) OTEC plant requires cold water pipes of 10 m diameter or more and a length of 1000 m, which need to be securely connected to the platforms. So far, only OTEC plants up to 1 MW have been built. Although it is technically feasible to build 10 MW plants using current design, manufacturing, deployment techniques and materials, the actual operating experience is still lacking. It is therefore important to learn and share the experience from the 10 MW plants under construction to ensure continuous and accelerated deployment.
Performance and Costs – OTEC provides electricity on a continuous (nonintermittent) basis and has a high capacity factor (around 90%). Although, small-scale applications have been tested and demonstrated since the late 1970s, most components have already been tested and are commercially available in the offshore industry. There are considerable economies of scale. Small scale OTEC plants (<10 MW) have high overheads, and installation costs lie between USD2010 16400 and USD2010 35400 per kilowatt (/kW). These small-scale OTEC plants can be made to accommodate the electricity production of small communities (5000-50 000 residents), but would require the production of valuable by-products – like fresh water or cooling – to be economically viable. For island states with electricity prices of USD 0.30 per kilowatt-hour (/kWh), OTEC can be an economically attractive option if the high up-front costs can be secured through loans with low interest rates. The estimated costs – based on feasibility studies – for larger scale installed OTEC plants range between USD2010 5000-15 000/kW, and the costs for large scale floating OTEC plants could be as low as USD2010 2500/kW that results in a levelised cost of electricity of around USD 0.07-0.19/kWh. These cost estimates are highly dependent on the financing options. Furthermore, these cost projections require large-scale deployment and a steep learning curve for OTEC deployment costs.
Potential and Barriers – OTEC has the highest potential when comparing all ocean energy technologies, and as many as 98 nations and territories have been identified that have viable OTEC resources in their exclusive economic zones. Recent studies suggest that total worldwide power generation capacity could be supplied by OTEC, and that this would have no impact on the ocean’s temperature profiles. Furthermore, a large number of island states in the Caribbean and Pacific Ocean have OTEC resources within 10 kilometres (km) of their shores. OTEC seems especially suitable and economically viable for remote islands in tropical seas where generation can be combined with other functions e.g., air-conditioning and fresh water production. The existing barriers are high up-front capital costs, and the lack of experience building OTEC plants at scale. Most funding still comes from governments and technology developers, but for large scale deployment, suitable finance options need to be developed to cover the upfront costs. From an environmental perspective, OTEC plants at scale will require large pipes to transport the volumes of water required to produce electricity, which might have an impact on marine life, as well as the infrastructures to transfer the water (for land-based systems) or electricity (for off-shore systems) to and from the coast line. Also because it is not a tried and tested technology at large scale, there are unknown risks to marine life at depth and on the seabed where there is large scale upward transfer of cold water with high nutrient content. From a technical perspective, the large-scale pipes, bio-fouling of the pipes and the heat exchangers, the corrosive environment, and discharge of seawater are still being researched.