Ocean Thermal Energy Conversion
Ocean thermal energy conversion, or OTEC, uses seawater to turn solar energy into electricity. It relies on the ocean’s thermal gradient - the temperature decline from the sun-warmed waters on the surface to the cold waters found at great depths. OTEC plants pipe in hot and cold seawater and run them through heat exchangers and water condensers, in the process spinning turbines that generate electricity. It can only be done efficiently where the thermal gradient within the upper 1,000 meters of the ocean is more than 20° Celsius. These conditions occur in most of Earth’s tropical seas.
The concept of OTEC power is enormously appealing. Sunlight is free and renewable every morning. And scientists estimate that OTEC has the potential to generate billions of watts of electricity. Yet only a few, mainly experimental, plants have been built. One of the problems that restrict OTEC is that the necessary thermal gradient is found at sea, but the power it can generate is needed on land. In this activity, you will examine some of the issues involved in this dilemma by comparing onshore and offshore OTEC facilities.
Examine and compare the two diagrams of OTEC facilities. You can click on each diagram to see a larger image. Once you understand how each of the OTECs work, answer the questions below.
OTEC plants built on land can easily transfer their electricity into commercial power grids, but don’t have immediate access to the thermal gradient.
- How do these plants tap into the thermal energy in the ocean? How might this impact the cost and efficiency of power generation?
The land based plants run long pipes out into the ocean, one close to the surface for warm water, and one deep to collect cold water. These pipes would have to be very long to reach far from shore into the two extremes of the thermal gradient. Building and maintaining the system is probably expensive since the pipes cross through the very energetic shore zone. And it must take a lot of power to pump the water that far, which would reduce the net electricity production.
- What are some of the physical considerations (such as geography, geology, and topography) that could affect where land-based OTEC facilities can be situated?
The thermal gradient is steep enough for OTEC only in tropical areas. The seafloor would have to drop off fairly quickly close to shore to allow access to deep cold water. Oceanfront land that is stable, large, and flat enough for a large power facility must be available.
- Based on the above, where do you think land-based OTEC plants will be most cost effective?
Land-based OTEC is likely to be a practical power source on tropical islands – not only do they tend to have the characteristics described above, they often lack other sources of energy such as oil and coal deposits.
OTEC plants built on ships at sea sit right on top of the thermal gradient but face challenges in transmitting the power they produce to shore.
- How could electricity generated by an ocean-going OTEC plant get to shore? How might this impact the cost and efficiency of power generation?
The electricity would have to be run through submerged power cables. This will reduce the efficiency of the plant, because of energy loss during transmission. And the cables would be expensive to maintain, since they are surrounded by corrosive saltwater and subject to waves and current activity.
- Some proposed floating OTEC plants don’t send electricity ashore, but use it onboard to produce fuels such as hydrogen that are then carried to land. What are some of the benefits and drawbacks of adding this extra step in making power?
The expense and inefficiencies of underwater power lines would be avoided, and the ships would be mobile, so they could travel to areas with the steepest thermal gradient or move out of the path of dangerous storm or sea conditions. The fuels that are produced could be shipped great distances, so that even landlocked countries could utilize OTEC power.
But the process of manufacturing fuels will use up some of the power, reducing the net energy production. Transporting the finished fuels will require ships and manpower and add to the costs. Many of these fuels are dangerous to produce and handle – hydrogen, for example, is quite explosive.
- What regions are most likely to utilize offshore OTEC plants?
The ships themselves would need to remain in tropical waters, and would be best suited for areas that are sheltered by nearby landmasses or that tend to have calmer sea conditions. Moored ships would be a solution for equatorial nations that don’t have suitable land for onshore facilities.
Mobile ships with onboard chemical plants could be built by and supply fuel to any country that could afford them, even those located far from the sea and the tropics.
OTEC is often hailed as environmentally friendly power, because it is "clean" and renewable. Describe some of the potential ecologic benefits of this energy source. But there’s no such thing as a "free lunch" - speculate on some of the environmental drawbacks of OTEC plants.
Scientists seek to understand and explain how the natural world works. Many of the questions raised in this endeavor have no absolute answers.
OTEC plants pull hot and cold water from the sea, but these resources are continuously replenished by natural, geologic processes. Unlike conventional fuels such as petroleum and coal, OTEC power doesn’t require mining or drilling operations, and produces no toxic byproducts.
But onshore facilities must be built along shorelines, which are often ecologically fragile. Plants and power lines are unsightly. Associated facilities, such as refrigeration plants or large-scale agriculture or aquaculture may have a large footprint and interfere with native species.
Ships at sea may spill or dump chemicals, fuel, or other toxic substances overboard.
Both types of facilities discharge large amounts of used cold and warm water back into the sea after use. If this water has a markedly different temperature than the surrounding seawater, it may harm marine life. This water may have become tainted by dangerous substances during the OTEC process.