Ocean thermal energy conversion

Figure 1. Makai Ocean Engineering's 105 kW OTEC plant in Kona, HI.

Ocean thermal energy conversion, or simply OTEC, is the process of using the ocean itself as a solar collector. This technology (seen in figure 1) utilizes the slight temperature gradient between the warm surface of the ocean and the cooler water deeper down. This technology operates in a similar way that solar ponds do but on a much larger scale. Although there is a temperature gradient, the overall temperature difference with depth is fairly small. In waters deeper than 1000 meters the temperature change might only amount to 20°C.

In the 1990s, the Natural Energy Laboratory of Hawaii Authority was one of the world's leading test facilities for OTEC technology. In 2015, the facility became operational and now supplies electricity to the local electricity grid. Advances in deep-water piping technology makes OTEC more feasible as a reliable power solution for coastal communities.

How it Works

As shown in figure 2, OTEC power plants make use of the ocean's vertical thermal gradient, where the temperature differential between surface water and deep ocean can drive a heat engine. In tropical oceans specifically, surface water often exceeds temperatures of 25°C (77°F), while the water 1000 meters down is generally 5°C (41°F). OTEC power plants operate like any other thermal power plant, however they require the use of an organic Rankine cycle.

Figure 2. A diagram showing how an open-cycle OTEC facility uses ocean water to generate electricity.

Closed-cycle vs. open-cycle

In closed-cycle plants, the warm surface sea water heats up some fluid with a low boiling point (that is not the sea water itself). The fluid needs a low boiling point because the surface water itself isn't very warm, so generally ammonia is used (which has a boiling point of -33°C at atmospheric pressure). The warm water heats this fluid, turning it into a gas that drives a turbine to generate electricity. This vapour then comes into contact with a cold reservoir, chilled with cold water from below the ocean surface. This condenses the vapour and the process begins again.

In open-cycle systems, the seawater itself is the working fluid. To drive the turbine, warm surface water is flash-evaporated in a low-pressure environment. Then, the steam is condensed by the cold deep water. This system yields both electricity and desalinated water as a byproduct.

Environmental Impacts

The primary concern with open-cycle OTEC facilities is the potential harm to marine life that could occur as a result of the mixing of warm and cool ocean waters. This mixing changes the temperature gradient of the ocean in a specific area, disrupting the ecosystem. For example, nutrients that are brought to the surface with the cold deep water could increase the number of fish or lead to algae blooms. If these facilities are large scale, the average ocean surface temperature could even be affected and this would lead to changes in the weather, climate, and ocean circulation.

For Further Reading

• Solar collector
• Organic Rankine cycle
• Ocean heat
• Ammonia
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