The Ocean, an Underused Heat Reservoir
5 min read
Using from ocean waters to produce energy is not a new idea. However, a number of major technical obstacles must be overcome and there are few suitable sites for these facilities. Heat pumps can sometimes also offer an alternative solution for harnessing the thermal energy of the ocean.
© D. MCNEW / AFP - The temperature differential between surface waters and deeper layers can be tapped.
Using Temperature Differentials
Harnessing the thermal energy of the oceans involves using the temperature differential between warm surface waters (22°C and above) and deeper cold waters (2 to 4°C at 1,000 meters) to vaporize a fluid. The resultant vapor then expands to drive a turbine generator.
To achieve vaporization, the fluid must have a very low boiling point, somewhere between the temperature of the warm seawater and that of the cold seawater. One option is ammonia (NH3), which, at normal pressure, changes from liquid to gas at 15°C.1
When it comes into contact with warm seawater pumped from the surface, the fluid changes state in an evaporator. The vapor then drives a turbine generator to produce , before returning to a liquid state in a condenser in contact with cold seawater pumped up from lower layers.
Low Efficiency, High Investment Costs
Using the thermal energy of oceans — an abundant, stable resource — produces no direct emissions. But this energy source has a number of disadvantages:
- It can only be tapped in intertropical regions, where the surface waters are warm enough and the deep waters cold enough to obtain a temperature differential of about 20°C.
- Efficiency — the between the energy used to produce electricity and the energy eventually recovered — is very low, only 2 to 3%.
- The investment cost is very high. An ocean thermal energy (OTEC) plant needs a very high water flow, which in turn necessitates huge pipes measuring at least 10 meters in diameter.
- Natural sea flows can be disrupted locally.
On a global scale, the potential that could be technically achieved ranges from 10,000 to 80,000 TWh per year. However, given both the R&D spending and specific geographical conditions required, the United States (in Hawaii) and Japan are the only two countries that have recently launched experiments to tap the thermal energy of the oceans. Commercial-scale solutions are not expected to come on stream before 2030.
Heat Pumps, a Modest but Realistic Application
On a smaller scale, experiments using heat from warm seawater, such as in the Mediterranean, have been carried out utilizing heat pumps.
- Since 1960, the Principality of Monaco has generated 15% of its electricity using heat pumps that take their energy from the sea. Many buildings located on the coast use this energy source for heating in winter and air conditioning in summer.
- In 2007, La Seyne-sur-Mer in southeastern France began heating some of its public buildings — including a 500-seat theater and municipal offices — and 500 new homes using thermal energy from the sea. The facilities cost the city €2.5 million. The solution is expected to reduce users' energy bills by two-thirds and cut annual greenhouse gas emissions by 1,300 tons.
Water from colder seas can also be used for heating and air conditioning. The public library in Visby, Sweden, on the Baltic Sea, uses seawater to supply an electric heat pump and maintain a constant, stable temperature that helps preserve its books. Propane is used as a .
Cold water from lakes can also be used to produce heat. In Zurich, Switzerland, some district heating is provided using lake water, which has a temperature of 4°C, as a source.
The Thermal Energy of the Oceans, a Century-Old Solution
French physicist Jacques-Arsène d’Arsonval first proposed ocean thermal energy conversion in the late 19th century, developing refrigerating machines.
In the 1930s, another Frenchman, Georges Claude, experimented with power generation using the temperature differential between warmer surface waters and cooler deep layers. In 1933, he transformed the cargo vessel La Tunisie into a floating plant producing 2,000 tons of ice per day.
