The Sea- an Underused Heat Reserve

Using heat from the oceans' waters to produce energy is not a new idea. However, we are still at the experimental stage. This is because there are still a number of technical obstacles and few sites that are suitable for setting up these facilities. Heat pumps offer an alternative whereby the sea's abundant energy can be used.

The oceans' thermal energy
© Idé 

An Abundant but Unevenly Distributed Energy Source

Using the thermal energy of the oceans involves using the temperature difference between surface water (22°C and above) and deep water (2-4°C at 1000m) to vaporize a fluid and drive a turbo generator. Because of its high speed, steam can drive a turbine.

To obtain steam, the fluid selected must have a condensation point (the moment when it changes state from liquid to gas and vice versa) equivalent to the temperature of relatively cold water. For example, ammonia changes state at 15°C¹.

The fluid changes from a liquid to a vaporized state in an evaporator that is in contact with warm sea water pumped at the surface. The steam then passes through a turbo generator to produce electricity, and cools down in a condenser that is in contact with cold sea water pumped up from below. On land, a transformer recovers the electricity produced by the turbo generator and sends it to the grid.

The advantages of the thermal energy of the oceans are as follows:

   • No greenhouse gas emissions

and

The are various disadvantages, which are:

   • It can only be tapped in intertropical regions, where the surface water is warm enough and the deep water cold enough to obtain a temperature differential of about 20°C;

  • Economically, efficiency (the ratio between the energy spent to produce electricity and the energy eventually recovered) is very low- only 2-3%;

 • To operate, an ocean thermal energy conversion (OTEC) power plant needs a very high flow of water and therefore huge pipes measuring at least 10 meters in diameter. Thus, facility investment costs are very high;

   • It can disrupt natural sea flows locally.

Given the investment and geographical conditions required, only the United States and Japan have launched experiments.

On a global scale, the seawater's technically workable potential is about 10-80000 TWh per year. However, given the investment in research and development needed and the geographical conditions required, only the United States (in Hawaii) and Japan have recently launched experiments involving the thermal energy of the oceans. Industrial solutions are not expected to come on stream before 2030².

The thermal energy of the oceans is a century-old solution

True. Towards the end of the 19^th century, French physicist Jacques Arsene d'Arsonval became aware of the oceans' potential thermal energy while developing refrigerating machines. In the 1930s, another Frenchman, Georges Claude, experimented with the production of electricity using the temperature difference in the sea between warmer surface water and cooler deep water. In 1933, he transformed the cargo vessel La Tunisie into a refrigerating plant, producing 2000 tons of ice per day!

Heat Pumps- a Modest and Realistic Application

On a smaller scale, there have been some experiments involving using the heat of water from warm seas (such as the Mediterranean) using heat pumps³.

   • Since 1960, the principality of Monaco has produced 15% of its electricity using heat pumps that take heat directly from the sea. Many buildings located on the coast benefit from this for heating in winter and air-conditioning in summer.

Since 1960, the principality of Monaco has produced 15% of its electricity using heat pumps that take heat directly from the sea.

  
   • In 2007, La Seyne-sur-Mer in Southeastern France started heating some of its public buildings, including a 500-seat theatre, the town hall, and 500 new housing units using thermal energy from the sea. To be economically viable, the buildings must be located less than 600m from the coast. The facilities cost the town € 2.5 million. In exchange, this solution should reduce users' energy bills by two thirds and will reduce the amount of annual greenhouse gas emissions by 1300 tons.

   • The water from colder seas can also be used for heating (in winter) or air-conditioning (in summer). The 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 the books. In this case, the heat exchange fluid (the temperature of which varies when it comes into contact with water and enters the pump circuit) is propane, a gas derived from oil.

   • Cold water from lakes can also be used to produce heat. In Zurich, Switzerland, some of the urban heating is provided by pumping lake water (4°C).

[1] http://www.inter-mines.org/docs/0904140804PR_090319_DeLaleu.pdf%20pp.40-49
[2] http://www.nanodata.com/sdn76/epr3/doc/analyse-energie-des-mers.pdf p.35
[3] http://www.nanodata.com/sdn76/epr3/doc/analyse-energie-des-mers.pdf