Ocean thermal energy conversion for Japan?

Introduction

On march 11, 2011, Japan was hit by an earth quake and a tsunami which resulted in the Fukushima Daiichi nuclear disaster. Consequently the public opinion in Japan turned 180 degrees against nuclear power. Even their government began to consider a nuclear free future. But Japan is so heavily dependent on nuclear power, that last summer two nuclear power plants had to be restarted in the face of massive public opposition. The question of this post is what are the alternatives for Japan? I will discuss solar power, wind power and Ocean Thermal Energy Conversion (OTEC). [However, both wind power and OTEC are in fact indirect forms of solar energy since both winds and the oceans are powered by the Sun.]

Wind and Solar power

These are the “classical” kinds of alternative energy sources. Both options require a lot of space, and the intensity of solar radiation depends on altitude (lowest at the poles, highest at the equator). Japan is roughly located between 30 and 45 degrees north, which is a good point to start with. Currently Japan, together with Germany and China, is one the biggest producers of photo-voltaic cells. Yes, solar power is clearly a major alternative for Japan, but there are some other issues. First Japan is a mountainous country and has high population density, so relatively few land area is available for massive solar farms. To some extent this can be solved by placing solar arrays on roof tops, however this will only be a partial, but very important, solution for Japan’s energy problem.

Another important potential energy source is wind power. There are two main problems with wind power. First it is notorious because it is very irregular, secondly in order to be efficient wind turbines have to place at a minimum distance from each other. This means that in a given area only a fixed number of turbines can be placed. As we know in Japan land is a scarce commodity, although this can be solved by placing wind turbines at sea. In theory Japan has the potential to meet its entire energy demand through wind power alone, but then they have to deal with the irregular supply of energy. Some kind of grid energy storage is needed. Note that solar power has the disadvantage that it is not available during the night.

What is Ocean Thermal Energy Conversion?

Surface water of the oceans has a higher temperature than water a thousand meters below the surface, this difference in temperature represents a difference in energy (as heat), which can be used to power a heat engine. The basic idea is to heat a working fluid with the heat from surface water, subsequently this working fluid is vaporized. Then we can use this to drive a turbine, which produces electricity. Thereafter we let the working fluid condense by cooling it with cold water, which we have pumped up from a thousand meters below the surface of the ocean. In a closed cycle we let the fluid flow back to the evaporator, while in an open cycle we “dump” it somehow into the environment. A more detailed explanation how OTEC works can be found here.

Other proposed applications of OTEC are for instance mariculture, desalination and mineral extraction. Regardless whether these applications are practical, we should ask our selves where the best location for OTEC power plants are. This map shows that Japan is relatively close to one of the best (i.e. the locations with the highest difference between surface water and at a debt of a thousand meters), and this spot also covers a large area. Therefore this area contains an enormous amount of power.

Synthetic fuels

Traditional power lines are impracticable, so energy has to stored somehow. One way to do this, is by producing hydrogen through electrolysis. But hydrogen has some severe drawbacks. First the very low density of hydrogen gas requires either storage under high pressure or liquefaction to very low temperatures,  which might cost more energy than can be delivered. The storage problem of hydrogen is one of the greatest obstacles for the transition to a hydrogen economy.

An alternative for hydrogen would be the production of synthetic fuels through the Fischer-Tropsch process from hydrogen and carbon monoxide gas. CO gas can be obtained by electrolysis of CO2 from the atmosphere or sea water. There is also current research of creating fuels directly from water and CO2. Both methods will produce hydrocarbons, like methane gas [main component of natural gas], or alcohols like methanol. These synthetic fuels can easily be transported and because the synthesized fuels are chemically similar to “mineral” gasoline, they do not suffer from the transition paradox. This is the problem that no one will buy hydrogen cars if there are no hydrogen gas station, but no one will build hydrogen gas station if no one drives hydrogen cars.

Once the synthetic fuel is produced, it can be transported by tankers to where it is needed. This also solves another problem: the energy production can now be fine tuned to the demand. During peak hours more fuel is burned, and when energy demand is low we can simply turn off some generators.

Conclusion

Ocean Thermal Energy Conversion is an option, which deserves consideration if Japan really wants to phase out nuclear power. Unlike wind power OTEC is a reliable and predictable source of power. In combination with synthetic fuels it also offers the possibility to make Japan self-sufficient in respect of its energy supply. Of course Ocean Thermal Energy Conversion in combination with the production of synthetic fuels is not only a solution for Japan, but it is also interesting for emerging economies like China and India.

See also:

http://www.nrel.gov/otec/

http://en.wikipedia.org/wiki/Synthetic_fuel

The next link is a critique of the hydrogen economy, and prefers an “electron” economy.

http://phys.org/news85074285.html

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