Frequently asked questions about heat engines and traditional OTEC.

OTEC is organic Rankine cycle with sea water as a renewable heat source, why isn’t it developed already?

Of course, for some people OTEC is obviously viable this is just organic Rankine cycle (ORC) with very low-grade heat. However, this is where the controversy begins; does very low-grade heat work in ORC or not and if so, how well? OTEC is doubted for this reason, and equipment costs are high enough to have stalled private development of traditional OTEC.

Can’t you add more heat?

There is one simple fix can be applied and that is to use additional heat, With the use of the additional heat the grade of heat can match that of an untold number of sites where ORC. is working and the exact same ‘bankable’ machine can be purchased. Problem solved!

Doesn’t this make the use of sea water pointless?

This does NOT make the use of seawater pointless, far from it. The organic Rankine cycle requires for working fluid phase changes. It is the phase change from gas to liquid and liquid to gas that takes allot of energy.

The quantity of energy required to change the state from gas to liquid and liquid to gas is relatively large. Much greater than the net power that can be produced by natural seawater temperatures, there is a ratio of about sixty waste heat to one net power produced at sites near the edge of the tropics like the Bahamas.

How much heat must be added for what benefit?

In a traditional OTEC ORC system the working fluid can be made to vaporise and condense at different temperatures. The working fluid vapor is condensing around 12 degrees, this is to meet with the deep-sea-cold-water temperature of around 4 degrees. Vaporising working fluid from 12 degrees to under 20 degrees? Given this example the system is not economically viable, this is equivalent to subtropical. Once over about 23 degrees from 12 degrees and the system becomes worthwhile economically, this is equivalent to just inside the tropics.

However, if you can get up to around 30 degrees from 12 degrees, equivalent to the Equator, the user will already have doubled the net power to capital cost ratio when compared with sites just on the inside of the tropics.

What sites are eligible then?

This depends on whether you mean natural OTEC or not, heat engines and natural sources of hot and cold can be made to work in many ways. Natural hot and cold is a selling point of OTEC.

OTEC is traditionally associated with polar meltwater, so definitions may obscure feasibility, so too the assumption that OTEC is using a rigid pipe and the cost problems associated with that method.

Can you add as much heat as you like?

There might be no need for a NEW-OTEC system to go over 30 degrees for the user to be happy with the economics of the plant, however, the system can go on to add as much heat as is practical using traditional heat transfer methods just as traditional power plants do.

Does this mean that a NEW-OTEC plant can be the best way to produce power from any heat source? 

It certainly looks like it.

As surface water temperatures go up in the summer less additional heat may need to be added to generate the same amount of net power as in the winter. That means greater fuel economy through warmer months. Even geothermal sites can benefit by reducing the amount of work that the geothermal heat source must do.

Use of NEW-OTEC even with dirty fuels is a better way of creating electrical power; eligible sites can use the same equipment as traditional ORC., but with a vastly improved fuel economy.

 

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