OTEC - Ocean Thermal Energy Conversion

March 2005

This month I would like to tell you about a form of generating electricity that is used throughout the world, but not on a large scale. I'm talking about geothermal electricity production and Ocean Thermal Energy Conversion (OTEC). Both of these forms of power generation use the same principles. Basically, they convert a "working fluid" into a vapor and use the expansion to turn a turbine which turns the generator. Nothing new, right? Well I bet there are a few things you didn't know about this process... OTEC uses the temperature difference between the warmer upper layer and colder lower layer of the ocean to heat & cool the working fluid. The upper layer of ocean water is not hot enough to boil water and make steam, so how is the steam made in these systems? The answer is two ways - 1) put the working fluid under a vacuum thus lowering its boiling point and 2) using a working fluid with a lower boiling point to start with like ammonia. Geothermal systems are usually a lot hotter, so it is easier to flash the working fluid into steam. Here are two system diagrams of an "open cycle" and a "closed cycle" system...

Open Cycle System

Closed Cycle System

The purpose of this newsletter is not to educate you about the minute details of how these systems work, but to give you a general idea how they work and then give you some ideas on how you can utilize this technology to generate electricity in your own back yard. After all, who is going to install an OTEC or geothermal system in their back yard? Not a whole lot of people - that's for sure! But... you can use some of these ideas to generate your own steam. And steam = power, right? Most of today's electricity is generated using steam turbines. Even nuclear power plants use the heat from the nuclear reaction to generate steam that turns a turbine. It's technology that is 100's of years old. This country was built with steam and it still runs on steam!
In "Electricity - Make it, Don't Buy it" I explain to you how you can make water boil at room temperature or a lower temperature by submitting it to a vacuum. Wouldn't it be easier to make steam if water would boil at 150 deg F instead of 212 deg F? Where can you get a source of heat in your back yard? The sun of course! If you pull a vacuum on a closed cycle system with water as the working fluid, you can make that water flash evaporate at a lower temperature. That means you don't need a huge, fancy, expensive parabolic solar collector or high temperature fresnel lens to make steam. A simple solar water heater working on the thermosiphon principle will do as described in the book. Now I'm not going to give you a detailed schematic of a system you can build because I have never done it myself. I'm just going to give you some ideas and let you design your own system the way you want to do it. Here are a few tips...

Use water as the working fluid because it is less toxic, less corrisive, easier to work with, and more available than other working fluids with a lower boiling point.
Vacuum pumps can be found on eBay at a fairly cheap price.
Small steam turbines are almost non-existant, so look into building your own turbine or maybe even using/building a Tesla turbine (bladeless turbine). I even thought about using a pneumatic motor like the ones used in air tools.
A venturi nozzle between the evaporator and turbine will accelerate the steam to supersonic speeds.
You will have to condense the steam back to a liquid after it runs through the turbine - try using cheap automobile transmission oil coolers as heat exchangers.
Use a water pump to pump the condensed water back into the flash evaporator.
The more water surface area exposed to the vacuum, the faster it will flash evaporate - so use a nozzle to spray the water into the flashing chamber.
I would use a DC generator instead of an AC generator head so you can charge the batteries directly without having to use an innefficient AC to DC battery charger.
Try to have about a 50 deg difference between the hot side (evaporator) and cold side (condenser).
The cold side can be air, shaded air, water source, underground, etc.

Here is a table showing the boiling point of water at different levels of vacuum:

Inches Hg         Deg F
-------------------------
5                     203.80
10                   192.37
15                   179.14
20                   161.49
25                   133.76
27                   101.96
29                     79.03
29.5                  58.80
29.8                  34.57

The genius Nikola Tesla wrote about a very similar system in his "Our Future Motive Power" published in Everyday Science and Mechanics in December 1931. Here are some pictures containing the original article. I must warn you up front - the pictures are not the best quality. I wouldn't even think about giving you such low quality, but this is a very, very interesting article. Just click on the links or "right click" on them and save them to your hard drive. They can be viewed with Windows Picture Viewer or resized within Internet Explorer. If you zoom in several times, they will be easier to read. If you print them out, use a magnifying glass to help with the reading...

Our Future Page 1
Our Future Page 2
Our Future Page 3
Our Future Page 4
Our Future Page 5
Our Future Page 6
Our Future Page 7

I tell you in "Electricity - Make it, Don't Buy it" how to access the US Patent database. Here are a few patents relating to these types of systems that are very interesting...

Open Cycle OTEC Plant     4,430,861
Method and Means for lifting water and generating power therefrom     4,475,342
Atmospheric Latent Heat Engine     4,878,349
Thermodynamic Process and Latent Heat Engine     4,106,294

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