OTEC

OTEC (Ocean Thermal Energy Conversion), involves taking advantage of the considerable difference in temperature between the top of the ocean and the deep water. In effect it is recovering the suns energy that has been stored. A very large amount of research has been done on Hawaii. Commercial plants have also been produced. Some methods interestingly produce fresh water as a bi product.

Research on Hawaii has been looking for a cooling depth of 2,000 ft, and temperature shift of 40 degrees Fahrenheit, they have plenty of hot water but their largest difficulty was accessing deep water. We have no difficulty with access to deep water, but the surface water is cooler at our chosen location.

The basic idea is not new, a French engineer came up with the idea in the early 1880's, and a lot of work was done in the 1970's and 80's, with the first system to produce usable amounts being a mini OTEC producing around 15kw net on a naval barge off Hawaii in 1979. By 1982 a Japanese company had developed a plant producing 35kw net. Since then, developments have continued with some requiring a working depth of just 90 metres.

OTEC -The Basic Process

There are basically three types of OTEC processes: closed-cycle, open-cycle, and hybrid-cycle.

The closed-cycle system, heat transferred from the warm surface sea water causes a working fluid (such as ammonia, which boils at a temperature of about 78°F at atmospheric pressure), to turn to vapor. The expanding vapor drives a turbine attached to a generator which produces electricity. Cold sea water passing through a condenser containing the vaporized working fluid turns the vapor back into a liquid which is then recycled through the system. All the early systems were of this type.

Open-cycle uses the warm surface water itself as the working fluid. The water vaporizes in a near vacuum at surface water temperatures. The expanding vapor drives a low-pressure turbine attached to a generator which produces electricity.

The vapor, which has lost its salt and is almost pure fresh water, is condensed back into a liquid by exposure to cold temperatures from deep ocean water. If the condenser keeps the vapor from direct contact with sea water, the condensed water can be used for drinking water, irrigation or agriculture.

A "direct contact" condenser produces more electricity, but the vapor is mixed with cold sea water and the discharge water is salty. That mixture is returned to the ocean. The process is repeated with a continuous supply of warm surface sea water.

Hybrid systems use parts of both open- and closed-cycle systems to optimize production of electricity and fresh water.

Our ideas

From what you have seen above you will see that the open cycle system without a direct contact condenser would appear to be the answer in that we get electricity without pollution and fresh water as well.

We therefore are looking to make the operation more efficient by using extra heat using glass domes, to warm some water more and by recovering heat and using it to raise the water temperature.

In effect we are proposing adding two extra stages, in that we are taking the top sea water, putting it through an extra process to add extra heat to it to increase the temperature difference, and then using one or more of the OTEC processes. We then look to another extra stage in that we use the surface water to part cool, and transfer in some heat, before using the deep sea water for final cooling. Some researchers have suggested a far smaller temperature shift is economic.

As we will have a fair amount of recovered heat that can be used, at first sight we may appear to have totally free energy. However there is one running cost, electricity, or some other energy is required to operate pumps to pull the water up from a depth, and to drive the heat recovery pumps. It does not require a lot of energy to bring up the cold water which has only a fractional direct weight than the surface water, and it has even been suggested that wave, bobbing float pump or current paddle wheel may do this. In one small experimental setup it was found that pulling water up from 2,000 feet on a 100 megawatt OTEC pumping and operation consumed 41 megawatts leaving a profit on the exercise of 59 megawatts.

Studies by others have suggested OTEC's are fully practical, but due to the need to get deep water and installation/building costs the electricity produced could be produced cheaper by burning oil. With the exception of Hawaii, where oil is not easily available, very little further has been done, especially looking longer term, at reduced pollution and as an overall package. There are however small commercial plants available, see links below.

Another study showed a 2 megawatt (net) system produced 4,300 cubic meters of desalinated water a day, as a bi product. It also produced the balance of the contents of the water 57 elements, including gold, platinum, and a wide range of other items that may be able to be profitably separated. Other work has been done before on this and given up due to the cost of pumping the large amount of water involved, however we have already covered this above.

We believe this is very much a technology in its infancy and that when other power sources are used for pumping when available, energy is stored, heat recovery is used to boost the operation and more yet to be proven or discovered the operating profit will be far greater as a percentage. The plant may also be more efficient as it becomes larger. The one point to remember is that most if not all studies so far have tended to be looking at producing energy for on land use from the sea, where we are on the sea, which requires less pipes and pumping, and greater depth available.

So where do you go from here:-

Our page under seacrete and its supplementary consider page looks at some of the other bi products of OTECs, including their use in protein production.

T OTEC Fact Sheet.

www.nrel.gov/otec/ covers history, possibilities etc around OTEC's.

www.hawaii.gov/dbedt/ert/otec_hi.html Hawaii developments and research.

www.seasolarpower.com/ Company producing small commercial plants.

Entering OTEC into AOL's search engine brought up over 76,000 links so there is quite a bit to explore!!!

http://www.luf.org/wiki/view/Project/HowMuchIsTheOceanWorth a project looking at harvesting minor metal contents from the sea.

http://www.seafriends.org.nz/oceano/seawater.htm is a very detailed breakdown of the contents within seawater. Although it may not be economic to go after these items on their own, as a bi-product of OTEC's there may be some economic recovery possible.