To understand the details of the design and operation of a CC-OTEC plant, it is useful to consider a specific example given by the 5 MW (nominal) floating hybrid-OTEC.  The author conceived this plant, as the pre-commercial plant needed to demonstrate the technical and economical viability of OTEC and to assess the environmental impact.  Unfortunately, funding was not secured.

A simplified flow diagram of the power cycle is shown in Figure 4.  The plant is based on a closed-cycle for electricity production and on a second stage, using the effluent water streams from the power cycle, for desalinated water production.  The baseline is for a floating plant, i.e., the power and water cycles are housed in a barge or ship with the electricity transmitted to shore via a 15 cm submarine power cable and the desalinated water via a small, 15 to 16 cm diameter hose pipe.

Assuming temperatures of 26 °C and 4.5 °C for the surface and deep ocean waters, in the electricity production mode, a gross power output of 7920 kW, using off-the-shelf technology, is sufficient to produce 5260 kW-net with an in-plant consumption of 2660 kW. The power output for this cycle varies as a function of surface water temperature (the cold water temperature is essentially constant) by 860 kW per °C.  For example, for 28 °C temperature (average summer conditions in Hawaii) the output would be 6980 kW-net.  With the combined production of desalinated water and electricity, the baseline outputs would be 5100 kW-net (160 kW required for the second stage plant) and a daily production of  2281 m³ of desalinated water.  This water output is only 20 percent of the amount that can be produced with the second stage.

The proposed baseline facility will employ pressurized ammonia as the working fluid in the power cycle.  The baseline seawater flow rates are:  26.4 m³ per day of warm water and 13.9 m³ per day of cold water.  These flow rates can be supplied using validated technologies.  A 2.74 m (inside diameter) glass fiber reinforced plastic (FRP) cold water pipe will be suspended from the barge to a depth of 1000 m.


Warm seawater will be drawn in through a 4.6 m  FRP pipe from a depth of 20 m.  The mixed effluent will be discharged through a 5.5 m FRP pipe at a depth of 60 m.  This discharge depth has been selected to minimize the environmental impact.  The baseline design employs compact heat exchangers for the evaporator and condenser.  A chlorinating unit will be installed to minimize biofouling of the evaporator passages.  It is known that biofouling from cold seawater is negligible and that evaporator fouling can be controlled effectively by intermittent chlorinating (50-100 parts per billion chlorine for 1 hour every day).  Monitoring of the effluent water for elevated concentrations of ammonia or chlorine will be performed on a regular basis.


The seawater effluents from the power cycle exhibit a temperature difference of 12 °C.  This residual thermal gradient could not be used in an additional power stage, but it allows the production of significant amounts of desalinated water through a Desalinated Water Cycle (DWC) or second stage water production.  The DWC is an OC-OTEC cycle without the turbine.  In a low-pressure vessel, or evaporator, the warm seawater is partially flashed into steam.  The evaporator is connected to two surface condensers, where the steam is converted into desalinated (fresh) water by exchanging heat with the cold seawater.  During this process, dissolved gases, mainly nitrogen and oxygen, are released from the warm seawater when pressures as low as 2 percent of an atmosphere are reached.  These non-condensable gases must be evacuated continuously from the second condenser, or vent condenser, by a vacuum compressor to prevent accumulation and sustain the required low operating pressures.  Non-condensables also adversely affect condensation performance through a blanketing effect at the heat exchanger walls.  To reduce the impact of released non-condensable gases, a pre-deaeration chamber at about 17 kPa is installed below the flashing chamber, so that much outgassing occurs before steam generation, and at a higher pressure more suitable for compression.  Moreover, gases are discharged into the warm seawater effluent at subatmospheric pressures of about 30 kPa, a procedure that not only saves power, but also restores the gas content of the warm seawater before it returns to the ocean.