The news source for Ocean Thermal Energy Conversion (OTEC)

 

  by L. A. Vega, Ph.D., Hawaii, USA.

Previous: Engineering Challenges

Open Cycle OTEC

The open cycle consists of the following steps:  (i) flash evaporation of a fraction of the warm seawater by reduction of pressure below the saturation value corresponding to its temperature (ii) expansion of the vapor through a turbine to generate power; (iii) heat transfer to the cold seawater thermal sink resulting in condensation of the working fluid; and (iv) compression of the non-condensable gases (air released from the seawater streams at the low operating pressure) to pressures required to discharge them from the system.  These steps are depicted in Figure 1.  In the case of a surface condenser the condensate (desalinated water) must be compressed to pressures required to discharge it from the power generating system. 

The evaporator, turbine, and condenser operate in partial vacuum ranging from 3 percent to 1 percent atmospheric pressure.  This poses a number of practical concerns that must be addressed. First, the system must be carefully sealed to prevent in-leakage of atmospheric air that can severely degrade or shut down operation.  Second, the specific volume of the low-pressure steam is very large compared to that of the pressurized working fluid used in closed cycle OTEC.  This means that components must have large flow areas to ensure that steam velocities do not attain excessively high values.  Finally, gases such as oxygen, nitrogen and carbon dioxide that are dissolved in seawater (essentially air) come out of solution in a vacuum.  These gases are uncondensable and must be exhausted from the system. In spite of the aforementioned complications, the Claude cycle enjoys certain benefits from the selection of water as the working fluid.  Water, unlike ammonia, is non-toxic and environmentally benign.  Moreover, since the evaporator produces desalinated steam, the condenser can be designed to yield fresh water.  In many potential sites in the tropics, potable water is a highly desired commodity that can be marketed to offset the price of OTEC-generated electricity.

Flash evaporation is a distinguishing feature of open cycle OTEC.  Flash evaporation involves complex heat and mass transfer processes.  In the configuration tested by a team lead by the author, warm seawater was pumped into a chamber through spouts designed to maximize the heat-and-mass-transfer surface area by producing a spray of the liquid.  The pressure in the chamber (2.6 percent of atmospheric) was less than the saturation pressure of the warm seawater.  Exposed to this low-pressure environment, water in the spray began to boil.  As in thermal desalination plants, the vapor produced was relatively pure steam.  As steam is generated, it carries away with it its heat of vaporization.  This energy comes from the liquid phase and results in a lowering of the liquid temperature and the cessation of boiling.  Thus, as mentioned above, flash evaporation may be seen as a transfer of thermal energy from the bulk of the warm seawater to the small fraction of mass that is vaporized to become the working fluid.  Approximately 0.5 percent of the mass of warm seawater entering the evaporator is converted into steam.


A large turbine is required to accommodate the huge volumetric flow rates of low-pressure steam needed to generate any practical amount of electrical power.  Although the last stages of turbines used in conventional steam power plants can be adapted to OC- OTEC operating conditions, existing technology limits the power that can be generated by a single turbine module, comprising a pair of rotors, to about 2.5 MW.  Unless significant effort is invested to develop new, specialized turbines (which may employ fiber-reinforced plastic blades in rotors having diameters in excess of 100 m), increasing the gross power generating capacity of a Claude cycle plant above 2.5 MW will require multiple modules and incur an associated equipment cost penalty.  Condensation of the low-pressure working fluid leaving the turbine occurs by heat transfer to the cold seawater.  This heat transfer may occur in a DCC, in which the seawater is sprayed directly over the vapor, or in a surface condenser that does not allow contact between the coolant and the condensate.  DCCs are relatively inexpensive and have good heat transfer characteristics due to the lack of a solid thermal boundary between the warm and cool fluids.  Although surface condensers for OTEC applications are relatively expensive to fabricate they permit the production of desalinated water.  Desalinated water production with a DCC requires the use of fresh water as the coolant.  In such an arrangement, the cold seawater sink is used to chill the fresh water coolant supply using a liquid-to-liquid heat exchanger.

Effluent from the low-pressure condenser must be returned to the environment.  Liquid can be pressurized to ambient conditions at the point of discharge by means of a pump or, if the elevation of the condenser is suitably high, it can be compressed hydrostatically.  Non-condensable gases, which include any residual water vapor, dissolved gases that have come out of solution, and air that may have leaked into the system, must be pressurized with a compressor.  Although the primary role of the compressor is to discharge exhaust gases, it usually is perceived as the means to reduce pressure in the system below atmospheric.  For a system that includes both the OC-OTEC heat engine and its environment, the cycle is closed and parallels the Rankine cycle.  Here, the condensate discharge pump and the non-condensable gas compressor assume the role of the Rankine cycle pump.

The analysis of the cycle yields (Figure 1):

Heat (added) absorbed from seawater (J/s)    q_w  =  m^.  _ww Cp (T_wwi - T_wwo )
Steam generation rate (kg/s)                           m^. _s   =  q_w/^hf_g
Turbine work (J/s)                                            w_T  =  m ^. _s  (h₃ - h₅) = m^.   _s  nT (h₃ - h _5s)   
Heat (rejected) into seawater (J/s)                  q_c  =  m^.   _cwC_p (T_cwo - T_cwi)

where,

m^._ww is the mass flow rate of warm water; Cp the specific heat; T_wwi and  T_wwo the seawater temperature at the inlet and outlet of the heat exchanger; hfg the heat of evaporation; and the enthalpies at the indicated points are given by h, with the subscript  s referring to constant entropy.  The turbine isentropic efficiency is given by  nT.   The subscript  cw  refers to the cold water.

Next: The 210 kW OC-OTEC Experimental Apparatus

© 1999. L. A. Vega. All rights reserved.
Published here with the kind permission of the author.