The news source for Ocean Thermal Energy Conversion (OTEC)

 

  by L. A. Vega, Ph.D., Hawaii, USA.

Previous: The 210 kW OC-OTEC Experimental Apparatus

Design of a Small Land-Based OC-OTEC Plant

To understand the details of the design and operation of a  Claude Cycle plant, it is useful to consider a specific example given by the design of a small land-based plant.  The design philosophy reflects an emphasis on feasibility: a state-of-the-art cold water supply pipeline, a 1.6 m high-density-polyethylene (HDPE) conduit, was selected as the design starting point, rather than a prescribed net power output.  Consistent with this choice, some critical hardware components were preferred over alternative configurations because they are presently available off-the-shelf.  The major conclusion is that cost-effective electricity and desalinated water could be supplied to small South Pacific Island communities if appropriate financing is available.  Depending on the importance of desalinated water for these communities, two global design options were identified:

1. OC-OTEC power plant proper, with a net power production capability of about 1.2 MW and desalinated water production of 2200 m³ per day;

1. OC-OTEC power plant fitted with a second-stage water flashing unit, with a net power production capability of about 1.1 MW and desalinated water production of 5150 m³ per day.

In addition, a 300-room air conditioning system can be included for resort development.

These configurations are cost competitive with conventional power plants if investment loans with interest rates of less than 5 percent are available and if credit for desalinated water and air-conditioning byproducts are included.  However, the transfer of OTEC to small Pacific Islands should not be exclusively assessed from the perspective of present-day cost effectiveness since it offers these isolated communities some degree of energy independence while preserving their environment.  In this regard the most important finding is not related to technical matters but rather to financial considerations: the announcement by industrialized countries, after the Kyoto Conference, that low interest loans would be made available to developing countries for renewable energy projects with minimal environmental impact.

The heat and mass balance is given in Figure 2.  A 6156 kg s^-1 flow rate of warm seawater at 26 °C is supplied via a 2.5 m ID FRP pipe.  The pipe has an intake depth of 25 m and is 120 m long.  Five inline dry motor vertical turbine propeller pumps (three operational, two standby) supply the flow to an intake pool below the first stage evaporator.  The intake pool has a nominal operating level of 2.78 m from mean sea level (MSL).  This level is selected to provide enough head (level) in the mixed flow discharge pool for gravity discharge into the ocean.  Three inline submersible propeller-type pumps (two operational, one standby) bring 3175 kg s ^-1 of cold seawater through a 1.6 m OD pipe from a depth of 1000 m.  The pipe length is 2590 m.  A 3085 kg s^-1 flow rate of 4 °C cold seawater is available for the OTEC system, whereas 90 kg s^-1 is reserved for air-conditioning applications.  An upriser takes the warm water into the evaporator.  A pre-deaeration nozzle removes a portion of the non-condensables from the warm water accumulated below the spout plate.  The evaporator spout plate has 122 spouts and the warm water flashes through the spouts into the evaporation chamber at a pressure of 2.76 kPa.  A small fraction (26.1 kg s^-1) of supply water is changed into steam and the rest is discharged into the first stage discharge pool at a temperature of 23.4 °C.  The discharge pool at a level of 1.76 m MSL also acts as the supply pool for the second stage evaporator.  The evaporation pressure in the second stage is 2.22 kPa.  No pre-deaeration is provided in the second stage as the water has been deaerated in the first stage.  The steam flow from the second stage evaporator is 34 kg s^-1.  The effluent water from the second stage evaporator at 20 °C goes into the mixed water discharge pool.

Steam from the first stage evaporator enters the turbine at 2.74 kPa and leaves the turbine diffuser system at 1.29 kPa.  The turbine generator system gives a gross output of 1838 kW.  Steam exhausted from the turbine-diffuser system enters the first stage main surface condenser.  The main condenser receives 2702 kg s^-1  of cold seawater at 4 °C and condenses  92 percent of the incoming steam.  The vent condenser gets 281 kg s ^-1 of 4 °C cold seawater supply and condenses 90 percent of the steam leaving the main condenser.  The remaining steam and the non-condensables are evacuated by the vacuum compressor system.  Steam generated by the second stage evaporator enters the second stage main condenser at 2.18 kPa and 19.2 °C.  This steam is expected to be virtually free of non-condensables.  The condenser receives 3085 kg s^-1 of cold seawater at 9.4 °C and discharges it at 16.2 °C.  The minimal amount of uncondensed steam, along with any non-condensables, goes to a vent condenser.  A hook-up is provided to let the vapor compressor system remove any non-condensables and water vapor from the vent condenser.

Non-condensables and steam vapor from the first and second stage vent condenser enters the vacuum compressor system through a counter-current direct contact precooler.  The precooler receives 4 °C cold seawater and ensures that the mixture temperature at the first stage inlet of the compressor system is not more than 5 °C and that the entire vapor is condensed until its partial pressure becomes equal to the seawater saturation pressure at 5 °C.  The basic compressor system has four stages with direct contact coolers in-between.  The fourth stage compressor takes the non-condensables from the third stage.  The discharge from the fourth stage is re-injected at 30 kPa into the warm water effluent returning from the second stage evaporator.  A fifth stage compressor is also provided to alternately bypass the re-injection scheme and discharge into the atmosphere.  The first four stages are centrifugal, whereas the fifth stage is a positive displacement type.


The cold and warm water effluents from the second stage combine into a mixed discharge pool with a nominal level of 0.75m MSL.  A 3 m ID, 190 m long and 60 m deep pipe provides gravity discharge recourse for the mixed water system.  The net power from the system is 1126 kW obtained after subtracting 334 kW for cold water supply pumping; 284 kW for warm water supply pumping; 80 kW for compressor system; and 14 kW for desalinated water pumping from the gross power produced.  Without second stage water production the combined pumping losses will be reduced by approximately 100 kW (net increase) and the desalinated water production would be 25.8 kg s^-1.

Next: Closed Cycle OTEC

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