by L. A. Vega, Ph.D.,
Hawaii, USA.
Keywords
Ocean Temperature Differences, Ocean Thermal Energy
Conversion, OTEC, Renewable Energy, Solar Energy
Short content list
Summary
1 - Background
2 - Technical Limitations
3 - OTEC and the Environment
4 - Engineering Challenges
5 - Open Cycle OTEC
6 - The 210 kW OC-OTEC Experimental Apparatus
7 - Design of a Small Land-Based OC-OTEC
Plant
8 - Closed Cycle OTEC
9 - Design of a Pre-Commercial Floating
Hybrid-OTEC Plant
10 - Potential Sites
11 - Economic Considerations and Market
Potential
12 - Hydrogen Production
13 - Externalities
Bibliography
The Vision for
Hawaii
Glossary
COE Cost of Electricity
Production
CWP Cold Water Pipe
DCC Direct Contact Condenser
DWC Desalinated Water Cycle
EEZ Exclusive Economic Zone
kW kilowatt, 103
watts
MSL Mean Sea Level
MW Megawatt, 106
watts
OTEC Ocean Thermal Energy Conversion
CC-OTEC Closed Cycle OTEC
OC-OTEC Open Cycle OTEC
SIDS Small Island Developing States
Summary
The vertical temperature distribution in the open ocean can be simplistically
described as consisting of two layers separated by an interface. The upper
layer is warmed by the sun and mixed to depths of about 100 m by wave motion.
The bottom layer consists of colder water formed at high latitudes.
The interface or thermocline is sometimes marked by an abrupt change in temperature
but more often the change is gradual. The temperature difference between
the upper (warm) and bottom (cold) layers ranges from 10 °C to 25 °C,
with the higher values found in equatorial waters. To an engineer this implies
that there are two enormous reservoirs providing the heat source and the
heat sink required for a heat engine. A practical application is found
in a system (heat engine) designed to transform the thermal energy into electricity.
This is referred to as OTEC for Ocean Thermal Energy Conversion.
Several techniques have been proposed to use this ocean thermal
resource; however, at present it appears that only the closed cycle (CC-OTEC)
and the open cycle (OC-OTEC) schemes have a solid foundation of theoretical
as well as experimental work. In the CC-OTEC system, warm surface
seawater and cold seawater are used to vaporize and condense a working fluid,
such as anhydrous ammonia, which drives a turbine-generator in a closed
loop producing electricity. In the OC-OTEC system seawater is flash-evaporated
in a vacuum chamber. The resulting low-pressure steam is used to drive a
turbine-generator. Cold seawater is used to condense the steam after it
has passed through the turbine. The open-cycle can, therefore, be configured
to produce desalinated water as well as electricity.
Records available from experimental plants demonstrate technical viability
and provide invaluable data on the operation of OTEC plants. The economic
evaluation of OTEC plants indicates that their commercial future lies in
floating plants of approximately 100 MW capacity for industrialized nations
and smaller plants for small-island-developing-states (SIDS). Unfortunately,
the size of the experimental plants (< 0.3 MW) is about two orders of
magnitude less than the size required for commercial (i.e., cost competitive)
systems in industrial nations. Data extrapolation of this order is
not acceptable to banking institutions or developers. The records that are
available, however, are sufficient to design an OTEC plant sized at approximately
1.5 to 2 MW. This size range is appropriate for the smaller markets
encountered in SIDS.
To proceed beyond experimental plants and towards commercialization
in developed nations, a scaled version of a 100 MW plant must be designed
and operated. The operational data is needed to earn the support required
from the financial community and developers. Considering a 4-module
system, a 1/5-scaled version of a 25 MW module is proposed as an appropriate
size. The 5 MW pre-commercial plant is also directly applicable in
some SIDS.
Next: Background
© 1999. L. A. Vega. All rights reserved.
Published here with the kind permission of the author.