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5
Ocean Thermal Energy Conversion
Resource Assessment
Ocean thermal energy conversion (OTEC) is the process of deriving
energy from the difference in temperature between surface and deep
waters in the tropical oceans. The OTEC process absorbs thermal energy
from warm surface seawater found throughout the tropical oceans and
ejects a slightly smaller amount of thermal energy into cold seawater
pumped from water depths of approximately 1,000 m. In the process,
energy is recovered as an auxiliary fluid expands through a turbine.
There are two basic OTEC plant design types—open cycle and closed
cycle. Open-cycle plants use vacuum to flash evaporate warm surface
seawater, and the resulting steam is used to drive a low-pressure turbine-
generator. Cold seawater drawn up from depth is used to condense the
steam. Desalinized fresh water is a by-product produced in an open cycle
system. Closed-cycle designs use an intermediate working fluid, such as
ammonia, to run a higher pressure turbine-generator system and require
an additional heat exchanger.
Fundamentally, OTEC systems are similar to most other heat engines.
There are, however, significant practical aspects of the technology that
make it difficult to implement, largely resulting from the small available
temperature difference DT (~20ºC) between the warm and cold seawater
streams (Figure 5-1). The theoretical maximum thermodynamic efficiency
of a heat engine is proportional to DT, with a DT of 20ºC being fairly low.
Because of the low efficiencies, OTEC plants require very large equipment
(e.g., heat exchangers, pipes) and seawater flow rates (~200-300 m3/s
for a typical 100-MW design) that would exceed those from any existing
56
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1
OCEAN THERMAL ENERGY CONVERSION RESOURCE ASSESSMENT 57
FIGURE 5-1 Barriers and concerns for OTEC deployment. OTEC plants work
with a small temperature difference that necessitates large physical plants with
high seawater flow rates. Such large flow rates may cause a decrease in the avail-
able temperature resource due to flow disturbance from the plant and may lead
to significant environmental impacts.
industrial process in order to generate a significant amount of electric-
ity. The cold-water pipe is one of the largest expenses in an OTEC plant.
As a result, the most economical OTEC power plants are likely to be
open-ocean designs with short vertical cold-water pipes. However, these
designs face the issue of bringing power to shore. The earliest practical
OTEC plants are likely to be based on or near tropical islands that have
steep topography, which will make it easier to reach deep cold water and
transmit power to shore. In the future, OTEC plants could also use the
generated energy to produce hydrogen or extract carbon dioxide from
seawater in order to produce synthetic fuel using a modified Fischer-
Tropsch process in remote ocean locations. A side benefit could be in using
pumped-up cold seawater for air conditioning systems, with costs of one-
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58 DOE’S MARINE AND HYDROKINETIC RESOURCE ASSESSMENTS
tenth or less that of running a regular air-to-air heat pump (Jagusztyn and
Reny, 2010). Stand-alone seawater air conditioning systems modeled on
this idea are already in use on some tropical islands.
The potential for harnessing power from the open ocean is attrac-
tive, especially for low-latitude island populations. In the United States,
Hawaii has been a proving ground for OTEC, with several test plants built
in the past four decades. These include a barge-based mini-OTEC (50 kW)
in 1979, a ship-based OTEC-1 (1 MW) in 1980, and a shore-based Open
Cycle 210-kW plant operated from 1993 to 1998 (Vega and Evans, 1994).
PROJECT DESCRIPTION
The most favorable sites for OTEC can be identified using local water
temperature and depth data in a simple calculation of power density,
adapted from Nihous (2007a). In the following equation, the power den-
sity per unit of upwelled cold water (W/(m3/s))—where Pnet is net power
and Qcw is the deep seawater flow rate—is expressed as a function of the
environmental temperature difference (DT), the surface seawater tempera-
ture (Ts), the average density of seawater (ρ), the specific heat of seawater
(CP), the turbogenerator efficiency (TGE), and the fractional losses to
pumping (PL) (both head losses and drag):
ρCP × TGE × ( ∆ T ) (1 − PL )
2
Pnet (1)
=
QCW 8 ( 273 + TS )
The quadratic dependence on DT arises from a linear heat transfer depen-
dence and the expression for the Carnot efficiency (~DT/absolute T).
Nonlinearity is weak within the limited temperature range of the ocean,
so the assessment group used a linearized approximation of plant perfor-
mance to simplify the calculations (Lockheed Martin Mission Systems &
Sensors, 2012).
The OTEC resource assessment group conducted its study by using
the output from a 2-year model run of the global HYbrid Coordinate
Ocean Model (HYCOM) to determine the ocean’s temperature structure.1
HYCOM, maintained by the Naval Research Laboratory, is a real-time
1/12-degree global nowcast/forecast ocean model with 32 vertical levels.
The group chose HYCOM because the model uses density as the vertical
coordinate below the surface layers, which would provide realistic simu-
lations of deepwater ocean physics. The model provides an approximately
7 km resolution, and the 2-year model run included strong El Niño and
1 H.P. Hanson, Florida Atlantic University, “Global OTEC resource assessment,” Presenta-
tion to the committee on September 27, 2011.
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OCEAN THERMAL ENERGY CONVERSION RESOURCE ASSESSMENT 59
La Niña events in order to assimilate extremes into the dataset. Within
their database, sites can be evaluated for annual average and inter and
w
summer temperatures.
The group chose the cold water source to be the temperature at either
the bottom of the ocean, the depth at which the temperature gradient
is less than 7°C/km, or 1,000 m, whichever was shallowest. The group
chose to use a specific OTEC plant model that is proprietary to Lockheed
Martin as the basis for its resource assessment.2 This is a nominal 100-MW
plant, a size generally considered to be large enough to be economically
viable and of utility-scale interest yet small enough to construct with
manageable environmental impacts (Whitehead and Gershenfeld, 1981).
However, since no plants this large have yet been built, there are many
technical and environmental challenges to overcome before even larger
plants are attempted.
COMMITTEE COMMENTS
Methodology
In its approach, the assessment group noted that the theoretical
resource and the technical resource are inextricably linked. The tem-
perature differentials generated by the HYCOM model are essentially
the theoretical assessment; evaluating the temperature differential via an
assumed plant model leads to a technical resource assessment. Therefore,
a key imperative for the OTEC resource assessment is to evaluate global
ocean surface temperatures and their seasonal fluctuations, along with
temperature gradients as a function of depth and location. The commit-
tee views the use of the HYCOM model for assessment of the theoretical
resource to be inadequate and also regards the application of a specific
proprietary Lockheed Martin plant model with a fixed pipe length to
be unnecessarily restrictive. A more generic plant model—for example,
Nihous (2007a)—would have been preferable to make it easier for devel-
opers to evaluate different plant models by varying pipe lengths or turbo
generator efficiencies.
The DOE funding opportunity for OTEC was the only one to specify
that the assessment should include both U.S. and global resources, and
the assessment group chose to focus on the global resource. The commit-
tee believed, however, that more emphasis should have been placed on
potential OTEC candidates in U.S. coastal waters. To demonstrate this
point, the committee evaluated equation 1 and used the National Oceano-
graphic Data Center of the National Oceanic and Atmospheric Adminis-
2 See preceding footnote.
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60 DOE’S MARINE AND HYDROKINETIC RESOURCE ASSESSMENTS
tration’s World Ocean Atlas data to map this function for a 1,000-m pipe
length, a TGE efficiency of 0.85, and PL of 30 percent (Figure 5-2). This
simple exercise immediately shows that within United States territory, the
coastal regions of the Hawaiian Islands, Puerto Rico and the U.S. Virgin
Islands, Guam and the Northern Mariana Islands, and American Samoa
would be the most efficient sites for OTEC.
The committee is also concerned that the 2-yr HYCOM run will
not provide proper statistics on the temporal variability of the thermal
resource. Although it does include both El Niño and La Niña events,
2 years is not sufficient to characterize the global ocean temperature field
with any reliability. Longer datasets are widely available, so it is not clear
why the assessment group limited itself in this way. Ocean databases that
extend for more than 50 years are readily available; these data would
allow assessment of the interannual variability in thermal structure due to
El Niño/Southern Oscillation (ENSO) to be evaluated. The advantage of
HYCOM’s higher resolution over earlier estimates from coarser climatolo-
gies may vanish if HYCOM is used without appropriate boundary con-
ditions near the coasts, resulting in inaccurate seasonal and inter nnual
a
statistics on thermal structure. Without these abilities, this study is not
much more valuable than prior maps of global ocean temperature differ-
ences (Avery and Wu, 1994), which already identified OTEC hot spots.
As an alternative to HYCOM, the committee notes that the U.S.
Navy maintains the Generalized Digital Environmental Model (GDEM),
5-2
FIGURE 5-2 Global map of the annual OTEC power density available with a
1,000-m cold-water pipe, a turbogenerator efficiency of .85, and pumping losses
of 30 percent. NOTE: The power density is expressed in kW/(m3/s) of cold water
pumped up. The entirety of the World Ocean Atlas data (1955-2006) was used.
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OCEAN THERMAL ENERGY CONVERSION RESOURCE ASSESSMENT 61
a state-of-the-art gridded monthly full-depth climatology of temperature
and salinity and their standard deviations (Teague et al., 1990; Carnes,
2009; Carnes et al., 2010). GDEM represents the monthly climatologi-
cal averages of ocean temperature and salinity as a function of depth
and location around the globe and is based on analyses of quality-
controlled in situ profile observations throughout the historical record.
These measurements rely primarily on expendable bathythermographs
(XBTs, nstruments that measure temperature at depth), conductivity-
i
temperature-depth (CTD) data, and Argo float data sets. Moderate Reso-
lution Imaging pectroradiometer (MODAS) satellite records may also
S
provide a better source for estimates of the sea surface temperature (SST)
climatology than HYCOM, as the MODAS SST is a daily analysis of infra-
red satellite estimates of surface temperature (Barron and Kara, 2006; Kara
and Barron, 2007; and Kara et al., 2009).
As an alternative to the methodology put forth by the assessment
group, the committee used GDEM and MODAS to construct fields of
the mean monthly temperatures averaged over all years from 1993 to
2010. The combined GDEM and MODAS data sets can be queried to find
not only mean monthly SSTs over regions with subsurface temperatures
below 4°C, but also average 4°C isotherm depth in regions where monthly
mean SST exceeds certain thresholds (21°C, 24°C, and 27°C, for example).
Performance predictions on monthly and seasonal timescales could be
done with HYCOM (Hurlburt et al., 2008; Chassignet et al., 2009). How-
ever, the OTEC assessment group has not made such statistics accessible
in its GIS.
The committee feels that the resource assessment should include an
investigation of temperature variability that accounts for tidal variations,
seasonal variations, and ENSO timescales. The assessment group’s data-
base currently contains only a summer and winter season contrast that
was constructed from averaging two anomalous (El Niño/La Niña) years.
Including more years in the model run, especially years without El Niño
or La Niña, would allow for a better representation of the ocean environ-
ment. El Niño and La Niña occur on 3- to 6-year timescales, so approxi-
mately a decade of data would be needed to catch both instances. It is
these longer-term signals that a planner would need for evaluation of a
site beyond the seasonal cycle, and the 2-year average from the assess-
ment group does not even allow exploration of the seasonal signal. The
committee also notes that variations in isotherm depth due to internal
tides can be significant near islands. For example, deep isotherm displace-
ments of as much as 50 or even 100 m are common near the Hawaiian
Islands (Klymak et al., 2008), which could induce a 5-10 percent varia-
tion in power output over the tidal cycle from an OTEC plant situated
there. In addition, areas with strong internal tides will also impose strong
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62 DOE’S MARINE AND HYDROKINETIC RESOURCE ASSESSMENTS
shear currents on the cold-water pipe. With regard to seasonal variations,
estimates based on equation 1 and the annual cycle of temperature sug-
gest that a 20 percent variation in power output can be expected with a
1,000-m pipe at Hawaii over the course of the year (Rand Corporation,
1980; Cohen, 1982). Even more dramatic changes result from the SST
fluctuations due to El Niño or La Niña in the central tropical Pacific,
where the committee estimates variations in power production as high
as 50 percent. The assessment group largely fails to address the tempo-
ral variability issue. The GIS database would be of much more use if it
included at least monthly resolution, which for the present 2-year run
would at least allow evaluation of specific El Niño or La Niña conditions
that are important for OTEC in the tropical Pacific. It would also be useful
to have some measure of internal tidal displacements, if only for high-
priority sites like Hawaii.
Given the substantial seawater requirements of OTEC plants, the
number and spatial density of plants would be a major consideration
when considering available power. Plants need to be scaled and designed
to minimize their own back effects so they do not adversely affect the
locally available temperature contrast. There will also be a maximum
plant spatial density beyond which plant discharges would begin to inter-
fere with one another. At regional and global scales there could be a
variety of impacts on the ocean arising from widespread deployment of
OTEC. Since OTEC is essentially a mixing process, promoting the flux
of heat down the vertical temperature gradient, massive deployment of
OTEC could actually enhance thermohaline circulation. The potential
impacts of these effects, such as decreased tropical surface temperatures
or increased primary productivity due to an influx of nutrients from deep
cold water, would require careful modeling and would remain specula-
tive until actual plant operations commenced.
Instead of looking at plant spacing issues or the size of individual
plants, the OTEC assessment group focused on the supply of cold water
as the resource limit. They used the flow speeds at the depth of the cold-
water pipe in the HYCOM model to estimate possible plant densities.
The size of the ultimate resource available with massive deployment
of OTEC plants is a highly speculative question worthy of significant
study on its own. The assessment group chose to adopt a figure from the
literature (Nihous, 2007b) that was developed by assuming that the net
cold water upwelling from all OTEC plants would not be too large a frac-
tion of the net thermohaline overturning circulation. The volume of cold
water required by the plant was met by a specified change in the deep
layer thickness, which was adjusted to meet the Nihous global estimate
of OTEC potential. However, this assumes that the cold-water supply is
limited in the ocean, an idea that is not universally accepted. Most mod-
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OCEAN THERMAL ENERGY CONVERSION RESOURCE ASSESSMENT 63
ern theories and models (Bryan, 1987; Zhang et al., 1999) recognize that
the thermohaline circulation is controlled by the mixing rate, not the cold
water supply. Indeed, the ocean is certainly not lacking in cold water—
its average temperature is ~3.5°C (Worthington, 1981). Nevertheless, it
is not unreasonable to assume that OTEC’s impact on global circulation
should not be too large, and using the Nihous (2007b) limit is a plausible
approach in the absence of a proper modeling study. However, the assess-
ment group’s use of it to address plant packing density is misguided.
The committee is disappointed that the OTEC assessment group did
little to address device spacing requirements, individual plant size, or the
limits of the ocean thermal resource. Clearly, a key question for determin-
ing the OTEC technical resource would be how closely plants could be
spaced without interfering with each other or excessively disturbing the
ocean thermal structure. A related issue would be how spacing might dif-
fer in coastal and open-ocean environments. Another issue would be the
size limit of a single OTEC plant, due to back effects on the ocean hermal
t
structure such as smaller temperature gradients owing to decreased
t
hermal stratification. While a global resource assessment is difficult to
constrain, the committee had hoped the assessment group would address
constraints such as plant spacing, tidal amplitudes, and anchoring in deep
water or strong currents.
Validation
The group focused its validation efforts on the Lockheed Martin
OTEC plant operating model while neglecting validation of the thermal
resource.3 Focusing the validation process on the proprietary plant model
seems inappropriate and not at all transparent to this committee. In fact,
it appears rather to be a reverse engineering of their plant model, as the
agreement on performance seems remarkably perfect (Lockheed Martin
Mission Systems & Sensors, 2012). Assuring the accuracy of the tempera-
ture gradient in the assessment group’s database would have been a more
valuable effort, especially with a focus on how well the 2-yr HYCOM run
represents the available temperature difference. The seasonal and inter
annual statistics and the model representation of nearshore deep tempera-
tures are of particular interest, especially as the group noted a problem
with deep temperatures off Florida.4 The validation effort should have
drawn on the many available hydrographic databases and compared sur-
face and deep temperature contrasts between observational data and the
3 H.P. Hanson, Florida Atlantic University, “Global OTEC resource assessment,” Presenta-
tion to the committee on December 12, 2011.
4 See preceding footnote.
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64 DOE’S MARINE AND HYDROKINETIC RESOURCE ASSESSMENTS
HYCOM model, which would have better paralleled the other resource
assessment efforts.
Estimate of Available OTEC Power
There are many interesting physics, chemistry, and biology prob-
lems associated with the operation of an OTEC plant. Whitehead and
G
ershenfeld (1981) suggested that an optimal plant size would be around
100 MW in order to avoid adverse effects on the thermal structure the
plant is designed to exploit. The ultimate size of the OTEC resource
itself is an interesting question and an issue which has been discussed
in both old (Isaacs and Schmitt, 1980) and new literature (Nihous, 2005;
2007a; 2007b). Previous work yielded a wide range of estimates for the
global OTEC resource of between 3 TW and 1,000 TW (Nihous, 2005
and references therein), which compares favorably to the current global
energy consumption of about 16 TW (IEA, 2011). If the committee uses
its own estimate of the power density of ~500 kW/(m3/s) of cold water
upwelling, then a total added upwelling of 10 Sv5 is equivalent to a total
power of 5 TW, in agreement with Nihous (2007a). This would represent
a 100-MW plant spaced approximately every 50 km in the tropical ocean.
While this suggests that OTEC is a very substantial ocean energy source,
the many technical and environmental obstacles to its deployment, espe-
cially the challenge of utilizing the power produced at sea, means that this
concept is still quite far from such large-scale implementation.
The GIS created by the OTEC assessment group was a good way to
visually identify sites that might be optimal for OTEC plant placement.
However, despite the large global potential, the U.S. OTEC resource esti-
mate provided by the assessment group seems unrealistically high. The
assessment group arrives at a figure of 4,642 TWh/yr for the United States,
but the majority of the resource is found near Micronesia (1,134 TWh/yr)
and Samoa (1,331 TWh/yr) (Lockheed Martin Mission Systems & Sensors,
2012). Unfortunately, there is a serious mismatch between the supply
and demand at those locations, as low population densities and levels
of industrialization will not create a market for the electricity produced
through OTEC. In addition, the 200-mile Exclusive Economic Zone was
used as a limit for energy production. This does not fully address the DOE
funding opportunity, which requested a discussion of both “resources
available with near-shore, grid-connected ocean thermal energy systems
and those requir[ing] floating offshore systems” (DOE, 2009). A more
realistic limit would be needed to address nearshore options.
5 A sverdrup (Sv) is a unit of volume transport used in physical oceanography, equivalent
to 106 m3/s.
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OCEAN THERMAL ENERGY CONVERSION RESOURCE ASSESSMENT 65
The total OTEC resource for the continental United States was
394 TWh/yr, less than 9 percent of the total U.S. resource estimated
( ockheed Martin Mission Systems & Sensors, 2012). The Florida Straits
L
and the East Coast account for 87 percent of the continental U.S. resource.
The Gulf of Mexico, which accounts for the other 13 percent, is not a
viable source in winter. The continental U.S. resource is very seasonal
and limited, and it is unlikely that plant owners would want to operate
only part of the year. According to the assessment, Hawaii could gener-
ate 143 TWh/yr, the Mariana Islands (including Guam) could generate
137 TWh/yr, and Puerto Rico and the U.S. Virgin Islands could gener-
ate 39 TWh/yr ( ockheed artin Mission Systems & Sensors, 2012). A
L M
further focus on these areas where the thermal resource and the societal
need coincide would be worthwhile.
CONCLUSIONS AND RECOMMENDATIONS
The OTEC assessment group’s GIS database provides a visualiza-
tion tool to identify sites for optimal OTEC plant placement. However,
the resource assessment falls short in other ways. The proprietary plant
model used does not allow other plant designs to be optimized. Too little
information is available on the temporal variability of the thermal resource,
with only seasonal averages from two anomalous El Niño/La Niña years
available. In addition, too few isotherm depths are available to allow users
to optimize the cold-water pipe length.
Recommendation: The OTEC GIS should be modified to display
monthly resolution over a longer time period (at least a decade) to
allow for evaluation of the thermal resource for the full seasonal
cycle as well as for special periods such as El Niño and La Niña.
Isotherm depths (at 1°C intervals) should be included in the data-
base so other pipe lengths can be evaluated for OTEC and seawater
air conditioning.
The validation effort, which was focused on the plant model instead
of the thermal resource represented in their model output, is obviously an
issue of great concern. There are plentiful, widely available oceanographic
databases available for comparison of the thermal resource. Because the
ocean’s thermal stratification is the key input for the OTEC resource
assessment, it would have been more useful for the validation to have
focused on its representation in the model rather than on a specific
plant design.
The group’s estimate of the limit on plant spacing based on cold water
supply was also physically unjustified. Instead, the focus should have
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66 DOE’S MARINE AND HYDROKINETIC RESOURCE ASSESSMENTS
been on using the numerical models to estimate the back effects of plant
operation on the thermal resources. As the back effects on the thermal
resource will be the limiting factor on OTEC plant spacing in both the
coastal and open ocean environments, models of the flow around OTEC
plants must be developed to understand potential impacts on the ocean.
Site-specific studies would be needed to evaluate current (including tidal)
and storm vulnerability, as well as distance from shore.
Recommendation: Any future studies of the U.S. OTEC resource
should focus on Hawaii and Puerto Rico, where there is both a
potential thermal resource and a demand for electricity.
