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 AT (∼20°C) between the warm and cold seawater streams (Figure 5-1). The theoretical maximum thermodynamic efficiency of a heat engine is proportional to AT, with a AT of 20°C 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
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 available 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 electricity. 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-
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 attractive, 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).
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 density 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 (AT), the surface seawater temperature (Ts), the average density of seawater (p), the specific heat of seawater (CP), the turbogenerator efficiency (TGE), and the fractional losses to pumping (PL) (both head losses and drag):
The quadratic dependence on AT arises from a linear heat transfer dependence and the expression for the Carnot efficiency (∼AT/absolute T). Nonlinearity is weak within the limited temperature range of the ocean, so the assessment group used a linearized approximation of plant performance 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 simulations 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,” Presentation to the committee on September 27, 2011.
La Niña events in order to assimilate extremes into the dataset. Within their database, sites can be evaluated for annual average and winter and 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.
In its approach, the assessment group noted that the theoretical resource and the technical resource are inextricably linked. The temperature 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 committee 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 developers to evaluate different plant models by varying pipe lengths or turbogenerator 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 committee 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 Oceanographic Data Center of the National Oceanic and Atmospheric Administration’s
2 See preceding footnote.
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 climatologies may vanish if HYCOM is used without appropriate boundary conditions near the coasts, resulting in inaccurate seasonal and interannual statistics on thermal structure. Without these abilities, this study is not much more valuable than prior maps of global ocean temperature differences (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),
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.
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 climatological 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, instruments that measure temperature at depth), conductivity-temperature-depth (CTD) data, and Argo float data sets. Moderate Resolution Imaging Spectroradiometer (MODAS) satellite records may also provide a better source for estimates of the sea surface temperature (SST) climatology than HYCOM, as the MODAS SST is a daily analysis of infrared 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). However, 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 database 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 environment. El Niño and La Niña occur on 3- to 6-year timescales, so approximately 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 assessment 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 displacements 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 variation in power output over the tidal cycle from an OTEC plant situated there. In addition, areas with strong internal tides will also impose strong
shear currents on the cold-water pipe. With regard to seasonal variations, estimates based on equation 1 and the annual cycle of temperature suggest 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 temporal 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 interfere 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 speculative 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 fraction 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 modern
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 assessment 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 determining 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 differ 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 thermal structure such as smaller temperature gradients owing to decreased thermal 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.
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 temperature 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 temperatures 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 surface and deep temperature contrasts between observational data and the
3 H.P. Hanson, Florida Atlantic University, “Global OTEC resource assessment,” Presentation to the committee on December 12, 2011.
4 See preceding footnote.
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 problems associated with the operation of an OTEC plant. Whitehead and Gershenfeld (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, especially 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 estimate 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
The total OTEC resource for the continental United States was 394 TWh/yr, less than 9 percent of the total U.S. resource estimated (Lockheed Martin Mission Systems & Sensors, 2012). The Florida Straits 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 generate 143 TWh/yr, the Mariana Islands (including Guam) could generate 137 TWh/yr, and Puerto Rico and the U.S. Virgin Islands could generate 39 TWh/yr (Lockheed Martin Mission Systems & Sensors, 2012). A further focus on these areas where the thermal resource and the societal need coincide would be worthwhile.
The OTEC assessment group’s GIS database provides a visualization 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 database 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
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.