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| Fig. 1: A typical ocean temperature profile in the tropics. Green and blue layers represent regions with constant temperatures of 28 °C and 4.4 °C, respectively. |
As part of the response to the energy crisis in the 1970s, the US government began exploring ocean energy technologies, which derive renewable energy from the ocean, as alternatives to traditional energy resources. [1] Given that the world's oceans constitute 71% of the earth's surface and serve as large reservoirs for collecting and storing solar radiation, an effort was made to extract the stored solar energy by utilizing the naturally occurring thermal gradient found in tropical ocean waters. [2] Ocean thermal energy conversion (OTEC) is a process that exploits the temperature difference between the warm surface ocean water and the cold deep ocean water to drive a heat engine to generate electric power. [3]
Fig. 1 shows the "layered" temperature profile typically found in tropical oceans, located between 15° north and 15° south of the equator. [3] The topmost layer in the profile (green), up to a depth of 100 m, absorbs all of the incident sunlight and maintains an average temperature of 28 °C. [4] Beyond 100 m, the temperature profile drops rapidly as depth increases. At a depth of ~1000 m, the temperature of the ocean levels off to ~4.4 °C. [3] The temperature difference that exists between the topmost and bottommost layers in tropical oceans is greater than that found in any other ocean environment worldwide. [3] In addition, the temperature gradients remain relatively constant throughout the year, making equatorial ocean environments best suited for OTEC operations. [1]
OTEC power plants can be constructed onshore or offshore on large floating platforms. [1] The central element in an OTEC power facility is the heat engine. Two heat engine configurations proposed for OTEC are shown in Fig. 2. These configurations are classified as either open cycle or closed cycle depending on how the working fluid, which is used to drive the turbine, is utilized in the system. [3] In an open cycle system, the working fluid is removed from the system after the energy conversion process is complete. In contrast, in a closed cycle system the working fluid is conserved and cycled back through the system.
The open cycle system depicted in Fig. 2(a) operates by drawing warm surface water (the working fluid) into a partially evacuated chamber (evaporator) maintained at a reduced pressure by a vacuum pump. The lower pressure in the chamber causes the warm ocean water to boil which generates steam to drive a turbine connected to a generator. Once the steam passes through the turbine, it enters a heat exchanger (condenser) cooled by cold water pumped from deep below the ocean's surface where it is condensed back into a liquid. The condensate, which is now desalinated water, is then vented from the system either to the ocean or to an isolated storage tank.
In the closed cycle system shown in Fig. 2(b), warm surface water is used to heat an evaporator containing a working fluid with a low-boiling point, such as ammonia. The heat from the surface water causes the working fluid to boil and evaporate. The expanding vapor from the boiling working fluid drives the turbine which is connected to a generator. After passing through the turbine, the vapor enters a condenser cooled by cold ocean water pumped from deep below the surface where it is condensed back into a liquid and recirculated through the system.
While the main objective of both heat engine configurations is to convert the heat stored in the warm surface water into mechanical work in order to create electricity, many ancillary benefits can also be derived from OTEC. For example, the desalinated water generated as a byproduct of the open cycle system could be used to supply neighboring municipalities with a source of freshwater. [5] In addition, the cold water pumped from deep in the ocean could be utilized in air conditioning systems or for aquaculture. [6]
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| Fig. 2: Schematics of (a) open cycle and (b) closed cycle OTEC systems. |
It is worth noting that while the temperature difference between the topmost and bottommost layers in tropical regions of the ocean is appreciable when compared with other ocean environments, it is still quite small. The ideal Carnot efficiency (η) for a heat engine operated with a heat source at Tw = 28 °C (301.16 K)and a heat sink at Tc = 4.4 °C (277.56 K) is 0.0784 [4]:
| 301.16 K - 277.56 K
301.16 K |
= | 0.0784 |
This value, when compared with traditional heat engines, is low. [3] Given that the actual efficiency of a heat engine driven by this small temperature difference would be even lower than the maximum theoretical value due to heat and friction losses in the system, it would appear that OTEC is not a feasible alternative energy source. [3] However, since the fuel used to drive the heat engine is ocean water, which is essentially free and available in large quantities, many evaluators believe that these factors offset the low efficiency and make OTEC a viable candidate for large scale use in tropical regions where electricity is primarily imported. [7]
One other major criticism of OTEC is the massive volumes of ocean water required to generate a substantial amount of electricity. [4] Consider for example, the closed cycle system shown Fig. 3. For this system, the warm ocean water used to heat the working fluid in the evaporator undergoes a temperature change (ΔT) of 2 °C. The temperature drop that occurs in the working fluid across the turbine is 12 °C which leads to a Carnot efficiency of η = 0.0371. Assuming a turbogenerator efficiency of 85%, the net efficiency for this system η’= 0.0315. Given that the water flow rate (Ω) required to generate a power P can be determined by
| Ω | = | P
η'ρCPΔT |
where ρ is the density (106 gm/m3) and CP is the specific heat (4.2 J/gm°C) of water, the flow rate required at the input of the evaporator to generate 100MW of electricity is 377.93 m3/s. [4] This flow rate is equivalent to filling an Olympic-size swimming pool (375 m3) in less than 1 second. Similarly, the flow rate required to supply the world's energy demands in 2008 (~15 TW) is 56.69 x 106 m3/s , which is equivalent to ~283 times the average amount water flowing out of Amazon River into the Atlantic Ocean per second (200 x 103 m3/s). [2,8] Clearly, the enormous flow rates required for substantial electric power generation by OTEC would introduce a myriad of engineering challenges.
While government funded pilot plants constructed and tested during the late 1970s successfully generated net power, economic studies found that the cost of generating electricity by OTEC was not competitive when compared with the cost associated with fossil fuels. [5] As a result, funding for OTEC research was drastically cut in the 1980s. [1] To date, there are no large-scale net-power-generating OTEC systems operating within the US. [5-6] However, in recent years interest in OTEC power generation appears to be making a comeback. In 2006, Lockheed Martin began developing a more economically efficient OTEC power system and was recently awarded government contracts to continue OTEC development. [7,9] A construction date for the company's new OTEC power plant, to be built off the coast of Oahu, HI, has not yet been released.
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| Fig. 3: Temperatures for an example OTEC system. |
© Sara E. Harrison. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] R. A. Ristinen, Energy and the Environment (Wiley, 1999).
[2] A. V. Da Rosa, Fundamentals of Renewable Energy Processes (Academic Press/Elsevier, 2009)
[3] W. H. Avery and C. Wu, Renewable Energy From the Ocean : A Guide to OTEC (Oxford University Press, 1994).
[4] A. Lavi, "Ocean Thermal Energy Conversion: A General Introduction," Energy, 5, 469 (1980).
[5] R. Pelc and R. M. Fujita, "Renewable Energy From the Ocean," Marine Policy 26, 471 (2002).
[6] M. Paillard, D. Lacroix, and V. Lamblin, Marine Renewable Energies: Prospective Foresight Study for 2030. (Quae, 2009).
[7] K. Galbraith, "Generating Energy from the Deep", New York Times, 29 Apr 09.
[8] J. E. Richey, C. Nobre, and C. Deser, "Amazon River Discharge and Climate Variability - 1903 To 1985", Science 246, 101 (1989).
[9] R. Bedard, P. T. Jacobson, M. Previsic, W. Musial, and R. Varley, "An Overview of Ocean Renewable Energy Technologies," Oceanography 23, 22 (2010).