Author: Gérard C. Nihous, Hawaii Natural Energy Institute, University of Hawaii, U.S.A.
In the standard formulation of OTEC, electricity would be produced by circulating a working fluid through a Rankine thermodynamic cycle. Because of the moderate temperatures involved, ordinary refrigerants such as ammonia typically have been considered for such systems. Available seawater temperature differences, of the order of 20ºC, must be used not only to define the boundaries of the cycle (evaporation and condensation temperatures), but also to maintain adequate temperature differentials between seawater streams and working fluid as heat is transferred. All issues related to – and hurdles impeding the development of OTEC stem from this fact.
OTEC systems require cold seawater flow rates of about 2.5 to 3 m3/s per net megawatt, with usually greater warm surface seawater flow rates. Large and efficient heat exchangers are thus necessary. Because of a need to also minimize seawater pumping losses, very large conduits also must be envisioned. The Cold Water Pipe (CWP) in particular represents a technological frontier, at least for OTEC plant designs beyond 10 MW [9]. Difficulties with the OTEC power block have been tackled differently. To be able to replace costly metal heat exchangers with simple hardware, Claude invented the Open-Cycle (OC-OTEC) [1] where steam generated from surface seawater in a low-pressure chamber continuously provides the working fluid. Unfortunately, the benefits gained with simpler robust evaporator and condenser designs are offset by the needs for very large low-pressure turbines and multi-stage vacuum compression systems. This would effectively limit OC-OTEC to plants smaller than 10 MW.
More recently, there have been efforts to improve the low efficiency of OTEC Rankine cycles by using a mixture of ammonia and water through the heat exchangers. This concept is embodied in the Kalina and Uehara cycles. The behavior of the mixture during evaporation and condensation differs from that of pure fluids. It theoretically allows a better match of heat loads during heat transfer since the temperatures of working fluid and seawater can remain closer. A plant based on this cycle requires additional hardware, i.e., a separator before the turbine inlet and an absorber after the turbine outlet. Also, the heat carried by the water in the mixture can be partly recuperated through a regenerator. The Kalina cycle reportedly can boost the Carnot efficiency of an OTEC system by 50% or so, but it also imposes increased demands on the evaporator and condenser. Hence, the viability of OTEC cycles departing from the standard Rankine cycle probably hinges on the availability of better heat exchangers [10].
The greatest technological (and credibility) challenges facing OTEC remain in the realm of ocean engineering, as OTEC field experimentation critically depends on whether a CWP can be deployed and how long it survives. From Claude’s hardships in the 1930s [1, 2] to recent trouble in Indian waters [11], the history of OTEC development is rife with CWP failures. The state-of-the-art for operating deep cold seawater pipelines consists of seafloor-mounted high density polyethylene (HDPE) conduits. The largest to date (1.4 m in diameter and 2.8 km long) was deployed off the west coast of Hawaii to a depth of 900 m in 2001 [12]. While HDPE CWPs would be ideal for small megawatt-class systems, OTEC plants of much greater capacity would have to rely on other choices. On the other hand, the exploitation of vast remote offshore areas with floating platforms poses specific challenges that are not addressed with land-based systems.
The most ambitious program designed to resolve ocean engineering problems specific to large floating OTEC plants remains the comprehensive effort led by the U.S. National Oceanic and Atmospheric Administration (NOAA) in the late 1970s and early 1980s.
The large size of OTEC components and the demands imposed by offshore environments on equipment survival and power production logistics result in high projected capital costs. From an economic point of view, this is exacerbated by relatively low power outputs so that standard analyses based on the levelized cost of electricity generation have consistently resulted in uneconomical projects. Even though the cost-effectiveness gap between OTEC and the most expensive fossil-fuel power generation technologies (e.g., oil) has steadily declined, OTEC market penetration has not yet succeeded. When considering estimates of capital costs per unit power as a function of rated power, OTEC systems exhibit a considerable expected economy of scale as one would move from small pilot plants to larger commercial units. Because of a lack of experimental and operational data in running OTEC systems, however, taking advantage of this purported economy of scale has not been possible. Various strategies aimed at leveraging market resources have been attempted. A common approach has been to identify niche markets where the local cost of electricity is sufficiently high and the overall power demand sufficiently low to make OTEC potentially attractive at the modest power outputs suitable for first-generation projects (e.g. 1 to 10 MW).
REFERENCES
[1] Claude G. (1930). Power from the tropical seas. Mech. Eng., 52, 1035-1044.
[2] Gauthier M. (1991). The pioneer OTEC operation: “La Tunisie”. Club des Argonautes, Newsletter 2. http://www.clubdesargonautes.org/otec/vol/vol2-1-10.htm
[3] Vega L.A., (1992). Economics of Ocean Thermal Energy Conversion (OTEC). Chap. 7, Ocean Energy Recovery: the State of the Art (R.J. Seymour ed.), ASCE, New York, 152-181.
[4] Vega L.A., Nihous G.C., (1988). At-sea test of the structural response of a large diameter pipe attached to a surface vessel. Proc. Off. Tech. Conf., Houston, U.S.A., Paper 5798, 473-480.
[5] Vega L.A., Evans D.E. (1994). Operation of a small open-cycle OTEC experimental facility. Proc. Oceanology Int. Conf., Brighton, U.K., 5(7), 16 p.
[6] Cousteau J.Y., Jacquier H. (1981). Énergie des mers: plan-plan les watts. Chap. 9 in Français, on a volé ta mer (R. Laffont ed.), ISBN 2221007654, Paris.
[7] Nihous G.C., (2005). An order-of-magnitude estimate of Ocean Thermal Energy Conversion resources. J. Energy Res. Tech., 127(4), 328-333.
[8] Nihous G.C., (2007). A preliminary assessment of Ocean Thermal Energy Conversion (OTEC) resources. J. Energy Res. Tech., 129(1), 10-17.
[9] Brown M.G., Hearn G.E., Langley R.S. (1989). A new design of cold water pipe for use with floating OTEC platforms. Proc. Oceans ’89 Conf., 1, 42-47.
[10] Kobayashi H., Jitsuhara S., Uehara H., (2001). The present status and features of OTEC and recent aspects of thermal energy conversion technologies. http://www.nmri.go.jp/main/cooperation/ujnr/24ujnr_paper_jpn/Kobayashi.pdf
[11] Comptroller and Auditor General of India, (2008). Chap. 7, Report No. CA 3 of 2008, 39-48. http://www.cag.gov.in/html/reports/civil/2008_3SD_CA/chap_7.pdf[12] Daniel T.H., (2001). 55" seawater system CIP project update. NELHA Pipeline, 10, October 2001. http://www.nelha.org/pdf/PLiss10.pdf
