SERl/TP -252-2331 UC Category: 64 DE85000504
Open-Cycle Ocean Thermal Energy Conversion (OTEC) Research: Progress Summary and a Design Study
T.Penney D. Bharathan J. Althof B. Parsons
August 1984
To be presented at the ASME Winter Meeting New Orleans, Louisiana 9 - 14 December 1984
Prepared under Task No. 4001.22 FTP No. 456-84
Solar Energy Research Institute A Division of Midwest Research Institute
1617 Cole Boulevard Golden, Colorado 80401
Prepared for the U.S. Department of Energy Contract No. DE-AC02-83CH10093 Printed in the United States of America Available from: National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road Springfield, VA 22161 Price: Microfiche A01 Printed Copy A02
NOTICE
This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Department of Energy, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights, SERI/TP-252-233 1
OPEN-CYCLE OCEAN 'l'BERMAL ENERGY CONVERSION (OTEC) RESEARCH: PROGRESS SUMKARY AND A DESIGN STUDY
T. Penney* D. Bharathan* J. Althof* B. Parsons
Solar Energy Research Institute 1617 Cole Blvd . Golden, Colorado
*Member ASME
ABSTRACT publications that present a well rounded and complete view of both open- and closed-cycle systems can be In 1980, the Westinghouse Corporation completed an found in Cohen ( 6, 7), Lavi (_!!), and Apte et al. (1). extensive Claude open-cycle ocean thermal energy con In July 1983, the Department of Energy (DOE) version (OC-OTEC ) system design study . Since that completed an ocean energy technology assessment (10 ) time, the Solar Energy Research Institute (SERI) has and identified the unknowns in the various produced concep ts and da ta bases that have reduced the approaches . Specific issues related to heat and mass technical uncertainties associated with the evaporator transfer in OC-OTEC are addressed in the 1983 overview and condenser design and performance, seawa ter sorption paper by Bharathan et al . Cl.!.)· kinetics and gas removal systems, low pressure turbine design wi th novel materials, and low co st system The reasons for pursuing closed- rather than open containment and structural design. This paper cycle OTEC sys tems were, however, quite clear; indus try describes an integrated system design case study using was better equipped to deal with the problems of bio the improved data base and summarizes an assessment of fouling and corrojion of heat exchangers than to the relative thermodynamic performance of advanced develop, for the Claude cycle system, an entirely new technologies, drawing parallels with Claude's early product line of large, low-pressure turbines and low work in the 1930s . Projections from these latest loss (e.g ., liquid and vapor-side pressure drop) advances imply that OC-OTEC systems can be cost direct-contact heat exchangers of which lit tle was effective in sizes less than 10 MW . known. e Analyzing the research needs for OC-OTEC systems Problems generic to both cycles include ocean engineer reveals that an experimental facility integrating all ing questions of sea-state survivance and aasociated essential components of a system is required. This risks. The large ca pital inves tments needed to build paper describes a facility for conducting advanced an OTEC system drove policy makers to pursue a research research and verifying cycle feasibility in terms of and development path with lower technological risks so performance, reliability, and co st. The thermodynamic indus try might begin to commercialize OTEC systems performance of this integrated design is projected within the next decade. using an analytical system model incorporating the With the foresight to keep the less advanced OTEC highly coupled component interactions. options in the long term horizon and to identify the issues needing resolution for a more definitive assess BACKGROUND ment, DOE commissioned the Westinghouse Corporation to complete an alternative OTEC power system study (12 ) to Since the filing of Georges Claude's patent (1) on address the open cycle and hybrid power system options . - 17 June 1932, a rather limited set of open-cycle OTEC Drawing upon work on OTEC in the 1970s by Boot and designs for basic components or total sys tems ·have been McGowan (13), Heronemus (14), Brown and Wechsler (15), scrutinized. Most papers in the OTEC literature deal Anderson (16 ), Zener (i?), Watt et al . (18 ), and superficially with open-cycle systems, mainly focusing others, Westinghouse showed that an integratedOC-OTEC on closed-cycle systems . Some references consider open plant with a 100-� net capacity could produce an e and the Claude cycle1 synonomous, ignoring other impor acceptably low cost per kilowatt hour . In a later tant open-cycle options proposed by Beck (2) (steam paper (19 ), Westinghouse indicated that a full-scale lift pump), Ridgway (3,4) (m ist lift), Zener (5) (foam n s ha ° t lift), and others . Although somewhat outdated, OTEC ;��:�� �����o:;::: ::�ec:;::s :n :er:: ��P:�:�, �o �::= tial funding, and timing of its participation as a much needed energy resource. They hypothesized that the 1The Claude open cycle is referred to in this paper as best approach would be to design and build a 2 .5- to OC-OTEC, for convenience . 3.0-MW, OC-OTEC plant to provide power and potable
1 SERI/TP-252-2331 water to any one of several island communities that the process of converting ocean thermal gradients in a were largely dependent on imported fossil fuels. They direct-contact open cycle method. Claude and further postulated that the plant would be designed for Boucherot's id eas we re explicit and detailed based upon a 20- to 30-year life, and after extensive initial their own experime nts (36-41 ) conducted in earlier testing, it would be used as a commercial water and trials at Ougree, Belgium and Ma tanzas Bay, Cuba . power production facility. Incurred plant cost would A recent systems analysis of the Claude open-cycle be recoverable during its commercial application phase. system was completed by Parsons et al. ( 42) and subse During the experimental phase, the plant could serve as quently used to give design guidance for our small the demonstration model to evaluate the availability/ scale conceptual OC-OTEC facility. We felt it would be reliability criteria as required by regulations govern worthwhile to compare Claude 's projections with our ing utility company acquisitions of unconventional model to identify research issues that complement or power plants. deviate from current knowledge and to reflect on Consistent wi th that philosophy and in concert with unknowns in the feasibility of this power cycle concep the timing of the deployment ( 20) of the reconfigured tual design. OTE C-1 (21 ) 4-ft polyethylene cold wa ter pipe at the The statements by Claude and Boucherot in their Natural Energy Laboratory in Hawaii (NELH), SERI, patent contained details about evaporation, use of the assisted by Creare Research and Development, Inc. of vapor in the turbines, condensation of vapor, extrac Hanover, New Hampshire, Science Applications, Inc. of tion of air from the degasifiers and the condenser, Hermosa Beach, Calif., and T. Y. Lin International of pumping of warm and cold seawater, and itemization of San Francisco, Calif., developed a small-scale (nominal developed (gross) and absorbed (gross minus net) power. l MW ) preliminary co nceptual design of a shore-based Using their quoted technical specifications for the oc-ofEc research facility (22) that incorporated the categories cited in their patent, we were able to cross
latest developments on the heat exchangers (23��--35) and check and independently compare performance values, other components. water flow rates, temperature distributions, noncon There are several identified, unresolved issues densable gas and facility leak rate assumptions, auxil related to large-scale OC-OTEC development. One of the iary motor efficiencies, water and steam side effec primary issues that a research facility would help tiveness for heat exchangers, head losses in the resolve is of the highly coupled nature of the open hydraulic circuits, pressure drops in the intercom cycle system. This coupling is the result of using ponent passages and net-to-gross power ratios. Fur steam produced from the warm resource water as the thermore, we completed a short thermodynamic working fluid to drive the power generator and then optimization study to evaluate how well Claude's condensing it directly on the cold resource water. Any predictions matched a design criterion for maximum net changes in the resource temperature, tidal and wave power (and hence minimal auxiliary losses) . fluctuations, or speed of the power train (turbine/ Claude and Boucherot' s concept for an integrated generator) would cause a highly coupled interaction 50-MW (gross) OTEC plant is shown in Fig. l (taken e within the sys tem, changing the operation and perfor from their patent filing). Warm seawater, entering via mance of the subcomponents. A research facility could pipe Nq. 62, is partially deaerated by the traps in the also be used to: upcomer, and is distributed to the evaporator by way of • develop experimental data bases for validating design a slotted overhead wa ter box. Low density steam is methods of large-scale direct contact heat exchangers produced in the vacuum chamber and expanded through a set of turbines (84). A series of generators produce • investigate various subcomponent options including electricity with fe�dback loops (108, 110, 1 12) that con evaporators, condenser subsystems, and turbines trol turbine speed and load. Cold water from the ocean • evaluate turbine performances for dual-flow hori depth is fed into a direct-contact condenser maintain zontal and vertical axis turbines ing low pressure on the downstream side of the turbine . Pumps ( 82 and 102) are required to overcome the fric • investigate suitable composite structures for devel tional head losses in the warm and cold circuits and oping low-cost turbine rotor options maintain the water level in chamber 96 at the correct • evaluate structural material options for vacuum height. A vacuum exhaust system (104) removes noncon containment densable gases evolved from the feedstreams or leaked .uo • investigate int eraction of sea states wi th the baro metric heat exchangers " Lt11 o/!\P:.oe " "� " t-'""' 'i'// '# �· .:J 0 • evaluate the effects and extent of gas desorption 0 from seawater in the various heat exchanger options fi.S��_;;�"" • investigate material and subcomponent degradation · " l :� . 88 caused by seawater corrosion and biofouling. ru . . 'r;:;'�f �o ··· -'f=I :"' · .. :�r ... <··'- IA ;�-- r:��ii•._._;.�_·_:. ----.-��-- �"'ft;_!:j;:'."'""__ �ttF!ff:F - ;· · -· · � · J:; }""''. ,_o fJ6 A rather comprehensive list of open research is sues re - lated to OTEC development may be found in Ref. (..!.Q). · ) · 60 A fresh look at smaller size OC-OTE C systems for (}� 1::1 . Jr� ·: advancing the state of the art combined with advances through recent research establishes a greater potential for OC-OTEC systems sooner than previously anticipated. A few of the key technical aspects of our new concep tual design are discussed here and compared with Claude's early vision of his open-cycle plant. =--.::..._--:::..._-__::- = ===F=-.: -t·f� .-=Joz-- -�";;;:. · - - -- CLAUDE'S PREDICTIONS AND WHAT WE KNOW TODAY =:__ -=--=- -=-::::if}-:_-::._� i:::�-100--�1-� .:::--: --:: -=- On 2 July 1935, the United States patent office SOURCE: Claude and Baucherot (1) granted Georges Claude and Paul Boucherot (.!) patent Pig. 1. Claude's 50-MWe Conceptual Open-Cycle Plant number 2006985 in which they had 12 claims regarding Design
2 SERI/TP-252-2331
from the atmosphere into the vacuum chambers and a condensers result in an actual/minimum water flow ratio small amount of uncondensed steam from the condenser . of 1.2 and a HTU of 0.3 m. The large-scale plant design is based on the Claude projected a rather low total-to-static effi results from their onshore experimental apparatus shown ciency of 0. 71 for the performance of a single turbine in Fig . 2. !!ere the incoming seawater follows a tor in his 50-MW plant compared with today's improved e tuous path to free dissolved noncondensable gases technology capability with projections by Westinghouse before entering the heat exchangers. Rather than dis of nearly 0.81. The eight tu rbines (nominally 6 MW e tribute the warm seawater to the evaporator from above, gross each) in Claude's design are quite large (roughly as in the conceptual design, they chose a vertical 10 m in diameter) with rotational speeds of nominally spout (160) arrangement. After the steam expands 840 rpm and would require engineering and manufacturing through the tu rbine (166), it condenses in a three development even today . 2 stage direct-contact condenser . The first two stages Claude's design includes passive degasification of of the condenser are illustrated in Fig. 3. Cold sea the seawater in the water supply. upcomer to remove water spills in a controlled manner onto the inside of inert gases before the streams enter the heat ex a circular pipe forming a falling film (188, 192) . In changers. The pressure in the degasification chambers the first stage the steam flows downward in a cocurrent are held below atmospheric pressure, yet higher than fashion. The steam is then turned 18 0 degrees and the pressure required to flash (i .e., boil) the sea passes through the second stage a countercurrent water. Possible benefits of degasification include falling film section. Finally, the remaining less power required to remove the no ncondensable gases uncondensed steam and inert gases enter the third (which are compressed from a higher pressure) and s tage, a countercurrent packed column condenser shown improved condenser performance due to a reduction of in Fig. 2 as item 196 . gas-side diffusional resistance . The plant design parameters and performance listed The condenser exhaust system identified by Claude by Claude and Boucherot in their patent are rather could handle all the noncondensable gases dissolved in detailed, but we note that their calculations were the seawater (primarily nitrogen and oxygen) and an limited to OTEC geographic locations with an average available resource temperature difference of 24°c ° 2 (e.g., Guam, Manila) (43), 2 c higher than Cuba's Recent reports of the 198 0s by Hydronautics (46 ), ORNL resource . By using the available specific design (!!!.), Westinghouse (..!1}, Parsons, et al . ( 42), and parameters listed in Claude's patent as inputs to the Lewandowski et al . (48) indicated no significant computer program and by assuming and adjusting values benefit (in terms of cost and net power) results from of parameters not listed, we were able to closely match active deaeration of the seawater. Most of these conditions of their 50-MW i;ecent investigators had reason the e design as shown in to believe that gas Table 1. Important outputs and results related to this would not readily come out of solution without the comparison are discussed next. additional seawater head loss imposed by the active Even before the 1940s when Leon Nisolle experimen deaeration systems (i .e., packed columns) . Recently, tally quanti fied the performance of various evaporator Krock at the University of Hawaii under subcontract to geometries (44,45), Claude recognized that efficient SERI, communicated results (49) indicating dissolved evaporation :LS"easier to achieve than efficient conden gases readily desorb in the upcoming seawater supply sation. This is demonstrated by his assumption of 100% pipes nearly independent of seawater flow rates at an evaporator effectiveness (very close to our fresh and exchange rate of twice that of fresh water data . This seawater data with verified values of 90-98%) and his new result may warrant inclusion of passive seawater very conservative condenser design with its deaeration coGlponehts (similar to Claude 's ideas) in actual/minimum water flow ratio (minimum thermodynamic current OC-OTEC plant designs. Final confirmation of requirement) of 1.65 and a height of transfer unit these data and the physical explanations of the (HTU) of 3 .45 m. Current designs of direct-contact phenomena will be reported in the near future.
SOURCE: Claude and Baucherot (1) Fig. 2. Claude's Experimental Apparatus
3 SERI/TP-252-2331
Clau:le Progran Progran Canponent Description Design Inp.its Results
Evaporator wann water inlet tanpera ture (0c) 29 29 fairly high warm water resource stean production temperature (°G) 26 26 temperature, limita:I locatioas; effectiveness 1.0 actual evaporator effectiveness -3 walill seawater flow rate x 10 (kg/s) 142 143 closer to 0.95 (30) stean production rate (kg/s) 710 703 'l\Jrbines stean inlet temperature (0c) 26 26 no evaporator--turbin e vapor stean ootlet temperature (°C) 13.S 13.S pressure loss is assuned; generator generator effici ency 0. 96 0.96 efficiency has not changed Ill.ICh si nce (Mol ) so so 1932; Westinghouse (12) used a turbine gross power e turbin e to tal-to-static efficiency 0.71 efficiency of o.81; turbine dianeter number of turbines 8 8 of up to S m possible withcurrent turbine dianeter (m) 9. 7 technology (SS). lbndenser cold water inlet temperature (0c) s s no turbine-condeaser pressure loss ° cold water outlet tanperature ( C) 10.S 10.s or diffuser recovery is asSl.llEd; ° stean inlet tanperature ( C) 13.S 13.S measuranents in CD 4 SERI/TP-252-2331 Components � Jl586 Evaporators. Four po tential evaporator geometries were reviewed; open channel flow (23,24,44,45), falling ver l -·- films (25,31, 33), downward falling tica jets (30- . . - - I 32 ,34,3 55": and upward flowing vertical spouts (26 ,30, 44,45)-.- Several criteria, required for an effective I . evaporator design, were defined as: SOURCE: Claude and Bauch•rot (1) Fig. 3. Claude's Two-Stage Direct-Contact Condenser • Low liquid-side pressure loss Detail • Low vapor-side pressure loss cold water intake pipe. Reducing the diameter and increasing the allowable seawater velocity in the cold • Simple inlet and exit manifolds water pipe (which results in higher pumping power) are • High effectiveness economic trade-offs well wo rth considering in modern designs. Claude also recognized the importance of the • Small volume cold water pipe in the overall engineering evaluation • Low liquid entrainment as demonstrated by his two unsuccessful attempts to deploy a cold water pipe for his experimental apparatus • Low gas desorption ·in Cuba. • Low fabrication cost When he finally did succeed in laying a 2-km-long, 1. 7 5-m-diameter pipe and operating the power cycle, the • Low susceptibility to biofouling and corrosion. pipe was destroyed after only 11 days in rough seas. Claude 's calculations of the 50-MWe gross design Using analytical projections and experimental data, did no t include the losses associated with vapor pas we evaluated the four contending evaporator geometries. sages between the evaporator and turbine, between the A summary of the evaluation, shows that vertical spouts turbine and condenser, and within the condenser itself. have more merits and fewer negative aspects (high He, however, had a qualitative feel for their impor liquid entrainment (53)] than the other geometries. tance since his design has smooth vapor paths with few Preliminary results with seawater on sm all scale (54) sharp bends. We find these pressure drops can be sig vertical spouts confirm fresh water test data obtained nificant in the thermodynamic performance evaluations . from full scale evaporators. Further testing on In addition, current designs provide for a mist removal alternative components at a larger scale, at NELH, will device between the evaporator and turbine to prevent provide final seawater test results for the comparison salt droplet carry-over, which causes erosion and among contending evaporator geometries. Our last five stress corrosion problems (purity of the fresh wa ter years of research show that seawater evaporation can be by-product is a concern as well) . These pressure done in evaporators with approximately half the volume losses, which detract from the useful work of the and using less than 30% of the hydraulic seawater system, along with increased seawater intake pumping power disclosed in the 1979 Westinghouse velocities, result in a lower net-to-gross power assessment (12). - ratio. A current 50-MW (gross) plant with seawater Turbine. The turbine is perhaps the single most e resource temperatures of 25°C and s0c, accounting for important component in the OC-OTE C system. The re- vapor pressure drops and 2 m/ s seawater intake . quired large volumetric flow of steam coupled with the velocities, would yield a net-to-gross ratio of around low amount of available specific energy results in tur 75% (�. Claude's 50-MW plant calculations projected bine designs that are relatively large and expensive � a net-to-gross ratio of 80%. compared with conventional steam turbines. Small-scale OC-OTEC turbo-machinery (less than 5 MW ) is w�..:iiin e A NEW CONCEPTUAL DESIGN: SMALL-SCALE APPLICATIONS WITH state-of-the-art capability because the ro tor is simi INTEGRATED FLEXIBILITY lar to designs for the low-pressure stages of the con ventional steam turbines. Costs of these uni ts can be The SERI OTEC team, supported by its industrial and R&D reduced by using composite materials ( 19), but several subcontractors worked to combine the latest advances in years of research and development effort is needed OC-OTEC research to formulate and define a conceptual before they can be used . design of a small-scale (1 �!W ) facility. Component A comparison of the last stage of a conventional e design guidance wa s initially provided by the systems power plant turbine and a turbine designed by the analysis program of Ref. (� • Table 2 summarizes the French for their conceptual OC-OTEC facility design for 5 SERI/TP-252-2331 Component Description Caapoaent Descrip tion 1\Jrbine (dooble rotor) generated power aoo.o kW cold seawater fl.ow rate 1590.0 kg/s generator effici ency 0.97 actual/ininim.m water fl.ow rate 1.2 turbine efficiency 0.81 height of a transfer unit (!IlU) 0.3 m stean inlet tanperature 20.0 Oc fraction of equilibriun stean ootlet tanperature 12.0 Oc gas release 0.8 stean inlet velocity 60.0 mis non::orrlensable gas evolution 0.028 kg/s stean outlet velocity 97.5 mis air leakage rate 0.004 kg/s ste an enthalpy drop 65.5 k.J/kg water loading 60.4 kgfm 2 s stean flow rate 14.7 kg/s OindemerExhaust a.upessor Train maxinun tip speed 450.0 mis mmber of canpressio n stages 4.0 hub-to-tip ra tio 0.44 intercooler pressure drop 276.0 Pa ootside di aneter 2.8 m caopressor pressure ratio 3.1 rotational speed 3000.0 rpn intercooler exit tanpera ture 7.0 Diffuser caopressor efficiency 0.80 recovery factor 0.62 power requiranent 75.9 inlet stean speed 97.5 mis liD:lll SysteaSeailater Flair outlet stean velocity 60.0 mis inlet + dischar ge pipe leng th 1120.0 m stean inlet tenperature 12.0 OC seawater veloc ity in pi pes 1.78 mis stean outlet tanperature 12.4 Oc nunber of 9CP berrls 10.0 Evaporator pi pe diameter 1.22 m mist raooval pressure loss pi pe frictio n factor 0.0114 coefficient 10.0 total inlet + discharge mist raooval stean velocity 30.0 mis pressure loss 23.45 kPa mist raooval. pressure drop 81.7 Pa evaporato r height 0.5 m evap-mi.st pressure loss evaporator pre ssure loss 9.4 kPa coefficient o.s density pressure loss 0.5 kPa stean generati on velocity 23.0 mis total wann seawater pressure loss 33.4 kPa evap-mi.st pressure drop 2.4 Pa ccmbined punp-1110tor effici ency 0.786 stean generation tanperature 20.54 Oc punp di.aneter 0.84 m evaporator effectiveness 0.95 puq> speed soo.o wann seawater inlet tanpera ture 25.0 OC punp powerrequiranent 88.4 warm seawater outlet tenperature 20.76 OC QWl Seam.erSystea Flair . wann seawater fl.ow rate 2130.0 kg/s inlet + di scharge pi pe length 3370.0 m fraction of equil ibriun seawater velocity in pi pes 1.33 mis gas rele ase 0.9 nunber of 9CP berrls • 10.0 noncorrlensable gas evolution 0.034 kg/s pi pe di aneter 1.22 m air leakage rate 0.004 kg/s pi pe friction factor 0.0123 spoot seawater velocity 2.0 �s total inlet + di scrarge evaporator planfo nn area 35.2 pressure loss 34.2 kPa 2 mist remval area 27.0 m average:i corrleaser height 0.46 m Direct Coot.act o.n, ISE'T" co rrlenser pressure loss 8.7 kPa diff .-corrl. pressure loss density pressure loss 4.2 kPa coefficient 1.0 total cold seawater pressure loss 47.1 kPa diff.-corrl pressure loss 19.6 Pa canbined punp-1110tor effici ency 0.786 cocurrent pressure loss o.o Pa punp di aneter 0.66 m countercurrent pressure loss 200.0 Pa puq> speed 750.0 rpn cold seawater inlet tanperature 5.0 Oc punp power requiranent 92.7 kW cold seawater ootlet tenperature 10.6 OC o � st�as outlet tenperature 6.0 c total parasitic power requirement 257.0 kW outlet stean flow rate o.08 kg/s net power production 543.0 kW Tahiti is given in Table 3. The major difference in Condensers. The condenser research effort at SERI operating conditions is the inlet steam pressure, which has primarily focused on direct-contact heat exchange. is considerably higher for the conventional turbine. Direct-contact condensers are inherently more efficient However, the pressure ratio of the conventional turbine than standard surf ace type exchangers because the tem is only about 50% higher than the OC-OTEC turbine. perature drop across the solid surface interface is Because of a lower power density, which corresponds to eliminated. Without water production3 the proposed lower pressure, the Tahiti design yields a gross power condenser configuration (based on criterion similar to output of about S MW compared to 20-25 MW for the last that for the evaporator design) is a direct-contact stage of a conventional power plant turbine. Based on cocurrent region followed by a direct-contact counter this comparison, existing turbine rotors are perhaps current final stage, somewhat similar to Claude and best suited for prototype designs to reduce risks. Because of the nature of their design, these are hori 3 zontal axis, double-ended turbines. Typical deli very However, the production of potable fresh water from an times quoted by a reputable manufacturer for these con OC-OTEC system requires a surface condenser or a ventional types of turbines adapted for OC-OTEC, are water-to-water heat exchanger with a fresh water or about two years wi th performance guarantees (2.2_)· nontoxic imiscible fluid direct-contact condenser. 6 SERI/TP-252-2331 Table 3. Comparison Between Conventional Last Stage the water feed and discharge streams and provide for and Tahiti OC-OTEC Turbine Conditions damping of water level fluctuations. The water columns in the intake and drain passages reduce the pressure Parameter OC-OTEC Last Stage difference the structure must withstand. A surface condenser with a small fresh water pro Inlet pressure (Pa) 2700 15,000 duction capacity of 0.63 kg/s is located outside the enclosure at ground level for easy access and main Exit pressure (Pa) 1100 4000 tenance as merits which were traded off against the additional pressure drops caused by steam routing Isentropic enthalpy obvious in this arrangement. The gas exhaust system, drop (kJ/kg) 120 200 located at ground level, has obvious merits with no foreseen compromises. * Gross power Provisions are also made to deaerate the intake (MWe ) 4-5 20-25 water passively in the upcomer as shown. Speed (rpm) 1500 1500 Degasification. The effect of evolved noncon- densable gases on the evaporator and turbine perfor Tip diameter (m) 5.6 5.6 mance is inconsequential. Evolution of noncondensables in the evaporator feed stream contributes to generation * nouble pass turbine and reformation of the phase interface enhancing evapo ration. However, at the evaporator saturation pres Boucherot's design (1). Evaluation of various sure, the noncondensable gases at a volume fraction of condenser geometries ( 56 ,57) indicates that this less than 0.3% are unlikely to have any major impact on condenser subsystem is able to achieve low vapor-side surface renewal or liquid mixing. Noncondensables from pressure losses (<200 Pa) with water- and steam-side the evaporator flow through the turbine and make a effectiveness of over 0.8 at low cost. Approach steam slight but neglible contribution to the power output. velocities to the cocurrent region, where 90% of the The reduction in condenser effectiveness due to steam is condensed, should be about 20 m/s for a cost noncondensable gases and the accompanying increase in effective subsystem design. This condenser confi the condenser pressure strongly affects the turbine guration represents a significant improvement over output by 10-15%. However, the trade-off in system Claude's designs requiring 25% less water flow and parasitic power loss between degasification and no about an order of magnitude lower liquid-side pressure degasification shows only marginal improvements wnen loss yet with a higher number of transfer units for the active degasifiers (e.g., packed columns) are used. same height. Research on passive barometric degasifiers (e.g., riser Noncondensable Exhaust Handling. The warm and cold column direction changer) is in progress to quantify seawater feed streams subjected to a reduction in pres their potential for reducing the overall noncondensable sure desorb dissolved noncondensable gases (primarily exhaust pumping power. nitrogen and oxygen). These noncondensables along with Structure. For structural needs many new mate- any containment leaks must be continuously exhausted rials, such as petrochemical derivatives, polyethylene, from the facility to maintain a low system pressure. vinyl, and carbon-based graphite epoxy were considered Several options for gas exhaust systems were examined along with the more conventional building materials, based on a conservative design point of 100% evolution such as steel, steel alloys, other metals, and various of dissolved gases and an equivalent atmospheric air types of concrete.• A comparison based on permeability, in-leakage. Calculations indicate that conventional corrosion protection, leaching, and cost leads to con designs for hardware using four to six centrifugal com crete as the recommended choice for our small-scale pressor stages with interstage coolers would limit OC-OTEC structural design (58). Several architectural losses to about 10% of system gross power output. options were examined, an�a vertical cylinder was Although not off-the-shelf equipment, centrifugal com identified as the most cost effective. Test facility pressors are readily available from many manufacturers operation needs dictated that internal components such in the size range appropriate for this application. as evaporators, condensers, gas removal, and turbine The operating conditions, however, would require rede areas remain accessible to allow for experimental signing components such as bearings and seals. changes. Construction of the relatively large vacuum An integrated 1- vessel (15 m diameter 30 m height) will use slip-form System Integration. MWe OC-OTEC x facility is shown in Fig. 4. The entire vacuum enclo techniques to minimize construction joints. The sure and water feed and drain systems are integrated construction materials and procedures are commonly into a concrete cylindrical enclosure of 15-m diameter applied techniques in the construction industry. and 30 m height. The evaporator and condenser are Flow Hydraulics. To obtain an understanding of located at their respective barometric levels. The liquid side losses and the associated pumping power turbine (horizontal axis, double pass) is located in requirements, calculations were performed to identify the center directly above the evaporator. The top areas of significant loss contributions. The primary hatch of the enclosure allows easy access for instal pressure loss occurring in the seawater inlet pipes is lation and removal of the turbine and for inspection, caused by friction and is proportional to the square of maintenance, and repair. A round dome cover, although the seawater velocity. Aside from the inlet pipe loss, more condusive for vacuum vessels, was eliminated for hydrostatic pressure losses dominate. The interactions cost reasons. associated with waves were found to be sufficient!� Warm seawater enters the vacuum enclosure through damped by relatively small holding pools of about 10 m an inner annulus. Warm water effluent is drained planform area. Ocean dynamics associated with tides through a central discharge pipe. Cold water enters are of a quasisteady state nature due to the relatively through an outer annulus and is distributed to the long periods (12 hours and more) of tidal fluctuations. direct-contact condensers located around the periphery. To achieve continuous system operation, flow must be Cold water effluent like the warm water effluent dis maintained at low and high tide. We realize that the charges through a middle annular passage. Four sepa hydrostatic loss at low tide being a maximum is a pri rate pools directly underneath the enclosure separate mary design constraint. 7 SERI/TP-252-2331 Vapor Side Flow Analysis. Calculations were per indicates th at if the cold water pipe is a dominate formed to evaluate the vapor passage losses. A typical cost the best operating conditions would be that of pressure distribution through the system yie lds about maximum power output. This, of course, does not 3% (40 Pa) across the mi st removal device, 23% (315 Pa) correspond to the best net/gross ratio (0. 851) but for turbine losses, 62% (930 Pa) across the turbine for rather a value of about Q.60 for this configuration. useful work, and 13% (200 Pa) across the condenser. Overall system economic optimization, however, requires This emphasizes the importance of maintaining low further consideration. Efforts are underway to combine losses th rough the evaporator, condenser, and the thermodynamic systems model with economic analysis intercomponent passages to maximize the work out of the to arrive at the best system designs based on various turbine. optimization criteria beyond the results presented System Analysis. A systems model with a series of below. subroutines that predict the pe rformance of the various Instrumentation. Several basic measurements are components or subsystems and an executive routine that required, in cluding: flow rates (warm seawater, cold integrates the appropriate input/ output subroutine seawater, fresh potable water, effluent discharges); values to ensure system compatability was developed to pressure (evaporator, condenser, turbine, exhaust examine th e th ermodynamic performance of OC-OTEC sys system); temperature (warm seawater, cold seawater, tems (42). Thermodynamic optimization to minimize evaporator vapor, turbine exhaust, condenser exhaust, parasitic losses for the research facility results in exhaust system); dissolved gas content (warm seawater, turbine inlet and outlet temperatures of about 20°c and cold seawater), and power (generator output, pumping 11°c, respectively. For an inlet cold water pipe power, exhaust power). Further instrumentation details diameter of 1. 2 m and length of 27 SO m the net/ gross may be found in Ref. (22). power ratio and net power production as a function of All measurements are routine and within state-of- cold water flow rate are shown in Fig. S. This graph the-art techniques. The objective of the instrumen- i.---- 7.0 m --- .i I Pneumatic seal Removable steel hatch 23.0 Pres tressed / concrete cylinder vacuum chamber E 0 Manhole cci access _,- Noncondensable gas annular duct 15.4 Vertical 1-m Dia. baffle-'--'-----' condenser \ Cold cw Warm water out water in in L� 3.0 \ � I 2.0 Noncondensable gas exhaust system 0.0 11 11 !I It I II II 11 -3.0 I II II I Fig. 4. Sectional View of the Integrated 1-MWe Research Facility Design 8 SERI/TP-252-2331 1.0 � 0.50 500 "' 0"' 0 0.9 450 0.45 0.8 400 z .c 0.40 Ci) !2. s: "' 0.7 350 "'O .¥ 0 ...... 0.35 .... 0 Cl � � ..... 0.6 (1) Qi 300 ... ;;; 0.30 "'O .s ... 0 0.5 250 0 u 0 Q. >- 0.25 � c: Cl .... 0.4 ... 200 $l Q) a; 5· c: 0.20 ;: 0.3 ::J Q) 150 .... 0 ;1l:" al 0.15 CL .0 0.2 � en 100 ::l � ID 0.10 0.1 50 0.05 0.0 0 0.0 0.5 1. 0 1.5 2.0 2.5 3.0 3.5 4.0 0.00 0 2 4 6 8 1 0 12 14 16 18 20 Cold water flow rate (m3/s) Fig. 5. Typical Variation of the Net-to-Gross Power Net power production (MWe) Ratio and Net Power Output With Cold Water Fig. 6. Recommended Research Facility Instrumentation Flow Rate for a 1.2 m Diameter Inlet Pipe Requirements and Their Locations tation is to provide researchers with data to evaluate In addition, if certain components have projected life the performance of various component options. Some of spans less than overall plant life, the energy cost the measurements may also be used for monitoring, con must be proportionally increased to account for the trolling, and optimizing overall facility operations. added capital cost. It is clear that the economic Economics. Values solicited from various manufac parameters that are constantly changing from year to turers and engineering projections show OC-OTEC plant year must be carefully evaluated prior to assessing the capital investment costs ranging from off-the-shelf overall economic feasibility of an OTEC plant. equipment at $12/W to a projected improved cost of Presently, the dominant costs of OC-OTEC plants are about $3/W for routinely produced facilities at large related to the seawater feed pipes (especially driven scales (i.e., )20 MW net). Fig. 6 shows our estimate e by length) and associated deployment and the vacuum of busbar energy cost as a function of net power containment structure. Recent research has reduced the production over the range of 1 to 20 MWe for two fixed heat exchanger costs to an extent where the cost is charge rates (0. 17 and 0. 06). This energy cost is insignificant at the level of uncertainty in cost anal found by levelizing the capital cost of the plant over ysis (iQ_). its lifetime and accounting for return on investment, operatioh and maintenance, and actual operating time. FUTURE DIRECTIONS Specifically, the amortization of plant capital cost is done using the fixed charge rate, an economic parameter Although this paper reemphasizes the Westinghouse defined by conclusion that OC-OTEC is a technically feasible cost effective renewable energy alternative today, further candidate areas of performance and cost improvements __ ,�,, , , quantities of noncondensable gases may evolve must be quantified to design the appropriate heat exchanger This parameter accounts for the escalation rate i the size (direct contact or surface). Composite blading rate of return on investment r, and plant life N. For for the turbine can further improve the economics of our study we have used fixed charge rates that give the fixed capital investment costs. Instrumentation reasonable bounds for escalation and return on required to quantify component performance was inves t:nent. (For an escalation of 8%, rate of return described. The experimental data needs for OC-OTEC of 30%, and 30-year plant life the fixed charge rate is reveal that experimental facilities with a flow 0 .17. For an escalation of 6%, rate of return of 10%, capability of only a few thousand gallons of seawater and a plant life of 30 years the fixed charge rate is per minute could evaluate heat exchangers for modularly 0. 06). Further, we have assumed that the operation and scalable units to sizes in excess of MW · Although 5 e maintenance costs are 1% of the total plant capital the practicality of producing fresh water using surface cost per year, the plant produces electricity 90% of condensers in an OC-OTEC plant is not new (61), achiev the year (capacity factor of 0. 9). The busbar energy ing dual product outputs is important to improve the cost can be defined as cost effectiveness of OTEC plants. Two recently com pleted analytical studies (62,63) have explored the BBEC = ((FCR x K/CF) + OM x K/CF]0. 114 commercial viability of this scheme. Both conclude a near-term potential for commercial viability of the OC where BBEC = busbar energy cost, $/kWh OTEC concept for production of fresh water and elec FCR fixed charge rate, $/$-yr tricity at relatively small (<10 MW) sizes. K = plant capital cost, $/W CF = capacity factor CONCLUSIONS OM = operation and maintenance factor 0. 114 = unit conversion constant (W-Yr to This paper has kW-h} 9 SERI/TP-252-2331 • Assessed the impact of recent research results on the (3) Ridgway, s. and Lee, C. K. B., "Installation and thermodynamic performance of a small-scale, shore Operation of the Mist Lift Ocean Thermal Energy Conver based OC-OTEC system sion (OTEC) Process at Keahole Point Hawaii," Research and Development Associates, Inc., RDA-TRP126200-001, Provided limited economic evaluations of key OC-OTEC • Marina Del Ray, CA, October 1983. components integrated into a design concept • Id entified issues that require further investigation ( 4) Ridgway, S. L., "The Mist Flow Ocean Thermal to reduce the uncertainties in projected system per Energy Conversion Plant, " Proceedings of the Fourth formance. Ocean Thermal Energy Conversion Conference, New Orleans, LA, 1977. Progress since the time of Claude has been slow, but, more recently, steady because of the facilities in the (5) Zener, c. and Fetkovich, J., "Foam Solar Sea Power continental United States testing with fresh wa ter Plant, " Science, Vol. 189, 1975. (31,64), and in Hawaii (65), using warm and cold seawater from the ocean on acontinuous basis. Efforts (6) Cohen, R., "Electric Power Generaion--Ocean in these facilities have resolved a number of unknown Thermal Energy Conversion (OTEC)," Chapter 13, Solar issues and uncertainties in low-temperature direct Energy Handbook, Marcel Dekker, Inc. contact heat exchange. Combining these technical results with the econom (7) Cohen, R. "Energy from the Ocean, " Phil Trans. R. ical projections we can conclude that: Soc. Land, A 307, PP• 405-437. • Most of the eco nomies of scale for an OC-OTEC plant (8) Lavi, A., Energy, The International Journal, can be realized for plants less than 10 MWe in size Vol. 5, No. 6, Pergamon Press, New York, June 1980. • Plants of 10 MWe size can ·be built economically (within 20% of optimum estimates) using multiple (9) Apte, et al., "Ocean Thermal Energy Conversion state-o f-the-art turbines and cold water pipes while Reference Manual," PRC llR-4017, PRC System Services, using the new experimental data bases as the design McLean, VA, August 1981. strategy for cost-effective, direct-contact heat exchangers. (10) "Ocean Energy Technology Assessment, " Meridian Corporation, Falls Church, VA, July 1983. • There are significant commercial opportunities for 10-Ml< plants producing electricity and fresh water e (11) Bharathan, D., Kreith, F., and Owens, w. L., "An because of the cost reductions from dual product Overview of Heat and Mass Trans fer in Open Cycle OTEC outputs Systems," SERI-TP-252-1854, Solar Energy Research • The technological risks still appear to be high for Institute, Golden, CO, 1983. OC-OTEC because of a lack of applicable seawater data at a scale amenable to extrapolation for commercial (12) "100 MWe OTEC Alternate Power Systems, Final ventures. Report, " Westinghouse Electric Corporation, Power Generation Divisions, Lester, PA., March 1979. An analysis of the research data needs for the next step of meaningful open-cycle OTEC development plan (13) Boot, J. L. and McGowan, J. G., "Feasibility Study reveals th at an experimental fa't:ility integrating all of a 100 Megawatt �pen Cycle Ocean Thermal Difference the essentials of a complete system is required. This Power Plant, " Report NSF/RANN/SE/GI-34979/TR/ 74/3, design case study describes such a facility that could Mechanical Engineering Department, University of provide advanced research data and verify the cycle Massachusetts, Amherst, August 1974. feasibility in terms of performance, cost, and relia bility of key components and system s. ( 14) Heronemus, W. E., et al., "Technical and Economic Feasibility of the Ocean Thermal Differences Process, " Acknowledgments Third Quarterly Progress Report, NSF/RANN/SE/GT- 34979/PR/74/3, University of Massachusetts, Amherst We would like to express our gradi tude to Benjamin 1 December 1974. Shelpuk, manager of the SERI Oceans Program for pro viding guidance over the last five years of this (15) Brown, c. 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