NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Staging Rankine Cycles Using 

Ammonia for OTEC Power 

Production 

D. Bharathan

Technical Report

NREL/TP-5500-49121

March 2011
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401

303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

Staging Rankine Cycles Using 

Ammonia for OTEC Power 

Production 

D. Bharathan 

Prepared under Task No. WA10.4010

Technical Report

NREL/TP-5500-49121

March 2011
NOTICE

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Acknowledgements 

The author would like to acknowledge the support of the DOE Program Manager Alejandro 

Moreno of the Wind and Hydro Technologies Program of the Office of Energy Efficiency and 

Renewable Energy. Support by Bob Thresher and Al LiVecchi of the National Wind Technology 

Center of NREL is also appreciated.
iv

Table of Contents

Introduction ..................................................................................................................................... 1

Approach ......................................................................................................................................... 1

Baseline plant .............................................................................................................................. 2

Staged Power Plants .................................................................................................................... 6

Concluding remarks ...................................................................................................................... 10

References ..................................................................................................................................... 11
v 

List of Figures

Figure 1 Cycle temperature-entropy (T-S) diagram for a single and two-staged Carnot cycles (not 

to scale) ........................................................................................................................................... 2

Figure 2 Detailed flow conditions for a single-stage 10 MWe plant .............................................. 4

Figure 3 Variation of net power versus ammonia circulation rate .................................................. 5

Figure 4 Temperature/entropy diagram for the single-stage ammonia OTEC power plant ........... 6

Figure 5 Two-stage Rankine cycle power plant showing details of the streams ............................ 7

Figure 6 Variation of gross, net and parasitic powers with the number of stages .......................... 9

Figure 7 Variation of heat exchanger (UA) product requirements with the number of stages ..... 10

List of Tables

Table 1 Summary of parameters for the baseline 10 MWe OTEC plant ........................................ 3

Table 2 Power system summary for staged OTEC cycles .............................................................. 8
1 

Introduction

Recent focus on renewable power production has renewed interest in looking into Ocean thermal 

energy conversion (OTEC) systems. Early studies in OTEC applicability indicate that the Island 

of Hawaii offers a potential market for a nominal 40-MWe system. However, a 40-MW system 

represents a large leap in the current state of OTEC technology with the associated risks, 

considering that the largest net-power producing system was tested at a power level of only 200 

kWe in the 1990s [Bharathan 1990]. Smaller sized plants on the order of 1 to 2 MWe should be 

pursued first. Lockheed Martin Inc., under US Navy funding, is currently developing a 10-MWe 

system design [Lockheed 2009]. With estimated capital cost per capacity ranging from 

10,000$/kWe to 15,000$/kWe [Vega 2003] or more, it is essential that the potential risks 

associated with the first-of-the-kind plant be minimized. Every means for cost reduction must 

also be pursued without adding potential risks. Considering that majority of the costs are 

associated with the seawater systems, maximum use of the resource water takes on a highimportance. It is with this in mind that we take on this short study to assess the potential for 

increasing return on the investment both in terms of effective use of the seawater and of reducing 

equipment costs. 

Approach

Many potential thermodynamic cycles have been investigated with the aim of reducing the 

overall cost for OTEC. Those include the familiar Rankine cycle, the open or Claude cycle, and 

others, such as mist-lift cycle [Ridgeway 1980], Kalina cycle [Kalina 1984] and Uehara [Uehara 

1999] cycle, among others. On account its well-established practice in the engineering 

community we confine our analyses to the closed Rankine cycle at this time.

With respect to the working fluid, ammonia remains the fluid of choice for OTEC closed-cycle 

systems, even though other fluids such as propylene and various refrigerants have been looked 

into in the literature. We analyze power systems that use ammonia as the working fluid. At 

OTEC conditions, there is considerable amount experimental data available for heat exchangers 

that use ammonia and seawater, both for boiling and condensation [see for example, Panchal 

1981]. Studies have also addressed qualification of aluminum alloys and methods for mitigation 

of biofouling [Panchal 1990].

Having made these practical and less-risky choices of ammonia and Rankine cycle, options to 

increase power yield from the seawater resources come in the form of staging the cycles. Staging 

allows maximum potential extraction of heat and power from a set of given resources. Many 

researchers have studied staging power cycles.

Figure 1 illustrates the nature of staging. A temperature-entropy (T-S) diagram shows both a 

single-stage and a two-stage Carnot cycles. Advantages of staging is best illustrated with Carnot 

cycles, without any loss of applicability. Both cooling and heating lines for the warm and cold 

seawater are indicated in this figure. The single stage working fluid state points are indicated by 

points ABCD. The area within this rectangle represents the amount of power that can be 

generated from that cycle. The temperature approach at points A and C are dictated by the 

minimal internal temperature approach (MITA) on the evaporator and condenser, respectively. If 

the cycle is staged as two separate cycles, indicated by the rectangles, AFJK and GHCI, one can 
2 

still maintain the same temperature approach in the two evaporators and condensers. However, 

the overall area covered by these two rectangles is substantially greater than the for the singlestage cycle. It is possible to increase the number of stages to increase power yield from a given 

set of resources while maintaining a given set of approach temperatures. However, cost tradeoffs 

will limit the number of stages that can be economically used.

Figure 1 Cycle temperature-entropy (T-S) diagram for a single and two-staged Carnot cycles (not 

to scale)

While Figure 1 illustrates the principle for staging for Carnot cycles, the same arguments apply 

for the Rankine cycle as well.

Baseline plant

For the baseline plant, which was sized at a nominal 10 MWe net electrical power, we used a 

warm seawater flow rate of 50,000 kg/s and a cold seawater flow rate of 28,450 kg/s. Economic 

analyses indicate that because of the larger costs and pumping power requirements for the cold 

seawater, a proportionately higher flow rate of warm seawater is beneficial [Vega 1995]. Other 

assumptions, based on many prior design studies, made for the rest of the presented analyses are 

summarized in Table 1. Once again, since cold seawater is a more costly resource, the condenser 

minimum internal temperature approach (MITA) is set at 1oC as opposed to 1.2oC for the 

evaporator. Condenser and evaporator pressure losses are based on earlier studies. Overall 

system loss is consistent with a floating platform for the plant. 
3 

Table 1 Summary of parameters for the baseline 10 MWe OTEC plant

All components of the system were modeled using commercially available software ASPEN Plus 

[Aspen 2010]. Ammonia flow rate and the evaporator and condenser pressures were varied to 

arrive at maximum power production for the system. Figure 2 shows the details of the simulation 

for a single-stage power plant.
4 

Figure 2 Detailed flow conditions for a single-stage 10 MWe plant

Optimization of the ammonia flow through the system indicates a peak net power yield of 11930 

kWe, at an ammonia circulation rate of about 585 kg/s. A plot of net power versus ammonia flow 

rate is shown in Figure 3. Despite the peak we note that in Figure 3, this peak is very shallow 

with the variation in the power of only about 100 kWe over the flow rate variation of about 15%. 

The system is very tolerant to variations in ammonia flow rate.
5 

Figure 3 Variation of net power versus ammonia circulation rate

Note that the parasitic power used in the system is large, over 30% of the gross electric power 

generated as shown in Table 2. The quoted pressure losses in Table 1 are consistent with an 

OTEC system mounted on a floating platform. Over 5 MWe power is used up in pumping the 

seawaters and circulating ammonia through the system. In case of a shore-based system, the 

parasitic power used within the plant may reach 40% of the gross power. 

Temperature-entropy diagram for the single-stage ammonia cycle with proper scales is shown in 

Figure 4. Note that the warm seawater water leaves the system at 22.62oC and the cold seawater 

leaves at 10.3o

C. There is still substantive potential left in these streams to extract usable energy 

using multiple stages in the cycle.
6 

Figure 4 Temperature/entropy diagram for the single-stage ammonia OTEC power plant

Staged Power Plants

Consider a two-stage Rankine cycle system for OTEC as shown in Figure 5. This system still 

maintains the same MITA as the single-stage system. Flow rates of ammonia through the two 

stages were optimized to yield the results shown in this figure. Ammonia flow rates turned out to 

be the same for both the stages at 390 kg/s. 
7 

Figure 5 Two-stage Rankine cycle power plant showing details of the streams

The two stages produce a combined gross shaft power of 24.4 MW or electrical power of nearly 

23 MWe. This gross power is substantially greater than that for the single stage. Parasitic power 

for the system also increases on account of the two heat exchangers through which the seawaters 

must flow. Despite increased parasitic power, we calculate the net power for the two stage 

system to be 16.17 MWe. This is substantially greater than that for the single stage system. We 

also note that the warm seawater leaves the system colder and the cold seawater warmer, than the 

single-stage system.

Heat exchanger requirements are also indicated in Table 2 in the form of an overall heat-transfer 

coefficient times the heat exchanger area product, or the (UA) product. Heat exchange area 

required for the two stage system is greater than that for the single stage.

We conducted a similar set of analyses for a three-stage and four-stage Rankine power cycles for 

OTEC. We summarize the results in accompanying Table 2. 
8 

Table 2 Power system summary for staged OTEC cycles

Figure 6 plots the gross, net and parasitic power(s) for the staged systems. The gross, net and 

parasitic powers increase with increasing number of stages. The net power levels off between 3 

and 4 stages. Most increase in net power is achieved by going from one to two stages. 
9 

Figure 6 Variation of gross, net and parasitic powers with the number of stages

Required heat transfer areas for the various staged cycles are indicated in Figure 7. Heat transfer 

product (UA) increases more or less linearly with increasing number of stages. Condenser area is 

about 12% smaller than that for the evaporator in all cases. Heat exchanger costs are the next 

highest, after the cold seawater system. It is essential to limit these costs as well, which in turn 

will limit the number of applicable stages for economic operation.
10 

Figure 7 Variation of heat exchanger (UA) product requirements with the number of stages

Concluding remarks

Increased number of stages makes better use of the seawater resource and yields higher net 

power. The increase in net power decreases with increasing number of stages. However, the 

required heat exchanger area also increases, in this case more or less linearly, with increasing 

number of stages. Considering that the cold seawater system remains the highest cost item for 

OTEC, economic optimization will lead to two or perhaps three as the preferred number of 

stages for the closed cycle OTEC system. The author recommends a similar study with other 

potential working fluids, such as, propylene. 
11 

References 

[Bharathan 1990] Bharathan, D., H.J. Green, H.F. Link, B.K. Parsons, J.M. Parsons, and F. 

Zangrando, "Conceptual Design of an Open-Cycle Ocean Thermal Energy Conversion Net 

Power Producing Experiment," Solar Energy Research Institute, SERI/TR-253-3616. 

[Kalina 1984] Kalina, A.L., “Combined cycle system with novel bottoming cycle,” ASME J. of 

Engineering for Power, October, pp 737-742. 

[Lockheed 2009] www.lockheedmartin.com/products/OTEC/index.html

[Panchal 1981] Panchal, C. B., D. L. Hillis, J. J. Lorenz, D. T. Young," OTEC Performance 

Tests of the Trane Plate-Fin Heat Exchanger," Argonne National Laboratory Report, 

ANL/OTEC-PS-8, April,1981. 

[Panchal 1990] Panchal, C.B., H. Stevens, L. Genens, A. Thomas, C. Clark, D. Sasscer, F. 

Yaggee, J. Darby, J.Larsen-Basse, B. Liebert, L. Berger, A. Bhargava, and B. Lee, "OTEC 

Biofouling and Corrosion Study at the Natural Energy Laboratory of Hawaii," Argonne National 

Laboratory Report, ANL/ESD-10, Oct. 1990. 

[Ridgeway 1980] Ridgeway, S.L., Mist flow ocean thermal energy process, U.S Patent 4, 

216,657. 

[Uehara 1999] Uehera, H, et. al., “The experimental research on ocean therma energy conversion 

using the Uehara cycle,” Proc. of the International OTEC/DOWA conference, Imari, Japan, pp 

132-141. 

[Vega 1995] Vega, L.A., “Ocean thermal energy conversion,” Encyclopedia of Energy 

Technology and the Environment, Jon Wiley & Sons, Inc., New York, NY, pp 2104-2119. 

[Vega 2009] Vega, L.A., OTEC Technology Status: Summary Outline,” presented at the NOAA 

workshop on Ocean Energy, held at Univ. of New Hampshire, November. 
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Staging Rankine Cycles Using Ammonia for OTEC Power 

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14. ABSTRACT (Maximum 200 Words)

Recent focus on renewable power production has renewed interest in looking into ocean thermal energy conversion 

(OTEC) systems. Early studies in OTEC applicability indicate that the island of Hawaii offers a potential market for a 

nominal 40-MWe system. A 40-MWe system represents a large leap in the current state of OTEC technology. 

Lockheed Martin Inc. is currently pursuing a more realistic goal of developing a 10-MWe system under U.S. Navy 

funding (Lockheed 2009). It is essential that the potential risks associated with the first-of-its-kind plant should be 

minimized for the project's success. Every means for reducing costs must also be pursued without increasing risks. 

With this in mind, the potential for increasing return on the investment is assessed both in terms of effective use of 

the seawater resource and of reducing equipment costs.

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Ocean thermal energy; staged cycles; rankine cycle; OTEC, ammonia

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