1ＯＴＥＣ
Vol. 27（2022），1 ～ 6

Explore the influence of intermediate extraction on
thermodynamic performance and economy of Uehara cycle
Keyu ZHU, Yongning BIAN, Yang LIU
School of Ocean Science and Technology, Dalian University of Technology
dlutzhukeyu@foxmail.com, Panjin, Liaoning, 124211, P.R.China

Abstract
The South China Sea contains abundant ocean thermal energy. This paper simulates an OTEC system
based on Uehara cycle and considers the actual situation in the South China Sea. The extra middle steam
extraction segment is examined to check its influences on the circulating thermal efficiency, the consumptionof cold and hot seawater and the circulating exergy efficiency. The results show that in the Uehara cycle Ocean
Thermal Energy Conversion System under the working condition of this paper, for the circulating thermal
efficiency and the circulating exergy efficiency, there is the best intermediate steam extraction capacity.
Different pressures have different optimum values. Concurrently, the circulating exergy efficiency is muchgreater than thermal efficiency. With the increase of intermediate steam extraction, the consumption of coldand hot seawater both decreased and there is a certain upper limit for steam extraction. The cost of equipment
foundation increases with the increase of additional intermediate steam extraction rate, and is mainly
concentrated in the heat exchanger. With other conditions unchanged, increasing the working fluid flow caneffectively reduce Levelized Cost of Energy (LCOE). When building an ocean thermoelectric power plant, theappropriate intermediate extraction pressure and extraction rate shall be selected according to the specificworking conditions.
Key words : OTEC；low-temperature heat；Uehara cycle；economic analysis；Aspen plus

1. Introduction
The ocean thermal energy is the ocean thermal energy stored in the form of the water temperature difference
between the surface hot sea water and the deep cold sea water, whose reserves are huge and is almost unaffected bytime. 71% of the earth's surface area is the ocean, which is also the largest solar storage device on the earth. At thecurrent technical level, about 10000YWh/a of ocean temperature difference energy can be converted into electricity [1].
In 1881, the concept of ocean thermal energy conversion was first proposed by French scientist Arsened Arsonva[2].
In 1930, Claude successfully used ocean thermal energy to generate electricity in the offshore of Cuba, proving thepractical feasibility of ocean thermal energy conversion theory[3]. In 1998, Professor Uehara H of Saga University inJapan first proposed the Uehara cycle, and through analyzing the physical properties of ammonia working fluid andcomparing it with Kalina cycle, it was proved that it can greatly improve the thermal efficiency of the cycle. Later, thefeasibility and rationality of the system were analyzed through experiments, and the system was evaluated [4 5].
In this paper, based on the Uehara cycle, 95.6% ammonia water mixture is used as the working fluid, and the initial
parameters are determined according to the geographical information and climatic conditions of the South China Sea.
Adjust the extra intermediate air extraction rate of Uehara cycle, observe the thermal performance change of its cycle
and conduct economic analysis, so as to provide a theoretical basis for the development of ocean thermal powergeneration technology applied in the South China Sea region in the future.

* Date of Manuscript Acceptance 2022.11.3

E-mail of corresponding author: ybian@dlut.edu.cn

2Keyu ZHU, Yongning BIAN and Yang LIU

2. Circulation process and control equation
2.1 Systemic circulation process
Figure 1 shows the flow diagram of Uehara cycle. The ocean surface warm seawater is recycled as the heat source,
and the deep cold seawater is used as the cold source. The ammonia solution is partially heated by the warm sea waterin evaporator and evaporated into a gas-liquid mixed state. It is separated into rich ammonia steam and lean ammonia
solution through separator. Among them, the ammonia rich steam enters turbine 1 for expansion and work, and thenenters turbine 2 to continue expansion with the liquid phase part extracted through the separator. After the leanammonia solution enters regenerator 1 for heat release, the pressure is adjusted by the throttle valve to be consistent
with the outlet pressure of turbine 2, and then it is mixed with the rich ammonia steam in mixer. The mixed working
medium is condensed into liquid phase by cold sea water in the condenser, pressurized to the initial pressure by pump
1, and heat exchange with the extracted liquid phase in regenerator 2. The extracted liquid passes through regenerator 2and tank, is pressurized by pump 2 to the initial pressure, and then mixed with the ammonia solution.
Figure 2 is the schematic diagram of the improved Uehara cycle system. The rich ammonia solution passing
through separator 2 does not directly enter steam turbine 2, but passes through the middle extraction, and then theremaining part enters steam turbine 2 for expansion and work. The extracted solution is mixed with stream 7 and thenenters regenerator 2 for heat release. After condensation, the ammonia working medium is pressurized by pump 1 to theoutlet pressure of turbine 1 after passing through the liquid storage tank. It is mixed with stream 16 in the buffer tankafter passing through the regenerator 2 and outputs pure liquid stream 17. Pressurize to the initial pressure throughpump 2, and then absorb heat in regenerator 1 to complete the whole cycle. Compared with the normal Uehara cycle,
the improved model increases the intermediate extraction, makes full use of the total heat of the extracted part, andimproves the thermal efficiency of the cycle.

Fig. 1

System flow chart of the Uehara cycle.

Fig. 2

System flow chart of the modified Uehara cycle.

Explore the influence of intermediate extraction on thermodynamic performance and economy ofUehara cycle

3

2.2 Model assumptions and initial parameters
To simplify the system model calculation, the following assumptions are made for Uehara cycle: (1) The system isan adiabatic system. (2) Ignore the pressure drop and heat loss in the pipeline and heat exchanger. (3) The system
operates stably, and the parameters at each point do not change with time. (4) Heat transfer end difference of heatexchanger is 2℃. The parameters of each point of the cycle are shown in Table 1:
Table 1

Initial parameters of the circulatory system modle.

Parameter
Surface temperature sea water inlet temperature WS1/℃
Surface temperature sea water return temperature WS2/℃
Deep cold sea water inlet temperature CS1/℃
Deep cold sea water return temperature CS2/℃
Mass fraction of ammonia working medium/%
Working medium pressure of evaporator/kPaTurbine 1 outlet pressure/kPa
Turbine 2 outlet pressure/kPa
Isentropic efficiency of steam turbine/%
Mechanical efficiency of steam turbine/%
Mechanical efficiency of pump/%
Working medium mass flow/kg·s
Ambient temperature T0/℃
Ambient pressure P0/kPa

Number29275795.6
900
780
630
87
96
98
2.06
25
101

2.3 Governing equation
The Uehara circulation system is composed of evaporator, steam turbine, condenser, pump and other equipment.
The model control equations involved are as follows: [m* is the mass flow of ammonia water, kg/s; h* is the enthalpyof ammonia, kJ/kg; s* is the entropy of ammonia.]
Heat absorption of ammonia working medium in evaporator:(1)
Qe =m1 ( h2 − h1 )Ammonia mass conservation:m=
m3 x3 + m4 x4
2 x2
m=
m6 x6 + m7 x7
5 x5=
m6 x6 m20 x20 + m21 x21

(2)
(3)
(4)

Heat release of ammonia working medium in condenser:

Qc =m11 ( h11 − h12 )(5)Heat exchange in regenerator 1:

Qh1 =m4 ( h4 − h9 ) =m1 ( h1 − h18 )(6)Heat exchange in regenerator 2:

Qh2 =m22 ( h22 − h16 ) =m14 ( h15 − h14 )(7)

Output power of turbine 1:

Wt1 =m3 ( h3 − h5 )(8)Output power of turbine 2:

Wt 2 =m20 ( h20 − h8 )

(9)

4

Keyu ZHU, Yongning BIAN and Yang LIU

Input power of pump 1:

Wp1 =m13 ( h14 − h13 )(10)

Input power of pump 2:

Wp2 =m17 ( h18 − h17 )Net output work:((11))(Wnet = Wt1 +Wt 2 - Wp1 +Wp2)(12)

Cycle thermal efficiency:

I =

Wnet
Qe(13)

Input heat exergy:

() ()

(14)

=
Ee mws  hws1 -hws2 -T0 sws1 -sws2 
Input cold exergy:

() ()

(15)

=
Ec mcs  hcs1 -hcs2 -T0 scs1 -scs2 Cycle exergy efficiency:

 =

Wnet
Ee +Ec(16)

Investment cost calculation formula:

log10 Cp =K1 +K 2log10 A* +K 3 ( log10 A* )

(17)

2

In the formula, A* represents the measurement parameter of equipment investment cost, and the subscript *represents the corresponding equipment: For the heat exchanger, it is the effective heat exchange area(m2). For theturbine, it is the output work(kW). For pumps, it is the input work(kW).
Effective heat exchange area of heat exchanger:(18)
Q

A=U  LMTD

Where LMTD is the average logarithmic temperature difference:(T -T ) - (T -T )LMTD =''
hot'cold'hot

(19)

''
cold'
T '' -Tcold
ln hot'''-Tcold
Thot

Investment cost after correction of material and pressure environment:

CBM= CP FBM= CP ( B1 +B2 FM FP )(20)In order to simplify the calculation, all heat exchangers adopt plate heat exchangers from the heat transfer
coefficient U=3100 W·m-2·K-1. The calculation coefficients are listed in Table 2 according to literature [6].
Table 2

Parameter
K1
K2
K3
B1
B2
FM
FBM

Parameters of cost calculation.

Heat exchanger/regenerator
4.6656-0.1557
0.1557
0.96
1.21
1.4/1
calculation

TurbinePump

2.70511.4398-0.1776\\\3.53.8696
0.31610.1220
1.89
1.35
2.2
calculation

5

Explore the influence of intermediate extraction on thermodynamic performance and economy ofUehara cycle

2.4 Economic evaluation index

In this paper, the system performance is analyzed from the perspective of thermal economy, and the Leveled Costof Energy(LCOE) is taken as the system economic index.
Investment recovery factor:
N(21)i i +1

CRF =()( i +1) -1
N

Leveled Cost of Energy:

LCOE =C  CRF +φ
Wnet  n(22)In the formula, i is the interest rate, the service life of the equipment N=20, the operation and maintenance cost φ
is 3% of the total investment cost, and the annual operation time n=8000h.

3. Result analysis and discussion
3.1 Thermal analysis
In Uehara cycle, the change of intermediate extraction volume will affect the mass flow of working medium
entering turbine 2, reducing the output work of turbine 2. At the same time, the working medium extracted is used toheat the condensed working medium, improve the temperature of the working medium at the evaporator inlet, reduce
the heat absorption of the evaporator, and thus increase the circulating thermal efficiency. In Fig.3, with the increase ofadditional extraction, the cycle thermal efficiency shows a trend of increasing first and then decreasing. When the extra
middle steam extraction is small, because the isentropic efficiency of the turbine is less than 1, the working medium
cannot fully expand. This part of heat is taken away by the cold seawater in the condenser and cannot be used. The
extracted part does not pass through the condenser, and the heat utilization rate is 100%. When the extra middle steam
extraction reaches a certain limit, continuous extraction will reduce the flow of working medium in turbine 2, resulting
in a decrease in output work.

thermal efficiency (%)

3.00

3.020

2.98

3.018

700kPa705kPa710kPa715kPa720kPa740kPa39.6

exergy efficiency (%)

700kPa705kPa710kPa715kPa720kPa740kPa

3.02

3.016

39.4

39.2
39.64

39.0

39.62

39.60
3.014

2.96

1.6

1.8

2.00

2.2

2.4

2

38.8

2.6

46

1.6

1.80

extra middle steam extraction (%)

2.0

2.2

2.42

2.6

46

extra middle steam extraction (%)

Effect of extra middle steam extraction on thecirculating exergy efficiency.
Exergy refers to the part of energy that can be converted into any other energy form theoretically when the systemchanges reversibly from any state to a state in equilibrium with the given environment. The relationship curve between
the cycle execution efficiency of Uehara cycle system and extra middle steam extraction is shown in Fig.4. In the cycle,
the main exergy loss is concentrated in the evaporator and condenser. With the increase of extraction, the heat exchange
in these two places decreases, and the corresponding exergy loss is also decreasing. At the beginning, the decrease of
the input exergy is greater than the decrease of the output work, and it reverses after reaching the limit.
Fig. 3

Effect of extra middle steam extraction on thecirculating thermal efficiency.

Fig. 4

3.2 Economic analysis
Take the extract pressure of 710kPa as an example, with the increase of additional intermediate air extraction rate,
the equipment cost increases slowly at first, and then the cost increases faster when it exceeds the limit. As shown inFig.8, the main cost is concentrated on the heat exchanger. With the increase of extraction, the working medium

6

Keyu ZHU, Yongning BIAN and Yang LIU

flowing through the condenser decreases, the heat exchange area required by the condenser decreases, and the cost of
the condenser decreases. Meanwhile, after heat recovery, the working medium temperature at the evaporator inletincreases, the logarithmic average temperature difference of the evaporator decreases, the required heat exchange area
increases, and the cost of the evaporator increases. When the cycle output power is increased by increasing the working
fluid flow, the LCOE decreases sharply first and then slowly, as shown in Fig.6.
1000000

800000LCOE ($/kWh)equipment foundation cost ($)

0.20

PumpTurbine
Heat exchanger

1200000

6000004000000.10

200000

00

1

2

3

40.05

5Effect of extra middle steam extraction onequipment foundation cost.

05101520working medium mass flow (kg/s)

extra middle steam extraction (%)

Fig. 5

0.15

Fig. 6

Effect of working medium flow on Levelized
Cost of Energy.

4. Conclusion
Based on Uehara cycle, this paper uses ammonia water mixture as working fluid, and Aspen Plus to simulate thecycle system. By changing a series of thermodynamic parameters and economic parameters obtained from extra middle
steam extraction and analyzing and comparing them, the following conclusions are drawn:(1) Compared with the approximate Karina cycle, the cycle thermal efficiency of the Uehara cycle has beensignificantly improved. The maximum thermal efficiency of 3.0183% is obtained when the ammonia massfraction is 95.6%, the surface temperature of sea water is 29℃, and the deep cold sea water is 5℃, the extractpressure is 710kPa, and the extra middle extraction is 1.9%.
(2) Considering the utilization of effective energy, there is an optimal value for extraction to maximize the exergy
efficiency. And because the ocean temperature difference energy is a low-grade heat source with lessefficiency, the exergy efficiency is far greater than the thermal efficiency.
(3) The equipment cost is mainly concentrated in the heat exchanger cost. Large scale can effectively reduce costs.

Acknowledgment
The cooperative Research Program of IOES (No.21A19) and the National Nature Science Foundation of China(No.11972105).

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