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Vol. 26（2021），25 ～ 33

A Combined Cooling and Power System
Based on Guohai Cycle
Yutong ZHANG, Yongning BIAN
School of Ocean Science and Technology, Dalian University of Technology
Panjin, Liaoning, 124211, P.R. China

Abstract
In this study, a combined cooling and power system is constructed based on the Guohai Cycle, andthe cooling and power output performance of the cycle is analyzed by numerical simulation. In the scopeof this study, it can be clearly seen that the improvement of thermal efficiency and primary energy ratein the combined cooling and power system. In addition, the effects of operating conditions on the cycle
performance were also investigated. The results show that the inlet temperature of seawater and thepressure of the expander inlet have obvious effects on the cycle performance.
Keywords: OTEC, Guohai Cycle, a combined cooling and power system

1. Introduction
Coal, petroleum and natural gas and other traditional fossil fuels are non-renewableenergies, which
will eventually be exhausted due to human exploitation. The continuous consumption of fossil energy
has been causing serious problems including global temperature rise, air pollution and changes of
ecological system. Therefore ocean energy as an renewable, abundant energy source is expected to
become a suitable alternative, which also has advantages such as non-polluting, recyclable and soon.(Junxiu Pan,2020)
The thermal energy resources in China are mainly concentrated in the South China Sea, and the flux
density of thermal energy is also increasing. It is expected that the generation of thermal energy using
the temperature difference between sea surface and deep sea to become a broad prospective in the
future.( Jisheng Zhang,et al.,2019)
In south China, the high temperature lasts throughout the year has generated a high demand of air
conditioning for daily life as well as the storage requirement of fishery industry. Currently, the energy
consumption of seafood refrigeration is very high, thus reducing the energy consumption of storage
systembecomes an important task of energy-saving.
The concept of ocean thermal energy was first proposed by D’Arsonval in 1881, and Anderson first
proposed the concept of a closed cycle system for ocean thermal energy in 1964. In 1979, Hawaii built
the world’s first truly closed cycle OTEC device “MINI-OTEC” For the study of OTEC, scholars also
proposed other improvement schemes.(Weimin Liu,2012) Kalina proposed a Kalina Cycle using nonazeotropic mixtures to reduce the irreversible loss during heat exchange. Uehara et al. also proposed the
Uehara Cycle for the low efficiency of the condenser when using non-azeotropic refrigerants. The
Guohai Cycle of the First Institute of Oceanography of China and the new cycle of Ocean University of
China have also built new cycles based on the Kalina cycle, and prototype tests have verified that they
have higher thermal efficiency than the Kalina Cycle.(Haifeng Xue et al.,2018)
Recently, the research direction of OTEC is more popular, which is the combined cooling and powersystem. A combination of power cycle system and refrigeration cycle system is used to improve theenergy utilization efficiency of the cycle system, and generate electricity and cooling capacity. Bian etal. proposed a combined cooling and power cycle system combining Kalina Cycle and ejection
refrigeration cycle. (Yongning Bian et al.2019) The cycle system mainly uses the waste heat of exhaust
Received Date: 2021.10.17
E-mail: 1219847620@qq.com*

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Yutong ZHANG, Yongning BIAN

steam discharged from the steam turbine to vaporize the working medium in the jet refrigeration cycle,
so that the jet refrigeration cycle can operate and output cooling. Yunjie Yang proposed the combined
cooling and power system of Rankine Cycle.(Yunjie Yang,2021) Some of the working fluids outflowedfrom the condenser were transported to the refrigeration cycle through the shunt for refrigeration andthen entered the compressor. The working fluids outflowed from the compressor were mixed withanother working fluid in the mixer and entered the power generation cycle, so as to realize the full
utilization of energy.
In this paper, the absorption refrigeration cycle driven by low-grade heat source (Xiaoyang
Hui,2014) is analyzed. Based on the Guohai Cycle, the absorption refrigeration with ammonia-water asworking fluid is combined to improve the energy utilization of the cycle system without affecting thepower generation performance.

2．
．Cycle description
2.1 Guohai Cycle
Wu Haoyu et al. from the first Institute of Oceanography has put forward a new thermodynamic
circulation mode for ocean thermal energy cycle, Guohai circulation, as shown in Fig.1. Compared with
Rankine Cycle, the power generation performance of Guohai Cycle is improved by 31.7%, and the cycle
thermal efficiency and turbine net output are significantly improved.( Haoyu Wu,2020)
According to their research of workflow, the surface warm seawater is used as a heat source of the
non-azeotropic mixed working medium (ammonia water solution)in the evaporator, while the ammonia
water solution enters the separator (process1), the separated gaseous ammonia working medium enters
the two-stage turbine to expand and do work, then the lean ammonia solution from the separator enters
the Regenerator1(Process3) to preheat the ammonia solution from the working fluid pump, and the steam
from the outlet of the first stage turbine is drawn out into the Regenerator2(process4). The remaining
steam goes into the second stage turbine to do work, and the steam outlet after work goes into the absorber(process5). In the absorber, there is the steam from the two stage turbine, and the preheat regenerator1
comes out through the throttle valve with a lean ammonia solution (process12), and the working medium
from the Regenerator2(process10), entering the condenser and the cold sea water together for heatexchange (process6), being condensed into a liquid solution into the ammonia tank (process7), entering
the Regenerator1(process8)and Regenerator2(process12)in turn through the pressure of the pump;
Finally enters the evaporator to absorb heat, completes the cycle (process11).

Fig. 1

Guohai CycleA Combined Cooling and Power System Based on Guohai Cycle

2.2 A combined cooling and power system based on Guohai Cycle
Based on the existing research of Guohai Cycle, the absorption refrigeration cycle is added. The
cooling capacity is produced simultaneously on the basis of power generation. The combined coolingand power system is shown in Fig. 2. A stream of lean ammonia solution is separated from the Separator
and fed into the refrigeration cycle by absorption refrigeration. The cooling capacity is released from the
Evaporator and returned to the Regenerator1.

Fig. 2

Combined cooling and power system based on Guohai Cycle

3. Thermodynamic modeling of systems
3.1 System assumption
Aspen Plus, which has been used for decades, is generally considered to have the most suitable andcomplete physical system for industry. This paper uses the modules of heat exchanger, separator andpressure transmission equipment in Aspen Plus V11 to build the cycle simulation and explore theperformance of the cycle system by changing different parameters. The state parameters of ammoniawater are available in Aspen Plus database, and their thermodynamic properties are determined by PengRobinson cubic equation. In order to simplify the calculation of the system model, following assumptions
are made for the simulation of the combined cooling and power system by Aspen Plus:(1) Ignore the heat loss of pipe fittings and parts;(2) The steam turbine and the working fluid pump are both isentropic processes(3) Ignore the power consumption of warm seawater supply pump and cold seawater supply pump;(4) The efficiency of the pump and turbine is an ideal value given.
In order to obtain better cold output performance, this study envisages the introduction of auxiliary
heating means ( such as the use of solar energy, hot seawater salt pool, etc. ) to increase the heat source
temperature.
The initial parameters of the Guohai circulation system are shown in Table 1.

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Yutong ZHANG, Yongning BIAN

Table 1 The initial parameters of the modelParameters
Inlet temperature of warm seawater/°C
Mass flow rate of the warm seawater/kg·s-1
Inlet temperature of cold seawater/°C
Basic solution concentration of ammonia-water/%
Inlet pressure of the turbine/MPa
Isentropic efficiency of the turbine/%
Mechanical efficiency/%
Pressure losses in heat exchanger/MPa

Value
99
89
5
89
2.2
87
98
0.1

3.2 Computing formula
Based on the first and second law of thermodynamics, the mathematical model of the proposed cycle
is expressed as follows.
∑���
(3.1)
�� 𝑚𝑚� = 0
where m is the mass flow of working medium in the system, and i = 1, 2, 3... are the state points at
different positions of the system.
The evaporator and condenser, the core equipment in the system, are simulated by the HeatX modelin Aspen Plus.
The surface warm seawater exchanges heat with the working medium in the evaporator. The warm
seawater releases heat. The ammonia-water absorbed heat which is the system heat absorption.
According to the law of energy conservation, the system heat absorption is:
𝑄𝑄� = 𝑚𝑚� (ℎ� − ℎ�� )(3.2)
where h represents the specific enthalpy of the working medium.
The deep cold seawater absorbs heat to cool the working medium in the condenser. The heat isreleased is the system heat release, which can be calculated as:
𝑄𝑄� = 𝑚𝑚� (ℎ� − ℎ� )(3.3)
The heat of the condenser in absorption refrigeration cycle can be calculated as follows:(3.4)
𝑄𝑄� = 𝑚𝑚�� （h�� − ℎ�� )The cooling capacity released by the evaporator in the system which can be calculated as:
𝑄𝑄��� = 𝑚𝑚�� (ℎ�� − ℎ�� )(3.5)
The double stage turbine in Guohai cycle is simulated by using two compr modules and flash module.
The output work of the steam turbine can be calculated as follows:𝑊𝑊� = 𝑚𝑚� (ℎ� − ℎ� ) + 𝑚𝑚� (ℎ� − ℎ� )(3.6)
The working medium pump is simulated by the pump module, and the input work of the working
medium pump can be calculated as:𝑊𝑊� = 𝑚𝑚� (ℎ� − ℎ� )(3.7)
After obtaining the output work W1 and WP , the net output work of the system can be calculated. In
this system, the net output work is the output work in the steam turbine minus the input work of theworking medium pump, which is:𝑊𝑊��� = 𝑊𝑊� − 𝑊𝑊�(3.8)
Thermal efficiency is an important index to measure the performance of marine thermal energy cycle
system. The thermal efficiency of Guohai Cycle can be calculated as:
η=

����
��

(3.9)

The Coefficient Of Performance (COP) is usually used to evaluate the performance of therefrigeration cycle which is defined as:

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A Combined Cooling and Power System Based on Guohai Cycle

����(3.10)

���� �����

(3.11)COP =��

Primary energy rate (per) can be used to evaluate the Guohai Cycle combined cooling and powersystem. PER is defined as the total energy output to the heat input and it represents the energy utilization
ratio as:
PER =

��

During the simulation, the built-in PENG-ROB method of Aspen Plus is employed to calculate thethermodynamic properties of working fluids. The standard Peng-Robinson cubic equation of state isexpressed by:
P=

��

���

�

− [�(���)��(���)]where R is the gas constant, a and b can be calculated as follows:�.�
a = ∑� ∑� 𝑥𝑥� 𝑥𝑥� �𝑎𝑎� 𝑎𝑎� � �1 − 𝑘𝑘�� �
b = ∑� 𝑥𝑥� 𝑏𝑏�
the ai, bi are:𝑎𝑎� =��� (�)⋅�.������ � ������𝑏𝑏� = 0.0778𝑅𝑅𝑅𝑅�� /𝑃𝑃��
where the αi is given by:𝛼𝛼� (𝑇𝑇) = [1 + 𝑚𝑚� (1 − 𝑇𝑇��.� )]�
where x is the mole fraction of component, Tr is the reduced temperature which is defined as:
𝑇𝑇� =

�

(3.12)(3.13)(3.14)(3.15)(3.16)(3.17)(3.18)

��The subscript i, j and c represent component i, j and critical state respectively. kij is binary interaction
coefficient between the component i and j in a mixture, whose value is set to zero in this study. mi is aconstant characteristic of each substance and it could be correlated against acentric factors ω, resultingin:𝑚𝑚� = 0.37464 + 1.54226𝜔𝜔� − 0.26996𝜔𝜔��(3.19)
where ω is the acentric factor, calculated according to:𝜔𝜔� = − log�� �

�∗

���

� − 1.0(3.20)

where P is the vapor pressure calculated at a reduced temperature T = 𝑇𝑇� × 0.7*

4. Model validation

4.1 Calculation results
According to the calculation formula, the results of the Guohai cycle combined cooling and powersystem in this paper are shown in Table 2.
Under the same conditions, the primary energy rate of single Guohai Cycle is the thermal efficiency
which is 2.60%. According to the calculation table, the thermal efficiency of Guohai cycle combined
cooling and power system is 2.57%, the primary energy rate is 55.11%. It is proved that the new system
greatly improves the energy rate.

4.2 Analysis and discussion
After the simulation experiment, this section selects to change the thermodynamic state parameters

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Yutong ZHANG, Yongning BIAN

Table 2

Simulated results

Parameters

Value

Qe/kJ

4471.51

Qg/kJ

780.58

Qref/kJ

2349.54

Wt/kW

170.50

Wp/kW

55.76

Wnet/kW114.74

η/%

2.57

COP

3.01

PER

55.11

Net output work(kW)such as inlet temperature of warm seawater and stream pressure to analyze and explore the best
performance of the system.
Fig.3 shows net output work under different inlet temperatures at 2.2MPa. Fig.4 shows therefrigeration capacity by changing different inlet temperatures at 2.2MPa.
450.00400.00350.00300.00250.00200.00150.00100.0050.00
0.00

381.17292.41203.60114.74
25.75
97

99

101

103

105

Inlet temperature of seawater(℃)
Fig. 3 Effect of inlet temperature of seawater on net output work

Refrigerating capacity(kJ)

2450.00

2416.74

2400.00
2350.00

2349.542300.00

2283.712250.00

2219.87

2200.00

2158.112150.00
2100.00
2050.00
2000.00

97

99

101

103

105

Inlet temperature of seawater(℃)
Fig. 4 Effect of inlet temperature of seawater on refrigerating capacity

A Combined Cooling and Power System Based on Guohai Cycle

It can be analyzed from the data that under the same pressure, when the inlet temperature is higher,
the more net output work system obtains and the less refrigeration capacity it obtains. The working
medium enters the separator, after that the gaseous ammonia enters the double stage turbine, and the leanammonia solution enters the condenser of refrigeration cycle. When the temperature of the heat source
rises, the mass flow of gaseous ammonia entering the double stage turbine increases and the mass flowof lean ammonia solution decreases. So the work of the steam turbine increases and the refrigeration
capacity in the refrigeration cycle decreases.
Under this condition, the primary energy rate (per) can be obtained, as shown in Fig.5:
80.00%
66.83%

70.00%
60.00%

55.11%

PER

50.00%

47.03%

40.00%

41.13%

30.00%

36.66%

20.00%
10.00%
0.00%

97

99

101

103

105

Inlet temperature of seawater(℃)
Fig. 5 Effect of inlet temperature of seawater on PER

By analyzing Fig.5, the primary energy rate decreases with the increase of heat source temperature
under the condition of constant working fluid flow pressure. When the heat source temperature is 97 ℃,
the primary energy rate is the highest. Therefore, the flow pressure of working medium was changed at
97 ℃ for data analysis.
Fig.6 shows the net output work by changing the stream pressure at 97 ℃. Fig. 7 shows therefrigerating capacity by changing the stream pressure at 97 ℃.
It can be analyzed from the data that under the same heat source temperature, when the flow pressureis greater, the net output work is less and the refrigeration capacity is more. When the working medium
stream pressure increases, the mass flow of gaseous ammonia entering the double stage turbine decreases
and the mass flow of lean ammonia solution increases. So the turbine work decreases and the refrigeration
capacity increases.

Net output work(kW)120.00103.81100.00

85.58

80.00
60.00

58.98

40.00
25.99

20.00
0.00

16

18

20

22

Stream pressure(bar)Fig. 6 Effect of stream pressure on net output work

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Yutong ZHANG, Yongning BIAN

Refrigerating capacity(kJ)

3000
2500

2,416.74
2,063.93

2000
1,698.92

1500

1,318.76

1000
500
0

16

18

20

22

Stream pressure(bar)Fig. 7 Effect of stream pressure on refrigerating capacity

Under this condition, the f primary energy rate (per) can be obtained, as shown in Fig.8:
80.00%
70.00%

66.84%

60.00%

58.01%

PER

50.00%

48.71%

40.00%

38.78%

30.00%
20.00%
10.00%
0.00%

16

18

20

22

Stream pressure(bar)Fig. 8 Effect of stream pressure on PER

According to the analysis of Fig.8, under the condition of constant heat source temperature, theprimary energy rate increases with the increase of working medium flow pressure. When the flowpressure is 22bar, the primary energy rate is the highest. Therefore, according to the current calculation
and analysis, the primary energy utilization rate is the highest under the conditions of heat source
temperature of 97℃ and working medium flow pressure of 22bar. However, the thermal efficiency isvery low at 97℃, it can not meet the expected power generation, so 99℃ is the most appropriate heatsource temperature.

5. Conclusion
In this paper, on the basis of the latest national sea cycle proposed by the ocean thermal energy cycle,
a combined cooling and power supply system for the Guohai Cycle is established. The absorption
refrigeration cycle is added to Guohai Cycle, and the output of cooling capacity is increased without
affecting the output of electricity.The new cycle is simulated by Aspen Plus, and the system performanceis analyzed by changing key thermal parameters. The main conclusions are as follows:

A Combined Cooling and Power System Based on Guohai Cycle(1) Under the same conditions, the thermal efficiency of the Guohai Cycle is 2.60 %, and theprimary energy rate is thermal efficiency, which is 2.60 %. The calculation table shows that the thermal
efficiency of the combined cooling and power system is 2.57 %, and the primary energy rate is 55.11 %,which proves that the combined cooling and power system greatly improves the energy utilization rate.
(2) The net output net work, refrigerating capacity and primary energy rate of the new systemvary with the change of heat source temperature and working medium pressure.
(3) According to the current simulation calculation, the results of Guohai cycle combined coolingand power system are more remarkable at 99 ℃ and 2.2MPa.

Acknowledgments
This study is sponsored by the National Key R&D Program of China (2019YFB1504301) the
National Natural Science Foundation of China (No. 11972105) and the Cooperative Research Programof IOES (No. 19A02).

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