7

ＯＴＥＣ
Vol. 27（2022），7 ～ 12

Evaluation of a novel OTEC System composed of Uehara
Cycle for power and refrigeration cogeneration
Yuming XUE, Yongning BIAN and Yang LIU
School of Ocean Science and Technology, Dalian University of Technology
Panjin, Liaoning, 124211, P.R. China

Abstract
In this paper, a cogeneration system based on Uehara cycle and compression refrigeration cycle is proposedto generate electricity by using temperature difference energy (OTEC). The surface warm seawater of 29℃ isused as the heat source, and the deep cold seawater of 8℃ is used as the cold source. The system can be used togenerate electricity and refrigeration for fishing boats, provide fresh-keeping technology for seafood, and cancontrol the refrigeration capacity according to demand. Ammonia and tetrafluoroethane (R134a) are used as theworking fluid of Uehara cycle and compression refrigeration cycle, with mass flow rates of 2.06kg/s and 2.7kg/s,
respectively. The ASPEN PLUS V11 is used to conduct performance simulation study on co-production cycle
system. The results show that the intermediate extraction rate of the steam turbine, the inlet temperature of thesteam turbine, the ammonia concentration, and the mass flow rate of R134a all have a certain influence on the
exergy efficiency, the primary energy saving rate, the primary energy utilization rate and the Coefficient Of
Performance of the refrigeration cycle. From an energy-saving perspective, the system has better energy-saving
effects than a single Uehara cycle. From an energy perspective, the primary energy utilization rate is higher thanthat of a single Uehara cycle.
Key words : Combined power and refrigeration cycle; OTEC; Uehara cycle; Compression refrigeration

1. Introduction
Energy is the basic support for economic development, and also the basis for the survival of human society. With therapid development of science and technology, while mankind creates material wealth, the demand for energy isincreasing. In the past 100 years, fossil energy has always occupied the main part of energy supply, and the problems ofenergy shortage and environmental pollution have become increasingly serious. With the concept of global carbon
neutrality put forward, the global development and application of clean energy using sustainable energy is rapidly
increasing. Ocean energy is a clean and renewable energy, with the characteristics of large reserves, stable temperature
difference, high density and comprehensive utilization. The surface seawater temperature and the deep seawatertemperature change very little, relatively stable, and the energy density is high (Wang et al.,2006), which is a very large
amount of energy and makes the ocean the largest energy storage body on the earth. If it can be used, it will bring hugeenergy benefits to mankind and provide greater impetus for human development.
After D'Arsonval first proposed the concept of ocean thermal energy conversion in 1881, a lot of research work on
ocean thermal energy conversion has been carried out (Uehara H and Ikegami Y,1990). In 1981, Kalina proposed the
Kalina cycle with ammonia mixture as working fluid to achieve low temperature differential power generation. In the
Kalina cycle, the evaporation and condensation temperatures of the mixture are used to improve the cycle thermal
efficiency (Matsuda Y et al., 2018). In order to obtain higher thermal efficiency, in 1994, Uehara adopted two-stage
expansion and intermediate extraction to improve thermal efficiency on the basis of Kalina cycle. Saga University inJapan established a 4.5KW test power station to test the Uehara cycle (Ikegami et al., 2006) system invented by Professor* Date of Manuscript Acceptance 2022.11.8

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

8Yuming XUE, Yongning BIAN and Yang LIU

Haruo Uehara. The thermal efficiency of Uehara cycle is 1%-2% higher than that of Kalina cycle (Liu et al., 2012).
Refrigeration is a necessity for human life and the dramatic increase in its use will force the refrigeration sector tocontinue its efforts towards sustainable development. Based on this basis, this paper combines the Uehara cycle andcompression refrigeration, uses the temperature difference energy to generate electricity, and realizes the development
and utilization of new energy. At the same time, according to the principle of "temperature matching and step utilization",the power cycle and refrigeration cycle are organically coupled, and the thermal energy is efficiently converted into workand cold, so as to realize the comprehensive utilization of temperature difference energy and improve the thermal
efficiency and exergic efficiency. In addition, the compression refrigeration system can be used for the seafoodpreservation technology on the fishing boat, and can adjust the energy generation and cooling capacity according to thework of the refrigeration compressor, so as to realize the self-sufficiency of the fishing boat in power generation andrefrigeration.

2. Simulation and computation
Figure 2.1 shows the system diagram of the combined cycle, which is the co-generation cycle of the Uehara cycle
and the compressed refrigeration cycle. OTEC technology is used to achieve simultaneous power generation andrefrigeration. The Uehara cycle adds a heat exchanger coupled to the compression refrigeration cycle, which operates byadding heat exchangers to absorb the heat released by the steam from the turbine in the Uehara cycle.

Fig.2.1 System diagram of the union cycle
The Uehara cycle consists of evaporator, condenser, steam turbine, generator, separator, absorber, heat exchanger,
pump and so on. Ammonia was chosen as the working fluid in this cycle. The warm seawater is used as the heat source,which is returned after heat release. The ammonia solution is heated by the heat source in evaporator 1 and turns intoa gas-liquid mixed stateand enters the separator. The ammonia steam separated from the separator enters the steam turbine
1 to expand and do work. Part of the exhaust steam enters the turbine 2 to expand and do work, and part flows into themixer 2 to release heat. The unevaporated ammonia solution separated from the separator enters the regenerator toreleases heat. After passing through the throttle valve , the ammonia solution is mixed with the lack steam from thesteam turbine 2 in the mixer 1. Cold seawater is a cold source, which absorbs heat in condenser 1 and then returns; The
basic solution of gas-liquid mixture enters the condenser 1 and is condensed into ammonia solution by cooling water;
After the pump 1 pressurized ammonia solution into the mixer 2 to absorb heat, the mixed solution by the pump 2pressure into regenerator to absorb heat. The heated solution enters evaporator 1 to complete the cycle.
The compressed refrigeration cycle(Huang et al., 2019) , uses less than one throttle valve, but uses evaporative
condenser to provide heat for the refrigeration cycle. The cycle consists of condenser 2, heat exchanger, throttle valve 2,
compressor and evaporator 2. The circulating working medium is R134a, and the cooling temperature is 5℃. After theEvaluation of a novel OTEC System composed of Uehara Cycle for power and refrigerationcogeneration9

refrigerant comes out of the evaporator 2, it is overheated by the heat exchanger, compressed and pressurized by thecompressor, then enters the condenser 2 for heat transfer and condensation. Subsequently, the working medium flows
into the evaporator 2 to absorb heat after the pressure reduction by throttle valve 2,which provides cold capacity for thecold storage and complete the refrigeration cycle.
In order to simplify the calculation of the system model, the following assumptions are made for the Ueharacompression refrigeration cycle:
1. Ignoring pressure drop and heat loss in heat exchangers and pipes;
2. Ignore the pump power consumption of cold sea water pump and warm sea water pump;
3. The equipment of the combined generation circulation system is in a stable state;
4. The working process of the working medium in the throttle valve is an isentropic process;5. The working medium at the outlet of the gas-liquid separator is saturated;
6.The working medium flowing out of the condenser is in the saturated liquid state;
7.The working medium flowed out of the evaporator is in the condition of saturated steam.
8.The working medium temperature of the heat exchanger outflow is 5℃;
9.The isentropic efficiency of the pump is 98%, and the isentropic efficiency and mechanical efficiency of the turbine
and compressor are 87% and 96%, respectively.
Table 1 Input date for the combined cycle.

Parameter
Inlet temperature of warm seawater/°C
Mass flow rate of the warm seawater/kg s-1
Mass flow rate of ammonia-water to evaporator 1/kg s-1
Basic solution concentration of ammonia-water/%Cold sea water temperature/°C
Mass flow rate of the cold seawater/kg s-1
Inlet temperature of the turbine 1/°C
Inlet pressure of the turbine 1/MPa
Outlet pressure of the turbine 1/MPa
Inlet pressure of the turbine 2/MPa
Outlet pressure of the turbine 2/MPa
Inlet temperature of evaporator 2 /℃
Outlet pressure of evaporator 2/MPa
Outlet temperature of heat source of condenser 2/℃
Outlet pressure of valve 1(MPa)
Mass flow rate of R134a/kg s-1

Number
29
80
2.06
95
8
50
27.26
0.9
0.78
0.78
0.63
0
0.35
10
0.201
2.7

To evaluate the overall performance of the combined cycle, some key evaluation indicators are defined as follows:
The performance of the separate Uehara cycle is evaluated in terms of its thermal efficiency which can be definedas:𝜂𝜂 = 𝑊𝑊𝑛𝑛𝑒𝑒𝑒𝑒 ⁄𝑄𝑄𝑔𝑔1(1)
𝑊𝑊𝑛𝑛𝑒𝑒𝑒𝑒 = 𝑊𝑊𝑇𝑇 − 𝑊𝑊𝑝𝑝(2)
where 𝑊𝑊𝑇𝑇 represents the total output power of the turbine 1 and turbine 2, 𝑊𝑊𝑝𝑝 represents the total input power of thepump 1 and pump 2, and 𝑄𝑄𝑔𝑔1 represents the heat exchange in the generator 1.
The Coefficient of Performance (𝐶𝐶𝐶𝐶𝐶𝐶) is usually used to evaluate the performance of the refrigeration cycle which
is defined as:
𝐶𝐶𝐶𝐶𝐶𝐶 = (𝑄𝑄𝑟𝑟𝑟𝑟𝑟𝑟 + 𝑊𝑊𝑛𝑛𝑒𝑒𝑒𝑒 )⁄𝑄𝑄𝑔𝑔1(3)
where 𝑄𝑄𝑟𝑟𝑟𝑟𝑟𝑟 represents the total heat exchange through evaporator 2.
The exergy efficiency is given as:

10Yuming XUE, Yongning BIAN and Yang LIU𝜂𝜂𝑒𝑒𝑒𝑒 = (𝑊𝑊𝑛𝑛𝑛𝑛𝑛𝑛 + 𝐸𝐸𝑥𝑥,𝑟𝑟𝑟𝑟𝑟𝑟 )⁄𝐸𝐸𝑥𝑥,𝑖𝑖𝑖𝑖(4)where 𝐸𝐸𝑥𝑥,𝑟𝑟𝑟𝑟𝑟𝑟 denotes the refrigeration exergy and 𝐸𝐸𝑥𝑥,𝑖𝑖𝑖𝑖 stands for the exergy of the heat source fluid.
It is not quite proper to use the thermal efficiency here to measure a combined cycle (Han Yuan et al, 2015) since it
is usually used to evaluate a single thermodynamic cycle. However, analog to its definition, a primary energy ratio and a
primary energy saving ratio are proposed in the current study. 𝑃𝑃𝑃𝑃𝑃𝑃 is defined as the total energy output to the heat input
and it represents the energy utilization ratio as:
𝑃𝑃𝑃𝑃𝑃𝑃 = (𝑊𝑊𝑇𝑇 + 𝑄𝑄𝑟𝑟𝑟𝑟𝑟𝑟 )⁄𝑄𝑄𝑔𝑔1(5)
Primary energy saving ratio (𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃) (Alelyani SM et al., 2015) is an indicator of the combined cooling and power
production system. It reflects the energy saved in the combined system compared with the energy consumption of thereferred separate systems from the energy saving perspective. It is defined as:
𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 = (Q′ − 𝑄𝑄𝑔𝑔1 )⁄𝑄𝑄𝑔𝑔1(6)Q′ = (𝑊𝑊𝑇𝑇 + 𝑄𝑄𝑟𝑟𝑟𝑟𝑟𝑟 ⁄3)⁄𝜂𝜂(7)′where, Q is the primary energy consumption in conventional separate systems, Uehara cycle and a conventional
compression chiller system, under the same energy output as the combined system. Take the calculation of Q′ in thecompression chiller (𝐶𝐶𝐶𝐶𝐶𝐶=3) of the refrigeration cycle for example.

3 Results and discussion
In order to study the combined cycle under different working conditions, this paper established the Uehara cycle
compression refrigeration co-generation system with reference to the data of references (Aphornratana S and Sriveerakul
T,1984) and (Goto S et al., 2011). ASPEN PLUS V11 software was used to simulate the co-generation cycle system. The
state parameters of ammonia and R134a were provided in ASPEN PLUS database, and PENG-ROB was selected as thephysical property method. The flow rate of warm sea water is 80 kg/s and the temperature is 29℃. The flow rate of cold
sea water is 50kg/s and the temperature is 8℃. Basic solution concentration of ammonia-water is 95%, and the flow rateis 2.06kg/s. Based on the input parameters, this paper analyzes the influence on the thermodynamic performance of thecogeneration system by changing the intermediate pumping rate of the turbine, the concentration of ammonia-water, mass
flow rate of refrigerant (R134a) and the inlet temperature of turbine 1. The results are shown in Figure 3.1-3.4.

Fig.3.1 Relationship between 𝑃𝑃𝑃𝑃𝑃𝑃, 𝐶𝐶𝐶𝐶𝐶𝐶, 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃, 𝜂𝜂𝑒𝑒𝑒𝑒
and pumping rate

Fig.3.2 Relationship between 𝑃𝑃𝑃𝑃𝑃𝑃, 𝐶𝐶𝐶𝐶𝐶𝐶, 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃, 𝜂𝜂𝑒𝑒𝑒𝑒
and concentration of ammonia-water

Fig.3.3 Relationship between 𝑃𝑃𝑃𝑃𝑃𝑃, 𝐶𝐶𝐶𝐶𝐶𝐶, 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃, 𝜂𝜂𝑒𝑒𝑒𝑒

Fig.3.4 Relationship between 𝑃𝑃𝑃𝑃𝑃𝑃, 𝐶𝐶𝐶𝐶𝐶𝐶, 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃, 𝜂𝜂𝑒𝑒𝑒𝑒

Evaluation of a novel OTEC System composed of Uehara Cycle for power and refrigerationcogeneration

and R134a flow

11

and inlet temperature

As can be seen in FIG. 3.1, exergy efficiency (𝜂𝜂𝑒𝑒𝑒𝑒 ), primary energy saving rate (𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃), primary energy rate
(𝑃𝑃𝑃𝑃𝑃𝑃) and the Coefficient Of Performance (𝐶𝐶𝐶𝐶𝐶𝐶) versus the pumping rate. When the extraction rate of the original
cycle is 0, turbines 1 and 2 are considered as a whole, and the regenerator 2 has no heat input and cannot work, whichis equivalent to a Kalina cycle. It is clearly demonstrated that the gas extraction increases with the increase of thepumping rate. Therefore, the steam entering turbine 2 decreases, the output power of turbine 2 decreases, and the total
output work of turbines 1 and 2 decreases. At the same time, the working fluid entering the condenser is reduced, andthe heat exchange of evaporator 1 is reduced. The total input power of pump 1 and 2 decreases slightly, while theinput power of the compressor remains unchanged. The decrease of the total output power of turbine 1 and 2 is larger
than that of pump 1 and 2. According to Eq. (2), the net output power of the system (𝑊𝑊𝑛𝑛𝑛𝑛𝑛𝑛 ) decreases. According toEq (3), the decrease of 𝑊𝑊𝑛𝑛𝑛𝑛𝑛𝑛 is much smaller than that of evaporator 1 heat exchange, so the COP increases.
According to Eq (2), the reduction of the sum of the total output work of turbines 1 and 2 and the cooling capacity of
the cold storage is less than the reduction of the heat exchange of evaporator 1, so the PER increases. The PESR isbigger than 0 and gradually increases, indicating that compared with the single generation cycle or refrigeration cycle,
the performance of the union cycle is better and has certain energy saving. With the increase of pumping rate, theperformance of the union cycle becomes better and better. As the 𝜂𝜂𝑒𝑒𝑒𝑒 gradually decreased, the qualitative pumping
rate of the system was about 2%.
Figure 3.2 shows that the relationship between 𝜂𝜂𝑒𝑒𝑒𝑒 , 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃, 𝑃𝑃𝑃𝑃𝑃𝑃, 𝐶𝐶𝐶𝐶𝐶𝐶 and the concentration of ammoniawater. With the increase of concentration, the mass flow rate of ammonia-rich steam separated from the separator,
𝑊𝑊𝑇𝑇 , 𝑊𝑊𝑛𝑛𝑛𝑛𝑛𝑛 , 𝑄𝑄𝑔𝑔1 all increase with the concentration of ammonia-water. Besides, the increase range is much larger
than 𝑊𝑊𝑇𝑇 . According to Eq. (3), COP decreases gradually. 𝐸𝐸𝑥𝑥,𝑟𝑟𝑟𝑟𝑟𝑟 remained unchanged and 𝑊𝑊𝑛𝑛𝑛𝑛𝑛𝑛 was greater thanthat of overall 𝐸𝐸𝑥𝑥,𝑖𝑖𝑖𝑖 . As per Eq. (4), the exergy efficiency 𝜂𝜂𝑒𝑒𝑒𝑒 gradually increased. The PESR, is greater than 0.
According to Eq. (5), although the system shows certain energy saving performance, the PER gradually decreases,
indicating that the ammonia concentration is not good if it is too high or too low, but reaches the best value at 95%.

Figure 3.3 illustrates the effect of the R134a flow on 𝑃𝑃𝑃𝑃𝑃𝑃, 𝐶𝐶𝐶𝐶𝐶𝐶, 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃, 𝜂𝜂𝑒𝑒𝑒𝑒 . The total heat exchange throughevaporator 2 (𝑄𝑄ref ) and the net output power of the compressor (𝑊𝑊𝑐𝑐𝑐𝑐𝑐𝑐 ) increases as the mass flow rate of R134a
increases. It is noted that as the heat exchange of evaporator 2 increases the heat exchange of condenser 1 and 𝑊𝑊𝑛𝑛𝑛𝑛𝑛𝑛
declines significantly. According to Eq. (3), 𝑄𝑄𝑔𝑔1 remains unchanged, and COP gradually increases. According to
Eq. (5), 𝑊𝑊𝑇𝑇 remains unchanged, and 𝑃𝑃𝑃𝑃𝑃𝑃 gradually increases. Since 𝑃𝑃𝐸𝐸𝐸𝐸𝐸𝐸 which is greater than 0 gradually
increases, the energy saving performance of the system gradually becomes better. According to Eq. (4), the decreaseof 𝑊𝑊𝑛𝑛𝑛𝑛𝑛𝑛 was larger than the increase of 𝐸𝐸𝑥𝑥,𝑟𝑟𝑟𝑟𝑟𝑟 , as a consequence, 𝜂𝜂𝑒𝑒𝑒𝑒 gradually decreased. As a result, there is acertain limit to the R134a flow rate.

Figure 3.4 describes the effect of the inlet temperature of the turbine 1 on 𝑃𝑃𝑃𝑃𝑃𝑃, 𝐶𝐶𝐶𝐶𝐶𝐶, 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃, 𝜂𝜂𝑒𝑒𝑒𝑒 . It is easily
deduced that as inlet temperature of the turbine 1 increases, the specific enthalpy difference between inlet and outletof the turbine rises accordingly. Therefore, these result in the increase of 𝑊𝑊𝑇𝑇 and 𝑄𝑄𝑔𝑔1 . It is also found that 𝑊𝑊𝑝𝑝decreased slightly. Based on Eq. (3) and Eq. (4), 𝐶𝐶𝐶𝐶𝐶𝐶 increases slightly accordingly and so does 𝑊𝑊𝑛𝑛𝑛𝑛𝑛𝑛 , which
together lead to mildly increase of 𝜂𝜂𝑒𝑒𝑒𝑒 as observed in Fig. 5.4. 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 is greater than 0, which means the system hasenergy saving property; According to Eq. (2), 𝑃𝑃𝑃𝑃𝑃𝑃 decreases gradually. The above results show that the inlet
temperature of the turbine reaches the optimum at 26.9℃.

The results show that under the condition of the same input parameters, compared with the power generation
system which uses the Uehara cycle, the input heat is reduced by 0.7%, outlet power of the turbine is increased by
4.6%. At the same time, the cooling output of 3502KW is increased, the net output power is increased by 4%, and thethermal efficiency is increased by 5%. When the parameters are changed, 𝐶𝐶𝐶𝐶𝐶𝐶 is almost unchanged and 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 is
positive. It means that this system has certain energy saving effect. However, as ammonia concentration and inlet
temperature increased, 𝜂𝜂𝑒𝑒𝑒𝑒 increases and yet 𝑃𝑃𝑃𝑃𝑃𝑃 decreases. As pumping rate and R134a flow increase, 𝜂𝜂𝑒𝑒𝑒𝑒
decreases while 𝑃𝑃𝑃𝑃𝑃𝑃 increases. The research results provide a useful reference for further optimizing theperformance and comprehensive utilization of thermal power generation system.

Acknowledgments

12

Yuming XUE, Yongning BIAN and Yang LIU

The cooperative Research Program of IOES (No.21A19) and the National Nature Science Foundation of China(No.11972105).
References
Aphornratana S, Sriveerakul T. Analysis of a combined Rankine–vapor–compression refrigeration cycle, Energy
Conversion & Management, Vol.51, No.12 (2010), pp. 2557-2564.
Alelyani SM, Sherbeck JA, Fette NW, Wang Y, Phelan PE. Assessment of a novel heat-driven cycle to produce shaft
power and refrigeration. Applied Energy, Vol.215, No.1 (2015), pp.809-819.
Goto S, Motoshima Y, Sugi T, Yasunaga T, Ikegami Y, Nakamura M. Construction of simulation model for OTEC plantusing Uehara cycle. Electrical Engineering in Japan, Vol.176, No.2 (2011), pp. 1-13.
Han Yuan, Peilin Zhou, Ning Mei. Performance analysis of a solar-assisted OTEC cycle for power generation and fisherycold storage refrigeration. Applied Thermal Engineering, Vol.90, No.5 (2015), pp.809-819.
Xiankun Huang, Han Yuan, Ning Mei. Performance analysis of solar- OTEC based hybrid refrigeration and powersystem. Acta Energiae Solaris Sinica, Vol.40, No.4 (2019), pp. 906-913. (in Chinese).
Ikegami Y Yasunaga T , Harada H . Performance Experiments on Ocean Thermal Energy Conversion System Using
the Uehara Cycle. Bulletin of the Society of Sea Water Science Japan, Vol. 60, No.1 (2006), pp.32-38.
Liu WM, Chen FY , Wang YQ, Jiang WJ, Zhang JG. Progress of Closed-cycle OTEC and Study of a new cycle of OTEC.
Advanced Materials Research, Vol. 354-355 (2012), pp. 275-278.
Matsuda Y, Yoshitake T, Sugi T, Goto S, Morisaki T, Yasunaga T, Ikegami Y. Construction of a Static Model for Power
Generation of OTEC Plant Using Uehara Cycle Based on Experimental Data. Journal of Marine Science andEngineering, Vol.6, No.1 (2018), pp.1-14.
Putri I M, Munaf D R. Increasing Fishery Economic Added Value through Post Fishing Program: Cold Storage Program.
Waset Org, Vol.7, No.8 (2013), pp.2391-2394.
Uehara H , Ikegami Y. Optimization of a Closed-Cycle OTEC System. Journal of Solar Energy Engineering-Transactionsof the Asme, Vol.112, No.4 (1990), pp. 247-256.
Zhong Wang, Chuankun Wang. Analysis on renewable oceanic energy use in China, Marine Environmental Science
Vol.25, No.4 (2006), pp.78-80. (in Chinese).