10 24-0490 WANG Wei-中低温发电介质选型问题

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Journal of Thermal Science Vol.35, No.1 (2026) 151167

Received: Aug 28, 2024 Corresponding author: WANG Wei E-mail: wang_wei@bjut.edu.cn

AE: WANG Liwei

www.springerlink.com

DOI: https://doi.org/10.1007/s11630-025-2200-8 Article ID: 1003-2169(2026)01-0151-17

CSTR: https://cstr.cn/32141.14.JTS-025-2200-8

Thermodynamic Analysis of Supercritical Organic Fluid Brayton Cycles for

Middle and Low Temperature Power Generation

WANG Wei*, WANG Longyu, LI Zhe, WU Yuting, MA Chongfang

Key Laboratory of Heat Transfer and Energy Conversion, Beijing Municipality, College of Energy and Power

Engineering, Beijing University of Technology, Beijing 100124, China

Nature 2025

Abstract: Middle and low temperature thermal energy widely exists in the natural world and many industrial

fields. Unlike fossil fuel power generation systems, the significant feature of middle and low temperature power

generation systems is the temperature constraint of the heat source. Exploring the potential of the cycle within a

limited temperature range is key to improve energy utilization efficiency. This study proposes the conception of

supercritical organic fluid Brayton cycles (SOFBC) and evaluates the feasibility and application potential. R116,

R23, R170 and N2O are selected as the working fluids for cycle analysis based on their thermal properties. Then,

thermodynamic models of the supercritical gas Brayton cycle based on simple regeneration and organic Rankine

cycles (ORCs) have been established. According to the calculation results, the performances of regenerative

Brayton cycles (RBCs) using four working fluids are better than that of CO2. The maximum thermal efficiencies

of R116, R23, and R170 are 41.9%, 20.2%, and 15.3% higher than that of CO2 at the highest temperature of 150°C.

Even at 300°C, the corresponding values of three organic fluids are 25.6%, 13.7%, and 13.7% higher than that of

CO2. By analyzing the variations in isobaric specific heat capacity (cp) of different working fluids, it is found that

the cp difference between the high and low pressure sides in the regenerator of CO2 is significantly higher than

that of organic working fluids. Additionally, the performance of RBCs using R116 is better than the sub-ORC

using R123 and the trans-RORC using R236fa at the same temperature range. The results can demonstrate that

the SOFBC is superior in middle and low temperature power generation compared with the sCO2 Brayton cycle

and ORCs. This study provides preliminary and rough evidence of the feasibility and potential for SOFBCs.

Keywords: SOFBC; middle and low temperature power generation; regeneration; pressure ratio of cycle;

temperature ratio of cycle

1. Introduction

For responding to global climate change, reaching

carbon neutrality has gradually become global consensus.

As of October 2023, 151 countries have made net-zero

pledges [1]. However, achieving the goal is an arduous

task, especially for industrialized countries. In 2020,

China committed to achieving carbon peak by 2030 and

carbon neutrality by 2060. In 2022, China’s total energy

consumption reached 5.41 billion tons of TCE (ton of

152 J. Therm. Sci., Vol.35, No.1, 2026

Nomenclature

Variables

cp isobaric specific heat capacity/kJkg–1K–1 π pressure ratio of cycle

h specific enthalpy/kJkg–1 τ temperature ratio of cycle

 mass flow rate/kgs–1 Subscripts

p pressure/MPa abs heat absorption

Q heat flux/kW C compression, Carnot, critical

T temperature/K com compressor

t temperature/°C dis heat dissipation

W power/kW E expansion

Acronyms ex exergy

ORC organic Rankine cycle max maximum

PCHE printed circuit heat exchanger P pump

RBC regenerative Brayton cycle re regenerative

RGBC real gas Brayton cycle S thermal decomposition

RORC regenerative organic Rankine cycle sub subcritical

SOFBC supercritical organic fluid Brayton cycle T turbine

Greek symbols th thermal

Δ difference trans transcritical

η efficiency 1,2… state point

standard coal equivalent) [2]. According to IEA statistics,

China’s total carbon dioxide emissions were 11.9 billion

tons in 2021, accounting for 33% of global emissions [3].

In China, the proportion of carbon dioxide emissions in

the power and industrial sectors is 68% [4]. The common

view is that renewable energy generation is the ultimate

solution for low-carbon transformation in the power

system, and the production and utilization of hydrogen

energy is the key to solving the problem of renewable

energy absorption and industrial decarbonization. Based

on the fundamental principles of energy transfer and

conversion, any other form of energy can be converted

into thermal energy, and the middle and low temperature

thermal energy is the secondary final state energy form

(before dissipating to environment). Therefore, middle

and low temperature thermal energy widely exists in

natural world and many industrial fields, such as

geothermal energy, solar energy collection and waste heat

of technological processes, and so on. Due to the huge

amount of resources, the large-scale application of

medium and low temperature power generation will

contribute to carbon peak and neutrality in the power and

industrial fields.

At present, the main power cycles applied to medium

and low temperature thermal power conversion include

organic Rankine cycle (ORC), Karina cycle, Rankine

cycle, Stirling cycle, and Erikson cycle, etc. In many

industrial fields, ORC has realized engineering

applications. However, according to the actual data of the

engineering cases, the thermal efficiency of the ORC

power generation system needs to be improved. In theory,

the isobaric heat-absorbing process of the ORC is

variable temperature heat transfer. Due to the use of

mostly isentropic or dry organic working fluids, the

isobaric heat-releasing process is also composed of

sensible and latent heat-releasing, which significantly

reduces the average heat absorption temperature and

slightly increases the average heat release temperature.

This means that within the same temperature range, the

theoretical thermal efficiency of ORC is difficult to

achieve Carnot efficiency; the exergy efficiency is not

high, and the exergy efficiency of the transcritical cycle

is lower than that of the subcritical cycle.

The supercritical carbon dioxide (sCO2) Brayton cycle

is considered a new generation of power cycle due to its

advantages of high efficiency, compactness, and simple

structure [5]. In the 1960s, the sCO2 Brayton cycle was

proposed for use in nuclear reactors, but it was not

actually applied due to multiple reasons, such as low

coolant discharge temperature in light water reactors,

lack of engineering experience in high-pressure operation

of power systems and poor regeneration capacity of

regenerators, etc. [6]. In recent years, the sCO2 Brayton

cycle has received widespread attention once again

because of technological advances. Except in nuclear

power [7–9], researchers and technicians attempt to apply

the sCO2 Brayton cycle to more fields, such as solar

thermal power generation [10, 11], geothermal

powergeneration [12], coal-fired power generation [13]

and waste heat recovery [14], etc. Due to the sCO2

WANG Wei et al. Thermodynamic Analysis of Supercritical Organic Fluid Brayton Cycles for Middle and Low… 153

Brayton cycle being close to the critical point, its cycle

characteristics are significantly different from traditional

air Brayton cycles. The closer to the critical point the

fluid state gets to, the more drastic the physical properties

of the fluid changes. In addition, regenerative heat can

reduce the original heat absorption of the cycle, so the

sCO2 Brayton cycle can have higher thermal efficiency.

Due to the pressure at the lowest point of the cycle must

be higher than the critical pressure of CO2, the pressure of

the entire cycle is very high. According to the articles of

the sCO2 Brayton cycle [15–22], the highest temperature

is about 400°C–700°C; the lowest temperature before

compression is about 32°C; the pre-compression pressure

is about 7.4 MPa–8 MPa, and the inlet pressure of the

turbine is about 14 MPa–30 MPa. During the regeneration

process, the isobaric specific heat capacity between the

high-pressure side and the low-pressure side differs by

several times. Improving the regeneration effect is vital

to ensuring high thermodynamic performance of the

cycle. Numerous cycle optimization studies are aimed at

this goal, such as re-compression, pre-compression,

intercooling, partial cooling, and split expansion [23–31].

For medium- and low-temperature thermal energy, the

regenerative effect of the sCO2 Brayton cycle will

decrease because of the significant difference in thermal

properties of CO2 on the high- and low-pressure sides.

Compared with CO2, many organic working fluids have

significantly lower critical pressures at nearly same

critical temperature (mostly between 3 MPa–4 MPa). If

supercritical organic fluid is used as the working fluid to

construct the Brayton cycle, the higher-pressure ratio

could be obtained under the same temperature ratio of the

cycle. Costante et al. [32, 33] analyzed the

thermodynamic performance of Brayton cycles with

different supercritical working fluids, and the results

proved the potential application of real gas Brayton

cycles. Moreover, many organic working fluids have

smaller changes in thermal properties near the critical

point compared with CO2. Supercritical Organic Fluid

Brayton Cycle (SOFBC) has considerable potential in the

field of medium and low temperature thermal energy.

This work discusses the feasibility and application

potential of SOFBC. Firstly, through literature review,

the physical and chemical properties of dozens of organic

fluids are collected and organized. Based on critical

parameters and decomposition temperatures, several

organic fluids are selected as working fluids for cycle

analysis. Secondly, Thermodynamic models of the

supercritical gas Brayton cycle based on simple

regeneration and ORCs have been established. Then, the

calculation results of SOFBC are compared with that of

the sCO2 Brayton cycle and ORCs at the same

temperature range. Finally, the influence of different

factors on thermodynamic performance of the SOFBC is

analyzed, including the pressure ratio, the temperature

ratio, the efficiencies of the compressor and the turbine,

pinch temperature, etc. This work could provide

theoretical basis and technical evidence for SOFBC in

practical applications.

2. Methodology

2.1 First principles of power cycles

Improving the thermal efficiency of actual cycles is

the main research objective of cycle optimization

analysis. There are many technical measures could

improve it, such as selection of working fluids,

increasing the inlet temperature or pressure of expanders,

reducing the exhaust pressure, regenerative, reheating,

split flow, re-compression and pre-compression, etc.

However, it is not easy to compare the effect of

optimization measures for different types of power cycles.

This work attempts to analyze how to improve the cycle

thermal efficiency based on mathematical logic. For any

power cycle, thermal efficiency ηth is the ratio of cycle

net work Wnet to heat absorption Qabs，and cycle net work

is the difference between expansion work We and

compression work Wc.

net abs th

net e c

th net abs th

abs abs

net abs th

(1) ,

(2) ,

(3) ,

WWW WQ

QQ WQ













 







(1)

As shown in Eq. (1), there are three typical cases for

variation in the thermal efficiency: (1) Both cycle net

work and heat absorption increase simultaneously, and

the thermal efficiency increases accordingly. It indicates

that the increase rate of cycle net work is faster than that

of heat absorption. Many technical measures could

achieve the objective, such as increasing the inlet

temperature or pressure of expanders, reducing the

exhaust pressure, and reheating, etc. (2) Both cycle net

work and heat absorption increase simultaneously, but

the thermal efficiency decreases accordingly. It indicates

that the increase rate of cycle net work is slower than that

of heat absorption. The reason for this situation is that the

deteriorating efficiency of compression and expansion

might lead to the decrease of the thermal efficiency. For

example, when the pressure ratio is higher than the

optimal pressure ratio, the thermal efficiency of Brayton

cycles will decrease. Certainly, the case should be

avoided. (3) Cycle net work increases and heat

absorption decreases simultaneously, and the thermal

efficiency increases accordingly. Various regenerative

cycles may achieve this effect.

In response to carbon neutrality, new requirements

have been proposed for the construction of thermal

power conversion systems. On the one hand, the

consumption of fossil fuels will gradually decrease. On

the other hand, the thermal power conversion system of

renewable energy and waste heat has temperature

154 J. Therm. Sci., Vol.35, No.1, 2026

constraints, and maximizing the exergy efficiency is the

first principle of an environmental protection energy

system. As shown in the following Eq. (2), the exergy

efficiency ηex of a power cycle is the ratio of the thermal

efficiency ηth to the Carnot efficiency ηc at the same

temperature range. Here, ηc is calculated based on the

maximum and minimum temperatures of the cycle.

ex th c





 (2)

Reducing compression power and improving

regenerative effect simultaneously are very effective

measures to improve cycle exergy efficiency, which is

adopted in the sCO2 regenerative Brayton cycle. Near the

critical point, the thermal properties of CO2 change

drastically, so this cycle is known as the real gas Brayton

cycle (RGBC). The compression power of real gases near

the critical point is small, and the regeneration effect

influenced on the thermodynamic performance of RGBC

is relatively greater. For the sCO2 Brayton cycle, the

isobaric specific heat capacity of CO2 in the

high-pressure state is several times higher than that in the

low-pressure state. Its regeneration effect is worse than

that of ideal gas, which may not be suitable for middle

and low temperature power generation. If a working fluid

with small thermal property changes near the critical

zone safety, environmental friendliness, appropriate

critical point parameters, and good thermal stability can

be found, its performance is expected to be higher than

that of CO2.

2.2 Selection of working fluids

Identifying suitable organic substances is a

prerequisite for constructing SOFBCs. This study has

Table 1 The thermodynamic, thermal stability and environmental properties of working fluids

Substances Structural formulas Physical data Thermal stability threshold Environmental data

tc/°C pc/MPa ts/°C Ref. ODP GWP/102 a

CO2 CO2 30.98 7.38 0 1

R116 CF3CF3 19.88 3.05 842 [34] 0 11 100

R170 CH3CH3 32.17 4.87 550 [35] 0 20

N2O N2O 36.37 7.24 550 [36] 298

SF6 SF6 45.57 3.76 204 [37] 0 22 800

R13I1 CF3I 123.29 3.95 102 [37]

R141b CCl2FCH3 204.35 4.21 90 [37] 0.11 0.09

R125 CF3CHF2 66.02 3.62 396 [37] 0 3170

R134a CF3CH2F 101.06 4.06 340 [38] 0 1300

R152a CHF2CH3 113.26 4.52 160 [38] 0 138

R123 CHCl2CF3 183.68 3.66 200 [38] 0.01 79

R1234ze(E) CF3CH=CHF 109.36 3.63 176 [39] 0 <1

Pentane CH3-3(CH2)-CH3 196.55 3.37 280 [40]

Hexane CH3-4(CH2)-CH3 234.67 3.03 260 [40]

Isopentane (CH3)2CHCH2CH3 187.20 3.38 260 [40]

Butane CH3-2(CH2)-CH3 151.98 3.80 300 [40] 0 20

Isobutane CH(CH3)3 134.66 3.63 300 [40]

Cyclopentane C5H10 238.57 4.57 255 [41]

Ethanol CH3CH2OH 241.56 6.27 172 [41]

Acetone (CH3)2CO 234.95 4.70 159 [41]

MM C6H18OSi2 245.60 1.94 240 [42]

MDM C8H24O2Si3 290.94 1.42 260 [42]

R1234yf CF3CF=CH2 94.70 3.38 170 [43] 0 <1

R32 CH2F2 78.10 5.78 270 [44] 0 677

R245fa CF3CH2CHF2 154.01 3.65 300 [45] 0 858

R236fa CF3CH2CF3 124.92 3.20 380 [45] 0 8060

R227ea CF3CHFCF3 101.75 2.92 425 [45] 0 3350

R23 CHF3 26.14 4.83 400 [45] 0 12 400

R143a CF3CH3 72.71 3.76 350 [45] 0 4800

R1233zd(E) CHCl=CH-CF3 165.60 3.57 250 [46] 0.000 34 1

R1336mzz(Z) CF3CH=CHCF3(Z) 171.35 2.90 270 [47] 0 2

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摘要：

JournalofThermalScienceVol.35,No.1(2026)151167Received:Aug28,2024Correspondingauthor:WANGWeiE-mail:wang_wei@bjut.edu.cnAE:WANGLiweiwww.springerlink.comDOI:https://doi.org/10.1007/s11630-025-2200-8ArticleID:1003-2169(2026)01-0151-17CSTR:https://cstr.cn/32141.14.JTS-025-2200-8ThermodynamicAnalysisofS...

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