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Manuscript Details

Manuscript number ATE_2017_6109_R2

Title Experimental analysis and comparison between CO2 transcritical power cyclesand R245fa organic Rankine cycles for low-grade heat power generations

Article type Research Paper

Abstract

In this study, experimental investigations were conducted on two different test rigs to investigate and compare theperformances of CO2 transcritical power cycles (T-CO2) and R245fa organic Rankine cycles (ORC) for low-grade heatpower generations. Each test rig consisted of a number of essential components including a turboexpander with a highspeed generator, finned-tube air cooled condenser, liquid pump and plate-type gas generator/ evaporator. Theexhaust flue gases from an 80 kWe micro-turbine CHP unit were utilised as heat sources for both T-CO2 and R245faORC power generation systems and hot thermal oil flow was applied commonly as a heat transfer medium. Both testrigs were fully commissioned and instrumented from which comprehensive experimental investigations were carriedout to examine the effects of various important operational parameters on system performance. These include workingfluid mass flow rate and heat source input etc. at constant heat sink (ambient) parameters. Results showed that with afixed heat source input, the turbine power generation and overall efficiency of the R245fa ORC or T-CO2 system couldbe improved significantly at higher working fluid mass flow rates. Quantitatively, when the CO2 and R245fa mass flowrates increased respectively from 0.2kg/s to 0.26kg/s and from 0.23kg/s to 0.27kg/s, the corresponding turbine powergeneration increased by 88.2% and 27.3% while the respective turbine overall efficiency enhanced by 35.4% and7.5%. On the other hand, the turbine power generation and overall efficiency of the R245fa ORC or T-CO2 systemincreased variably with higher heat source input when the working fluid mass flow rate is fixed. In percentage, whenthe heat source inputs of the T-CO2 and R245fa ORC systems increased respectively from 52kW to 60kW and 61kWto 68kW, the corresponding turbine power generation increased 47.7% and 63% while the respective turbine overallefficiency enhanced 8.65% and 1.08%. In addition, the cycle point temperatures and pressures of both systemsrevealed similar increments at higher working fluid mass flow rates or at higher heat source inputs. Furthermore, heattransfer analyses of both CO2 gas generator and R245fa evaporator can be used to set up efficient controls of workingfluid superheating at the heat exchanger outlet. The test results and analyses are essential in evaluating andcomparing both systems’ operations at different operating conditions, design structures and components, and cansignificantly contribute towards optimal component selections and system performance controls.

Keywords CO2 transcritical power cycle; R245fa organic Rankine cycle; experimentalinvestigation; performance comparisons.

Corresponding Author Yunting Ge

Order of Authors Liang Li, Yunting Ge, Xiang Luo, Savvas Tassou

Suggested reviewers Yaodong Wang, Changqing Tian, Jie Zhu

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Highlights.docx [Highlights]

Manuscript_with marked.docx [Manuscript File]

Figure 1.docx [Figure]











Table 1.docx [Table]




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Feb. 21, 2018Department of Mechanical, Aerospace and Civil EngineeringCollege of Engineering, Design and Physical SciencesBrunel University LondonUxbridge, MiddlesexUB8 3PH, UKTel: +44(0) 1895 266722Email: [email protected]

Dear Sir or Madam:

I would like to re-submit our paper with the titled “Experimental analysis and comparison between CO2 transcritical power cycles and R245fa organic Rankine cycles for low-grade heat power generations” after receiving the second comments from the reviewers.

According to the requirement, enclosed please find the following document files for the paper, Manuscript, Responses to reviewers, Figures and Tables. I wish our paper can be processed soon.

Please let me know if any further document is required.

Best regards

Dr. Yunting GeReader in Brunel University London

Dear Reviewers:Many thanks for your comments on our paper which are very helpful. We have read all the comments and tried to address them thoroughly. We wish our explanations could be acceptable. Please let us know if any further modification is needed.

Best regards

Dr. Yunting Ge Brunel University

Comments from the editors and reviewers:-Reviewer 1

-

After the review of this manuscript, the authors have taken into account the most comments and suggestions that I have raised and revised the paper accordingly. Thus, it can be accepted for publication also after minor revision and see the comments from other reviewers if achieved.

Reviewer's comments

- Introduction: it should be updated by some literature regarding small-scale axial turbine (theoretical and experimental studies).

The authors:

Thanks. To address the comments, the introduction has been updated by adding the following sentences:

A small scale R245fa ORC test rig with a radial-type turbine and a number of supersonic nozzles was developed to regulate the mass flow rate at dynamic thermal energy supplies [20]. As such, the ORC system can operate at optimal operating conditions by adjusting the total inlet temperature, mass flowrate, and rotational speed of the turbine. In addition, it is understood that the rotational speed, expansion ratio, mass flow rate and turbine size have significant effect on turbine performance. Accordingly, mean-line design and three-dimensional computational fluid dynamics analysis were integrated for both micro axial and radial-inflow turbines with five organic fluids and at low-temperature heat sources in ORC cycles [21]. The results showed that n-pentane had the highest performance at all design conditions in terms of maximum total-to-total efficiency and power output. Furthermore, a single stage axial turbine expander coupled with a permanent magnet synchronous generator was tested on a small-scale ORC system with working fluids of R245fa and HFE7100 [22]. It was found that the system evaporating pressure, pressure drop across the turbine and system mass flow rate had significant effect on the turbine and system performance.

Correspondingly, the following references are added:

[20] S. Cho, C. Cho, K. Ahn, Y. Lee, A study of the optimal operating conditions in the organic Rankine cycle using a turbo-expander for fluctuations of the available thermal energy, Energy, 64(2014) 900-911.

[21] A. Al Jubori, A. Baabo, R. Al-Dadah, S. Mahmoud, A. Ennil, Development of micro-scale axial and radial turbines for low-temperature heat source driven organic Rankine cycle, Energy Conversion and Management 130 (2016), 141-155.

[22] W. Pu, C. Yue, D. Han, W. He, X. Liu, Q. Zhang, Y. Chen, Experimental study on Organic Rankine cycle for low grade thermal energy recovery, Applied Thermal Engineering, 94(2016), 221-227.

- Experimental facility and methodology: more details and descriptions including the turbine dimensions should be added.

The authors:

Thanks.

To address these, the following sentence is added in section 2.

The radiuses of R245fa and CO2 turbines are 114mm and 72mm respectively while the overall lengths of the R245fa and CO2 turbines are 307 mm and 298 mm each.

-Reviewer 2

- All the comments have been properly modified according to the review. As a result, the paper has improved substantially.

The authors:

Thanks.

HIGHLIGHTS

Two test rigs of small scale low-grade power generation systems with CO2 transcritical

power cycles (T-CO2) and R245fa Organic Rankine Cycles (ORC) were developed and

measured;

The effects of various important operational parameters such as heat source input and

working fluid mass flow rate on the performance of T-CO2 and R245fa ORC were

analyzed and compared.

Heat transfer analyses of both CO2 gas generator and R245fa evaporator can be used to set

up efficient controls of working fluid superheating at the heat exchanger outlet.

The test results and analyses are essential in evaluating and comparing both systems’

operations at different operating conditions, design structures and components.

Experimental analysis and comparison between CO2 transcritical power cycles and

R245fa organic Rankine cycles for low-grade heat power generations

L. Lia, Y.T. Gea*, X. Luob, S.A. Tassoua

aRCUK National Centre for Sustainable Energy Use in Food Chains (CSEF), Institute of Energy Futures, Brunel

University London, Uxbridge, Middlesex, UB8 3PH, UK

bNational Key Laboratory of Science and Technology on Aero Engines Aero-thermodynamics, The

Collaborative Innovation Centre for Advanced Aero-Engine of China, Beihang University, Beijing, 10191,

China

ABSTRACT

In this study, experimental investigations were conducted on two different test rigs to

investigate and compare the performances of CO2 transcritical power cycles (T-CO2) and

R245fa organic Rankine cycles (ORC) for low-grade heat power generations. Each test rig

consisted of a number of essential components including a turboexpander with a high speed

generator, finned-tube air cooled condenser, liquid pump and plate-type gas generator/

evaporator. The exhaust flue gases from an 80 kWe micro-turbine CHP unit were utilised as

heat sources for both T-CO2 and R245fa ORC power generation systems and hot thermal oil

flow was applied commonly as a heat transfer medium. Both test rigs were fully

commissioned and instrumented from which comprehensive experimental investigations were

carried out to examine the effects of various important operational parameters on system

performance. These include working fluid mass flow rate and heat source input etc. at

constant heat sink (ambient) parameters. Results showed that with a fixed heat source input,

the turbine power generation and overall efficiency of the R245fa ORC or T-CO2 system

could be improved significantly at higher working fluid mass flow rates. Quantitatively, when

the CO2 and R245fa mass flow rates increased respectively from 0.2kg/s to 0.26kg/s and from

0.23kg/s to 0.27kg/s, the corresponding turbine power generation increased by 88.2% and

27.3% while the respective turbine overall efficiency enhanced by 35.4% and 7.5%. On the

other hand, the turbine power generation and overall efficiency of the R245fa ORC or T-CO2

system increased variably with higher heat source input when the working fluid mass flow

rate is fixed. In percentage, when the heat source inputs of the T-CO2 and R245fa ORC

systems increased respectively from 52kW to 60kW and 61kW to 68kW, the corresponding

turbine power generation increased 47.7% and 63% while the respective turbine overall

efficiency enhanced 8.65% and 1.08%. In addition, the cycle point temperatures and

pressures of both systems revealed similar increments at higher working fluid mass flow rates

or at higher heat source inputs. Furthermore, heat transfer analyses of both CO2 gas generator

and R245fa evaporator can be used to set up efficient controls of working fluid superheating

at the heat exchanger outlet. The test results and analyses are essential in evaluating and

comparing both systems’ operations at different operating conditions, design structures and

components, and can significantly contribute towards optimal component selections and

system performance controls.

Keywords: CO2 transcritical power cycle, R245fa organic Rankine cycle, experimental

investigation, performance comparisons.

* Corresponding author. Tel.: +44 1895 266722; fax: +44 1895 256392.

E-mail address: [email protected] (Y.T. Ge).

Nomenclature

h enthalpy (J/kg)

I exergy destruction (W)

mass flow rate (kg/s) 𝑚

s entropy (J/kg K)

T temperature (K)

W power generation (W)

Greek symbols

efficiency

Subscripts

all overall

c calculated

e electrical

ex exergy

f working fluid

is isentropic

m mechanical

t turbine

0 dead state

1 turbine inlet

2 turbine outlet

1. Introduction

Over the last few decades, there has been extensive waste thermal energy discharged into

the atmosphere in various forms of industrial waste or exhaust flue gases from engines and

turbines. This is accumulating in serious risks of global warming, environmental pollution

and fossil fuel crises. On the other hand, inexhaustible renewable energy such as solar,

geothermal, biomass energies can be utilized around the world. Notably, these waste heat and

renewable sources are categorized mostly into low grade thermal energy [1]. Therefore, there

is an urgent obligation to generate power using low-grade thermal energy and applicable

thermodynamic power cycles such as the organic Rankine cycle (ORC) [2], transcritical

power cycle (TPC) [3], Brayton cycle [4] and trilateral flash cycle [5]. Even so, for heat

resources such as industrial waste heat and renewable energy with low temperatures ranging

from 100oC to 350oC, the ORC and TPC systems prove to be promising thermodynamic

processes for converting low-grade heat to electricity.

The ORC functions similarly to a century-old steam Rankine power plant, but instead uses

an organic working fluid such as R245fa. When applied to a low-grade heat source, the

system with ORC is expected to generate power at a higher efficiency and greater cost-

effectiveness than that of steam Rankine cycle [6]. Nevertheless, the constant temperature

evaporating behaviour of a pure fluid in a conventional ORC results in a pinch point and a

mismatch between the temperature profiles of the working fluid and a sensible heat source

fluid, which can introduce significant irreversibilities [7, 8]. On the other hand, in a TPC , a

working fluid with relatively low critical temperature and pressure can be compressed

directly to its supercritical pressure and heated to its supercritical state before expansion.

Therefore, the heating process of a TPC does not pass through a distinct two-phase region

like a conventional organic Rankine cycle, resulting in a better thermal match in the heat

exchanger with less irreversibility [9]. Even so, challenges for the ORC and TPC systems

include the choice of the appropriate working fluids with high efficiency operation, low cost,

safety and less environmental impact etc. [10-12] and the particular design of the cycles. As it

is difficult to find the working fluids of ORC and TPC systems that can meet all of the above

criteria, four working fluids Benzene, Toluene, p-Xylene, R123 and R113 were investigated

in an ORC system for their thermal efficiency and irreversibility. The results of this research

showed that the ORC with p-Xylene had the highest thermal efficiency while the ORCs with

R123 and R113 had the least irreversibility in recovering a low temperature thermal energy

[13]. Although R1234yf and R1233ze are recently under consideration as working fluids with

lower level of GWP to replace R245fa, these refrigerants are still fairly new and most of the

ORC expansion machines still use R245fa as the working fluid in the current market [14]. In

the meantime, mixture working fluids have been examined in attempt to improve ORC

system performance and reversibility for low grade thermal energy conversions [15]. The

study indicated that the use of suitable zeotropic mixtures as working fluids for the ORC

system may lead to improved system performance and reversibility. In terms of system

components, the expander is the main component for an ORC system, as it can determine the

system’s final power generation, efficiency and cost effectiveness. Thus, various

experimental investigations have been carried out to evaluate and compare system

performance when different expanders were used. Compared with the screw [16] and scroll

[17] expanders, the turbine expander offered more advantages in terms of smaller size, lower

cost, compact structure and higher efficiency [18, 19]. A small scale R245fa ORC test rig

with a radial-type turbine and a number of supersonic nozzles was developed to regulate the

mass flow rate at dynamic thermal energy supplies [20]. As such, the ORC system can

operate at optimal operating conditions by adjusting the total inlet temperature, mass flowrate,

and rotational speed of the turbine. In addition, it is understood that the rotational speed,

expansion ratio, mass flow rate and turbine size have significant effect on turbine

performance. Accordingly, mean-line design and three-dimensional computational fluid

dynamics analysis were integrated for both micro axial and radial-inflow turbines with five

organic fluids and at low-temperature heat sources in ORC cycles [21]. The results showed

that n-pentane had the highest performance at all design conditions in terms of maximum

total-to-total efficiency and power output. Furthermore, a single stage axial turbine expander

coupled with a permanent magnet synchronous generator was tested on a small-scale ORC

system with working fluids of R245fa and HFE7100 [22]. It was found that the system

evaporating pressure, pressure drop across the turbine and system mass flow rate had

significant effect on the turbine and system performance. However, for the ORC system, the

turbine expander inlet superheat temperature needs to be precisely controlled to ensure the

expander’s dry operation as it will affect the system power generation.

It is worth noting that the working fluids used in ORC system such as R245fa are mostly

HFCs, which have relatively high Global Warming Potential (GWP). These will influence

long-term applications of ORC system in the near future. Alternatively, as listed in Table 1,

when compared to R245fa, CO2 is a natural working fluid that has been widely used in heat

pump and refrigeration systems due to its zero ODP and negligible GWP. In addition, it has

superb thermophysical properties, despite its low critical temperature and high critical

pressure, and features of being non-flammable, non-toxic and thermally stable. Kim et al. [23]

researched a comparison between supercritical Brayton (S-CO2) and CO2 transcritical power

(T-CO2) systems in terms of energy and exergy efficiencies. The results of this study showed

that T-CO2 was more applicable for low-grade heat sources due to its good thermal match in

heat transfer process of high pressure side. Due to the high critical pressure, the CO2 pressure

of gas generating processes in a T-CO2 would also be high, such that conventional heat

exchangers, expanders etc. cannot be directly used in the system. Consequently, up till now,

investigations on low-grade heat source energy conversion systems with T-CO2 have been

limited to small-scale laboratory work without measurement of actual power generation [24]

and mostly are on simulation analyses in various T-CO2 with the application of low-

temperature waste heat recovery [25]. Therefore, the comprehensive experimental analyses

for low-grade T-CO2 system operations and controls need to be further investigated and

developed. In addition, an experimental analysis and performance comparison between the T-

CO2 and conventional R245fa ORC systems at similar operating conditions need to be

conducted, which have not been investigated so far.

Subsequently, in this study, the small-scale T-CO2 and R245fa ORC system test rigs with

the CO2 and R245fa turbines were designed, fabricated and underwent experimentation. The

effect of working fluid mass flow rates and heat source input on the performances of turbine

and high-pressure side heat exchangers have been measured and analysed. The research

outcomes can contribute significantly to the component designs of T-CO2 and R245fa ORC

systems and systems controls.

2. Experimental facility and methodology

It is noted that the thermal and exergy analyses and comparisons between R245fa ORC and

CO2 power generation systems at various operating conditions have been described and

demonstrated from our previous publication [25]. In order to test and compare the T-CO2 and

R245fa ORC systems, two small scale test rigs have been set up in the laboratory at Brunel

University London, as shown in Fig.1. The experiment set up consisted of typical ORC and

TPC system components, such as turbo expanders with high speed generators, finned-tube

air-cooled condensers, working fluid liquid pumps and thermal oil-heated plate evaporator

and gas generator. It should be noted that for the heat addition heat exchanger in an ORC or

TPC system, if there is an evaporation or boiling process involved, the heat exchanger is

called evaporator otherwise named gas generator. Subsequently, in the evaporator, the

working fluid is heated from liquid to two-phase and then to superheat state while in the gas

generator, the working fluid is heated directly from liquid to superheat fluid. For the test rig

fabrications, copper connection pipes were installed in the ORC system while stainless steel

pipelines were applied in the T-CO2 rig. The R245fa plate-type evaporator or CO2 plate-type

gas generator was heated indirectly by exhaust flue gases of an 80 kWe CHP unit through a

thermal oil circuit and a thermal oil boiler installed inside the CHP exhaust, as shown in Fig.

1 and Fig. 2b. The thermal oil flow rate was controlled by a variable speed oil pump while its

temperature was modulated by the CHP power output controls [26]. Operationally, the

vapour working fluid (R245fa or CO2) with high pressure and high temperature expands in

the ORC or CO2 turbine (point 1). The R245fa and CO2 turbines are both axial type with

single stage. The radiuses of R245fa and CO2 turbines are 114mm and 72mm respectively

while the overall lengths of the R245fa and CO2 turbines are 307 mm and 298 mm each. Each

turbine stage is a reaction type with reaction degree or reaction ratio of 0.3 for R245fa turbine

or 0.5 for CO2 turbine. In addition, the designed turbine power generation, pressure ratio,

working fluid mass flow rate and diameter are 5kWe, 5, 0.4kg/s and 42mm for R245fa

turbine and 5kWe, 1.5, 0.281kg/s and 144mm for CO2 turbine respectively. The R245fa or

CO2 turbine was integrated separately with an individual high speed and permanent magnet

synchronous generator at rated rotation speed of up to 18,000 rpm. The electrical power

generated by each generator was connected and transmitted into the campus electric grid by

means of a smart inverter and transformer. The smart inverter in each turbine system,

provided by ABB, allowed the generator speed to be monitored and matched with the electric

power generated so that the R245fa and CO2 turbines could operate safely. Situated in

parallel to the CO2 and R245fa turbines, two by-pass valves were installed separately to

completely bypass the CO2 and R245fa flow whenever necessary. After expansion (point 2),

the low pressure R245fa and CO2 vapours separately enter two finned-tube air cooled

condensers to be condensed into their liquid states (point 4) and are drawn individually into

two liquid receivers. The air flow rate of the condenser was controlled by its variable speed

fan while the air inlet temperature was modulated by mixing warm exhaust and cold ambient

air flows through a number of recircular fans installed on two sides of the condenser outlet, as

shown in Fig. 1 and Fig2 (a). With the installation of a receiver in each system, the working

fluid state at liquid could be maintained at pump inlet. From the liquid receivers (point 5), the

liquid R245fa and liquid CO2 are pumped back to the evaporator and gas generator

respectively (points 7 and 8) and thus each continues another operation cycle. For the R245fa

ORC system, the working fluid pump installed was a seal-less diaphragm type pump and

direct coupled to a 1.1 kW asynchronous motor. For the T-CO2 system, a triplex plunger

pump with normal PTFE sealing was particularly designed and installed. The PTFE sealing

could last 6 months of continuous operation before being replaced with a new one.

Similar to the condenser fans, the speed of the liquid R245fa or CO2 pump was also

controlled by a frequency drive inverter which could modulate the R245fa or CO2 fluid mass

flow rate and operating pressures in each system.

Both test systems were fully instrumented with calibrated temperature sensors and

pressure transducers at the inlet and outlet of each main component, as shown in Fig. 1. The

thermal oil and air flow inlet and outlet temperatures and flow rates were measured

respectively at the R245fa evaporator, CO2 gas generator and condensers. The measurements

and calibrations also included the turbine power generation and working fluid mass flow rate

in each system. For the measurement instruments, all temperatures were measured by K-type

thermocouples with accuracy of ±0.5 oC. The pressure transducers with 0-25bar range and 0-

160bar range were selected for the pressure measurements in the ORC and T-CO2 systems

respectively. All pressure transducers were measured with ±0.3% accuracy and 0.5s response

time. A twin tube type mass flowmeter with 0-6500kg/h and ±0.15% accuracy, and a twin V-

shaped tube type mass flow meter with 0-1800kg/h and ±0.1% accuracy were applied to

measure the liquid mass flow of R245fa and CO2 respectively in each system. In addition, the

R245fa and CO2 turbine power outlets were measured directly using a power meter with

accuracy of ±0.8% installed at outlet electric wires of both power generators. A hot wire

anemometer was used to measure the ambient air velocity, with an accuracy class of ±0.15

m/s at full range of 1.27-78.7 m/s. The detailed characteristics (measuring range and accuracy

etc.) of each measurement instrument are summarised in Table 2. After the fabrication of the

ORC and T-CO2 test systems, nitrogen gas has been pressured into the systems to check for

any leakages. After that, 50kg liquid R245fa and CO2 fluids were charged into the vacuumed

ORC and T-CO2 systems respectively.

3. Results and analysis

Both systems described in section 2 were applied to examine the effects of working fluid

mass rates and heat source inputs on the system performance. Parameters of temperatures,

pressures and fluid mass flow rates for the T-CO2 working fluid (CO2), ORC working fluid

(R245fa) and heat source (thermal oil) were measured and logged by a National Instruments

data logger system and recorded by a computer with LabVIEW at each steady state.

Corresponding to Fig.1, sample T-S diagrams for the T-CO2 and R245fa ORC systems are

shown in Fig.3 (a) and (b) respectively. In the T-S diagram of the T-CO2 system, the gas

generator and condenser pressures are specified as 90.83bar and 66.81bar respectively.

Meanwhile, the T-S diagram of the R245fa ORC system is based on the specifications of

10.92bar and 3.128bar each for the evaporator and condenser pressures. For the T-CO2

system, the thermal oil inlet and ambient air temperatures are 139.6 oC and 26.8 oC

respectively. While, for the R245fa ORC system, the thermal oil inlet and ambient air

temperatures are 130.7 oC and 17.8 oC individually. All the thermophysical properties of CO2

and R245fa such as entropy and enthalpy etc. were calculated using REFPROP 8.0 software

[27] based on the average temperature and pressure at each measured point. During the

measurements, the CO2 and R245fa mass flow rates were controlled from 0.2 kg/s to 0.26

kg/s and 0.23 to 0.27 kg/s respectively by changing the corresponding pump motor

frequencies, while heat source inputs were controlled from 52 kW to 60kW and from 61kW

to 68kW for T-CO2 circuit and R245fa ORC system individually. For each test group, only

one operational parameter was changed and all others were controlled to remain

approximately constant. These settings were designed to ensure that the inlet temperatures

and pressures of CO2 and R245fa turbines were within their maximum limitations during the

tests. The maximum inlet fluid temperature was set as 110 oC (120 oC for a short operational

period) for both turbines, and maximum fluid inlet pressures at 110bar for CO2 turbine and

14bar for the R245fa turbine as required by the turbine manufacturer. In addition, the

thermal stability of R245fa has been researched by Juhasz et al. [28]. R245fa has a very low

degradation at a temperature of 250 oC and an exposure time of 7 days. On the other hand, the

CO2 is a very stable working fluid. Since both working fluids in the test systems operated

always at temperatures much below 250 oC, there was no any risk of decomposition of each

fluid.

Subsequently, the effects of working fluid mass flow rates and heat source inputs on the

system performances can be evaluated and compared experimentally.

3.1 The effect of the working fluid mass flow rate swings

To achieve the targets, a test matrix as listed in Table 3 was planned for the working fluid

mass flow rate swings of the T-CO2 and R245fa ORC systems. As listed in the Table, the

working fluid mass flow rates were controlled to vary from 0.2kg/s to 0.26kg/s and from

0.23kg/s to 0.27kg/s for T-CO2 and R245fa ORC systems respectively. In the meantime,

other parameters such as heat source inputs, condenser air velocities and ambient air

temperatures were maintained as constant for both systems.

The effect of varying cycle point pressures for the T-CO2 and R245fa ORC systems with

different working fluid mass flow rates were measured and plotted, as illustrated in Fig. 4. It

should be noted that the working fluid pressures can be classified into two groups of high and

low-pressure sides for both systems. The pressure ranges on the high-pressure side in the T-

CO2 and R245fa ORC systems were changed from 80 to 90 bar and from 9 to 12 bar

respectively, and the low-pressure side from 60 to 65 bar and from 1.8 to 4 bar respectively

for T-CO2 and R245fa ORC systems. The high pressure side of each cycle includes the cycle

locations at pump outlet, gas generator/ evaporator inlet and turbine inlet, while the low

pressure side includes the cycle locations at the turbine outlet, condenser inlet and outlet, and

pump inlet. The experimental results show that the R245fa pressure at each cycle point

increases almost linearly with higher R245fa mass flow rates. Similarly, a greater CO2 mass

flow rate increases CO2 cycle point pressures, especially when the flow rate is higher than

0.23 kg/s. At the low pressure side, however, the CO2 pressure at each cycle point decreases

slightly when the CO2 mass flow rate increases from 0.2 kg/s to 0.23 kg/s. This can be

explained as the effect of CO2 mass flow rates on the supercritical CO2 pressure and varied

CO2 thermophysical properties at the supercritical region. It must be noted that there were

significant pressure drops between the pump outlet and gas generator inlet at different

working fluid mass flow rates for the T-CO2 system due to the fittings and bends of pipework.

This could lead to higher CO2 pressures at the pump outlet and thus higher pump power

consumption. For the R245fa ORC system, a significant pressure drop occurred between the

turbine outlet and condenser inlet at each working fluid flow rate also due to associated

fittings and pipework bends. This could lead to a reduced power generation and overall

system thermal efficiency. The fittings included two or three-way connections. Considering

the limited lab space, a number of fittings and bends were installed between two neighbour

components of each test system and thus more pressure drop was added for each system

component. Cumulatively, when the CO2 mass flow rate increased from 0.2kg/s to 0.26kg/s,

the T-CO2 cycle point pressures of pump outlet, gas generator inlet, turbine inlet and outlet,

condenser inlet and outlet and pump inlet amplified 11.5%, 11.58%, 11.56%, 6.82%,

6.85%,6.75% and 6.38% respectively. For the R245fa ORC system, when the R245fa mass

flow rate increased from 0.23kg/s to 0.27kg/s, the cycle point pressures of pump outlet,

evaporator inlet, turbine inlet and outlet, condenser inlet and outlet and pump inlet each

increased 9.82%, 9.6%,9.1%, 12.33%, 10.25%, 8.88% and 6.42%.

On the other hand, the effects of the CO2 and R245fa mass flow rates on CO2 and R245fa

temperatures at the primary components inlets and outlets have all been measured and

presented in Fig. 5. The figures show that higher working fluid mass flow rates resulted in

lower working fluid temperatures at either the turbine inlets or outlets of both systems due to

the heat transfer behaviours in the gas generator/ evaporator and the specified power

generation for each turbine. Similar effects have been observed for the working fluid mass

flow rates on the gas generator/ evaporator outlet and condenser inlet temperatures. For both

systems, the gas generator/ evaporator outlet and turbine inlet temperatures varied between

120 oC and 100 oC, and the turbine outlet and condenser inlet temperatures between 100 oC

and 80 oC. On the other hand, the working fluid temperatures at condenser outlet, pump outlet

and gas generator/ evaporator inlet increased with working fluid mass flow rates and varied

between 20 oC and 30 oC, because of the constant sink (ambient) parameters for both systems.

In general, when CO2 mass flow rate increased from 0.2kg/s to 0.26kg/s, the temperatures at

gas generator outlet, turbine inlet and outlet and condenser inlet dropped by approximately

20.99K, 20.97K, 23.79K and 23.19K respectively. Meanwhile, at the same CO2 mass flow

increase range, the T-CO2 temperatures at the condenser outlet, pump outlet and gas

generator inlet each increased by only 2.79K, 4.62K and 4.71K. On the other hand, when

R245fa mass flow rate increased from 0.23kg/s to 0.27kg/s, the cycle point temperatures of

the evaporator outlet, turbine inlet and outlet and condenser inlet decreased by 18.5K, 19.75K,

26.66K and 26.86K individually. However, at the same R245fa mass flow rate increase rate,

R245fa temperatures at condenser outlet, pump outlet and gas generator inlet increased only

2.35K, 2.86K and 2.88K respectively.

The turbine power generations at increased working fluid mass flow rates for both systems

were also measured and depicted in Fig. 6. To make a comparison, for each system, the

turbine power generation, turbine exergy destruction, turbine isentropic efficiency, turbine

overall efficiency and turbine exergy efficiency are calculated with equations 1 to 5

respectively. These calculated parameters with increased working fluid mass flow rates are

also demonstrated in Fig. 6.

(1)𝑊𝑡,𝑐 = 𝑚𝑓(ℎ1 ‒ ℎ2)

(2)𝜂𝑡,𝑖𝑠 =(ℎ1 ‒ ℎ2)

(ℎ1 ‒ ℎ2,𝑖𝑠)

(3)𝜂𝑡, 𝑎𝑙𝑙 = 𝜂𝑡,𝑖𝑠𝜂𝑡,𝑚𝜂𝑡,𝑒 =𝑊𝑡,𝑚

𝑚𝑓(ℎ1 ‒ ℎ2,𝑖𝑠)

(4) 𝐼𝑡 = 𝑚𝑓𝑇0(𝑠2 ‒ 𝑠1)

𝜂𝑡, 𝑒𝑥 =𝑊𝑡,𝑐

𝑊𝑡,𝑐 + 𝐼𝑡=

ℎ1 ‒ ℎ2(ℎ1 ‒ ℎ2) + 𝑇0(𝑠2 ‒ 𝑠1)

(5)

It can be observed from the measurements and calculations that the measured turbine

power generation is far lower than the calculated particularly for the T-CO2 system. The ratio

of these two values is the product of turbine mechanical and electrical efficiencies which

need great improvement for future design and manufacture. Subsequently, the overall

efficiency of each turbine is relatively low compared with its isentropic efficiency as shown

in the Figure.

As observed from the measurements, the measured power generation for each turbine

increased with higher working fluid mass flow rates. Similar to the measured power

generation of each turbine, the turbine exergy destruction increased with higher working fluid

mass flow rates. However, the circumstances are different for the calculated turbine power

generations of both systems. For the T-CO2 system, the calculated turbine power generation

increased with higher CO2 mass flow rate. As to the R245fa ORC system, the calculated

power generation always decreases at higher R245fa mass flow rates. These can be explained

by the working fluid enthalpy differences between the turbine inlet and outlet which changes

according to working fluid mass flow rates. Correspondingly, for each system, the turbine

isentropic and overall efficiencies follow similar trends as the calculated and measured

turbine power generations. In addition, the exergy efficiency of both turbines decreased with

higher working fluid mass flow rates. Thus, both the turbine exergy efficiencies and turbine

isentropic efficiencies have similar variations. However, the turbine exergy efficiency is

higher than the turbine isentropic efficiency at different working fluid mass flow rates.

Quantitatively, the percentage increase rates of measured turbine power generation are 88.2%

and 27.3%, the turbine exergy destruction 64.9% and 33.3%, and the turbine overall

efficiency 35.4% and 7.5% when the CO2 mass flow rate increases from 0.2kg/s to 0.26kg/s

and R245fa mass flow rate increases from 0.23kg/s to 0.27kg/s respectively. Correspondingly,

the percentage decrease rates of turbine isentropic efficiency are 16.3% and 51.6%, the

turbine exergy efficiency 16.4% and 54.1% for the CO2 turbine and R245fa turbine

respectively.

3.2 The effect of the heat source input swings

To examine the effect of heat source input on the system performances, the heat source

inputs were controlled to vary from 52kW to 60kW and from 61kW to 68kW for the T-CO2

and R245fa ORC systems respectively by modulating the CHP power outputs and thermal oil

mass flow rates. In the meantime, as listed in Table 4, all other operating parameters such as

working fluid mass flow rate, condenser air velocity and ambient air temperature were kept

constant for both systems.

Subsequently, the variations of cycle point pressures and pressure ratios between the

turbine inlet and outlet with the increased heat source (thermal oil) inputs on the T-CO2 and

R245fa ORC systems were measured and recorded and are shown in Fig.7. Clearly, for both

the T-CO2 and R245fa ORC systems, the cycle point pressures can be categorised into two

groups of system high and low pressures. For further clarification, for each system, the higher

pressure group includes the cycle points at the pump outlet, gas generator/ evaporator inlet

and turbine inlet while the low pressure group consists of system locations at the turbine

outlet, condenser inlet and outlet and pump inlet. It can be seen from the measurements that

for each system the cycle point pressures of high pressure groups all significantly increase

with higher heat source input while the cycle point pressures of low pressure groups only

increase marginally. Meanwhile, the pressure ratios between the turbine inlet and outlet for

both systems all increase with higher heat source input. However, the turbine pressure ratios

of the T-CO2 system were much lower than those of R245fa ORC system. In addition, it

should be noted that the values of heat source inputs for the T-CO2 system were far lower

than those of the R245fa ORC. For both systems, the minimum and maximum pressure points

were respectively at the pump inlet and outlet. In percentage terms, when the heat source

input increased from 52kW to 60kW for the T-CO2 system and from 61kW to 68kW for the

R245fa ORC system, the pump outlet pressure amplified by 7.29% and 4.81%, gas generator/

evaporator inlet pressure by 7.33% and 4.79%, turbine inlet pressure by 7.31% and 4.72%

respectively.

The variations of both cycle point temperatures with heat source input on the T-CO2 and

R245fa ORC systems were recorded and are depicted in Fig. 8. For both systems, the

measurements demonstrated that the temperatures at gas generator/ evaporator outlet, turbine

inlet, turbine outlet and condenser inlet all increased with higher heat source inputs. However,

the working fluid temperatures at the condenser outlet, pump outlet and gas generator/

evaporator inlet were not significantly affected by the heat source inputs for the T-CO2 and

R245fa ORC systems owing to the constancy of heat sink parameters and working fluid mass

flow rate. More precisely, when the heat source input was increased from 52kW to 60kW for

the T-CO2 system, the temperatures at gas generator outlet, turbine inlet, turbine outlet and

condenser inlet increased around 40.65K, 41.18K, 40.34K and 40.01K respectively. In

addition, when heat source input increased from 61kW to 68kW for the R245fa ORC system,

the cycle point temperatures of the evaporator outlet, turbine inlet, turbine outlet and

condenser inlet each amplified by 23.18K, 19.54K, 20.37K and 20.37K.

As shown in Table 4, the fixed working fluid mass flow rates were applied for both the T-

CO2 and R245fa ORC systems. However, the CO2 mass flow rate was kept constant at

0.25kg/s for the T-CO2 system and the R245fa mass flow rate at as 0.26kg/s for the ORC

system. Based on the measured parameters at each turbine inlet and outlet and working fluid

mass flow rates, the actual turbine power output for both systems were calculated at different

heat source inputs, while the turbine power outputs were also measured directly by a power

meter installed at outlet wire of the turbines. For both turbines, the isentropic and overall

efficiencies of each system turbine are calculated with Equations (2) and (3) respectively

based on the measurements. In addition, each turbine overall efficiency can be solved using

the ratio of the respective measured turbine power generation and the turbine isentropic

power output. The turbine overall efficiency can also be calculated as a production of

isentropic, mechanical and electrical efficiencies of each turbine, as listed in Equation (3).

For both systems, the above calculated and measured parameters are presented in Fig. 9.

Clearly, it is shown that the actual and measured turbine power outputs increase with higher

heat source input for both turbines. This can be explained in that the higher heat source input

increased the pressure ratio between the turbine inlet and outlet, as shown in Fig. 7, and thus

the power output of each turbine. However, the pressure ratios of the T-CO2 system were

much smaller than those of the R245fa ORC system and thus the subsequent turbine power

output. It should be noted that power generation for both turbines are much lower than the

designed value of 5kW. Theoretically, power generation can be achieved by further

increasing working fluid mass flow rate, heat source input, turbine inlet temperature and

pressure ratio at the turbine inlet and outlet. Nevertheless, practically there is an increased

limitation for power generation due to the limited pressure and temperature at both turbine

inlets set by manufacturers. Further design improvements for the CO2 and R245fa turbines

need to be considered in the near future. For the CO2 turbine as shown in Fig. 9, the turbine

exergy destruction increased with higher heat source input. However, the effect of heat

source input on the turbine exergy destruction is insignificant for the R245fa turbine. In

addition, the variations of isentropic, overall and exergy efficiencies for CO2 and R245fa

turbines with heat source inputs on both systems were calculated and are depicted in Fig. 9.

Higher heat source input can reinforce the turbine isentropic, overall and exergy efficiencies

in both systems. Quantitatively, when the heat source input increased from 52kW to 60kW

for the T-CO2 system, 47.7%, 50.74%, 15.13%, 10.89%, 8.65%, 18.14% were increased

respectively for the turbine measured power output, turbine calculated power output, turbine

exergy destruction, turbine isentropic efficiency, turbine overall efficiency and turbine exergy

efficiency. For the R245fa ORC system, when the heat source input increased from 61kW to

68kW, the percentage increase rates of the turbine measured power output, turbine calculated

power output, turbine isentropic efficiency, turbine overall efficiency and turbine exergy

efficiency were 63%, 28.59%, 14.38%, 1.08% , 17.89% individually.

4. Performances of oil-heated CO2 gas generator and R245fa evaporator

The temperature vs. heat transfer (TQ) diagrams of the CO2 gas generator and R245fa

evaporator at different heat source inputs and working fluid working rates are presented in

Figs. 10 and 11 respectively. As seen from the Figures, there are two coloured lines in each

section diagram: the red line represents the thermal oil temperature change from the gas

generator/ evaporator inlet to outlet via gas generator/ evaporator heat capacities, while the

blue line indicates the working fluid temperature variation from the gas generator/evaporator

inlet to outlet temperatures via gas generator/ evaporator heat capacities. The temperature

difference circled with dotted lines in each section diagram of Fig. 10 is the calculated

temperature difference between the thermal oil inlet and the CO2 gas generator outlet.

However, the pinch point temperature of the R245fa evaporator shown in Fig. 11 is

calculated based on the measured thermal oil inlet and outlet temperature profiles and R245fa

side measurements of temperatures and pressures.

An example of TQ diagram of T-CO2 system gas generator is presented for heat source

input 56kW and CO2 flow rate 0.26kg/s in Fig.10a. As seen from this Fig., at a fixed heat

source input of 56 kW and CO2 mass flow rate of 0.26 kg/s, the temperature difference at the

thermal oil inlet end of the CO2 gas generator is 44.13 K. When the heat source input is kept

constant at 56 kW but the CO2 mass flow rate is reduced from 0.26kg/s to 0.2kg/s, as shown

in Fig.10b, the temperature difference at the same heat exchanger end is reduced to 25.41 K .

On the other hand, when the CO2 flow rate is kept constant at 0.26kg/s (compared to Fig.10a)

and the heat source input is increased from 56kW to 60kW, the fluid temperature difference

at the thermal oil inlet end decreases from 44.13K to 27.43K, as shown in Fig. 10c. It should

be noted that in Figs. 10a, 10b and 10c only the heat source input and CO2 mass flow rate are

variable and the heat sink (air flow) flow rate and temperature are all kept constant.

Subsequently, the demonstrations from the measurements can thus reveal that higher CO2

mass flow rate will effectively increase the temperature difference between the heat source

inlet and CO2 flow outlet and thus decrease the CO2 turbine inlet temperature. Meanwhile,

higher heat source input will reduce the temperature difference between the heat source inlet

and CO2 flow outlet and therefore increase the CO2 turbine inlet temperature.

In addition, the TQ diagram of R245fa ORC system evaporator is depicted in Fig. 11a at

fixed heat source input of 64kW and R245fa mass flow rate of 0.25 kg/s. The pinch point

temperature is calculated as 33.07 K. As opposed to the TQ diagram of CO2 gas generator, in

the ORC evaporator, the R245fa flow undergoes three distinguished heating processes: liquid

preheating, two-phase evaporating and vapour superheating. As shown in Fig. 11b, when the

heat source input is kept constant and the R245fa flow rate is decreased from 0.25kg/s to

0.23kg/s, the pinch point temperature decreases to 26.17K. Meanwhile, the superheating at

the evaporator outlet and the transferred heat in the vapour phase both increase. However,

when R245fa flow rate is kept constant at 0.25kg/s (compared with Fig.11a) and the heat

source input is increased from 64kW to 68kW, the pinch point temperature is increased to

36.43K, as shown in Fig. 11c. However, the amount of superheating and the transferred heat

in the vapour phase are also increased. It can be observed that higher R245fa flow rates will

increase pinch point temperatures and decrease the superheat temperatures at the R245fa side.

In addition, a higher heat source input will increase both the pinch point temperature of the

evaporator and superheating at the evaporator R245fa outlet. These will further help to

understand the controls of CO2 and R245fa parameters at both turbine inlets.

5. Conclusions

The test rigs of small-scale T-CO2 and R245fa ORC low-grade power generation systems

were designed, built up and tested to investigate the effects of two important operating

parameters including heat source input and working fluid mass flow rate on the performance

of each system. Hot thermal oil fluid, which was heated by the exhaust flue gases from an

80kWe CHP unit, was used as a common heat transfer medium and heat source for both of

the T-CO2 and R245fa ORC systems. Several useful research outcomes have been obtained.

At higher working fluid (CO2 or R245fa) mass flow rates, the working fluid pressures at the

pump outlet, gas generator/ evaporator inlet, turbine inlet and out, condenser inlet and outlet

and pump inlet all increased differently in each system but the working fluid temperatures of

gas generator/evaporator outlet, turbine inlet and outlet and condenser inlet all decreased in

each system. In addition, for either the CO2 or R245fa turbine, the measured turbine power

generation and overall turbine efficiency all increased with higher working fluid mass flow

rates. In percentage, when the CO2 mass flow rate increased from 0.2kg/s to 0.26kg/s, the

turbine power generation and turbine overall efficiency increased by 88.2% and 35.4%

respectively and when R245fa mass flow rate increased from 0.23kg/s to 0.27kg/s, the turbine

power generation and turbine overall efficiency increased by 27.3% and 7.5% individually.

The tested overall efficiencies of each turbine proved to be smaller than its isentropic

efficiency, indicating that the turbine mechanical and electrical efficiencies need further

improvement.

With increased heat source input, the working fluid (CO2 or R245fa) pressures of pump

outlet, gas generator/ evaporator inlet and turbine inlet and pressure ratios of each system

turbine were all increased. Meanwhile, the working fluid (CO2 or R245fa) temperatures of

gas generator/evaporator outlet, turbine inlet and outlet and condenser inlet all increased

differently but the temperatures of condenser outlet, pump outlet and gas generator inlet did

not change significantly for each system. Furthermore, with increased heat source input, the

measured and calculated power generations, turbine isentropic, overall and exergy

efficiencies were all increased for each system turbine. Quantitatively, when heat source

input increased from 52kW to 60kW for T-CO2 system and from 61kW to 68kW for R245fa

ORC system, the turbine power generation increased by 47.7% and 63% and turbine overall

efficiency 8.65% and 1.08% respectively.

Ultimately, the ORC working fluid superheat at turbine inlet and CO2 turbine inlet

temperature and pressure were found to be important parameters which can be controlled

alone with R245fa and CO2 mass flow rates and heat source inputs respectively. Further

investigations will be carried out on the experimental test rigs in the future. These include the

effects of heat source flow rates, heat sink parameters and controls on the system

performance. In addition, overall system dynamic models will also be developed to simulate

the system performance at a broad range of operating conditions.

Acknowledgements

The authors would like to acknowledge the support received from GEA Searle, Research

Councils UK (RCUK) and Innovate UK for this research project

References

[1] T. Morisaki, Y. Ikegami, Maximum power of a multistage Rankine cycle in low-grade

thermal energy conversion, Applied Thermal Engineering 69 (2014)78-85.

[2] L. Li, Y.T. Ge, X. Luo, S.T. Tassou, Experimental investigations into power generation

with low grade waste heat and R245fa Organic Rankine Cycles (ORCs), Applied

Thermal Engineering 115(2017)815-824.

[3] E. Cayer, N. Galanis, M. Desilets, H. Nesreddine, P. Roy, Analysis of a carbon dioxide

transcritical power cycle using a low temperature source, Applied Energy

86(2009)1055-1063.

[4] J. Ortega, S. Khivsara, J. Christian, C. Ho, P. Dutta, Coupled modeling of a directly

heated tubular solar receiver for supercritical carbon dioxide Brayton cycle: Structural

and creep-fatigue evaluation , Applied Thermal Engineering 109(2016)979-987.

[5] A. Date, S. Vahaji, J. Andrews, A. Akbarzadeh, Experimental performance of a rotating

two-phase reaction turbine, Applied Thermal Engineering 76(2015)475-483.

[6] O. Badr, S.D. Probert, P.W. O'Callaghan, Selecting a working fluid for a Rankine-cycle

engine, Applied Energy 21(1985)1-42.

[7] H. Chen, D.Y. Goswami, E.K. Stefanakos, A review of thermodynamic cycles and

working fluids for the conversion of low-grade heat, Renew Sustain Energy Rev

14(2010)3059–67.

[8] K. Kim, H., Perez-Blanco, Performance analysis of a combined organic Rankine cycle

and vapor compression cycle for power and refrigeration cogeneration, Applied


[9] S. Karellas , A. Schuster, Supercritical fluid parameters in organic Rankine cycle

applications, Int J Thermodynamics 11(2008)101–108.

[10] J.L. Wang, X.D. Wang, L. Zhao, An experimental study on the recuperative low

temperature solar Rankine cycle using R245fa, Applied Energy 94(2012)34-40.

[11] Z. Shengjun, W. Huaixin, G. Tao, Performance comparison and parametric optimization

of subcritical Organic Rankine Cycle (ORC) and transcritical power cycle system for

low-temperature geothermal power generation, Applied Energy 88(2011)2740-2754.

[12] H. Chen, D.Y. Goswami, M.M. Rahman, E.K. Stefanakos, Energetic and exergetic

analysis of CO2- and R32-based transcritical Rankine cycles for low-grade heat

conversion, Applied Energy 88 (2011) 2802-2808.

[13] T. Hung ,Waste heat recovery of organic Rankine cycle using dry fluids. Energy

Conversion and Management 42( 2001)539-553.

[14] J. Zhang, A. Desideri, M. Kaern, T. Ommen, J. Wronski, F. Haglind, Flow boiling heat

transfer and pressure drop characteristics of R134a, R1234yf and R1234ze in a plate

heat exchanger for organic Rankine cycle units, International Journal of Heat and Mass

Transfer108(2017)1787-1801.

[15] M. Chys, M. van den Broek, V.B. anslambrouck, M. De Paepe, Potential of zeotropic

mixtures as working fluids in organic Rankine cycles, Energy 44(2012)623-632.

[16] G. Xia, Y. Zhang, Y. Wu, C. Ma, W. Ji, S. Liu, H. Guo, Experimental study on the

performance of single-screw expander with different inlet vapor dryness, Applied


[17] Z. Wu, D. Pan, N. Gao, T. Zhu, F. Xie, Experimental testing and numerical simulation

of scroll expander in a small scale organic Rankine cycle system, Applied Thermal

Engineering 87(2015)529-537.

[18] W. Pu, C. Yue, D. Han, W. He, X. Liu, Q. Zhang, Y. Chen ,Experimental study on

Organic Rankine cycle for low grade thermal energy recovery, Applied Thermal

Engineering 94(2016)221-227.

[19] S.H. Kang, Design and experimental study of ORC (organic Rankine cycle) and radial

turbine using R245fa working fluid, Energy 41(2012)514-524.

[20] S. Cho, C. Cho, K. Ahn, Y. Lee, A study of the optimal operating conditions in the

organic Rankine cycle using a turbo-expander for fluctuations of the available thermal

energy, Energy, 64(2014) 900-911.

[21] A. Al Jubori, A. Baabo, R. Al-Dadah, S. Mahmoud, A. Ennil, Development of micro-

scale axial and radial turbines for low-temperature heat source driven organic Rankine

cycle, Energy Conversion and Management 130 (2016), 141-155.

[22] W. Pu, C. Yue, D. Han, W. He, X. Liu, Q. Zhang, Y. Chen, Experimental study on

Organic Rankine cycle for low grade thermal energy recovery, Applied Thermal

Engineering, 94(2016), 221-227.

[23] Y. Kim, C. Kim, D. Favrat, Transcritical or supercritical CO2 cycles using both low-

and high-temperature heat sources, Energy 43(2012)402-415.

[24] H. Yamaguchi, X.R., Zhang, K. Fujima, M. Enomoto, N. Sawada, Solar energy

powered Rankine cycle using supercritical CO2, Applied Thermal Engineering

26(2006)2345-2354.

[25] L. Li, Y.T. Ge, X. Luo, S.T. Tassou, Thermodynamic analysis and comparison between

CO2 transcritical power cycles and R245fa organic Rankine cycles for low grade heat

to power energy conversion, Applied Thermal Engineering 106(2016)1290-1299.

[26] Y.T. Ge, S.A. Tassou, I. Chaer, N. Suguartha, Performance evaluation of a tri-

generation system with simulation and experiment, Applied Energy 86(2009)2317-

2326.

[27] E. Lemmon, M. Huber, M. McLinden, NIST REFPROP standard reference data based

23. Version 8.0. User’s guide, NIST. 2007.

[28] J. R. Juhasz, L.D. Simoni, A review of potential working fluids for low temperature

organic Rankine cycles in waste heat recovery, in: 3rd International Seminar on ORC,

Brussels, Belgium, 12-14 October, 2015.

Fig.1. Test facilities of integrated CHP, T-CO2 and ORC systems.

Fig.2. Photographs of the systems: (a) T-CO2 and ORC systems, (b) 80 kWe CHP unit.

a b

(a) T-S diagram of T-CO2 (b) T-S diagram of R245fa ORC

Fig.3. T-S diagrams of T-CO2 and R245fa ORC systems.

0.2 0.21 0.22 0.23 0.24 0.25 0.2650556065707580859095

100Turbine inlet

Turbine outlet

Condenser inlet

Condenser outlet

Pump outlet

Pump inlet

Gas generator inlet

T-CO2 system

Cyc

le P

ress

ure

(bar

)

CO2 mass flow rate (kg/s)

0.23 0.24 0.25 0.26 0.27 0.280

2

4

6

8

10

12Turbine inlet

Turbine outlet

Condenser inlet

Condenser outlet

Pump outlet

Pump inlet

Evaporator inlet

R245fa ORC system

Cycl

e Pr

essu

re(b

ar)

R245fa mass flow rate (kg/s)

Fig.4. Variations of cycle point pressures with CO2 and R245fa mass flow rates.

0.2 0.21 0.22 0.23 0.24 0.25 0.260

20

40

60

80

100

120

140Turbine inlet

Turbine outlet

Condenser inlet

Condenser outletPump outlet

Gas generator inletGas generator outlet

T-CO2 system

Cycl

e Te

mpe

ratu

re (⁰

C)


0.23 0.24 0.25 0.26 0.27 0.280

20

40

60

80

100

120

140Turbine inlet

Turbine outlet

Condenser inlet

Condeser outlet

Pump outlet

Evaporator inlet

Evaporator outlet

R245fa ORC system

Cyc

le T

empe

ratu

res (

⁰C)


Fig.5. Variations of cycle point temperatures with CO2 and R245fa mass flow rates.

0.2 0.21 0.22 0.23 0.24 0.25 0.260

600

1200

1800

2400

3000

3600

0

10

20

30

40

50

60 Turbine power generation (measurement)Turbine power generation (calculated)Turbine exergy destruction

Turbine isentropic efficiency

Turbine overall efficiency

Turbine exergy efficiency

T-CO2 system

Pow

er g

ener

atio

n &

exer

gy d

estr

uctio

n (W

)

Turb

ine

effic

ienc

ies (

%)


0.23 0.24 0.25 0.26 0.27 0.280

600

1200

1800

2400

3000

3600

0

10

20

30

40

50





R245fa ORC system

Pow

er g

ener

atio

n &

exer

gy d

estr

uctio

n (W

)

Turb

ine

effic

ienc

ies (

%)


Fig.6. Variations of turbine power generation and turbine isentropic and overall efficiencies

with CO2 and R245fa mass flow rates.

50 52.5 55 57.5 60 62.550556065707580859095

100

1.31

1.32

1.33

1.34

1.35

1.36

1.37Turbine inlet

Turbine outlet

Condenser inlet

Condenser outlet

Pump outlet

Pump inlet

Gas generator inlet

Pressure ratio

T-CO2 system C

ycle

Pre

ssur

e (b

ar)

Pres

sure

Rat

io

Heat source input (kW)

60 62.5 65 67.5 700

2

4

6

8

10

12

2.5

2.51

2.52

2.53

2.54

2.55

2.56

2.57

2.58

2.59Turbine inlet

Turbine outlet

Condenser inlet

Condenser outlet

Pump outlet

Pump inlet

Evaporator inlet

Pressure ratio

R245fa ORC system

Cycl

e Pr

essu

re(b

ar)

Pres

sure

Rat

io


Fig.7. Variations of cycle point pressures and pressure ratios with heat source inputs.

50 52.5 55 57.5 60 62.50

20

40

60

80

100

120

140Turbine inlet

Turbine outlet

Condenser inlet

Condenser outletPump outlet

Gas generator inletGas generator outlet

T-CO2 system

Cycl

e Te

mpe

ratu

re (⁰

C)


60 62.5 65 67.5 700

20

40

60

80

100

120

140Turbine inlet

Turbine outlet

Condenser inlet

Condeser outlet

Pump outlet

Evaporator inlet

Evaporator outlet

R245fa ORC system

Cyc

le T

empe

ratu

res (

⁰C)


Fig.8. Variations of cycle point temperatures with heat source inputs.

50 52.5 55 57.5 60 62.50

600

1200

1800

2400

3000

3600

0

10

20

30

40

50





T-CO2 system P

ower

gen

erat

ion

&

exer

gy d

estr

uctio

n (W

)

Turb

ine

effic

ienc

ies (

%)


60 62.5 65 67.5 700

600

1200

1800

2400

3000

3600

0

10

20

30

40

50





R245fa ORC system

Pow

er g

ener

atio

n &

exer

gy d

estr

uctio

n (W

)

Turb

ine

effic

ienc

ies (

%)


Fig.9. Variations of turbine power generation and turbine isentropic and overall efficiencies

with heat source inputs.

0 5 10 15 20 25 30 35 40 45 50 55 60 65 700

20406080

100120140160

Thermal oil (Heat source input=56kW)

CO2 (CO2 flow rate=0.26kg/s)

T-CO2 System Gas Generator (a)Ga

s Gen

erat

or &

Oil

Tem

pera

ture

(⁰

C)

Heat transfer rate (kW)

0 5 10 15 20 25 30 35 40 45 50 55 60 65 700

20

40

60

80

100

120

140

160



T-CO2 System Gas Generator (b)

Gas G

ener

ator

& O

il Te

mpe

ratu

re

(⁰C)


0 5 10 15 20 25 30 35 40 45 50 55 60 65 700

20406080

100120140160



T-CO2 System Gas Generator (c)

Gas G

ener

ator

& O

il Te

mpe

ratu

re

(⁰C)


Fig.10. Temperature vs. heat transfer rate diagrams of (a) lower heat source input and higher

CO2 flow rate, (b) lower heat source input and lower CO2 flow rate and (c) higher heat source

input and higher CO2 flow rate.

0 5 10 15 20 25 30 35 40 45 50 55 60 65 700

20

40

60

80

100

120

140

160

R245fa (R245fa flow rate=0.25kg/s)


ORC System Evaporator(a)Ev

apor

ator

& O

il Te

mpe

ratu

re (⁰

C)


0 5 10 15 20 25 30 35 40 45 50 55 60 65 700

20

40

60

80

100

120

140

160



ORC System Evaporator(b)

Evap

orat

or&

Oil

Tem

pera

ture

(⁰C)


0 5 10 15 20 25 30 35 40 45 50 55 60 65 700

20

40

60

80

100

120

140

160



ORC System Evaporator(c)

Evap

orat

or&

Oil

Tem

pera

ture

(⁰C)


Fig.11. Temperature vs. heat transfer rate diagrams of (a) lower heat source input and higher

R245fa flow rate, (b) lower heat source input and lower R245fa flow rate and (c) higher heat

source input and higher R245fa flow rate.

Table 1The properties for R245fa and CO2 (R744).

Substance Thermophysical data Environmental data

Molecular

mass

Tb

(oC)

Tc

(oC)

Pc

(Mpa)

Atmospheric

(yr)

ODP GWP

R245fa 134.05 15.1 154 3.65 7.6 0 1030

CO2 (R744) 44.01 -78.4 31.1 7.38 >50 0 1

Table 2 Main measurement instruments used in the experimental systems.

Parameters Type Range Accuracy

Temperatures K-type thermocouple (-10) to 1100 oC ±0.5 oC

Pressures (ORC) AKS 32 0~25bar ±0.3%

Pressures (T-CO2) MBS 33 0~160bar ±0.3%

Flowmeter (ORC) Twin tube type 0~6500kg/h ±0.15%

Flowmeter (T-CO2) Twin V-shaped tube type 0~1800kg/h ±0.1%

Power meters Digital multimeter 1mW~8kW ±0.8%

Air flow meters TA465 1.27~78.7 m/s ±0.15 m/s

Table 3 The operating conditions for the experiment set up of mass flow rate swings

Power

cycle

Heat source

input (kW)

Working fluid mass

flow rate (kg/s)

Condenser air

velocity (m/s)

Ambient air

temperature (oC)

T-CO2 56 0.2~0.26 3.67 24

R245fa

ORC64 0.23~0.27 3.67 17

Table 4 The operating conditions for the experiment set up of heat source input swings

Heat source

inputs (kW)

Working fluid mass

flow rates (kg/s)

Condenser air

velocity (m/s)

Ambient air

temperature (oC)

T-CO2 52~60 0.26 3.67 26

R245fa

ORC61~68 0.25 3.67 16

ManuscriptDetails - University of Wales, Newport · Feb. 21, 2018 Department of Mechanical,...

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