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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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Table 3.docx [Table]
Table 4.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
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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).
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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
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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
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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
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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
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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
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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)
CO2 mass flow rate (kg/s)
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)
R245fa mass flow rate (kg/s)
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 (
%)
CO2 mass flow rate (kg/s)
0.23 0.24 0.25 0.26 0.27 0.280
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
R245fa ORC system
Pow
er g
ener
atio
n &
exer
gy d
estr
uctio
n (W
)
Turb
ine
effic
ienc
ies (
%)
R245fa mass flow rate (kg/s)
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
Heat source input (kW)
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)
Heat source input (kW)
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)
Heat source input (kW)
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
60 Turbine power generation (measurement)Turbine power
generation (calculated)Turbine exergy destruction
Turbine isentropic efficiency
Turbine overall efficiency
Turbine exergy efficiency
T-CO2 system P
ower
gen
erat
ion
&
exer
gy d
estr
uctio
n (W
)
Turb
ine
effic
ienc
ies (
%)
Heat source input (kW)
60 62.5 65 67.5 700
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
R245fa ORC system
Pow
er g
ener
atio
n &
exer
gy d
estr
uctio
n (W
)
Turb
ine
effic
ienc
ies (
%)
Heat source input (kW)
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
Thermal oil (Heat source input=56kW)
CO2 (CO2 flow rate=0.2kg/s)
T-CO2 System Gas Generator (b)
Gas G
ener
ator
& O
il Te
mpe
ratu
re
(⁰C)
Heat transfer rate (kW)
0 5 10 15 20 25 30 35 40 45 50 55 60 65 700
20406080
100120140160
Thermal oil (Heat source input=60kW)
CO2 (CO2 flow rate=0.26kg/s)
T-CO2 System Gas Generator (c)
Gas G
ener
ator
& O
il Te
mpe
ratu
re
(⁰C)
Heat transfer rate (kW)
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)
Thermal oil (Heat source input=64kW)
ORC System Evaporator(a)Ev
apor
ator
& O
il Te
mpe
ratu
re (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
R245fa (R245fa flow rate=0.23kg/s)
Thermal oil (Heat source input=64kW)
ORC System Evaporator(b)
Evap
orat
or&
Oil
Tem
pera
ture
(⁰C)
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
R245fa (R245fa flow rate=0.25kg/s)
Thermal oil (Heat source input=68kW)
ORC System Evaporator(c)
Evap
orat
or&
Oil
Tem
pera
ture
(⁰C)
Heat transfer rate (kW)
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