Tổng quan về chu trình làm lạnh và chu trình bơm nhiệt của môi chất Co2 trên tới hạn Môi chất CO2: Là chất làm lạnh tự nhiên, an toàn, bền vững và thân thiện môi trường, không làm suy giảm tầng ozone và có tiềm năng làm nóng toàn cầu (GWP) thấp. Ứng dụng: Được dùng trong điều hòa không khí ô tô, bơm nhiệt nước nóng, và hệ thống làm lạnh thương mại, đặc biệt là trong các hệ thống sử dụng chu trình siêu tới hạn. Chu trình siêu tới hạn: CO2 vận hành ở áp suất cao vượt quá điểm tới hạn, cho hiệu suất nhiệt cao hơn so với các chất làm lạnh truyền thống. Lịch sử phát triển: CO2 từng phổ biến trong các hệ thống làm lạnh nhưng bị thay thế bởi các chất tổng hợp. Gần đây, nó được nghiên cứu và sử dụng trở lại nhờ những ưu điểm về môi trường. Tính chất nhiệt động: CO2 có nhiệt dung riêng thay đổi mạnh gần nhiệt độ giả tới hạn, mang lại khả năng trao đổi nhiệt cao. Công nghệ cải tiến: Bao gồm các chu trình có trao đổi nhiệt bên trong (IHE), nén hai giai đoạn, và sử dụng máy giãn nở để cải thiện hiệu suất. Thách thức: Áp suất vận hành cao và yêu cầu kỹ thuật phức tạp hơn so với các hệ thống truyền thống.
Trang 1A review of transcritical carbon dioxide heat pump and refrigeration
cycles
Yitai Maa, Zhongyan Liua, Hua Tianb,*
a Key Lab of Efficient Utilization of Low and Medium Grade Energy, Ministry of Education of China, Tianjin University, China
b State Key Lab of Engines, Tianjin University, China
a r t i c l e i n f o
Article history:
Received 26 November 2012
Received in revised form
5 March 2013
Accepted 7 March 2013
Available online 13 April 2013
Keywords:
Transcritical cycle
Carbon dioxide
Expander
Natural refrigerant
a b s t r a c t
As a natural refrigerant, carbon dioxide is safe, economic and environmentally sustainable which can be used in heat pump and refrigeration systems especially in transcritical cycles From the early 1990s, in which the carbon dioxide transcritical cycle began, theoretical and experimental researches, as well as commercial system development, has improved to make the transcritical system performance to a level similar to that of conventional heat pump systems This paper presents an overview of transcritical carbon dioxide heat pump and refrigeration systems The paper introduces a summary of the history and main application of carbon dioxide’s use as a refrigerant firstly Secondly, the properties of supercritical pure carbon dioxide and that containing polyalkylene glycol (PAG) lubricants are analyzed and reviewed
In Section3the paper began with an analysis of some special characteristics of the basic carbon dioxide transcritical cycle such as the optimum system high pressure and so on, and then followed by a per-formance analysis and comparison of several novel transcritical cycles The studyfinally presents a re-view of research on transcritical carbon dioxide heat pump systems, which covers the main components and research hotspots, such as heat transfer and expander
Ó 2013 Elsevier Ltd All rights reserved
0 Introduction
Climate change is a major worldwide concern with potentially
dramatic impact on developing and industrialized countries alike
In heating, ventilation, air conditioning and refrigeration (HVAC&R)
industry, due to the limitation of ozone depletion effect and
greenhouse effect, refrigerants have faced upgrading again Ozone
depletion potential (ODP) is a relative measure of a kind of gas’s
ozone depletion effect in comparison to an equal mass of R11
Global warming potential (GWP) is a relative measure of the heat
trapping effect of a gas in comparison to an equal mass of carbon
dioxide over a given period in the atmosphere[1] In 1974, Rowland
and Molina announced their research result that chlorine and
bromine discharged with compounds will move into stratosphere
and destroy ozone, which can shield harmful ultraviolet ray in solar
radiation[2] The result drew global attention Finally, 1987 saw the
signing of Montreal Protocol, which was about reducing the
pro-duction and application of ozone depleting substance (ODS) The
protocol only incorporated chlorofluorocarbons (CFCs) at first, and later expanded to hydrochlorofluorocarbons (HCFCs) Most devel-oped countries had eliminated the application of CFCs in new refrigeration equipment until 1996, while developing countries had completed the elimination until 2010 According to the latest reg-ulations of the 19th Annual Conference of Montreal Protocol held in
2007[3], for developing countries, the consumption and produc-tion of HCFCs is freezed to baseline in 2013, HCFCs consumpproduc-tion is reduced by 10%, 35% and 67.5% from baseline in 2015, 2020 and
2025 respectively, and completely phased out by 2030 but with 2.5% maintenance quantity As a substitute of CFC and HCFC, hydrofluorocarbons (HFCs), such as R134a, has been widely used in HVAC&R systems However, it’s reached a consensus globally that the effort of mitigating climate change is focused on the reduction
of greenhouse gas emissions The goal of reducing greenhouse gas emissions has become a major driver of technology Many re-frigerants used in HVAC&R systems are potent greenhouse gases R134a, for instance, has a GWP of 1300 over a 100 year time span [4] When securely contained in a properly operated system, re-frigerants do not impact climate change However, system leakage and improper recovery of refrigerants during repairs or at the end
of life result in these harmful gases entering the atmosphere Some climatologists have called for a complete worldwide phase-out of
* Corresponding author.
E-mail addresses: ytma@tju.edu.cn (Y Ma), thtju@tju.edu.cn , jeviwntian@
tju.edu.cn (H Tian).
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j o u r n a l h o me p a g e : w w w e l s e v i e r c o m/ l o ca t e / e n e r g y
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Energy 55 (2013) 156e172
Trang 2refrigerants with high GWP similar to the phase-out of ozone
depleting substances enacted under the Montreal Protocol in 1987
The European Union already approved the scheduled phase-out of
mobile air conditioning systems using refrigerants with GWP
higher than 150 This directive was ratified in 2007 and went into
effect at the beginning of 2008[5,6] In addition, some countries
banned the use of HFCs in large systems, explicitly prohibiting the
use of HFCs in cold water chiller, or imposing free tax on HFCs of
low GWP Florida enforced the use of refrigerants with low GWP in
new traffic systems, and banned the use of leak system through
granting no registration to technology
Therefore, refrigerants face a new round of upgrading In 2005,
in thefield of new synthetic organic refrigerants, globally at least
three refrigerant manufacturers issued the patents of new
refrig-erant with the GWP upper limit of 150[7e10] In the new synthetic
refrigerants that have been popularized, hydrofluoroolefins (HFOs),
like R1234yf with zero ODP and low GWP and a substitute for
R134a, and derivative R1234ze, is main representative Among
them, R1234yf is a propylene derivative with 0 ODP and 4 GWP,
featuring non-toxic, micro-combustion and thermodynamic
prop-erties including working temperature, pressure, cooling capacity
and coefficient of performance parameters similar to R134a, and
also completely compatible with polyalkylene glycol (PAG) oil The
newfluid can be directly changed, and has been tested on
pre-liminary air conditioning vehicle, while the original air
condition-ing system for R134a almost doesn’t need to change It seemed to us
that HFO as a new hope interfered with CO2application So that,
many companies want to use this new direct alternative refrigerant
in near future However, R1234yf has many isomers, which lead to
complex production process, high cost and limitation of system
performance Besides, its micro-combustion is also a thorny
prob-lem Since most synthetic organic refrigerants are eventually
dis-charged to the environment, it’s hard to predict its long-term
impact on the earth ecosystem environment In view of long-term
environmental safety, one potential substitute refrigerant is carbon
dioxide, a natural refrigerant that has negligible impact on climate
change Carbon dioxide (CO2) used in HVAC&R systems has a zero
impact on climate change, since it’s recovered from other industrial
processes[11] Furthermore, CO2is non-toxic, non-flammable and
no-corrosive, and it has no impact on the ozone layer It is
inex-pensive and readily available CO2’s performance as a refrigerant in
heat pump systems is also competitive compared with refrigerants
currently in use[11e13] The unreliability of HFO could be fulfilled a
basic principle: Anything not existing in nature environment will
be harmful to nature
1 History and main application of CO2’s use as a refrigerant
The concept of CO2vapor compression refrigeration system was
first proposed by Alexander Catlin Twining in 1850, but CO2was
first used actually in a vapor compression system to produce ice by
Thaddeus Lowe in 1866 Later on, there had significant progress in
using CO2 as a refrigerant In Brucevick, Germany, Franz
Wind-hausen designed a CO2compressor Then British J&E Hall Company
bought the patent, improved, and began producing CO2compressor
in 1890 Thefirst product was successfully installed on a British
merchant ship called Highland Chief According to statistics, in
1900, 37% of the worldwide 356 ships used air refrigeration, 37%
used ammonia absorption refrigeration, and the rest 25% used CO2
vapor compression refrigeration After World War I, Voorhees[14]
developed a multipurpose CO2refrigeration system, and Linde also
applied CO2to two-stage compression unit By 1930, 80% ships had
used CO2compressor refrigeration, and the remaining 20% used
ammonia absorption refrigeration It was relatively late for CO2
compressor to be used in air-conditioning systems In around 1919,
CO2compressor began to be widely used in air-conditioning sys-tems, making the environment comfortable Then in 1919 it was used in the air conditioning system of theaters and department stores; in 1920 it was expanded to churches; in 1925 solid CO2cycle was used in air conditioning; in 1927, it was used in the air con-ditioning system of offices; in 1930 it extended to residential air conditioning; and later on in various commercial buildings and public facilities[15e17]
However, due to poor technology at that time, CO2’s low critical temperature (31.1C) and high critical pressure (7.37 MPa), and its cooling medium often being low-temperature groundwater or seawater, it turned out to be a subcritical cycle for CO2system and resulted in low refrigeration efficiency In 1928, when Midgley, etc [18], presented CFCs refrigerants such as R12, the use of CO2 in refrigeration finally ended up replaced by synthetic refrigerants R12 and R11 werefirst introduced for commercial use in 1931 and
1932 respectively [19] They were non-toxic, non-flammable and being operated efficiently over a range of temperatures Synthetic refrigerants had begun to replace CO2and come to dominate non-industrial systems by the 1950s and the 1960s
Interest in CO2was renewed in the early 1990s in part due to the phase-out of ozone depleting refrigerants Gustav Lorentzen from Norway received high credit for the new attention to CO2, and considered it an irreplaceable refrigerant Lorentzen published a patent application for a transcritical CO2 automotive air condi-tioning system in 1990[20] Lorentzen’s transcritical cycle solved the problem of capacity and efficiency loss that subcritical systems have when operating with heat rejection temperatures near the critical point Technological and manufacturing improvements make it possible now achieve the high pressures required for transcritical operation One of thefirst transcritical CO2 systems was a prototype automotive air conditioning system built and tested by Lorentzen and Pettersen [21] The system was further reported by Pettersen[22] Its performance was similar to that of a R12 system, which encouraged further development of transcritical
CO2system
Researches on CO2 refrigeration, air-conditioning and heat pump systems continue, while some CO2 systems have already been successfully commercialized in three aspects as follows The first aspect is the application in automotive air conditioning Applying CO2supercritical cycle in automotive air conditioning has unique advantages: compression ratio is very low, so compressor
efficiency is relatively high; volumetric refrigeration capacity is as high as 5 times of R22, which makes the system size small and suitable for automotive; supercritical fluids have excellent heat transfer and thermodynamic properties, heat exchanger has high
efficiency and the irreversible exergy loss of high side is low, making the whole air conditioning highly efficient and fully com-parable with traditional refrigerants (such as R12, R22, etc.) and its alternatives (such as McQuay, R410A, etc.) In addition, CO2’s properties used in heat pump can also fill the gap that modern automotive air conditioning can’t provide enough heat to carriage
in winter Besides thefirst researches by Lorentzen and Pettersen [21,22] described above, J Kohler, etc., researchers in the Kassel university of Germany[23e26], carried out research on the appli-cation of CO2in car air conditioning and heat pump, and in August,
1996, they performed prototype experiment on thefirst bus air conditioning The researches showed that, considering weight, safety, reliability, tightness, system performance and refrigeration
efficiency together, CO2supercritical cycle system can be a reliable system for air conditioning system of vehicles Brown J S etc.[27] evaluated performance merits of CO2 and R134a automotive air conditioning systems using semi-theoretical cycle models The CO2 system was additionally equipped with a liquid-line/suction-line heat exchanger The entropy generation calculations indicated
Trang 3that CO2 has a somewhat better performance than R134a in the
evaporator With the supports of many American refrigeration and
air conditioning companies, C.W Bullard et al.[28e30], researchers
in Air Condition Refrigeration Center (ACRC) of Illinois University
(UIUC), established a homologous experiment bench of automotive
air conditioning, researched properties and layout characteristics of
system regenerator, compared with other substitutes such as
R134a, R410A, and acquired good result J Hols et al [31] of
Denmark established an experiment bench of CO2 supercritical
automotive air conditioning in Danfoss and made an analysis of and
a research on regulation component of system H Quack et al.[32],
researchers in Dresden University of Germany, researched the
feasibility of applying CO2 supercritical cycle in train air
condi-tioning refrigeration system
The second aspect is the application to heat pump water
heater In CO2supercritical cycle, there is a great temperature glide
in CO2 exothermic process, so it’s for water heating, and outlet
water temperature is relatively high As a result, CO2 heat pump
has a high efficiency P Neksa, J Petterson et al [33,34],
re-searchers in SINTEF Research Institute of Norway, first made
theoretical and experimental researches on the properties of
wa-terewater heat pump and system design In 1996, they completed
thefirst experiment bench of CO2heat pump water heater with a
power of 50 kW in SINTEF Research Institute The experimental
results showed when the evaporating temperature was 0C, water
temperature could be heated from 9 to 60C and the coefficient of
heat pump could reach 4.3 Meanwhile, compared with electric
water heater and gas water heater, its energy consumption could
be reduced by 75% Besides, CO2 heat pump water heater has a
more prominent advantage over traditional heaters, since it’s easy
to provide hot water of 90C The CO2heat pump water heater has
been developed most rapidly in Japan, known as the“ecological
cute” (ECO CUTE) Introduced in 2001, over one million Eco Cute
water heaters had been sold by 2007[35]and sales topped two
million in October, 2009[36]
The third aspect is the application to commercial refrigeration
system Vending machine using a transcritical CO2 refrigeration
cycle is becoming more common in Japan and throughout Europe
In December, 2009, the Coca Cola Company announced it would
phase outfluorinated refrigerants in all new vending machines by
2015, switching primarily to CO2systems[37] Besides, due to its
great transport performance, security and thermodynamic
prop-erties, CO2 can be used as a coolant or used in cascade system,
which can solve the problems other flammable refrigerants face
and reduce pump power to achieve the effect of energy saving
Therefore, CO2commonly serves as low temperature refrigerant in
cascade type industrial refrigeration systems, and has been used
increasingly as a secondaryfluid in food display applications where
harmful refrigerants must be kept separate for safety reason[19] In
Europe, using CO2as a low temperature refrigerant, some cascade
refrigeration systems were established in supermarkets, and
operation conditions showed it was technically feasible
2 Properties of supercritical CO2
On account of CO2’s great vaporizing latent heat and quite high volumetric refrigeration capacity (at 0C, its volumetric refrigera-tion capacity is 1.58 times of that of NH3, 5.12 times of R22 and 8.25 times of R12), the size of compressor and other components can be smaller The operation viscosity of CO2is low, equal to only 5.2% of
NH3, 23.8% of R12 at 0C, which leads to the result that even at a relatively lowflow rate, it’s easy to produce turbulent flow, thus leads to a good heat transfer performance Due to its high con-ductivity coefficient, small thermal resistance and ratio of liquid density to vapor density, after being throttled, the distribution of refrigerants in each circuit is uniform, and all tubes can be used effectively As a result, it can significantly improve the flow and heat transfer properties, greatly reduce the size of pipeline and heat exchanger, so that the whole system is very compact In addition, adiabatic index K is high, equal to 1.30, and compression ratio is about 2.5e3.0, lower than that of other refrigeration system Vol-ume efficiency is relatively high Table 1 shows thermophysical properties of several common refrigerants
2.1 Basic properties of supercritical CO2 Supercritical CO2is a kind offluid with high density and double-characteristic of both gas and liquid, namely its density is higher than that of gas and close to that of liquid Its viscosity is similar to that of gas but far less than liquid viscosity Its diffusion coefficient
is close to that of gas and far greater than the coefficient of liquid, so
it has goodflowability and transmission characteristics CO2
spe-cific heat varies abruptly near pseudocritical temperature but changes slowly with temperature away from pseudocritical point Fig 1shows the relation between specific heat and temperature of
CO2under different supercritical pressures As can be seen, under different supercritical pressures, the position where specific heat changes greatly is basically near pseudocritical temperature, so it’s wise to choose the region with high specific heat when designing
CO2 gas cooler Moreover, as the pressure increasing, the corre-sponding temperature of achieving the highest specific heat in-creases and the value of highest specific heat decreases
Fig 2shows that CO2 density and enthalpy change with the change of temperature Obviously, within the same temperature range, both change rates of density and enthalpy are very rapid Under a given supercritical pressure, density declines with the in-crease of temperature, while under a given temperature, density increases with increasing pressure When CO2 fluid is near the critical point, its density is very sensitive to the changes of pressure and temperature, namely, tiny change of pressure or temperature can result in a dramatic change of density A lot of researches confirmed that there exists heterogeneity of local density among supercritical fluid, whose unique properties stem from high pressure-sensitivity of density Conductivity coefficient has the same change rule with density The rule of viscosity change is a Table 1
Comparison among thermophysical properties of several common refrigerants.
Parameter of saturated state 0 C 10 C 0 C 10 C 0 C 10 C 0 C 10 C 0 C 10 C
Liquid density (kg/m 3 ) 928.1 861.5 1295.2 1261.5 1396.9 1363.8 1281.5 1246.7 1177 1144
Specific heat of liquid (kJ/kg/K) 2.54 3.00 1.31 1.34 0.92 0.93 1.17 1.20 1.26 1.29 Specific heat of vapor (kJ/kg/K) 1.87 2.56 0.90 0.96 0.65 0.68 0.74 0.79 0.93 0.97 Liquid viscosity (mPa s) 104.5 86.4 330.53 294.21 247.97 231.16 217.8 195.5 219.7 193.7 Vapor viscosity (mPa s) 14.8 16.0 10.95 11.42 11.70 12.18 11.4 11.9 10.84 11.23 Thermal conductivity of liquid (W/m/K) 0.11 0.0993 0.0994 0.0947 0.0783 0.0746 0.1000 0.0947 0.0751 0.0713 Thermal conductivity of vapor (W/m/K) 0.0208 0.0255 0.0121 0.0130 0.0858 0.0917 0.095 0.0101 0.0091 0.0098
Y Ma et al / Energy 55 (2013) 156e172 158
Trang 4little different from that but very close They all change greatly at
some temperature and pressure intervals By calculation, the
tem-peratures at which thermal conductivity, viscosity and density
possess the greatest change are almost equal
2.2 Properties of supercritical CO2containing PAG lubricants
Lubricants play some role in lubricating working parts, reducing
compressor noise, sealing and cooling friction surface in
refriger-ation system, and have a significant impact on working
perfor-mance and the life of cooling equipment Generally speaking,
lubricants need good thermal stability, chemical stability, low
temperature flowability and proper viscosity Falex test [38]
showed that in order from good to bad, performances of
lubri-cants containing 10% CO2 are: PAG > polyol ester (POE) >
alkylbenzene> polyalphaolefin (PAO) Therefore, PAG/CO2has the
best lubrication performance and the lowest wear level
Mutual solubility with refrigerant is an important measure to
choose lubricants At the same temperature, the solubility of CO2
increases as pressure increases, while at the same pressure, it
de-creases with the increase of temperature So at low temperature,
the solubility of CO2in PAG is high, and the mixture has a poor
lubrication performance In contrast, at high temperature, the
solubility is low, giving good lubrication performance Thus it can
be seen, the lower the pressure as well as the higher the temper-ature is, the better lubrication performance PAG/CO2has
Formula (1)proposed by Baustian[39]tells how to acquire the density of refrigerant-oil mixture Using it, the densities of super-critical CO2eoil mixture in different states can be obtained, as shown inFig 3 Experimenting on R113/naphthenic oil, Jensen and Jackman[40]compared the experiment data with theoretical data, and found the formula is feasible, which was also recommended in ASHRAE handbook (1984) Adding lubricants in supercritical CO2, the density of the mixture becomes slightly higher, meaning trace amounts of lubricant has minimal effect on density Besides, den-sity change trend of the mixture agrees with that of pure super-critical CO2 Near the critical point, the density is choppy
1 ð1 uoÞð1 rr=roÞ uo ¼ mo=ðmoþ mrÞ (1)
ByFormula (2)proposed by Jensen and Jackman[40]to calcu-late the specific heat of mixture., constantepressure specific heat of supercritical CO2eoil mixture in different states can be obtained, as shown in Fig 4 Constantepressure specific heat of supercritical
CO2is slightly higher than that of mixture, and the change trend that specific heat of mixture changes with the change of temper-ature and pressure is basically consistent with the pure one That is
to say, trace amounts of lubricant has little effect on constante
100
300
500
700
900
1100
-350 -300 -250 -200 -150 -100 -50 0 50
Temperature ( o C)
Density Enthalpy
Fig 3 Variable of the density of pure CO 2 and CO 2 /PAG with temperature.
0 2 4 6 8 10 12 14 16
PAG/CO2-7.38MPa CO2-7.38MPa PAG/CO2-8MPa CO2-8MPa PAG/CO2-10MPa CO2-10MPa PAG/CO2-12MPa CO2-12MPa
Temperature ( ) Fig 4 Specific heat of supercritical pure CO and COePAG mixture.
2
4
6
8
10
12
Temperature(°C)
8MPa 9MPa 10MPa
Fig 1 Variable of the specific heat with temperature.
Trang 5pressure specific heat of supercritical CO2, so the change trend of
mixture specific heat cannot be determined by it
ByFormula (3)proposed by Filippov to calculate conductivity
coefficient of mixture, the conductivity coefficient of supercritical
CO2eoil mixture in different states can be obtained, as shown in
Fig 5 Both Baustian[39]and Conde[41]recommended the above
calculation formula Jensen and Jackman[40]carried out thermal
conductivity experiment on lubricanterefrigerant mixture
con-taining 10% lubricant, and found the conductivity of mixture
increased by less than 3%, generally in agreement with the
calcu-lated value of empirical formula FromFig 5, the thermal
conduc-tivity of supercritical CO2 is slightly worse than that of mixture
containing trace amounts of PAG Thus trace amounts of lubricant
have little effect on the thermal conductivity of supercritical CO2
km ¼ krurþ koilð1 urÞ 0:72ð1 urÞurðkoil krÞ (3)
3 Analysis of CO2transcritical cycle
3.1 Characteristics analysis of basic CO2transcritical cycle (BCTC)
Fig 6 shows the Tes diagram of basic CO2 transcritical cycle
system Process 1e2 is an isentropic compression process, process
2e3 an exothermic process under a constant pressure, process 3e
4 h an adiabatic expansion process and process 4h-1 an evaporation
process under certain pressure Compared with traditional
subcritical cycle, CO2does give out heat not in two-phase region
but in the region where it approaches or exceeds critical point The
temperature in this process is constantly changed, and has a biggish slippage The process can be matched with changing heat source, forming a special Lorentz cycle, and it has a good efficiency 3.1.1 Optimum system high pressure
In traditional subcritical cycle, the enthalpy value of condenser outlet is just the function of temperature (or pressure), without the case of subcooling That is to say, as long as evaporation tempera-ture and condensing temperatempera-ture are determined, the performance
of this cycle can be basically defined However, in supercritical re-gion of CO2transcritical cycle, temperature and pressure are two independent variables When the outlet temperature of gas cooler
is constant, the high pressure of the system also affects fluid enthalpy With the increase of high pressure, isotherm becomes more and more steep Therefore, the increment of refrigerating capacity decreases with the increase of high pressure And accordingly, isentropic line is almost straight line, which leads to the linearly increase of compressor work as the high pressure in-creases So when other parameters are constant, along with the change of the system high pressure, there must exist maximum point in COP of circulation system, and the corresponding high pressure is called optimum high pressure Popt.Fig 7 shows the refrigerating capacity of CO2transcritical basic cycle qe, compressor consumption work wcand cycle COP curve changing with high pressure Similarly, several simulations revealed that an optimum system high pressure exists for a given outlet temperature of gas cooler[42e45] Kauf[42]correlated the optimum pressure in terms
of ambient temperature of air, which is the heat recoveryfluid for gas cooler Liao et al.[43]derived a correlation of optimum pressure based on evaporator temperature and gas cooler outlet tempera-ture Moreover, Sarkar et al.[44]used gas cooler outlet temperature and evaporator temperature to develop correlations of optimum pressure, system COP and optimal gas cooler inlet temperature Zhang et al.[45]analyzed the optimum discharge pressure using a system with two-stage expansion via throttling valves
3.1.2 Refrigerant temperature glide The difference between the CO2temperature at the inlet and outlet of gas cooler is typically greater than that during heat rejection by condensation This temperature difference is known as refrigerant temperature glide Several researches focused on the effect of refrigerant temperature glide on basic transcritical system performance An experimental study by Jiang et al.[46]indicated that COP increases as water inlet temperature decreases A watere water CO2 transcritical heat pump system was tested by heating water from 15C, 20C, 25C to 65C The results showed that COP
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
PAG/CO2-7.38M Pa CO2-7.38M Pa PAG/CO2-8M Pa CO2-8M Pa PAG/CO2-10M Pa CO2-10M Pa PAG/CO2-12M Pa CO2-12M Pa
Temperature ( ) Fig 5 Conductivity of supercritical pure CO 2 and CO 2 ePAG mixture.
2
3
1 4h
T
s Fig 6 Tes diagram of CO
70000 8000 9000 10000 11000 12000 20
40 60 80 100 120 140
0 0.5 1 1.5 2 2.5 3 3.5
q e
q e
q e
COP
Y Ma et al / Energy 55 (2013) 156e172 160
Trang 6increased with 15% when water inlet temperature decreased from
25C to 15C A theoretical and experimental study by Cecchinato
et al.[47]showed that the COP of a basic transcritical heap pump
system increased significantly as water temperature glide
increased An air-source CO2heat pump water heater was tested by
heating water from 15C to 45C and from 40C to 45C The COP
was 56e84% higher for the 15Ce45C temperature rise The
re-sults from a simulation by Laipradit et al.[48]showed that COP
increased as water inlet temperature decreased The reduced water
inlet temperature corresponds with a reduced refrigerant
temper-ature at the outlet of gas cooler and hence a greater refrigerant
temperature glide Fernandez et al.[49]tested the overall
perfor-mance of a CO2system connected to a storage tank under three
different heating conditions, which are initial heating of a tank
from 15 C to 57.2C, reheating to 57.2C after cooling due to
standby loss, and reheating to 57.2C after cold make-up water is
added to the tank Overall performance was the best under initial
heating condition, which has the greatest water temperature glide
since the entire tank must be heated from 15C to 57.2C, and the
worst under standby loss reheating Compared with initial tank
heating, the COPs were 30e40% lower under standby loss
reheating
3.2 Researches on novel CO2transcritical cycle
3.2.1 BCTC with an internal heat exchanger (IHE)
In the researches on novel CO2transcritical cycle, internal heat
exchanger (IHE) has drawn lots of concern IHE brings the discharge
vapor of gas cooler into thermal exchange with the discharge vapor
of evaporator Jiang and Ma[46]presented the comparison of CO2
transcritical cycle with and without IHE when the inlet
tempera-ture of gas cooler was 20C, as shown inFig 8 The heating COP of
the cycle with IHE is about 5e10% higher than the cycle without
IHE When the inlet and outlet temperatures of cooling water are
20C and 65C, the heating COP of the cycle with IHE can reach 2.7,
while that of the cycle without IHE can attain 2.53 At the same
time, the heating capacity of the system with IHE increases with
9%e13% Chen and Gu[50]declared that as IHE effectiveness
in-creases, optimum pressure decreases and COP increases Robinson
and Groll [51]obtained a 7% increase in COP when an IHE was
added to a basic transcritical CO2heat pump system Kim et al.[52]
simulated the effect of an IHE to optimize IHE size with respect to
gas cooler pressure The results indicated that under a certain gas
cooler pressure, COP improved up to 4% on average as IHE length
increased White et al.[53]compared the performance of a system
with an IHE and a system with a larger heat exchange area of gas cooler The increased area of gas cooler was equal to that of IHE The modification gave rise to the optimum gas cooler pressure from
11 MPa to 12.4 MPa, which in turn affected a 20% increase in heating capacity, though COP maintained unchanged The mass flow rate also increased, which is due to the higher density (due to lower temperature) of the vapor at the compressor inlet The au-thors attribute the capacity increase primarily to the increased CO2 flow rate
3.2.2 Two-stage compression transcritical cycle Two-stage CO2transcritical cycle can overcome the excessively high temperature of discharge in single-stage cycle, which essen-tially is the shortage of an over high “equivalent condensation temperature”; secondly, using two-stage cycle with intermediate cooling can reduce the work input of compressor; in addition, it can also solve the problem of low volumetric efficiency of compressor due to the serious workingfluid leakage caused by great pressure differential Consequently the performance of the system is improved[54] Cecchinato et al.[55]analyzed the performance of two-stage compression transcritical CO2 air conditioning system with intercooling Compared with BCTC system, two-stage compression cycle improved cooling COP with 9% The results also showed that the benefits of inter stage cooling are greatly improved by using a line heat exchanger Cavallini et al.[13]tested the cooling performance of a two-stage compression transcritical experimental system at different intercooler temperatures Compared with BCTC system, the two-stage compression cycle improved cooling COP by 21.1% Flash intercooling is an alternative way of cooling the refrigerant between compression stages, in which the inter-stage CO2temperature is reduced by mixing with expansion vapor in aflash tank Tian H and Ma YT et al.[56]did theoretical and experimental study on CO2trans-critical cycle with flash intercooling The calculated results indicated that there exists optimal high pressure and optimal intermediate pressure The tests
on the system were shown asFig 9 Under the optimal interme-diate pressure of 6.5 MPa, cooling COP and heating COP were both
at their maximum values, which were 2.5 and 3.5 respectively Sarkar and Agrawal [57] tested a two-stage compression tran-scritical cycle with an economizer The cooling COP improved by 47.3% over a basic conventional transcritical system Cecchinato
et al.[55]theoretically analyzed the same system with the results that a cooling COP increases 16.8e28.7% compared with a basic cycle Cho et al.[58]obtained an increase in cooling COP up to 16.5%
by the addition of an economizer to an experimental two-stage
45 50 55 60 65 70
1.6
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3.0
3.1
3.2
3.3
3.4
3.5
COPc(with IHX) COPc(without IHX)
COPh(with IHX) COPh(without IHX)
Outlet temperature of cooling water
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
tw_in=19.5 0.5 Gw=1.50 0.01m3/h
Intermediate pressure (MPa) Fig 9 Variable of COP with optimal intermediate pressure of two stage compression
Trang 7compression transcritical CO2system Agrawal and Bhattacharyya
[59]determined that, unlike other intercooling methods, two-stage
compression withflash intercooling reduced COP compared with
that of an analogous system with single-stage compression This is
due to the fact that mass flow rate through the second stage
compressor increases significantly Though the specific work of
compression at the second stage is reduced, the actual compression
work in the second stage increases Intermediate pressure was
found to have little impact on COP
3.2.3 CO2transcritical cycle with expander
The throttling loss of CO2transcritical cycle is much higher than
that of conventional working medium, so it is more meaningful to
use expansion machine to recover expansion work CO2
tran-scritical cycle with expander has more advantages than traditional
workingfluid cycle with expander, which is mainly decided by the
physical characteristics of refrigerant If assuming the calculated
conditions as follows: inlet and outlet temperatures of expander
are 40 C and 5 C, inlet pressure of expander is 10 MPa, the
calculation of the system using CO2and R134a as workingfluids
individually are compared inTable 2
As the table shows, under the given conditions, the expansion
ratio of R134a expander is higher than 16, which makes expander
design become very difficult and only the speed type expander can
be used While the expansion ratio of CO2expander is only 2.6,
which is closer to that of compressor It can be realized by using low
cost displacement expander, and it’s easier to connect compressor
with expander coaxially, which contributes to the improvement of
recovery work efficiency At the same time, the recovery work using
R134a expander accounted for only about 15% of compressor
con-sumption work, while the expansion work recovered by CO2
expander accounted for about 37% of compressor work, and it
in-creases as expander inlet temperature inin-creases Tian H and Ma YT
et al [60] calculated the CO2 transcritical cycle with expander,
which showed that COP has an increase tendency as the increase of
evaporating temperature The COP of CO2trans-critical cycle with
expander was about 6e10% higher than that of CO2trans-critical
cycle without expander averagely Yang JL and Ma YT et al.[61]
determined that the use of an expander in place of a
conven-tional expansion valve produced 50% decrease in exergy loss,
resulting in 30% improvement in system exergy efficiency The
expander reduced optimum gas cooler pressure and led to a 33%
higher cooling COP Huff and Radermacher [62] simulated the
performance of a CO2system with expander relative to a system
using R22 with expander and with expansion valve under the
conditions of 28C and 50C ambient temperatures and 100%, 80%,
60% and 0% expander efficiencies CO2system generally performed
better than R22 system does at lower ambient temperatures Kim
et al.[63]calculated a two-stage compression cycle with scroll type
expander, whose shaft was directly coupled to thefirst stage scroll
compressor At gas cooler exit temperature of 35C and compressor
inlet and outlet pressures of 3.5 MPa and 100 MPa respectively, the
total efficiency of the expander was 54.4% This resulted in an
in-crease of 8.6% in the cooling capacity and an inin-crease of 23.5% in
cooling COP A test of a CO2transcritical cycle with a two-cylinder
reciprocating piston expander under transcritical cooling condi-tion, carried out by Baek et al [64], showed that the isentropic
efficiency of the expander was 10.27%, and the expander improved cooling COP by 6.6% compared with the system using an expansion valve Nickl et al.[65]presented that cooling COP increased by 40% when using a three-stage piston expander to directly power the second stage compressor, compared with a transcritical CO2cycle with throttling valve
3.2.4 Comparison of several novel CO2transcritical cycles Table 3lists the structure components and the performance parameters of several novel CO2transcritical cycles The simulation conditions in the table are: system high pressure is 10 MPa; evaporating pressure under refrigerating condition and heating condition are 4.5 MPa and 3.5 MPa; superheat degree is 10C; outlet temperature of the gas cooler under refrigerating condition and heating condition are 35 C and 20 C; the efficiencies of compressor and expander are 0.7 and 0.6 respectively; theflow loss
in pipes is neglected In the table, it can be seen that if only simple structure is required, the SCE (Single-stage CO2transcritical cycle with expander) system can be used in small system to obtain higher
efficiency The two-stage CO2 transcritical cycle (STCEM) with intercooler system using expander to drive low stage compressor has the greatest performance But its structure is relatively com-plex, so it can be used in large and medium-sized systems
4 Researches on heat transfer in CO2transcritical cycle
CO2transcritical cycle is significantly characterized by the fact that exothermic process occurs in a supercritical pressure, and the heat transfer characteristics of CO2under supercritical pressure are greatly different from conventional workingfluids, which is mainly attributed to the vicinity of the critical region of the CO2thermal physical parameters changing sharply with the change of temper-ature and pressure, as shown in Section2 Unique heat transfer characteristics are determined by its special thermophysical properties
4.1 Supercritical CO2cooling heat transfer
Wei Dong [66] studied the supercritical CO2 cooling heat transfer performance of shell-and-tube gas cooler, and obtained oil-containing supercritical CO2cooling heatflux correlations ac-cording to the experimental results Pettersen et al.[67]studied the heat transfer and pressure drop characteristics of supercritical CO2
when being cooled in 0.79 mm inner diameter aluminum micro-channel, analyzed the impact of mass flow rate, heat flux and pressure on heat transfer coefficient and pressure drop, and compared with the commonly used correlation Olson[68], with experimental measurement, acquired the heat transfer coefficient
of CO2under supercritical pressure in a 10.9 mm diameter hori-zontal pipe The results showed that the massflow rate, heat flux, pressure and cooling waterflow rate all affect CO2heat transfer coefficient Liao et al.[69], using experimental measurement, ob-tained the heat transfer coefficient of supercritical CO2when being cooled in a miniature stainless steel pipe with different directions (horizontalflow, upward flow and downward flow) (as shown in Figs 10and11) The test tube diameter is respectively 0.5, 0.7, 1.1, 1.4, 1.55 and 2.16 mm; pressure range: 7.4e12 MPa, temperature:
20e110 C; massflow rate: 0.02e0.2 kg/min, the corresponding
Re¼ 104e2 105; Pr¼ 0.9e10 The research results showed that the difference between the heat transfer correlations applied to large diameter and the experimental data of small diameter is very great Therefore, a new heat transfer guideline correlation of
Table 2
Comparison of system using CO 2 and R134a as working fluids.
Working
fluid
Expansion ratio
of expander
Compression ratio
of compressor
The ratio of expansion work to compression work
Y Ma et al / Energy 55 (2013) 156e172 162
Trang 8Table 3
Comparisons of several novel CO 2 transcritical cycles.
1 Single-stage CO 2
transcritical cycle
with expander (SCE)
2 Single-stage CO 2
transcritical cycle
with expander for
power generation
(SCE&GE)
3 Two compressors-single
expander CO 2 transcritical
cycle without splitter
(TCSCE)
4 Two compressors-single
expander CO 2 transcritical
cycle with splitter (TCSCES)
5 Two-stage CO 2 transcritical
cycle without intercooler
using expander to drive low
stage compressor (TCEL)
6 Two-stage CO 2 transcritical
cycle with intercooler using
expander to drive low stage
compressor (TCIEL)
(continued on next page)
Trang 9convective heat transfer about the thin pipe of supercritical CO2is
put forward based on experimental data
Yoon et al.[67]studied the cooling heat transfer of supercritical
CO2in a brass with 7.73 mm inner diameter The results showed
that, with the ongoing of cooling process, the heat transfer coef
fi-cient increased gradually and reached the maximum value, and
then declined, as shown in Fig 12 This is mainly because the
specific heat changes violently in the vicinity of near-critical region
Therefore, the heat transfer coefficient reaches the maximum value
in the vicinity of the quasi-critical temperature under different
pressures, and the peak value of the heat transfer coefficient
decreased with the increase of the pressure Based on the
experi-mental results, a new correlation was proposed, which was a
modification to the Baskov correlation, shown as Equation(4) As
many correlations require properties to be evaluated at the bulk
temperature and at the wall temperature, this correlation is
considered more applicable for engineering purposes, since it uses
the bulk temperature for evaluation of all properties
Pitla et al.[71], based on the numerical predictions and
exper-imental data, obtained a new heat transfer correlation calculation
of supercritical CO2cooling Their study about the effect of pressure
on heat transfer coefficient shows that the heat transfer coefficient
increases with the increase of the pressure whenfluid temperature
is higher than the quasi-critical temperature under higher
pres-sure; conversely, heat transfer coefficient drops with the increase
of the pressure Pitla et al.[72]also used the standard model to
carry out numerical heat transfer analysis of CO2heat exchanger
Liao and Zhao[73]used numerical calculation method to study the
heat transfer of laminarflow when supercritical CO2is heated and
cooled in vertical micro tube, and obtained the trend of velocity
distribution, temperature distribution, heat transfer coefficient,
Nusselt number and surface friction coefficient Hoo-Kyu Oh and
Tae-Guen Yu[74]carried out experimental study the
characteris-tics of heat transfer and pressure drop of supercritical CO2in casing
heat exchanger with spiral inner tube Internal diameter of brass:
4.55 mm; pressure range: 7.5e10 MPa; mass flow rate: 200e
800 kg/m2 s The results showed, within a certain temperature range, the heat transfer coefficient of supercritical CO2increases with the decrease of cooling pressure and increases with the in-crease of massflow; pressure drop decreases with the increase of cooling pressure and increases with the increase of massflow rate
He et al.[75]used the vortex viscousflow of low Reynolds number
to carry out the simulation calculation of supercritical CO2in ver-tical thin tube with diameter being 0.948 mm Simoes et al.[76] established a model of supercritical CO2 heat exchanger under turbulent conditions, considering the heat transfer resistance of hotfluid in inner stainless steel tube, and new correlation was proposed based on experimental data to control the error between calculation value and test value to be about 2.3% Shan et al.[77], using CFD, calculated the temperature distribution andflow-type
of the cold-wall reactor supercritical CO2 under natural convec-tion condiconvec-tions Asinari et al.[78] proposed a turbulence model based on the consideration of variable physical properties, and
calculated the heat transfer characteristics of CO2 under super-critical pressure when being cooled in thin tube or micro-channel, and found densityfluctuation showed less impact on heat transfer, but showed significant impact during the experiment Zoggia et al [79]analyzed air-cooled heat exchanger of micro-finned tube, and the study showed that the use of micro-finned air-cooled heat exchanger can 5%e7% increase of heat exchanger performance Spindler[80]reviewed and analyzed heat transfer correlation of supercritical CO2cooling heat exchange in single tube and multi-channel heat exchanger with various diameters The results showed that, the buoyancy affects a lot and the change in thermal properties are great with high heatflow density and near critical point Ishihara[81]studied the natural convection of su-percritical CO2at vertical cylindrical outer surface experimentally, and observed that cooling heat exchange and subcritical conden-sation heat transfer shared similar characteristics through visual means
Table 3 (continued )
7 Two-stage CO 2 transcritical
cycle with intercooler using
expander to drive High stage
compressor (TCIEH)
8 Two-stage CO 2 transcritical
cycle with splitter using two
stage expander to drive low
stage compressor (TCF-TEL)
Remarks: C-compressor; EX-expander; EV-evaporator; G-gas cooler; GE-generator; IC- intercooler; M-motor; S-segregator.
Nub ¼ aReb
bPrcb
rpc
rb
n
when Tb> Tpc; a ¼ 0:14; b ¼ 0:69; c ¼ 0:66; n ¼ 0 when Tb&Tpc; a ¼ 0:013; b ¼ 1:0; c ¼ 0:05; n ¼ 1:6
(4)
Y Ma et al / Energy 55 (2013) 156e172 164
Trang 10Dang and Hihara[82], after calculating the heat transfer
coef-ficient of supercritical CO2when being cooled and comparing them
with experimental measurements, proposed a new correlation as
shown inFig 13 Through the results we can see that, under certain
pressure, the heat transfer coefficient of supercritical CO2changes
quickly with the change of temperature, and drops with the
decrease of temperature after reaching its peak near quasi-critical
point, whose change trend is the same with the result of
litera-ture[70] Dang and Hihara[83]performed numerical simulation
analysis about laminarflow heat exchange of supercritical CO2in
tube The results showed that under the conditions of constant heat
flux density, when the fluid temperature Tf> Tccritical
tempera-ture, Nu reaches the peak; Nu reaches the minimum when Tf< Tc;
when Tf¼ Tc, the product of Nu and f (f-friction factor) reaches the
peak Oh and Son[84]compared the correlations of several recent
studies and found the correlation of Yoon et al to be one of the
most accurate for macro-channels Cheng et al.[85]reviewed the
pressure dropfindings of several authors and compared the results
Cheng et al concluded that the Blasius equation for friction factor
predicts the pressure drop of cooling supercritical CO2 in both
micro and macro channels with sufficient accuracy Huai et al
[86,87]measured heat transfer andflow characteristics on
super-critical CO2in 10 horizontal aluminum tubes with 1.31 mm inner
diameter, 7.4e8.5 Mpa pressure, 22e53C temperature and 113.7e
418.6 kg/(m2s) massflow rate The results showed that operating pressure and massflow rate are of great influence on the charac-teristics offlow and heat transfer Jiang Peixue et al.[88]performed numerical simulation and experimental study about heat transfer
of supercritical CO2in porous tube The numerical simulation re-sults showed that the numerical calculation of the convective heat transfer coefficient under the condition of local thermal equilib-rium is higher than that of local thermal non-equilibequilib-rium condi-tions; convective heat transfer coefficient increases with the increase of the particle diameter of the porous medium Experi-mental results showed that, when wall temperature is higher than critical temperature, the local heat transfer coefficient in porous tube along CO2direction gradually decreased, and inletfluid tem-perature, pressure, heatflux density and flow direction are of great impact on the convective heat transfer performance in the porous tube
4.2 Two-phase boiling heat transfer of CO2
An experiment was conducted to study the evaporation heat transfer of CO2in a horizontal smooth aluminum tube with 7 mm diameter by Bredesen et al [89] They found the heat transfer
Fig 10 Supercritical CO 2 heat transfer coefficient at various flow directions [69]
(d ¼ 1.4 mm, m ¼ 0.10 kg/min).
Fig 11 Supercritical CO 2 heat transfer coefficient at various flow directions [69]
¼ 0.7 mm, m ¼ 0.05 kg/min).
Temperature (°C) 0
5 10 15 20 25
-2 K
`-1 )
P MPa
7.5 7.7 8.0 8.2 8.5 8.8
Fig 12 Variable of heat transfer coefficient with different temperatures [67]
Fig 13 Heat transfer coefficient under different pressures