A review of carbon dioxide as a refrigerant in refrigeration technology

Abstract

Tough environmental laws and stringent government policies have revolutionised the refrigeration sector, especially concerning the cycle fluid known as the refrigerant. It has been observed that only natural refrigerants are environmentally benign. When other refrigerant qualities are considered, especially those relating to toxicity and flammability, carbon dioxide emerges as the best among the natural refrigerants. However, carbon dioxide based refrigerants are not without drawbacks. Even though the use of R744 - a carbon dioxide based refrigerant gas - has solved the direct effect of emissions on the environment, studies to investigate the indirect effects of these systems are needed. Improvement in existing technical solutions and the formulation of additional solutions to existing R744 refrigeration problems is paramount if this technology is to be accepted by all, especially in areas with warm climates. National policies geared to green technologies are important to clear the way and provide support for these technologies. It is clear that carbon dioxide is one of the best refrigerants and as environmental regulations become more intense, it will be the ultimate refrigerant of the future.

Full text

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South African Journal of Science
http://www.sajs.co.za
Volume 111 | Number 9/10
September/October 2015
Review Article Carbon dioxide as a refrigerant
Page 1 of 10
© 2015. The Author(s).
Published under a Creative
Commons Attribution Licence.
A review of carbon dioxide as a refrigerant in
refrigeration technology
AUTHORS:
Paul Maina1,2
Zhongjie Huan1
AFFILIATIONS:
1Department of Mechanical
Engineering, Tshwane University
of Technology, Pretoria, South
Africa
2Department of Mechanical
Engineering, Moi University,
Eldoret, Rift Valley, Kenya
CORRESPONDENCE TO:
Paul Maina
EMAIL:
mainap@tut.ac.za
POSTAL ADDRESS:
Department of Mechanical
Engineering, Tshwane University
of Technology, Private Bag X680,
Pretoria 0001, South Africa
DATES:
Received: 31 July 2014
Revised: 13 Nov. 2014
Accepted: 19 Jan. 2015
KEYWORDS:
environment; safety; heat pump;
energy efficiency; transcritical
HOW TO CITE:
Maina P, Huan Z. A review of
carbon dioxide as a refrigerant
in refrigeration technology. S
Afr J Sci. 2015;111(9/10),
Art. #2014-0258, 10 pages.
http://dx.doi.org/10.17159/
sajs.2015/20140258
Tough environmental laws and stringent government policies have revolutionised the refrigeration sector,
especially concerning the cycle fluid known as the refrigerant. It has been observed that only natural
refrigerants are environmentally benign. When other refrigerant qualities are considered, especially those
relating to toxicity and flammability, carbon dioxide emerges as the best among the natural refrigerants.
However, carbon dioxide based refrigerants are not without drawbacks. Even though the use of R744 –
a carbon dioxide based refrigerant gas – has solved the direct effect of emissions on the environment,
studies to investigate the indirect effects of these systems are needed. Improvement in existing technical
solutions and the formulation of additional solutions to existing R744 refrigeration problems is paramount if
this technology is to be accepted by all, especially in areas with warm climates. National policies geared to
green technologies are important to clear the way and provide suppor t for these technologies. It is clear that
carbon dioxide is one of the best refrigerants and as environmental regulations become more intense, it will
be the ultimate refrigerant of the future.
Introduction
Most refrigerators use a liquefiable vapour to transfer heat. This fluid is known as the refrigerant. Refrigerant
selection is a key design decision that influences the mechanical design of the refrigeration equipment. Factors
that must be considered in refrigerant selection include performance, safety, reliability, environmental acceptability
and cost. However, the primary requirements are safety, reliability and, nowadays, environmental friendliness (in
terms of ozone depletion and global warming potential). Table 1 summarises the properties of some refrigerants
and indicates that no progress has been made in terms of global warming potential (GWP) when switching from
hydrochlorofluorocarbons (HCFCs) to the hydrofluorocarbon (HFC) family. When securely contained in a properly
operating system, refrigerants do not impact climate change; however, system leaks and improper recovery of
refrigerants during repairs or at end of life result in these harmful gases entering the atmosphere. Furthermore,
during production of refrigerants, toxic and harmful wastes are released into the environment, which cause air,
water and land pollution in addition to releasing greenhouse gases. An alternative to HFCs is to apply naturally
occurring and ecologically safe substances, the so-called natural working fluids. The most important substances in
this category are hydrocarbons, ammonia and carbon dioxide, although when safety concerns are raised (toxicity
and flammability), R744, a carbon dioxide based refrigerant gas, becomes the best substitute.
Carbon dioxide (CO2) is a clear gas (at atmospheric conditions) without a particular smell when the concentration is
below suffocation level. When the concentration reaches toxic levels, it has a slightly pungent smell and somewhat
acidic taste. It has a higher density than air, which has its own advantages with respect to refrigeration and
disadvantages with respect to safety. CO2 is made both naturally and artificially – artificially through the burning of
fuel and other industrial processes.1,3 Approximately 0.04% of atmospheric air is CO2, thus CO2 is at a concentration
of approximately 380 parts per million (ppm) in air. Exhaled air from the body has a CO2 concentration of about 4%.
History of R744 as a refrigerant
Since the invention of the vapour-compression cycle by Evans and Perkins in 1834, R744 has been a candidate for
a refrigerant. Documented studies state that Alexander Twining was the first to propose R744 refrigeration using a
steam compression system in his British patent of 1850. However, Thaddeus Lowe was the first to actually build
a refrigerator running on R744 for ice production in 1866 after discovering its potential while using it in military
balloons. Carl Linde followed suit and built a better refrigerator running on R744 in 1881, just after Windhausen
had built the first R744 compressor in 1880. In 1884, W Raydt built a R744 refrigeration system for making ice
using a vapour compression mechanism while, at the same time, J Harrison was the first person to build a device
for manufacturing R744 purely for refrigeration use. The British company J and E Hall built the first R744 marine
refrigerator in 1890 using Windhausen’s compressor designs, while in the USA, continuous production of these
refrigerators was started in 1897, mainly by Kroeschell Bros. Ice Making Company. Owing to its safety aspects
when compared to other refrigerants during this period, R744 refrigerators grew in number, especially in the marine
sector. At the same time, its technology was improving. For example, in 1889, J and E Hall created a two-stage
R744 compressor which was more efficient, and in 1905, Voorhees created a flash chamber which was very
similar to a liquid-vapour separator.4-7
Calcium chloride solution was used in most refrigerators as a secondary fluid. The salt solution was cooled to
around -10 ºC (evaporation temperature of -15 ºC). Originally, the evaporator and condensers used galvanised steel
pipes, 32 mm in diameter for small refrigerators and 51 mm in diameter for large cold rooms. Tank and coil heat
exchangers were the first to be used, before tube in tube (double pipe) technology was introduced in 1902. The
shell and tube type were invented in the early 1930s and fin technology in the 1920s. Copper replaced steel pipes
during this decade too, with pipe diameters being reduced to 13 mm because of the increased heat transfer offered
by the fins and copper. Air circulating fans were introduced around this time for improved cooling, especially in
cold rooms. R744 used to cost around 9 cents per kg but the price increased to 12 cents per kg in the late 1920s.

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Volume 111 | Number 9/10
September/October 2015
Table 1: Properties of some refrigerants1,2
Refrigerant Critical temperature
(ºC)
Critical pressure
(bar)
Ozone depletion
potential
Global warming
potential (100 years)
Flammable or
explosive Toxicity
CFCs and HCFCs
R12 100.9 40.6 0.9 8100 No No
R22 96.2 49.8 0.055 1500 No No
Pure HFCs
R32 78.4 58.3 0 650 Yes No
R134a 101.1 40.7 0 1200 No No
R152a 113.5 45.2 0 140 Yes No
HFC mixtures
R404A 72.1 37.4 0 3300 No No
R407C 86.8 46.0 0 1600 No No
R410A 72.5 49.6 0 1900 No No
Natural refrigerants
Propane (R290) 96.8 42.5 0 3 Ye s No
Isobutane (R600a) 135.0 36.5 0 3 Yes No
Ammonia (R717) 132.2 113.5 0 0 Yes Ye s
Carbon dioxide (R744) 31.0 73.8 0 1 No No
CFC, chlorofluorocarbon; HCFC, hydrochlorofluorocarbon; HFC, hydrofluorocarbon
Figure 1: An old single cylinder R744 compressor.3
With the invention of compressors, vertical, cylinder-type compressors
of up to 42 kW (325 rpm) were first used but were later replaced by
horizontal compressors of up to 176 kW (120 rpm) in size. Both these
constructions were similar to the steam engine design. R744 required
heavy duty parts for valves, fittings, compressors and heat exchangers
as a result of its associated pressure. Refrigerator size increased up to
704 kW by 1916.7
The use of R744 air conditioners for comfort cooling began in the
1900s. Because of the toxicity and/or flammability of NH3 and SO2, R744
gained in popularity, especially in food-related industries (food markets
and eateries) and human comfort applications, e.g. in theatres, bars,
hospitals, ships and hotels. In 1900, only 25% of all ships were using
R744 as the refrigerant, but by the 1930s, this proportion had increased
to 80%. Although the ships used old technology, R744 equipment still
worked, albeit inefficiently, especially because they used a convectional
subcritical refrigeration cycle. In addition, there were sealing and capacity
loss problems related to R744 high pressures. These disadvantages
encouraged a search for safe and efficient (especially at high discharge
temperatures in warm climates) refrigerants which ended with the
discovery of chlorofluorocarbons (CFCs) in the 1930s. The invention of
CFCs, coupled with a lack of technological improvements from the R744
refrigeration industry, caused the decline in the use of R744. The last
large R744 refrigeration system was installed in 1935 for Commonwealth
Edison Company headquarters. The system was replaced by CFC
refrigerators 15 years later. CFCs eliminated the problems encountered
when using R744, such as the need for high pressure sealing, capacity
and efficiency loss and the high cost of components. Eventually by the
1950s, R744 refrigeration was completely phased out.4-7
After the discovery of the adverse effects of synthetic refrigerants in
the late 1980s, there was a renewed interest in R744. Professor Gustav
Lorentzen was the pioneer of the revival of R744 refrigeration in the
early 1990s, with many studies and ideas dedicated to its improvement.
He suggested that, because of the properties of R744, motor vehicle
air-conditioning systems (the leading sector in refrigerant leakage –
60% of all leakages8) and water heat pumps are best suited for R744
refrigeration.9 His idea was positively acknowledged with many motor
vehicle and water heat pump companies investing in R744 research.
Leading car manufacturers, such as Nissan, Bavarian Motor Works
AG (BMW) and DaimlerChrysler, have installed R744 air-conditioning
systems in new cars.4,10 Many R744 domestic heat pumps are
manufactured and marketed in Asia and Europe.11-13 Recently, there has
been a keen interest in using R744 in supermarkets and other commercial
refrigeration applications.14,15 Leading beverage companies like Coca-
Cola and PepsiCo have embarked on converting their vending machines
to use R744 as the refrigerant of choice.12 Also, R744 refrigeration has
been applied in residential and commercial buildings’ air-conditioning
systems with great promise.16 In short, there are currently many
application prospects for R744 refrigeration under investigation, most of
which are quite promising.
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R744 properties
General introduction
As indicated in Table 1, R744 is a non-toxic, non-flammable natural
refrigerant with an ozone depletion potential of zero and a GWP of 1.
Furthermore, it is widely and cheaply available as a constituent of
the atmosphere and as a result of industrial processes, especially
the ones involving fuel. Its critical temperature, however, is 31.1 ºC.
Low critical temperature means that R744 cannot be used effectively
in a convectional (subcritical) refrigeration cycle simply because
the condenser will not transfer heat above the critical temperature.
Therefore, the condenser will be ineffective and many losses may occur.
Furthermore, at temperatures that are near the critical temperature but
less than it, there is a drastic reduction on the vaporisation enthalpy
which leads to a reduction in heating capacity and reduced system
performance. Therefore, as Lorentzen has suggested, R744 can be
effectively used only in a transcritical cycle.9
Transcritical cycle
A transcritical cycle (Figure 2) is not limited by the critical temperature
because heat output is through a temperature glide. The condenser in
a transcritical cycle is replaced by a gas cooler because there is no
condensation taking place, but rather a gas cooling process. This
temperature glide is advantageous especially in applications such
as water heating and air heating (e.g. drying processes) because of
the associated efficiencies. The temperature range in which R744
refrigeration can operate in transcritical operation is the highest when
compared to other convectional refrigerants, i.e. -50 ºC to 120 ºC.4 The
only drawback with the transcritical cycle for R744 is the high pressure.
The critical pressure for R744 is 73.7 bars. If R744 is operated through a
transcritical cycle, then its high pressure will be above 73.7 bar, which is
quite high. This necessitates an equipment design that can handle such a
high pressure. The high pressure has its own advantages (e.g. compact
equipment and design) and disadvantages (costly equipment and safety
issues). However, with current technological advances, this pressure is
not a big concern.
Thermophysical properties
The high latent heat of vaporisation and volumetric heat transfer caused
by the high pressures involved means that R744 equipment components
can be designed in smaller size. Coupled with the fact that R744 has
very low viscosity, Reynolds number is reached even with a low flow
rate, which means that most flows are turbulent in nature and the heat
transfer rate is high. Apart from the flow nature, R744 has a small liquid
to vapour density ratio (especially near the critical point) which leads to
uniform and homogeneous distribution of the refrigerant in channels.
This homogeneity also adds to the high heat transfer rate. Furthermore,
proximity to the critical point and less pressure loss, especially in the gas
cooler, contributes to improved convective heat transfer. When the heat
transfer rate is high, the size of the heat exchangers can be drastically
reduced for the same amount of heat transfer to occur.6
The compressor size can also be reduced because of the associated
compression ratio involved. Even though the R744 transcritical cycle
deals with high operational pressures, the ratio between the high pressure
and low pressure is low when compared to other refrigerants. The ratio
is also closely associated with the high adiabatic index of R744 (which is
approximately 1.3). These factors lead to a compact, smaller and more
efficient compressor, ideal for applications for which space is limited,
e.g. in mobile air conditioners in cars.3 The R744 compressor efficiency
increase is also brought about by the low effect of valve pressure drops
and the re-expansion ratio it experiences. Furthermore, internal leakages
and piston blow-by losses are negligible when compared to those of
other compressors. Even though the compressor walls must be thicker
because of the pressures involved, R744 volumetric capacity dictates
small parts in the compressor and therefore the overall size of the
compressor is smaller when compared to compressors of convectional
refrigerants of the same capacity.5,6
Properties of R744 at a supercritical state are always in between those
at liquid state and those at gaseous state. The critical point, defined as
the point at which no liquefaction occurs above the critical pressure and
no gas is formed above the critical temperature, is peculiar in nature
because, near the critical point, there is always a sudden variation in
the properties. Specific heat, thermal conductivity, enthalpy, entropy,
density and viscosity undergo a major change as the critical point is
approached.17 Figure 3 shows the variation of specific heat at constant
pressure (cp) against temperature for R744 at several pressures.18 As
can be seen from Figure 3, the highest value of cp occurs at a pseudo-
critical temperature for that pressure. This property can be incorporated
into the design of gas coolers to maximise on their output.
R744’s surface tension is smaller than that of other refrigerants. The
significance of this observation is that surface tension affects the wetting
characteristics, flow characteristics and evaporation characteristics of
a fluid. Low surface tension might be positive in that the temperature
required to initiate and maintain nucleate boiling is reduced, but it can
also be negative because of drop formation and entrainment, especially
when there is reduced surface stability. On the other hand, the thermal
conductivity of R744 is considerably higher than that of most synthetic
refrigerants, while its viscosity is lower. A high thermal conductivity
means a higher heat transfer rate, while viscosity affects flow properties,
which in turn affects heat transfer and pressure drop. Therefore, the
thermophysical characteristics of R744 are favourable and encourage its
use as a refrigerant.5 Still, being relatively inert, R744 is compatible with
most lubricants and equipment materials, as documented in numerous
studies (even ones not related to refrigeration).
High (gas cooler) pressure
At supercritical state, the temperature and pressure of R744 are
independent of each other and thus can be regulated independently to
optimise output. Still, compressor input power is proportional to the high
pressure. As shown in Figure 2, if the gas cooling process occurs at a
constant pressure (process 2–3), the magnitude of the pressure will affect
the specific enthalpy. This pressure is not controlled by the cooling fluid
conditions (temperature and flow rates) as is the case with convectional
refrigerants; it is mostly controlled by the amount of refrigerant charge
present.8 As the pressure increases, there will be an initial increase in
heat output with a moderate increase in compressor power input, thus
there will be an overall increase in system efficiency. As the pressure is
increased further, it will reach a point at which the additional work input
is more than the additional heat output, and thus the efficiency of the
2
1
4
3
T
S
Figure 2: A theoretical transcritical cycle characterised by isentropic
compression (process 1–2), isobaric heat output (process
2–3), isenthalpic expansion (process 3–4) and isobaric heat
intake (process 4–1).
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refrigerator will begin to decrease. This behaviour is attributed to the
shape of isotherms (which affect the heat output) and isentropes (which
affect the compressor power) at pressures above the critical point. There
is always an optimum high pressure value which corresponds to certain
operational conditions. The optimum pressure depends on the gas
cooler outlet temperature, the evaporation temperature, the compressor
isentropic efficiency and the amount of refrigerant.19,20 Therefore, it is
paramount that there is a means of controlling the high pressure. The
optimum pressure can approximately be given by:
Popt = 2.6Texit + 8 Equation 1
where Popt is expressed in bars and Texit is the gas cooler exit temperature
in ºC.
Another important characteristic of R744 is the relationship between
its pressure and temperature. The vapour pressure of R744, apart
from being higher than other refrigerants (Table 2), also has a greater
variation per unit temperature change, especially near the critical point.
The slope of change in vapour pressure to change in temperature is
much steeper for R744 than for other refrigerants. This means that for
every unit change in pressure, there is a lesser change in temperature
with R744 as compared to other refrigerants. Therefore, the effect of
pressure loss is less severe and more tolerable in R744 than in other
refrigerants. The high vapour pressure also causes high vapour density
(Clausiu–Clapeyron’s relation). Because volumetric heat transfer is the
product of vapour density and latent heat of evaporation, a high vapour
density results in a high volumetric heat transfer.5 Furthermore, a high
vapour density results in a low velocity of R744 in pipes, thus resulting in
less of a pressure drop. In addition to high thermal conductivity of R744
(Table 2), high vapour density also allows the use of small components,
e.g. tubes, which results in lower radiation losses.21 Phase separation
characteristics between vapour and liquid phases are directly controlled
by phase density differences. R744 has a low density ratio, which is
necessary for a homogenous two-phase flow.4
Even though R744 has very favourable properties, especially when
used as a heat pump, its cycle is still affected by many losses. Given
an evaporation temperature and minimum heat rejection temperature,
the transcritical cycle is affected by higher throttling losses when
compared to the convectional subcritical cycle. These losses increase
the theoretical work done on the transcritical cycle. The throttling losses
in refrigeration are normally caused by the temperature difference in the
throttle device and the refrigerant properties. With the temperature being
set at a specific value, R744’s unique properties, especially near the
critical point (i.e. high liquid specific heat and low evaporation enthalpy),
increase the throttling losses and thus increase the compressor power
required. Therefore, even though compressor losses are lower in R744
machines, other factors tend to increase power consumption. Thus, it is
paramount to reduce the losses as much as possible if R744 technology
is to be fully embraced.5
Table 2: Properties of several common refrigerants21
Refrigerant
Evaporator temperature = -30 ºC Evaporator temperature = 0 ºC
Saturation pressure
(bar)
Liquid thermal
conductivity (Wm-1K-1)Vapour density (kg/m3)Saturation pressure
(bar)
Liquid thermal
conductivity (Wm-1K-1)Vapour density (kg/m3)
R22 1.64 0.1084 7.38 4.98 0.0947 21.23
R407C 1.62 0.1187 7.21 5.13 0.102 21.88
R134a 0.84 0.1058 4.43 2.93 0.092 14.43
R410a 2.79 0.1293 10.57 7.99 0.1099 30.63
R404a 2.05 0.0862 10.69 6.05 0.074 30.72
R744 14.28 0.1469 37.1 34.85 0.1104 97.65
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Vapour
saturated line
Cp (kJ/kg.K)
Temperature (°C)
20
15
10
5
0
20 30
60 bar 70 bar 80 bar
90 bar
100 bar
40 50 60
Figure 3: Variation of specific heat with temperature.

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Safety
Finally, R744 is considered non-toxic, although at concentrations
above 2% (about 5 kg of R744 in a 120-m3 room) it can start to
become harmful. It is colourless and odourless so it cannot easily be
detected; therefore, in installations for which a high amount of charge is
used (25 kg by European standards) and the ventilation is poor, R744
detectors should be installed. Furthermore, having a higher density than
air (R744 relative density is about 1.53 at atmospheric conditions), it will
tend to occupy lower areas so can prove to be more deadly when spilled
in a non-ventilated room. Therefore, the detector should be installed near
the floor. Table 3 gives the effects of R744 at various concentrations in
the air. To prevent an accumulation of R744 in an enclosed space with
a big installation:
• sufficient ventilation must be provided in the equipment room
• R744 containers must always be in an upright position, especially
when charging the system
In the event of a large R744 release, the equipment room should be
avoided until the concentration in the room is within allowable limits
(measured either through sensors or other means). In case of exposure,
the person should be taken into open air immediately.
Apart from toxicity issues, explosions as a result of the operational
pressures experienced in R744 equipment is also a safety concern.
The shock and flying fragments caused by a blast might cause injury and
harm. Apart from explosions caused by the pressure in R744 systems,
there is another severe type of explosion known as boiling liquid
expanding vapour explosion (or BLEVE), which usually occurs when a
vessel containing pressurised saturated liquid is rapidly depressurised as
a result of a crack or rupture and can cause a blast which is more severe
than the pressure blast. The rapid depressurisation leads to explosive
vaporisation and a sudden overpressure in the tank which might blow the
vessel.21 To prevent these catastrophes, the safety design of the R744
equipment should include the following5:
• The equipment should have an over-pressure release valve on both
the high pressure side and low pressure side.
• Components should be pressure tested at twice (or more) the
amount of normal operation pressure and temperature.
• The pressure test should incorporate the other operational effects
like fatigue due to pressure cycles, creep, vibration and corrosion
(especially if there is water present).
• All components should be designed with the highest pressure and
temperature (including a safety factor) the system may encounter
even in standstill mode.
When considering the size of the system versus the pressures involved,
the relative explosion energy in R744 systems is approximately the same
as in convectional systems. This is mainly because R744 systems tend
to have a smaller system size and less charge for the same capacity.8
R744 applications
Apart from being a refrigerant, R744 is used in many processes and
applications. However, in refrigeration, there are numerous applications
for R744, some of which have been commercialised.
Water heat pumps
Production of hot water is the best application for R744 heating
refrigerators (heat pumps) as the temperature slide in the transcritical
cycle suits the thermodynamic properties of water well.5 Very efficient
heat transfer and very high water temperatures are achieved with water
heating applications, especially when using a counterflow gas cooler.13
With heat pumps being the preferred water heating method when
compared to electricity or fuel-fired systems, governments and other
energy efficiency conscious bodies are encouraging their use. This
promotion comes after it was realised that water heat pumps with an
average coefficient of performance (COP) of 3 can reduce energy usage
by 67% when compared to electric heating, and by even more when
compared to fuel heating. The percentage energy saving of a heat pump
(ΔE) when compared to another heating system with an efficiency of η
is given by21:
ΔE = 1 1
η
COP Equation 2
where η for electric heaters is approximately 1 while η for fuel-fired
heaters varies from 0.5 to 0.95, depending on the equipment and the
type of fuel.
In addition, environmentalists are encouraging the use of heat pumps
which are environmentally friendly, such as R744 heat pumps.22 This is
one of the applications that the ‘father’ of R744 refrigeration re-invention,
Prof. Lorentzen, suggested for R744 as a refrigerant.9 In fact, it is this
sector that has seen major commercialisation of R744 refrigerators.
Since 2001, the production of commercial R744 heat pump units has
taken place under the general name of ECO CUTE in Japan (Figure 4).
These units, which are marketed both in Asia and Europe, have now
surpassed an annual production rate of 1 million, a growth encouraged
both by their high efficiency and by incentives from government and
environmental bodies.3,11,23 Other manufacturers have introduced similar
systems for residential, commercial and industrial use. The possibility
of efficiently producing hot water at 90 ºC and above is encouraging the
Table 3: Effect on humans of R744 at various concentrations1,21
Concentration (%) Effects on humans
0.1 Human comfort limit
0.5 8 h per day exposure limit
2 50% increase in breathing rate
3 100% increase in breathing rate; 10 min short-term exposure limit
5300% increase in breathing rate; headache and sweating may begin after about an hour. This is the immediate danger to life and health
concentration. An escape within 30 min will avoid irreversible health effects.
8–10 Headache after 10 to 15 min; dizziness, buzzing in the ears, blood pressure increase (because of high blood R744 content and lowered pH), high
pulse rate, excitation and nausea. Pungent smell and irritant to both nose and throat. This is the lowest lethal concentration.
10–18 Cramps similar to epileptic fits, loss of consciousness and shock after exposure of a few minutes
18–20 Stroke symptoms. Death can easily occur.
30 Rapid unconsciousness and convulsions
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industrial and commercial use of R744 heat pumps, especially in hotels,
hospitals and the food industry.5
With water heating being one of the highest energy uses, especially in
the residential sector (approximately 20%), coupled by the environmental
and energy efficiency advantages, R744 heat pump technology has great
future prospects.22 Lorentzen has described the possibility of using R744
heat pumps for both heating and cooling applications simultaneously.
These systems have high overall system efficiencies and can find
application in places where both refrigeration and hot water are needed,
for example, in hospitals, supermarkets, hotels, and the food processing
and chemical industries.9
Mobile air conditioning
Mobile air conditioning is the second application that Lorentzen
envisioned for R744 refrigeration systems. Compact R744 systems
coupled with good heat transfer characteristics between air and R744
encourage the use of these systems in a sector in which equipment
space and weight is limited while energy efficiency is paramount.9 Mobile
air conditioning is the largest consumer of refrigerants in the world,
followed by commercial refrigeration. Mobile air conditioning consumes
31% of the world’s refrigerant, which adds up to more than 150 000
t/year.15,24 On the same note, mobile air conditioning has the highest
leakage rate.21 This means a complete change to R744 will be highly
advantageous. Recent studies demonstrated the superiority of R744
systems when compared to commonly used HFC systems.25 With the
phasing out of HFC in progress, especially in European countries, R744
is the best alternative for mobile air conditioning. R744 air conditioning
will work especially well with fuel-efficient hybrid or electrical cars with
little waste heat available. With electrical cars specifically, if the air-
conditioning system is efficient enough, more energy will be used to
drive the car and thus more travelling distance will be covered before the
electricity runs out.8
The greatest disadvantage of R744 in mobile air-conditioning
applications is its high heat rejection temperature. However, in the new
technologically advanced vehicles, in which there is little or no excess
heat generation, this high temperature heat output can be effectively
used, especially in cold season, not only for human comfort but also for
heating essential vehicle parts like engine fluids. Common HFCs used in
mobile air conditioning do not perform well as heat pumps because of
their thermophysical properties.
As mobile air conditioning systems are prone to more maintenance
problems than stationary systems, logistical concerns arise as R744
systems are fairly new and are different from convectional systems.
There is a need for qualified technicians to handle the new technology,
which is made more complex in vehicles because of the limited weight
and space requirements coupled with integration of the air-conditioning
system and other electrical and mechanical systems of the car. These,
among other minor economic issues, need to be addressed before
R744 mobile air conditioning is fully embraced. Furthermore, frost
accumulation on evaporators presents a complication which has not
yet been solved effectively, especially in a mobile application. Despite
these challenges, R744 is proving to be an ideal refrigerant, capable
of providing high temperature heat instantly while requiring less air to
convey the heat. This makes it a hot research topic for complex mobile
environment control with improved efficiency.5
Commercial refrigeration
Commercial refrigeration is the equipment used by retail outlets to
display, hold or prepare food and beverages that customers purchase.
This equipment includes refrigerated display counters in supermarkets,
refrigerated vending machines, water coolers/heaters and ice generating
machines. Commercial refrigeration consumes about 28% of worldwide
refrigerants, thus is the second largest user of refrigerants.15,24 This
makes it one of the largest emitters of refrigerants into the environment
and accounts for approximately 37% of worldwide emissions. In 2002,
commercial refrigeration was responsible for more than 185 000 t
of leaked refrigerant into the atmosphere.26 Furthermore, the energy
utilisation in this sector is usually very high, necessitating a need for
efficient refrigeration systems.
Ironically, until the year 2000, R744 applications in commercial
refrigeration were not considered viable. The perception has since
changed with its use either as a heat transfer fluid, in a cascade
system, or on its own in either a transcritical cycle or a subcritical
cycle, depending on the environmental temperature. External factors
like safety requirements, extra tax on HFC systems and limitations
on the maximum amount of HFC charge that can be used on a single
system were the main reasons for R744 acceptability in commercial
refrigeration. It was first purely used indirectly as a heat transfer fluid,
then in cascade systems in conjunction with HFC at a reduced charge
or with hydrocarbons (HCs). With time, more skills and knowledge were
acquired and the cascade systems were replaced with fully transcritical
R744 systems. With the possibility of heat recovery (for space heating or
tap water heating), R744 commercial refrigeration has a great potential.
As the world accepts the use of R744 in supermarkets, studies show
that its associated costs and energy consumption are comparable to
Figure 4: Examples of EcoCute heat pumps.1
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other convectional refrigeration systems. Actually, it has been suggested
that R744 systems are the dominant technology of the future because of
their good thermophysical and safety properties.8
R744 technology is even being utilised in stand-alone bottle coolers
and both hot and cold vending machines. Major investment in the light
commercial sector is directed to R744 technology, with more than
85 000 units in operation worldwide.8 Most of these vending machines
utilise the transcritical cycle.
Other applications
Residential air conditioning
R744 air in air refrigerators has been the focus of investigations by
both research institutions and industry because of the high demand for
such equipment and the requirement for HFC alternatives. The annual
demand for residential air-conditioning units is more than 40 million
units and further market growth is expected.21 Air conditioning is
the second largest consumer of energy after water heating in most
residential areas. Environmental concerns in this application are more
focused on the indirect impacts of emissions due to energy use, than
on the direct impacts of refrigerant leaking. Therefore, energy efficiency
is paramount.5
There have been promising results with the application of R744 in
stationary air-conditioning systems. Units with a one-way refrigerant
circuit are working reliably and are more efficient than those with
complicated switching devices for redirecting refrigerant flow in different
seasons. Redirecting refrigerant flow involves additional valves and fittings
which increase the capital costs and efficiency losses through leakages
and pressure loss. One-way refrigerant flows (with the air flow redirected
in different seasons) are simple, flexible, compact and cost efficient.
Heat pump dryers
Compared to convectional dryers, heat pumps may reduce energy
consumption of food dryers by up to 80%. Other products which
need drying include wood, laundry and sewage sludge. The product
quality can be optimised more easily by using a heat pump because
of the availability of greater control options for different drying chamber
conditions. Water vapour from the products is absorbed by warm air,
which is heated by the gas cooler prior to the drying chamber. As this
air passes through the evaporator, it is dehumidified and cooled down
before returning to the gas cooler to be reheated. This closed air cycle
provides remarkable energy conservation, contributes tremendously
high energy efficiency (by participating in the heat transfer in both the
gas cooler and evaporator) and reduces environmental contaminants
and the unpleasant odour experienced in some drying processes, e.g.
drying of sewage sludge.21 Owing to the gliding temperature and better
temperature adaptation of heat exchangers, R744 heat pumps can
achieve substantial energy savings when used as a dryer. Higher air
temperatures can be easily and efficiently achieved in these systems,
thus enhancing the moisture extraction rate.27 As more efficient R744
equipment parts are produced (e.g. compressors and heat exchangers),
commercial heat pump dryers using R744 as the refrigerant are
becoming a possibility.21
Transport refrigeration
The efficient, reliable and compact characteristics of R744 equipment
encourage their application in the transport sector.8 R744 refrigeration
systems are considered as a replacement for HFC refrigeration systems,
both in public and goods (especially perishable) transport. These
transport modes include perishable goods trucks, public and goods
trains and ships. The relatively high density and capacity of R744 is
an added advantage in this sector. Furthermore, because of the global
nature of transport refrigeration, an environmentally benign alternative
is required which is available everywhere (even in rural areas where
the refrigerated trucks operate) and acceptable to all (some countries
in which the refrigerated ships dock have stringent environmental rules
which need to be strictly adhered to5).
Environment control units
Military operations usually require space conditioning for their temporary
shelters, command modules and vehicles, which should be able to
withstand the unique operational environment. The compactness of R744
equipment in addition to its availability globally has led to an increased
interest in R744 space-conditioning systems for the military.5,28
Future applications for R744
The potential of R744 refrigeration is far wider than the applications
discussed above. Experts asser t that R744 as a refrigerant shows
promise of capturing more markets, even outside the refrigeration
industry, although currently, it is the natural refrigerant with the widest
range of use.21 Some future potential uses of R744 as a refrigerant are
information technology (IT) equipment cooling, industrial heat pumps
and industrial waste heat recovery.
Information technology equipment cooling
The necessity of high performance data centres is increasing as the need
for information to be made available at anytime from anywhere to anyone
grows. For this to be possible, high-density data storage and processing
environments are required, and must be coupled with an efficient heat
absorption system which ensures that working environments are not
overheated by processes and that a smooth operation and flow of
information can exist. A high-efficiency cooling system with high COP
and automatic control is therefore paramount for both energy-saving
purposes and smooth operation of the data centre. R744 cooling
systems can provide the heat absorption process efficiently and reliably
without interfering with the IT equipment because R744 is electrically
benign when compared to traditional water-based systems. Another
advantage of R744 cooling systems is their compactness, which eases
their integration with the IT equipment and surrounding structures.
Furthermore, it is a form of waste heat recovery with the possibility of
simultaneous heating and cooling.21
Industrial heat pumps
R744 can be used for recovery of useful waste heat while at the same
time providing low temperature heat (up to 130 ºC) for industrial
processes. The application of R744 heat pumps in this sector therefore
has potential while providing energy savings. Heat pump drying is an
example of such an application already introduced into the market. Other
sectors with future potential are21:
• Washing processes: Warm to hot water is required in some
industrial processes, for example, textile washing, washing of food
and cosmetics production facilities. This hot water can easily and
efficiently be provided by R744 heat pumps.
• Process water: Warm process water is required in cer tain industries,
for example, the production of starch and other viscous chemicals.
• Process air: Some industries use warm to hot air instead of
water, for example, in the production of flake boards and some
plastic containers.
• Steaming processes: Steam is required in most manufacturing
processes as a heating media or just for cleansing purposes. A good
example where R744’s simultaneous heating and cooling can be
effectively applied is in the regeneration of activated carbon filters
in order to recover solvents. In this process, steam generated by the
R744 gas cooler vaporises the activated carbons which are loaded
with solvents. Subsequently, the steam absorbs the solvent, which is
condensed in the evaporator and extracted. Even though it might be
difficult to produce steam at the required temperature and pressure
with the current R744 heat pump technology, studies are being
conducted to investigate this possibility. Still, R744 can be used to
preheat feed water to the boilers, thus reducing the amount of energy
used in the boiler while improving overall system efficiency.
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Industrial waste heat recovery
Many industrial processes produce heat that is released to the
environment as waste because it cannot be reused effectively. This
waste heat can be used efficiently by a R744 heat pump to produce
useful heat, either for air conditioning or for tap water applications. This
waste heat recovery will reduce the energy costs of generating hot tap
water and/or building air. In addition, the industry will reduce its waste
product treatment by meeting temperature regulations concerning waste
products released into the environment and will reduce expenditure on
waste products while providing useful heat.
R744 applications in South Africa
South Africa is ranked 16th in the world in terms of total primary energy
consumption. This makes it the highest consumer of commercial energy
per capita in Africa and thus a relatively energy intensive country.
However, in terms of energy efficiency, South Africa performs poorly.
In fact, it was ranked among the bottom 50 of the 150 countries
compared in a study.29 Therefore, energy costs form a large part of
total production costs because the efficiency of utilisation is low. South
Africa produces about 2% of the world’s carbon emissions and thus
requires a check in its carbon emissions.30 With the threat of less energy
created than the current energy demand, an energy crisis is looming in
South Africa and energy efficiency is essential through incentives from
the government and carbon tax adoption. South Africa committed itself
at the Copenhagen Accord in 2009 that by 2020 it would reduce its
greenhouse gas emissions to 34% below its projected emission value
and by 2025 by 42%. By the look of its energy, environmental and
industrial policies, South Africa is trying to achieve this commitment
even though it is not legally binding, simply because of the numerous
economic and resource advantages that can be achieved by being a low
carbon economy.31
Refrigeration equipment (i.e. industrial, commercial and residential
equipment) accounts for a sizeable chunk of national energy
consumption. Unfortunately, in addition to the indirect effect of this
equipment on the environment, most of the equipment in South Africa,
and Africa in general, also still requires synthetic refrigerants. Ozone
depleting and global warming refrigerants like R22 are still common in
most refrigerators, while new equipment uses high GWP refrigerants like
R134a and R404a.32 Between 2005 and 2009 in South Africa, HCFCs
such as R22 had the highest consumption of 25 759 t (81.4%), HFCs
such as R134a had a consumption of 3.439 t (10.9%), HFC blends
such as R404a of 1089 t (3.4%), methyl bromide of 747 t (2.4%) and
bromochloromethane of 624 t (2%).33 With high refrigerant leakage rates
reported in the literature (between 10% and 15%),26,34 it can be assumed
that system leaks are also relatively high in South Africa. Therefore,
direct emission effects are high too. South Africa, being a signatory to
the Montreal Protocol, needs to reduce its HCFC consumption to 90%
of baseline (2010 amount) by 2015, to 65% by 2020, to 32.5% by
2025, to 2.5% by 2030 and be completely phased out by 2040, while
its methyl bromide consumption is supposed to be completely phased
out by 2015.33 To reduce the environmental effects due to refrigerant
leakages, it is paramount that these synthetic refrigerants are replaced
by more efficient natural and environmentally friendly ones. While these
environmentally friendly refrigerants are getting the required attention
and acceptance in Europe and Asia, application is still at the infancy
stage in South Africa and Africa in general.32
Currently in South Africa, there are approximately 30 industrial and
commercial installations of R744 refrigeration equipment and a
negligible number of R744 residential and transport refrigeration
installations. In a country where industrial and commercial refrigeration
installations exceed 2000 and there are millions of residential and
commercial refrigerators, it is clear how far behind we are in terms of
green technology. Still, compared to the rest of Africa, South Africa leads
with this green technology, therefore emphasising how the continent is
lagging behind. The country’s main power generating company (Eskom)
has also encouraged the adoption of heat pumps and environmentally
friendly refrigerators. This was brought about by its agreement to the
carbon tax regime of reducing carbon emissions by 20% by 2025. The
Department of Energy of South Africa forecast that for this target to be
easily and smoothly achieved, alternative and efficient technologies
need to be adopted.35 To achieve carbon emission targets, Eskom has
encouraged all stakeholders to reduce electricity usage by 40% by
2015 while for ozone depletion substances, the government introduced
a policy of eliminating the use of R22 in all new commercial and
industrial refrigerators.36
As per Eskom estimates, for every 1 kWh of electricity produced at the
power station, 1.4 L of water and 530 g of coal are consumed. The
pollutants emitted from the generation of 1 kWh of electricity include:
7.75 g of SO2, 4.18 g of NOx, 990 g of CO2 and 157 g of ash. These
estimates do not include the pollutants emitted while mining the coal,
i.e. directly from the mine (methane which has a GWP of 20 is normally
released during mining), indirectly from the mining equipment, from
transporting the coal to the power station, and from establishing and
maintaining the power station and mine infrastructure. By Eskom
estimates, if a single household converts from electrical geysers to a
normal water heat pump with a COP of about 3, approximately 355 kWh
of electricity will be saved in 1 month. If the above pollutant estimates
are used, it is possible to protect the environment from a large amount of
pollutants. This becomes clearer if the estimated 5.4 million households
which use electric geysers in South Africa all convert to heat pumps.
Still, this is a conservative estimate of pollutants saved because it only
considers power station production. In addition, if heat pumps with
higher efficiencies are used, fewer pollutants will be released.37
In addition to being environmentally friendlier, the efficiency of operation
of R744 refrigerators is also comparable (if not better) to conventional
systems, thus they are also competitive in terms of indirect emissions,
as reported in the literature.32 In the first R744 refrigeration supermarket
in South Africa (Woolworths), a 35% reduction of electricity consumption
was achieved, resulting in much fewer pollutants being released.36
The only setback is that most studies concerning R744 refrigeration and
other environmentally benign alternatives have been done in countries
with cold climates. Studies in warm tropical climates like in South Africa
and Africa in general are scarce, especially in the open literature. It is
therefore paramount that more studies are conducted in this field so
as to ascertain the advantages of these systems in warm regions. With
the existing R744 installations, the commercial and industrial owners
have reported satisfactory performance to date and are motivated to
install more of these refrigerators. Still, because of the perceived low
efficiencies of R744 refrigerators in warm climates owing to their
low critical point, additional studies are required in order to further
improve the performance of these refrigerators and make them even
more attractive.
Therefore, even though the use of R744 has solved the direct effect
of emissions on the environment, if there are no studies to investigate
the indirect effects of these systems, we might end up with inefficient
systems consuming much energy, thus still affecting the environment.38
Improvement of existing technical solutions and the formulation of more
solutions to existing R744 refrigeration problems is vital if this technology
is to be accepted by all, especially in areas with warm climates. Their
installation and operating costs should be lower than that for conventional
systems. Theoretical and experimental studies should be conducted
on existing and new R744 systems in order to perfect this technology.
System optimisation and modification are paramount if this technology
is to completely replace conventional synthetic refrigerants. Also,
national policies geared to encourage R744 refrigeration and other green
technologies are important so as to clear the way and provide suppor t
for these technologies. In addition to research and industrial input, other
stakeholders like the government and other policy organisations are
important in facilitating the widespread use of these technologies.
Conclusion
Carbon dioxide as a refrigerant was explored from its historical
background to specific properties which affect its performance in the
refrigeration industry. As a result of its superior properties, especially
concerning refrigeration, we believe R744 will be a dominant refrigerant
in many applications of the refrigeration technology in the future.
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Table 4: Some existing practical examples of NH3 refrigeration1,23,39-42
Country Application Organisation and/or manufacturer
Global
All
Commercial
refrigeration Coca-Cola vending machines since 2000
Industrial
refrigeration
Green and cool chillers
Nestle new food processors since 2012
Heat pumps EcoCute
Europe
All Commercial
refrigeration Tesco supermarket new facilities since 2009
United
Kingdom
Commercial
refrigeration
Marks and Spencer’s retail market (White City)
Harrods supermarkets (plus heat pump)
Asda supermarkets new facilities since 2002
Sainsbury’s supermarkets new facilities since
2010
Booths supermarkets new facilities since 2010
Industrial
refrigeration
ABN Amro bank data systems coolers (London)
Netto fresh meat warehouse
Denmark
Commercial
refrigeration
Metro supermarket new facilities
Fakta supermarket new facilities since 2011
Industrial
refrigeration
Netto fresh meat warehouses
Netto central cold store (Arhus)
Switzerland
Commercial
refrigeration
COOP supermarket new and rebuilt facilities
since 2009
Heat pumps Le Locle hospital
Germany
Air-
conditioning
system
Konvekta buses since 1996
Berliner Verkehrsbetriebe (BVG) buses since
1996 (Berlin)
Commercial
refrigeration
Tengelmann Group supermarket (plus heat pump
in Mulheim an der Ruh), and all the new facilities
since 2008
Tegut…gute Lebensmittel supermarket (Lorsch),
and all the new and rebuilt facilities since 2010
Edeka supermarket (North)
Metro supermarket new facilities
REWE supermarket new facilities since 2008
Aldi Sud food discount chain new facilities since
2010
Industrial
refrigeration
Carrier transicold NaturaLINE container since
2010
Hapag–Lloyd carrier container since 2010
Sweden Commercial
refrigeration
City gross supermarket (Rosengard, Malmo)
ICA Kvantum supermarket (Varberg) and all other
new and rebuilt facilities since 2010
Turkey Commercial
refrigeration Carrefour hypermarket (Izmir and Istanbul)
Switzerland Commercial
refrigeration
Prodega Cash and Carry supermarket (plus heat
pumps in St Blaise)
Migros supermarket new facilities since 2002
Austria Commercial
refrigeration
Eurospar supermarket refrigerators (Klangenfurt
and St. Gilgen)
France Commercial
refrigeration Carrefour supermarket (Beaurans-les-Arras)
Country Application Organisation and/or manufacturer
Asia
All Commercial
refrigeration Tesco supermarket new facilities since 2009
China
Air-
conditioning
system
2008 Olympic buses (Beijing)
Commercial
refrigeration AEON retail markets new facilities since 2012
Industrial
refrigeration Zhangzi sea food processing centre (Dalian)
Heat pumps Bumade railway station
Wuhan University
Malaysia Commercial
refrigeration AEON retail markets new facilities since 2012
Jordan Industrial
refrigeration Jordan poultry plant (Amman)
Hong Kong Commercial
refrigeration AEON retail markets new facilities since 2012
United States of America
All Commercial
refrigeration Tesco supermarkets new facilities since 2009
Philadelphia Commercial
refrigeration Star Market supermarket (Chestnut Hill)
California Commercial
refrigeration Supervalu supermarket (Albertsons, Carpinteria)
Maryland Commercial
refrigeration Wegmans supermarket (Woodmore)
Canada
Quebec Commercial
refrigeration
Sobeys supermarket new and rebuilt facilities
(since 2006)
South America
Brazil
Commercial
refrigeration
Condor hypermarket (Curitiba)
Verdemar supermarket and food distribution
(Nova Lima)
Domestic
refrigeration Metalfrio Solutions plug-ins
Oceania
Australia Commercial
refrigeration
Drakes supermarket (Angle Vale and North
Adelaide)
Woolworths supermarket (Sydney, Melbourne
and all the new stores)
Coles supermarket (Ropes Crossing)
Foodland IGA store (Adelaide)
New
Zealand
Commercial
refrigeration
Countdown Auckland supermarket
Warehouse supermarket
Africa
South Africa Commercial
refrigeration
Woolworths supermarket (Grey Owl, Midrand,
2009 and Claremont, Cape Town, 2010 and all the
new stores and rebuilt stores)
Pick n’ Pay supermarket (Johannesburg and
Cape Town)
Makro supermarket, Polokwane and Vaal (plus heat
pump) and all the other new stores since 2011
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Acknowledgement
We acknowledge the financial and logistical assistance of Eskom and the
Tshwane University of Technology.
Authors’ contributions
P.M. conducted the literature review under the guidance of Z.H. who was
also the project leader. Both authors wrote the paper.
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