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EXPERIMENTAL INVESTIGATION OF A CARBON DIOXIDE REFRIGERATION UNIT
DESIGNED FOR MEDIUM SIZE REFRIGERATED TRUCKS
Wasim Shah(1)*, F ancesco Fab is(2), Sil ia Mine o(2), An onio Rosse i(2), Se gio
Ma ine i(2)
(1) Uni e si y o Pado a, Depa men o Indus ial Enginee ing, Via Venezia 1, 35131
Pado a, I aly
(2) Na ional Resea ch Council, Cons uc ion Technologies Ins i u e, Co so S a i Uni i 4,
35127 Pado a (I aly)
*Co esponding au ho : [email p o ec ed]
Abs ac
The EU p ojec ENOUGH is ocusing on s eng hening he sus ainabili y o EU's ood
supply chain by o e ing echnological, inancial, and poli ical esou ces o lowe
g eenhouse gas emissions by 2030 and a ain ca bon neu ali y by 2050 wi hin he ood
sec o . As he p ojec is app oaching i s conclusion in Sep embe 2025, se e al
demons a o s ac oss Eu ope a e collec ing esul s o demons a e easible solu ions o
deca bonize he EU ood indus y. In pa icula , o help educing he ca bon oo p in o
anspo ope a ions wi hin he cold chain, a new CO₂ e ige a ing uni o medium-sized
e ige a ed ucks has been de eloped and designed a CNR (Pado a), and an
expe imen al campaign o e alua e he pe o mance o such sys em has been conduc ed
on a s a iona y p o o ype ins alled in he lab.
The s eady-s a e pe o mance o he uni has been expe imen ally assessed o di e en
alues o he e ige a ed space empe a u e (-5 ºC ,0 ºC and 5 ºC) and ambien
empe a u e ( a ying be ween 20 and 40 ºC, wi h a s ep o 5 °C). Resul s demons a e
good ene gy pe o mance o he de eloped CO2 uni compa ed o baseline syn he ic
e ige an s-based solu ions cu en ly employed in he ma ke .
INTRODUCTION
App oxima ely 70% o ood in ended o human consump ion equi es e ige a ion a
some s ages o main ain eshness and sa e y (IIR, 2021). The cold chain is essen ial o
keeping and deli e ing pe ishable ood by main aining empe a u e, hence slowing
biological decay p ocesses. An es ima ed 14% o all ood p oduced globally o human
use is los , while 17% is was ed (UNEP and FAO, 2022). The absence o e icien
e ige a ion signi ican ly con ibu es o his issue, esul ing in a loss o 12% o wo ldwide
p oduced ood only in 2017 (IIR, 2021). Imp o ing cold chain is hen essen ial o suppo
UN Sus ainable De elopmen Goals (Uni ed Na ions, 2024), which means ex ending he
cold chain in a sus ainable way, as he cu en cold chain is al eady esponsible o 4%
o he o al g eenhouse gases emissions (UNEP and FAO, 2022), ela ed o ene gy
consump ion and e ige an leakages. IIR (2021) s a es ha an imp o ed global cold
chain would allow a educ ion o almos 50% o he CO2 emissions o he cu en cold
chain.
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Unde Ho izon 2020, he Eu opean Union- unded ENOUGH p ojec aims a suppo ing
he EU Fa m o Fo k s a egy (Eu opean Commission, 2020) by p oposing s a egies and
solu ions o each ca bon neu ali y in he ood sec o by 2025. While ENOUGH adop s
an holis ic app oach, analyzing also inancial and poli ical bounda ies, om a echnical
poin o iew i p oposes echnologies which op imize ene gy use, inc ease enewables,
p omo e elec i ica ion, imp o e p ocessing and p ese a ion condi ions and use only
na u al e ige an s, o imp o e he o e all sus ainabili y o ood sys ems. F om ha es ing
o consump ion, a se e al Eu opean si es, demons a o s o di e en p oduc ca ego ies
and o di e en applica ions wi hin he ood chain ha e been de eloped employing
inno a i e echnology solu ions like high empe a u e hea pumps and e ige a ion
sys ems using na u al e ige an s ac oss impo an pa s o he ood chain o educing
emissions. Mo eo e , ENOUGH also aims o c ea e sma da a sys ems, digi al ools, and
s a egic oadmaps o maximize ope a ions and educe emissions.
T anspo e ige a ion is one o he essen ial pa s in cold chain o p ese ing he quali y
and sa e y o pe ishable goods and ood h oughou hei supply chain. I ensu es ha
hese goods emain wi hin speci ic empe a u e anges o p e en spoilage and
deg ada ion. Howe e , anspo e ige a ion aces se e al cons ain s compa ed o
s a iona y e ige a ion such as limi ed space and weigh , limi ed powe sou ce, ib a ions
and shocks du ing anspo a ion, empe a u e and en i onmen al luc ua ions and
main enance and se iceabili y issues (Mine o e al., 2023). Add essing hese cons ain s
equi es specialized design, echnology, and ope a ional conside a ions. Besides hese
cons ain s, e ige an leakage ep esen s a majo challenge in he cu en anspo
e ige a ion sec o , as i is s ill mainly based on he employmen o syn he ic e ige an s
(HFCs, HFOs), cha ac e ized by a high Global Wa ming Po en ial (GWP) and by nega i e
en i onmen al impac s. Howe e , hese issues can be add essed by using na u al
e ige an s wi h op imized componen s and con ol echniques. In addi ion, new policies
and egula ions ha e been in oduced in ecen yea s o educe emissions, ensu e sa e y,
and ansi ion owa ds mo e sus ainable and en i onmen ally iendly subs ances in
anspo e ige a ion sys ems.
To ace he abo e-men ioned challenges o his c i ical s age o he ood supply chain,
one o he demons a o s de eloped wi hin he EU ENOUGH p ojec consis s o a new
e ige a ing uni employing a na u al e ige an (CO2) as he ope a ing luid, speci ically
de eloped and designed a CNR in Pado a, I aly, o medium-sized e ige a ed ucks.
To enhance sys em pe o mance in di e en clima e condi ions, his no el CO₂ cooling
uni is able o ope a e in a ious con igu a ions by including o excluding a wo-phase
ejec o and an auxilia y e apo a o , in addi ion o a simple back-p essu e con igu a ion
which is u he desc ibed in he ollowing sec ion. This s udy desc ibes he uni design
and layou and p esen s he esul s o an expe imen al campaign conduc ed on a
s a iona y p o o ype ins alled in he lab. Expe imen al esul s a e use ul o de e mine he
pe o mance o such a sys em be o e componen s and layou op imiza ion and be o e
manu ac u ing o he inal mobile uni .
THE REFRIGERATION UNIT
This wo k p esen s a no el R744 (CO2) cooling uni layou designed o o e medium
empe a u e (MT) e ige a ion in oad empe a u e- egula ed anspo a ion applica ions.
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This uni is able o ope a e in bo h subc i ical and ansc i ical mode depending on he
ou side ambien condi ions. The simpli ied schema ic o he cooling uni is shown in Fig.1.
Figu e 1. Simpli ied schema ic o he R744 cooling uni .
The schema ic p esen ed in Fig. 1 allows o achie e ope a ion in di e en con igu a ions.
In back-p essu e con igu a ion, a simple high-p essu e al e (HPV) is used o expand he
e ige an and op imize he p essu e a he gas coole ou le . A e expansion h ough he
HPV, he e ige an lows o he main e apo a o , whe e i deli e s he equi ed cooling
e ec . F om he e apo a o ou pu , he e ige an goes o he liquid sepa a o , om which
he apo phase is ed o he suc ion o comp esso ia an in e nal hea exchange (IHX)
o p o ide he equi ed supe hea o a oid liquid e ige an a he comp esso suc ion.
Depending on ex e nal ambien condi ions, he e ige a ing sys em schema ic allows
modi ying he cycle o use he high-p essu e al e in pa allel wi h a ixed geome y wo-
phase ejec o , hus ope a ing in an ejec o ansc i ical cycle. In such con igu a ion, he
expansion wo k o he e ige an a he gas coole ou le can be pa ially exploi ed o
p o ide a p essu e li o he e ige an mass low a e om e apo a ion p essu e o he
liquid sepa a o p essu e, hus educing he comp ession wo k. An auxilia y e apo a o
be ween he liquid sepa a o and he ejec o exi can p o ide u he modi ica ion o he
ejec o cycle, allowing o ope a e wi h ejec o con igu a ion owa ds lowe ambien
empe a u es. A de ailed desc ip ion o he e ige a ion uni concep can be ound in
A uso e al. (2020).
A s a iona y e ige a ion uni based on he schema ic p esen ed in Fig.1 was
commissioned and ins alled in he CNR labo a o ies loca ed in Pado a, I aly. The
objec i e o his p elimina y lab ins alla ion was o allow expe imen al assessmen o he
pe o mance o such cooling uni concep in di e en con igu a ions and unde di e en
ope a ing condi ions be o e de ining he inal schema ic, wi h op imized componen s,
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encumb ance and con ols, o he mobile uni o be enginee ed and ins alled on a
e ige a ed ehicle.
Fig. 2 illus a es he s a iona y expe imen al acili y implemen ed in he CNR labo a o ies.
The gas coole and he e apo a o s o he e ige a ion uni a e placed in wo sepa a e
insula ed chambe s, in which he ai empe a u e can be con olled o expe imen ally es
he uni unde he desi ed ope a ing condi ions: di e en ambien empe a u e condi ions
can be achie ed by con olling he ai empe a u e a he gas coole (in he ho oom) and
di e en empe a u es o pe ishable goods p ese a ion can be achie ed by con olling
he ai empe a u e a he e apo a o s (in he cold oom).
Figu e 2. Expe imen al acili y (CNR Pado a): (a) Re ige a ion uni ; (b) Ho oom; (c)
Cold oom; (d) PC o da a acquisi ion.
A lis o he main componen s in eg a ed in o he expe imen al e ige a ion acili y is
shown in Table 1, along wi h hei co esponding main speci ica ions. I should be
speci ied ha he gas coole ins alled in he expe imen al acili y is o e sized compa ed
o he design cooling capaci y o app oxima ely 6 kW in nominal condi ions (𝑇𝑎𝑚𝑏 = 30 °C,
𝑇𝑖𝑛𝑡 = 0 °C) de e mined by he comp esso size. The e o e, he speed o he gas coole
ans has been educed in o de o ealize he design he modynamic cycle wi h a
empe a u e app oach o app oxima ely 2 K be ween ambien ai and e ige an a he
gas coole ou le .
Table 1. Componen s and hei main speci ica ions.
Componen s
Speci ica ions
Comp esso
Displacemen olume: 2.39 m3 /h @50 Hz
Gas coole
Ex e nal Hea ans e A ea: 78 m2
Main E apo a o
Ex e nal Hea ans e A ea: 39.4 m2
Auxilia y E apo a o
Ex e nal Hea ans e A ea: 19.7 m2
Liquid Recei e
Volume: 40 L
In e nal Hea Exchange
Numbe o pla es: 26
Two-phase ejec o
Th oa diame e : 1 mm
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THE EXPERIMENTAL SETUP
The expe imen al acili y's da a acquisi ion sys em in eg a es high-p ecision senso s o
moni o he mal and elec ical pa ame e s equi ed o assess he sys em pe o mance.
All measu emen s a e eco ded using Na ional Ins umen s (NI) da a acquisi ion
ha dwa e, which is in e aced wi h a LabVIEW-based moni o ing p og am.
T- ype he mocouples, calib a ed agains a e e ence P 100 RTD ( he mo- esis ance
de ec o ), a e moun ed a di e en poin s o he acili y o measu e he empe a u e o he
e ige an along i s he modynamic cycle and o he ai in he insula ed chambe s and a
inle /ou le o e e y ai hea exchange . P essu e ansduce s a e placed a comp esso
suc ion and discha ge, a gas coole ou le , a liquid sepa a o and a ejec o suc ion and
discha ge po s. Mass low a e o e ige an is moni o ed h ough Co iolis mass low
me e s ins alled a exi o he gas coole and a ejec o mo i e and suc ion po s. Cu en
and ol age o each o he h ee phases o he uni powe supply line a e moni o ed and
hen used o assess he e ige a ion sys em elec ical powe consump ion. Table 2
p esen s he measu ing equipmen ins alled in he expe imen al acili y oge he wi h he
co esponding accu acy.
Table 2. Da a acquisi ion equipmen wi h co esponding accu acy.
Ins umen s
Accu acy
The mocouples
±0.15°C
P essu e Senso s
±0.1% o se span
Mass low me e s
±0.50% o eading
Elec ical Cu en
±0.2% o se span
Elec ical Vol age
± (0.2% o eading + 0.005% o se span)
All he moni o ed pa ame e s a e measu ed wi h a sampling equency o 1 Hz, wi h he
excep ion o ol age and cu en alues. Speci ic en halpies, necessa y o he calcula ion
o he uni ene gy pe o mance, a e e alua ed om he measu ed alues o p essu e and
empe a u e h ough he REFPROP 9.1 da abase.
Fo each ope a ing condi ion conside ed in he expe imen al campaign, da a a e
measu ed o 15 minu es (960 samples) and hen a e aged o de e mine he s eady-s a e
pe o mance in hose speci ic condi ions.
RESULTS AND DISCUSSION
As p e iously s a ed, he e ige a ion uni is capable o ope a ing in a ious
con igu a ions; howe e , expe imen al esul s in back-p essu e con igu a ion ope a ion
will be p esen ed in his s udy. A simpli ied schema ic o he expe imen al acili y o back-
p essu e is shown in Fig 3.
To expe imen ally e alua e he s eady s a e pe o mance o he e ige a ion uni in back-
p essu e con igu a ion a condi ions ele an o he selec ed applica ion, expe imen s
we e pe o med o di e en alues o he e ige a ed space empe a u e 𝑇𝑖𝑛𝑡 (-5 ºC ,0
ºC and 5 ºC) and ambien empe a u e 𝑇𝑎𝑚𝑏 ( a ying be ween 20 °C and 40 ºC, wi h a
s ep o 5 °C).
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Figu e 3. Back-p essu e con igu a ion.
The desi ed se poin empe a u es o 𝑇𝑖𝑛𝑡
and 𝑇𝑎𝑚𝑏 we e main ained by con olling
he ai empe a u e in ho and cold ooms.
The e ec o ambien and in e nal
empe a u es on he Coe icien o
Pe o mance (COP) and on he cooling
capaci y o he e ige a ion uni is shown
in Fig 4. Conside ing he design condi ions
o 𝑇𝑖𝑛𝑡 = 0 °C, he expe imen al COP is in
he ange be ween 2.8 ( o 𝑇𝑎𝑚𝑏 = 20°C)
and 1.3 ( o 𝑇𝑎𝑚𝑏 = 40°C).
To compa e he pe o mance o he R744
uni p esen ed in his s udy o he
pe o mance o baseline anspo
e ige a ion uni s cu en ly a ailable in he
ma ke , da a om p io s udies a ailable in
li e a u e, e e ed o a R404A uni (Colbou ne e al., 2017) and o a R410A uni (Wu e
al., 2013), ha e been compa ed in Fig.5 o he expe imen al esul s o he R744 uni . A
20°C, he R744 sys em demons a es signi ican pe o mance imp o emen o 42.5%
while a 30°C and 40°C he imp o emen s a e app oxima ely 30.8% and 19.5%,
espec i ely. The employmen o ejec o con igu a ions, no conside ed in his s udy,
could esul in u he pe o mance imp o emen a high ambien empe a u es.
I should be poin ed ou ha , besides he abo e discussed signi ican pe o mance
imp o emen , he p oposed uni ep esen s a iable solu ion o ul ill he EU goals o
en i onmen al sus ainabili y o he ood chain also hanks o he eplacemen o syn he ic
e ige an s wi h he na u al e ige an R744 (GWP = 1).
(a)
(b)
Figu e 4. E ec o ope a ing condi ions on (a) COP ;(b) Cooling capaci y o he
e ige a ing uni .
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Figu e 5. Compa ison o cu en s udy wi h p e ious baseline s udies.
CONCLUSIONS
This s udy p esen ed he s eady s a e pe o mance o he R744 cooling uni in back
p essu e con igu a ion designed o medium sized e ige a ed ucks. An expe imen al
campaign was conduc ed o h ee e ige a ed space empe a u e 𝑇𝑖𝑛𝑡 (-5 ºC ,0 ºC and
5 ºC) and ambien empe a u e 𝑇𝑎𝑚𝑏 was a ied be ween 20 °C and 40 ºC, wi h a s ep o
5 °C.
Expe imen al esul s demons a ed he good pe o mance o he p oposed uni , assessing
a COP in design condi ions (𝑇𝑖𝑛𝑡 = 0 °C) anging be ween 2.8 and 1.3 o 𝑇𝑎𝑚𝑏 a ying
be ween 20 °C and 40°C, espec i ely, and highligh ing a signi ican COP imp o emen
(be ween +42.5% and +19.5% o 𝑇𝑎𝑚𝑏 a ying be ween 20 °C and 40 °C) compa ed o
syn he ic e ige an baseline solu ions epo ed in li e a u e.
ACKNOWLEDGEMENTS
The ac i i y desc ibed in his manusc ip has been pe o med wi hin he p ojec ENOUGH.
ENOUGH has ecei ed unding om he Eu opean Union’s Ho izon 2020 esea ch and
inno a ion p og amme unde g an ag eemen No 101036588.
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