The refrigeration and air conditioning market is more and more focusing its attention on problems related to the environmental impact of its systems in order to be in compliance with Montreal and Kyoto Protocol (respectively 1987 and 1997).
During last ten years the use of carbon dioxide (CO2) as a refrigerant has gained renewed interest due to the ecologic problems related to the use of HCFCs anf HFCs.
Infact carbon dioxide:
Is a natural refrigerant
has an ODP = 0 (ozone depletion potential)
has a GWP = 1 (global warming potential)
is not flammable
is not toxic
is world wide available
is a low cost product.
Additionally European Governments are planning the progressive phase out of synthetic refrigerant: for instance Norwegian installers have to pay taxes for HFC systems while Austria is going to ban HFC since from 2008. Danish and Swiss Governments are moving toward the same direction.
Anyhow it is a matter of fact that North-European Governments are deeply promoting the use of natural refrigerants, working closely NGOs such as GreenPeace and UNEP.
Nowadays the challenge of the HFC free systems has already been taken by famous multinational company who are conducting several thousands of field test all over the world. Some of them have been running successfully during recent Athens Olympic Games.
As it is for standard refrigerants, also with carbon dioxide the role of heart of the plant is acted by the compressor. The design and the development of such compressors lead to a lot of difficulties, mainly due to:
high working pressures (transcrtical conditions, fig.1)
high discharge temperature (fig.2)
high solubility of poliester oil
volumetric specific refrigerating capacity from 5 to 10 times higher than the one of standard refrigerants
reduction of pulsation phenomena and of noise level.
During last ten years there have been several attempts to realize open drive carbon dioxide compressors, capable to work in accordance to what described ed before, but the high pressure fields makes the shaft seal extremely critical.
Therefore the most suitable designs seems to be the hermetic and semi-hermetic ones.
So let’s deeper analyze which are the most critical issues related to the desing of carbon dioxide compressors.
Mechanical stresses:
As stated before carbon dioxide has a volumetric specific refrigerating capacity from 5 to 10 times higher than the one of standard refrigerants. This leads both to minor swept volumes of the same magnitude and to higher differential pressures across the piston of the same magnitude. Small bores can lead to small stresses on the connecting rods but, at the same time, they allow less space available, with consequent high specific loads on the moving parts. Very critical it has shown to be the dimensioning of the small end connecting rod bearing.
Thermal load:
Having started with the desing of single stage carbon dioxide compressors, one of the most challanging issue has revealed to be linked to the high discharge temperatures occuring with modest pressure ratio. This led to the choice of a valve plate assembly material capable to get to high temperature and to the choice of lubricants with flash temperature higher than 200°C and to
Lubrication:
Poliester oils are extremely miscible with carbon dioxide, especially at the thermodynamic conditions occurring in the crankcase while the compressor is operating in transcrtical conditions; therefore the lubricating power of the mixture composed by oil and refrigerant decrease sensibly. This of course is an additional strong challenge to the desing of such compressors.
Rotating speed:
Nowadays the most part of semi-hermetic piston compressors for refrigeration and air conditioning works @ 1450 (1750) rpm (50 or 60hz), therefore using four poles motor. Having consistently reduced pressure pulsation phenomena (see fig.3) it has been possible to validate the design of CO2 compressors with two poles motors using the same compressor platform; this led to a ratio - price over refrigeration duty - extremely favourable. Of course, switching to two poles motor with the same compressor platform led to slightly lower volumetric efficiencies [Neksa et al., 2000].
Head and valve plate:
Even though CO2 compressors work with low pressure ratio it’s very simple to get extremely high pressure differences. Consequently another key issue has been the desing of valve plate capable to get proper seal.
Two compressor ranges have designed, into two different bodies. 300 range (fig.4) has now become reliable thanks to extremely severe life test; performances are also very encouraging, for both single and double stage models. As a matter of fact carbon dioxide allows evaporating temperatures down to –55°C therefore calling for two stage solutions [Rekstad et al.]; 300 range is then characterized by:
Subcritical and transcritical working capability
Single and double stage models
Two and four poles electric motors.
Good performances
High reliability
Single stage compressors:
All compressors model have two cylinders, build with the same crankcase. Different swept volumes are obtained varying the stroke and keeping the same bore. Compressors are equipped with an oil pump in order to grant a correct lubrication; the use of an oil cooler is recommended in order to keep a correct lubricating power.
Figure 5 (fig.5) shows single stage model volumetric and isentropic efficiency versus pressure ratio.
Double stage compressors:
As stated previously extreme refrigeration applications (evaporating temperatures lower than –30°C) calls for two stage solutions. Those model are build using the same crankcase, increasing the dimensions of the low pressure cylinder in order to keep it working in subcritical conditions.
300 range includes compressors for subcritical and transcritical conditions; the first one are characterized by swept volumes varying from 5,1 m3/h e 18,8 m3/h and power installed within 6 kW and 18 kW. The second ones are characterized by swept volumes within 3,5 m3/h e 10,7 m3/h and power installed within 4 kW e 15 kW.
The very good results internally obtained led to a first phase of compressors divulgation. Nowadays several hundreds of compressors are running on the field for different applications, mainly in Northern Europe and Japan.
The main fields of use are:
Sanitary hot water heat pumps
Combined systems (where useful heat exchange is performed both on the low and the high pressure side of the system)
Dryers
Supermarket
First results achieved on the field are extremely encouraging; let’s analyze two examples:
1)SUPERMARKETS:
A comparison has been conducted between installation cost and utilization cost pertaining to two different systems able to provide the same refrigerating duty. The first one evolving R404A in direct expansion (the cheapest of its category) and one evolving CO2.
This analysis has been conducted during one year and results showed that both installation and utilization costs are totally comparable [Girotto et al., 2003]; big improvements are anyhow forecastable thanks to simple system design change.
2) RESTAURANTS:
Another comparison has been conducted in order to evaluate annual power consumption of two same size restaurants placed at the same latitude. One of the two restaurants was equipped with an HFC-free combined system (with Dorin CO2 compressors), used for food conservation and ambient climatization (both during summer and winter). The other one was equipped with a standard refrigerant system.
Results showed 12% lower annual power consumption for the HFC free systems with a 27% lower T.E.W.I. (Total Equivalent Warming Impact) coefficient.
Those results were shown by DTI (Danish Institute of Technology) representatives during “Refrigerants, naturally” conference, held in Bruxelles (june 2004,
Those are only two of the several positive feedbacks we’re receiving relatively to CO2 compression technology. This is one of the reason that makes us think to carbon dioxide as the best replacement for synthetic refrigerants. That’s why Dorin will continue developing new and innovative ideas to further improve performances and reliability of its CO2 compressors, making also use of:
Further tests intended to enlarge compressor application envelope and to improve its performances.
Customers feedback.
Partnership with research institutes worldwide leader in the refrigeration field.
Partnership with companies worldwide leader in the system controls market.
References:
1. P.Neksa, F.Dorin, H.Rekstad, A.Bredesen:Development of two-stage semi-hermetic CO2 compressor. [Proceedings from IIF-IIR, Purdue University, USA – 2000]
2. H.Rekstad, P.Neksa, G.Pisano:Measurements on a 2 stage CO2-compressor. [Proceedings from 6th Gustav Lorentzen Natural Working Fluids Conference 2004, Glasgow, UK]
3. S.Girotto, S.Minetto, P.Neksa:Commercial Refrigeration System with CO2 as refrigerant; Experimental Results. [Proceedings from International Congress of Refrigeration 2003, Washington, DC]
Figures:
Fig. 1: transcritical CO2 cycle
Fig.2: thermal loads on a CO2 compressor working with high pressure ratio
Fig.3: p-V diagram for 2 poles compressor working with pressure ratio of 4.2
Fig.4: CO2 Compressor, 300 range
Fig.5: Volumetric and isentropic efficiency for TCS362 compressor model