eco friendly refrigeration with carbon dioxide

8/8/2019 Eco Friendly Refrigeration With Carbon Dioxide

1/20

ECO FRIENDLYREFRIGERATIONWITH CARBON

DIOXIDE


2/20

WHY CO2 AS A REFRIGERANT?

Non-toxic

Non-flammable

Environmentally benign

The high vapour pressure leads not only to a low pressureratio with the advantage of high compressor efficiency, butalso to high heat-transfer coefficients and low relativepressure losses. Thus, despite the lack of efficiency of thetheoretical Tran critical cycle, the CO2 supercriticalrefrigeration cycle may still compete with the vapourcompression cycle using other refrigerants.

Its higher volumetric capacity due to its high workingpressures enabling small equipment components and small-diameter lines to be used


3/20

WHY CO2 AS A REFRIGERANT?

High refrigeration volumetric capacity

High heat transfer characteristics

Inexpensive

Readily available

gas cooler pressure and temperature are notlinked as in the sub critical two-phase region.Since the high-side pressure greatly affects, via

the pressure ratio, compressor work andefficiency, high temperatures can be achievedwith reasonable compressor power.


4/20

LEAKAGES OF R134a When you suspect a leak of R-134a in your air conditioning

system, detection can be done by using one of the following5 methods. The simplest method and cost effective is by theuse of soap solution. Workshops may use more sophisticatedequipments to do this. Fluorescent Dyes

Soap Solution

Electronic Leak Detectors

Halogen selective detectors

Ultrasonic leak detectors No problem as such of leakage in case of co2.


5/20

refrigerant R12 R22 R134a R290 Ammonia Carbon dioxide

Natural fluid no no no yes yes yes

ODP 0.82 0.055 0 0 0 0

GWP 10600 1900 1600 20


6/20

WHAT IS REQUIRED TO BE CHANGED IN

PRESENT DAY REFRIGERATOR? The major requirement of transcritical refrigerator is that it

requires higher pressure to operate. Since CO2 offers amuch higher volumetric capacity, the problem of the higherworking pressure can be overcome by optimal design

involving smaller, stronger components. Compressor compactness is another consequence of the

high pressure/low volume characteristics of CO2;displacement volumes are 6-8 times smaller than those of R-134a and sensitivity to valve losses is reduced by the highvapour density. In automotive compressors where

compactness is valued highly, smaller displacements mayenable reductions in component volume. Nevertheless, someearly prototypes were


7/20


PRESENT DAY REFRIGERATOR? The low pressure ratios have led to higher efficiencies, and

the high pressure differences experienced inside thecompressor have resulted in most prototypes being ofreciprocating or 2-stage rotary design

The assessment of lubricants for the transcritical cycle is stillunderway; with polyalkylene glycol (PAG) favoured for itslubricity, aromatic ester derivatives for their tolerance of hightemperatures and polyolesters (POE) for their miscibilitythroughout the transcritical cycle. The poor miscibility of

synthetic mineral oils is offset by their superior flow propertiesat low temperature, so the ultimate choice may involveblending two or more lubricants


8/20


PRESENT DAY REFRIGERATOR? The high pressures dictate the need for small-

diameter tubes to minimize stresses and material

requirements. Despite early concern that the high

pressures would present insurmountable safety

hazards, the development of ultra-compact

microchannel heat exchangers has reduced that

risk considerably, by confining the refrigerant

inside hundreds of parallel sub-millimetre

channels.


9/20


PRESENT DAY REFRIGERATOR? Transcritical CO2 system performance is

very sensitive to gas cooler refrigerant

outlet temperature, where a smalltemperature change causes a large

enthalpy change due to the high isobaric

specific heat. Fig shows the effect of

change of temp. on cop


10/20

Effect of gas cooler exit temperature

on transcritical cycle COP


11/20

Discharge pressure determination In the subcritical refrigeration cycle process, the heat rejection

process involves condensing of the refrigerant. A large part ofthe condenser volume on the refrigerant side will therefore beoccupied by a two-phase mixture of liquid and vapour. For a

two-phase mixture in thermal equilibrium, the pressure will bethe saturation pressure at the temperature of the mixture.

For a transcritical process, the heat rejection process does notinvolve condensing but only cooling of a gas in the fluid region.Consequently, the temperature of the refrigerant will changecontinuously throughout the entire heat rejection process.

Because this is not a phase change process, the temperatureand pressure become independent. So what then determinesthe pressure in the gas cooler?


12/20

Effect of gas cooler pressure on

system performance


13/20

Effect of gas cooler pressure on

system performance


14/20


15/20

Expander

Any compressor available now can be utilized as an expander operating in reverse.Typical candidates applicable for the CO2 expander consist of the turbine,reciprocating piston, free piston, scroll, screw, rotary vane and rolling piston and soon. Among these candidates, the turbine and twin/single screw machines are notsuitable for the commercial and household application with a small flow rate. For theturbine, the rotating speed is estimated to exceed 200,000rpm to meet this small flow

rate, which makes it a big challenge for the precise manufacture and reliableoperation of the expander, let alone the challenge caused by the two-phaseexpansion. For the twin screw and single screw machines, besides the high cost, thelow efficiency due to relatively large gas leakage is a disadvantage.

Among the other expander concepts left in the candidates, the scroll and the rotaryvane dont need the active inlet/outlet control valves, but the performance of the scrollmachine is very sensitive to a considerably small clearance owing to the largepressure difference [12], and the rotary vane will have a low efficiency if one takes

the gas leakage and friction loss into account. In fact, the efficiencies for these twoexpanders were predicted to be less than 43%

The free piston and rolling piston take disadvantages over the rotary type concepts inthat they need the active inlet/outlet control valves. .

If the challenge associated with the inlet/outlet control is put aside, the free piston canbe regarded as the most suitable expander concept in that it has the bestperformance potential owing to the good sealing, small friction and simple structure.


16/20

Schematic diagram of ejector-expansion Tran critical

refrigeration cycle


17/20

Condenser and evaporator heat transfer coefficients relative

to R507A.

0

50

100

150

200

250

300

350

400

450

CO2

N

3

R22

R23

R32

R41

R116

R125

R143a

R404A

R407C

R410A

R507A

R508A

R290

R1270

R218

Condenser heat transfer Evaporator heat transfer


18/20

Piping used for the system Due to the high volumetric capacity of CO2 tubes with a smaller diameter than in

conventional systems of similar capacity can be used.

Smaller diameters should be preferred as they decrease the refrigerant charge and

withstand higher pressures. For the existing range of TN compressors, in most

cases a tube of 6 x 1 mm will be sufficient. The maximum working pressure of thetubes must be observed as given in table 2.


19/20

Challenges Since CO2 offers a much higher volumetric capacity, the

problem of the higher working pressure can be overcome byoptimal design involving smaller, stronger components.

Nevertheless, newly designed components have to beproduced and can only be manufactured at reasonable pricesif mass produced in sufficient numbers. This can be a bighurdle to surmount before CO2 technology is introduced inrefrigeration, air-conditioning and heat-pump systems.

Fixed pressure performs worse than the optimal pressure

regarding COP and cooling capacity because of the highsensitivity of the ambient temperature.


20/20

Challenges To look to the long-term future and imagine what kinds of

future technological developments might be required fortranscritical CO2 systems to see widespread use in coolingapplications. One potentially plausible scenario would becharacterized by continued reductions in the cost of sensors,actuators and microprocessor controls, and embedding themin mass-produced items. Such R&D progress would diminishthe cost advantage now enjoyed by refrigerants havingfavourable standard-cycle thermodynamic properties, and

enable refrigerants to be selected for their environmentalsafety and compatibility

eco friendly refrigeration with carbon dioxide

Documents