by fermentation, via plant decay, plus it is released from the earth’s core through openings such as springs and volcanoes, and produced from acidic water on carbonated material such as limestone. Naturally-occurring CO2
emissions are
roughly balanced by both photosynthesis in plants and absorption at the surface of the world’s water masses. Atmospheric CO2
is thought to have risen about 40%
since the start of the industrial revolution. Internal environmental CO2
levels (in
the air) are typically considered to be at around 1,000 ppm and above, although many studies have measured long-term values that, in practice, are much higher, with the attendant concerns about the effect of this on mental performance and physical health3
. The phases of CO2 are shown in Figure
1. The triple point, at -56.6°C and 0.52MPa, indicates the point where CO2
may co-
exist as solid, liquid and gas – a slight perturbation in pressure or temperature can switch the state instantaneously. Above the critical point, 31.1°C at a pressure of 7.36MPa, there is no separated liquid or vapour – it is a homogenous ‘supercritical’ fl uid. Above this point, the latent heat of vaporisation is zero – it does not exist. If the supercritical fl uid was extremely compressed (far beyond anything in the commercial HVAC world) it would form a solid. CO2
may be distributed as a solid
(create d by pressurising and refrigerating carbon dioxide rich gases), and as it sublimes to a gas at -78.5° C (at standard atmospheric pressure) it will consistently provide a source of cooling (but with little opportunity to recycle the gas). Thus, it provides a portable source of cooling that requires no equipment at point of use. (It is also used theatrically as ‘dry ice’.) Each kilogram of solid CO2
will absorb 571 kJ
from its surroundings as it sublimes into a vapour. It is more often distributed as a liquid in pressure vessels and is used for industrial processes, fi re extinguishers, in the food industry and – increasingly – refrigeration. The cost of CO2
is very
low compared with other manufactured refrigerants.
CO2 in transcritical refrigeration
The outlines for both simple R134a (subcritical) and CO2
(transcritical)
systems are shown on the combined pressure enthalpy diagrams in Figure 2 – the term ‘transcritical’ simply meaning that the cycle passes across the critical
54 CIBSE Journal December 2012 5
Throttle (adiabatic)
20 10
2 5
1 0.5 R134a expansion evaporation
CO2 expansion evaporation condensation compression
gas cooling compression
Critical point 31°C, 7.3MPa
Critical point 101°C, 4.0MPa
lesser extent, industrial applications. Smaller CO2
systems tend to use unitary
transcritical systems, whereas larger, commercial and industrial systems are more likely to employ CO2
as a low Enthalpy (kJ/kg)
Figure 2: A basic comparison of simple refrigeration cycles – subcritical R134a refrigeration and transcritical CO2
cycle
point. Applications of transcritical refrigeration have taken off in the last 20 years, particularly with small refrigeration systems – initially for automotive and small marine applications – that are generally acknowledged to have developed from work undertaken by Gustav Lorentzen in the late 1980s. Conveniently, the low critical temperature sits in the middle of the ranges of temperature that are frequently found in HVAC and R applications. In transcritical refrigeration cycles, operates at much higher pressures
CO2
than traditional HFC and ammonia systems. Modern manufacturing methods have enabled the production of low-cost components capable of operating at the high pressures required for CO2
refrigeration. This includes
small domestic units, heat pumps, supermarket applications, and, to a
Subcritical and transcritical cycles Looking at Figure 2, the R134a cycle has the evaporating process starting off bottom left, where the low temperature refrigerant is a mix of vapour and liquid. As the refrigerant gains heat from the surrounding cooling load (or heat source for heat pump), its enthalpy rises with
temperature refrigerant in cascade systems, together with other refrigerants such as ammonia being used as the high temperature refrigerant. There have been developments in small scroll compressors and reciprocating compressors specifi cally for transcritical CO2 CO2
systems. has a higher volumetric refrigeration
capacity than traditional refrigerants (so requiring less displacement) but at much higher pressure differentials. The reduced volume fl ows of the refrigerant provides opportunities for smaller components. CO2
can also be used as a direct refrigerant, where liquid CO2 is simply
pumped under pressure to an evaporator supplying the cooling load, with the vaporised CO2
temperature heat exchanger (still at high pressure) and condensed, ready to be recirculated to the load.
then passed through a low
P3a = P2 T3a = 70°C
3a q space Space heater 3 T3 = 45°C
Refrigerant R744 (Carbon dioxide)
P4 = P3 = P2 4 q evap Evaporator P5 = P6 = P1
Figure 3 Outline CO2 transcritical heat pump operation schematic4 (example created by Israel Urieli
www.ohio.edu/mechanical/thermo)
www.cibsejournal.com 6
P6 = 3.5MPa sat. vapor
q hotw Hot water tank
Gas cooler section
2
P2 = 13MPa T2 = 160°C
Compressor (adiabatic)
w comp η c
T1 = 30°C 1
Internal heat exchanger (adiabatic)
Pressure (MPa)
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