CO2 As A Viable Working Fluid
CO2 has an exceptionally low GWP of 1, compared to other working fluids such as R410A (HFC) or R448A (HFO) – see table below.
Every system will lose some of its gas each year from service activities or from leakage between components. Manufacturers and those working with plant take considerable precautions to prevent loss, but a 15% loss is not unusual. This loss of gas leads to fugitive emissions which must be accounted for in carbon footprint calculations. An example comparison is given below.
F-gas regulations have been steadily reducing the quantity of high GWP gases available and some of the worst are now completely banned. This trend is expected to continue in order to reduce fugitive emissions and emissions created during the manufacture of synthetic gases. Not only are they being regulated out but there have been significant price rises, in some cases by over 300 %, as supplies become restricted.
There is no prospect of a regulatory change which could phase out or prevent the use of CO2 and supply is plentiful. Therefore, there is no stranded asset or cost increase risk associated with its adoption as a working fluid.
It should be noted that some working fluids (e.g. HFOs) are in fact blends of different chemicals. Losses from HFO blends can result in the need to replace the entire volume as it is impossible to determine which component chemicals have been lost. This results in a much higher loss of high GWP fluid. This does not apply to CO2.
||CO2e for annual losses*
|*Calculation based on like for like volume of 354kg
CO2 is non-flammable and non-toxic
CO2 is non-corrosive, non-toxic and non-flammable. It is a stable molecule which does not decompose either in the system or when accidentally released. In comparison, a recent study by the University of New South Wales in Sydney, Australia, suggests that elevated levels of high-GWP HFC-23 (R23) in the atmosphere could be linked to the uptake of HFO1234ze, which the study says produces R23 as it decomposes in the atmosphere.
R23 has a GWP of 14800 which makes it amongst the worst gases for climate change. CO2 is an asphyxiant in large concentrations therefore the use of detectors in confined spaces is normal. As the gas is heavier than air it drops to the floor where detectors should be placed. The best placement for a CO2 heat pump is outside where the gas disperses naturally in the highly unlikely event of a severe leak.
CO2 allows for higher temperatures
The index of compression is very high for CO2, so the discharge temperature is higher than for the HFCs. Index of compression, also known as the polytropic exponent, is 1.289 for CO2 and only 1.005 for R404A. A polytropic process is a thermodynamic process that obeys the relation:
Where p is the pressure, v is specific volume, n is the polytropic index, and C is a constant. The polytropic process equation can describe multiple expansion and compression processes which include heat transfer. The value of n is different in different thermodynamic processes. The polytropic index is a measure of the work done by the system. If you have a value for n, then you can determine the heat of compression by the equation below. It is common to look at the measured suction, discharge temperatures and compression ratios and determine the polytropic index.
T2 / T1 = (p2 / p1) [(n – 1)/n] where T is the thermodynamic temperature
Where the numbers 1 and 2 denote the states at the beginning and end of the compression process. It can be seen that a higher value of n gives a higher differential in temperatures, thus CO2 has a greater temperature difference than HFCs. This means CO2 can deliver useful temperatures for heating applications whilst drawing heat from air at normal ambient temperature ranges, all year round. The performance difference can be seen on the graph below where CO2 is in green against a typical synthetic fluid in blue, vs. COP.
CO2 systems producing high water temperatures have a higher efficiency than systems with other working fluids. However, there are many other factors in heat pump design which also influence this. Different manufacturers will control the heat pump using proprietary algorithms and careful component selection resulting in performance differences.
CO2 has Greater Density
CO2 is more dense when compared to other working fluids. This means that all the pipework, number of compressors, components and rack size in general is smaller. For example, the required suction pipe cross-section area for CO2 is approximately half that required for R404A (for the same volumetric capacity). This is especially valid for large capacity systems that would require too many compressors and much larger diameter pipework if designed for HFC/HFOs.