The use of natural refrigerants is certainly not new. Way back in the last century and even the century before that, air, ammonia (NH3), carbon dioxide (CO2), propane (R290), sulphur dioxide (SO2) and other refrigerants were used. Due to the introduction of the chemical refrigerants around 1930, the natural refrigerants have almost completely disappeared into the background. The refrigerant CO2 is currently again in the spotlight. Unlike many other natural refrigerants, this natural refrigerant is non-explosive and / or toxic. However, it is intoxicating and can be harmful to health in high concentrations.
The amount of energy required to produce CO2 is much lower than for chemical refrigerants. Most of the CO2 used in industry is released as a waste product in other processes (including beer breweries). Interesting to critically review the application in refrigeration technology. There are also disadvantages, such as higher pressures and a lower critical temperature. It is not easy to decide to choose a specific refrigerant and associated installation.
Assuming manufacturers authorize the use of the refrigerants in combination with their products, several criteria may play a part in this choice.
- Local applicability in a residential environment
- Fines for the use of chemical refrigerants
- Knowledge of own staff
- Energy consumption
- Possibility of heat recovery
- Reliability of the installation
- Maintenance costs
It is important for the entrepreneur to know the total costs (TCOO: Total Cost Of Ownership) of the installation before purchasing. The higher price of a particular installation can then easily be explained by a possibly lower TCOO. When using the refrigerant CO2, an optimized installation can save considerably on other energy sources (electricity and / or gas).
In this it is interesting to note that the image “green” will play an increasingly important role in the considerations. The (TEWI: Total Equivalent Warming Impact) formula, which gives us a total overview of the impact on the greenhouse effect, shows that the choice of a refrigerant based on the GWP value is of secondary importance and TCOO is leading.
Main features of CO2 (systems)
- CO2 systems have a high working pressure and the installations must be designed in such a way that they can withstand these high pressures.
- CO2 has a low toxicity. However, care must be taken to ensure that CO2 levels do not rise above a maximum value in manned spaces.
- CO2 is heavier than air, just like fluorocarbons, and therefore tends to accumulate in low-lying areas.
- Copper tubing is suitable for most commercial CO2 applications. The copper must have a higher wall thickness and must be suitable for CO2.
- The oil solubility of CO2 is good and therefore direct expansion (DX) applications are easy to realize.
- CO2 has a low critical temperature, unlike all other known refrigerants. This means that many CO2 cooling systems work in the so-called “trans-critical area”. This offers advantages, but also creates the necessary challenges.
- At pressures below 520 kPa, liquid CO2 cannot exist and solid “dry ice” is formed. This requires specific handling of issues related to maintenance procedures, the design and location / arrangement of the safety valves.
- CO2 is very suitable as a low-temperature refrigerant, both for industrial freezer applications and for commercial freezer showcases.
- CO2 is also exceptionally suitable for heat pump applications
- CO2 can replace HFCs in a cascade system in combination with a high-stage refrigerant or with a complete CO2 system.
Immediate, easily achievable / realizable applications include:
- Cold storage, (small and large scale)
- All supermarket cooling systems
- Blast, spiral and plate freezers
- Industrial, commercial and domestic heat pumps
Another important aspect for working with CO2 systems is investing in very good training. CO2 systems are often more complex in nature and specialist knowledge is necessary. Therefore, Companies must invest in sound training.