Natural and Certified CO₂: Past and Future
To guarantee maximum efficiency in a CO₂ project, it is not enough to compare nominal data from technical sheets, which are often similar among competitors.
Natural and certified CO₂: past and future
Historical and Regulatory Context
The use of carbon dioxide (CO₂ or R744) as a refrigerant is not a novelty, but rather a return to a historically established solution. Widely used until the first decades of the 20th century, CO₂ was gradually replaced by chemical fluorocarbons between 1930 and 1940.
The turning point occurred in 1987 with the Montreal Protocol: the discovery of the link between HFCs (hydrofluorocarbons) and the depletion of the ozone layer triggered a process of replacing synthetic gases. Starting in 2000, attention shifted toward natural refrigerants. CO₂ stands out due to its optimal intrinsic properties: it is non-flammable and has a GWP (Global Warming Potential) of 1, an infinitesimal value compared to traditional HFCs.
The Thermodynamic Challenge: The Transcritical Regime
Despite its environmental advantages, CO₂ poses unique technical challenges. Unlike HFCs, CO₂ systems operate at significantly higher pressures. With a critical point set at 31.1°C and 73.8 bar, vapor compression systems operating at standard ambient temperatures often function in a transcritical regime.
Under these conditions, discharge pressure is no longer solely linked to saturation temperature (as in subcritical cycles) but must be actively controlled to optimize the COP (Coefficient of Performance) of the plant. Heat rejection capacity depends directly on the CO₂ temperature at the gas cooler outlet: for every temperature, there is a specific pressure that maximizes efficiency. A system may meet the required cooling demand, but if it is not perfectly optimized, energy efficiency will be severely reduced.
The Eurovent white paper detailing the test results
Natural and Certified CO₂: Past and Future
The Impact of Sizing on Operational Efficiency
Component sizing is the critical factor in achieving performance consistent with design-phase estimates. Unlike traditional HFC fluids, CO₂ requires extra precautions: due to the unique properties of this fluid in the transcritical regime, heat exchange capacity varies considerably based on operating temperatures and pressure. It is therefore always advisable to verify performance at multiple operating points, and it is crucial to examine the conditions in which the gas cooler operates as a condenser.
For example, let us analyze the operation of a gas cooler under different environmental conditions.
- Maximum temperature condition (Ambient 40°C) :
- Approach (DT) = 2K
- Required capacity: 100 kW; Gas cooler capacity: 100 kW
- Lower temperature condition (Ambient 30°C) :
- Required capacity: 90 kW
- If the gas cooler is not correctly sized to reject the required 90 kW at lower temperatures and pressures, the approach increases to 3K.
This forced increase in operating temperature compels the entire system to raise the operating pressure, significantly increasing the total energy consumption of the plant.
The Importance of Performance Certification
- Reliability : Only 20% of the non-certified units tested confirmed their declared performance.
- Underperformance : In some cases, a gap of 53% was found between declared capacity and actual capacity.
- Systemic Consequences : An inefficient component can lead to overconsumption of more than 43,000 kWh/year, with an increase in operating costs exceeding €7,800 and the emission of an additional 14.7 tonnes of CO₂ equivalent per year of operation.
Conclusions for CO₂ System Design
To guarantee maximum efficiency in a CO₂ project, it is not enough to compare nominal data from technical sheets, which are often similar among competitors. The key to success lies in:
Independent Certification
Use components certified by Eurovent Certita Certification to ensure the veracity of heat exchange data.
Cycle Optimization
Design the system to operate at ideal pressures and temperatures, reducing the energy required for compression in the transcritical phase.
Research and Development
Rely on partners who invest in real performance testing (such as LUVE and its specialized climatic chamber) to ensure that the component is sized based on real environmental variables rather than generic data tables.