Key takeaways for CO2 system optimisation

The imperative of reducing compressor work – this is the fifth instalment in a five-part series.

Potgieter summarised his recommendations for CO2 cooling optimisation. © RACA Journal

Continued from part four…

At a FRIGAIR 2025 SAIRAC presentation, Jannie Potgieter, a consulting engineer with Thermologica, said the decision is complex, especially for plants designed with evaporative cooling to primarily operate subcritically. In such cases, carefully evaluating the worth of forcing transcritical mode for hot water supply, particularly for smaller loads, is crucial. While 30kW is a significant load for small applications, many end-users might require less. Optimising storage vessel sizing for heat recovery can also play a role, allowing for short bursts of high-capacity heat recovery rather than continuous transcritical operation.

Despite these caveats, Potgieter affirmed that, in most cases, adding a heat exchanger to a CO₂ system for heat recovery remains more efficient than using standalone heat pumps. The industry’s claims of savings are generally true, but professionals must account for specific operational contexts and not treat it as a universal, blanket solution.

Key takeaways for CO₂ system optimisation

Potgieter summarised his recommendations for CO₂ cooling optimisation:

• Parallel compressors are highly recommended for most applications due to their significant efficiency improvements.
• Internal heat exchangers (IHXs) are a strong recommendation, as they offer continuous efficiency gains across all operating modes (transcritical and subcritical) and protect compressors.
• Evaporative cooling provides a substantial leap in efficiency, especially in hot environments, but its cost-effectiveness must be carefully evaluated against the specific climate conditions and maintenance implications.
• Ejectors hold significant promise for future efficiency gains, particularly in enabling higher evaporating temperatures, but their current cost and complexity in implementation remain barriers, especially in markets like South Africa.
• Designing for higher evaporating temperatures at the outset is a powerful strategy for continuous efficiency improvements, provided all connected equipment can operate effectively at those conditions.
• Proper gas cooler/condenser placement is paramount to avoid air recirculation, which can significantly degrade performance and increase energy consumption over the system’s lifetime.

Combining cooling and heating: the synergistic advantage

CO₂‘s unique properties, particularly in the transcritical region, make it exceptionally well-suited for combined cooling and heating applications.

• Superior water heating profile: Unlike conventional refrigerants that condense at a constant temperature, transcritical CO₂ exhibits a temperature glide. This characteristic perfectly aligns with the linear temperature increase of water during heating, leading to more efficient heat transfer in counter-current heat exchangers and minimising pinch points.
• High water temperatures: CO₂ systems can produce very high hot water temperatures, easily reaching 80–85°C (or even higher, depending on system design). This surpasses the typical 55-60°C limit of conventional refrigerants, making CO₂ ideal for processes requiring elevated water temperatures.

Potgieter concluded by emphasising that the synergy between cooling and heating in CO₂ systems can unlock significant overall efficiency gains. While the initial focus might be on the cooling plant, the potential for high-temperature heat recovery adds immense value, especially for facilities with concurrent heating demands. He also advised against undersizing storage vessels for heat recovery, suggesting that slightly larger vessels could allow for more efficient, short-duration operation.

Q&A insight: high inlet water temperatures in heat recovery

During the Q&A, an attendee raised a pertinent point regarding heat recovery: the challenge of high inlet water temperatures (for example, going from 50°C to 70°C) in transcritical systems, which typically prefer lower inlet water temperatures (such as 30°C) for optimal heat transfer.

Potgieter acknowledged this:

While increasing the transcritical pressure can raise the discharge temperature and potentially enable more heat recovery, this is limited by component design.

It’s true that CO₂ systems are better aligned to match lower inlet water temperatures.

However, even with higher inlet temperatures (for example, 50°C to 70°C), heat recovery is still possible. The trade-off is that a larger portion of the heat might still need to be rejected to the atmosphere via the gas cooler, as the system’s profile may not perfectly match the higher water temperature rise, thus reducing the amount of recoverable heat.

This final exchange underscored the technical complexities and the need for meticulous design and analysis when implementing CO₂ systems, particularly when leveraging their heat recovery capabilities across varied operational scenarios.