The imperative of reducing compressor work – this is the fourth instalment in a five-part series.
The critical caveat is that all equipment in a given loop must be designed to operate efficiently. © RACA Journal
At a FRIGAIR 2025 SAIRAC presentation, Jannie Potgieter, a consulting engineer with Thermologica, emphasised that irrespective of ejectors, designing for higher evaporating temperatures wherever possible offers drastic, year-round savings. This directly aligns with the core principle of reducing the pressure ratio across the compressor. A higher evaporating temperature translates to a higher suction pressure, leading to a smaller pressure differential for the compressor to overcome.
The critical caveat, however, is that all equipment in a given loop must be designed to operate efficiently at these higher evaporating temperatures. If even one piece of equipment requires a lower evaporating temperature, the entire loop’s temperature will drop, negating the potential savings. This ‘weakest link’ scenario can erode the significant benefits gained from advanced components like ejectors.
Beyond the internal components, Potgieter stressed the importance of seemingly simple but often overlooked design considerations, such as the placement of gas coolers and condensers. He highlighted a common pitfall in South Africa where these units are often ‘squeezed in’ due to space constraints, leading to air recirculation. Even a slight increase of 2–3°C in the inlet air temperature due to recirculation can significantly impact the unit’s total power consumption over its lifetime. Proper planning to ensure adequate airflow and prevent hot air recirculation is crucial for maintaining design performance.
Heat recovery: CO2‘s transcritical advantage
Finally, Potgieter shed light on the advantages of integrating heat recovery with CO2 cooling plants, given CO2‘s excellent suitability for both heating and cooling. He compared CO2 heat recovery with a standard air-source heat pump (COP of approximately 4 at 15°C ambient, meaning 10kW power for 40kW heating capacity).
CO2‘s unique properties in the transcritical region make it particularly well-suited for water heating. Unlike traditional refrigerants, which condense at a constant temperature (leading to potential ‘pinch points’ with linearly increasing water temperatures in heat exchangers), transcritical CO2 undergoes a continuous temperature glide as it cools. This allows for a much better thermal match with water heating, especially in counter-current heat exchangers, minimising pinch points and maximising heat transfer.
Furthermore, CO2 can achieve much higher water temperatures than conventional refrigerants. While typical refrigerants are limited to 55–60°C on the water side, CO2 can easily push water temperatures to 80–85°C or even higher, making it ideal for applications requiring very hot water.
Forcing the CO2 system into transcritical mode is often necessary to achieve high heating capacities for heat recovery. The critical question then becomes the additional power consumed by the compressor when operating transcritically to generate this heating capacity, a topic that will likely be explored in the next segment.
The nuance of forcing transcritical operation for heating
A common belief is that incorporating heat recovery into a CO2 system is a ‘no-brainer’ due to its inherent efficiency. However, Potgieter challenged this notion, particularly when a system needs to be forced into transcritical mode solely to provide hot water. While CO2 is highly efficient for heating, the question becomes: does the energy penalty of operating transcritically outweigh the benefits of heat recovery for a specific application?
In a calculated example, forcing a system into transcritical mode at a 13°C ambient temperature (to enable 147kW of theoretical gas cooler heating capacity) resulted in an additional 10kW of compressor power consumption. For comparison, a standalone heat pump providing 40kW of heating might consume 10kW of power (a COP of 4).
Potgieter emphasised that for smaller heating demands (for example, around 20–30kW), it might not even be necessary to operate in transcritical mode, as sufficient preheating energy can often be extracted from the subcritical phase. There will be a ‘magic transition number’ where forcing transcritical operation becomes beneficial.
