Further optimisation strategies

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

A crucial advancement in CO2 system design involves the strategic incorporation of a parallel compressor. © RACA Journal

…continued from part one.

At a FRIGAIR 2025 SAIRAC presentation, Jannie Potgieter, a consulting engineer with Thermologica, briefly touched upon other optimisation techniques:

• Internal Heat Exchangers (IHX): Recommended for improving efficiency by subcooling the liquid before the expansion valve and superheating the suction gas.
• Evaporative Cooling: Proposed as a method to lower the gas cooler exit temperature, bringing it closer to the wet-bulb temperature and thus reducing the transcritical pressure.
• Injectors: Acknowledged for their potential to recover expansion work but noted their high cost as a barrier to widespread adoption.
• Finally, he highlighted CO₂‘s excellent properties for heating applications in transcritical mode, demonstrating that a 10kW increase in compressor power could yield a substantial 147kW of heating capacity, showcasing its versatility beyond just cooling.

Potgieter’s presentation set the stage for a deeper understanding of CO₂ system optimisation, emphasising that every added component serves a clear purpose in reducing compressor work, ultimately leading to more energy-efficient and resilient refrigeration solutions.

The power of parallel compression

A crucial advancement in CO₂ system design involves the strategic incorporation of a parallel compressor. As discussed previously, a significant challenge in transcritical CO₂ systems is the generation of substantial flash gas in the receiver. In a basic system, this flash gas is typically expanded to a lower pressure and then re-compressed by the main medium-temperature compressors, leading to increased compressor work.

By adding a parallel compressor, this flash gas can be directly drawn from the flash gas tank at its higher pressure, thereby bypassing the expansion to a lower pressure. This effectively creates a lower pressure ratio for the parallel compressor compared to the main compressors. The result is a notable reduction in the overall work required. Potgieter highlighted a specific example: the addition of a parallel compressor improved the Coefficient of Performance (COP) from 1.67 to 1.85. Consequently, parallel compressors are integrated into most of the advanced system configurations discussed.

However, Potgieter clarified that parallel compressors are not universally necessary. Their advantages are diminished in systems primarily operating subcritically or when the evaporating temperatures are already quite high. In such scenarios, the pressure difference between the medium evaporating temperature and the gas cooler, or the parallel compressor’s operating range, might be too small to justify the added complexity and cost. Therefore, careful evaluation is crucial.

Embracing subcritical operation as the ideal scenario

While much attention is given to managing transcritical CO₂, Potgieter emphasised that subcritical CO₂ operation represents the ideal scenario for efficiency. In a subcritical cycle, CO₂ behaves more like conventional refrigerants, condensing directly from gas to liquid. This drastically reduces the amount of flash gas, making the system inherently more efficient. The COP of a subcritical CO₂ system can reach 3.11, surpassing even R404A systems (which had a COP of 3.02 in the presented comparison).

The primary goal for CO₂ system optimisation, therefore, is to maximise subcritical operation and minimise time spent in transcritical mode. When transcritical operation is unavoidable, the focus shifts to optimising that specific part of the cycle.

Balancing complexity and savings with additional components

While significant improvements can be made, each additional component introduces complexity and capital cost. Potgieter stressed the importance of careful evaluation to determine if the efficiency gains outweigh these factors over the system’s lifespan.

1. Internal Heat Exchangers (IHX):

Adding an internal heat exchanger between the liquid line and the low-temperature suction line offers multiple benefits:

• Compressor protection: Subcooling the liquid line before the evaporator helps prevent liquid droplets from returning to the compressor, enhancing its safety and longevity
• Reduced mass flow: Subcooling the liquid reduces its enthalpy, meaning less mass flow is required through the evaporator for the same cooling capacity. This, in turn, reduces the work demanded of the low-temperature compressor
• Universal benefit: IHXs improve efficiency across the entire range of operational conditions, including subcritical mode, making them a generally worthwhile investment

2. Desuperheaters:

A desuperheater, typically placed after the low-temperature compressor, reduces the discharge temperature of the superheated gas. While primarily for compressor protection (preventing excessively high discharge temperatures from entering the medium-temperature compressor), it also offers a subtle power saving. By lowering the temperature of the gas entering the next compressor stage, its density increases, which can slightly improve the volumetric efficiency of the subsequent compressor. Desuperheaters are particularly important for systems with a large low-temperature load to prevent ‘running away’ discharge temperatures. Like IHXs, they offer small but continuous savings in both transcritical and subcritical modes, making them a recommended addition where feasible.

Continue to part three…