3.2 Improving CO2 Efficiency in Warm Climates

Concerns around the efficiency of CO2 refrigeration systems in warm climates have long been seen as a barrier to CO2 uptake worldwide, with a so-called “CO2 equator” dividing viable installations to the north from non-viable ones to the south. However, technology developments such as adiabatic condensing, ejectors and parallel compression have increasingly made CO2 technology viable in all climates, including regions previously deemed not suitable.

Ejectors and parallel compression make CO2 systems more efficient while operating in transcritical mode. Parallel compression compresses the excess gas at the highest possible pressure level. This leads to a significant increase of coefficient of performance (COP) in warm climates.65

Evaporative condensation, mechanical sub-cooling and adiabatic gas cooling decrease the outlet temperature of the gas cooler and force the system to operate longer in subcritical mode, thereby making it more efficient.

Manufacturers are continuing to come up with new ways to improve the efficiency of refrigeration systems, and there are multiple solutions from different companies already available on the market today.

For example, In a supermarket in Italy, 15–20°C (59 –68°F) groundwater is used as a cooling fluid to condense the CO2 instead of air, allowing the system to run in subcritical mode even during the hottest summer months when the ambient air temperature is 28°C (82.4°F) or more.66

Last year Belgian retailer Delhaize, part of the Ahold Delhaize group, reported installing an evaporative cooling system on existing air-cooled gas coolers serving transcritical CO2 systems at more than 100 corporate stores, effectively turning the gas coolers into energy-saving adiabatic systems.67

Delhaize's evaporative cooling system, called the chillBooster, from Italian component manufacturer Carel, consists of a cabinet with a pressurizing pump (10bar/145psi) and a system of nozzles mounted onto steel manifolds around the condensing unit.68

Delhaize installed the chillBooster retrofits in response to a series of heat waves during the summer of 2019, which caused refrigeration performance issues at some Delhaize stores.

Another new technology designed to improve the efficiency of transcritical CO2 systems is the PXG1300, a pressure exchanger device made by Energy Recovery, a San Leandro, California-based manufacturer of pressure-exchanger devices for several industries.

A new store in Europe will be the second retail location in the world to install a transcritical CO2 (R744) system that uses the integrated pressure-exchanger device. The device will also be employed by Vallarta Supermarkets, a California chain with more than 50 locations, as a retrofit of an existing CO2 system in a store located in Indio, California.69 Both installations are taking place in 2022.

Italian OEM Epta has designed its own technologies to improve the efficiency of transcritical CO 2 systems in warm climates, the FTE (Full Transcritical Efficiency) and ETE (Extreme Temperature Efficiency).70 ETE goes an extra step, enabling efficient operation in temperatures higher than 40°C (104°F), said Epta.

The FTE system, Introduced in Europe in 2017 (with version 2 released in 2020), employs a low-pressure liquid receiver to flood medium-temperature evaporators with liquid CO2. This eliminates superheat and allows the evaporation temperature in the cabinets – and ultimately the efficiency of the system – to increase.

Introduced in Europe in 2020, ETE uses a heat exchanger to subcool a portion of the CO2 coming out of the gas cooler and delivers the refrigerant – expanded to an intermediate pressure level by the EEV (electronic expansion valve) – to the high-pressure line.

CO2 anywhere

By removing the barrier that prevents CO2 systems from operating efficiently in higher ambient climates, these technologies make it possible to install CO2 systems in stores and warehouses anywhere in the world. This is leading to growth in CO2 systems in regions previously not considered suitable – such as the southern U.S. – and this positive trend in CO2 system growth is expected to continue.