A study by Dorin, an Italian multinational compressor manufacturer, shows that enhanced transcritical CO2 systems can achieve energy parity with an ammonia/CO2 cascade system in warm U.S.cities like Atlanta, Georgia, and Orlando, Florida.

The study – IIAR Technical Paper #3: Transcritical CO2 and Ammonia: Energy Efficiency and Basic Capital Cost Comparison for Industrial Refrigeration Systems –was discussed by Giacomo Pisano, Sales Manager at Dorin, in an online session on May 19. The session was part of the 2020 IIAR Online Conference & Virtual Exposition, hosted in May and early June by the Alexandria, Virginia (U.S.)-based International Institute of Ammonia Refrigeration. The original physical conference, scheduled for March 15-18, 2020, was cancelled due to the COVID-19 pandemic.

The efficiency comparison was made using Pack Calculation Pro, a product by IPU, a Denmark-based engineering consultancy firm.

The software is a simulation tool for calculating and comparing the yearly energy consumption of refrigeration systems and heat pumps in a variety of simulated locations and time periods, according to the IPU website.

This allows for a “very accurate” simulation of the energy efficiency of different refrigerants in various climate zones, said Pisano.

The study covered six North American cities – Montreal, Canada; Boston, Massachusetts; New York, New York; Washington, D.C.; Atlanta, Georgia; and Orlando, Florida. It compared the energy performance of six transcritical CO2 systems and four ammonia systems, including a baseline ammonia system.

The transcritical systems included a standard flash-gas bypass system (with and without evaporative condenser), a parallel condenser system (with and without evaporative condenser), and an ejector system (with and without evaporative condenser).

The ammonia systems included the baseline (NH3/CO2 cascade with evaporative condenser) and three others: NH3/CO2 with cooling tower, NH3 with evaporative condenser, and NH3 with cooling tower.

All CO2 and ammonia systems have a low-temperature capacity of 142TR (499kW) and a medium-temperature capacity of 708TR (2,490kW).

Standard transcritical systems perform less efficiently in warm ambient temperatures. But in the cities with the warmest ambient temperatures, Orlando and Atlanta, the study found that a transcritical CO2 system with an ejector and evaporative condenser achieved energy parity with the baseline NH3/CO2 system. The same CO2 system used 15% less energy than the baseline system in Montreal.

“CO2 outperforms [ammonia/CO2] in cold weather and is similar to it in warm weather,” said Pisano. An ammonia-only system with evaporative condenser in Orlando consumed 6% more energy than the baseline NH3/CO2 system and the ejector CO2 system with evaporative condenser.

Larger compressor sizes, combined with adding enhancements like parallel compression and ejectors, allows CO2 to be considered “a good alternative to synthetic refrigerants for industrial application, together with ammonia,” said Pisano.

Cost comparison

Another factor in favor of industrial transcritical CO2 systems is that they cost less than NH3-based systems.

Unlike ammonia systems, CO2 systems do not require stainless- or carbon-steel piping. The equipment is typically made pf brass or copper, and thus “you don’t need a welding procedure on site,” said Pisano. In addition, CO2 systems are typically smaller than their ammonia counterparts.

Compared with NH3/CO2 cascade, “a basic CO2 medium-temperature [system], featuring about 3,500 kbtu/h brings a cost saving between 30% to 50% in favor of CO2 transcritical units,” said Pisano.

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