Chillventa 2024: Free Piston Expander Cuts Energy Use of Transcritical CO₂ Systems by Up to 18%

Italy-based Turboalgor’s free piston expander, Freeex, can cut the energy use of a transcritical CO₂ (R744) refrigeration system by up to 18%, according to Innovation Technology Manager Maurizio Ascani.

Ascani explained the technology and the results seen in laboratory and field testing during a presentation at Chillventa 2024. The event ‒ hailed as one of the world’s largest HVAC&R trade shows ‒ was held October 8‒10 in Nuremberg, Germany.

“The free piston expander recovers energy otherwise lost at the expansion valve in a transcritical CO₂ system,” Ascani said. “With it, we can increase the pressure between the evaporator outlet and the inlet of the main compressor by up to 5bar [72.5psi].”

Italian-based Epta, an international manufacturer specializing in commercial and industrial refrigeration systems, has partnered with Turboalgor to demonstrate the feasibility and viability of the technology.

In 2023, Turboalgor received a grant of almost €1.8 million ($2 million) from the European Commission’s LIFE Programme to develop/test the CO₂ expander. The LIFE Programme, launched in 1992, serves as a funding mechanism for environmental and climate action.

The technology: According to Turboalgor, Freeex, covered by two patents, includes the free piston expander and an associated fluid control unit.

• “The pressurized gas enters a rotating valve that continuously switches the direction of the gas to move the piston,” Ascani said. “From this, we have the effect of a reciprocating compressor using the energy of the pressurized gas.”
• With an oscillation rate of 15 switches per second, Ascani noted that the free expander offers a speed similar to an electrically driven piston compressor.
• A 100W electric motor operates the rotating valve, providing fluid control. “The complementary device, not the primary one, consumes a minor amount of electricity,” Ascani said.
• In a product brochure, Turboalgor indicated that the technology works with new and old CO₂ systems, allowing retrofit applicability.

Testing results: According to Ascani, the energy savings and the capacity to generate additional pressure for the main compressor depend on the application.

• At a gas cooler outlet temperature of 30°C (86°F), the device cuts the energy consumption of a CO₂ system by 15%, Ascani said. At 40°C (104°F), the energy savings reach 18%. According to the company’s website, the device can also increase the cooling capacity of a CO₂ system by more than 40%.
• In one external test case, the free piston expansion technology raised the gas pressure received by the main compressor by 3.5bar (50.8psi). In another, it raised it by 4.5bar (65.3psi).

Similar concepts: Other devices on the market also offer “free compression” for transcritical CO₂ systems.

• U.S.-based Energy Recovery’s PX G1300 pressure exchanger can increase the energy efficiency of CO₂ refrigeration systems by up to 25% at ambient temperatures around 37°C (98.6°F) compared to a standard booster system, according to the company.
• Ricardo Freitas, Vice President and General Manager of CO₂ at Energy Recovery, told NaturalRefrigerants.com at Chillventa 2024 that the company expects racks with integrated PX G1300s manufactured by Epta, Fieuw Koeltechnik and Kysor Warren to hit the market in the coming months.
• Danish component manufacturer Danfoss offers a combination CO₂ ejector capitalizing on the Venturi effect. According to Key Account Manager Steve Renz, the unit uses free compression to increase CO₂ system efficiencies, with a pilot Pennsylvania store seeing an average 14% reduction in annual energy consumption.

Quotable: “The free piston expander makes it possible to increase the cooling capacity of a CO₂ refrigeration system while reducing the energy consumption,” Ascani said. “We are working on a production range with three models which can be used alone or combined in parallel to meet the needs of different sized systems.”