Luca Bisetto, Enex, at ATMO Europe 2022, presenting a case study on Enex's recent installation of CO2 heat pumps at an Italian hotel.
Luca Bisetto, Enex, at ATMO Europe 2022.

ATMO Europe: Enex’s CO2 Heat Pumps Provide AC, Space Heating and Hot Water for Sicilian Hotel

The four-pipe transcritical CO2 system can operate in cooling and heating modes simultaneously or independently of each other.

Four CO2 (R744) heat pump units made by Italian HVAC&R manufacturer Enex have been installed as part of a four-pipe stand-alone transcritical COsystem at a hotel spa resort in Sicily to provide space heating and air-conditioning, as well as domestic hot water for hotel guests.

According to Luca Bisetto, R&D Laboratory Manager at Enex, the system can operate in cooling and heating modes “simultaneously or independently of each other.”

The case study was presented by Bisetto during an HVAC and heat pumps session at the ATMOsphere (ATMO) Europe Summit on natural refrigerants. The conference took place November 15–16 in Brussels and was organized by ATMOsphere, publisher of

“The [end user] initially envisioned a mixed installation with Enex reversible heat pumps for space cooling and heating and natural gas boilers for domestic hot water,” explained Bisetto. “However, due to [the] complexity of [the] natural gas supply, the client chose a stand-alone full COsystem.”

The system is designed to provide a cooling capacity of 1.6MW (455TR) with water temperatures of 7–12°C (44.6-53.6°F) at an ambient temperature of 38°C (100.4°F). For heating, the system can provide 1.6MW in capacity with water temperatures of 35–70°C (95–158°F) at an ambient temperature of 3°C (37.4°F).

Bisetto added that the heat pumps would also work well at lower and higher temperatures, noting that Sicily experienced temperatures of around 42°C (107.6°F) this summer.

Four units for redundancy

The system includes four reversible heat pumps for redundancy and two separate heat exchangers for the production of chilled and hot water. To minimize noise levels, the system’s compressor racks are equipped with cladding and are located in the hotel’s plant room.

In chiller mode, a gravity-fed flooded evaporator rejects heat to a remote gas cooler, explained Bisetto. When in heat pump mode, the remote gas cooler can be reversed to act as an evaporator using direct expansion.

The system has been designed to offer simplified defrost using hot gas. There is also the option to reverse the fan during defrost mode to remove water-drop residue from the coil.

To manage the four heat pump units in parallel, Enex developed its own software program, said Bisetto. The main functions of the software include sequencing the compressors, rotation, defrost management, demand limit and centralization of information and setpoints, he explained.

The system, which was designed by Italian engineering consultancy Studio Delazer, has been operational since July 2022. Since then, the design temperature has been reached, and the customer is satisfied, said Bisetto.

Enex is currently monitoring the system’s energy consumption via its software, and results will be analyzed in 2023 after winter, he added.

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