A recent study of medium-sized cruise ships has shown that a transcritical CO2 (R744) system can be efficiently used for both air-conditioning and refrigeration (chilling and freezing) of food provisions, with the AC working in concert with a cold thermal energy storage (CTES) system to improve efficiency.

The study, supported by the Research Council of Norway, was conducted by a team of researchers from the Norwegian University of Science and Technology (NTNU) and SINTEF Energy Research, who performed simulations to examine the CO2 system’s effectiveness under different weather conditions. Cooling COPs of up to 5.99 for the AC system and 3.54 for the provision refrigeration system were calculated in varied ambient conditions.

One of the researchers, Muhammad Zaheed Saeed, a doctoral student at NTNU, explained the project in a paper, “Investigation of the CO2 refrigeration system and thermal energy storage for passenger ships,” presented at the 10th International Institute of Refrigeration (IIR) conference, held in Ohrid, North Macedonia, April 27–29. 

The worldwide push for environmental responsibility has driven the cruise ship industry towards natural refrigerants like ammonia (R717), hydrocarbons and especially CO2. CO2, being nontoxic and nonflammable, is often a refrigeration solution that fits seamlessly within the limited space of cruise vessels, noted Zaheed Saeed.

According to the study, the proposed CO2 AC system can achieve a maximum cooling COP of 2.89 in warm ambient conditions, 4.19 in medium conditions, and 5.99 in cold conditions compared to a traditional AC system. The CO2 refrigeration system for food and beverages (provision system) also demonstrated good performance efficiency, with COPs of 2.33, 2.98 and 3.54 for warm, medium and cold conditions, respectively.

Zahid Saeed demonstrated that the AC system can also provide heat recovery ranging from 400kW (113.7TR) to 5000kW (1421.7TR) in the three different scenarios. Likewise, the provision system can recover between 55kW(15.6TR) to 140kW(39.8TR) in heat.

Use of CTES

Many cruise ships are now implementing innovative energy-saving systems that use latent heat storage in compact thermal energy storage (CTES) systems. CTES systems store energy in water by utilizing its phase change properties.

While the ship is sailing, the air-conditioning system cools (charges) the water in the CTES system. Once the ship docks, the stored energy is released by circulating water through the ship’s AC system, thereby saving energy. Due to the limited space on cruise ships, there are typically multiple CTES systems that operate simultaneously. Additionally, the water is stored in specially designed “dual-bundle” containers to increase efficiency.

“During the two-hour port stop, in order to cool efficiently, a cold thermal energy storage system requires around 21,163kg [46,656lbs] of water to act as a phase-change material,” explained Zahid Saeed. “This amount of water is compactly housed within a 38m3(1341.95ft3)  space.”

The study explained how the CTES is charged by the AC system. During the charging process, the liquid CO2 from the receiver is throttled to 31bar (449.6psi) prior to entering the latent heat storage (LHS), or CTES system, where it is superheated to 5°C (41°F). The refrigerant is then compressed to receiver pressure before being mixed with the refrigerant in the internal heat exchanger (IHX). The LHS chills the water from the HVAC system, which is then returned.

“By implementing the CTES systems, we are not only providing a more environmentally friendly cruise experience but also improving the efficiency of our ships,” said Zahid Saeed. “It’s a mutually beneficial situation.”

Different SINTEF researchers recently explored how CTES can be used with a CO2 system for AC in a supermarket.

CO2 systems for passenger ships

AC system: The AC system has a two-stage evaporator setup with ejector support. Once compressed, the CO2 refrigerant flows through a gas cooler for heat recovery, which can be partially or fully bypassed depending on demand. The excess heat is then released into the seawater through another gas cooler.

The refrigerant is then subcooled in an IHX and expanded with the help of an ejector to move it from the second stage. It passes through two evaporators, aided by the ejector, before being superheated by another IHX and compressed. This process cools water from 12 to 6°C (53.6 to 42.8°F) for AC purposes.

Provision system: The cooling and freezing system is designed to work efficiently using parallel compression and ejector support. The CO2 refrigerant from the medium temperature (MT) compressor passes through two gas coolers, one for heat recovery using cooling water and the other using seawater. After cooling in the IHX, the refrigerant passes through an ejector. The low-temperature (LT) evaporator operates at low pressure for optimal performance.

The refrigerant used in the LT evaporator is compressed separately and then combined with the refrigerant used in the MT evaporator. The combined refrigerant is handled by a parallel compressor. In warmer ambient areas, the parallel compressor might be sufficient on its own, provided that the receiver pressure is adequate.

“By implementing the CTES systems, we are not only providing a more environmentally friendly cruise experience but also improving the efficiency of our ships”

Muhammad Zaheed Saeed, a doctoral student at Norwegian University of Science and Technology

Author Saroj Thapa