By combining a transcritical CO2 (R744) booster system with a cold thermal energy storage (CTES) system instead of a traditional glycol circuit for air conditioning, a Norwegian supermarket chain would be able to reduce peak power usage by 13% to 19%, according to simulations conducted by a Norwegian researcher.

The CTES system uses water/ice as the storage medium.

The proposed system was explained by Håkon Selvnes, a research scientist at SINTEF Energy, in a paper, “Cold thermal energy storage for air conditioning in a supermarket CO2 booster refrigeration system,” presented at the 10th International Institute of Refrigeration (IIR) conference, held in Ohrid, North Macedonia, April 27–29.

The research, proposing an innovative refrigeration system for Norwegian supermarkets, is part of a Norwegian program called the HighEFF Centre for an Energy Efficient and Competitive Industry for the Future. The project received financial support from the Research Council of Norway and user partners of HighEFF. It is also a part of the Novel and Innovative Emerging Concept (NIEC) project TES-AC, which focuses on the monitoring and analysis of a pilot thermal energy storage unit to supply air-conditioning in commercial refrigeration systems.

Integrated transcritical CO2 booster systems can be designed to supply cooling and freezing of food products and air-conditioning of the shop area. The systems are able to handle all refrigeration and AC loads during the warmest summer days, resulting in overcapacity and part-load operation for most of the year.

In his paper, Selvnes suggests a new more efficient model with a CO2 circuit and CTES for the AC, reducing the AC demand. “By using water/ice as the storage medium, the CTES provides a method to store ‘cold’ energy during the night and utilize it for AC during peak daytime hours,” he said.

“The integration of CTES technology into refrigeration systems empowers supermarket owners with the ability to decouple the supply and demand of refrigeration,” said Selvnes. “This flexibility means systems can adapt readily to future electricity markets, offering both significant savings and a sustainable path forward.”

Four cities and scenarios

Early numerical simulations using ambient conditions in four Norwegian cities (Oslo, Kristiansand, Tromsø and Trondheim) have shown “promising results,” noted Selvnes. The simulation with the new system was performed under four different scenarios:

Scenario 1: This approach mirrors a traditional system using a glycol loop. AC is produced in real-time based on immediate needs. The cost of electricity is determined by the market price.

Scenario 2: This is where the CTES comes into play. Instead of generating AC in real-time, the AC needed for the following day is produced the previous night over an eight-hour period when the shop is closed. This AC is stored as ice in the CTES unit, which is then used throughout the following day. “The assumption here is that CTES storage is dimensionally adequate to cover the AC load,” said Selvnes. The cost of electricity follows the market price.

Scenario 3: This approach retains real-time AC production as in Scenario 1 (using a glycol loop traditional system) but alters the cost of electricity. Instead of fluctuating market prices, it proposes fixed day and night prices, with the daytime prices set at 50% above the average and the night-time price at 50% below the average.

Scenario 4: This scenario integrates the best of scenario 2 (night-time production and storage of AC using CTES), and in terms of electricity it follows scenario 3. This combination is aimed at maximizing both energy and cost efficiency.

When comparing scenario 1 (traditional CO2 booster system) and scenario 2 (night-time production and storage of AC using CTES), peak power demand was found to drop by 13% in Tromsø, 15% in Kristiansand, 16% in Oslo, and 19% in Trondheim. “This represents a significant decrease in power grid pressure during peak hours,” said Selvnes.

Likewise, in comparison with scenarios 3 and 4, scenario 4 offers a potential savings of 2% in Tromsø, 2.2% in Trondheim, 6.8% in Kristiansand and 7.3% in Oslo. “This proves the potential savings from reducing the maximum power required by the supermarket,” Selvnes said. “Diminishing this demand directly impacts the consumption tariffs paid to the grid operator, and many operators now offer tariff reductions to customers who can demonstrate flexibility in power demand.”

The new proposed model by the researcher:

The novel concept for supplying AC by CTES unit based on water/ice as the storage medium. The left shows the system in charging mode, the right shows the system in discharging mode.

“The integration of CTES technology into refrigeration systems empowers supermarket owners with the ability to decouple the supply and demand of refrigeration.”

Håkon Selvnes, Research scientist at SINTEF Energy Research

Author Saroj Thapa