Heat recovery in CO₂ systems: refrigeration becomes an energy hub

In commercial CO₂ (R744) refrigeration systems, the heat produced during the refrigeration cycle does not necessarily need to be dispersed outdoors. More and more systems are being designed to recover it and use it for space heating, hot water production, dehumidification, and other thermal applications .

Heat recovery is not entirely new to refrigeration, but the characteristics of CO₂ open up particularly interesting possibilities. The high exhaust temperatures available under certain conditions allow thermal energy to be obtained at temperatures compatible with numerous building services.

For supermarkets, shopping centres and other facilities characterised by a continuous demand for refrigeration, the CO₂ rack can therefore evolve from a simple refrigeration unit to an integrated energy system capable of simultaneously producing cold and heat .

Why CO₂ is interesting for heat recovery

Every refrigeration system transfers heat from the refrigerated components to the outside. This energy is supplemented by the work done by the compressors.

Normally, heat is expelled through condensers or gas coolers. With heat reclaim , however, a portion is intercepted before dissipation and transferred to another user.

CO₂ has an important advantage: it can reach relatively high exhaust temperatures , making available useful quality heat for applications that may be more difficult to supply with other systems.

Different temperature levels may be present in centralized booster racks. Indicatively:

• low temperature circuits can deliver heat in the order of about 54-66°C ;
• medium temperature circuits can reach significantly higher exhaust temperatures;
• Through suitable exchangers, some systems can produce water at temperatures in the range of 60-88 °C , depending on the operating conditions.

The actual value naturally depends on the system configuration, the cooling load and the operating pressures.

From point-of-sale heating to hot water

The recovered heat can be used in different points of the building.

One of the most immediate applications is space heating . Through an exchanger, the heat from the refrigeration circuit can be transferred directly to the air treated by the HVAC system or to a hydronic circuit.

Possible applications include:

• space heating;
• preheating of domestic water;
• hot water production;
• dehumidification ;
• heating of glycol circuits;
• frost protection of cell floors;
• power or support for other thermal systems.

Water preheating is particularly interesting. The energy recovered during peak cooling hours can be transferred to a thermal storage system and used later when demand increases.

In this way , refrigeration and thermal storage can also help shift part of energy consumption over time.

Direct recovery or hydronic circuit: different possible configurations

There is no single architecture for recovering heat from a CO₂ plant.

One solution is to transfer the heat from the exhaust gas directly to a coil installed in the air handling unit , using it to heat the air intended for the rooms.

Another option uses a CO₂-water or CO₂-glycol exchanger . The secondary circuit can then distribute the heat to one or more users in the building.

This configuration offers greater flexibility because it allows you to connect different applications to the same heat source.

A system can therefore include:

• CO₂ refrigerator rack;
• recovery exchangers;
• hot water storage;
• hydronic circuit;
• HVAC unit;
• supervision systems;
• any additional heat pumps.

The design therefore becomes increasingly similar to that of an integrated power plant , in which cooling production and heating needs are analysed together.

Heat pumps and CO₂ can work together

A particularly interesting development consists in combining the heat recovery of refrigeration with heat pumps .

The recovered heat can become a favorable heat source for a heat pump, or be used directly when the available temperature is already sufficient.

This configuration allows to reduce the use of electrical resistors or generators powered by fossil fuels.

Solutions may include heat pumps:

• air-water;
• water-water;
• integrated with thermal accumulators;
• connected to other sources available in the building.

The potential benefit is particularly significant in facilities that have both a cooling demand and a heating demand .

Supermarkets and hypermarkets are a clear example: refrigerated cabinets and storage rooms operate year-round, while the building may require heating, dehumidification, and hot water.

How much can it affect consumption?

The benefit depends heavily on the climate, the size of the building, and the simultaneity between cooling production and heating demand.

In a simulation conducted on different supermarket and hypermarket configurations, the integration of R744 refrigeration, direct heat recovery, and heat pumps showed, in the most favorable configuration analyzed, a reduction of approximately 53% in energy used for space heating and 35% in overall energy compared to a reference solution with traditional refrigeration and gas heating.

These values ​​aren't automatically replicable in every installation. However, they demonstrate how heat recovery can impact the energy balance when refrigeration and HVAC are designed as a single system.

The important parameter therefore becomes not only the cooling COP, but the overall amount of energy used by the building .

Recovery must not penalize refrigeration

However, one priority remains unchanged: the system must continue to ensure the correct preservation of products.

Artificially increasing the condensing pressure or high side pressure can allow heat to be obtained at a higher temperature, but can also increase the workload of the compressors.

The strategy must therefore find a balance between the quantity and quality of recovered heat and cooling efficiency .

Controls capable of modulating become essential:

• pressures;
• valves;
• compressors;
• fans;
• pumps;
• secondary circuit temperatures.

The supervision system must always assign priority to refrigeration, activating heat recovery within operating conditions compatible with reliability and efficiency.

From the refrigeration plant to the building's energy system

The growth of heat recovery shows how the role of commercial refrigeration is changing.

A CO₂ rack no longer necessarily needs to be considered as a system that produces cold and disperses heat , but as a machine capable of transferring energy within the building.

If designed correctly, the same system can cool counters and cells, contribute to room heating and produce hot water.

This is a particularly interesting prospect in the decarbonisation process of large-scale retail trade: the low GWP of R744 affects emissions linked to the refrigerant, while heat recovery can reduce energy consumption associated with other building services .

The real evolution therefore becomes integration: refrigeration, HVAC, hot water production, storage and energy control are increasingly designed as parts of the same system.