For a decade or more, the potential of CO2 in supermarket refrigeration has been recognized in Northern Europe.Thanks to its unique properties, notably the high heat transfer coefficients and the low sensitivity to pressure losses, CO2 delivers high performance in supermarket applications.
In warmer climates, the advantages of CO2 in food retail applications have been disputed. In recent years, however, the interest from forward-thinking retailers has fueled the invention of new technologies that make transcritical CO2 systems in warmer climates both viable and profitable. New technologies are rapidly emerging as highly energy efficient solutions that help retailers reduce complexity and meet current and future regulation on traditional refrigerants.
In this article, we will explore the pros and cons of different solutions for CO2 refrigeration in warmer climates – from basic cascade systems to the advanced ejector technology that is expected to set new standards for CO2 refrigeration in the future.
CO2 is one of the most environmentally friendly refrigerants with a GWP of only 1; it is widely available as a by-product in a number of industries, and the price of CO2 is low.
CO2 has played a role in warmer climates for several years, typically in subcritical cascade systems that enable CO2 to perform at its best. A cascade system combines the benefits of CO2 in the secondary loop with another refrigerant in the primary loop, providing high energy efficiency in warm climates. However, the first cost and complexity of cascade systems are high and constitute a barrier to more widespread use of CO2 in food retail applications. This is particular evident in smaller systems where the complexity of cascade systems increases the first cost, to which smaller format stores are particularly sensitive to.
Figure 1: CO2 Transcritical Booster Plant with Intermediate Compressor
The transcritical booster system has become the most common CO2 solution today with more than 7,000 systems in place with Danfoss components alone. The system is particularly popular in temperate climates, whereas in warmer climates more development of the technology is needed in order to achieve similar or better energy efficiency than non-natural solutions. These technologies will be discussed in the following chapter.
Figure 2: Outline of CO2 transcritical booster system
Parallel Compression – Highly Efficient in Warm Climates
In the pursuit of simplification and energy efficiency, other solutions have surfaced to allow CO2 as the sole refrigerant in supermarket refrigeration. Parallel compression is the first step on the journey to make CO2 an attractive solution in warmer climates, especially for larger installations. Parallel compression is therefore steadily advancing as a highly energy efficient CO2 solution.
Parallel compression is a solution that compresses the excess gas at the highest possible pressure level to improve the energy efficiency. The solution yields significant improvement of COP in warm climates.
Evaporative condensation solutions use water to cool the gas in transcritical CO2 applications. The solution is well-suited for small refrigeration systems and most attractive when it can be combined with AC systems on site. The energy savings returned by the solution are about 5-10 % in warm climates. Furthermore, it is possible to save up to 50% on the compressor capacity; however, the capacity is typically needed on the auxiliary cooling unit instead.
The solution is only applicable in areas with no restriction on water supplies. Besides, several markets have restricted the use of evaporative condensation due to the risk of legionella contamination from the water used in the application. Evaporative condensation is regarded as a temporary solution filling the gap in CO2 refrigeration until other and safer solutions have matured.
Mechanical sub-cooling is yet another solution that has emerged in recent years in the quest to make transcritical CO2 refrigeration viable in warmer climates. Mechanical sub-cooling uses a small mechanical vapor-compression cycle coupled to the main cycle at the exit of the condenser in order to provide sub-cooling to the main refrigeration cycle.
The extra unit turns on only when the temperature exceeds a certain level and will therefore return energy savings during peak loads.
Figure 5: Outline of Evaporate Condensation unit for a small supermarket
Ejector is a well-known technology that has been used for more than 100 years in different applications, notably in water utilities, to retain the pressure in the system. In close cooperation with SINTEF, Danfoss has devised new ways to use the ejector technology in refrigeration application to increase the energy efficiency of parallel compression.
Today, the current experiments with the ejector technology return promising results proving the viability of transcritical CO2 system in warm climates. The high energy efficiency is achieved by recovering the energy taken out while reducing the pressure
from the gas cooler to the liquid line pressure. The ingenious solution reduces the workload of the compressors, while still ensuring that the cooling demand is met at any time.
The ejector is still in the prototype stage, but the initial trial set-ups in more than 10 supermarkets in developed countries have shown that the simple ejector technology can increase the efficiency of the parallel compression system as much as when going from no parallel compression to parallel compression. The energy saving potential compared to traditional HFC systems is significant. Furthermore, the ejector technology allows smaller and more compact compressor packs to be installed, reducing the first cost of the installation.
Next step in the development of the ejector technology for refrigeration is a liquid ejector that allows the MT evaporators to be flooded. This means added savings resulting from a higher suction pressure. Combined liquid and gas ejectors have run in trial set-ups since 2013 yielding energy savings of 20-25 % compared to HFC systems. The first release of commercial liquid ejectors is expected within 2-3 years.
