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Featured STBE Article, June 2019

Integrating Thermal Storage into Subcooling System of CO2 Booster Refrigeration Racks

From eSociety, June 2019

As transcritical CO2 supermarket refrigeration equipment is becoming a common near-zero global warming potential (GWP) solution, more attention is being given to improving these systems’ efficiencies, especially in transcritical operation, to counteract reduced efficiency during hot outdoor temperatures. Improving the cycle efficiency by using dedicated mechanical subcooling is one way to improve system efficiency.

A recent article in Science and Technology for the Built Environment examines further enhancement to the subcooler for the booster cycle by adding the use of thermal storage.

John Bush, Ph.D., Associate Member ASHRAE; Vikrant Aute, Ph.D., Member ASHRAE; and Reinhard Radermacher, Ph.D., Fellow/Life Member ASHRAE, wrote “Thermal Storage Subcooling for CO2 Booster Refrigeration Systems.”

Bush explained the research’s significance and challenges.

1. What is the significance of this research?

Our work examined integrating thermal storage into the subcooling system of CO2 booster refrigeration racks. Dedicated mechanical subcoolers are one way to greatly improve the efficiency of booster systems, particularly when they are operating in the transcritical range during hot outdoor temperature conditions.

In our research, we showed that integrating thermal energy storage into the subcooling system could allow the full benefits of mechanical subcooling to be maintained, while shifting the energy consumption of the subcooling system to off-peak hours.

We believe this could be part of a strategy to make supermarkets, or other large refrigeration systems, much more flexible.

2. Why is it important to explore this topic now?

One of the important changes that our industry is experiencing is the transition from high-GWP refrigerants to low-GWP refrigerants. The refrigeration world is feeling the push most of all because of their need for large quantities of refrigerant.

One of the solutions is CO2, but without careful design, CO2 systems can have low efficiency in hot outdoor conditions. At the same time, many utility companies are increasingly interested in ways of influencing when their customers use power—shifting energy away from peak periods with variable rates, demand charges or other strategies.

So as the industry looks to transition to different technologies such as CO2 booster systems, the time is right to look for innovative new ways to control how those systems use power.

3. What lessons, facts, and/or guidance can an engineer working in the field take away from this research?

One of the underlying motivators for our research is that CO2 booster systems, which are gaining in popularity, can be deployed in all climates. But they must be carefully engineered for hot climates to avoid potentially high demand charges or on-peak energy costs during hot weather.

There are a number of innovations in the industry, and our research supports the idea that dedicated subcoolers are one very useful solution for these systems. The integration of thermal storage into these systems, while not on the market today, is something we hope will be coming.

4. How can this research further the industry's knowledge on this topic?

We hope our research will increase awareness and attention paid to dedicated mechanical subcooling, as well as awareness of the potential for supermarket systems and other large-scale refrigeration systems to be more flexible in when they use energy.

The supermarket world is undergoing a period of major changes, so now is a great time to consider what other innovations we can be adopting.  

5. Were there any surprises or unforeseen challenges for you when preparing this research?

Researching new technologies in large-scale refrigeration systems is really hard. In the larger research effort leading to this paper, we were able to conduct testing on a laboratory-scale system. But the availability of detailed data on large-scale systems like this is relatively limited, and it’s difficult and expensive to perform such testing. Also, transient modeling of systems as complex as booster systems operating on both sides of the critical point can be really tricky.