Throughout the past decade the need for supermarkets to reduce the HFC refrigerants they use was become increasingly clear. By the end of the decade the next generation of Second Nature systems began entering the market and the key to these systems was their use of CO2.
The reasons for moving away from HFCs and to CO2 were numerous. Foremost among these were environmental concerns that lead initially to international agreements (i.e., Montreal and Kyoto Protocols), and then, more recently, to regulatory and statutory actions by the U.S. government (i.e., Clean Air Act and subsequent legislation) and abroad (i.e., European Union F-gases regulations) to curb the use of HCFCs and HFCs. The impact of regulatory pressures on HFC system operators added economic justifications to the move away from their use, as not just the penalties for emissions increased, but the scheduled phase-out of their production added to retailers’ costs.
Natural Refrigerants
The overarching need for sustainable approaches to refrigeration has strengthened as these forces upon the market have intensified. European grocers have for some time used different natural refrigerants, including hydrocarbons such as propane and iso-butane, and ammonia. But although these refrigerants have worked well there, for supermarkets here certain aspects of their use provide challenges. Hydrocarbons for instance, can be flammable, and ammonia has long been subject to restrictions on commercial use due to its toxicity. These approaches for North American retailers don’t appear to be practical. But another natural refrigerant — CO2 — does.
Carbon dioxide (CO2/R-744) is colorless, odorless, and naturally available in the atmosphere. CO2 is produced through natural processes, including the carbon cycle in which it occurs as a product of respiration in animals and from fermentation of organic compounds. It’s also produced by other natural phenomenon, including volcanic activity and as a product of combustion, although it is itself nonflammable; it doesn’t support combustion.
CO2 (R-744) Systems
CO2 as a secondary fluid has been used here in low and medium-temperature applications. Following on the success of these systems, cascade, or direct expansion (DX) systems have seen increasing use in low-temperature applications. As of 2011, fifteen low temperature CO2 cascade systems from different manufacturers in U.S. and Canada have been installed with retailers. Most of these systems include glycol secondary refrigeration for their medium-temp applications and they have all been certified under SNAP, UL, and ASHRAE.
How it works: liquid CO2 absorbs heat in the display case through coils similar to those used in traditional DX systems, but specially designed for use with CO2. The CO2 completely evaporates in the coils and the suction gas returns to the compressors. A highly efficient suction-liquid heat exchanger (SLHE) insures that all of the liquid CO2 is evaporated before returning to the compressors. The CO2 is then compressed and discharge gas from the compressors is condensed in a condenser-evaporator heat exchanger by the upper cascade of the system that operates in a similar manner to the primary side of secondary system. Liquid CO2 is then sent to a CO2 receiver and the SLHE before being distributed back to the liquid supply piping.
Significant features of all types of CO2 systems include the substantial HFC charge reductions and the lower leak rates that result due to the HFC piping being entirely contained within the machine room (for what HFCs the systems do use) and factory piped. Supermarkets have also found that both secondary and cascade systems operate at parity or better when compared on energy use with traditional DX systems. Another factor is the lower cost of CO2 compared to HFC refrigerants. Additional characteristics of all secondary systems include the use of simple solenoid control, the need for little or no balancing compared to single-phase fluids, and simple commissioning. It’s little wonder that CO2 is receiving the attention that it has.
Bill Katz is the technical writer and course developer for the Hill PHOENIX Learning Center (hillphoenixlc.com). Bill has more than a dozen years developing training for manufacturing and information technology organizations. He can be reached at [email protected]
New ASHRAE Whitepaper Released
A new whitepaper that serves as the first vendor neutral thermal guideline for liquid cooled data processing environments is available for free download from ASHRAE.
"2011 Thermal Guidelines for Liquid Cooled Data Processing Environments" creates data center classes for liquid cooling that can enable fulltime economizers for a number of applications in many climates, according to according to Don Beaty, chair of the Publications Subcommittee of ASHRAE's Technical Committee (TC) 9.9, Mission Critical Facilities, Technology Spaces and Electronic Equipment.
"2011 Thermal Guidelines for Liquid Cooled Data Processing Environments" can be downloaded for free from the ASHRAE TC9.9 website at tc99.ashraetcs.org.
The increasing heat density of modern electronics is stretching the ability of air to adequately cool the electronic components within servers as well as the data center facilities that house these servers. To meet this challenge, the use of direct water or refrigerant cooling at the rack or board level is now being deployed. This trend of increasing heat densities combined with the interest in energy and waste heat recovery created the need for liquid cooling guidelines to help bridge the gap between IT equipment design and data center facility design, according to Beaty.
Five liquid cooling classes have been created:
- W1 – Facility Water Supply Temperature of 2 to 17 C
- W2 – Facility Water Supply Temperature of 2 to 27 C
- W3 – Facility Water Supply Temperature of 2 to 32 C
- W4 – Facility Water Supply Temperature of 2 to 45 C
- W5 – Facility Water Supply Temperature of > 45 C
In addition to the classes, the whitepaper provides insight into other considerations for liquid cooling including condensation, operation, water flow rates, pressure, velocity and quality as well as information on interface connections and infrastructure heat rejection devices.