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©2015 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 57, no. 10, October 2015

Collin Weber, Associate Member ASHRAE; Harrison Horning, P.E.

About the Authors
Collin Weber is a managing consultant with Navigant Consulting in Washington, D.C. Harrison Horning, P.E., is a director of equipment purchasing, maintenance and energy at Delhaize America in Scarborough, Maine.

Hannaford Supermarkets opened a store in Turner, Maine, featuring the first transcritical (TC) carbon dioxide supermarket refrigeration system in the United States. The two-year-old, 35,000 ft2 (3252 m2) new-construction supermarket includes conditioned and refrigerated merchandising space, as well as food preparation areas and offices. The TC CO2 refrigeration system serves a total of 740 kBtu/h (217 kW) of combined low- and medium-temperature loads, while providing high-quality waste heat that meets much of the store’s space heating needs.

From Fall 2013 through Summer 2014, Hannaford partnered with the U.S. Department of Energy’s Better Buildings Alliance (http://tinyurl.com/qbxucjf) to conduct a study comparing the site to a legacy store in Bradford, Vermont, that uses a hydrofluorocarbon (HFC) refrigeration system. During the one-year data collection period, the TC CO2 system achieved approximate energy parity with the HFC system, while reducing storewide carbon equivalent emissions by 12%. The full report, Case Study: Transcritical Carbon Dioxide Supermarket Refrigeration Systems, is available at http://tinyurl.com/qbrx4hs.


Installation Overview and Study Background

TC CO2 systems, unlike carbon dioxide cascade systems, use carbon dioxide as the sole working fluid. The TC cycle, as the name suggests, involves cycling of the refrigerant between the subcritical and supercritical phases. When a refrigeration system is operating transcritically, heat rejection occurs above the critical pressure, while cooling takes place below the critical pressure. The use of carbon dioxide as the sole working fluid and the inherent high working pressure arising from it is the driver of many design differences between TC CO2 booster systems and conventional refrigeration systems. Compressor output pressure often exceeds 1,000 psia (6895 kPa).

While a mature technology in Europe and Canada, TC CO2 refrigeration is in the early stages of deployment in the United States. Motivated by the success of the technology in other cold-weather regions, Northeastern U.S. retailer Hannaford elected to pilot a TC CO2 refrigeration system in its new-construction supermarket in Turner, Maine. Hannaford worked with a supplier with significant experience in the sector to develop a system for its application.

The TC CO2 system at the Hannaford store in Turner consists of a single rack for both medium- and low-temperature applications, comprising three low-temperature compressors and six medium-temperature TC compressors. The store uses an air-cooled gas cooler (the equivalent of the condenser in a conventional system) mounted on the roof for heat rejection. For heat reclaim, an array of heat exchangers is connected to the system using steel piping, which is commonly used in TC CO2 systems due to the need to accommodate the extremely high compressor discharge pressures. The heat reclaim loop uses a propylene glycol and water mixtureas the working fluid.

In addition to TC CO2, the refrigeration system also uses hot-gas defrost and stepper type electronic expansion valves (EEVs). Hannaford estimates that the incremental cost of the TC CO2 system (over a prototypical brand-standard HFC system) was about 40% for the refrigeration equipment alone, in addition to a 10% to 15% incremental cost for piping and display cases. Additionally, due to the unique nature of this pilot project, installation and start-up costs were elevated above those associated with a prototypical HFC system.


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