Comparing different system types, Tristam Coffin, director of sustainability and facilities for Whole Foods Market, makes one point very clear: when it comes to installing natural refrigerant-based systems in supermarkets, there is no one ‘silver bullet’ solution for every store.
In a webinar hosted by the Environmental Protection Agency’s GreenChill Partnership on September 12, Coffin, along with Michael Harvey and Tom Wolgamot of DC Engineering, compared five natural refrigerant-based systems installed in Whole Foods supermarkets in California.
Their case study, called ‘Comparing the Performance of Different Advanced System Types’, looked at the way that transcritical CO2 and cascade systems perform in variable climates and stores around California.Coffin said that building type and climate zone are the two most important considerations when deciding which type of natural refrigerant system to use. He added that the many microclimates in California make the state an effective place to conduct case studies on how different systems work in varied climate types.
The five Whole Foods refrigeration systems that the webinar examined are as follows:
- A four-year old cascade system, with CO2 on the low side and R407A on the high side at a Castro (San Francisco) location.
- A three-year old transcritical CO2 system at a Berkeley location.
- A two-year old cascade system with R717 (ammonia) on the high side at a Dublin (Bay Area) location.
- A one-year-old cascade system with R290 (propane) on the high side at a Santa Clara location.
- A three-year old transcritical CO2 system with an absorption chiller for high side heat rejection at an Alameda location.
At the two locations where transcritical CO2 systems were installed, energy efficiency decreased significantly as the ambient temperature increased beyond the critical point of CO2: 88°F (31°C). However, at the Alameda location, the CO2 system was able to remain in subcritical mode when the absorption chiller was in operation.
Cascade systems, the study confirmed, are generally more efficient in climates with high ambient temperatures because transcritical operation is avoided. It was noted, however, that transcritical system design is improving, leading to increased efficiency. Technologies that enhance the efficiency of transcritical systems include parallel compression, ejectors, adiabatic condensers and subcooling.
The case study also showed that the cascade system, with R407A on the high side, at the Castro location, and the transcritical CO2 system with an absorption chiller at the Alameda location performed most efficiently in high ambient temperatures.
The comparison of energy efficiency provides a useful system comparison. However, a look at the Total Equivalent Warming Impact (TEWI) of the five systems paints a different picture. After direct refrigerant emissions are taken into account, the Castro system, using R407A (with a GWP of 2107) on the high side, proves to have the highest TEWI of the installations discussed.