Effect of door opening frequency and duration of an enclosed refrigerated display case on product temperatures and energy consumption

Retail display of highly perishable foods behind glass doors ensures uniform product temperatures below the FDA Food Code threshold of 5 degrees C, resulting in better-preserved foods while reducing energy costs. However, only a handful of studies have evaluated the effect of repeated door openings on product temperatures and energy consumption with contrasting reports. In this paper, we evaluated the effects of two frequencies (doors opened every 5 or 15 min) and four durations (doors held ajar for 5, 15, 30 or 60 s) on product simulator temperatures in a display case installed in our research supermarket. At ambient conditions (19.6-20.9 degrees C, 63% RH), with a case thermostat setting of 0.6 degrees C and a daily 30-min defrost cycle, the only statistically significant fluctuation in product simulator temperatures was found for the most aggressive opening schedule where the door was opened every 5 min for 60 s at each opening. Pairwise comparisons demonstrated that this treatment resulted in product simulator temperatures (up to 6.6 degrees C during defrost cycle) that were significantly higher (p < 0.001) or somewhat significantly higher (p < 0.03) compared to product exposed to all other combinations. Product exposed to all other treatment combination resulted in temperatures that either never exceeded 5 degrees C or briefly exceeded it only during the single 30-min defrost cycle. As a result, we selected an average opening sequence (every 10 min for 12 s) to perform an energy consumption assessment of the case. Energy consumption was determined to be 66% lower than that compared to an open-retail display case (same model, mark, size, operating schedule and thermostat setting). Even with the most extreme schedule where three of the six doors remained open continuously, there was still a measured 45% reduction in energy consumption as compared to that of the open-retail display case.