This report describes the process and results of a demonstration of solid-state lighting (SSL) technology combined with occupancy sensors in a set of upright grocery store freezer cases. The project was supported by the U.S. Department of Energy (DOE). A retrofit of freezer cases at a Eugene, OR Albertsons grocery store indicates a potentially successful application of light-emitting diode (LED) technology with additional energy savings from the use of occupancy sensors.

LED strip installation starting at one end of row.

The test location is an aisle of freezer cases storing a variety of frozen food items at or just below 0 degrees F. The retrofit side of the aisle includes four five-door cases and two three-door cases facing into the aisle capped by similar cases on each end. The opposite side of the aisle contains a similar case setup. Each case is lighted with vertically mounted linear fluorescent lamps between doors. The store aisles were lighted with standard overhead strip fluorescent luminaires aligned parallel with the aisles, which are typically on 24 hours a day due to cleaning and stocking activities. The replacement LED lighting strips and LED drivers were supplied by LED Power.

The uniformity of light distributed within the case was evaluated and found to be generally comparable to the fluorescent system. It is commonly believed that the directional nature of LED light emission means that much higher uniformity can be achieved compared with the omnidirectional light emission of a fluorescent system. However, in product cases where widely varying package graphics and colors are the norm, the limited personal observation from this study revealed little noticeable difference due to varying distribution uniformity.

Luminance measurement of white test surface inside of case, with measurement locations.

The logistics of arranging this comparison test did not allow for relamping of the existing fluorescent case lighting prior to the test and therefore a small portion of the apparent savings may be attributable to fluorescent lumen depreciation. It is more important to note that while the power consumed by the LED system showed a 61% reduction from that of the fluorescent system, a significant portion of the savings has resulted from a 36% reduction in illuminance as determined by relative measured levels before and after the retrofit. This 36% light level reduction accounts for more than half of the savings, which may have been alternatively accomplished with a retrofit of different fluorescent technology in the form of a lower output lamp and/or lower driving ballast. However, the complete retrofit demonstrated here that incorporates step dimming based on occupancy would be difficult with fluorescent technology, given the limited market offerings of specific dimming ballast technology.

The simplest energy reduction is the difference in wattage between the fluorescent and new LED lighting. The average fluorescent-lighted five-door case has measured amperage of 2.6, while the replacement LED lighting pulls only 1.0 amp. The assumption of a uniform voltage of 120 for both existing and retrofit cases is used and the average wattage reduction per five-door case is calculated at 192 watts. Based on 24 hour operation of the case lighting (store is open 18 hours per day but the case lighting not turned off after hours), the estimated total yearly energy use of a typical five-door case lighted with the fluorescent system used at the Albertsons store is 2,733 kWh. The corresponding yearly energy use for the retrofitted LED system is 1,051 kWh. The estimated savings from one five-door case lighted with LED is 1,682 kWh per year (61% savings — based in part on a relative 36% reduction in case illuminance).

Side-by-side comparison of fluorescent (left) to LED (right) lighting.

Total energy savings (including more efficient light source, reduced consumption from occupancy sensors, and reduced demand on the compressor) is estimated to be 2,659 kWh per year for a typical five-door case. At Eugene electricity rates ($0.0827/kWh), the calculated simple payback from energy savings alone is approximately 6.3 years. Maintenance savings are typically specific to each application and will not be the same for other applications. For this demonstration retrofit, maintenance savings are estimated to further reduce the payback to approximately 5.4 years. A complete simple payback analysis that might also include utility rebates and maintenance savings could be much shorter. Current Eugene-area Utility rebates and Oregon tax credits could further reduce the payback period to about 1.6 years.

It is also important to consider that other potential attributes of the LED application in freezer cases make it a potentially better retrofit than adjusted fluorescent lighting. These include potentially longer life and reduced freezer case heat load that contributes to additional savings with LED technology. The potential for savings from reduced freezer case heat load due to lighting is difficult to measure because of the number of variables affecting individual freezer case installations, so was only estimated in this analysis.

This work was done by E.E. Richman and J.R. Tuenge of the Pacific Northwest National Laboratory.


Lighting Technology Magazine

This article first appeared in the November, 2010 issue of Lighting Technology Magazine.

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