Over the last two decades, residential HVAC systems in the US have gone from 10 SEER to 15 SEER — a 25% improvement. For commercial buildings, the source and site-energy-use efficiency improvements in ASHRAE 90.1 standards have increased more than 40% — with corresponding reductions in CO2 emissions. But, HVAC and refrigeration equipment use 50-60% of all electricity a supermarket consumes.
Thus, it’s reasonable to ask: “Can HVACR technology cost-effectively boost supermarket energy efficiency and further reduce emissions in the future? Or, are we approaching a technological limit where efficiency gains will demand extraordinary investments — investments that may have a better payoff elsewhere in the facility?”
Option 1: Incremental efficiency improvements
Along with residential and commercial HVAC applications, supermarkets are also taking advantage of improved efficiency in components and systems.
Technological advancements have resulted in numerous efficiency improvements. For example, the introduction of NEMA Premium® efficiency motors for compressors help reduce energy consumption. In heat exchangers, advanced design reduces the refrigerant charge while increasing the heat transfer ability. Another example is the application of variable-frequency drive (VFD) technology. In a refrigeration system, the typical compressor rack employs multiple fixed-speed compressors. When a VFD and related controls are applied to one of those compressors, that compressor can reduce energy consumption by modulating its speed to match the refrigeration load more precisely. For mid-range rooftop HVAC equipment that supermarkets use for space cooling, a VFD can cut energy consumption by reducing compressor motor speed during 99% of operating hours when full cooling capacity is not needed. And applying VFDs may also help the store qualify for utility incentives.
More energy can be saved by using advanced controls. For example, implementing advanced superheat case control in the refrigeration plant boosts system efficiency by ensuring the evaporator is optimally charged, even when the load and suction pressures vary. Case control can also provide an adaptive defrost function that skips the energy-consuming defrost cycle when not needed. In CO2 racks, as an example, gas ejectors paired with advanced control algorithms can save close to 12% of system energy consumption on an annual basis. These devices exploit the work lost during initial expansion of the CO2 that otherwise would be regarded as a loss. As another example, liquid ejectors are being developed to pump liquid exiting from evaporators in CO2 systems and will be able to adapt to specific working conditions to better utilize evaporators. In both examples, the captured energy will result in less compressor work and, thereby, more energy savings.
At the store level, energy savings can be even more substantial than the efficiency gains that accrue from improving components and systems alone. But to get bigger savings, technologies must be employed that can manage and monitor all major energy-consuming appliances in the store.
The concept behind holistic store energy management is that managing everything together saves more energy than optimizing components and systems individually. Communication between all components is directed to a central hub, the system manager. Functioning as the “brains” of the system, a system manager exchanges data with individual refrigeration controllers and optimizes suction pressure, defrosting and other critical refrigeration functions. A single system manager can also manage lighting and HVAC.
The result is an integrated solution that unlocks the full potential of each device and increases the efficiency of the entire store. Depending on prior energy-consumption levels and scope of implementation, “smart store” solutions have achieved from 10-50% annual energy savings. The solution also simplifies equipment monitoring, HACCP reporting and other management functions, which reduces labor costs.
At a strategic level for the entire store, it’s possible to make decisions that take advantage of major energy-saving developments. For example, more and more store owners are making a strategic move to CO2 refrigerant. For years, CO2 has been recognized as the greenest, most environmentally-friendly refrigerant due to an ozone depletion potential (ODP) of zero and a global warming potential (GWP) of one. But, because of high pressure levels and relatively high implementation costs, CO2 has rarely been considered a cost-effective option, especially in warmer climates.
Now, CO2 has become cost-effective — even in warm climates — due to two technological developments: multi-ejector and compatible rack controller technology that optimizes the warm climate performance of a CO2 system. When applied in a transcritical CO2 system, multi-ejector technology can reduce energy consumption from 9-18% compared to R-404A systems. Additional savings are available when a CO2 booster system is used for heat reclaim. Heat reclaim boosts the seasonal performance factor (SPF) and the coefficient of performance (COP) of the system above most heat pumps of similar capacity. Stores can take advantage of heat reclaim to produce hot water for cleaning, food prep and lavatories. An innovative store in Carol Stream, Illinois, is using the hot water for under-floor heating in walk-in freezers.
However, improving energy efficiency inside the store through components and systems — although certainly worthwhile — is not the only way to boost energy savings and reduce environmental impact.
We’ll discuss these options in a future post.
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