Supermarkets use tremendous amounts of electricity.  A 50,000 square foot store can have peak electrical loads greater than 500 kW.  Approximately half this power is used to cool refrigerated display cases. 

A store's refrigeration system must continually pump the heat that the cool display case gains from the warmer store to the outdoors.  In effect the refrigerated display cases are air conditioning the store providing both sensible and latent cooling.  But they provide this cooling at a very poor efficiency.  This is because almost all the evaporators in these display cases are running at a much lower temperature than a conventional HVAC cooling system (e.g., the evaporators for display cases for frozen foods can run as low as -25 F and for non-frozen product, between 20 F and 35 F versus 45 F to 50 F for the evaporator of the store's air conditioner).  A refrigeration system's compressor must work harder, and less efficiently, when it pumps heat over a larger temperature rise. 

Any measures that reduce the heat gained by the refrigerated display cases will save electricity for the store.  As shown in tests by the Refrigeration and Thermal Test Center (RTTC) at Southern California Edison, a 1 F drop in store temperature will reduce the heat gained by the display case and lower compressor power by 1%.  However, saving energy by reducing the store's temperature has its limits since customers must be comfortable in the store. 

In addition to inefficiently providing sensible cooling, the refrigerated display cases are inefficiently providing latent cooling.  Reducing the humidity within the store can dramatically reduce the latent cooling provided by the cases while making the store a more comfortable place to shop..

In addition to reducing the latent loads on the refrigerated display cases, lowering store humidity saves energy two other ways.  Evaporators that operate below 32 F must be periodically defrosted (typically one to four times a day).  Defrosting almost always imposes penalties.  When defrosting is done with electric resistance heaters, the store's electric demand goes up.  When defrosting is done by cycling the refrigeration off, the cases lose their control of temperature, which can affect product quality.

The ice that melts off these evaporators during defrosting is the latent load on the display case (i.e., it is the water vapor that has been removed from the air). When a store's indoor humidity is decreased to reduce the latent load on the display cases, the frequency for defrosting low temperature evaporators goes down and the store saves energy.

In addition to dealing with ice accumulation on their evaporators, low temperature display cases also must avoid ice and condensation on the cooler surfaces of the case.  Electric resistance heaters, commonly called anti-sweat heaters, keep surfaces of cases above the dewpoint of the surrounding air.  The power to the anti-sweat heaters can be dramatically decreased when the humidity in the store is lowered.  The peak demand for the anti-sweat heaters in a 50,000 square foot can be over 25 kW, so the potential energy savings are great. 

A 6,000-cfm LDAC that deeply dries the ventilation air to a 50,000 square foot supermarket in a humid climate could reduce the store humidity from 70 grains to 45 grains (from 75 F, 55% rh to 75 F, 35% rh).  Average power (kW) and energy (kWh) savings with the store would be on the order of:

This total demand reduction would be about 25% of the store's power for its HVAC and refrigeration system.