Limp, shriveled produce or frost-caked frozen orange juice! These are the opposing problems for grocery stores in their battle to maintain an optimum humidity level. While produce is spayed with misters that inject humidity into the air, freezer cases deposit ice on the frozen produce as a response to that humidity. And as if that wasn’t enough of a problem, the added refrigeration load on the freezer cases drives electricity costs through the roof.

Balancing consumer aesthetics and electricity bills is a major headache for grocery store managers. Fresh milk, cold meats and cheeses and frozen fruit and vegetables can only be stocked if a supermarket operates a constant and effective cooling system. In a typical supermarket, the refrigeration system consumes approximately 60% of total energy used. HVAC accounts for a further 15-20% of the energy used and is the single most important factor affecting energy usage of the refrigeration system.

Most of the refrigerated display cases are designed for 55% relative humidity (RH) and 75F. The necessary defrost frequency is based on these conditions. If we can lower moisture content, display cases accumulate less frosting and operate more efficiently. Reduced frosting also results in less energy used to defrost the display case coils.

EPRI (Electric Power Research Institute, California) conducted extensive studies on the effect of humidity on refrigeration energy consumption in a typical supermarket environment. The research concluded that one percent reduction in relative humidity reduces electricity consumption by 10-20kWh/day in an average store. Accumulated savings in operating costs on the refrigeration system alone can amount to $2,000 per year, depending on store configuration, location and energy pricing. Energy savings from lowering humidity will be affected by several additional factors, such as defrost method (Electric vs. Hot gas), use of anti-sweat heater and control strategies.

A lower relative humidity leads to decreased operating costs for the refrigeration system, but the actual capital and operating cost for the HVAC system increases since the cooling system must be larger and must operate for more hours. Careful design can bring the grocer to the break-even point between refrigeration energy savings and capital/operating cost of the HVAC system. Since the capital and operating cost of an HVAC system for maintaining a particular humidity level greatly depends on ambient design conditions; each climactic zone will have different optimal humidity points.

For climates with high ambient humidity and heating loads, the optimum point for moisture is around 50-grains/lbs. of dry air (42% RH at 75F dry bulb temp). This optimum point may vary, depending on the refrigeration system design and store configuration.

To ensure minimal overall energy consumption in supermarket buildings, getting a detailed study of proper design conditions by an expert is always recommended.