DENNIS R . HELDMANDALE A . SEIBERLING FOOD ENGINEERING

RESEARCH LABORATORY

Using Off-Peak Power Rates to Reduce Refrigeration Costs

Introduction

Challenge -- Managing a refrigerated warehouse during periods of increasing energy costs

Can operating costs be controlled through effective and managed use of off-peak energy rates?

Frozen food quality and shelf-life is sensitive to elevated and fluctuating storage temperatures

Can changes in frozen food temperatures and quality during periods when a refrigeration system is not functioning be predicted?

Objectives

Overall -- to evaluate the relationships between storage temperatures and frozen food shelf-life

Specific –1. to review factors with influence on the temperature of

air in the warehouse 2. to translate changes in warehouse air temperature

into changes in frozen food temperature3. to predict changes in frozen food temperature into

losses of product quality and shelf-life

Frozen Food Quality

Quality Attributes of Frozen Foods1. Texture – impacted primarily by ice crystal size2. Color, flavor, nutrients – impacted primarily by temperature dependent reactions3. Changes in all attributes are influenced by amount

of water in liquid state at any temperature

Frozen Food Shelf-Life

0 50 100 150 200 250 300 350 4000

50

100

150

200

250

300

350

400

-11.5 °C (11.3 °F)-15.5 °C (4.1 °F)-18 °C (-0.4 °F)

Actual Storage Time (Day)

Shel

f Life

(Day

)

112 240 360

Shelf-life is reduced by any temperature above -18 ᵒC (-0.4 ᵒF) during storage and distribution.

Warehouse Air Temperature

Factors with influence on air temperature in the warehouse

1. Refrigeration system; on or off2. Insulation of walls, ceiling and floor3. Openings to air outside storage space4. Total volume within warehouse5. Amount of frozen product in warehouse

The Refrigerated Storage Space

Frozen Product Volume

Vs.

Walls, Ceiling and Floors

Fundamental energy transferEffects from solar radiationWalls and Ceilings

Insulation values Highly investigated materials

Floors Need attention Flow under for product Higher energetic cost

ConcreteConcrete

Recirculated air

Warehouse Air Temperature

Changes when refrigeration system is not functioning -- 1. Air temperature increases more rapidly on a

smaller volume warehouse 2. Good insulation reduces rate of temperature

increase3. Rate of temperature rise is tempered by

larger quantities of frozen product4. Frequency of openings to outside warmer air

causes air temperature to increase

Air Temperature as Function of Warehouse Volume

0 1 2 3 4 5 6 7-20

-10

0

10

20

30

10 x 100 x 1050 x 50 x 10100 x 100 x 10

Time (Day)

Bulk

Tem

pera

ture

(ᵒC

)

Air Movement

Complex in nature CFD Modeling

Adding variability Storage regime Tertiary package

Making universal Simulation tangible

to warehouse managers

Ho, S. H., Rosario, L., & Rahman, M. M. (2010). Numerical simulation of temperature and velocity in a refrigerated warehouse. International Journal of Refrigeration, 33(5), 1015–1025.

Product Temperature

Factors influencing the product temperature when refrigeration system is not operating –

1. Difference in air temperature as compared to the product temperature

2. Air movement over product surfaces3. Percent of space occupied by frozen product4. Configuration of product in warehouse space5. Product properties

Frozen Product Storage Configuration

Tertiary structure

Secondary structure Bulk material Packaged goods

Open configuration not good for system down time

Frozen Product Temperature as Function of Location

0 1 2 3 4 5 6 7-20

-10

0

10

20

30

Bulk air Surface temp

25% in 50% in

Time (Day)

Tem

pera

ture

(ᵒC

)

Frozen Product Quality

Factors influencing product quality when refrigeration system is not operating –

1. Product temperature and all factors influencing product temperature

2. Sensitivity of frozen food quality to temperature changes

3. Location of product within storage and product configuration

Sensitivity of Frozen Foods

Food Category Shelf-Life EA

Seafood 200 50

Vegetables 400 60

Fruit 350 64

Meat 215 65

Poultry 230 75

Ice Cream 300 100

Frozen Food Quality

The loss of frozen food shelf-life is accelerated by the following:

1. An increase in the product temperature2. Location of product near an interface of warm

air and frozen product 3. Configuration of frozen product stacking with

more product surface area exposed to warm air

4. Frozen food quality attributes with greater sensitivity to changes in temperature

Frozen Food Shelf-life as Function of Location

0 1 2 3 4 5 6 70

10

20

30

40

50

60

70

80

90

100

Surface temp25% of Dc50% of Dc75% of DcCenter

Time (Day)

Shelf

-Lif

e R

em

ain

ing (

%)

A Warehouse Management System

A Program for Reducing Warehouse Energy Costs While Maintaining Product Quality

A. Inputs – 1. Total capacity of the warehouse; volume of space2. Portion warehouse space occupied by product3. Insulation quality4. Air movement within the warehouse5. Sensitivity of product quality to temperature6. Local energy rates; both peak and off-peak; $/kWh7. A proposed number of hours per 24 hour period that the refrigeration is not operating

Warehouse Management System

Outcomes from System – 1. The cost savings for energy due to defined

peak rate periods of refrigeration system down time

2. The number of days with the defined refrigeration system down time before a defined loss of frozen food shelf-life is evident

Summary

A management system for estimating potential cost savings due to refrigeration system down-time during peak energy costs without significant loss of frozen food shelf-life has been proposed.

The system is based on the capability to predict changes in air temperature, frozen food temperature and product shelf-life during a period when the refrigeration system is not functioning.