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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.