Question

What energy change takes place in the freezing chambers?

Answer 1

The use of industrial cooling for food preservation has been revealed to be an efficient and widely employed technique, from harvest time to final consumption by the customer. However, the most used method to generate that cold (based on the compression refrigeration cycle) requires a considerable amount of electric energy, especially if no appropriate energy efficiency measures are implemented in cold storage chambers. This fact contributes to the increased costs in electricity bills, reduction of competitiveness among companies and also to a negative impact in terms of global warming. To help companies define and implement the right efficiency measures for cold production, this work aims to develop a methodology for simulation and optimization of energy consumption in cold chambers by improving both constructive and operating parameters (external temperature, enclosure insulation, door opening time, etc.), which contribute to the infiltration of heat energy. On the other hand, parameters that had the greatest influence in energy consumptions were those directly related with thermal insulation of enclosures and entry of warm air within. Total contribution of these two parameters in the global consumption was about 95 %.

Keywords
Cold storage chamber Energy efficiency Horticultural industry Simulation
List of symbols
Aent
Surface area of chamber entrance (m2)

Ai
Surface area of the ith enclosure (m2)

Cenergy
Average cost per unit of electric energy [€ (kW h)−1]

COP
Coefficient of performance for the refrigerant circuit

cp1
Average specific heat of packaged product above the initial freezing point (J kg−1 °C−1)

cp2
Average specific heat of product below freezing point (J kg−1 °C−1)

cp dry air
Specific heat of dry air (J kg−1 K−1)

cp vap
Specific heat of water vapor (J kg−1 K−1)

Df
External air flux factor

E
Factor of barrier efficiency against the passage of external air

Echamber
Electric energy consumption by the chamber (kW h)

eij
Thickness of jth material’s layer belonging to the ith enclosure (m)

Esaving
Simulated energy saving per year conferred by a specific efficiency measure (kW h year−1)

g
Acceleration of gravity (9.81 m s−2)

H
Height of chamber entrance (m)

hair
Specific enthalpy of air (J kg−1)

hext
Specific enthalpy of external air (J kg−1)

hint
Specific enthalpy of internal air (J kg−1)

hlat vap
Latent heat of evaporation of water (J kg−1 K−1)

Iinitial
Initial investment predicted for a specific efficiency measure (€)

Lfreeze
Average product’s latent heat of freezing (J kg−1)

macom
Total mass of product accommodated in the chamber (kg)

Mdry air
Molar mass of dry air (0.02897 kg mol−1)

min
Mass of product that was introduced in the chamber (kg)

Mvap
Molar mass of water vapor (0.018 kg mol−1)

npeople
Average number of people inside the chamber

P
Number of times that the door is opened during measurements

patm
Atmospheric pressure (101,325 Pa)

psat vap
Pressure of saturated water vapor contained in air (Pa)

Pte i
Thermal power released by ith electric device inside the chamber (W)

Pte lighting
Total electric power used by lighting (W)

Qair
Thermal load from infiltrated air (J)

Qele
Thermal load released by internal electric equipment (J)

Qin i
Thermal load i that gets into the chamber (J)

Qout
Total heat to be removed from the chamber (J)

Qpeople
Thermal load released by the metabolic activity of people (J)

Qresp
Thermal load of metabolic respiration of packaged fruits and vegetables (J)

q˙resp
Average specific heat flux released by metabolic respiration of product (J kg−1)

Qtemp red
Thermal load to be removed from products to decrease their temperature (J)

Qtrans
Total thermal load transmitted through the enclosures (J)

Qtrans i
Thermal load transmitted by the ith enclosure (J)

Rid gas
Universal constant of ideal gasses (8,314 Pa m3 mol−1 K−1)

Rsi i
Internal superficial thermal resistance of ith enclosure (m2 °C W−1)

Rse i
External superficial thermal resistance of ith enclosure (m2 °C W−1)

Tair
Temperature of air (K)

Tcondenser
Average temperature of the condenser (ºC)

tdoor
Average door opening time in each access (s)

Text i
External temperature to the ith enclosure (°C)

te i
Working time of the ith electric device inside the chamber (s)

Tevaporator
Average temperature of the evaporator (ºC)

Tini prod
Initial temperature of product before entrance in the chamber (ºC)

Tini freeze
Average initial freezing point of product (ºC)

Tint
Internal temperature of chamber (°C)

twork
Total working time of cold chamber (s)

tpayback
Payback time of an initial investment predicted for a specific efficiency measure (year)

ρair
Mass density of air (kg m−3)

ρext
Mass density of external air (kg m−3)

λi;j
Thermal conductivity of jth material’s layer belonging to the ith enclosure (W m−2 °C−1)

ρint
Mass density of internal air (kg m−3)

φair
Relative humidity of air (%)