ICE-E Simple model – assumptions and
parameters
Assumptions
Model calculations
The model is steady state (no change with time). Therefore all heat loads are
averaged over one day.
Geometry
The shape of the cold store is a rectangular box.
Infiltration
There is only 1 door and the cold store is otherwise fully sealed.
The cold store has enough thermal mass such that door openings do not change the
temperature in the cold store.
The temperature of the ambient outside the cold store is not changed by the door
openings.
Transmission
There is only one layer of insulation on the walls, roof and floor. Any metal cladding
is ignored as the resistance to heat transfer from this is considered negligible.
Lighting
The luminous flux (Lumen) from the lights is divided by the area of the floor and walls
to give a uniform luminance (lux). In reality there will be more lumens near the lights
and shadows from racking etc. Some of the lumens will illuminate the ceiling and
some will be absorbed by product and reflectors.
Fork lift trucks
The thermal mass of the trucks are ignored. Therefore if they move from a warm
environment into the store, they do not give up this heat to the store. There is no
energy from charging battery trucks given up to the store.
Product load
Any product which changes temperature when loaded into the store does not have a
latent load (e.g. freezing and thawing) only a sensible load.
Respiration is included for all vegetable and fruit product above 0°C.
Parameters used by the model
Surface heat transfer coefficient inside and outside cold store = 9.3 W/m2K
If wind, surface heat transfer coefficient outside cold store increases to 34 W/m2K.
Insulation
Insulation
Thermal conductivity(W/mK)
Polystyrene foam
0.036
Polyurethane foam
0.024
Glass/mineral wool
0.044
Concrete
1.1
Corkboard
0.043
Polyisocyanurate
0.027
Vacuum insulation panel (VIP)
0.005
Allowance for sun effect (solar radiation) is taken from ASHRAE Refrigeration
Handbook. These values, shown in the table below are in °C and are added to the
ambient temperature to allow for the increased surface temperature due to the sun.
These calculations assume that the cold store is in the Northern hemisphere.
Surface type
East and
West wall
South wall
Flat roof
Dark coloured e.g. slate, tar, black
paint
5
3
11
Medium coloured e.g. wood, brick,
tile, medium coloured paint
4
3
9
Light coloured e.g. stone, white paint
3
2
5
Effectiveness of door protection
Traffic
Strip curtain
No protection
Air curtain
Low
0.92
0
0.71
Medium
0.77
0.28
0.7
High
0.62
0.56
0.69
Efficiency of defrosts
Electric
0.4
Gas
0.45
Off-cycle
1
Power of Fork lift trucks (W)
Size
Electric
Internal
combustion
Small
1000
14000
Medium
5000
22000
Large
6000
44000
Personnel
Heat load due to people is taken from ASHRAE Refrigeration Handbook.
Qp = (272 – 6T)*1.25
Where T is the cold store set point temperature.
The heat load has been adjusted (factor of 1.25) to allow for personnel entering and
leaving every few minutes which gives a higher heat load than when they are in the
site for a long time. The latent load from personnel has been ignored.
Other parameters
Atmospheric pressure = 1.01 bar
Relative humidity inside cold store = 90%
Refrigeration parameters
Compressor power is calculated using the following equation taken from Cleland
(1994).
Ecomp = [Q(Tc-Te)] / [(273+Te)(1- x)n
c
Q is the Total heat load on cold store (W)
Tc is the condensing temperature (°C)
Te is the evaporating temperature (°C)
is the refrigeration coefficient
x is the fractional vaporisation
n is the stage coefficient
c is the Isentropic efficiency of compressor
Difference between air and condensing temperature = 15°C
Difference between air and evaporating temperature = 7°C
Level of sub-cooling = 2°C
References
Ashrae Handbook: Refrigeration systems and applications. American Society of
Heating, Refrigeration and Air-Conditioning Engineers, Inc. Atlanta, USA.
Cleland, A.C. Polynomial curve-fits for refrigerant thermodynamic properties:
extension to include R134a. Rev. Int. Froid 1994 Volume 17, Number 4. Page 245249.