From: black.hole.4.spam{at}gmail.com (Don Hills)

In article , "Geo."
 wrote:
>
>Well if that's true, then just put stuff into the freezers at off peak
>electrical usage times.

Costs more in delivery scheduling. You need more trucks etc. Twice as many
trucks if you can only deliver for 12 hours of the day instead of 24, for
example.

But now you've started me thinking: Frozen goods are normally "blast
frozen" at point of manufacture, then carried in refrigerated
containers / trucks to the cool stores if they aren't geographically
adjacent. (Even if they are, they have to be later transported from the
factory to the distribution depots.)

A lot of energy is used to quick freeze the goods (literally, blast them
with extremely cold air) to get them over the phase change. Assuming goods
with a high water content, and assuming an initial temperature of 20
degrees C and a final temperature of -20 degrees C, the enrgy required is
divided up roughly as follows:

17% to drop the temperature from 20C to 0C. 66% to freeze the goods.
17% to drop the tempearature from 0 to -20C.

The goods are usually transported at about this temperature or a bit
higher, truck/container insulation is kept as thin as practical to maximise
load space.

Long term storage warehouses keep the goods at -24 or less in some cases to
maximise storage life. That's only a drop of 4 to 6 degrees from the truck,
not a huge energy difference.

The other type of "cool store" is that for fruit and vegetables,
especially the ones used for keeping apples etc for months at a time. They
run above freezing, but usually do the full pull-down at the store rather
than at the packing house.

So it looks like a significant part of the energy consumption is at the
blast freezing stage for frozen storage. It would make sense to do that
"off peak", except that again you run into problems with
inefficient utilisation of plant.

--
Don Hills    (dmhills at attglobaldotnet)     Wellington, New Zealand
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