Agriculture Reference
In-Depth Information
Energy use minimisation
The percentage of the total production cost attributed to
energy costs can vary from effectively zero with a natuar-
ally ventilated system to over 10% with a sophisticated
refrigerated store in a year of low prices. It must be stressed
that the total cost of energy, virtually always electricity,
and the quantity of energy used are not synonymous as
there may be different tariffs. These tariffs may depend
on the time of day, time of year or day of the week or all of
these factors.
Devres and Bishop developed a computer model for
analysis of various factors on total energy use (Devres &
Bishop 1995a). The computer model was verified over a
two year period against actual energy usage in a 1500
tonne store located in the UK (Devres & Bishop 1995b).
The effect of changes in insulation, infiltration and target tem-
perature are given in Table 16.5. The computer model gave an
energy consumption of +1.0% in the first year and −2.3%
in the second year.
Figure 16.4
Box filler (courtesy of Herberts).
In all cases the ventilation system can be used with or
without refrigeration although for ease of understanding
the airflow patterns no evaporator is shown.
The non-ducted system uses boxes with slatted sides
and the cool air is blown out at high level and sucked
back at low level. The ventilation system works on the
principle that the cooler ventilating air drops into the
mass of boxes and is then sucked back to a low level
intake. It is a low cost system but if the ventilating air is
very much cooler or the fan is not powerful enough there
is little or no ventilation at the far end of the store from
the fan.
The differential system again uses the boxers with slat-
ted sides and the cooler ventilating air is ducted to the far
end of the store, sometimes a sheet of plastic is put on top
of the boxes instead of actual ducting. The air is then
sucked back through the boxes but so as to avoid the air
being channeled through some boxes but not others there
should be a gap of 350 mm after every five or six boxes.
The gap means that the air flow evens out.
The letterbox system uses boxes with solid sides and
slatted bases with only one-way entry pallet bases. The
arrangement of the boxes is that the pallet base actually
forms a duct or lateral and the air is positively ventilated
through the boxes. The air comes out of the main duct or
plenum chamber and passes down the lateral formed by
pallet bases and then through a box of tubers and is
exhausted through another set of pallet bases either above
or below. This system means that every box is positively
ventilated and so a uniform temperature can be main-
tained, however the system depends on careful stacking of
the boxes.
Low-cost storage structures
For hundreds of years potatoes were stored in simple low
cost structures which often lasted for one storage season.
The most basic objectives were to prevent greening, frost
damage, damage from rodents or birds and to avoid theft.
If a limited number of tubers had sprouted or rotted it may
have been possible to remove them. These requirements
either involved storing in cellars or caves or protection by
straw or soil. Particularly in the case of caves or cellars the
extremes of temperature were reduced or eliminated.
Clamps
Above ground piles or clamps of potatoes were the
traditional method of potato storage in Europe and are still
used in various forms in many countries. They have a low
capital cost but high labour input. Although there are a
number of different types they all require potatoes to be
heaped in a long narrow pile of typically 1-2 m in width
with a height dictated by the angle of repose of the tubers
of 0.4-1 m. Straw of a thickness of 0.3 m or more (a greater
thickness of straw will be needed in particularly hot or cold
climates) is laid over the top of the tubers to act as an
insulant, stop greening and absorb moisture given off by
the tubers. The straw is normally then covered with soil or
a plastic sheet to protect from rain or strong winds blowing
the straw away.
There is a more sophisticated version often referred to as
a “Dickie Pie” in which one or two “A” ducts are placed
parallel to the long sides. These A ducts are open at each
end and can provide some ventilation of the tubers and
