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re: taking solar power to new heights
7 nov 2000
sno wrote:
>i believe money is used as a counter for the amount of energy...
>total energy....that is used in a product... plus profit...
we can use a crop of out-of-season lettuce as an example. if lettuce is
planted in a 30 by 100 foot greenhouse at a spacing of 10 by 10 inches,
that amounts to 4200 lettuces. it requires a sheet of polyethylene
plastic 42 by 100 feet to cover that greenhouse. therefore, we can
assign one square foot of plastic to each lettuce.
all the energy used in raw materials, production, and delivery of a
manufactured item is defined by the term "embodied energy." the embodied
energy in one square foot of 6 mil polyethlyene greenhouse plastic...
is 1221 btu.
if, on the other hand, the lettuce were grown in california, it would
have to be transported 3200 miles from california to the east coast.
we need to know the energy consumption of a semi-tractor-trailer driven
across the country and divide by the number of lettuces per load. i found
three different figures from three different transportation energy data
sources. based on the average from those figures the energy consumption
per 12 oz. head of lettuce to transport a semi-tractor-trailer load
3200 miles from california to the east coast is 3034 btu.
according to those calculations, the lettuce grown in my maine greenhouse
requires only 40% as much energy. however, since the greenhouse
polyethylene lasts three years and since a grower can get both a spring
and a fall crop each year with no other energy input, six lettuces can
be produced over the lifespan of that square foot of polyethlyene. each
local greenhouse grown lettuce thus consumes only 6% of the energy
required by each trucked-in lettuce.
for the sake of simplicity, only the energy in the plastic cover and
the energy used in transportation have been considered. they are the
key factors in each system. i have not included the greenhouse pipe
frame, nor the truck and its refrigeration unit, nor the highway
construction. i am sure that analysis of these energy figures would
make the ratio even more favorable to the maine grown lettuce. nor
have i considered any pollution from the plastic manufacturing and
recycling process compared to the pollution from the burning of the
average 582 gallons of diesel fuel used in cross-country trucking.
these are obviously problems that need to be solved but they are
outside this grown-in-maine vs grown-in-california comparison.
at the moment polyethylene covered greenhouses are the economic
choice for me as a grower. i would love to avoid "plastic" but a
comparable glass covered greenhouse would cost more than ten times
as much as a plastic house. glass also contains ten times the
embodied energy of plastic--12,000 btu per square foot. having said
that, do i believe greenhouses covered with plastic made from
non-renewable petroleum feedstocks make any more long-term sense
than transportation based on those same non-renewable feedstocks?
obviously not. we need to address the use of non-renewable resources
as a factor in any out-of-season produce discussion.
since greenhouse plastic is made from petroleum feedstocks, the
question is whether this useful material can be made from some
renewable raw material when our economy's present fixation with
petroleum comes to an end. i am assured by friends who understand
polymer chemistry that greenhouse plastic can indeed be made from
renewable materials and that many plant-based compounds would be
suitable feedstocks. maybe that is an option that could be demanded
by growers. however, i suspect it is an option that will likely
happen on its own someday when dictated by the comparative cost of
feedstocks.
from the winter harvest manual by eliot coleman, 1998
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