Don't think of it as a cow, think of it as a "mobile cereal
compression/archive device." (one that happens to be mighty tasty if
you marinate it just right and then toss it onto the bbq :) The food
problem you allude to is not one of production but of distribution.
It costs too much to move the grain to where it is needed. We are
much better at growing food than we are at processing and
distributing it; we are so good at it that we have to pay people not
to grow food to prevent over-production. All of those resources go
into producing meat because the animal converts the grain into a much
denser package which is easier to transport. The conversion is not
even close to 100% efficient, but it is cheaper than piling the grain
up at the elevator and then watching it rot.
The whole "diet for a small planet" mantra reminds me of Karl Marx
spending time in England and coming to the conclusion that production
was everything because the transportation and distribution system was
so well-developed (comparatively) that he didn't even notice it. If
you examine the food production system closely you will see that we
have actually become damn good at it; at the beginning of this
century the overwhelming majority of Americans worked on a farm or at
a farm-related job, now fewer than one in a hundred have a job in
agriculture and we still produce too much food.
The urban greenhouse idea is nice, but it will require a major
breakthrough to beat the efficiency of the current system. You are
adding all sorts of complexity to cut out a single step, namely the
long-haul transportation. Even if you can make the engineering work
there is nothing to prevent the farmer from using the same methods to
increase the efficiency of her farm and she will always have you beat
on the energy costs (unless the government figures out how to
regulate sunlight :)
jim
p.s. skip the veggie flames please. I eat very little meat and have
almost completely given it up for health reasons, but spent most of
my childhood on a farm I know that the only reason cattle, sheep, and
most livestock exist in the world today are because we keep them
alive. If it were not to become burgers and steaks then cows would
have long since become another evolutionary dead-end (you cannot
quite appreciate just how dumb these animals have been bred until you
watch sheep trying to get out of the rain by hiding under the other
sheep.)