Index Home About Blog
From: Paul Dietz <>
Subject: Re: Depopulation
Date: Sat, 02 May 1998 06:19:28 -0500

Larry Caldwell wrote:
> In article <>,
> Paul Dietz <> wrote:
> > PV cells are much more efficient than photosynthesizing plants
> > at capturing energy, especially when one considers that they will
> > work in environments in which plants will not collect much, or any,
> > energy (in deserts, in deep subzero temperatures).
> Only to the social theorist, who somehow dings the plants for their
> inedible infrastructure, but neglects to ding the solar cells for the
> same thing.

There are PV cells that are in excess of 30% efficient.
The theoretical limit is well above that.  Moreover, the PV
cells can achieve these efficiencies in subzero conditions,
and in completely dry climates, and the field on which they
are placed needn't waste the solar energy that falls on
it after plants are harvested, or while the plants are still
small and intercepting only a small part of the incident

The *best* agricultural efficiency (light to food calories)
that has ever been achieved in the field is around 2%
(in a winter wheat crop in the US Pacific Northwest;
this crop was not growing during the summer).  The normal
efficiency is well under 1%.

> Engineers have a different perspective.  You have to manufacture,
> transport, install and maintain the things, transport the inedible
> power, manufacture, transport and install the synthesis equipment,
> and operate it.  You can write the actual power used in synthesizing
> food off against the energy requirements of agriculture.

The ancilliary energy expenditures, in a chemical plant, are
typically small compared to the energy required to operate the

> > If food synthesis can be made even moderately efficient (say 25%)
> > then the area needed to feed one person with PV cells + synthesis
> > is remarkably small, a few tens of square meters.
> There's no way you're going to get to even 2% efficient.  You are
> going to need whole technological revolutions even to move efficiency
> off of dead zero.

You are being unthinking in your pessimism.  Even today, we could
do better: use the solar energy to make hydrogen, synthesis methanol
with the hydrogen and CO2, then grow bacteria on the methanol
(this last is a proven technology, from ICI, for production of
their "Pruteen" feed supplement from natural gas-derived methanol).
As a complete diet it is inadequate (too much DNA in bacteria, gives you
gout), and it's nowhere near economical, but the efficiency would be
in excess of 2%.


Index Home About Blog