"The Natural Philosopher" wrote in message
...
Franz Heymann wrote:


So with a large area of Thermolux you might get to 200% more easily than
with other panels?
Please, please understand that there is no such concept as "efficiency
per
square foot" in either engineering or in physics. Efficiency is usually
simply the ratio between the output power and the input power of a
system.



Actually that is not totally so. Efficency is a term that can be applied
to more things than power.

For example, one could define the efficiency of a roof in terms of the
amount of water that runs off versus the total amount that falls on it.


One can define an efficient business as one that has the highest sales
value, or margin value, per employee.

Efficiency is a measure of the efficacy against a theoretically perfect
system,

That is the beginning of a circular argument.

of something doing the job it is designed to do. As normally
measured by how much it produces of the desired output versus how much
input it needs.

If we for example take solar energy, it is not menaingful to say that
e.g. civering every roof in lonbdon with a .3% efficient solar panel is
inefficient, if the cost of so doing would actually be less than
building and running an equivalent power station over the same .
timescales.

One could argue that in terms of various resources one or the other is
more efficient.

The power station takes up less space, but uses more fossil fuel. The
electric panel is inefficient in overall thermodynamic terms, but maybe
more efficient in the actual use of sunlight, since we don't have to
wait a couple of million years for the trees to turn back into oil...The
power station has far less labour content involved, but perhaps uses
more materials.

uppose fo an instant that we cracked fusion power. Who cares about
efficiency, since the actual waste products - helium and heat - are
totally insignificant in a global context. At that point electcity would
become the cheapest form of energy, subject to no taxes at all probably,
and we would all be driving electric cars, and heating our houses
electrically, immediately :-)

Thanks for the homily.

I agree that in general usage, "efficiecy" is bandied around with gay
abandon. However, the discussion about solar panels was a
scientific/engineering one. To talk about "efficiency per unit area" in
such a context is pure nonsense.


In engineering and scientific parlance, efficiency is to do *only* with
energy and power.
The efficiency of a sytem for converting energy from one form to another is
uniquely defined as the ratio between the output and the input power.
Since energy is simply the integral of power with respect to time, the same
definition for efficiency will then also determine the ratio between the
input and the output energies.

Franz