[mythtv-users] [Slightly OT] solar power for all our gadgets

[Raphael]

Paul Gardiner wrote:

> 
> All devices output 100% of the energy they consume (other than ones that
> involve nuclear reactions). It's a physical law: the conservation of
> energy. 
No one is arguing that conservation of energy isn't real. We are arguing 
that that a light bulb is not as efficient as an electric heater at 
heating a room. I will put a qualifier and say that statement may only 
be true when it's colder outside the room than inside.

> The efficiency figures are because a light bulb is intended to
> produce light, and only that fraction of the energy output is in the
> form of light. The rest is mainly heat, and what ever it is it ends up
> as heat, as does the light, when it has bounced around the room enough
> times partly being absorbed on each bounce. So it is true that 100W bulb
> puts 100W of energy out, 
Right, no one said otherwise.

> that that energy ends up as heat, and that that 
> heat almost all ends up in your house. You in effect have a 100W heater 
> that lights your room as a side effect for free.
There you go: "almost all", that is my only point. Thermodynamics 
guarantees that once the photons enter the walls, and eventually are 
converted into heat, that it is impossible to transfer 100% of that heat 
back to the air in the room if the air exterior to the room is colder. 
Some of that heat will also be transferred to colder parts of the walls, 
and eventually to whatever is on the other side of them. I'm not saying 
any other mechanism is 100% efficient either.
> 
> Practically, though, I think you can ignore this fact. The case where
> it really doesn't matter that a light bulb is inefficient at generating
> light is a rare one:
> 
> 1) The whether is cold enough to have the heating on.
> 
> 2) The heating is electric (and not a heat pump).
> 
> 3) The heating is thermostatically controlled.
> 
> 4) The bulb is in a room that is in the range of the thermostat.
> 
> Only then will the heat produced by the bulb cause the heating to cut
> back by a similar 100W. If your heating is gas, it makes an enormous
> difference. 
It still makes a difference even when the heater is electric. The 
mechanism of transferring the heat to the air is vastly different from 
the way the light from your bulb heats the room. It uses convection to 
transferring the vast majority of heat to the air, whereas the energy 
from the light is deposited in the walls.
It's obvious that an electric heater is also not 100% efficient at 
heating a room, either, of course.
I agree with your statements about gas vs electric, however.


> Using ordered energy to make heat (other than with a heat
> pump) is extremely inefficient. The inefficiency is at the power station
> and is an unavoidable consequence of another physical law to do with
> entropy. You can't generate oredered energy (electricity) from
> disordered energy (heat) without a huge proportion ending up in the
> outside as heat, where it is of no use. Use of gas doesn't involve
> that inefficiency (hmmm, as far as I know).
> So, if your bulb heats
> the room a bit, and your gas central heating cuts back in response,
> you are trading inefficient heating for efficient heating, which
> isn't so good.
> 
> Cheers,
>     Paul.
> 
Sounds good to me.
cheers,
Raphy