sci.energy.hydrogen

On Thu, 14 Feb 2008 18:56:55 -0500, daestrom wrote:

>
> "Bill Ward" wrote in message
> news:@...
>> On Wed, 13 Feb 2008 08:53:37 -0500, Morris Dovey wrote:
>>
>>> fragma wrote:
>>>
>>>> THE point is that the energy (carbon-based) involved to produce a PV
>>>> is more than the panel will ever deliver back until it dies.
>>>>
>>>> Basically independent of present price if you will...
>>>>
>>>> Please think about the complex and energy expensive production methods
>>>> (refinery systems)
>>>
>>> [ Extending discussion to news: ]
>>>
>>> Ok. I'm _still_ thinking about that - and about questions that your
>>> posting raises:
>>>
>>> (1) Has anyone produced any verifiable numbers for the amount of energy
>>> required to produce a PV cell - and if so, where can the numbers be
>>> seen?
>>>
>>> (2) How much of that production energy is _heat_, and at what
>>> temperatures?
>>>
>>> Because of my thermal solar focus, I'm inclined to consider the
>>> possibilities of reducing the carbon-based energy content of PV cells
>>> by substituting solar heat for electrical (resistance) heat.
>>>
>>> (3) Might we now - or at some point in the future - develop a
>>> solar-based production technology that could produce a more favorable
>>> energy equation?
>>
>> One clue would be when you see solar cells manufactured using only solar
>> energy.
>>
>>
> Hmm, no not really. Suppose it takes 105 MJ to create 1 watt of solar.
> Would you use electricity for all the heating processes, or would you use
> a much cheaper fossil fuel?
>
> For all stages of production that involve just thermal heating, you would
> use the direct heat input, not electricity. IIRC, that is a significant
> portion of the energy used to produce the PV cell. The energy from a
> solar cell in the form of electricity is more valuable than an equivalent
> amount of energy coming from natural gas (some would say that the exergy
> of a MJ of electricity is higher than the exergy of a MJ of natural gas).
> (and yes, I know that electricity *is* used in the final stages of
> refining to further purify the Si)

My impression is that most thermal energy in the final stages of
manufacture (after initial smelting) is electric, because of the
precision control and cleanliness needed. I guess one could look at the
relative sizes of the gas and electric bills at a plant and get a better
idea of the breakdown. I probably should have specified electrical
energy, rather than just "energy", as that's what I was thinking.

> So, while the government reports say solar cells can pay back the energy
> required to produce them in just a few years, it still wouldn't be
> economically smart to use solar electricity to produce them when thermal
> energy from fossil is so much cheaper.

That's pretty much my point. Until and unless they get cheap enough to
compete with grid power, solar cells will be economically restricted to
those few areas where their advantages outweigh their drawbacks. If the
manufacturers can't use them to replace grid power, who can?

>
> daestrom
>
> . Mind you, Dan Lancaster will still insist that all previous energy
> used in all R&D everywhere, including the incandenscent light used to
> light the drawing boards that scientists like Stefan and Boltzman used
> to work out radiation theory and quantum mechanics be included in that
> payback.

I wonder if nuclear could meet that test yet?

Thanks for your comments, as always.