Energy to build a PV cell?

[Tom Bruhns]

Anyone here have any educated guesses how much energy it takes to build
a square centimeter of photovoltaic cell? The real question comes down
to how long it takes for a cell in a place on Earth with high
insolation (high average sunlight) to pay back the energy it took to
make that cell. Associated with that, what it the expected service
life of a photovoltaic cell in normal sunlight at the Earth's surface?
I suppose all this depends on the cell technology, but the answers
could help pick an optimal technology.

Cheers,
Tom

 

[Luhan]

Anyone here have any educated guesses how much energy it takes to build
a square centimeter of photovoltaic cell? The real question comes down
to how long it takes for a cell in a place on Earth with high
insolation (high average sunlight) to pay back the energy it took to
make that cell. Associated with that, what it the expected service
life of a photovoltaic cell in normal sunlight at the Earth's surface?
I suppose all this depends on the cell technology, but the answers
could help pick an optimal technology.

You will soon be hearing from Don Lancaster on this one!!!

Luhan

 

[Tom Bruhns]

You will soon be hearing from Don Lancaster on this one!!!

Luhan

Well, I was hoping more for someone like Jim Thompson to reply. But it
wasn't difficult to find, at www.ases.org, the following:

-->7 What is the energy payback time for PV?
-->Typically, the energy payback time (the time it
-->takes the system to generate the same amount
-->of energy that it took to manufacture the system)
-->for PV systems is two to five years. Given that a
-->well-designed and maintained PV system will
-->operate for more than 20 years, and a system
-->with no moving parts will operate for close to 30
-->years, PV systems produce far more energy over
-->their lives than is used in their manufacture.

Cheers,
Tom

 

[Macgyver]

Tom Bruhns wrote:

Well, I was hoping more for someone like Jim Thompson to reply. But it
wasn't difficult to find, at www.ases.org, the following:

-->7 What is the energy payback time for PV?
-->Typically, the energy payback time (the time it
-->takes the system to generate the same amount
-->of energy that it took to manufacture the system)
-->for PV systems is two to five years. Given that a
-->well-designed and maintained PV system will
-->operate for more than 20 years, and a system
-->with no moving parts will operate for close to 30
-->years, PV systems produce far more energy over
-->their lives than is used in their manufacture.

Cheers,
Tom

This "typical" energy payback time is misleading (in that it does not
appear to take into account the energy required to pull the resources
out of the ground to make the system, payback time for additional
required system equipment such as the batteries, regulator, goverment
discounted pricing incentives, etc).

Payback time is much, much too low.

Solar panels have there place, but at the moment the only place I see
them being useful is in powering remote applications where I have no
access to a mains supply.


Where is our resident solar guru when we need him !

 

[[email protected]]

Anyone here have any educated guesses how much energy it takes to build
a square centimeter of photovoltaic cell? The real question comes down
to how long it takes for a cell in a place on Earth with high
insolation (high average sunlight) to pay back the energy it took to
make that cell. Associated with that, what it the expected service
life of a photovoltaic cell in normal sunlight at the Earth's surface?
I suppose all this depends on the cell technology, but the answers
could help pick an optimal technology.

Cheers,
Tom

http://www.nrel.gov/ncpv/energy_payback.html

Reference (2) from http://en.wikipedia.org/wiki/Solar_cell (also quite
good)

Michael

 

[BobG]

PV cells are 6" silicon wafers like integrated circuits are made
from... the pure crystal of silicon is 'grown' from a vat of molten
silicon in an electric furnace about 1500 deg C. Then the cylindrical
ingots are sawed into wafers. The cells are big diodes, so a 'boat' of
N type wafers will get put in a diffusion furnace at 1500 deg C for
some precise number of minutes with a P type gas blowing across it.
Then the PV cells are assembled into panels. You can see that both
processes are energy intensive. There are only a few companies that
make the wafers.... IC manufacturers have the wafer fabs to make ICs,
so there is a supply and demand force at work on the silicon itself. If
a 150 watt PV panel sells for $750 (about $5 a watt), you can bet that
the cost of the electricity to make it plus some profit is in that
price.

 

[Rich Grise]

PV cells are 6" silicon wafers like integrated circuits are made
from... the pure crystal of silicon is 'grown' from a vat of molten
silicon in an electric furnace about 1500 deg C. Then the cylindrical
ingots are sawed into wafers. The cells are big diodes, so a 'boat' of
N type wafers will get put in a diffusion furnace at 1500 deg C for
some precise number of minutes with a P type gas blowing across it.
Then the PV cells are assembled into panels. You can see that both
processes are energy intensive. There are only a few companies that
make the wafers.... IC manufacturers have the wafer fabs to make ICs,
so there is a supply and demand force at work on the silicon itself. If
a 150 watt PV panel sells for $750 (about $5 a watt), you can bet that
the cost of the electricity to make it plus some profit is in that
price.

Whatever happened to "Edge-defined film growth"? That was supposed to
solve the problem of sawing the ingot, because it grows a very thin
ribbon of single-crystal silicon. But I haven't heard of it for quite
some time. Anybody even ever heard of such a thing?

Thanks!
Rich

 

[Tom Bruhns]

BobG wrote:
....

If
a 150 watt PV panel sells for $750 (about $5 a watt), you can bet that
the cost of the electricity to make it plus some profit is in that
price.

Hmmm...not necessarily. It may be that the semiconductor manufacturers
have already paid for the cost in buying the high grade part of it, and
the "castoffs" used to make the PV cells are being sold below the cost
of the refining energy, but above the value as scrap. If YOU had a big
pile of something left over that didn't meet the requirements of your
prime customer, but had little intrinsic value, and you found a
customer willing to pay something above the scrap value for your scrap,
what would you do?

Of course, not all the PV cells are made that way. I suppose it will
only be when (if?) people get serious enough about using PV as an
electrical energy source that we'll be able to tell for sure just what
the real cost is, but it seems like most sources right now are saying
that's it's around four to five years to pay back the energy, with some
reasonable belief that it could become shorter. Even at four to five
years, it appears that it's not out of line with the cost per watt of
other facilities built to produce electric power.

Cheers,
Tom

 

[BobG]

PV isnt a commodity yet evidently. The price seems to be about $5 a
watt, because everyone is selling as much as they can make at that
price. Others on this newsgroup have said that about the time some
warehouse manager starts complaining about 'all those solar panels from
last year filling up the warehouse... move em out!' Then the price will
start coming down. There is some Dept of Energy paper that says they
take about $3 a watt to make, so 40% profit right now. That theoretical
150 watt panel that cost $750... at $.15 per KWH it would need to put
out 1000 KWH to pay for itself. .15KW x 5H a day is .75KWH a day for
1333 days... about 3.5 years. Another back of the envelope sketch by a
master!

 

[CC]

BobG wrote:
...

Hmmm...not necessarily. It may be that the semiconductor manufacturers
have already paid for the cost in buying the high grade part of it, and
the "castoffs" used to make the PV cells are being sold below the cost
of the refining energy, but above the value as scrap. If YOU had a big
pile of something left over that didn't meet the requirements of your
prime customer, but had little intrinsic value, and you found a
customer willing to pay something above the scrap value for your scrap,
what would you do?

Of course, not all the PV cells are made that way. I suppose it will
only be when (if?) people get serious enough about using PV as an
electrical energy source that we'll be able to tell for sure just what
the real cost is, but it seems like most sources right now are saying
that's it's around four to five years to pay back the energy, with some
reasonable belief that it could become shorter. Even at four to five
years, it appears that it's not out of line with the cost per watt of
other facilities built to produce electric power.

Cheers,
Tom

Aren't solar concentrators an obvious way to improve this situation?

 

[Keith]

PV isnt a commodity yet evidently. The price seems to be about $5 a
watt, because everyone is selling as much as they can make at that
price. Others on this newsgroup have said that about the time some
warehouse manager starts complaining about 'all those solar panels from
last year filling up the warehouse... move em out!' Then the price will
start coming down. There is some Dept of Energy paper that says they
take about $3 a watt to make, so 40% profit right now. That theoretical
150 watt panel that cost $750... at $.15 per KWH it would need to put
out 1000 KWH to pay for itself. .15KW x 5H a day is .75KWH a day for
1333 days... about 3.5 years. Another back of the envelope sketch by a
master!

The interest/expected ROR on that $750 panel is about $.20/day (10%
of $750), so the first 8ish hours of sunlight are the bank's. How
many hours of sunlight do you get on an average day?

 

[BobG]

Aren't solar concentrators an obvious way to improve this situation?

==========================================
I've heard they put out a little less when hot, so maybe a concentrator
would work well in Maine, but not so good in Arizona?

 

[Zak]

Aren't solar concentrators an obvious way to improve this situation?

They do not work well in the shade. And in bright and sunny conditions,
a mechanical generator might be more efficient and a whole lot less
expensive per watt.

A different perspective is teh allowed size of the panel.

Monocrystalline cells may have high efficiency but are expensive.

Thin film on glass may be less efficient, but if space is no issue the
size doesn't matter and cost to output or energy expenditure to output
could matter.

What is desirable on a satellite is not the ame as on a residential rooftop.



Thomas

 

[Glen Walpert]

BobG wrote:
...

Hmmm...not necessarily. It may be that the semiconductor manufacturers
have already paid for the cost in buying the high grade part of it, and
the "castoffs" used to make the PV cells are being sold below the cost
of the refining energy, but above the value as scrap. If YOU had a big
pile of something left over that didn't meet the requirements of your
prime customer, but had little intrinsic value, and you found a
customer willing to pay something above the scrap value for your scrap,
what would you do?

There really isn't any scrap involved. There is a finite amount of
silicon being made, all existing silicon manufacturing plants are
running at full capacity. Most of production goes to semiconductor
manufacturing (which pays a higher price for higher purity and more
precise doping - exact same raw material with further processing by
the material mfgr.) and essentially all of the rest goes to solar cell
production, which is currently limited by inadequate silicon supply.
The silicon manufacturers are however reluctant to add capacity,
remembering as they do the last industry downturn and figuring that
the next one will coincide with any new capacity coming on line. So
you should not expect to see any drop in solar cell prices until the
next major semiconductor industry downturn.

This is per one of my sisters who has been selling solar power systems
for over 20 years. She also claims that grid connected solar only
pays back the investment per standard accountant analysis if it is
used for long term emergency backup power instead of a diesel
generator set, otherwise buyers are either making an environmental
contribution and/or statement, or betting that the price of
electricity will go up substantially over the life of the equipment.

BTW she paid about $4 a peak watt for the 8 kW system on her house,
including batteries, inverter, automatic transfer switch and panel.
That was wholesale cost of material only, curiously around the same
price for the one on her old house ~15 years ago and her current house
~5 years ago.

 

[Rich Grise]

There really isn't any scrap involved. There is a finite amount of
silicon being made, all existing silicon manufacturing plants are
running at full capacity.

You must mean "being refined". There's a finite amount of silicon, like
15% of the mass of the planet[1]. ;-)

But, I have heard that the silicon in chips and stuff doesn't come from
sand - I wonder where they actually get it? Silicon ore?

Thanks!
Rich
[1] http://www.nineplanets.org/earth.html

 

[Glen Walpert]

There really isn't any scrap involved. There is a finite amount of
silicon being made, all existing silicon manufacturing plants are
running at full capacity.

You must mean "being refined". There's a finite amount of silicon, like
15% of the mass of the planet[1]. ;-)

But, I have heard that the silicon in chips and stuff doesn't come from
sand - I wonder where they actually get it? Silicon ore?

AFIK all silicon used in semiconductors is produced from silane, SiH4,
which is produced in bulk at "3 nines" (99.9% pure) and then purified
to 6 or 7 nines and usually doped before epitaxial deposition or
conversion to bulk Si for wafer growth. Per my hazy recollection of a
tour of a Praxair semiconductor gasses plant a few years ago.

Per http://mattson.creighton.edu/SiH4/Silane_info.html the bulk silane
production is:

"Tetrahalosilanes, such as SiCl4, are reduced to SiH4 by hydrides such
as LiH, NaH, or LiAlH4."

The MSDS for SiH4 is interesting, for example, if there is a silane
leak it will usually ignite immediately on contact with air, but
occasionally it will build up and then explode.

 

[Charlie Edmondson]

They do not work well in the shade. And in bright and sunny conditions,
a mechanical generator might be more efficient and a whole lot less
expensive per watt.

A different perspective is teh allowed size of the panel.

Monocrystalline cells may have high efficiency but are expensive.

Thin film on glass may be less efficient, but if space is no issue the
size doesn't matter and cost to output or energy expenditure to output
could matter.

What is desirable on a satellite is not the ame as on a residential
rooftop.



Thomas

But at some point, the cost of your support structure starts to
dominate, and low efficiency panels eat you up in the installation
costs, esp. when you amortize the costs...

Concentrators can get you more energy to work with, but they also vastly
increase the heat, which as most of gurus here now, rapidly age the
cells and reduce the lifetime of the panel.

One guy was using thin film concentrators and non-imaging optics to only
gather the 'usuable' wavelengths, to get that boost in efficiency
without the heat gain, but he still mounted his cells on big water
cooled heat sinks!

Charlie

 

[[email protected]]

BobG wrote:

That theoretical
150 watt panel that cost $750... at $.15 per KWH it would need to put
out 1000 KWH to pay for itself. .15KW x 5H a day is .75KWH a day for
1333 days... about 3.5 years. Another back of the envelope sketch by a
master!

Flip that envelope over!

$750 / $0.15 per KWH = 5,000 KW hours' output for payback.

At 0.75 KWH/day, that's 6,670 days, or 18 years for the panel alone.
Add the fixed costs (installation, inverters) and consumables
(maintenance,
batteries) and it looks even less attractive.

OTOH, 5hrs a day insolation seems a bit light.

Even so, every time I do the extended calculations, the payback
period
for solar in an urban locale is .... never. Not to mention the
environmental
impact of periodically recycling all those lead acid batteries.

Wind power pays back very quickly though, if you don't mind the bird
strikes.

Cheers,
James Arthur

 

[Tim Williams]

But, I have heard that the silicon in chips and stuff doesn't come from
sand - I wonder where they actually get it? Silicon ore?

AFAIK, it starts with white sand -- clean enough for clear glass, about 99
to 99.5% pure -- which is smelted with carbon and sometimes iron to make
silicon or ferrosilicon, respectively. This requires an electric arc
furnace. (This can also make silicon carbide, but using more sand prevents
it, since 2SiC + SiO2 = 3Si + 2CO.) Most goes to metallurgy (all those 2 to
4% Si alloys for transformers don'tcha know..oh and cast iron....), but some
goes elsewhere.

You can react silicon with hot chlorine gas (tasty..) and fractionally
distill the silicon tetrachloride. This gets it to reasonable purity. The
gas can be decomposed on a hot wire, forming chlorine gas and silicon again.
Or does it need hydrogen (to reduce it, 2H2 + SiCl4 = 4HCl + Si), I forget.
You can run a continuous process where halogens come in and spirit away
metal, then deposit it on a hotwire. Since different chlorides decompose at
different temperatures, you can precisely control purity.

Afterwards, you get a polycrystalline rod of rather pure silicon, with a
thin impurity of say, tungsten in the middle, which can be thin enough
inside a thick bar of silicon that it doesn't matter (else you could...bore
it out, or, something). This stock is pure enough to melt in a silica bowl
and pull crystals from (Czochralski(sp?) method or zone refining).

Speaking of zone refining, you can also start with only somewhat pure
silicon and draw a melt zone across it (with fire or resistance or induction
heating). I don't know how they do that in a band, through the entire
thickness of a rod, without it falling, or if they just heat up a section so
impurities *diffuse* along, rather than actual melting. It might be they
fit the silicon inside a fused quartz tube, then remove it with hydrofluoric
acid to get bare silicon.

Tim

 

[Jim Thompson]

AFAIK, it starts with white sand --

[snip]

Naaah! It comes from outer space and is brought here by alien space
travelers ;-)

...Jim Thompson