B
B1ackwater
Guest
Getting more bang for your solar-energy buck
- - - - - - - - - - - - - - - -
By far, the greatest problem with using solar energy to make
household electricity is the cost of silicon solar cells.
These are relatively high-tech items requiring expensive
facilities and processes to manufacture. As a result, the
price of cells has not changed very much in relation to
the volume of production. Make 100 times as many per year,
but the price is still almost the same as when you made
far fewer.
Although 'space' applications are limited by weight
considerations and thus must use lots of silicon
cells, ground-based applications can afford to to
use approaches that swap weight for price.
The idea is to use as few actual solar cells as possible
yet get the same amount of energy you'd get by completely
tiling your roof in the things. The solution is to
concentrate sunlight using lenses or mirrors.
There are numerous ways to go about this, but a few stand
out as being both practical and inexpensive.
For simply collecting and concentrating sunlight, a "lens"
doesn't have to be a particularly GOOD lens. "Fresnel"
type lenses - those flat sheets that look like they have
a bunch of concentric lines pressed into them - will
serve nicely. (actually, the "lines" form circular
prisms of increasing angle that bend a strip of light
towards the center of the lens)
http://en.wikipedia.org/wiki/Fresnel_lens
Fresnel lenses can be made of plastic or glass. Plastic
is lighter, cheaper and more shatter-resistant - but it
also absorbs IR and UV a solar cell could utilize plus
plastics 'yellow' after a few years exposure to the
sun, ozone and elements. Individual glass lenses are
not practical because of the expense - but Fresnel
patterns CAN be pressed into a large sheet of glass
using nothing more complex than a metal mold that
stamps down. You'd get, say, a 2x4 foot piece of
plate glass with maybe 4-inch lenses all up and
down its surface. This wouldn't cost much more than
an ordinary sheet of window glass.
Depending on your design, you might have each 4-inch
lens focus on a half-inch solar cell. Using a short
"F" lens, the cell might be just two inches below
the lens sheet. This geometry will yeild a 64:1
concentration ratio.
What this means is that instead of having 64 half-inch
silicon cells, you can get the same power from just one.
That's a BIG cost saving - you've drastically reduced
the need for the most expensive element in the system.
There's a company that makes such solar panels :
http://www.treehugger.com/files/2006/04/holographic_sol_1.php
Although they call them "holographic" concentrators. These
aren't quite the same as a Fresnel lens, but an actual
hologram OF a lens. Holograms of lenses ARE lenses. There
are two drawbacks though ... holograms require extremely
fine patterns on the glass, meaning each sheet has to be
custom etched with a laser. Holograms also work by
"interference" between light waves and that can mean
some of the energy gets lost. All in all, the pressed-in
Fresnel lens seems the most economical solution.
An easy alternative way to concentrate sunlight is to
use a reflective "half-pipe" ... just cut a 4" tube
in half longways so it forms a long "trench" and then
chrome-plate the inside. A strip of solar cells run
along a metal rod just above the "pipe". Again, a
64:1 concentration ratio is easily achievable, and
you don't have any big glass sheets to worry about
breaking. However, the whole assembly may be five
inches thick instead of just two. On a roof, that
may not be a big deal. The biggest concern might be
if you lived in hurricane country.
Both designs benifit from a small "F"-ratio. With a
lens that focuses far away, the image of the sun
will move a great deal during the day. With a very
short-focus lens or mirror it doesn't travel very
far from mid-morning to late-afternoon. This means
there's little reason to use tracking panels that
try to stay pointed exactly at the sun. Yet another
expense eliminated.
There IS a problem with concentrating panels - heat.
At 64:1 you can fry a silicon cell. This means you
must build in some way to keep the cell cool. The
"half-pipe" design lends itself to a row of radiator
fins on the strip holding the cells, thus using
air-cooling. How well this would work on a still
summer day would require some simple experiments.
The lens-panels don't lend themselves to air cooling
however. The only viable approach is to circulate water
or some other fluid through channels in the base-plate
holding the cells. The 'half-pipe' system could also
employ liquid cooling if necessary.
Now this isn't necessarily BAD. One VERY practical
use of solar energy is to take advantage of its HEAT.
Your typical family of 4.5 (plus pets) uses a LOT of
hot water every day. During the winter it consumes
a LOT of energy to simply keep the house warm. The
heat collected when cooling the solar cells can easily
and inexpensively be stored and put to use heating
bath/laundry water and/or directed through radiators
to warm the house. A two-fer.
So, what does all this MEAN ?
It means that solar NEED NOT be too expensive anymore.
Instead of the cell-filled panels you buy now for about
ten bucks a watt you can spend one buck a watt and
get hot water as part of the bargain.
It means that anyone having a new house designed, or
an old one thorougly renovated, should insist that
appropriate taps and tubes for solar hot water be
included in the plumbing. It means that your
southern-facing roof should have strong attachment
points for standard 2x4 footprint solar panels built
in from the get-go.
It also means that you should opt for a DC lighting
system instead of 120v AC so less power needs to be
converted to AC. (a DC lighting system can be run
off the power grid during 'dark' days simply by
using a battery charger). LED lights - the coming
thing - are happiest with low DC voltages anyway.
The reason most LED lights fail nowdays is because
120v AC has to be converted to just a few volts DC
and the converters tend to fail. If you started
with 10v-12v DC then everything is simplified.
It means that you should have your main electrical
panel set up for dual power sources. These are not
expensive or elaborate modifications. That way
when you GET your solar-electric panels they
will smoothly integrate with the utility-company
power ... and you can even sell your excess power
back to the utility.
Such solar-friendly tweaks are cheap at construction
time but not as cheap afterwards. So, think ahead.
You may not want, or be able to afford, solar-electric
TODAY ... but why not make it extra easy to add
tomorrow ?
The home of tomorrow need not be "deep green", but
it SHOULD aim to be "green-ish". If it can provide
25% or even 50% of its own power and heating needs
then you've made a huge amount of progress towards
lowering your "carbon footprint" and your utility
bills. It's "good enough".
- - - - - - - - - - - - - - - -
By far, the greatest problem with using solar energy to make
household electricity is the cost of silicon solar cells.
These are relatively high-tech items requiring expensive
facilities and processes to manufacture. As a result, the
price of cells has not changed very much in relation to
the volume of production. Make 100 times as many per year,
but the price is still almost the same as when you made
far fewer.
Although 'space' applications are limited by weight
considerations and thus must use lots of silicon
cells, ground-based applications can afford to to
use approaches that swap weight for price.
The idea is to use as few actual solar cells as possible
yet get the same amount of energy you'd get by completely
tiling your roof in the things. The solution is to
concentrate sunlight using lenses or mirrors.
There are numerous ways to go about this, but a few stand
out as being both practical and inexpensive.
For simply collecting and concentrating sunlight, a "lens"
doesn't have to be a particularly GOOD lens. "Fresnel"
type lenses - those flat sheets that look like they have
a bunch of concentric lines pressed into them - will
serve nicely. (actually, the "lines" form circular
prisms of increasing angle that bend a strip of light
towards the center of the lens)
http://en.wikipedia.org/wiki/Fresnel_lens
Fresnel lenses can be made of plastic or glass. Plastic
is lighter, cheaper and more shatter-resistant - but it
also absorbs IR and UV a solar cell could utilize plus
plastics 'yellow' after a few years exposure to the
sun, ozone and elements. Individual glass lenses are
not practical because of the expense - but Fresnel
patterns CAN be pressed into a large sheet of glass
using nothing more complex than a metal mold that
stamps down. You'd get, say, a 2x4 foot piece of
plate glass with maybe 4-inch lenses all up and
down its surface. This wouldn't cost much more than
an ordinary sheet of window glass.
Depending on your design, you might have each 4-inch
lens focus on a half-inch solar cell. Using a short
"F" lens, the cell might be just two inches below
the lens sheet. This geometry will yeild a 64:1
concentration ratio.
What this means is that instead of having 64 half-inch
silicon cells, you can get the same power from just one.
That's a BIG cost saving - you've drastically reduced
the need for the most expensive element in the system.
There's a company that makes such solar panels :
http://www.treehugger.com/files/2006/04/holographic_sol_1.php
Although they call them "holographic" concentrators. These
aren't quite the same as a Fresnel lens, but an actual
hologram OF a lens. Holograms of lenses ARE lenses. There
are two drawbacks though ... holograms require extremely
fine patterns on the glass, meaning each sheet has to be
custom etched with a laser. Holograms also work by
"interference" between light waves and that can mean
some of the energy gets lost. All in all, the pressed-in
Fresnel lens seems the most economical solution.
An easy alternative way to concentrate sunlight is to
use a reflective "half-pipe" ... just cut a 4" tube
in half longways so it forms a long "trench" and then
chrome-plate the inside. A strip of solar cells run
along a metal rod just above the "pipe". Again, a
64:1 concentration ratio is easily achievable, and
you don't have any big glass sheets to worry about
breaking. However, the whole assembly may be five
inches thick instead of just two. On a roof, that
may not be a big deal. The biggest concern might be
if you lived in hurricane country.
Both designs benifit from a small "F"-ratio. With a
lens that focuses far away, the image of the sun
will move a great deal during the day. With a very
short-focus lens or mirror it doesn't travel very
far from mid-morning to late-afternoon. This means
there's little reason to use tracking panels that
try to stay pointed exactly at the sun. Yet another
expense eliminated.
There IS a problem with concentrating panels - heat.
At 64:1 you can fry a silicon cell. This means you
must build in some way to keep the cell cool. The
"half-pipe" design lends itself to a row of radiator
fins on the strip holding the cells, thus using
air-cooling. How well this would work on a still
summer day would require some simple experiments.
The lens-panels don't lend themselves to air cooling
however. The only viable approach is to circulate water
or some other fluid through channels in the base-plate
holding the cells. The 'half-pipe' system could also
employ liquid cooling if necessary.
Now this isn't necessarily BAD. One VERY practical
use of solar energy is to take advantage of its HEAT.
Your typical family of 4.5 (plus pets) uses a LOT of
hot water every day. During the winter it consumes
a LOT of energy to simply keep the house warm. The
heat collected when cooling the solar cells can easily
and inexpensively be stored and put to use heating
bath/laundry water and/or directed through radiators
to warm the house. A two-fer.
So, what does all this MEAN ?
It means that solar NEED NOT be too expensive anymore.
Instead of the cell-filled panels you buy now for about
ten bucks a watt you can spend one buck a watt and
get hot water as part of the bargain.
It means that anyone having a new house designed, or
an old one thorougly renovated, should insist that
appropriate taps and tubes for solar hot water be
included in the plumbing. It means that your
southern-facing roof should have strong attachment
points for standard 2x4 footprint solar panels built
in from the get-go.
It also means that you should opt for a DC lighting
system instead of 120v AC so less power needs to be
converted to AC. (a DC lighting system can be run
off the power grid during 'dark' days simply by
using a battery charger). LED lights - the coming
thing - are happiest with low DC voltages anyway.
The reason most LED lights fail nowdays is because
120v AC has to be converted to just a few volts DC
and the converters tend to fail. If you started
with 10v-12v DC then everything is simplified.
It means that you should have your main electrical
panel set up for dual power sources. These are not
expensive or elaborate modifications. That way
when you GET your solar-electric panels they
will smoothly integrate with the utility-company
power ... and you can even sell your excess power
back to the utility.
Such solar-friendly tweaks are cheap at construction
time but not as cheap afterwards. So, think ahead.
You may not want, or be able to afford, solar-electric
TODAY ... but why not make it extra easy to add
tomorrow ?
The home of tomorrow need not be "deep green", but
it SHOULD aim to be "green-ish". If it can provide
25% or even 50% of its own power and heating needs
then you've made a huge amount of progress towards
lowering your "carbon footprint" and your utility
bills. It's "good enough".
