Posted by Morris Dovey on September 21, 2009, 12:47 pm
Ken Maltby wrote:
> "The maximum intensity of sunlight on earth is 0.089
> w/cm2 at normal incidence on a bright day." -S.L.Soo
> Ref; W.G.Pfann and W.VanRoosbroeck, "Radioactive
> and Photoelectric p-n Junction Power Sources", J.of
> Appl. Phys., 25. No.11 (1954),1422 [and other]
Interesting - that scales to 890 W/m^2, but the "maximum" warns me that
the value may only apply on the ecliptic at solar noon with ideal
atmospheric conditions. I assume that Soo's value applies at sea level.
I would expect that, all other things being equal, an increase in
altitude might increase the actual value, while increases in either
displacement from the ecliptic or time displacement from solar noon
would decrease the actual value.
Hmm - it might make an interesting leisure time play activity to try
putting together a model to approximate maximum insolation at any given
altitude, latitude, and date-time...
> A Silicon solar cell, with an energy gap of 1.1ev, has
> a practical efficiency of 14 to 15 per cent.
>
> As to the use of the spectrum, a 35% theoretical
> efficiency is the limit for silicon. It doesn't use 65%
> of the solar wavelengths. [Not that this matters to
> the energy installer, only to the designers of the
> solar cells.]
Well, it might matter if we're trying to work from total insolation to
calculate usable input to a cell, so that there's a meaningful value to
which the rated efficiency can be applied, yes?
> So far, you can't do better than the silicon cell on a
> theoretical basis. All the differences in performance
> seen over time have been developments that do
> something to overcome the many obstacles that
> keep the silicon cell from meeting its theoretical
> potential.
That makes sense.
> The above relates to a single cell, it takes many cells
> to make a panel and a number of panels to provide
> any practical amount of energy. A panel's conversion
> efficiency can only be a fraction of the cell efficiency.
I'll take your word for this (it seems intuitively right) and recognize
a need for me to learn more about this.
> But the efficiency of the PV panels isn't the main
> problem with establishing what is possible, in the way
> of energy production/extraction/conversion. The main
> factor rapidly becomes; How much of that solar energy
> actually arrives at the panels, with enough energy to
> drive the output of the panels. There are some things
> you can do to help here as well, like having the panels
> pointed directly at the sun, all the time it is shining.
Agreed, although I suspect that in the foreseeable future the huge
majority of panels will be in fixed or semi-fixed installations.
> There are a number of other factors that you can't
> control which effect "Solar Energy Per Square Foot In
> Temperate Zone". Hopefully most of them are taken
> into consideration in the official insolation tables.
Yuppers. One of my "back burner" projects has been a solar
(non-electric) pump for water-cooling PV panels. My focus has been
irrigation and remote-area water supply, but I understand that PV panel
performance can be improved by cooling the cells.
Thanks!
--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/
Posted by (PeteCresswell) on September 21, 2009, 2:23 pm
Per Martin Riddle:
>The amount of wattage that strikes 1 square meter is about 800w.
>Solar cell efficiency is about 15-20%.
>Wiring and inverter losses (grid tie) is another 10%.
>So worst case, 160w*0.70= 112w per square meter.
>2000^2ft = 185^2m
>185 * 112 = 20720watts
Seems like my 'joker-in-the-deck' is at least possible then -
unless some basic law(s) of physics preclude higher efficiencies.
Double that efficiency and you've got 4kw possible....
Seems like I read about a project somewhere (in Europe?)
dedicated to exploring something like "The two-kilowatt
lifestyle" where everybody on the planet lives a reasonable life
on a total (sustainable?) energy consumption of two
kilowatts/hour/24-7.
--
PeteCresswell
Posted by bw on September 22, 2009, 6:06 am
> Per Martin Riddle:
>>The amount of wattage that strikes 1 square meter is about 800w.
>>Solar cell efficiency is about 15-20%.
>>Wiring and inverter losses (grid tie) is another 10%.
>>
>>So worst case, 160w*0.70= 112w per square meter.
>>
>>2000^2ft = 185^2m
>>
>>185 * 112 = 20720watts
> Seems like my 'joker-in-the-deck' is at least possible then -
> unless some basic law(s) of physics preclude higher efficiencies.
> Double that efficiency and you've got 4kw possible....
> Seems like I read about a project somewhere (in Europe?)
> dedicated to exploring something like "The two-kilowatt
> lifestyle" where everybody on the planet lives a reasonable life
> on a total (sustainable?) energy consumption of two
> kilowatts/hour/24-7.
> --
> PeteCresswell
You don't specifiy your current consumption. Common value for that is over
10 kWh per day.
http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/serve.cgi
Shows you will get about 3 kWh/m2 per day of solar in December and January.
You will need 10 square meters of panels to convert solar input to the same
value electrical.
You will never recover the cost of the panels if you live outside the
southwest USA, unless the cost of your electrical power from the grid starts
to get close to one dollar per kWh.
Posted by (PeteCresswell) on September 22, 2009, 11:38 pm
Per bw:
>Shows you will get about 3 kWh/m2 per day of solar in December and January.
>You will need 10 square meters of panels to convert solar input to the same
>value electrical.
>You will never recover the cost of the panels if you live outside the
>southwest USA, unless the cost of your electrical power from the grid starts
>to get close to one dollar per kWh.
Understood.
I was thinking in terms of what hope there might be globally for
the future - and it sounds to me like if somebody comes up with
really-cheap, really-efficient photovoltaics there's some hope
there of making a significant dent in the energy situation.
The operative word, of course is "If".... but at least it sounds
like the energy per square meter is there.
--
PeteCresswell
Posted by Roberto Deboni on September 23, 2009, 12:04 pm
(PeteCresswell) wrote:
> It's always been in the back of my mind that one of the jokers in
> the energy-shortage deck is the prospect of somebody coming up
> with some really-cheap, really-efficient means of converting sun
> to electricity.
>
> But; engineering question, I guess; how much power is actually
> available on, say, a 2,000 square foot roof in, say Pennsylvania?
When looking about solar energy, power (Watt's) is not very meaningful.
It's better to look the available daily energy (kWh's or BTU's).
For Philadelphia:
http://www.eosss.com/pdfs/SolarWhitePaper_CondoHOAinPA.pdf
that is, at optimal tilt, the following solar energy availability:
kWh/m2/day kWh/sf/day BTU/sf/day
Jan 3.5 0.325 1.11
Feb 4.1 0.381 1.30
Mar 4.6 0.427 1.46
Apr 4.7 0.437 1.49
May 4.7 0.437 1.49
Jun 4.8 0.446 1.52
Jul 4.8 0.446 1.52
Aug 5.0 0.465 1.58
Sep 4.8 0.446 1.52
Oct 4.5 0.418 1.43
Nov 3.5 0.325 1.11
Dec 3.1 0.288 0.98
Year 4.3 0.399 1.36
giving your roof a solar energy availability from 2100 BTU/day (winter)
to 3100 BTU/day (summer).
At 15% electrical conversion efficiency, you may gather from 90 kWh/day
(winter) to 130 kWh/day (summer). That's more than 35'000 kWh/yearly
But PV it's a very costly solution. And storing electricity is also costly.
Heat capture and storage is better.
In winter, concentrating at useful temperatures, you get at least 50%
efficiency, that's about 1050 BTU/day at say 170 F
And solar thermal collectors are cheaper:
http://en.wikipedia.org/wiki/Solar_hot_water
and vacuum tube solar collectors give hot water also in cold climates
http://www.siliconsolar.com/solar-evacuated-tube-collectors.html
Heat storage is more affordable (you store hot water), and if you have
space problems, Glauber's Salt tanks can reduce your needs:
http://www.allanstime.com/SolarHome/Eutectic_Salt/index.html
I would start covering your household heating budget.
For example 600-700 BTU/day ?
About 1400 sf of roof would be covered with thermal collectors.
And 600 sf of roof with PV collectors (about 8 kWp) producing about
10'000 kWh/year
Summer air conditioning could be made with an absorption heat pump
http://en.wikipedia.org/wiki/Absorption_heat_pump
Remember: you have a lot of heat (1500 BTU/day in summer)
> i.e. Is there enough solar energy falling on that roof to make a
> significant dent in electric consumption if the efficiency of the
> photovoltaics is sufficient?
The answer is Yes.
But an only PV solution is not the best one.
A mix of 2/3 to 3/4 of thermal collectors and the remaining as PV is a
better choice, returns more useful energy. And cheaper (or less costly
... :-)
--
Roberto Deboni
> w/cm2 at normal incidence on a bright day." -S.L.Soo
> Ref; W.G.Pfann and W.VanRoosbroeck, "Radioactive
> and Photoelectric p-n Junction Power Sources", J.of
> Appl. Phys., 25. No.11 (1954),1422 [and other]