Transcript Solar - UVA Virtual Lab

Solar Energy
Today I will start out by addressing a couple of (ultimately crucial) questions
What is sunlight?
What (exactly) is "'electricity?"
I will then move on to:
Photovoltaics = Direct conversion of sunlight into electricity which involves:
The essential difference between photoconductors and photovoltaics
And how to create that difference based on material and design
Solar Thermal = Capture sunlight's heat => boil something => Indirect Electricity
Prompting a short discussion of: Molten Salt Heat Storage
Which could give solar thermal an edge over all other renewables
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
What is Sunlight?
It's a very BROAD range of wavelengths
Resembling Black Body Spectrum (vibrating things randomly sharing energy)
Significant portions of which are absorbed in the atmosphere (yellow => red)
Source: en.wikipedia.org/wiki/Sunlight
How much power can sunlight deliver?
Above earth's atmosphere, total power is ~ 1350 Watts / square meter
This value is referred to as "AM0" (air mass zero)
Atmosphere absorbs ~ 25% of this => ~ 1000 Watts / square meter
Referred to as "AM1.5" (air mass 1.5)
But this = MAXIMUM solar power intensity on earth's surface
Because this is the value when the sun is DIRECTLY overhead
Which happens only in certain locations, in certain seasons, once a day
AND clouds / haze / fog will further reduce intensities!
How do you convert wavelengths to photon energy?
Start with fact that light's energy is proportional to its frequency:
Elight = h f
h = Plank's constant, f = frequency (in Hz = cycles /sec)
Add in fact that, in one cycle, light travels one wavelength (= "l")
So velocity of light = c = l / (cycle time) = l f
Plug second relationship into first relationship:
Elight = h f = h (c / l) = hc / l
Then, agree to express light energies in eV, and wavelengths in microns
Yielding relationship: Elight (in eV) = 1.24 / l (light in microns)
Where a Joule = (coulomb of charge) (crossing 1 Volt potential)
But eV = (electron charge) (crossing 1 Volt potential) = 1.6 x 10-19 Joules
Using this to revise the scale of our earlier sunlight plot:
From formula above (and fact that 1000 nm = 1 micron):
Wavelength
Energy
5 eV
2 eV
1 eV
0.5 eV
And calling out spectral ranges:
UV
Infrared
We now need to know how these colors interact with matter:
Because we want to exploit these interactions to CAPTURE the light's energy!
1) INFRARED (IR) LIGHT:
700 nm < Wavelength
(> 0.7 microns)
Energy < 1.7 eV
If absorbed by matter => heat = atomic & molecular vibrations
Symmetric
Stretch
Asymmetric
Stretch
Scissoring
Rocking
Wagging
Uniqueness of atom/molecule's vibrational energies => absorption bands
=> "IR" spectroscopies used by chemists AND
to absorption bands seen in AM1.5 spectrum:
Animated GIFs from: en.wikipedia.org/wiki/Infrared_spectroscopy
Twisting
IR vibrations may be amusing (and quite useful elsewhere)
But, in the context of this lecture, the important conclusion is that:
Infrared light lacks the energy necessary to liberate electrons from atoms/bonds
So Infrared light cannot DIRECTLY produce electricity
But Infrared's heat energy CAN be transferred (absorbed) by other things
And if a liquid captures enough of that heat
It can then boil => Huge volume expansion => Pressure
Which can then be used to propel an electrical turbine-generator
So Infrared light can INDIRECTLY produce electricity
As opposed visible light that CAN directly produce electricity:
1) VISIBLE (Vis):
400 nm < Wavelength < 700 nm
(0.4-0.7 micron)
1.7 eV < Energy < 3 eV
Visible light CAN knock an electron free from an atom ("ionization")
Visible light CAN knock one electron out of a covalent bond
Probably why eyes use it: Eye's sensor output = Liberated electrons / ions
Infrared would just have caused atoms in eye to vibrate
Vibrations CAN be transferred to other atoms (a.k.a. heat flow)
But it’s hard to imagine a heat-directing "nerve fiber"
In contrast, visible light can liberate electrons producing => electricity
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
What about ultraviolet light?
1) ULTRAVIOLET (UV): Wavelengths < 400 nm
(< 0.4 micron)
3 eV < Energy
UV has MORE than enough energy to liberate electrons!
So we CAN also use UV to directly produce electricity
HOWEVER it also has enough energy to BREAK MANY ATOMIC BONDS
Distinction: "Liberating" = Removing one of a covalent bond's paired electrons
Or extracting one electron from an unbonded pair (=> "free radical")
In both cases, pair can re-form later by capturing an electron
Whereas: "Bond breaking" = wiping out bond / changing molecular structure
So, over time, UV LIGHT can even DESTROY solar cell materials
Particular problem for more weakly bonded "organic" solar cell materials
Moving on: What (exactly) is electricity?
This may sound like a very simple, or even a dumb question – In which case:
I've read an incredible number of "dumb" news stories
And "dumb" university press releases
And even the occasional "dumb" comment from a research scientist
That/who imply:
"Electricity" = THING that can just ooze out of a lump of material
WRONG!!
Electricity is not a thing – It is a process:
Of electrons being driven in a flow
But why CAN'T we just squeeze electrons out (and then USE them)?
Revisiting James Clerk Maxwell:
Maxwell's 1st Equation: Electric Field builds in proportion to net charge
"Net charge" = Positive charge density – Negative charge density
Electric force is then proportional to the strength of that electric field
So just a TINY ACCUMULATION of net charge => HUGE FORCE
For a second or two:
Then there is a loud snap
as charge build-up dissipates
BOTTOM LINE:
On scales much greater than molecular dimensions
Nature will not LET you remove or add significant net charge!
(because resulting HUGE force would then EXPEL that net charge)
Photo from : /joyerickson.wordpress.com/2012/08/05/pull-up-something-cool/
So "electricity" is almost all about pumping charge
PUMP charge in one end of something and out the other end:
"Something" = Generator, solar cell, battery . . .
That's WHY it's called electrical current: An analogy to incompressible water:
Can pump water THROUGH pipes, but if try to increase water IN pipe => Explosion!
Generator, solar cell, battery, . . . are all CHARGE PUMPS
And pumps are judged on basis of the flow and pressure they can generate:
Water Power = Flow x Pressure
which is analogous to:
Electrical Power = "Current" x "Voltage"
So what we are NOW looking for is a solar-powered electron pump!
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Solar-powered electron pumps = Photovoltaics (a.k.a. "solar cells")
What happens when light strikes a material? From above:
Case 1) Photon energy < Material's bond energy:
Photon can't shake anything loose, most just proceed on through
That is, material is ~ transparent to these too low energy photons
Case 2) Photon Energy ~ Material's bond energy
Photon IS now absorbed and its energy used to kick an electron out of a bond
Case 3) Photon Energy > Material's bond energy
Photon is absorbed:
Part of its energy kicks an electron out of a bond
Rest of its energy also goes to that electron in the form of kinetic energy
That is, photon kicks it out of the bond, then kicks it in the butt!
So when we shine light having bond energy onto a material:
(Remembering that I want is power = current x voltage out of a solar cell)
This light is (at least eventually) going to be absorbed by a bond in our material:
Before: Atom cores (positive nuclei + inner electrons) + bonding electrons
In 1D:
~
+2
-2 +2
-2
+2
-2 +2
0
0
0
0
After: One negative electron is liberated, leaving behind a positive region:
0
+1/2
+1/2
-1
0
0
0
0
But electron is drawn BACK to positive region => falling back into bond
(or a FEW might wander out the left or right end)
This gives me only a "photoconductor" and NOT a solar cell!
(Wake up all of you so-called science journalists!)
Most liberated electrons just wander around until pulled back into bonds
Or ones that DO exit are equally likely to exit right or left
Nothing is pumping (pushing) electrons to flow in one direction!
Application? ADD external battery/power supply and use as a light detector:
No light: All electrons in bonds, no current through sample (despite battery)
Light:
Freed electrons.
Battery can now suck them out one end and push back into other
But where did the light's energy go? Into the atoms
Freed electrons later fell back into atoms' clutches
Then giving those atoms a kick => atomic vibrations (a.k.a. heat)
To produce power we've ALSO got to drive (PUMP) electrons somewhere!
Classic Technique:
START with fully-bonded electrically neutral material, most commonly silicon
It sets the bonding rules with its crystal structure: Rule with Si = four bonds
ADD atom of almost same size but with one less bonding electron (e.g. boron)
Fits into crystal, steals electron from elsewhere, making it an Acceptor (thief?)
Bond where electron stolen from now becomes a positive Hole
Add neutral Acceptor atoms to Si => Negative ions + Liberated holes:
Silicon atoms = Grey (fixed neutral atoms)
Acceptor ions also FIXED in position
Holes = MOBILE Why?
ANSWER: Hole grabs electron from neighbor, leaving hole in a NEW place . . .
And holes don't fill with electrons from outside because that would add net charge
Can also add things that will shed electrons
Donor = Similar to Si in size, but with one additional bonding electron (e.g. P, As)
Fits into crystal but final electron has nothing to pair with and bond. Thus:
It easily loses that electron (ionizes), becoming a positive Donor:
With that last, now liberated, electron free to wander:
Add neutral Donor impurity atoms to Si => Positive ions + Liberated electrons:
Here only liberated electrons are MOBILE
And, as in other material, net charge is still zero!
So James Clerk Maxwell is still happy
And if mobile electrons return home, heat will eventually kick them back out!
NOTE: Acceptor and Donor impurities are called "DOPANTS"
Payoff comes when you put two such "doped" regions side by side:
Acceptor ions + Mobile Holes:
Donor ions + Mobile Electrons:
At intersection ("junction”) mobile electrons are going to rush across to FILL mobile holes!!
(Because holes ARE just bonds that have lost one of the normal paired electrons!)
Mobile electrons filling the holes (in the bonds) is called "recombination"
Central junction thus becomes depleted of ALL mobile charges (liberated electrons or holes):
But this leaves uncompensated FIXED acceptor ions (-) / donor ions (+) at the junction
Which produces a growing Electric field at that junction
Migration / recombination continues UNTIL field is strong enough to block further migration
Because Electric field pushes positive charges left and negative charges right
Electric field thus locks remaining mobile holes and electrons on their respective sides
NOW add light to knock electrons out of background silicon:
Light photon knocks an electron
out of a bond, creating a
wandering electron + hole
(traveling together ="exciton")
New electron and hole can both wander, but if reach "junction:"
"Built-in" electric field traps new electron on right, but propels hole to left
If instead created on left, hole trapped on left, but electron swept to right
= A CHARGE PUMP
(BTW this is also a DIODE: Can only force current through it in ONE direction)
More general way of creating boundary charge-separating electric field:
ABOVE: ONE MATERIAL but divided it in TWO DIFFERENTLY BEHAVING REGIONS
Made two regions different by adding acceptor OR donor impurity atoms
ALTERNATIVE: Just put two DIFFERENT MATERIALS side by side
Electrons at higher energies on one side may try to cross over to other side
NET RESULT (again) = Build up of electric field at boundary
It's analogous to diatomic bonding in molecules:
Atoms of two different materials:
Possibility 1) Covalent Bond = Equal sharing of electrons in bond:
Possibility 2) Polar Covalent Bond = Unequal sharing of electrons in bond:
Slight Electric Field
Possibility 3) Ionic Bond = Transfer of electron from one atom to other:
Big Electric Field!
+
-
Solar cell materials MUST allow some electron movement, thus:
At junction of two different materials, interfacial bonding can be polar or ionic:
OR:
Both => Interfacial Electric field
+
-
UNLESS the electron energy levels of the two materials are too similar
Then electron in one material may not find a lower energy state in the other
And the interfacial bonding will remain covalent (and E field =>0)
If layers are of same material there's no reason for electrons to shift (and E =>0)
But we can then, instead, add different impurities to layers (as in Si cells):
Impurities => Interfacial Electric field
But remember, charge only shifts near the interfaces:
Materials are composed of atoms which are intrinsically charge neutral
So natural state of any single layer is ALSO neutral:
And junction between two materials also "starts out" neutral
But if energy levels in materials are different enough, charge can cross interface:
However, charge shift builds electric field eventually blocking further charge transfer

So charge DEEPER in layers
will not get chance to cross!
Leading to common rules for almost all photovoltaics (solar cells):
You must have at least one set of paired materials:
Be it two distinctly different materials OR
One basic material (e.g., silicon) modified into two differently acting layers
In that pair, one layer/material must cling onto electrons more tightly
So that electrons will flow into it from second material
Until shift of charge across boundary builds up ELECTRIC FIELD at interface
To a level that it stops further shifting of charge
That "interfacial" electric field will then provide the critical push
Light energy => breaks electron bonds
But ELECTRIC FIELD then pushes freed electrons all in one direction
But what is the amount of POWER (= current x voltage) PRODUCED?
Current comes from the number of electrons liberated by light / second
- Function of how strongly that material absorbs photons of that color
- AND of how much material is doing the absorbing (e.g. its layer thickness)
Voltage comes from charge driving/separating junction ELECTRIC FIELD
Which was created by process of bond filling/liberating. Leading to fact that:
Photo-electrons/holes are driven out of cell by ~ 60-70% of liberation energy =>
Solar cell voltage ~ (0.65) (liberation energy) / (electron charge = "e")
For Si solar cell, "Voc" ~ (0.65) (Si electron liberation energy = 1.1 eV) / e ~ 0.7 Volt
Larger the liberation-energy (a.k.a. bandgap) => More VOLTAGE
But Less current: Why?
For answer, we must go back to solar spectrum
Energy
Wavelength
If this strikes a solar cell made of a material having Small liberation energy =>
MOST colors liberate electrons, but they're driven out of cell by small voltages
If this strikes a solar cell made of a material having Large liberation energy =>
Only HIGH ENERGY light liberates electrons
But fewer electrons that ARE liberated will be driven by higher voltages!
So now try to find optimum combination (based on choice of optimum material):
Energy absorbed by cells made of different materials:
Energy absorbed by solar cell
Cell with small "bandgap"
VIS
Electron Liberation
0 eV
Light's Energy
5 eV
VIS
Cell with large "bandgap"
Butt Kicking
Electron Liberation
0 eV
Light's Energy
Blue Triangles => Energy passing right thru the cell!
Yellow Triangles => Energy lost to heating of the cell
Rectangles (only!) => Electricity out of the cell
5 eV
Energy absorbed by solar cell
Butt Kicking
Energy absorbed by solar cell
Cell with medium "bandgap"
VIS
Butt
Kicking
Electron
Liberation
0 eV
Light's Energy
5 eV
Plotting backward, as percentages (preparing to blend in solar intensities):
Sun's wavelengths (in nm)
250
500
750
1000
1500
2000
2500
100%
Electron Butt Kicking
For solar cell using
material with 0.5 eV
electron liberation energy
(bandgap):
50%
Electron Liberation
0%
5 eV
2
1.5
1.25
1 eV
0.75 eV
0.5 eV
Sun's wavelengths (in nm)
250
500
750
1000
1500
2000
2500
100%
Electron Butt Kicking
For solar cell using
material with 0.75 eV
electron liberation
energy (bandgap):
50%
Electron Liberation
0%
5 eV
2
1.5
1.25
1 eV
0.75 eV
0.5 eV
Now for solar cells with a LOT of different bandgaps:
100%
PERCENTAGE of light's
energy captured as
electricity by different
bandgap materials:
50%
0%
5 eV
2
1.5
Sun's Energy Spectrum:
1.25
1 eV
0.75 eV
0.5 eV
Sun's AM0 Power Spectrum
Energy captured by
different bandgap cells:
250
(I really should've used
Sun's AM1.5 spectrum)
500
750
1000
1500
2000
Wavelength in nm (or above, equivalent energy in eV)
2500
Larger the area under a bottom curve => More solar energy captured
BIGGEST area comes between 1 eV and 1.5 eV curves, for ~ 1.3 eV material
Material of this bandgap could capture & convert ~ 35% of Sun's energy
It's called the Shockley-Queisser Limit after William Shockley & Hans Queisser
So fact that single-material solar cells efficiencies top out at 35% is
NOT because we are doing a poor job of engineering!
It is instead because:
We ONLY CAPTURE part of light energy liberating electron from bond,
REST of light energy is wasted giving liberated electron kick in the butt
All because photons refuse to divide their energy between multiple electrons!
(Sure would be nice if we could weasel our way around that!)
Another way of visualizing what's behind the Shockley-Quiesser Limit:
VERY energetic photon is absorbed by bonding electron:
Electron is freed and, with extra energy, crashes around semiconductor crystal:
Crashed into atoms absorb energy => atomic vibrations (= heat)
+ now slowed down electron
Shockley-Quiesser: With energy spread of of solar photons
2/3 of solar power => atomic vibration
Si has ALMOST perfect bandgap to reach S-Q Limit!
So it can approach ~ 30% efficiency. But silicon is also fragile and expensive
Fragility: It's tougher than OTHER semiconductors, but it is still brittle/breakable
Expense: In solar cell need light-liberated electrons/holes to wander a long time
So have good chance of wandering into electric field at junction (essential!)
But wandering electrons/holes tend to STOP at impurities
If both stop there, likely that electrons will fill holes (effectively vanishing)
So solar cell grade Si must be about 1000X more pure than electronic grade
About 1 part in 1012 pure! => Much more expensive than normal Si
So we'd really like some sort of breakthrough!
To beat the Shockley-Quiesser Limit, minimize the electron butt kicking:
How? By stacking solar cells of DIFFERENT materials atop one another
Materials with different bond energies/"bandgaps"
Top: Material with large bond energies ~ purple/blue light:
High energy photons use ALMOST ALL of their energy liberating electrons
While less energetic photons pass right through!
Middle: Material with medium bond energies ~ green light:
Medium energy photons use ALMOST ALL of their energy liberating electrons
While less energetic photons pass right through!
Bottom: Material with low bond energies ~ red light:
Low energy photons use ALMOST ALL of their energy liberating electrons
~ ALL photon energy => electron liberation => ~ 100% energy capture!!
These are called "Multi-Junction Solar Cells" or "Tandem Solar Cells"
BIG REMAINING PROBLEM:
Shockley-Quiesser Limit was ~ 33% energy capture efficiency
Multi-junction cell can approach ~ 3X (= 100%) energy capture efficiency
but requires the combination of ~ 3 different cells to get there
So it likely costs (at least) 3X times as much!!!! (!$#!$@$%)
We must, instead, produce stack of three cells for ~ cost of single cell
To radically cut cost, we need materials that are so similar that
they can be grown by one continuous processes
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
How to select those candidate semiconductor materials:
Bandgap (electron liberation energy) of semiconductors vs. their "lattice constant"
Shockley-Quiesser
optimum bandgap
for single material cell
Strained Layer Epitaxy of
Germanium-Silicon Alloys,
John C. Bean, Invited Review,
Science Magazine 230, p127 (1985)
"Lattice constant" = Size of fundamental atomic grouping => Spacing of atoms
Connecting lines => "Alloy" mixtures of one semiconductor with another
With connecting line giving the bandgap of the resultant material mixture
To grow atop one another, materials must be along vertical line:
Shockley-Quiesser
optimum bandgap
for single material cell
Strained Layer Epitaxy of
Germanium-Silicon Alloys,
John C. Bean, Invited Review,
Science Magazine 230, p127 (1985)
Why? So lattices match well enough that bonds between layers are not interrupted:
That is, so that this is possible:
Layers of cell #1
Layers of cell #2
(and so on)
Because if bonds between cell materials do NOT match up . . .
You'll end up with something like this:
Many/most bonds at interface are incomplete:
Incomplete bonds will try to grab ("trap") passing electrons or holes
INCLUDING our photo-generated electrons or holes BEFORE they get out of cell
And if they don't make it out, we don't get photovoltaic power out!
Broken bonds can also be a problem in single thin film cells:
Because "thin film" = They are not made from one big monolithic crystal
Instead consist of a lot of micro-crystals packed together = "polycrystal"
OR from something that is not crystalline at all = "amorphous"
Thus, while still only one material, there are LOTS of screwed up interfacial bonds:
And those screwed up bonds can also trap photo-carriers!
Trying to take ALL of this into account:
HAVE solar cells (of ANY type) broken through the S-Q limit?
Following figure:
Compilation by U.S. National Renewable Energy Lab (NREL) of
latest,
greatest,
one of a kind,
possibly never reproduced
(or horrendously expensive),
solar cell efficiency records:
Best RESEARCH solar cells (1976 – 2015):
At lower resolution but with some guidance as to cell types:
Multi-junction / Tandem
Single crystal GaAs
Single crystal Si
Polycrystal thin film Si
Other thin films
"Hero" (best in lab / single shot) efficiencies, top to bottom:
Multi-junction solar cells: Highest at almost 45%
So they’ve beat, but not shattered, the Shockley-Quiesser Limit
Crystalline GaAs solar cells (more exotic/$ crystal than Si): Hair over 34%
Crystalline silicon solar cells: Highest at 27.6%
Thin-film cells (e.g. polycrystalline/amorphous Si and CdTe): Highest at 23%
Perovskite cells: Highest just over 20%
Dye-sensitized, organic . . . cells: Highest at 12%
Quantum Dot solar cells: Highest at 9.2%
I will explain the differences between all of these cell types in my upcoming
Next Generation Solar Power lecture
Solar Thermal Power: How to use all of that infrared sunlight!
Wavelength
Energy
5 eV
UV
2 eV
1 eV
0.5 eV
Infrared (IR)
Solar Photovoltics: Ignored IR because infrared couldn't knock electrons loose
Solar Thermal: Exploits fact that IR CAN still excite vibrations (a.k.a "heat")
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Concentrate infrared light with mirrors – then boil something:
One way of doing this is to build from small single mirror / liquid pipe units:
http://www.esolar.com/business/w
here-esolar-fits-in-the-industry/
And to then combine large numbers of these into solar thermal "farms:"
www.treehugger.com/renewableenergy/us-army-goes-solar-500-mwsolar-thermal-power-plant-to-bebuilt-at-fort-irwin.html
Or use mirrors (only) to send COMBINED IR to central tower:
Big:
http://blog.zintro.com/cleantech-alt-energy/page/2/
Or "supersized" - California's recently completed "Ivanpah" Mojave Desert farm:
"Tower of Power"
Time Magazine
June 24, 2013
1600 Hectares (4000 acres)!
But goal is to "boil something" and drive turbine–generator
Which leads to advantages and disadvantages for each alternative
DISADVANTAGE of small units:
They don't incorporate individual mini-turbines
Instead, tubes are all plumbed together then
eventually all routed to turbine-generator(s)
=> Lot of piping and lot of pumping
=> Lot of heat loss (before reaching turbine)?
http://en.wikipedia.org/wiki/Solar
_thermal_energy
ADVANTAGE of small units:
East to west line can “automatically” track sun as it moves across the sky
Because, even as sun moves, IR stills hits somewhere on the tube
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Versus central tower design:
DISADVANTAGE of central tower farms:
For IR from ALL of the mirrors to hit top of tower
EVERY mirror MUST CONINUOUSLY steer!
And tilt of EVERY mirror is DIFFERENT
=> Large investment in 2-Axis steering gear for each mirror + control!
ADVANTAGE of central tower farms:
No distributed (mirror to mirror) plumbing
Thus no heat loss in such plumbing
Instead ALL IR goes directly (as light) to the top of central tower
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Which solar thermal design is better?
Both sides (naturally) claim their design is better
So I usually turn to (nominally?) objective government studies
In particular, to data gathered by U.S. Energy Information Agency (EIA)
However, EIA data (to date) do not differentiate between two alternatives
But their "levelized cost" 1 data for solar thermal power (all types)
IS currently 2X cost of solar photovoltaic power (all types)
However, I DO note that over last ~ half dozen years:
I've read more worldwide reports on central tower solar thermal farms
To which one might cynically respond: "But Mega-projects => Mega PR!"
1) I'll explain "Levelized costs" in the Economic Analysis lecture
Frequent problem with both types of current solar thermal:
WATER
Many projects to date have used water as the liquid
Which boils at 100°C => Expands => Turns turbine => Generates electricity
But what happens next?
Get more water? Not in deserts where these plants are built!
Not in California! Not in Spain! (locations of some of biggest solar thermal farms)
Reuse water? Fine, but you must first cool and re-condense it
Even 50°C desert air can eventually cool pipes/water
But to be expedite: Use FORCED air cooling (using up some of your power)
OR use new COOL WATER to cool old water (but cool water from where?)
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Better answer: Replace water with an oil
Choosing an oil that, instead of boiling at 100°C, might boil at say 400°C
Get the top of that tower REALLY HOT (in order to boil that oil!)
But then, by contrast, desert air is so cool that:
Even passive (non-forced) air suffices to cool / re-condense oil
This alternative IS now used in some solar thermal designs
(Such as the newer Crescent Dunes concentrating solar thermal plant in Nevada)
And oil could become the STANDARD in solar thermal!
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
But there may be an even better coolant: Molten salt
Keep water or oil as the fluid to be eventually boiled, powering the generator
But in the solar tower itself, have molten salt absorb IR heat
AND add big reservoir to store large amounts of heated salt
Tower
Hot salt reservoir
Salt loop to
reservoir
Salt to H2O or Oil
heat exchanger
Salt loop from
reservoir
Boiling H2O or Oil
loop to generator
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
But why add in another layer of complexity?
Because with enough molten salt (heated in the middle of the day)
That molten salt can be used to boil water/oil all night
Producing around-the-clock-solar energy!
Why use molten salt reservoirs?
- Non-toxic / Non-flammable
- Melts at a low temperature (~130 °C for sodium/potassium/calcium salt mixture)
- Mass => Its heating sucks in a huge amount of energy (= high "heat capacity" )
Our only other, non-nuclear, sustainable 24/7 power source is
Hydroelectric Power
Which also operates 24/7 based on its use of big storage reservoirs!
Credits / Acknowledgements
Some materials used in this class were developed under a National Science Foundation "Research
Initiation Grant in Engineering Education" (RIGEE).
Other materials, including the "UVA Virtual Lab" science education website, were developed under even
earlier NSF "Course, Curriculum and Laboratory Improvement" (CCLI) and "Nanoscience Undergraduate
Education" (NUE) awards.
This set of notes was authored by John C. Bean who also created all figures not explicitly credited above.
Copyright John C. Bean (2016)
(However, permission is granted for use by individual instructors in non-profit academic institutions)
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm