Silicon, Semiconductor, LED, Photon, Photovoltaic Effect, Photocell, Solar Panel, Converter, Battery and Solar Power Plant

Posted by PITHOCRATES - July 4th, 2012

Technology 101

A Photocell basically works like a Light Emitting Diode (LED) in Reverse

Solar power is based on the same technology that that gave us the electronic world.  Silicon.  That special material in the periodic table that has four electrons in its valence (i.e., outer most) shell.  And four holes that can accept an electron.  Allowing it to form a perfect silicon crystal.  When these silicon atoms come together their four valence electrons form covalent bonds with the holes in neighboring silicon atoms.  These silicon atoms share their valence electrons so that each silicon atom now has a full valence shell of eight electrons (with four of their own electrons and four shared electrons).  Making that perfect crystal structure.  Which is pretty much useless in the world of semiconductors.  Because you need free electrons to conduct electricity.

When we add impurities (called ‘doping’) to silicon is where the magic starts.  If we add a little bit of an element with five electrons in its valence shell we introduce free electrons into the silicon crystal.  Giving it a negative charge.  If we instead add a little bit of an element with 3 electrons in its valence shell we introduce extra holes looking for an electron to fill it.  Giving it a positive charge.  When we bring the positive (P) and the negative (N) materials together they from a P-N junction.  The free electrons cross the junction to fill the nearby holes.  Creating a neutrally charged depletion zone between the P and the N material.  This is a diode.  If we apply a forward biased voltage (positive battery terminal to the P side and the negative battery terminal to the N side) across this junction current will flow.  Like charges repel each other.  The negative charge pushes the free electrons on the N side of the junction towards the junction.  And the positive charge pushes the holes on the P side of the junction towards the junction.  Where they meet.  With free electrons filling available holes causing current to flow.  A reverse bias does the reverse.  Pulls the holes and electrons away from the junction so they can’t combine and cause current to flow.

It takes energy to move an electron out of its ‘hole’.  And when an electron combines with a hole it emits energy.  Typically this energy is not in a visible wavelength so we see nothing.  However, with the proper use of materials we can shift this wave length into the visible spectrum.  So we can see light.  Or photons.  This is the principle behind the light emitting diode.  Or LED.  An electric current through a P-N junction causes electrons to leave their holes and then recombine with holes.  And when they recombine they give off a photon in the visible spectrum of light.  Which is what we see.  A photocell basically works the other way.  Instead of using voltage and current to create photons we use photons from the sun to create voltage and current.

A Solar Array that could Produce 12,000 Watts under Ideal Conditions may only Produce 2,400 Watts in Reality

When we use the sun to bump electrons free from their shells we call this the photovoltaic (PV) effect.  This produces a small direct current (DC) at a low voltage.  A PV cell (or solar cell) then is basically a battery when hit with sunlight.  Electric power is the product of voltage and current.  So a small DC current and a low voltage won’t power much.  So like batteries in a flashlight we have to connect solar cells together to increase the available power.  So we connect solar cells into modules and modules into arrays.  Or what we commonly call solar panels.  Small panels can power small loads.  Like emergency telephones along the highway that are rarely used.  To channel buoys that can charge a battery during the day to power a light at night.  And, of course, the electronics on our spacecraft.  Where PV cells are very useful as there are no utility lines that run into space.

These work well for small loads.  Especially DC loads.  But it gets a little complicated for AC loads.  The kind we have in our homes.  A typical 1,000 square foot home may have a 100 amp electric service at 240 volts.  Let’s assume that at any given time there could be as much as half of that service (50 amps) in use at any one time.  That’s 12,000 watts.  Assuming a solar panel array generates about 10 watts per square foot that means this house would need approximately 1,200 square feet of solar panels (such as a 60 foot by 20 foot array or a 40 foot by 30 foot array).  But it’s not quite that simple.

The sun doesn’t shine all of the time.  The capacity factor (the percentage of actual power produced divided by the total possible it could produce under the ideal conditions) is only about 15-20%.  Meaning that a 1,200 square foot solar array that could produce 12,000 watts under ideal conditions may only produce 2,400 watts (at a 20% capacity factor).  Dividing this by 120 volts gives you 20 amps.  Or approximately the size of a single circuit in your electrical panel.  Which won’t power a lot.  And it sure won’t turn on your air conditioner.  Which means you’re probably not going to be able to disconnect from the electric grid by adding solar panels to your house.  You may reduce the amount of electric power you buy from your utility but it will come at a pretty steep cost.

Solar Power Plants can be Costly to Build and Maintain even if the Fuel is Free 

Everything in your house that uses electricity either plugs into a standard 120V electrical outlet, a special purpose 240V outlet (such as an electric stove) or is hard-wired to a 240V circuit (such as your central air conditioner).  All of these circuits go back to your electrical panel.  Which is wired to a 240V AC electrical service.  A lot of electronic devices actually operate on DC power but even these still plug into an AC outlet.  Inside these devices there is a power supply that converts the AC power into DC power.  So you’ll need to convert all that DC power generated by solar panels into useable AC power with a converter.  Which is costly.  And reduces the efficiency of the solar panels.  Because when you convert energy you always end up with less than you started with.  The electronics in the converters will heat up and dissipate some of that generated electric power as heat.  If you want to use any of this power when the sun isn’t shining you’ll need a battery to store that energy.  Another costly device.  Another place to lose some of that generated electric power.  And something else to fail.

We typically build large scale solar power plants in the middle of nowhere so there is nothing to shade these solar panel arrays.  From sun up to sun down they are in the sunlight.  They even turn and track the sun as it rises overhead, travels across the sky and sets.  To maximize the amount of sunlight hitting these panels.  Of course the larger the installation the larger the maintenance.  And the panels have to be clean.  That means washing these arrays to keep them dirt and bird poop free.  Some of the biggest plants in service today have about 200 MW of installed solar arrays.  One of the largest is in India.  Charanka Solar Park.  When completed it will have 500 MW of PV arrays on approximately 7.7 square miles of land.  With a generous capacity factor of 30% that comes to 150 MW.  Or about 19 MW/square mile.  The coal-fired Robert W. Scherer Electric Generating Plant in Georgia, on the other hand, generates 3,520 MW on approximately 18.75 square miles.  At a capacity factor of about 90% for coal that comes to about 3,168 MW.  Or about 169 MW/square mile.  About 9 times more power generated per square mile of land used.

 So you can see the reason why we use so much coal to generate our electric power.  Because coal is a highly concentrated source of fuel.  The energy it releases creates a lot of reliable electricity.  Day or night.  Summer or winter.  A large coal-fired electric generating facility needs a lot of real estate but the plants themselves don’t.  Unlike a solar plant.  Where the only way to generate more power is to cover more land with PV solar panels.  To generate, convert and store as much electric power as possible.  All with electronic equipment full of semiconductors that don’t operate well in extreme temperatures (which is why our electronics have vents, heat sinks and cooling fans).  So the ideal conditions to produce electricity are not the ideal conditions for the semiconductors making it all work.  Causing performance and maintenance issues.  Which makes these plants very costly.  Even if the fuel is free. 

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Silicon, Semiconductor, Electrons, Holes, PN Junction, Diode, LED, Photon, 7-Segment LED and Full-Color Flat Panel LED Displays

Posted by PITHOCRATES - May 30th, 2012

Technology 101

Applying a Voltage across a PN junction to Create a Forward Bias Pushes Electrons and Holes towards the Junction

There’s gold in them thar Hills.  And silicon in the valley.  California has been a fountain of wealth.  Much of which they built from two materials located on the periodic table.  Atomic number 79.  Gold.  Or ‘Au’ as it appears on the periodic table.  And atomic number 14.  Silicon.  Or ‘Si’ as it appears on the periodic table.  Both of these metals proved to be valuable.  One by its scarcity.  One by what we could do with it.  For it was anything but scarce.  Silicon is the second most common element behind only oxygen.  But this commonly found material proved to be a greater font of wealth for California.  For it fueled the semiconductor industry.  For when we doped it with impurities we produced negatively (N-type) and positively (P-type) charged material.  Bringing the N and the P together gave us the PN junction.  Giving us the diode, transistor and integrated circuit.

The miracle of semiconductors occurs at the atomic level.  Down to the electrons orbiting the atom’s nucleus.  The nucleus contains an equal number of positively charged protons and neutrally charged neutrons.  The number of protons gives us the atomic number.  Changing the number of neutrons gives us isotopes.  Radioactive material has more protons than neutrons.  Uranium-235 is an isotope.  The stuff that made the atomic bomb dropped on Hiroshima.  Electrons orbit the nucleus.  In discrete energy levels.  The orbits closest to the nucleus have the lowest energy levels.  The orbits father away from the nucleus have higher energy levels.  Most of these orbits are ‘full’ of electrons.  The outer electron shell when ‘full’ is inert.  An outer shell that isn’t ‘full’ or has extra electrons is active.  And can chemically react.  Forming molecules.  When chemicals come into contact with each other and form molecules it is these electrons in the outer orbits (or valence electrons) that move into and out of the orbits of the different chemicals.  That is, the different elements share these valence electrons.

This is what we do when we dope silicon with impurities.  We either remove electrons from the valence shell to create a net positive charge.  Or we add electrons to the valence shell to create a net negative charge.  Giving us P-type and N-type material.  At the PN junction the N-type material loses its excess electrons to the P-type material across the junction as the empty holes in the valence shell attract the excess electrons.  As electrons leave the valence shells in the N-type material they leave holes in the valence shell where they once were.  Or, in the world of electronics, as electrons flow one way holes flow the other.  When we apply a voltage across a PN junction to create a forward bias (negative voltage applied to N-type and positive voltage applied to P-type) we push electrons and holes towards the junction.  If the forward bias is great enough they will continue all the way through the junction and into the material on the far side.  Where electrons will combine with excess holes.  And holes will combine with excess electrons.  Creating an electric current.  If we apply a voltage to create a reverse bias we will pull electrons and holes away from the PN junction.  And there will be no electrical current. We call such a PN device a diode.  A very important and indispensible device in electronics.

Placing Seven LEDs into a Figure-Eight Pattern created the Seven-Segment LED

Now back to those discrete energy levels.  There is another useful property we get when electrons move between these energy levels.  Electrons absorb energy when they move to a higher energy level.  And emit energy when they move to a lower energy level.  We make use of this property in fluorescent lighting.  A charged plasma field in a fluorescent lamp excites a small amount of mercury in the lamp.  As electrons fall into lower orbits in the mercury atoms they release invisible short-wave ultraviolet radiation.  The phosphor coating on the inside of the lamp absorbs this radiation and fluoresces.  Creating visible light.  By using different materials, though, we could see the energy (a photon) emitted by an electron falling into a lower energy level.  We have been able to move the wavelength of this photon into the visible spectrum.  The first commercial application to convert these photons into visible light was a device that gave us a red light.  That device was that important and indispensible PN-junction.  The diode.  And the use of different materials other than silicon moved these photons into the visible spectrum.  Giving us the light-emitting diode.  Or LED.

The first LEDs were only red.  Then we developed other colors using different materials.  Shifting the wavelength of the photon through all colors of the visible spectrum.  Being low-power devices, though, the intensity of light emitted was limited.  So an LED required careful mechanical construction and optics.  To direct the light out of the material forming the PN junction.  With a reflector behind the junction.  And a lens above.  To aim and diffuse the light.  And to prevent it from reflecting back into the material where it may be dissipated as heat.  Early use of LEDs was for indicator lights.  The low power consumption meant little heat was generated as with an incandescent lamp.  Which worked well in the temperature sensitive computer world.  Placing 7 LEDs into a figure-eight pattern created the seven-segment LED display.  With a rectangular shaped piece of translucent plastic above each LED you could see a bar of light for each light emitting diode.  Creating a forward bias on certain bars in the seven-segment display created the numbers we saw on our first calculators and digital watches.

An LED could produce a similar radiation like in the fluorescent lamp.  Using that radiation to fluoresce a phosphor coating inside a lamp to produce white light.  Similar to the fluorescence lamp.  Only while using less power.  Mixing the emitted light from red, green and blue (RGB) LEDs also produced white light.  Further improvements allowed us to emit whiter and brighter lights.  Allowing brighter lamps that consumed less power than the compact fluorescent lamps which were energy saving alternatives to the incandescent lamps.  With the lower power consumption of LEDs creating less heat we expanded the lifespan of lighting sources made from LEDs.  Using them to increase the service life in lamps inconvenient to change.  Like in traffic signal lights over busy intersections.  To the taillights in tractor trailers.  Where anytime spent not hauling freight was lost revenue.

We made Full-Color Flat Panel Displays from LEDs by combining Red, Green and Blue LEDs into Full-Color Pixel Clusters

The market didn’t demand these developments in semiconductors or LEDs.  For the most part the market didn’t even know this technology existed.  But the entrepreneurs gathering in Silicon Valley did.  They had some great ideas of what they could do with this new technology.  All they needed was the capital to bring these ideas to market.  It was risky.  The technology was good.  But could they use it to make useful things at affordable prices?  And would the people be so enamored with the things they built that they would buy them?  There were just too many unknowns for conservative bankers to take a risk.  But thanks to venture capitalists those entrepreneurs got the capital they needed.  Brought their ideas to market.  Created Silicon Valley.  And the modern world we now take for granted.

They continue to advance this technology.  Creating full-color flat panel displays.  By combining red, green and blue LEDs into full-color pixel clusters.  Which, unlike an LCD flat panel display, does not need a backlight as they produce their own light.  So these panels are thinner and use less power than LCD displays.  Making them ideal for the displays in our cellular devices for they allow more battery life between charges.  They also have wide viewing angles.  People looking at these displays from near perpendicular viewing angles see nearly the same quality of picture as those viewing directly in front.  Making them ideal for use in stadiums.  The video replays you see on that mammoth flat panel display in the new Dallas Cowboy stadium is an LED flat panel display.

All of this from joining two differently-charged semiconductor materials together.  Creating that all important and indispensible PN junction.  The foundation for every electronic device.  And of Silicon Valley itself.

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