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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