Energy Storage

Posted by PITHOCRATES - September 18th, 2013

Technology 101

Our First Energy Storage Devices helped us Kill each other in Battle

There’s something very important to today’s generation.  Stored energy.  It’s utmost on their minds.  As they are literally obsessed with it.  And get downright furious when they have none.  Because without stored energy their smartphones, tablets and laptop computers will not work.  And when they don’t they will disconnect them from the Internet.  And social media.  A fate so horrible that they carry spare batteries with them.  Or a power cord to plug into an electrical outlet or cigarette lighter in a car.

Energy storage devices go back millennia.  Of course, back then there was no Internet or social media.  People just talked to each other in person. Something unimaginable to today’s generation.  For it was a simpler time then.  We ate.  We procreated.  Sometimes talked.  And we killed each other.  Which is where that energy storage comes in.

An early use of energy storage was to make killing each other easier.  Early humans used rocks thrown by slings and spears thrown by hand in hunting and war.  But you had to get pretty close to your prey/enemy to use these things.  As the human body doesn’t have the strength to throw these things very far or hard.  But thanks to our ingenuity we could use our tools and make machines that could.  Such as the bow and arrow.

The Bow and Arrow and the Crossbow use Tension and Compression to Store Energy

We made early bows from wood.  They had a handgrip and two limbs, one above and one below the handgrip.  Attached to these limbs was a bowstring.  The limbs were flexible and could bend.  And because they could they could store energy.  The archer would draw back the bowstring, bending the two limbs towards him.  This took a lot of strength to bend this wood.  The farther the archer pulled back the bowstring the more strength it took.  Because it was not the natural state for those limbs.  They wanted to remain unbent.  And were ready to snap back to that unbent position in a fraction of a second.  Much quicker than the archer pulled back the bowstring.

As the limbs bent the inside of the limb (towards the archer) was under compression.  The outside of the limb (facing away from the archer) was under tension.  The compression side was storing energy.  And the tension side was storing energy.  Think of two springs.  One that you stretch out in tension that will snap back to an un-stretched position when released.  And one that you push down in compression that will push back to an uncompressed position when released.  These are the two forces acting on the inside and the outside of the bending limbs of a bow.  Storing energy in the bow.  When the archer releases the bowstring this releases that stored energy.  Snapping those limbs back to an unbent position in a fraction of a second.  Bringing the bowstring with it.  Very quickly.  Launching the arrow into a fast flight toward the archer’s prey/enemy.

The stronger the bow the more energy it will store.  And the more lethal will be the projectile it launches.  Iron is much harder to bend than wood.  So it will store a lot more energy.  But a human cannot draw back a bowstring on an iron bow.  He just doesn’t have the strength to bend iron like he can bend wood.  So they added a couple of simple machines—levers to turn a wheel—at the end of a large wooden beam to draw back the bowstring.  At the other end of this beam was the iron bow.  What we call a crossbow.  With the wheel increasing the force the archer applied to the hand-crank the iron bow slowly but surely bent back.  Storing enormous amounts of energy.  And when released it could send a heavy projectile fast enough to penetrate the armor of a knight.

The Mangonel uses Twisted Rope to Store Energy while a Trebuchet uses a Counterpoise

Most children did this little trick in elementary school.  The old rattlesnake in the envelope trick.  You open up a large paperclip and stretch a small rubber band across it.  Then you slide a smaller paperclip across the taut rubber band.  And then you turn that small paperclip over and over until you twist the rubber band up into a tight twist.  Storing energy in that twist.  Slip it into the envelope.  And let some unsuspecting person open the envelope.  Allowing that rubber band to untwist quickly.  With the paperclip spinning around in the envelope making a rattlesnake sound.

We call this type of energy storage torsion.  An object that in its normal state is untwisted.  When you twist it the object wants to untwist back to its normal state.  On the battlefield we used this type of energy storage in a catapult.  The mangonel.  Which used a few simple machines.  We used a lever inserted into a tight rope braid.  In its normal state the lever stood upright.  A lever turned a wheel a cog at a time to pull the large lever down parallel to the ground.  Twisting the rope.  Putting it under torsion.  Storing a lot of energy.  When they released the holding mechanism the rope rapidly untwisted sending the large lever back upright at great speed.  Sending the object on it hurling towards the enemy.

The problem with the mangonel is that it took a long time to crank that rope into torsion.  Another catapult did away with this problem.  The trebuchet.  Perhaps the king of catapults.  This was a large lever with a small length on one side of the pivot and a large length on the other side of the pivot.  Think of a railroad crossing arm.  A long arm blocking the road with a counterweight at the other end.  We balance this so well that we need very little energy to raise or lower it.  The trebuchet, on the other hand, is not perfectly balanced.  It has a very heavy counterweight—a counterpoise—that in its normal state is hanging down with the long end of the lever pointing skyward.  They pull the long end of the lever down close to the ground.  Pulling up the counterweight.  Attached to the far end of the lever is a rope.  At the end of the rope is a rope pouch to hold the projectile.  When released the counterweight swings back down.  Sending the long end of the lever up quickly.  With the far end traveling very quickly.  Pulling the rope with it.  Because the length of the rope adds additional distance to the lever the projectile travels even faster than the end of the lever.  Which is why the stored energy in the hanging counterweight can launch a very heavy projectile great distances.


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