Lead–Acid Battery, Nickel–Cadmium Battery (NiCd), Nickel–Metal Hydride Battery (NiMH) and Lithium-Ion Battery

Posted by PITHOCRATES - January 9th, 2013

Technology 101

The Chemical Reactions in a Zinc-Carbon Battery are One Way

A battery uses chemistry to make electricity.  An electric current is a flow of electrons that can do useful work.  The chemical reaction inside a battery creates that flow of electrons to produce an electric current.  In a common zinc-carbon battery, for example, a zinc electrode dissolves in an electrolyte.  As it does atoms release free electrons and become positive ions (cations) in the electrolyte.  Giving this solution a positive charge.  At the same time a carbon electrode is in a different electrolyte solution.  One filled with negative ions (anions).  Giving this solution a negative charge.

With no electrical load attached to the battery these electrodes and electrolytes are in equilibrium.  When we attach an external circuit across the battery terminals they provide a pathway for those free electrons.  As the free electrons travel through the external circuit the cations and anions travel through a porous membrane from one electrolyte to the other.  The positive cations (atoms with room for an additional electron) flow towards the carbon electrode.  And combine with the free electrons on the surface of the carbon electrode and become electrically neutral.

We can stop this chemical reaction.  Say by turning a flashlight or a portable radio off.  But we can’t reverse it.  This is a one-way chemical reaction that eventually dissolves away the anode.  A Zinc-carbon battery is inexpensive.  The amount of battery life we get out of it more than offsets the price.  And they’re easy to change.  But sometimes an application calls for a battery that isn’t easy to change.  Like a car battery.  Imagine having to change that a few times a year when it ran down.  No, that would be far too inconvenient.  Difficult.  And costly.  So we don’t.  Instead, we recharge car batteries.

The Chemical Reactions in a Lead-Acid Battery are Reversible allowing these batteries to be Recharged

A car battery is a lead-acid battery.  Each cell of a lead-acid battery has a positive electrode (i.e., plate) of lead dioxide.  A negative electrode of lead.  And an electrolyte of a sulfuric acid-water solution containing sulfate ions.  The lead chemically reacts with the sulfate ions to produce lead sulfate on the negative electrode while producing positive ions.  The lead dioxide chemically reacts with the sulfuric acid to produce lead sulfate on the positive electrode while giving up free electrons.

When we attach an external circuit to the battery (such as starting a car) the free electrons leave the positive electrode, travel through the external circuit and return to the battery.  Where they combine with those positive ions.  Lead sulfate forms on both electrodes.  These reactions consume the sulfuric acid in the electrolyte and leave mostly water behind.  Reducing the available charge in the battery.  But unlike zinc-carbon batteries these chemical reactions are reversible.  After a car starts, for example, the alternator provides the electric power needs of the car.  While applying a charging voltage to the battery.  This voltage will ionize the water in the battery which will break down the lead sulfate.  Deposit lead oxide back onto the positive electrode.  And deposit lead back onto the negative electrode.  Giving you a charged battery for the next time you need to start your engine.

A lead acid battery can provide a strong current to spin an internal combustion engine.  Which takes a lot of energy to fight the compression of the pistons.  And it can work in some very cold temperatures.  But it’s big and heavy.  And works best in things bigger and heavier.  Like cars.  Trucks.  Trains.  And ships.  But they don’t work well in things that are smaller and lighter.  Like cordless power tools.  Cell phones.  And laptop computers.  Things where battery weight is an important issue.  Requiring an alternative to the lead-acid battery.  One of the earliest rechargeable battery alternatives was the nickel–cadmium battery.  Or NiCad battery.

The Chemical Reactions produce Heat in a Lithium Ion Battery and can Catch Fire or Explode

The nickel–cadmium battery works like every other battery.  With chemical reactions that produce electrons.  And chemical reactions that consumes electrons.  The NiCad battery uses nickel (III) oxide-hydroxide for the positive electrode.  Cadmium for the negative electrode.  And potassium hydroxide as the electrolyte.  A NiCad battery may look like a zinc-carbon battery.  But the electrodes are different.  Instead of the zinc canister and a carbon rod the electrodes in a NiCad battery are long strips.  One is placed onto the other with a separator in between.  Then rolled up like a jelly-roll.

NiCad batteries have a memory effect.  If they were recharged without being fully discharged the battery ‘remembers’ the amount of charge it took to recharge the partially discharged battery.  So even if you fully discharged the battery it would only recharge it as if you partially discharged it.  Reducing the battery capacity over time.  The nickel–metal hydride battery (NiMH) eliminated this problem.  And improved on the NiCad.  Giving it 2-3 times the capacity of a NiCad battery.  NiCad and NiMH batteries are very similar.  They use the same positive electrode.  But instead of the highly toxic cadmium NiMH batteries use a mixture of a rare earth metal mixed with another metal.

Today battery technology has evolved into the lithium-ion battery.  Where the positive electrode is a compound containing lithium.  The negative electrode is typically graphite.  The electrolyte is a lithium salt.  Lithium ions travel between the electrodes through the electrolyte.  And electrons flow between the electrodes via the external circuit.  They have a greater capacity, no memory effect and hold their charge for a long time when not being used.  Making the lithium ion battery ideal for cell phones and other consumer electronics.  These chemical reactions produce heat, though.  And can catch fire or explode.  Trying to prevent this from happening increases their manufacturing costs, making them expensive batteries.  So expensive that people will buy cheaper generic brands.  Cheaper because they are not built to the same quality standards of the more expensive ones.  And are more prone to catching fire or exploding.

Something to think about when you feel the heat of your cell phone after a long conversation.  Only use a battery recommended by the manufacturer.  Even if it costs a small fortune.  It may be expensive.  But probably not as expensive as your monthly airtime charges.  So don’t skimp when it comes to lithium ion batteries.  For those cheap ones do have a tendency to catch fire.  Or explode.

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Neutrons, Electrons, Electric Current, Nuclear Power, Nuclear Chain Reaction, Residual Decay Heat and Pressurized Water Reactor

Posted by PITHOCRATES - July 18th, 2012

Technology 101

We create about Half of our Electric Power by Burning Coal to Boil Water into Steam

An atom consists of a nucleus made up of protons and neutrons.  And electrons orbiting around the nucleus.  Protons have a positive charge.  Electrons have a negative charge.  Neutrons have a neutral charge.  In chemistry and electricity the electrons are key.  When different atoms come together they form chemical bonds.  By sharing some of those electrons orbiting their nuclei.  In metals free electrons roam around the metal lattice of the crystalline solid they’re in.  If we apply a voltage across this metal these free electrons begin to flow.  Creating an electric current.  The greater the voltage the greater the current.  And the greater the work it can do.  It can power a television set.  Keep your food from spoiling in a refrigerator.  Even make your summers comfortable by running your air conditioner. 

We use electric power to do work for us.  Power is the product of voltage and current.  The higher each is the more work this power can do for us.  In a direct current (DC) system the free electrons have to make a complete path from the power source (an electric generator) through the wiring to the work load and back again to the power source.  But generating the power at the voltage of the workload required high currents.  Thick wires.  And a lot of power plants because you could only make wires so thick before they were too heavy to work with.  Alternating current (AC) solved this problem.  By using transformers at each end of the distribution path to step up and then step down the voltage.  Allowing us to transmit lower currents at higher voltages which required thinner wires.  And AC didn’t need to return to the power plant.  It was more like a steam locomotive that converted the back and forth motion of the steam engine into rotational power.  AC power plants generated a back and forth current in the wires.  And electrical loads are able to take this back and forth motion and convert it into useful electrical power.

Even though AC power allows us to transmit lower currents we still need to move a lot of these free electrons.  And we do this with massive electric generators.  Where another power source spins these generators.  This generator spins an electric field through another set of windings to induce an electrical current.  Sort of how transformers work.  This electrical current goes out to the switchyard.  And on to our homes.  Simple, really.  The difficult part is creating that rotational motion to spin the generator.  We create about half of our electric power by burning coal to boil water into steam.  This steam expands against the vanes of a steam turbine causing it to spin.  But that’s not the only heat engine we use to make steam.

To Shut Down a Nuclear Reactor takes the Full Insertion of the Control Rods and Continuously Pumping Cooling Water through the Core

We use another part of the atom to generate heat.  Which boils water into steam.  That we use to spin a steam turbine.  The neutron.  Nuclear power plants use uranium for fuel.  It is the heaviest naturally occurring element.  The density of its nucleus determines an element’s weight.  The more protons and neutrons in it the heavier it is.  Without getting into too much physics we basically get heat when we bombard these heavy nuclei with neutrons.  When a nucleus splits apart it throws off a few spare neutrons which can split other nuclei.  And so on.  Creating a nuclear chain reaction.  It’s the actual splitting of these nuclei that generates heat.  And from there it’s just boiling water into steam to spin a steam turbine coupled to a generator.

Continuous atom splitting creates a lot of heat.  So much heat that it can melt down the core.  Which would be a bad thing.  So we move an array of neutron absorbers into and out of the core to control this chain reaction.  So in the core of a nuclear reactor we have uranium fuel pellets loaded into vertical fuel rods.  There are spaces in between these fuel rods for control rods (made out of carbon or boron) to move in and out of the core.  When we fully insert the control rods they will shut down the nuclear chain reaction by absorbing those free neutrons.  However there is a lot of residual heat (i.e., decay heat) that can cause the core to melt if we don’t remove it with continuous cooling water pumped through the core. 

So to shut down a nuclear reactor it takes both the full insertion of the control rods.  And continuously pumping cooling water through the core for days after shutting down the reactor.  Even spent fuel rods have to spend a decade or two in a spent fuel pool.  To dissipate this residual decay heat.  (This residual decay heat caused the trouble at Fukushima in Japan after their earthquake/tsunami.  The reactor survived the earthquake.  But the tsunami submerged the electrical gear that powered the cooling pumps.  Preventing them from cooling the core to remove this residual decay heat.  Leading to the partial core meltdowns.)

Nuclear Power is one of the most Reliable and Cleanest Sources of Power that leaves no Carbon Footprint

There is more than one nuclear reactor design.  But more than half in the U.S. are the Pressurized Water Reactor (PWR) type.  It’s also the kind they had at Three Mile Island.  Which saw America’s worst nuclear accident.  The PWR is the classic nuclear power plant that all people fear.  The tall hyperboloid cooling towers.  And the short cylindrical containment buildings with a dome on top housing the reactor.  The reactor itself is inside a humongous steel pressure vessel.  For pressure is key in a PWR.  The cooling water of the reactor is under very high pressure.  Keeping the water from boiling even though it reaches temperatures as high as 600 degrees Fahrenheit (water boils into steam at 212 degrees Fahrenheit under normal atmospheric pressure).  This is the primary loop.

The superheated water in the primary loop then flows through a heat exchanger.  Where it heats water in another loop of pumped water.  The secondary loop.  The hot water in the primary loop boils the water in the secondary loop into steam.  As it boils the water in the secondary loop it loses some of its own heat.  So it can return to the reactor core to remove more of its heat.  To prevent it from overheating.  The steam in the secondary loop drives the steam turbine.  The steam then flows from the turbine to a condenser and changes back into water.  The cooling water for the condenser is what goes to the cooling tower.  Making those scary looking cooling towers the least dangerous part of the power plant.

The PWR is one of the safest nuclear reactors.  The primary cooling loop is the only loop exposed to radiation.  The problem at Three Mile Island resulted from a stuck pressure relief valve.  That opened to vent high pressure during an event that caused the control rods to drop in and shut down the nuclear chain reaction.  So while they stopped the chain reaction the residual decay heat continued to cook the core.  But there was no feedback from the valve to the control room showing that it was still open after everyone thought it was closed.  So as cooling water entered the core it just boiled away.  Uncovering the core.  And causing part of it to melt.  Other problems with valves and gages did not identify this problem.  As some of the fuel melted it reacted with the steam producing hydrogen gas.  Fearing an explosion they vented some of this radioactive gas into the atmosphere.  But not much.  But it was enough to effectively shut down the U.S. nuclear power industry. 

A pity, really.  For if we had pursued nuclear power these past decades we may have found ways to make it safer.  Neither wind power nor solar power is a practical substitution for fossil-fuel generated electricity.  Yet we pour billions into these industries in hopes that we can advance them to a point when they can be more than a novelty.  But we have turned away from one of the most reliable and cleanest sources of power (when things work properly).  Using neutrons to move electrons.  Taking complete control of the atom to our make our lives better.  And to keep our environment clean.  And cool.  For there is no carbon footprint with nuclear power.

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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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