Batteries and Fuel Cells Portable Electric Energy
The Battery • A cell consists of two electrodes of different metals immersed in a weak acid • Multiple cells can be stacked in series to make a battery • The positive terminal is called the anode and the negative terminal the cathode
Connecting Batteries in Series • Batteries connected end to end will have a voltage equal to the total voltage of the individual batteries • Disposable dry cell batteries have a typical voltage of 1.5 V 1.5 V 3 V + + +
Amp-hours • The total energy contained within a battery can be described using Amp-hours • Example: A battery that can provide 4 A-hrs can generate 4 A for 1 hour, 2 A for 2 hrs., etc. • Example: A 12 volt car battery can provide 60 A-hrs. of energy. How many joules is this? • Solution: If it drew 60 A of current at 12 V, that would be (60 A) x (12 V) = 720 W. And 1 hr = 3600 s, so (720 W) x (3600 s) = 2.59 x 106 J • In other words: Energy (J) = (Amp-hours) x (Volts) x (3600)
How a Battery (Cell) Works • Both electrodes slowly dissolve in the acid • At the anode, electrons are used in chemical reactions as the metal dissolves • At the cathode, electrons are absorbed into the electrode as the metal dissolves • The net result is a buildup of electrons at the cathode
Disposable and Rechargeable Batteries • A rechargeable battery can be connected to an electric current so that dissolved metals reform on the electrodes • Examples: lead acid, nickel cadmium, lithium, etc. • The chemical reactions that power a disposable battery cannot be reversed • Examples: alkaline dry cells, etc.
The Lead Acid Battery • Two electrodes, one of lead, the other of lead dioxide (PbO2) immersed in sulfuric acid • Lead ions (Pb++) dissolve, leaving two electrons behind • Two electrons flow through the circuit and are used to help lead dioxide dissolve
Disposable Batteries • A typical disposable battery contains a carbon (graphite) and a zinc electrode • The electrolyte is a paste of ammonium chloride • Disposable batteries may leak if too much of the zinc can is dissolved
How Disposable Batteries Work • Both electrodes generate electrons when they dissolve • The cathode generates more than the anode • To remove electrons from the anode, positive ions “plate” or stick to the anode • A membrane separates the A+ and B+ ions • Eventually, positive ions accumulate near the cathode and are depleted near the anode
Electric Vehicles • Electric vehicles use electric motors powered by rechargeable batteries • Both Honda and GM (among others) manufacture electric vehicles • Electric vehicles have been around as long as gas powered cars! The GM EV-1
How and Electric Vehicle Works • An electric motor replaces the gas engine • Electric motors are extremely reliable • During braking, electric motors can act as generators and recharge the batteries • A rechargeable battery pack in the trunk provides electric power • Battery packs are heavy and costly • They must be replaced eventually Electric Motor Battery Pack
Why Nobody Buys Electric Cars “The battery challenge is vast. Even with our most advanced experimental power pack, operating costs in 1998 would be unacceptable to the vast majority of drivers. Essentially, it’s like asking the customer to buy a car with a $15,000 gas tank--a $15,000 gas tank that holds the range equivalent to 3 gallons of gasoline; a 3 gallon tank that takes 8 hours to refill, compared to a few minutes at a self-service gas station.” D. Wilkie, 1994
Pros Pollution occurs at the power plant where it can be more easily contained Lower operating expenses (repairs, refueling) than gas engines Cons Limited range Batteries must be replaced frequently Very expensive Same total amount of pollution (when including the power plant that generates electricity) Pros and Cons of Electric Cars
Fuel Cells • In a fuel cell hydrogen is “burned” by mixing with oxygen in such a way that it creates a voltage across two electrodes • Only water is produced as a by-product • Hydrogen can be fed in directly or as part of larger molecules such as methane (natural gas)
How They Work • Hydrogen molecules give up their electrons to the first electrode • Electrons pass through the circuit to the second electrode • Electrons are returned to the molecules when hydrogen and oxygen combine to make water
Comments on Fuel Cells • Fuel cells have been around for 100 years • Fuel cells can be made to burn other molecules, such as methane, propane, etc. • Hydrogen can be extracted from gasoline before being fed into a fuel cell • Fuel cells cannot store energy, so they must be used in conjunction with a storage battery
Fuel Cells in Electric Cars • Prototype cars have been developed that generate electricity using fuel cells • Pros • Very little pollution • Cons • Expensive • Hydrogen gas is explosive
Fuel Cells to Replace Batteries • Fuel cells can be used to power a laptop • Micro fuel cells have been developed that are small enough to fit into a cell phone • Fuel cells weigh less and last much longer than rechargeable batteries
Why are Fuel Cells so Uncommon? • Methods are being developed to store hydrogen in a porous material rather than as compressed gas • Fuel cells require expensive catalysts • How do you pump compressed hydrogen at a self-service gas station? • What happens to the hydrogen tank in an accident?
Flywheels? • A flywheel (i.e. a heavy disk) spinning in a vacuum can store a large amount of energy • Electrical energy can be extracted (and stored) using magnetic fields • What happens to a disk spinning at 100,000 rmp when you hit a bump?