Take a Closer Look at Cascaded Efficiency Consider the complete system,not individual components,when determining energy savings

摘要

ONE OF my friends-a lawyer-was the first person in our neighborhood to buy a Tesla.I admired the car,and he gave me a test ride,which impressed me even more.Then he told me how amazingly efficient the vehicle was.That was a dangerous thing to say to an engineer,especially one who specializes in energy efficiency.I tried to be gentle with him.He was only a lawyer,after all;some electric motors can convert 98% of the incoming electrical energy to useful power.Unfortunately,that is only part of the story.The power for the motor(98%)comes from a batrery(90%),which draws power from a charger(90%),which gets its power from the electric grid(85%),which,in turn,gets its electricity from a power plant(35%).The numbers in parentheses are indicative energy efficiencies of each subsystem within what is actually a very large,cascaded system;the overall system efficiency is 0.98x0.90x0.90x0.85x0.35 = 0.24,or 24%.Suddenly,rhe electric car doesn’t look so efficient.Note: Actual efficiencies can differ significantly from the indicative values shown above,especially for rhe power plant,which can use many different technologies and energy sources.