Efficiency….engineer speak

“Ah! You’re a mechanical engineer,” She said. This was social first conversation, and the subject of energy had somehow come up. She, being a civil engineer, quickly recognized the dialect of a cousin tribe.

She was correct, sort of. I earned the degree…a long time ago. Like everybody else in the program I studied thermodynamics. In fact “thermo” is one of the required subjects that’s a struggle for some. Thermodynamics is about heat energy and converting it into some other useful form, like motion. How well the conversion is done, that is, how much you move compared to how much you burn, is called thermal efficiency. Engineers are always looking for and get paid to find and reduce the part that’s wasted, in other words, improve efficiency.

I never worked as an engineer. Instead, I went into the U.S. Navy and learned to fly. After that as a professional pilot, I turned heat energy (produced by large quantities of jet fuel) into motion, for me and a couple hundred people riding along behind me.

Pilots generally don’t think much about the efficiency of what’s going on inside their engines, at least modern jet engines. These are efficient and pretty much automatic. Pilots though are very concerned about how energy efficiently they do everything else, that is, use and conserve the kinetic energy derived from fuel. Part of the job is to get from point A to point B using the least amount of fuel. Early in a career efficiency of operation becomes part of a mindset essential for success, sometimes for survival. After all, if fuel is wasted and then later what remains isn’t enough, consequences are serious.

Engine designers certainly care about efficiency inside the combustion chamber though. And they’ve had success in reducing the amount of fuel wasted very significantly. The engines on the B-787 that is just now entering service will consume 40% less fuel for a given amount of thrust than did their predecessors a few decades ago.

Airplane engine design is key to the much improved efficiency of electricity generation too. Big gas turbines that are the legacy of aircraft engine development are at the core of modern combined cycle electricity generating plants. The very hot exhaust from these machines, the ones that spin a first generator, is used to make steam that powers a second generator. It’s like two bites out of the apple, and it allows capture of as much as 60% of the available energy.

60% efficiency? How good is that? Very good, at least compared to the other combustion machines we use. For example, the coal fired power plants that are being rapidly phased out achieve less than 40% generally. The other one we use every day, the car engine, is awful by comparison. The gasoline engine could theoretically come close to 40% (the theoretical maximum), but in reality seldom reaches 20% in actual use. Most of what burning gasoline in a car engine produces is waste heat. Only about 15% of it actually pushes you down the road.

What about the efficiency of electric motors? Very high, 95% in some cases. That means that almost all the electrical energy gets converted to motion. All things being equal, wouldn’t it be much better to waste just a tiny part rather than the biggest part? Of course. Aha! Electric cars.

You bet, because they waste less. The total efficiency of fuel use is far superior to that of cars with gasoline engines. Taking into account the 60% power plant efficiency and then some loss as the electricity flows through the distribution grid and then a little loss as it goes into and then back out of the electric car’s battery and finally to the motor, the end result is probably three times as good. Since efficiency is a number that’s better if bigger, what’s not to like about 45% total efficiency compared to the 15% of a gasoline burner?

There’s three more pieces to the story that really make the case for electrics. First, over 50% of electricity generation in Florida is now powered by natural gas. Natural gas is cheap. On a BTU (unit of energy) basis it’s currently about one sixth the price of petroleum. Natural gas is cleaner too. There are essentially no particulate emissions or the release of other bad stuff like mercury or lead. And the carbon dioxide output is significantly less. Finally, at some point in the future, the electricity can come from the sun. Wouldn’t you like that, zero fuel expense and zero pollution?


About wattnextblog

I'm Bill Ferree, a chief officer of WattNext, Inc.
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2 Responses to Efficiency….engineer speak

  1. Angelo Perciballi says:

    Bill–Joanie and I were in Denver area a couple of weeks ago, visiting new grandbaby. We went to Park Meadows Mall (near Littleton) with son, and there was a TESLA showroom in the Mall!. Went in to look at the sedan coming available this Sept—very nice looking! They also had a naked chassis there, so you could see the guts. I picked up a brochure, if you want it, I will gladly drop it by.

    • wattnextblog says:

      I think the Tesla S is a potential game changer. It will compete well against the 5 Series BMW, Mercedes 300 and similar from Toyota, Honda and Nissan. I want one.

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