Elkhorn Creek Lodge

It’s the Thermodynamics, Stupid

Posted in uncategorized by Eugene Podrazik on September 21, 2008

Here’s my contribution to the debate over ways to be energy independent. And, commentary on new “carbon free” alternatives (you’ll see why I put carbon free in quotes).

First, a BTU is a BTU. It doesn’t matter what it source or how it’s used. It credits and debits much like your check book. Energy can be neither created or destroyed. I can only change form. In this case, the use of energy for the purpose of transportation, we’re talking about heat and mechanical work.

Second, transportation is a matter of converting heat to work. This conversion is subject to a number of constraints and laws of nature. And, waving a wand (or a congressional vote) isn’t going to change a thing. You simply cannot legislate water into flowing uphill. 

So, lets work through the issues:

First, as I mentioned, you need to convert heat to work. Some how you need to burn something to generate energy that can be converted to work. One of the biggest concepts that needs to be understood about this conversion is it’s not and cannot be 100 percent efficient. That is for every BTU derived from burning, you will only get a fraction of that BTU in actual work–turning the wheels on your car. This efficiency is limited by the the temperature differential between the heat source and the cold “sink.” The greater the differential, the greater the efficency in converting heat to work. The physical limitations are generally the maximium operating temperature of the materials comprising the engine and, for the heat sink, atmospheric temperature. Above certain temperatures, metals will melt or degrade, for example. For a Rankine cycle engine, the efficiency is also limited by the properties of the working fluid; generally water. That is you can only heat to certain maximum values at certain pressures. Then you have to factor such variables as wind resistance, friction and so on. To paraphrase Thatcher, real world facts are conservative.

For an internal combustion engine, you can get a 20 percent, maybe a 25 percent efficiency (theoretical maximum of about 37 percent). A Rankine cycle engine (a steam turbine engine), most commonly found in power generation plants maybe works at an efficiency of 40 percent (theoretical maximum of about 60 percent). If you got 100 percent, you’d have a perpetual motion machine (which is why the patent office requires a working model for any perpetual motion machine).

Now we come to the concept of work and its relationship to mass. Work is a function of force and distance. And, force is a function of mass and velocity (squared). The more mass, all other things being equal, the more work is required and therefore the more energy is required. That’s why the guy who installs your gourmet kitchen granite counter tops comes to your house in a full size pickup truck and not a Prius.

So, we need, in order to be portable, a portable power plant and a portable source of energy. It’s no accident that this is the reviled internal combustion engine. Weight for weight and volume for volume, a tank of gas is the most efficient was to carry around the BTU’s you need to move yourself and your stuff around. And, the internal combustion engine is the most efficient way to convert that tankful of BTU’s into work to move your stuff around.

There’s nothing magical about the Prius. Fundamentally, it is small light-weight car powered by a gasoline internal combustion engine. That is, the ultimate source of the energy for conversion to work is gasoline burned in an internal combustion engine. Other efficiencies are derived by using the kinetic energy of deceleration to recharge the batteries. But, you’re burning gas. And, your burning gas in a updated econobox (remember the Gremlin?) with the addition of batteries. 

One wonders, if fact, if you couldn’t get an equally efficient car by getting rid of the weight of the batteries and just putting in small engine and solely relying on gasoline alone (we’ll pass on the fact that now you just have an updated Gremlin without batteries–not very vogue). Thermodynamically, about the only real benefit you’re getting is turning the energy of deceleration into electrical power. Braking your car into a stop is converting work into heat; you just put some of that energy back into the batteries. You’re still at the mercy of an internal combustion engine and its thermodynamic efficiency of 25 percent. And, remember in order to get the kinetic energy of deceleration to convert to electrical charge, you have to accelerate the car in the first place–courtesy the gas engine.

Or, okay, you can get a car that charges up by plugging it in at night. But, that only means that your burning coal at a power plant to power your car. And, we all know that “clean coal” is just a Rovian catch word to get the “bitter” coal miner rubes in West Virginia to vote for Bush.

Of course, none of this considers real world issues. A small econobox probably works in urban settings for short distances. But, batteries being less efficient sources of power that a tank of gasoline–heavy and bulky–tend to fall down when you need to go farther. You need to dispose of those batteries at the end of their useful life; and batteries are full of all sorts of heavy metals that we don’t want leaching into the environment.

Weight equals safety. Yes, we could airbag our cars till the cows come home. But, pound for pound, you add another 100 pounds in structural weight, you’re safer.

We’re not a densely populated country. Our population density is about 80 persons per square mile as opposed to polulation densities of hundreds of persons per square mile in Europe. That means, on average, a typical trip for an American versus his European counterpart will be longer. It’s simple harder to make mass transit work here for that reason. Mass transit will work along densely populated corridors where there’s a high probability that a lot of people want to go to roughly the same places at roughly the same time.

But, a doctor making an emergency visit to the hospital at two in the morning is not taking the bus. Nor, did the ambulance crew pick you up on your trip to the emergency room. In fact, the ambulance is probably a converted Ford F350. Take a look in the back of a modern ambulance; there’s a lot of stuff that won’t fit into a Prius either.

Finally, there’s freedom. Freedom from being at the beck and call of transportation run at the pleasure of the government (intercity buses and light rail; Amtrak). Philosophically, if you look at every fortune amassed in America, all carry the common denominator of bringing to the common man something heretofore reserved for the rich. Henry Ford’s Model T did that in the arena of personal transportation; a coach, horses and livery was simply out of reach for all but the rich. A fact, by the way, not lost on our bi-coastal elite. 

And, contrary to every conspiracy theory, what we have on the road today is reflection of the transportation needs unique to this country and constrained by the laws of thermodynamics.



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