The Second Law Again

Let’s start by summarizing what we know so far (see previous page):
The Second Law of Thermodynamics tells us that energy always moves from a more concentrated condition to a less concentrated, or more spread out, condition. It gets “diluted” (my word – not official).

Energy will flow or move, making things happen, as long as there is a difference in concentration. When there is no longer a difference in concentration, the system is in equilibrium. When things are in equilibrium, energy stops changing and moving, things stop happening.
Peaceful…and…really boring.

Finally, we explained that one of the most important aspects of the 2nd Law is that energy becomes less “useful” as it moves to a less concentrated condition. It is an inescapable fact that becoming less concentrated also makes energy less useful.

The First Law tells us that the total amount of energy never changes no matter how many transformations the energy makes.

But the Second Law tells us that the usefulness of energy decreases as it moves and makes things happen.

Engineers say the energy “deteriorates”. They talk about high quality energy (concentrated), and low quality energy (spread out or less concentrated).

Still another term for deteriorated or low quality energy is “low-grade” energy. Low-grade energy is difficult and expensive to use, or even impossible. Nature can’t use it either. Without a fresh dose of concentrated solar energy everday, life would stop on our blue marble spaceship earth.

When Everything Stops 
As far as we know this is one of those inescapable realities of physics. The ultimate result of the Second Law of Thermodynamics is that energy in the whole universe is steadily deteriorating, or “un-winding”, to lower and lower quality. In other words, the universe is steadily moving toward total equilibrium.

When the universe finally does reach total equilibrium, way off in the very far distant future, everything will be at the same temperature, and nothing will happen anymore. Everything will stop. Well, not everything. The atoms and molecules will still be moving and colliding, vibrating and spinning, but all at the same average energy level. There won’t be anymore net heat transfer. No more energy flows, because there won’t be any difference in energy concentrations.

Scientists, in their happy way, like to cheer us up by calling this final state of total equilibrium the “HEAT DEATH OF THE UNIVERSE”!!!
It’s best when this is said dramatically in a deep booming voice.

Based on what we know so far, the “HEAT DEATH OF THE UNIVERSE” (don’t forget the deep booming voice) is inevitable.
But maybe we don’t know everything yet. After all, everything got started once.
So don’t worry about it.
There is a lot we don’t know.
And it really is a long long way off.

What the Second Law Says About Machines 

All Work Becomes Heat
We’ve talked about how energy moves and changes. As we’ve said elsewhere, there are only two ways that energy can move or be transformed to make things happen. One way is through a “work process”. The other way is through a “heat process”, or what we also call “heat flow“. We also use the word “heat” to refer to a type of energy that might better be called thermal energy.

Work can be converted to heat. In fact all work eventually ends up as heat through friction. The energy, converted through work processes, that pushes cars down the road and locomotives down the track, all ends up as heat. In other words, 100% of work can be, and is eventually, converted to heat.

But Not All Heat Can Become Work
Heat can also be converted to work.
Engineers convert heat to work in a type of engine they call heat engines. Steam engines are a good example of a heat engine. The heat in the boiler is used to turn water into steam, then the steam is used to drive a turbine or push a piston. Heat energy is transformed into work energy.

Other engines, like jet engines, gasoline (petrol), and diesel engines can also be considered to be heat engines. Though they don’t fit the exact text book description of a heat engine, they also convert heat into work. And they follow the same Laws of Thermodynamics.

Ah, but there is another rub here.
The Second Law tells us something about heat engines:
It tells us that not all the heat energy available to a heat engine can be converted into work energy. If all the heat energy could be turned into work energy, then our heat engine would have a thermal efficiency of 100 percent (%).

That’s what we engineers want. We want to make our engines 100% efficient. Car drivers want that too. The more efficient the engine, the farther the car can go on a gallon of gas. Dream on! The Second Law tells us we can’t do that. In reality we don’t even get close. Most car engines have thermal efficiencies well below 40%.

The heat energy flows through the engine from concentrated high temperature to less concentrated low temperature.

Sound familiar? Just as a wind turbine cannot be 100% efficient, because some energy must remain in the air just to move it out of the way of the air behind it, some heat must also be rejected out of the engine to keep it running. The engine cannot operate in a continous cycle if no heat is rejected.

The Best Type of Heat Engine That Can Exist…
Back in another century, a very smart French man named Carnot figured out what the best thermal efficiency possible is for a heat engine, and it’s not that great. The best type of heat engine is called a Carnot engine (guess where the name came from). It is such a perfect engine that we can only imagine it. We can’t really make one. We can make something close, but it is pretty useless because it has to move slow to approach the Carnot cycle. (Carnot is pronounced kar-no, with a long o sound).

Anyway, the efficiency of this perfect heat engine is not 100%. That’s another way mechanical engineers like to say the 2nd Law, “You can’t make a heat engine with an efficiency of 100%. Sorry.”

We always apologize after we say it. We feel bad. Someone is always dissappointed by this statement. Some people don’t believe it, and try to make perpetual motion machines to prove us wrong. But in the end they have all had one thing in common – they have all been the ones that are wrong.

There is a simple formula to predict the efficiency of a Carnot engine. The formula only needs temperature. It is based only on the ratio of the high temperature going into the engine and the low temperature leaving the engine.

This means the best possible efficiency of an ideal best-possible heat engine is based only on the operating temperatures of the engine. It’s not quite that simple in real engines. But it is also generally helpful in real heat engines to have high combustion temperatures in order to achieve the highest efficiencies.

Alas, engine parts tend to burn or melt when temperatures get really hot. So one of the things that limits the efficiency of real engines is the ability of the engine materials to withstand high temperatures.

…Still Can’t Convert All the Heat to Work
But even if we could get much higher temperatures, we still could not convert all the heat into mechanical energy (2nd Law).

Combined Heat and Power (CHP)
Sometimes we can use the “waste” heat for something useful like heating our living spaces. Your car’s heater most likely uses waste heat from the engine’s cooling water to heat the passenger compartment. At a place I used to work, exhaust energy from big diesel generator sets was used to make steam. The steam was then piped around the plant to heat the factories and office spaces. This is called Combined Heat and Power or CHP for short. An engine is used to make electricity by powering a generator, while the “waste heat” from the engine is used to provide heat for other uses. It is a very good way to use energy efficiently.
To paraphrase an old saying, “One person’s waste heat is another person’s useful energy”.

To summarize… 
All mechanical energy can and does eventually become less concentrated heat energy. But we can’t go the other way. We can’t convert all of the heat energy into mechanical energy. A large percentage of the heat just passes through and comes out the other side less concentrated and less useful. Engineers often call it waste heat, because to them it is energy that was wasted. Sometimes we can still use the low-grade heat to heat our homes and offices in a type of application called Combined Heat and Power