On many occasions posts are placed in vortex that discuss the efficiency of heat engines and cycles as well as whether or not all the heat can be extracted from a system and so on. A thought occurred to me earlier today which made much of the confusion go away and I wanted to share that concept with the others in the group. It is my hope that this unusual way of looking at these types of problems will simplify these outwardly complex looking systems.
The first thing that needs to be considered is that the conservation of energy is preserved in these machines and systems. When we speak of efficiency, it is should not be considered a loss of energy at all, but the lack of ability to extract all of the energy that is available from the source. The energy that is not turned into work by the machine is simply returned to the environment and could be released under the right circumstances. My thought experiment followed an interesting path. First, think of having an isolated system such as a resistor in empty space that is at essentially zero Kelvin and kinetic energy that matches. This resistor has leads attached to it and we connect a voltage source. According to standard electrical rules, energy will be given to the resistor at a rate proportional to the power applied. For example, if 1 watt is being delivered to the resistor, then it is absorbing energy at a rate of 1 joule per second. I am quite confident that everyone reading vortex posts understands that the increase in resistor internal energy is mainly going to be in the form of thermal energy. This is just a way to characterize kinetic energy of the molecules of the material. And kinetic energy just means that the atoms are in motion relative to each other, which can also be measured by the temperature of the device. So, after a period of time with power applied to the resistor, it will heat up and contain a well defined number of joules of energy. I realize that someone could chase down every last joule of energy in what ever form it takes, but this is a discussion to help simplify the concept for people wishing a better understanding of the principles so lets not bring in the secondary processes at this time. Someone asked the question as to whether or not the heat within a system could be mostly extracted and I think we can shed light upon that issue. First of all, if the external voltage source is removed and the system monitored for a very long time, it will be seen to radiate heat energy according to the Stefan-Boltzmann law until it ultimately has none left to radiate. This energy is in the form of IR radiation initially and eventually changes over to lower frequency peak emissions until there is no more energy available. Second, the resistor atoms slowly loose some of their energy of motion (kinetic), so they move slower. There is no theoretical law that prevents us from capturing most the radiated energy by one method or the other with a super conductor antenna - energy conversion device. Of course this would be impractical, but the principle is there. The end result of this complicated activity would be that the heat energy has been recovered in some other form and none is lost so the conservation of energy prevails. Now, it seems strange that we always speak in terms of two heat sinks when we talk of the efficiency of a heat engine. If you think carefully about the processes at work, you will see that this is just a short hand way of saying that you begin with kinetic energy of the source driving your heat engine, which is measured by the temperature of the source, and end up by not extracting all of the kinetic energy. The low temperature sink is the place where your engine allows the kinetic energy to escape that was not converted into mechanical work. This is a simple way to think of the engine. It's design is imperfect since the input kinetic energy of the source does not all get converted. Thermal radiation can behave as a perfect heat engine, except that its output is in the form of electromagnetic radiation instead of mechanical work. Always remember that energy is energy and that heat energy is just one of many types available. Generally, there is a process that will convert one form into another, and some are easier to work with than others. Raw heat is not the ideal energy form to work with, especially when compared to an easily converted type such as electrical energy. The heat can be converted into electrical energy, but the process does not typically function without allowing some of the input heat energy to escape simple conversion. And, if you convert electrical energy into mechanical work, such as raising a heavy load into the air with an electrical motor, some of the input electrical energy will be converted into that less useful form of heat energy. It is not lost, but harder to put into use after that process. Thinking of the operation of a device such as a heat pump is a bit tricky. In this case, kinetic energy is extracted from one region which is the source and transferred over to another location by using some neat processes. Notice that the source has kinetic energy that is reduced by the extraction process. This is like diverting some of that energy that we placed into the resistor earlier in this discussion and allowing it to be deposited into another location. The new place where the heat pump placed that energy now has more kinetic energy than before. The overall total energy is almost the same as before, with the exception that you need a device to perform the magic and it is not perfectly designed. An electrical motor and other paraphernalia is required and that motor and other components exhaust waste heat into the mix. I hope that my brief explanation of heat related topics is helpful and sheds light upon some of the mysterious thermodynamic relationships that have long plagued us. Anyone who wants to add additional inputs is welcome to do so. And, if you think that my description is in error, please post a correction since the intention is to educate others. Dave
