Energy Quality

Quality of Energy

Energy takes different forms and all forms of energy are not of the same quality. For example, mechanical energy can be converted with one hundred percent efficiency to thermal energy (heat). But thermal energy cannot be converted one hundred percent to mechanical energy. Therefore, the quality of mechanical energy can be considered superior to that of thermal energy (heat).
We use thermal systems (engines, turbines, and rockets) to convert thermal energy to mechanical energy. In these processes, thermal energy at a higher temperature is used and when some of that energy is converted to mechanical energy, there is a requirement that the rest of the thermal energy is rejected to another body, which is necessarily at a low temperature.
That all thermal energy cannot be converted to mechanical energy and some of that thermal energy must be rejected to another body at a low temperature are observed in nature. In Thermodynamics, such requirements are defined as the Laws of Thermodynamics.
If we have thermal energy in a body at a low temperature and if we wish to transfer it to another body at a higher temperature, it is not possible unless aided by another device with mechanical energy. This case is what we observe with items in a refrigerator. The food is kept at a low temperature and the thermal energy from it is taken out and transferred to outside air (at a higher temperature) with the use of an electric motor that requires the expense of mechanical energy. It may be noted that mechanical and electrical energy are of the same quality.
Chemical energy can be converted to electrical energy in a battery with one hundred percent efficiency in a theoretical sense. Thus, Chemical, electrical, and mechanical energy are of the same high quality.
Let us look at two hypothetical constructs.
Let us assume that we have a certain amount of energy in a body at an infinitely high temperature. We will require no mechanical energy to transfer it to a higher temperature because it is already at the highest possible temperature (Infinity).
Let us assume that a certain amount of energy is at a temperature Zero degrees absolute (Kelvin). Such energy cannot be converted to mechanical energy as there is no other body at a temperature below it to take rejected heat when we try to convert that thermal energy (heat) to mechanical energy (work).
Absolute Zero degrees temperature is defined thusly.
These two scenarios can be used to state that thermal energy (heat) can have a quality depending on the temperature at which it exists.
a) The quality of thermal energy (heat) can be as high as that of mechanical (or chemical or electrical) energy when thermal energy is at temperature, Infinity.
b) Thermal energy is of quality Zero (lowest) when it is at Zero degrees absolute (Kelvin).
c) When, thermal energy is at any temperature between these two limits (Infinity and Zero), its quality is in between the highest and lowest.

Combustion:
Let us consider the case of chemical energy in a fuel. When a fuel is burned, the chemical energy in the fuel is converted to Thermal energy in the products of combustion. Invariably, we get temperature of products much less than Infinity and greater than Zero degrees absolute. Hence, combustion degrades the quality of energy. The degradation depends on how low the temperature of the products of combustion is. This is the reason why in engines we try to obtain temperatures that are as high as practically possible.

Friction:
Friction will be there whenever there is motion caused by force. Frictional forces oppose motion. Friction causes mechanical energy to be converted to thermal energy (heat).
Since friction converts mechanical energy to be converted to thermal energy, it degrades the quality of energy. Friction does not cause thermal energy to be converted to mechanical energy.
We try to minimize friction so that energy degradation is minimized.

Heat transfer:
We know that thermal energy (heat) moves from a body at a higher temperature to a body at a lower temperature when the two bodies are allowed to come in contact. Say, certain amount of thermal energy is transferred from a body at a higher temperature to another body at a lower temperature. Since that thermal energy (heat), which got transferred, is now at a lower temperature, its quality has degraded. So, we state that heat transfer across a temperature gradient degrades energy.

Summary:
Thus, while designing and operating engineering systems, we try to make sure that the quality of energy is not unduly degraded.