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Hidden energy in thermodynamic cycles: Do you know how much waste heat is left after the fuel is burned?
In today's context of increasing energy demand, fuel efficiency has become an important research area. Energy conversion efficiency, that is, the ratio of the useful output energy of a machine to the input energy, has become a core issue in energy utilization. Depending on the nature of the different outputs, this ratio can cover forms such as chemical, electrical energy, mechanical work, light (radiation), or heat. However, during the process of fuel combustion, a large amount of heat energy is inevitably wasted, which makes us have to think about how much hidden energy is not utilized in these processes?
The understanding of energy conversion efficiency relies on the usefulness of the output. The heat energy produced by burning fuel is likely to become rejected waste heat if it is not used for the desired work.
Energy conversion and its efficiency
Energy conversion efficiency (η) is closely related to the usefulness of different energy sources. Generally speaking, this ratio ranges between 0 and 1, with the closer to 1 indicating a more efficient conversion. An example of this is a light bulb that converts electrical energy into light energy, but it does not convert all of the electrical energy into light efficiently and some of the energy is lost as heat.
It is worth noting that there is a difference between energy efficiency and effectiveness. Efficiency only describes the physical conversion ratio, while effectiveness focuses more on the realization of tasks or the achievement of goals.
Chemical conversion efficiency
During a chemical change, the Gibbs free energy change can be used to evaluate the minimum energy required or the maximum energy that can be obtained. For example, an ideal fuel cell can produce the equivalent of 0.06587 kWh of electrical energy under operating conditions of 25°C, and the process requires the removal of the equivalent of 0.01353 kWh of heat energy to maintain the reaction.
When understanding thermodynamic cycles, it should be noted that under the set experimental conditions and input energy requirements, the actual energy efficiency often cannot fully reach the theoretical maximum value.
Heating value and efficiency of fuel
In places such as Europe, the available energy of a fuel is usually calculated using the lower heating value (LHV). This value assumes that the steam produced after the fuel is burned does not condense, so its latent heat is not taken into account. However, in the United States and other regions, the high heating value (HHV) is used, which includes latent heat, preventing the maximum efficiency from exceeding 100%. The complexity of these calculations, and the differences in their results, illustrate the real challenge of fuel energy efficiency.
Efficiency measurement in lighting systems
In optical systems, energy conversion efficiency, often referred to as "wall efficiency," is the ratio of output radiant energy (watts) to total input electrical energy. In addition, the luminous efficiency further considers the sensitivity of the human eye to different wavelengths, and the two seem to be very different, because the wall-plug efficiency only targets direct energy conversion, while the luminous efficiency reflects the visual perception of the human eye. performance.
Due to the strong perception of blue and green light waves, the luminous efficiency of many lighting systems is often greater than their wall plug efficiency, causing us to rethink the actual effectiveness of lighting equipment.
Multiple factors affecting energy conversion
It is known that many factors affect the efficiency of energy conversion, including but not limited to equipment design and material selection. During every conversion process, energy is often lost as heat or other forms of loss, whether in the glow of a light, the cooling of a refrigerator, or the conversion of power in a vehicle.
Conclusion
The combustion of fuel and the subsequent waste of energy is an extremely important topic in energy science. In future technological development, we need to utilize these hidden energies more effectively and find ways to improve energy conversion efficiency. As fuel prices fluctuate and environmental issues emerge, how will we face future energy challenges?
