Summary of First and Second Laws

From the second law, it follows that with each process, interaction, reaction, or exchange of a stream of energy, some of its potential to do work is lost due to process irreversibilities, characterized by an increase in molecular disorder (increased entropy). In practice, in addition to these losses, processes may experience energetic losses, which are essentially heat losses (i.e., radiation, conduction, and convection).

When a fuel is combusted, it automatically loses energy. From the moment heat is collected on a solar plate, or rushing water is used to generate power, the stream of energy continues to decline in its total useful value to do work. The first law shows that on a global level, no energy is lost. It is only lost from within a given system to another. The second law states that even on a global level, availability to do work is lost, since entropy increases throughout the universe with no compensation. Whereas work and all other forms of energy can be wholly converted to heat, the converse is not true.

When these concepts are applied to a heat engine, there is a range of possibilities. At one end of the spectrum, a large portion of the input energy can be converted to prime power (useful shaft work) and most of the rest can be converted to useful thermal energy. At the other end of the spectrum, a small fraction of the input may be converted to prime power and excess energy may be a costly nuisance requiring rejection through the use of additional energy in such forms as pumps, fans, and cooling equipment. From an economic perspective, the same quantity of energy resulting from the same process can have a positive value on a cold day as a heat source and a negative value on a hot day as an excess heat load.

Power producing equipment (i.e., prime movers) operate on several different cycles. Generally, reciprocating engines operate on either the Otto cycle or the Diesel cycle, combustion turbines operate on the Brayton cycle, and steam turbines operate on the Rankine cycle.

In each of these power cycles, the initial and final states of the system are identical. At the end of the cycle, all of the properties have the same value they had at the beginning of the cycle, except that heat has been added, heat has been rejected, and work has been done. An analysis of the power cycle involves an accounting of all of the energy exchanges occurring at each of the processes, so that the sum of all energy inputs equals the sum of all outputs.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

Get My Free Ebook


Post a comment