Question

Consider natural gas, electric resistance, and heat pump heating systems. For a specified heating load, which one of these systems will do the job with the least irreversibility? Explain.

Short answer

A heat pump heating system generally has the least irreversibility for a specified heating load. Its main advantage is that it moves heat instead of generating it, resulting in less entropy generation. Furthermore, heat pump systems can have a coefficient of performance greater than 1, making them more efficient than electric resistance and natural gas heating systems under certain conditions. However, it is essential to consider specific operating conditions, system design, and location to determine the most suitable system for a particular application.

Step-by-step solution

Understand the concept of irreversibility

Irreversibility in a system is the measure of how much the system deviates from an ideal, reversible process. A reversible process is one where no entropy is generated, meaning it can be reversed without any change in the surroundings. The more irreversibility present, the less efficient a process will be. To minimize irreversibility in a heating system, we want to minimize entropy generation.

Analyze Natural Gas Heating System

A natural gas heating system burns natural gas to produce heat. The efficiency of this system is relatively high, typically around 80-98%. Combustion processes typically have some irreversibility due to heat transfer across a finite temperature difference and chemical reactions. Although natural gas heating systems have high efficiency, they do generate some entropy.

Analyze Electric Resistance Heating System

An electric resistance heating system converts electrical energy directly into heat. Its efficiency is essentially 100%, as all of the electrical energy is turned into heat. However, this is only true for the conversion process, not accounting for the energy losses during the generation and transmission of electricity. Since this type of heating system operates by resisting the flow of electric current, it produces entropy due to the Joule heating effect, which is a form of irreversibility.

Analyze Heat Pump Heating System

A heat pump heating system works by transferring heat from a low-temperature source (typically the outdoor air) to a higher temperature space (inside the building). Heat pumps are not 100% efficient, but their "coefficient of performance" can be higher than 1. The advantage of a heat pump is that it moves heat instead of generating it, which usually results in less irreversibility compared to other systems. They generate entropy mainly through the work input in the compression and expansion processes.

Compare the systems

Comparing the three systems, we can conclude that the heat pump heating system generally has the least irreversibility. The main reason is that it moves heat rather than generating it, typically resulting in less entropy generation. Additionally, heat pump systems can have a coefficient of performance greater than 1, making them more efficient than electric resistance and natural gas heating systems under certain conditions. In summary, for a specified heating load, a heat pump heating system will likely have the least irreversibility. However, it is essential to consider specific operating conditions, system design, and location to determine the most suitable system for a particular application.