Heat pumps are used for cooling applications and heat engines are used for producing work. Let us see if a heat pump is heat engine or not.
Heat pump cannot be referred to as a heat engine as they both are different. One extracts heat from low temperature reservoir and other extracts heat from a higher temperature reservoir. Both are used for different applications.
A heat engine is considered a work-producing device whereas a heat pump uses work to perform the heat transfer process. Let us discuss their working, equations and differences in detail in this article.
Heat pump vs heat engine
Heat engine is completely different from heat pump. Let us discuss about the differences between a heat engine and a heat pump.
| Parameter | Heat pump | Heat engine |
|---|---|---|
| Direction of heat flow | Heat flows from low temperature reservoir to high temperature reservoir. This can be related to a water pump in which the water goes from low head to high head. | Heat flows from higher temperature reservoir to low temperature reservoir. |
| Working | Heat engine works by converting heat energy to mechanical energy. Heat is added in the heat engine from a hot reservoir. The unused heat is transferred to a cold reservoir called as sink. This mechanical work is obtained generally in the form of shaft work of a turbine which generates electricity. | The heat pumps work by combining the external work with the heat taken from cold reservoir and is transferred to a reservoir with a higher temperature. |
| Applications | Heat pumps are used in refrigeration cycles. | Heat engines are used to generate electricity |
| Measurement of performance | The measure of efficiency is generally measured by a term called coefficient of performance.The equation is given by Q2/ (Q1-Q2). | The measure efficiency is done by measuring work produced per unit of heat added. Efficiency is given by η = W/Q1 where,η is the thermal efficiency of heat engine. |
| Sign of work done | The work done is considered negative as the work is utilised in this case. | The work done is positive as the work is produced in this case. |
| Heat transfer equation | Q2+ W = Q1. Where Q2 is the heat absorbed from the cold reservoir, W is the external work and Q1 is the total heat transferred to the hot reservoir. | Q1 – W = Q2. Q1 is the heat absorbed from the hot reservoir, W is the work produced by the heat engine, Q2 is the heat rejected to the sink or cold reservoir. |
| Components used | Compressor is the major component of a heat pump | A piston cylinder arrangement is commonly used in heat engines. |
| Law of thermodynamics applied | The second law of thermodynamics is applied in a heat pump | Both first and second law of thermodynamics are applied in a heat engine. |
Can we use heat pump for producing work?
Heat pumps are analogous to water pumps. Let us discuss whether we can use heat pump to produce work.
Heat pumps can not be used for producing work as heat pumps are used to transfer the heat in a direction opposite to natural flow, this needs external work as the heat cannot go in opposite direction by itself.
Image credits : Gonfer at English Wikipedia, Heat engine, CC BY-SA 3.0
Why does a heat engine need to reject heat?
Heat engine is a device used for producing work. Let us discuss why does a heat engine need to reject heat.
Heat engine needs to reject heat because second law of thermodynamics states that no engine can be work with only one reservoir or it cannot be 100% efficient. It needs a source and a sink. Heat is absorbed from the source and rejected to the sink.
What temperature is a heat pump not effective?
Heat pumps are reverse heat engines. Let us discuss about the temperature range at which the heat pumps are not effective.
Heat pumps will not work if the temperature difference or temperature gradient between both the reservoirs is very large.
Conclusion
In this article we discussed about heat engine and heat pump. We cleared the misconception that they are same. They are completely opposite to each other. One produces work and the other uses up work to function.
Read more about Is Heat Pump Gas Or Electric?
Hi ….I am Abhishek Khambhata, have pursued B. Tech in Mechanical Engineering. Throughout four years of my engineering, I have designed and flown unmanned aerial vehicles. My forte is fluid mechanics and thermal engineering. My fourth-year project was based on the performance enhancement of unmanned aerial vehicles using solar technology. I would like to connect with like-minded people.