Heat pumps and their application in industrial processes

Introduction

Heat pumps have become a preferred technology in the industrial energy sector due to their high efficiency. As energy costs rise, industries are looking to reduce their energy consumption, and heat pumps offer a viable solution. These machines transfer heat from a lower temperature system to a higher temperature system through a refrigeration cycle, capturing heat from a natural source such as air, water, or soil.

This technology has applications ranging from air conditioning and district heating to industrial processes such as drying, washing, and pasteurization, consolidating itself as a key option for optimizing energy consumption and reducing emissions.

Heat pump operation

Its operation is based on a thermodynamic cycle involving four key components: the evaporator, the compressor, the condenser, and the expansion valve.

The cycle begins with the absorption of heat from the environment in the evaporator, where a special refrigerant captures that thermal energy. Subsequently, the refrigerant is compressed, raising its temperature and then transferring the heat to the system that requires it through the condenser. Finally, the refrigerant expands, lowering its temperature and pressure, and is ready to repeat the cycle.

This cycle does not contradict the second law of thermodynamics, since heat transfer to a warmer source is achieved through the use of mechanical energy, provided by the compressor. Substances with low boiling points are used as refrigerants, including CO₂, a natural refrigerant with a low environmental impact and excellent performance in high-efficiency systems.

Heat pump classification

Heat pumps are classified according to the heat transfer mechanism:

1. Compression heat pumps: They use the refrigerant compression-expansion cycle to ensure an efficient transfer of thermal energy.
2. Absorption heat pumps: Incorporate an absorber and a chiller, improving efficiency in large-scale applications.

Advantages of CO₂ in heat pumps

The use of CO₂ as a refrigerant in high-temperature heat pumps, operating in transcritical cycles, is an attractive alternative in industrial applications. With a Coefficient of Performance (COP) that can exceed 5, these pumps offer much higher efficiency than conventional boilers, whether electric, gas, or coal-fired.

In addition, being a natural refrigerant, the CO₂ does not contribute to global warming or affect the ozone layer, which reinforces its role in the transition to a low-carbon economy.

Transcritical cycle with CO₂ in heat pumps

The term transcritical refers to a thermodynamic cycle that operates both below and above the critical point of the refrigerant used, in this case, carbon dioxide (CO₂).

Critical point: The point at which a substance cannot exist simultaneously in liquid and gas phase. For CO₂, this critical point is at about 31 °C and 73.8 bar pressure.

In a transcritical cycle:

1. Subcritical phase: The refrigerant (CO₂) behaves as a gas and a liquid in the initial stages, below its critical point.
2. Supercritical phase: In the most advanced stages of the cycle, CO₂ reaches temperatures and pressures above its critical point, where it no longer behaves as either a gas or a liquid, but as a supercritical fluid.

This type of cycle is advantageous in high-temperature heat pump applications because CO₂ can transfer heat very efficiently when in its supercritical state. In the transcritical cycle, CO₂ is compressed to high pressures, generating temperatures high enough for demanding industrial applications, such as steam generation or process heating.

In this cycle, CO₂ acts as a supercritical fluid when it reaches pressures and temperatures above its critical point. This allows it to absorb and release large amounts of heat with high energy efficiency.

Unlike traditional cycles, which operate in well-defined liquid and gas phases, the transcritical cycle allows CO₂ to work in a state where it behaves neither strictly as a gas nor as a liquid, but as a combination of both.

Transcritical cycle with CO₂: Advantages for industrial applications

1. High outlet temperatures: It is ideal for processes requiring temperatures above 100 °C, such as industrial steam generation.
2. High efficiency: The Coefficient of Performance (COP) can exceed 5, which translates into significant energy savings compared to conventional boilers.
3. Low environmental impact: By using CO₂ as a refrigerant, a natural substance with a Global Warming Potential (GWP) of 1, the problems associated with synthetic refrigerants that damage the ozone layer or contribute to global warming are eliminated.

Thanks to these characteristics, the transcritical cycle has established itself as a key technological option in the decarbonization of industrial processes and in the transition to a more sustainable use of energy.

Industrial applications

Heat pumps are increasingly used in industrial processes that require thermal energy, such as generating steam, drying materials, or heating large volumes of water. They are especially useful in industries such as food, paper, and textiles, where energy efficiency is key.

For example, a large heat pump, with capacities up to 30 MW, can harness waste heat sources – such as groundwater or heat from rivers and seas – to supply energy for district or industrial heating processes, improving the sustainability of the energy system.

Success stories

In Europe, projects such as SPIRIT, supported by the European Union, have shown remarkable results in the decarbonization of the food and paper industry through the use of high-temperature heat pumps. In addition, the company Geelen Counterflow has reported a 99% reduction in CO₂ emissions thanks to the use of heat pumps for industrial drying processes.

Conclusion

Heat pumps represent one of the most promising solutions for emission reduction and energy savings in industry. Replacing traditional systems with high-efficiency heat pumps not only optimizes energy consumption, but also makes a significant contribution to global decarbonization goals.

With its growing adoption, heat pump technology is poised to play a pivotal role in the transition to a more sustainable and cleaner energy future.

References

1. https://www.carrier.com/residential/en/us/products/heat-pumps/what-is-a-heat-pump-how-does-it-work/
2. https://en.wikipedia.org/wiki/Heat_pump
3. https://www.nationalgrid.com/stories/energy-explained/how-do-heat-pumps-work
4. https://greenenergysolution.org/en/heat-pumps-en/working-principle/
5. https://unacademy.com/content/nda/studymaterial/physics/applications-of-heat-pump/
6. https://www.ehpa.org/news-and-resources/news/large-heat-pumps-the-future-proof-technology-for-the-new-industrial-revolution/