Exploring Various Types of Heat Pumps: An Essential Guide

In today's world, where energy efficiency and sustainability are paramount, heat pumps have emerged as indispensable solutions for heating and cooling needs. These innovative devices utilize the principles of thermodynamics to transfer heat from one location to another, offering both environmental benefits and economic savings. However, within the realm of heat pumps, there exists a diverse array of technologies, each tailored to specific applications and environments.

As established in the previous article, heat pumps are devices that transfer heat from a low-temperature source (space or media from which the heat is extracted) to a high-temperature sink (space or media from which the heat is rejected) using very little mechanical work. We discussed the vapor compression heat pumps in detail, but other principles can be applied to achieve the heat-pumping effect. [1]

In this comprehensive guide, we delve into the world of heat pumps, exploring the various types of heat pumps based on their working principles and their unique characteristics.

1. Vapour Compression Cycle:

It is the most common working principle of the heat pumps available in the market. It consists of four major components: an evaporator, compressor, condenser, and expansion device, along with a working fluid/refrigerant moving through the system in a closed loop. Picking up the heat (thermal energy) from the source & reducing its temperature (cooling effect), the liquid refrigerant evaporates in the evaporator. This vaporized gas is compressed in the compressor, raising its pressure using electricity. This high-pressure gas is condensed in the condenser, giving out heat to the sink and increasing its temperature (heating effect). The expansion device placed in between the condenser & the evaporator allows the high-pressure liquid to come to the evaporator pressure (which is less than the condenser pressure) without changing from liquid to gas. This process repeats itself again and again to achieve the desired temperature, either on the cooling side or the heating side.

The working fluids that are used are mostly HFOs, HCs, & HFCs, along with natural refrigerants like ammonia. For a temperature lift of 35–40 °C between the source & sink, a single-stage cycle can be used, but if the lift is more than 40 °C, a multi-stage/cascade system is used.

2. Vapour Absorption Cycle

This working principle is similar to vapour compression cycle, but the compressor is replaced by an absorber & generator, & the input energy to the system is thermal energy?[1]. The working fluid is an aqueous solution of Li-Br (low-temperature lift) or ammonia. The absorbent (Water for ammonia & Li-Br for water) is used to absorb refrigerant vapours coming from the evaporator. Using a pump, this concentrated solution is pumped to the condenser pressure & sent to the generator. In the generator, the concentrated solution is distilled, separating the absorbent & the refrigerant by utilizing the heat from the thermal source given to the generator?[2]. The refrigerant vapours are passed to the condenser to reject heat, while the absorbent is sent back to the absorber.

With COP less than VCC systems, these systems can be utilized only when there is a requirement for simultaneous cooling and heating (low-temperature heating), with heat recovery done using a thermal source to drive the system.

3. Mechanical Vapour Recompression:

When there is a high-temperature source (50–80 °C), this system can be applied. It applies a shell-tube arrangement named Calandria. The fluid to be heated is passed in the tubes, while the heating source is passed in the shell. Heat transfer from the heating source to the fluid takes place, and unsaturated steam is generated in the tubes. This unsaturated steam is transferred to the separator, where, due to density difference, steam and water are separated. Vapour is recompressed using a re-compressor driven by an electric motor and provided at the usage site?[3]. It requires a high potential heat source, and since the lift achieved is low, the COP is high—about 10–30?[1]. It is mostly used coupled with VCC heat pumps to achieve higher lifts and efficiency optimization.

4. Thermal Vapour Recompression:

Thermal Vapour Recompression uses a sonic nozzle jet & high-pressure steam to recompress lower-pressure steam/vapour?[4]. In the live steam nozzle, the pressure of the in-flowing steam is converted into velocity. A jet is created, which draws in low-pressure vapour. In the diffuser, a fast-flowing mixture of live steam & vapours is formed, the speed of which is converted into pressure (temperature increase) by deceleration. These MVRs are open-cycle, and the source required is steam. These systems can be used in heat recovery applications to generate high-potential steam using low-pressure steam?[1].

5. Stirling Cycle:

Based on the Stirling heat engine, along with a working fluid, it has one cold cylinder & one hot cylinder with pistons synced such that when compression occurs in one, expansion occurs in the other. The working fluid doesn’t change the phase, as in VCC or VAC systems. When the pistons are driven using external work, the pressure of the working fluid rises as it is compressed in the hot cylinder. This hot, high-pressure fluid is passed through the regenerator/heat exchanger, where it gives out its energy isothermally to the sink & cools. This cool fluid, now in the cold cylinder, expands & is pushed through the regenerator, again absorbing energy isothermally from the source. The heat transfer from the working fluid to the regenerator occurs at constant volume both while cooling & heating. The working fluid currently applied to this system is helium, owing to its high thermal conductivity?[5].

It can be applied where the temperature lift required is about 100 °C. Though the COP would be in the range of 1.5–2.5, using an environment-friendly working fluid makes it an attractive choice.

As we look to the future, advancements in heat pump technology continue to push the boundaries of efficiency and performance. With ongoing research and development, we can expect even more innovative solutions to emerge, further solidifying the position of heat pumps as indispensable tools in the fight against climate change.

As consumers, businesses, and policymakers increasingly prioritize sustainability, the adoption of heat pump technology is poised to accelerate, driving us toward a more resilient and environmentally conscious future. By understanding the different types of heat pumps and their respective applications, we can make informed decisions that not only benefit us but also contribute to a healthier planet for generations to come.

References

[1] U. D. o. Energy, “Industrial Heat Pumps for Steam and Fuel Savings,” Energy Efficiency and Renewable Energy, 2003.

[2] “Module 10: Absorption Refrigeration—CIBSE Journal,” 2009. [Online]. Available: https://www.cibsejournal.com/cpd/modules/2009-11/.

[3] Howden, “Mechanical Vapor Recompression | Blower and Compressor Technology—YouTube,” 2018. [Online]. Available: https://youtu.be/9lm2ubpOoL4?feature=shared.

[4] “Thermocompressors for Steam Recovery - Transvac" [online]. Available: https://www.transvac.co.uk/thermocompressors/.

[5] “Olvondo Technology | Make your energy green,” 2021. [Online]. Available: https://olvondotech.no/.

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