Ionocaloric refrigeration

Last updated June 15, 2024

The ionocaloric refrigeration cycle is an advanced cooling technology that utilizes the ionocaloric effect, driven by an electrochemical field, to achieve efficient and eco-friendly refrigeration. By manipulating the electrochemical potential through ion addition or removal, significant temperature changes and entropy variations are achieved. This cycle offers a sustainable alternative to traditional refrigeration systems, with potential applications in various industries. Ongoing research is focused on optimizing ionocaloric materials and system design to enhance its performance and viability.

Overview

The ionocaloric refrigeration cycle is a cutting-edge cooling technology that offers high efficiency and zero global warming potential. This novel cycle utilizes the ionocaloric effect, which is driven by an electrochemical field, to achieve significant adiabatic temperature changes and isothermal entropy changes. Developed as a solution to the pressing need for sustainable and environmentally-friendly refrigeration systems, the ionocaloric refrigeration cycle shows promising results in terms of performance and energy efficiency.

Traditional refrigeration technologies, such as vapor-compression (VC) systems, have relied on hydrofluorocarbons (HFCs) as refrigerants. However, HFCs have a high global warming potential and contribute significantly to greenhouse gas emissions. To address these environmental concerns, researchers have explored solid-state caloric materials that exhibit refrigeration effects under external fields. While previous caloric materials have shown limited performance and low coefficient of performance (COP), the ionocaloric cycle demonstrates remarkable improvements.

The ionocaloric effect operates by manipulating the electrochemical field surrounding a solid phase through the addition or removal of ions. This electrochemical mixing of species induces significant energetic changes, resulting in a thermal response and temperature variation within the system. Unlike other caloric effects, where the applied field interacts with the material's conjugate field pair, the ionocaloric effect operates in reverse. The control of the electrochemical potential is achieved by altering the concentration of chemical species through various field variables such as temperature, pressure, and voltage.

The ionocaloric refrigeration cycle incorporates the ionocaloric effect into a thermodynamic cycle to provide continuous and efficient refrigeration. The cycle involves four steps: isentropic mixing, isocompositional and isothermal melting via heat absorption, isentropic separation, and isocompositional and isothermal crystallization via heat rejection. By following these steps, the cycle achieves Carnot-like behavior and enables efficient cooling.

Among various ionocaloric systems, the ethylene carbonate-sodium iodide system has shown particular promise. It exhibits high latent heat of fusion, a melting point above room temperature, and environmental compatibility, making it an attractive option for practical applications. The ionocaloric effect in this system surpasses other caloric effects reported to date, demonstrating significantly higher adiabatic temperature changes and entropy changes per unit mass and volume.

The practical implementation of the ionocaloric refrigeration cycle involves the use of desalination techniques, such as electrodialysis, to separate the solution and regenerate the system. While the theoretical properties of the EC/NaI system show competitive performance, real-world efficiency will depend on the details of the separation process. Electrochemical techniques like electrodialysis offer high efficiencies without requiring high operating pressures or fields, making them suitable for practical application.

Ideal ionocaloric materials require substantial enthalpy of fusion, elevated cryoscopic constant, and large dielectric constant, indicative of high salt solubility. The ethylene carbonate-sodium iodide system is a promising candidate for experimentation. While salts like 𝑍𝑛𝐶𝑙₂ and 𝐻𝑔𝐶𝑙₂ have high solubilities, their passage through cation exchange membranes is impeded, and they risk producing toxic chlorine gas. Conversely, 𝑁𝑎𝐼 and 𝐾𝐼, with high solubilities and ionic conductivity, are suitable for ionocaloric devices, NaI being preferred for superior salt solubility.

In conclusion, the ionocaloric refrigeration cycle represents a promising approach for efficient and eco-friendly cooling systems. Notably, the use of specific ionocaloric materials such as the ethylene carbonate-sodium iodide system and NaI, known for their high solubilities and ionic conductivity, further enhances this innovative technology's effectiveness. By harnessing the ionocaloric effect and integrating it into a carefully designed thermodynamic cycle, we are steering towards zero global warming potential and a more sustainable future. Continued research and development in the selection and application of these ionocaloric materials are crucial to fully realize this new cooling technology's potential.

Principle of operation

The ionocaloric heat pump is a solid-liquid based heat pumping technology with high efficiencies over very-high temperature spans. It utilizes the ionocaloric effect by changing the concentration of a salt in a mixture to modulate a material's melting point, and therefore heat content. The ionocaloric effect is defined as a thermal response to an applied electrochemical field (i.e. ionic field). Ionocaloric heating/cooling utilizes the ionocaloric effect within an appropriate thermodynamic cycle (e.g. Reverse Carnot or Stirling cycle).

Ionocaloric cooling works by surrounding a solid phase with ions (i.e. applying an electrochemical field to the solid), which makes the solid more stable as a liquid . To transition to a liquid, it must melt, which means it must absorb energy. If melted adiabatically (and therefore isenthalpically), it will increase its energy so that it can become a liquid by stealing energy from itself, which cools the whole material down. Once the material is cooled down, the solid can continue melting, but at a now lower temperature, thus absorbing energy from its surroundings (i.e. refrigeration).

Ionocaloric heating works in reverse. By removing the ions from a liquid, the liquid -- if chosen carefully -- becomes more stable as a solid. To transition to its solid phase, it must crystallize and release energy. If crystallized adiabatically, it will heat itself up (i.e., “recalescence”). Once the material is heated up, it will continue releasing energy by crystallizing, but now releasing heat to the environment (i.e. heat pumping).

By stepping through these four processes, a Carnot-like heat pump cycle emerges with zero global warming potential. The first demonstrated prototype was published in Science^[2] in 2022, and is CO₂-negative, environmentally benign, non-hazardous, zero-GWP, non-toxic, and non-flammable.

Advantages

Ionocaloric refrigeration has several advantages over traditional refrigeration technologies.

It does not require any harmful refrigerants, such as chlorofluorocarbons (CFCs) or hydrofluorocarbons (HFCs), which are known to contribute to global warming and ozone depletion.
Has the potential to offer high energy efficiency, which can lead to significant cost savings and reduced environmental impact.

Related Research Articles

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Refrigeration is any of various types of cooling of a space, substance, or system to lower and/or maintain its temperature below the ambient one. Refrigeration is an artificial, or human-made, cooling method.

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<span class="mw-page-title-main">Magnetic refrigeration</span> Phenomenon in which a suitable material can be cooled by a changing magnetic field

Magnetic refrigeration is a cooling technology based on the magnetocaloric effect. This technique can be used to attain extremely low temperatures, as well as the ranges used in common refrigerators.

The Stirling cycle is a thermodynamic cycle that describes the general class of Stirling devices. This includes the original Stirling engine that was invented, developed and patented in 1816 by Robert Stirling with help from his brother, an engineer.

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A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor.

<span class="mw-page-title-main">Chiller</span> Machine that removes heat from a liquid coolant via vapor compression

A chiller is a machine that removes heat from a liquid coolant via a vapor-compression, adsorption refrigeration, or absorption refrigeration cycles. This liquid can then be circulated through a heat exchanger to cool equipment, or another process stream. As a necessary by-product, refrigeration creates waste heat that must be exhausted to ambience, or for greater efficiency, recovered for heating purposes. Vapor compression chillers may use any of a number of different types of compressors. Most common today are the hermetic scroll, semi-hermetic screw, or centrifugal compressors. The condensing side of the chiller can be either air or water cooled. Even when liquid cooled, the chiller is often cooled by an induced or forced draft cooling tower. Absorption and adsorption chillers require a heat source to function.

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Vapour-compression refrigeration or vapor-compression refrigeration system (VCRS), in which the refrigerant undergoes phase changes, is one of the many refrigeration cycles and is the most widely used method for air conditioning of buildings and automobiles. It is also used in domestic and commercial refrigerators, large-scale warehouses for chilled or frozen storage of foods and meats, refrigerated trucks and railroad cars, and a host of other commercial and industrial services. Oil refineries, petrochemical and chemical processing plants, and natural gas processing plants are among the many types of industrial plants that often utilize large vapor-compression refrigeration systems. Cascade refrigeration systems may also be implemented using two compressors.

An air source heat pump (ASHP) is a heat pump that can absorb heat from air outside a building and release it inside; it uses the same vapor-compression refrigeration process and much the same equipment as an air conditioner, but in the opposite direction. ASHPs are the most common type of heat pump and, usually being smaller, tend to be used to heat individual houses or flats rather than blocks, districts or industrial processes.

Thermodynamic heat pump cycles or refrigeration cycles are the conceptual and mathematical models for heat pump, air conditioning and refrigeration systems. A heat pump is a mechanical system that transmits heat from one location at a certain temperature to another location at a higher temperature. Thus a heat pump may be thought of as a "heater" if the objective is to warm the heat sink, or a "refrigerator" or “cooler” if the objective is to cool the heat source. The operating principles in both cases are the same; energy is used to move heat from a colder place to a warmer place.

Natural refrigerants are considered substances that serve as refrigerants in refrigeration systems. They are alternatives to synthetic refrigerants such as chlorofluorocarbon (CFC), hydrochlorofluorocarbon (HCFC), and hydrofluorocarbon (HFC) based refrigerants. Unlike other refrigerants, natural refrigerants can be found in nature and are commercially available thanks to physical industrial processes like fractional distillation, chemical reactions such as Haber process and spin-off gases. The most prominent of these include various natural hydrocarbons, carbon dioxide, ammonia, and water. Natural refrigerants are preferred actually in new equipment to their synthetic counterparts for their presumption of higher degrees of sustainability. With the current technologies available, almost 75 percent of the refrigeration and air conditioning sector has the potential to be converted to natural refrigerants.

<span class="mw-page-title-main">Pumpable ice technology</span> Type of technology to produce and use fluids or secondary refrigerants

Pumpable icetechnology (PIT) uses thin liquids, with the cooling capacity of ice. Pumpable ice is typically a slurry of ice crystals or particles ranging from 5 micrometers to 1 cm in diameter and transported in brine, seawater, food liquid, or gas bubbles of air, ozone, or carbon dioxide.

The term subcooling refers to a liquid existing at a temperature below its normal boiling point. For example, water boils at 373 K; at room temperature (293 K) liquid water is termed "subcooled". A subcooled liquid is the convenient state in which, say, refrigerants may undergo the remaining stages of a refrigeration cycle. Normally, a refrigeration system has a subcooling stage, allowing technicians to be certain that the quality, in which the refrigerant reaches the next step on the cycle, is the desired one. Subcooling may take place in heat exchangers and outside them. Being both similar and inverse processes, subcooling and superheating are important to determine stability and well-functioning of a refrigeration system.

In refrigeration, flash-gas is refrigerant in gas form produced spontaneously when the condensed liquid is subjected to boiling. The presence of flash-gas in the liquid lines reduces the efficiency of the refrigeration cycle. It can also lead several expansion systems to work improperly, and increase superheating at the evaporator. This is normally perceived as an unwanted condition caused by dissociation between the volume of the system, and the pressures and temperatures that allow the refrigerant to be liquid. Flash-gas must not be confused with lack of condensation, but special gear such as receivers, internal heat exchangers, insulation, and refrigeration cycle optimizers may improve condensation and avoid gas in the liquid lines.

Heat engines, refrigeration cycles and heat pumps usually involve a fluid to and from which heat is transferred while undergoing a thermodynamic cycle. This fluid is called the working fluid. Refrigeration and heat pump technologies often refer to working fluids as refrigerants. Most thermodynamic cycles make use of the latent heat of the working fluid. In case of other cycles the working fluid remains in gaseous phase while undergoing all the processes of the cycle. When it comes to heat engines, working fluid generally undergoes a combustion process as well, for example in internal combustion engines or gas turbines. There are also technologies in heat pump and refrigeration, where working fluid does not change phase, such as reverse Brayton or Stirling cycle.

References

↑ Biron, Lauren (3 January 2023). "Berkeley Lab Scientists Develop a Cool New Method of Refrigeration". News Center. Lawrence Berkeley National Laboratory. Retrieved 14 January 2023.
1 2 Lilley, Drew; Prasher, Ravi (23 December 2022). "Ionocaloric refrigeration cycle". Science. 378 (6626): 1344–1348. Bibcode:2022Sci...378.1344L. doi:10.1126/science.ade1696. PMID 36548416. S2CID 254998229.

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[1] Biron, Lauren (3 January 2023). "Berkeley Lab Scientists Develop a Cool New Method of Refrigeration". News Center. Lawrence Berkeley National Laboratory. Retrieved 14 January 2023.

[:0-2] 1 2 Lilley, Drew; Prasher, Ravi (23 December 2022). "Ionocaloric refrigeration cycle". Science. 378 (6626): 1344–1348. Bibcode:2022Sci...378.1344L. doi:10.1126/science.ade1696. PMID 36548416. S2CID 254998229.

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