Heat naturally flows across a temperature differential. Many devices (like those using fluid circulation, air circulation, and heat pipes) utilize or enhance this natural flow. However, these methods only move heat and cannot produce a temperature lower than the outlet temperature.
I differentiate this from an “active cooling” device, which does more than just move heat around. An active device can produce cooler temperatures than the ambient air. Its performance is measured by its Carnot efficiency, as these devices operate based on a Carnot cycle. Second Law of Thermodynamics: The Refrigerator
The Second Law of Thermodynamics, specifically the Clausius statement (the "second form"), states: It is not possible for heat to flow spontaneously from a colder body to a warmer body without work having been done to accomplish this flow. Energy will not flow spontaneously from a low temperature object to a higher temperature object. This principle precludes a perfect refrigerator. The same principles apply to air conditioners and heat pumps.
Source: hyperphysics.phy-astr.gsu.edu Physical Phenomena that can cause cooling
All devices that can cool below ambient temperature use one or more of these phenomena in conjunction with other effects to drive the cooling:
- Expansion and Compression: Changing the pressure of solids, liquids, or gases.
- Phase Change (Evaporation): Expansion from a phase change, usually from liquid to gas (e.g., evaporation). The Joule-Thomson effect is the basis for Freon refrigeration.
- Crystal Structure Change: Changes in a crystal's structure caused by pressure, electric, or magnetic fields.
- Magnetocaloric Effect: A change in the specific heat of a material caused by exposure to a magnetic field, often used in Adiabatic Demagnetization.
- Thermoelectric Effect (Seebeck/Peltier): Heat transfer that occurs when electrons flow across two dissimilar materials.
- Thermo Tunnel Effect: Uses electron quantum tunneling across a very small vacuum gap.
- Thermionic Effect: Heat transfer from electrons "boiling" into a gas or vacuum.
- Laser Cooling: Uses laser interference to kick out warm atoms; only works near absolute zero.
- Nernst Effect: An electromotive force is observed perpendicular to the direction of heat flow and magnetic force lines. The Ettinghausen Effect is the reverse.
- Maxwell's Demon: A theoretical abstraction involving a demon that separates hot and cold molecules.
Carnot Efficiencies for Selected Technologies:
- Peltier: 5%
- Compressor: 45%
- Thermo Tunnel Effect (Cool Chip): 55% theoretical
- Magnetic Cooling: 30% to 60%
Compressor-based Refrigeration
This method uses a piston compressor to compress a gas and then allows it to expand through a nozzle, which cools the gas.
Modern refrigerators use Freon, a CFC (carbon fluorine) gas that is non-toxic but damages the ozone layer. Other gasses like ammonia ($\text{NH}_3$), methyl chloride ($\text{CH}_3\text{Cl}$), and sulfur dioxide ($\text{SO}_2$) have been used but are flammable or poisonous.
Vortex Cooling
The Vortex Tube (discovered in 1930 by French physicist Georges Ranque) uses the compression and expansion of a gas, usually air. Ambient temperature air enters the middle of the Vortex Tube, and hot air comes out one end while cold air comes out the other.
The Thermoelectric Effect
When two wires of dissimilar metals are joined together at each end and the junctions are at different temperatures, a thermoelectric EMF is generated, causing a current to flow (the Seebeck effect, discovered in 1826).
The Peltier effect is the converse: an electric current flowing across the junction of two dissimilar metals either produces or absorbs heat, depending on the direction of the current.
Thermionic Effect
This effect is the engine of a vacuum tube. A heated metal (the cathode) releases electrons, which form a cloud around it. A current will flow to a second electrode (the anode or plate) if a battery is connected between the two. The heated metal is positively charged due to the loss of electrons.
Thermo Acoustic Cooling
Acoustic cooling uses a sound generator inside a closed tube to vibrate a gas, causing alternate compression and expansion, and therefore heating and cooling. Prototypes have shown lower efficiency than vapor compression systems and are physically large for the amount of cooling they produce. Pulse Tube Refrigeration
Pulse tube cooling is similar to acoustic cooling, but it uses a compressor instead of a sound generator to induce the oscillation and alternate compression and expansion of an inert gas in a tube.
Stirling Cycle
The Stirling refrigeration cycle compresses and expands an inert gas in a single cylinder. Heat is rejected at one end of the cylinder and absorbed at the opposite end. While the Coefficient of Performance (COP) should theoretically be higher than vapor compression systems, technical difficulties have limited its use, primarily to small prototype domestic refrigerators. It has no circulating refrigerant fluid, and the small heat areas create heat exchange difficulties, often requiring heat pipes. Malone Refrigeration
Malone refrigeration is a variant of the Stirling cycle that uses a liquid instead of an inert gas as the refrigeration medium.
Current research in America at Los Alamos Laboratories is exploring innovative cooling technologies, considering either the Brayton cycle or the Stirling cycle. The team’s prototype has been based on the Stirling cycle principle. Air Cycle Refrigeration
Air cycle refrigeration is a tried and tested technology, long used for aircraft cabin cooling. Historically, low energy efficiency and high cost prevented its use in buildings. However, recent studies suggest air cycle systems could be viable for buildings requiring simultaneous heating and cooling. Although they have low COPs, they can provide relatively high-temperature heat recovery without the efficiency penalty of vapor compression systems.
Magnetocaloric Materials and Magnetic Cooling
Magnetic cooling is based on the principle that a metal heats up when it is magnetized and cools when it is demagnetized. The technology offers several advantages over conventional gas compression cooling, including potentially higher efficiency, the elimination of ozone layer-depleting chemicals, and reduced noise and vibrations.
Currently, expensive and rare gadolinium is used for its good magnetocalorific properties. However, rare earth-based materials (like gadolinium) possess high magnetic entropy change ($\Delta S_M$) but have a very low potential for large-scale commercialization due to their limited availability, high cost, and poor corrosion resistance.
The search for affordable magnetocaloric materials for near room temperature applications has gained momentum. Iron and manganese based magnetocaloric materials (MCM) are promising alternatives. Low-cost and readily available Fe-based magnetic materials (such as Fe-Ni and $\text{Fe}_{17}\text{R}_2$ based nanoparticles) are particularly attractive for magnetic cooling applications. The development of iron and manganese-based MCM involves the study of their magnetic phase transitions, processing techniques, performance, and applications.
In terms of performance, a prototype 500-Watt system with a super-conducting magnet achieved a COP of over 5, surpassing an equivalent vapor compression system. Magnetic cooling is thought to achieve significantly higher energy efficiency than vapor compression systems, with some studies showing it can reach 60% of Carnot (ideal) efficiency, while the best gas compression systems reach only 40%. A magnetic refrigerator was successfully tested in 2001.
Sources:
Sorption Refrigeration
This method uses cyclic heating and cooling with an absorbing material in a closed system to produce a cooling effect.
Other Cooling Methods
- Evaporative Cooling / Desiccant Cooling
- Elastic Refrigeration (Sokol Idea): Cooling from the stretching of an elastomer belt and heating when it contracts can be used to refrigerate. Some rubber-like materials act in reverse, heating when stretched and cooling when allowed to relax. This reverse effect could also be utilized.
