The Physics and Future of Cooling

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:

1. Expansion and Compression: Changing the pressure of solids, liquids, or gases.
2. 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.
3. Crystal Structure Change: Changes in a crystal's structure caused by pressure, electric, or magnetic fields.
4. Magnetocaloric Effect: A change in the specific heat of a material caused by exposure to a magnetic field, often used in Adiabatic Demagnetization.
5. Thermoelectric Effect (Seebeck/Peltier): Heat transfer that occurs when electrons flow across two dissimilar materials.
6. Thermo Tunnel Effect: Uses electron quantum tunneling across a very small vacuum gap.
7. Thermionic Effect: Heat transfer from electrons "boiling" into a gas or vacuum.
8. Laser Cooling: Uses laser interference to kick out warm atoms; only works near absolute zero.
9. Nernst Effect: An electromotive force is observed perpendicular to the direction of heat flow and magnetic force lines. The Ettinghausen Effect is the reverse.
10. 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%

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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.

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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.

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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.

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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.

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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.

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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.

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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:

• Ames Laboratory news release, Dec. 7, 2001
• V. Chaudhary and R.V. Ramanujan on Fe and Mn MCM

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Sorption Refrigeration

This method uses cyclic heating and cooling with an absorbing material in a closed system to produce a cooling effect.

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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.

DIY Supermaterial Could Save You From Heatstroke: Salt based PCMs

https://www.youtube.com/watch?v=Nqxjfp4Gi0k

The most useful resource I found while investigating phase change materials was the following paper which explores several different types of low temp salt based PCMs: https://www.researchgate.net/publicat... The key to reducing the melting point of a PCM is to make what is called a eutectic mixture ( https://en.wikipedia.org/wiki/Eutecti... ). In this case that is a mixture of two different hydrated salts. When the two come together the freezing point of both is lowered to a temperature which depends on the particular salts used and the ratio between them. Sodium sulfate and table salt (sodium chloride)

 

Ths is very similar to the Study on solidification process of sodium acetate trihydrate for seasonal solar thermal energy storage


sodium acetate hot ice packs.

Sodium acetate trihydrate (SAT) has a phase change temperature of 58°C (136°F),

Review of Inorganic Salt Hydrates with Phase Change Temperature in Range

of 5°C to 60°C and Material Cost Comparison with Common Waxes

Space Powered Cooling May Be the Future of Energy

Passive Radiator Cools by Sending Heat Straight to Outer Space

http://spectrum.ieee.org/tech-talk/green-tech/solar/passive-radiators-cool-by-sending-heat-straight-to-outer-space





Illustration: Nicolle R. Fuller/Sayo-Art

In this illustration a panel coated with a multilayered material designed by Stanford engineers helps cool buildings without air conditioning. The material works in two ways. It reflects incoming sunlight [yellow] that would otherwise heat the panel. More importantly, it sends heat from inside the structure directly into space as infrared radiation of a particular wavelength [red]. The result is a cooler [blue] roof.

Conventional cooling is all about moving heat from a place where you don’t want it to a place that you care about slightly less. Your refrigerator, for example, cools itself by pumping heat into your house. Your house cools itself by pumping heat into the outdoors. It takes a significant amount of energy to keep this up—15 percent of the energy consumption of most buildings is spent just on air conditioning—meaning that the work put into transferring the heat generates even more heat. And then it’s not like the heat just vanishes when it gets outside: in urban areas, all of this waste heat builds up to increase local temperatures as part of the urban heat island effect.

In Nature this week, Stanford researchers describe a passive radiator system that can lower the temperature of anything that it’s placed on by up to five degrees Celsius by absorbing heat and sending it directly into outer space, and it even works in direct sunlight.

Radiative cooling is a way of passively moving heat from one place to another through thermal radiation, without the need for any additional energy (like electricity). If you have a hot thing, it will radiate its heat into whatever cooler thing is most convenient. In your house, this is probably the air outside, and in your car, it’s also the air outside, by way of the water in your radiator.
Since the general approach here is to use the atmosphere as the final heat sink, radiative cooling doesn’t work if you’re trying to end up at a temperature lower than the ambient temperature outside, which is why completely passive air conditioning isn’t a thing.

The clever thing about the passive radiative cooling system that Stanford came with is that it skips the atmosphere completely, and uses the entire Universe as a place to dump heat. The entire Universe, being mostly empty space, has an average temperature of just under three Kelvin, meaning that it’ll happily absorb just about as much heat as you can possibly throw at it, making it a heat sink that’s nearly, you know, universal.

Photo: Norbert von der Groeben/Stanford Engineering

Stanford electrical engineering professor Shanhui Fan [center] gazes into the pizza-sized prototype with colleagues Linxiao Zhu [left] and Aaswath Raman [right]. The high-tech mirror reflecting their faces beams heat directly into space.

To use outer space as a heat sink, you need to have access to outer space, which sounds like it’s probably a difficult thing to achieve. But fundamentally, it just means being able to transfer heat straight through Earth’s atmosphere. Stanford’s cooling system emits thermal radiation in a very specific infrared wavelength that the Earth’s atmosphere is completely transparent to, between 8 and 13 micrometers.

So, this is great, but the other part of the problem with radiative cooling is that we really need it to work during the day, when the sun is out and it’s hot. But if the sun is warming the radiator more than the radiator can cool itself, the system isn’t going to accomplish much. Stanford’s radiator also functions as a mirror that can reflect 97 percent of incident sunlight, enabling the radiator to cool itself (or something underneath it) by up to five degrees Celsius even during the heat of the day.  In a three-story commercial building with a 1600 square meter roof, using the radiative cooler would save an estimated 118,500 kWh annually, the engineers calculate.

The radiator itself is composed of seven layers of silicon dioxide and hafnium oxide on top of a thin layer of silver. The structure has been tuned to only radiate at the specific infrared wavelengths that can pass through the atmosphere. It’s just 1.8 microns thick in total, and the researchers say that it can be fabricated at production scales in existing facilities. Otherwise, the only remaining issue is to figure out how to conduct the heat from inside a building through to the exterior walls, to where the radiator could do its job.

These problems both seem surmountable, and even surmountable in the near future, as opposed to the “five to ten years” void that many technologies like this fall into. If this radiative cooler material can in fact be produced inexpensively and efficiently, it could have a significant impact on energy usage, especially in the developing world where off-grid cooling is often the only option in rural areas.

SLASHDOT: Using the Sea To Cool Your Data Center

Cooling data centers accounts for almost 50% of the power consumption. This is a massive amount of energy used for cooling.


Article on Slashdot:  Using the Sea To Cool Your Data Center

"We haven't yet seen signs of the Google Navy of seagoing data centers that use the ocean for power and cooling. But data center developers are planning to use sea water air conditioning in a new project on the island nation of Mauritius in the Indian Ocean. Cold water from deep-sea currents would be piped ashore to be used in a heat exchanger for the data center facility. A similar system has been used to replace the chillers at Cornell University, which draws cold water from Lake Cayuga. The Cornell system cost $50 million, but has slashed cooling-related energy usage by 86 percent."

Sea water has several disadvantages mostly keeping the system clean, barnacles, muscles and other small plants and animals will get sucked in to the system, and eventually clog up everything. It's also very corrosive. In addition hot water discharged from the system will hurt local ecosystems in both salt and freshwater systems.

Using the Hull of a ship would solve the clogging problems, where there is a large mass of metal in which to dissipate heat without having to pump seawater.

Still using seawater is still not a very good solution. Even is it's cost effective in reducing energy consumption.


When I had my start-up Nisvara Inc.(2002 to 2006 RIP) we worked out that we could accomplish the same using nothing but chiller towers that just used evaporative cooling. In cooler climates like where we were based at NASA Ames Research Center in Mountain View Ca, we worked out that we could cool the largest computer cluster what would have been built at that time using nothing just large truck style radiators and fans. No compressors or any active cooling just circulating water or cooling fluid.

A lot of data centers objected to the use of water because it would damage equipment. The Nisvara solution kept water in continuous copper tubes without any joints or seals.  Still that wasn't enough to belay their fears of water contacting electricity, so we also found other suitable coolants such as using 3M Novec 1230 Fire Protection Fluid. It's amazing stuff. Totally green and safe also known as "Dry Water" and "Waterless Water", will not harm equipment and just happened that it could be used as a coolant too.

It may even be useful as a refrigerant because it can phase change at a lower temperature then water, but this would have required more research.

More about Novec 1230