In thermodynamics, nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture.
Nucleation is typically defined to be the process that determines how
long an observer has to wait before the new phase or self-organized
structure appears. For example, if a volume of water is cooled (at atmospheric pressure) significantly below 0°C, it will tend to freeze into ice, but volumes of water cooled only a few degrees below 0°C often stay completely free of ice for long periods (supercooling).
At these conditions, nucleation of ice is either slow or does not occur
at all. However, at lower temperatures nucleation is fast, and ice crystals appear after little or no delay.
Nucleation is a common mechanism which generates first-order phase transitions,
and it is the start of the process of forming a new thermodynamic
phase. In contrast, new phases at continuous phase transitions start to
form immediately.
Nucleation is often very sensitive to impurities
in the system. These impurities may be too small to be seen by the
naked eye, but still can control the rate of nucleation. Because of
this, it is often important to distinguish between heterogeneous
nucleation and homogeneous nucleation. Heterogeneous nucleation occurs
at nucleation sites on surfaces in the system. Homogeneous nucleation occurs away from a surface.
Characteristics
Nucleation is usually a stochastic (random) process, so even in two identical systems nucleation will occur at different times.
A common mechanism is illustrated in the animation to the right. This
shows nucleation of a new phase (shown in red) in an existing phase
(white). In the existing phase microscopic fluctuations of the red phase
appear and decay continuously, until an unusually large fluctuation of
the new red phase is so large it is more favourable for it to grow than
to shrink back to nothing. This nucleus of the red phase then grows and
converts the system to this phase. The standard theory that describes
this behaviour for the nucleation of a new thermodynamic phase is called
classical nucleation theory.
However, the CNT fails in describing experimental results of vapour to
liquid nucleation even for model substances like argon by several orders
of magnitude.
For nucleation of a new thermodynamic phase, such as the formation of ice in water below 0°C, if the system is not evolving with time and nucleation occurs in one step, then the probability that nucleation has not occurred should undergo exponential decay. This is seen for example in the nucleation of ice in supercooled small water droplets. The decay rate of the exponential gives the nucleation rate. Classical nucleation theory
is a widely used approximate theory for estimating these rates, and how
they vary with variables such as temperature. It correctly predicts
that the time you have to wait for nucleation decreases extremely
rapidly when supersaturated.
It is not just new phases such as liquids and crystals that form
via nucleation followed by growth. The self-assembly process that forms
objects like the amyloid aggregates associated with Alzheimer's disease also starts with nucleation. Energy consuming self-organising systems such as the microtubules in cells also show nucleation and growth.
Heterogeneous nucleation often dominates homogeneous nucleation
Three nuclei on a surface, illustrating decreasing contact angles. The contact angle
the nucleus surface makes with the solid horizontal surface decreases
from left to right. The surface area of the nucleus decreases as the
contact angle decreases. This geometrical effect reduces the barrier in classical nucleation theory
and hence results in faster nucleation on surfaces with smaller contact
angles. Also, if instead of the surface being flat it curves towards
fluid, then this also reduces the interfacial area and so the nucleation
barrier.
Heterogeneous nucleation, nucleation with the nucleus at a surface, is much more common than homogeneous nucleation.
For example, in the nucleation of ice from supercooled water droplets,
purifying the water to remove all or almost all impurities results in
water droplets that freeze below around −35°C,whereas water that contains impurities may freeze at −5°C or warmer.
This observation that heterogeneous nucleation can occur when the
rate of homogeneous nucleation is essentially zero, is often understood
using classical nucleation theory. This predicts that the nucleation slows exponentially with the height of a free energybarrier
ΔG*. This barrier comes from the free energy penalty of forming the
surface of the growing nucleus. For homogeneous nucleation the nucleus
is approximated by a sphere, but as we can see in the schematic of
macroscopic droplets to the right, droplets on surfaces are not complete
spheres and so the area of the interface between the droplet and the
surrounding fluid is less than a sphere's .
This reduction in surface area of the nucleus reduces the height of the
barrier to nucleation and so speeds nucleation up exponentially.
Nucleation can also start at the surface of a liquid. For example, computer simulations of gold nanoparticles show that the crystal phase sometimes nucleates at the liquid-gold surface.
Computer simulation studies of simple models
Classical nucleation theory
makes a number of assumptions, for example it treats a microscopic
nucleus as if it is a macroscopic droplet with a well-defined surface
whose free energy is estimated using an equilibrium property: the
interfacial tension σ. For a nucleus that may be only of order ten
molecules across it is not always clear that we can treat something so
small as a volume plus a surface. Also nucleation is an inherently out
of thermodynamic equilibrium phenomenon so it is not always obvious that its rate can be estimated using equilibrium properties.
However, modern computers are powerful enough to calculate
essentially exact nucleation rates for simple models. These have been
compared with the classical theory, for example for the case of
nucleation of the crystal phase in the model of hard spheres. This is a
model of perfectly hard spheres in thermal motion, and is a simple model
of some colloids. For the crystallization of hard spheres the classical theory is a very reasonable approximate theory.
So for the simple models we can study, classical nucleation theory
works quite well, but we do not know if it works equally well for (say)
complex molecules crystallising out of solution.
The spinodal region
Phase-transition processes can also be explained in terms of spinodal decomposition,
where phase separation is delayed until the system enters the unstable
region where a small perturbation in composition leads to a decrease in
energy and, thus, spontaneous growth of the perturbation.
This region of a phase diagram is known as the spinodal region and the
phase separation process is known as spinodal decomposition and may be
governed by the Cahn–Hilliard equation.
The nucleation of crystals
In
many cases, liquids and solutions can be cooled down or concentrated up
to conditions where the liquid or solution is significantly less
thermodynamically stable than the crystal, but where no crystals will
form for minutes, hours, weeks or longer; this process is called supercooling.
Nucleation of the crystal is then being prevented by a substantial
barrier. This has consequences, for example cold high altitude clouds
may contain large numbers of small liquid water droplets that are far
below 0°C.
In small volumes, such as in small droplets, only one nucleation event
may be needed for crystallisation. In these small volumes, the time
until the first crystal appears is usually defined to be the nucleation
time. Calcium carbonate crystal nucleation depends not only on degree of
supersaturation but also the ratio of calcium to carbonate ions in
aqueous solutions.
In larger volumes many nucleation events will occur. A simple model for
crystallisation in that case, that combines nucleation and growth is
the KJMA or Avrami model.
When sugar is supersaturated in water, nucleation will occur, allowing sugar molecules to stick together and form large crystal structures.
Although
the existing theories including the classical nucleation theory explain
well the steady nucleation state when the crystal nucleation rate is
not time dependent, the initial non-steady state transient nucleation,
and even more mysterious incubation period, require more attention of
the scientific community. Chemical ordering of the undercooling liquid
prior to crystal nucleation was suggested to be responsible for that feature by reducing the energy barrier for nucleation.
Primary and secondary nucleation
The
time until the appearance of the first crystal is also called primary
nucleation time, to distinguish it from secondary nucleation times.
Primary here refers to the first nucleus to form, while secondary nuclei
are crystal nuclei produced from a preexisting crystal. Primary
nucleation describes the transition to a new phase that does not rely on
the new phase already being present, either because it is the very
first nucleus of that phase to form, or because the nucleus forms far
from any pre-existing piece of the new phase. Particularly in the study
of crystallisation, secondary nucleation can be important. This is the
formation of nuclei of a new crystal directly caused by pre-existing
crystals.
For example, if the crystals are in a solution and the system is
subject to shearing forces, small crystal nuclei could be sheared off a
growing crystal, thus increasing the number of crystals in the system.
So both primary and secondary nucleation increase the number of crystals
in the system but their mechanisms are very different, and secondary
nucleation relies on crystals already being present.
Experimental observations on the nucleation times for the crystallisation of small volumes
It
is typically difficult to experimentally study the nucleation of
crystals. The nucleus is microscopic, and thus too small to be directly
observed. In large liquid volumes there are typically multiple
nucleation events, and it is difficult to disentangle the effects of
nucleation from those of growth of the nucleated phase. These problems
can be overcome by working with small droplets. As nucleation is stochastic, many droplets are needed so that statistics for the nucleation events can be obtained.
The
black triangles are the fraction of a large set of small supercooled
liquid tin droplets that are still liquid, i.e., where the crystal state
has not nucleated, as a function of time. The data are from Pound and
La Mer (1952). The red curve is a fit of a function of the Gompertz form
to these data.
To the right is shown an
example set of nucleation data. It is for the nucleation at constant
temperature and hence supersaturation of the crystal phase in small
droplets of supercooled liquid tin; this is the work of Pound and La
Mer.
Nucleation occurs in different droplets at different times, hence
the fraction is not a simple step function that drops sharply from one
to zero at one particular time. The red curve is a fit of a Gompertz function to the data. This is a simplified version of the model Pound and La Mer used to model their data.
The model assumes that nucleation occurs due to impurity particles in
the liquid tin droplets, and it makes the simplifying assumption that
all impurity particles produce nucleation at the same rate. It also
assumes that these particles are Poisson distributed
among the liquid tin droplets. The fit values are that the nucleation
rate due to a single impurity particle is 0.02/s, and the average number
of impurity particles per droplet is 1.2. Note that about 30% of the
tin droplets never freeze; the data plateaus at a fraction of about 0.3.
Within the model this is assumed to be because, by chance, these
droplets do not have even one impurity particle and so there is no
heterogeneous nucleation. Homogeneous nucleation is assumed to be
negligible on the timescale of this experiment. The remaining droplets
freeze in a stochastic way, at rates 0.02/s if they have one impurity
particle, 0.04/s if they have two, and so on.
These data are just one example, but they illustrate common
features of the nucleation of crystals in that there is clear evidence
for heterogeneous nucleation, and that nucleation is clearly stochastic.
Ice
The freezing of small water droplets to ice is an important process, particularly in the formation and dynamics of clouds. Water (at atmospheric pressure) does not freeze at 0°C,
but rather at temperatures that tend to decrease as the volume of the
water decreases and as the concentration of dissolved chemicals in the
water increases.
Survival curve for water droplets 34.5 μm in diameter. Blue circles are data, and the red curve is a fit of a Gumbel distribution.
Thus small droplets of water, as found in clouds, may remain liquid far below 0°C.
An example of experimental data on the freezing of small water
droplets is shown at the right. The plot shows the fraction of a large
set of water droplets, that are still liquid water, i.e., have not yet
frozen, as a function of temperature. Note that the highest temperature
at which any of the droplets freezes is close to -19°C, while the last droplet to freeze does so at almost -35°C.
Examples
Nucleation of fluids (gases and liquids)
Nucleation of carbon dioxide bubbles around a finger
Clouds form when wet air cools (often because the air rises) and many small water droplets nucleate from the supersaturated air. The amount of water vapour that air can carry decreases with lower temperatures.
The excess vapor begins to nucleate and to form small water droplets
which form a cloud. Nucleation of the droplets of liquid water is
heterogeneous, occurring on particles referred to as cloud condensation nuclei. Cloud seeding is the process of adding artificial condensation nuclei to quicken the formation of clouds.
Bubbles of carbon dioxide nucleate shortly after the pressure is released from a container of carbonated liquid.
Nucleation in boiling can occur in the bulk liquid if the pressure is reduced so that the liquid becomes superheated
with respect to the pressure-dependent boiling point. More often,
nucleation occurs on the heating surface, at nucleation sites.
Typically, nucleation sites are tiny crevices where free gas-liquid
surface is maintained or spots on the heating surface with lower wetting
properties. Substantial superheating of a liquid can be achieved after
the liquid is de-gassed and if the heating surfaces are clean, smooth
and made of materials well wetted by the liquid.
Some champagne stirrers
operate by providing many nucleation sites via high surface-area and
sharp corners, speeding the release of bubbles and removing carbonation
from the wine.
The Diet Coke and Mentos eruption
offers another example. The surface of Mentos candy provides nucleation
sites for the formation of carbon-dioxide bubbles from carbonated soda.
The most common crystallisation process on Earth is the formation of ice. Liquid water does not freeze at 0°C unless there is ice already present; cooling significantly below 0°C
is required to nucleate ice and for the water to freeze. For example,
small droplets of very pure water can remain liquid down to below -30 °C
although ice is the stable state below 0°C.
Many of the materials we make and use are crystalline, but are made
from liquids, e.g. crystalline iron made from liquid iron cast into a
mold, so the nucleation of crystalline materials is widely studied in
industry.
It is used heavily in the chemical industry for cases such as in the
preparation of metallic ultradispersed powders that can serve as
catalysts. For example, platinum deposited onto TiO2nanoparticles catalyses the decomposition of water.
It is an important factor in the semiconductor industry, as the band
gap energy in semiconductors is influenced by the size of nanoclusters.
Nucleation in solids
In
addition to the nucleation and growth of crystals e.g. in
non-crystalline glasses, the nucleation and growth of impurity
precipitates in crystals at, and between, grain boundaries is quite
important industrially. For example in metals solid-state nucleation and
precipitate growth plays an important role e.g. in modifying mechanical
properties like ductility, while in semiconductors it plays an
important role e.g. in trapping impurities during integrated circuit
manufacture.
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 (while the removed heat is ejected to a place of higher temperature). Refrigeration is an artificial, or human-made, cooling method.
Refrigeration refers to the process by which energy, in the form
of heat, is removed from a low-temperature medium and transferred to a
high-temperature medium. This work of energy transfer is traditionally driven by mechanical means (whether ice or electromechanical machines), but it can also be driven by heat, magnetism, electricity, laser, or other means. Refrigeration has many applications, including household refrigerators, industrial freezers, cryogenics, and air conditioning. Heat pumps
may use the heat output of the refrigeration process, and also may be
designed to be reversible, but are otherwise similar to air conditioning
units.
Refrigeration has had a large impact on industry, lifestyle, agriculture, and settlement patterns. The idea of preserving food dates back to human prehistory, but for thousands of years humans were limited regarding the means of doing so. They used curing via salting and drying, and they made use of natural coolness in caves, root cellars, and winter weather, but other means of cooling were unavailable. In the 19th century, they began to make use of the ice trade to develop cold chains. In the late 19th through mid-20th centuries, mechanical refrigeration was developed, improved, and greatly expanded in its reach. Refrigeration has thus rapidly evolved in the past century, from ice harvesting to temperature-controlled rail cars, refrigerator trucks, and ubiquitous refrigerators and freezers
in both stores and homes in many countries. The introduction of
refrigerated rail cars contributed to the settlement of areas that were
not on earlier main transport channels such as rivers, harbors, or
valley trails.
These new settlement patterns sparked the building of large
cities which are able to thrive in areas that were otherwise thought to
be inhospitable, such as Houston, Texas, and Las Vegas, Nevada. In most developed countries, cities are heavily dependent upon refrigeration in supermarkets in order to obtain their food for daily consumption. The increase in food sources has led to a larger concentration of agricultural sales coming from a smaller percentage of farms. Farms today have a much larger output per person in comparison to the late 1800s.
This has resulted in new food sources available to entire populations,
which has had a large impact on the nutrition of society.
The seasonal harvesting of snow and ice is an ancient practice estimated to have begun earlier than 1000 BC. A Chinese collection of lyrics from this time period known as the Sleaping,
describes religious ceremonies for filling and emptying ice cellars.
However, little is known about the construction of these ice cellars or
the purpose of the ice. The next ancient society to record the
harvesting of ice may have been the Jews in the book of Proverbs, which
reads, "As the cold of snow in the time of harvest, so is a faithful
messenger to them who sent him." Historians have interpreted this to
mean that the Jews used ice to cool beverages rather than to preserve
food. Other ancient cultures such as the Greeks and the Romans dug large
snow pits insulated with grass, chaff, or branches of trees as cold
storage. Like the Jews, the Greeks and Romans did not use ice and snow
to preserve food, but primarily as a means to cool beverages. Egyptians
cooled water by evaporation in shallow earthen jars on the roofs of
their houses at night. The ancient people of India used this same
concept to produce ice. The Persians stored ice in a pit called a Yakhchal
and may have been the first group of people to use cold storage to
preserve food. In the Australian outback before a reliable electricity
supply was available many farmers used a Coolgardie safe, consisting of a room with hessian
(burlap) curtains hanging from the ceiling soaked in water. The water
would evaporate and thereby cool the room, allowing many perishables
such as fruit, butter, and cured meats to be kept.
Ice harvesting in Massachusetts, 1852, showing the railroad line in the background, used to transport the ice.
Before 1830, few Americans used ice to refrigerate foods due to a
lack of ice-storehouses and iceboxes. As these two things became more
widely available, individuals used axes and saws to harvest ice
for their storehouses. This method proved to be difficult, dangerous,
and certainly did not resemble anything that could be duplicated on a
commercial scale.
Despite the difficulties of harvesting ice, Frederic Tudor
thought that he could capitalize on this new commodity by harvesting ice
in New England and shipping it to the Caribbean islands as well as the
southern states. In the beginning, Tudor lost thousands of dollars, but
eventually turned a profit as he constructed icehouses in Charleston,
Virginia and in the Cuban port town of Havana. These icehouses as well
as better insulated ships helped reduce ice wastage from 66% to 8%. This
efficiency gain influenced Tudor to expand his ice market to other
towns with icehouses such as New Orleans and Savannah. This ice market
further expanded as harvesting ice became faster and cheaper after one
of Tudor's suppliers, Nathaniel Wyeth, invented a horse-drawn ice cutter
in 1825. This invention as well as Tudor's success inspired others to
get involved in the ice trade and the ice industry grew.
Ice became a mass-market commodity by the early 1830s with the
price of ice dropping from six cents per pound to a half of a cent per
pound. In New York City, ice consumption increased from 12,000 tons in
1843 to 100,000 tons in 1856. Boston's consumption leapt from 6,000 tons
to 85,000 tons during that same period. Ice harvesting created a
"cooling culture" as majority of people used ice and iceboxes to store
their dairy products, fish, meat, and even fruits and vegetables. These
early cold storage practices paved the way for many Americans to accept
the refrigeration technology that would soon take over the country.
Refrigeration research
William Cullen, the first to conduct experiments into artificial refrigeration.
The history of artificial refrigeration began when Scottish professor William Cullen designed a small refrigerating machine in 1755. Cullen used a pump to create a partial vacuum over a container of diethyl ether, which then boiled, absorbing heat from the surrounding air. The experiment even created a small amount of ice, but had no practical application at that time.
In 1758, Benjamin Franklin and John Hadley,
professor of chemistry, collaborated on a project investigating the
principle of evaporation as a means to rapidly cool an object at Cambridge University, England.
They confirmed that the evaporation of highly volatile liquids, such as
alcohol and ether, could be used to drive down the temperature of an
object past the freezing point of water. They conducted their experiment
with the bulb of a mercury thermometer as their object and with a
bellows used to quicken the evaporation; they lowered the temperature of
the thermometer bulb down to −14 °C (7 °F), while the ambient
temperature was 18 °C (65 °F). They noted that soon after they passed
the freezing point of water 0 °C (32 °F), a thin film of ice formed on
the surface of the thermometer's bulb and that the ice mass was about a 6.4 millimetres (1⁄4 in)
thick when they stopped the experiment upon reaching −14 °C (7 °F).
Franklin wrote, "From this experiment, one may see the possibility of
freezing a man to death on a warm summer's day". In 1805, American inventor Oliver Evans described a closed vapor-compression refrigeration cycle for the production of ice by ether under vacuum.
In 1820, the English scientist Michael Faraday liquefied ammonia and other gases by using high pressures and low temperatures, and in 1834, an American expatriate to Great Britain, Jacob Perkins,
built the first working vapor-compression refrigeration system in the
world. It was a closed-cycle that could operate continuously, as he
described in his patent:
I am enabled to use volatile fluids for the purpose of producing
the cooling or freezing of fluids, and yet at the same time constantly
condensing such volatile fluids, and bringing them again into operation
without waste.
His prototype system worked although it did not succeed commercially.
In 1842, a similar attempt was made by American physician, John Gorrie,
who built a working prototype, but it was a commercial failure. Like
many of the medical experts during this time, Gorrie thought too much
exposure to tropical heat led to mental and physical degeneration, as
well as the spread of diseases such as malaria.
He conceived the idea of using his refrigeration system to cool the air
for comfort in homes and hospitals to prevent disease. American
engineer Alexander Twining took out a British patent in 1850 for a vapour compression system that used ether.
The first practical vapour-compression refrigeration system was built by James Harrison, a British journalist who had emigrated to Australia.
His 1856 patent was for a vapour-compression system using ether,
alcohol, or ammonia. He built a mechanical ice-making machine in 1851 on
the banks of the Barwon River at Rocky Point in Geelong, Victoria,
and his first commercial ice-making machine followed in 1854. Harrison
also introduced commercial vapour-compression refrigeration to breweries
and meat-packing houses, and by 1861, a dozen of his systems were in
operation. He later entered the debate of how to compete against the
American advantage of unrefrigerated beef sales to the United Kingdom. In 1873 he prepared the sailing ship Norfolk
for an experimental beef shipment to the United Kingdom, which used a
cold room system instead of a refrigeration system. The venture was a
failure as the ice was consumed faster than expected.
The first gas absorption refrigeration system using gaseous ammonia dissolved in water (referred to as "aqua ammonia") was developed by Ferdinand Carré of France in 1859 and patented in 1860. Carl von Linde, an engineer specializing in steam locomotives and professor of engineering at the Technological University of Munich
in Germany, began researching refrigeration in the 1860s and 1870s in
response to demand from brewers for a technology that would allow
year-round, large-scale production of lager; he patented an improved method of liquefying gases in 1876. His new process made possible using gases such as ammonia, sulfur dioxide (SO2) and methyl chloride (CH3Cl) as refrigerants and they were widely used for that purpose until the late 1920s.
Thaddeus Lowe,
an American balloonist, held several patents on ice-making machines.
His "Compression Ice Machine" would revolutionize the cold-storage
industry. In 1869, he and other investors purchased an old steamship
onto which they loaded one of Lowe's refrigeration units and began
shipping fresh fruit from New York to the Gulf Coast area, and fresh
meat from Galveston, Texas back to New York, but because of Lowe's lack
of knowledge about shipping, the business was a costly failure.
An 1870 refrigerator car design. Hatches in the roof provided access to the tanks for the storage of harvested ice at each end.Icemaker Patent by Andrew Muhl, dated December 12, 1871.
In 1842, John Gorrie
created a system capable of refrigerating water to produce ice.
Although it was a commercial failure, it inspired scientists and
inventors around the world. France's Ferdinand Carre was one of the
inspired and he created an ice producing system that was simpler and
smaller than that of Gorrie. During the Civil War, cities such as New
Orleans could no longer get ice from New England via the coastal ice
trade. Carre's refrigeration system became the solution to New Orleans'
ice problems and, by 1865, the city had three of Carre's machines.
In 1867, in San Antonio, Texas, a French immigrant named Andrew Muhl
built an ice-making machine to help service the expanding beef industry
before moving it to Waco in 1871. In 1873, the patent for this machine
was contracted by the Columbus Iron Works, a company acquired by the
W.C. Bradley Co., which went on to produce the first commercial
ice-makers in the US.
By the 1870s, breweries had become the largest users of harvested
ice. Though the ice-harvesting industry had grown immensely by the turn
of the 20th century, pollution and sewage had begun to creep into
natural ice, making it a problem in the metropolitan suburbs.
Eventually, breweries began to complain of tainted ice. Public concern
for the purity of water, from which ice was formed, began to increase in
the early 1900s with the rise of germ theory. Numerous media outlets
published articles connecting diseases such as typhoid fever with
natural ice consumption. This caused ice harvesting to become illegal in
certain areas of the country. All of these scenarios increased the
demands for modern refrigeration and manufactured ice. Ice producing
machines like that of Carre's and Muhl's were looked to as means of
producing ice to meet the needs of grocers, farmers, and food shippers.
Refrigerated railroad cars were introduced in the US in the 1840s
for short-run transport of dairy products, but these used harvested ice
to maintain a cool temperature.
Dunedin, the first commercially successful refrigerated ship.
The new refrigerating technology first met with widespread industrial
use as a means to freeze meat supplies for transport by sea in reefer ships from the British Dominions and other countries to the British Isles. Although not actually the first to achieve successful transportation of frozen goods overseas (the Strathleven had arrived at the London docks on 2 February 1880 with a cargo of frozen beef, mutton and butter from Sydney and Melbourne ), the breakthrough is often attributed to William Soltau Davidson, an entrepreneur who had emigrated to New Zealand. Davidson thought that Britain's rising population and meat demand could mitigate the slump in world wool markets that was heavily affecting New Zealand. After extensive research, he commissioned the Dunedin to be refitted with a compression refrigeration unit for meat shipment in 1881. On February 15, 1882, the Dunedin
sailed for London with what was to be the first commercially successful
refrigerated shipping voyage, and the foundation of the refrigerated meat industry.
The Times
commented "Today we have to record such a triumph over physical
difficulties, as would have been incredible, even unimaginable, a very
few days ago...". The Marlborough—sister ship to the Dunedin – was immediately converted and joined the trade the following year, along with the rival New Zealand Shipping Company vessel Mataurua, while the German Steamer Marsala
began carrying frozen New Zealand lamb in December 1882. Within five
years, 172 shipments of frozen meat were sent from New Zealand to the
United Kingdom, of which only 9 had significant amounts of meat
condemned. Refrigerated shipping also led to a broader meat and dairy
boom in Australasia and South America. J & E Hall of Dartford, England outfitted the SS Selembria with a vapor compression system to bring 30,000 carcasses of mutton from the Falkland Islands in 1886. In the years ahead, the industry rapidly expanded to Australia, Argentina and the United States.
By the 1890s, refrigeration played a vital role in the
distribution of food. The meat-packing industry relied heavily on
natural ice in the 1880s and continued to rely on manufactured ice as
those technologies became available.
By 1900, the meat-packing houses of Chicago had adopted ammonia-cycle
commercial refrigeration. By 1914, almost every location used artificial
refrigeration. The major meat packers,
Armour, Swift, and Wilson, had purchased the most expensive units which
they installed on train cars and in branch houses and storage
facilities in the more remote distribution areas.
By the middle of the 20th century, refrigeration units were
designed for installation on trucks or lorries. Refrigerated vehicles
are used to transport perishable goods, such as frozen foods, fruit and
vegetables, and temperature-sensitive chemicals. Most modern
refrigerators keep the temperature between –40 and –20 °C, and have a
maximum payload of around 24,000 kg gross weight (in Europe).
Although commercial refrigeration quickly progressed, it had
limitations that prevented it from moving into the household. First,
most refrigerators were far too large. Some of the commercial units
being used in 1910 weighed between five and two hundred tons. Second,
commercial refrigerators were expensive to produce, purchase, and
maintain. Lastly, these refrigerators were unsafe. It was not uncommon
for commercial refrigerators to catch fire, explode, or leak toxic
gases. Refrigeration did not become a household technology until these
three challenges were overcome.
Home and consumer use
An early example of the consumerization of mechanical refrigeration that began in the early 20th century. The refrigerant was sulfur dioxide.A modern home refrigerator
During the early 1800s, consumers preserved their food by storing
food and ice purchased from ice harvesters in iceboxes. In 1803, Thomas
Moore patented a metal-lined butter-storage tub which became the
prototype for most iceboxes. These iceboxes were used until nearly 1910
and the technology did not progress. In fact, consumers that used the
icebox in 1910 faced the same challenge of a moldy and stinky icebox
that consumers had in the early 1800s.
General Electric (GE) was one of the first companies to overcome
these challenges. In 1911, GE released a household refrigeration unit
that was powered by gas. The use of gas eliminated the need for an
electric compressor motor and decreased the size of the refrigerator.
However, electric companies that were customers of GE did not benefit
from a gas-powered unit. Thus, GE invested in developing an electric
model. In 1927, GE released the Monitor Top, the first refrigerator to
run on electricity.
In 1930, Frigidaire, one of GE's main competitors, synthesized Freon. With the invention of synthetic refrigerants based mostly on a chlorofluorocarbon
(CFC) chemical, safer refrigerators were possible for home and consumer
use. Freon led to the development of smaller, lighter, and cheaper
refrigerators. The average price of a refrigerator dropped from $275 to
$154 with the synthesis of Freon. This lower price allowed ownership of
refrigerators in American households to exceed 50% by 1940.
Freon is a trademark of the DuPont Corporation and refers to these
CFCs, and later hydro chlorofluorocarbon (HCFC) and hydro fluorocarbon
(HFC), refrigerants developed in the late 1920s. These refrigerants were
considered — at the time — to be less harmful than the commonly-used
refrigerants of the time, including methyl formate, ammonia, methyl
chloride, and sulfur dioxide. The intent was to provide refrigeration
equipment for home use without danger. These CFC refrigerants answered
that need. In the 1970s, though, the compounds were found to be reacting
with atmospheric ozone, an important protection against solar
ultraviolet radiation, and their use as a refrigerant worldwide was
curtailed in the Montreal Protocol of 1987.
Impact on settlement patterns in the United States of America
In the last century, refrigeration allowed new settlement patterns to
emerge. This new technology has allowed for new areas to be settled
that are not on a natural channel of transport such as a river, valley
trail or harbor that may have otherwise not been settled. Refrigeration
has given opportunities to early settlers to expand westward and into
rural areas that were unpopulated. These new settlers with rich and
untapped soil saw opportunity to profit by sending raw goods to the
eastern cities and states. In the 20th century, refrigeration has made
"Galactic Cities" such as Dallas, Phoenix and Los Angeles possible.
Refrigerated rail cars
The refrigerated rail car (refrigerated van or refrigerator car),
along with the dense railroad network, became an exceedingly important
link between the marketplace and the farm allowing for a national
opportunity rather than a just a regional one. Before the invention of
the refrigerated rail car, it was impossible to ship perishable food
products long distances. The beef packing industry made the first demand
push for refrigeration cars. The railroad companies were slow to adopt
this new invention because of their heavy investments in cattle cars,
stockyards, and feedlots.
Refrigeration cars were also complex and costly compared to other rail
cars, which also slowed the adoption of the refrigerated rail car. After
the slow adoption of the refrigerated car, the beef packing industry
dominated the refrigerated rail car business with their ability to
control ice plants and the setting of icing fees. The United States
Department of Agriculture estimated that, in 1916, over sixty-nine
percent of the cattle killed in the country was done in plants involved
in interstate trade. The same companies that were also involved in the
meat trade later implemented refrigerated transport to include
vegetables and fruit. The meat packing companies had much of the
expensive machinery, such as refrigerated cars, and cold storage
facilities that allowed for them to effectively distribute all types of
perishable goods. During World War I, a national refrigerator car pool
was established by the United States Administration to deal with problem
of idle cars and was later continued after the war.
The idle car problem was the problem of refrigeration cars sitting
pointlessly in between seasonal harvests. This meant that very expensive
cars sat in rail yards for a good portion of the year while making no
revenue for the car's owner. The car pool was a system where cars were
distributed to areas as crops matured ensuring maximum use of the cars.
Refrigerated rail cars moved eastward from vineyards, orchards, fields,
and gardens in western states to satisfy Americas consuming market in
the east.
The refrigerated car made it possible to transport perishable crops
hundreds and even thousands of kilometres or miles. The most noticeable
effect the car gave was a regional specialization of vegetables and
fruits. The refrigeration rail car was widely used for the
transportation of perishable goods up until the 1950s. By the 1960s, the
nation's interstate highway system was adequately complete allowing for
trucks to carry the majority of the perishable food loads and to push
out the old system of the refrigerated rail cars.
Expansion west and into rural areas
The
widespread use of refrigeration allowed for a vast amount of new
agricultural opportunities to open up in the United States. New markets
emerged throughout the United States in areas that were previously
uninhabited and far-removed from heavily populated areas. New
agricultural opportunity presented itself in areas that were considered
rural, such as states in the south and in the west. Shipments on a large
scale from the south and California were both made around the same
time, although natural ice was used from the Sierras in California
rather than manufactured ice in the south.
Refrigeration allowed for many areas to specialize in the growing of
specific fruits. California specialized in several fruits, grapes,
peaches, pears, plums, and apples, while Georgia became famous for
specifically its peaches. In California, the acceptance of the
refrigerated rail cars led to an increase of car loads from 4,500
carloads in 1895 to between 8,000 and 10,000 carloads in 1905.
The Gulf States, Arkansas, Missouri and Tennessee entered into
strawberry production on a large-scale while Mississippi became the
center of the tomato industry.
New Mexico, Colorado, Arizona, and Nevada grew cantaloupes. Without
refrigeration, this would have not been possible. By 1917,
well-established fruit and vegetable areas that were close to eastern
markets felt the pressure of competition from these distant specialized
centers.
Refrigeration was not limited to meat, fruit and vegetables but it also
encompassed dairy product and dairy farms. In the early twentieth
century, large cities got their dairy supply from farms as far as 640
kilometres (400 mi). Dairy products were not as easily transported over
great distances like fruits and vegetables due to greater perishability.
Refrigeration made production possible in the west far from eastern
markets, so much in fact that dairy farmers could pay transportation
cost and still undersell their eastern competitors.
Refrigeration and the refrigerated rail gave opportunity to areas with
rich soil far from natural channel of transport such as a river, valley
trail or harbors.
Rise of the galactic city
"Edge city" was a term coined by Joel Garreau, whereas the term "galactic city" was coined by Lewis Mumford.
These terms refer to a concentration of business, shopping, and
entertainment outside a traditional downtown or central business
district in what had previously been a residential or rural area. There
were several factors contributing to the growth of these cities such as
Los Angeles, Las Vegas, Houston, and Phoenix. The factors that
contributed to these large cities include reliable automobiles, highway
systems, refrigeration, and agricultural production increases. Large
cities such as the ones mentioned above have not been uncommon in
history, but what separates these cities from the rest are that these
cities are not along some natural channel of transport, or at some
crossroad of two or more channels such as a trail, harbor, mountain,
river, or valley. These large cities have been developed in areas that
only a few hundred years ago would have been uninhabitable. Without a
cost efficient way of cooling air and transporting water and food from
great distances, these large cities would have never developed. The
rapid growth of these cities was influenced by refrigeration and an
agricultural productivity increase, allowing more distant farms to
effectively feed the population.
Impact on agriculture and food production
Agriculture's role in developed countries has drastically changed in
the last century due to many factors, including refrigeration.
Statistics from the 2007 census gives information on the large
concentration of agricultural sales coming from a small portion of the
existing farms in the United States today. This is a partial result of
the market created for the frozen meat trade by the first successful
shipment of frozen sheep carcasses coming from New Zealand in the 1880s.
As the market continued to grow, regulations on food processing and
quality began to be enforced. Eventually, electricity was introduced
into rural homes in the United States, which allowed refrigeration
technology to continue to expand on the farm, increasing output per
person. Today, refrigeration's use on the farm reduces humidity levels,
avoids spoiling due to bacterial growth, and assists in preservation.
Demographics
The
introduction of refrigeration and evolution of additional technologies
drastically changed agriculture in the United States. During the
beginning of the 20th century, farming was a common occupation and
lifestyle for United States citizens, as most farmers actually lived on
their farm. In 1935, there were 6.8 million farms in the United States
and a population of 127 million. Yet, while the United States population
has continued to climb, citizens pursuing agriculture continue to
decline. Based on the 2007 US Census, less than one percent of a
population of 310 million people claim farming as an occupation today.
However, the increasing population has led to an increasing demand for
agricultural products, which is met through a greater variety of crops,
fertilizers, pesticides, and improved technology. Improved technology
has decreased the risk and time involved for agricultural management and
allows larger farms to increase their output per person to meet
society's demand.
Meat packing and trade
Prior to 1882, the South Island
of New Zealand had been experimenting with sowing grass and
crossbreeding sheep, which immediately gave their farmers economic
potential in the exportation of meat. In 1882, the first successful
shipment of sheep carcasses was sent from Port Chalmers in Dunedin, New Zealand, to London. By the 1890s, the frozen meat trade became increasingly more profitable in New Zealand, especially in Canterbury,
where 50% of exported sheep carcasses came from in 1900. It was not
long before Canterbury meat was known for the highest quality, creating a
demand for New Zealand meat around the world. In order to meet this new
demand, the farmers improved their feed so sheep could be ready for the
slaughter in only seven months. This new method of shipping led to an
economic boom in New Zealand by the mid 1890s.
In the United States, the Meat Inspection Act of 1891 was put in
place in the United States because local butchers felt the refrigerated
railcar system was unwholesome. When meat packing began to take off, consumers became nervous about the quality of the meat for consumption. Upton Sinclair's 1906 novel The Jungle
brought negative attention to the meat packing industry, by drawing to
light unsanitary working conditions and processing of diseased animals.
The book caught the attention of President Theodore Roosevelt, and the 1906 Meat Inspection Act
was put into place as an amendment to the Meat Inspection Act of 1891.
This new act focused on the quality of the meat and environment it is
processed in.
Electricity in rural areas
In the early 1930s, 90 percent of the urban population of the United States had electric power,
in comparison to only 10 percent of rural homes. At the time, power
companies did not feel that extending power to rural areas (rural electrification) would produce enough profit to make it worth their while. However, in the midst of the Great Depression, President Franklin D. Roosevelt
realized that rural areas would continue to lag behind urban areas in
both poverty and production if they were not electrically wired. On May
11, 1935, the president signed an executive order called the Rural Electrification Administration,
also known as REA. The agency provided loans to fund electric
infrastructure in the rural areas. In just a few years, 300,000 people
in rural areas of the United States had received power in their homes.
While electricity dramatically improved working conditions on
farms, it also had a large impact on the safety of food production.
Refrigeration systems were introduced to the farming and food distribution processes, which helped in food preservation and kept food supplies safe.
Refrigeration also allowed for shipment of perishable commodities
throughout the United States. As a result, United States farmers quickly
became the most productive in the world, and entire new food systems arose.
Farm use
In
order to reduce humidity levels and spoiling due to bacterial growth,
refrigeration is used for meat, produce, and dairy processing in farming
today. Refrigeration systems are used the heaviest in the warmer months
for farming produce, which must be cooled as soon as possible in order
to meet quality standards and increase the shelf life. Meanwhile, dairy
farms refrigerate milk year round to avoid spoiling.
Effects on lifestyle and diet
In
the late 19th Century and into the very early 20th Century, except for
staple foods (sugar, rice, and beans) that needed no refrigeration, the
available foods were affected heavily by the seasons and what could be
grown locally.
Refrigeration has removed these limitations. Refrigeration played a
large part in the feasibility and then popularity of the modern
supermarket. Fruits and vegetables out of season, or grown in distant
locations, are now available at relatively low prices. Refrigerators
have led to a huge increase in meat and dairy products as a portion of
overall supermarket sales.
As well as changing the goods purchased at the market, the ability to
store these foods for extended periods of time has led to an increase in
leisure time.
Prior to the advent of the household refrigerator, people would have to
shop on a daily basis for the supplies needed for their meals.
Impact on nutrition
The introduction of refrigeration allowed for the hygienic handling and storage of perishables,
and as such, promoted output growth, consumption, and the availability
of nutrition. The change in our method of food preservation moved us
away from salts to a more manageable sodium level. The ability to move
and store perishables such as meat and dairy led to a 1.7% increase in
dairy consumption and overall protein intake by 1.25% annually in the US
after the 1890s.
People were not only consuming these perishables because it
became easier for they themselves to store them, but because the
innovations in refrigerated transportation and storage led to less
spoilage and waste, thereby driving the prices of these products down.
Refrigeration accounts for at least 5.1% of the increase in adult
stature (in the US) through improved nutrition,
and when the indirect effects associated with improvements in the
quality of nutrients and the reduction in illness is additionally
factored in, the overall impact becomes considerably larger.
Recent studies have also shown a negative relationship between the
number of refrigerators in a household and the rate of gastric cancer
mortality.
Current applications of refrigeration
Probably the most widely used current applications of refrigeration are for air conditioning
of private homes and public buildings, and refrigerating foodstuffs in
homes, restaurants and large storage warehouses. The use of refrigerators and walk-in coolers and freezers in kitchens, factories and warehouses for storing and processing fruits and vegetables has allowed adding
fresh salads to the modern diet year round, and storing fish and meats
safely for long periods.
The optimum temperature range for perishable food storage is 3 to 5 °C
(37 to 41 °F).
In commerce and manufacturing, there are many uses for refrigeration. Refrigeration is used to liquefy gases – oxygen, nitrogen, propane, and methane, for example. In compressed air purification, it is used to condense water vapor from compressed air to reduce its moisture content. In oil refineries, chemical plants, and petrochemical plants, refrigeration is used to maintain certain processes at their needed low temperatures (for example, in alkylation of butenes and butane to produce a high-octane
gasoline component). Metal workers use refrigeration to temper steel
and cutlery. When transporting temperature-sensitive foodstuffs and
other materials by trucks, trains, airplanes and seagoing vessels,
refrigeration is a necessity.
Dairy products are constantly in need of refrigeration,
and it was only discovered in the past few decades that eggs needed to
be refrigerated during shipment rather than waiting to be refrigerated
after arrival at the grocery store. Meats, poultry and fish all must be
kept in climate-controlled environments before being sold. Refrigeration also helps keep fruits and vegetables edible longer.
One of the most influential uses of refrigeration was in the development of the sushi/sashimi industry in Japan.
Before the discovery of refrigeration, many sushi connoisseurs were at
risk of contracting diseases. The dangers of unrefrigerated sashimi were
not brought to light for decades due to the lack of research and
healthcare distribution across rural Japan. Around mid-century, the Zojirushi
corporation, based in Kyoto, made breakthroughs in refrigerator
designs, making refrigerators cheaper and more accessible for restaurant
proprietors and the general public.
Methods of refrigeration
Methods of refrigeration can be classified as non-cyclic, cyclic, thermoelectric and magnetic.
This refrigeration method cools a contained area by melting ice, or by sublimating dry ice.
Perhaps the simplest example of this is a portable cooler, where items
are put in it, then ice is poured over the top. Regular ice can maintain
temperatures near, but not below the freezing point, unless salt is
used to cool the ice down further (as in a traditional ice-cream maker). Dry ice can reliably bring the temperature well below water freezing point.
This consists of a refrigeration cycle, where heat is removed from a
low-temperature space or source and rejected to a high-temperature sink
with the help of external work, and its inverse, the thermodynamic power cycle.
In the power cycle, heat is supplied from a high-temperature source to
the engine, part of the heat being used to produce work and the rest
being rejected to a low-temperature sink. This satisfies the second law of thermodynamics.
A refrigeration cycle describes the changes that take place in the refrigerant as it alternately absorbs and rejects heat as it circulates through a refrigerator. It is also applied to heating, ventilation, and air conditioning HVACR work, when describing the "process" of refrigerant flow through an HVACR unit, whether it is a packaged or split system.
Heat naturally flows from hot to cold. Work
is applied to cool a living space or storage volume by pumping heat
from a lower temperature heat source into a higher temperature heat
sink. Insulation is used to reduce the work and energy
needed to achieve and maintain a lower temperature in the cooled space.
The operating principle of the refrigeration cycle was described
mathematically by Sadi Carnot in 1824 as a heat engine.
The vapor-compression cycle is used in most household refrigerators as well as in many large commercial and industrial refrigeration systems. Figure 1 provides a schematic diagram of the components of a typical vapor-compression refrigeration system.
The thermodynamics of the cycle can be analyzed on a diagram as shown in Figure 2. In this cycle, a circulating refrigerant such as a low boiling hydrocarbon or hydrofluorocarbons enters the compressor as a vapour. From point 1 to point 2, the vapor is compressed at constant entropy and exits the compressor as a vapor at a higher temperature, but still below the vapor pressure at that temperature. From point 2 to point 3 and on to point 4, the vapor travels through the condenser
which cools the vapour until it starts condensing, and then condenses
the vapor into a liquid by removing additional heat at constant pressure
and temperature. Between points 4 and 5, the liquid refrigerant goes
through the expansion valve (also called a throttle valve) where its pressure abruptly decreases, causing flash evaporation and auto-refrigeration of, typically, less than half of the liquid.
That results in a mixture of liquid and vapour at a lower
temperature and pressure as shown at point 5. The cold liquid-vapor
mixture then travels through the evaporator coil or tubes and is
completely vaporized by cooling the warm air (from the space being
refrigerated) being blown by a fan across the evaporator coil or tubes.
The resulting refrigerant vapour returns to the compressor inlet at
point 1 to complete the thermodynamic cycle.
The above discussion is based on the ideal vapour-compression
refrigeration cycle, and does not take into account real-world effects
like frictional pressure drop in the system, slight thermodynamic irreversibility during the compression of the refrigerant vapor, or non-ideal gas behavior, if any. Vapor compression refrigerators can be arranged in two stages in cascade refrigeration systems, with the second stage cooling the condenser of the first stage. This can be used for achieving very low temperatures.
More information about the design and performance of vapor-compression refrigeration systems is available in the classic Perry's Chemical Engineers' Handbook.
In the early years of the twentieth century, the vapor absorption cycle using water-ammonia systems or LiBr-water
was popular and widely used. After the development of the vapor
compression cycle, the vapor absorption cycle lost much of its
importance because of its low coefficient of performance
(about one fifth of that of the vapor compression cycle). Today, the
vapor absorption cycle is used mainly where fuel for heating is
available but electricity is not, such as in recreational vehicles that carry LP gas. It is also used in industrial environments where plentiful waste heat overcomes its inefficiency.
The absorption cycle is similar to the compression cycle, except
for the method of raising the pressure of the refrigerant vapor. In the
absorption system, the compressor is replaced by an absorber which
dissolves the refrigerant in a suitable liquid, a liquid pump which
raises the pressure and a generator which, on heat addition, drives off
the refrigerant vapor from the high-pressure liquid. Some work is needed
by the liquid pump but, for a given quantity of refrigerant, it is much
smaller than needed by the compressor in the vapor compression cycle.
In an absorption refrigerator, a suitable combination of refrigerant and
absorbent is used. The most common combinations are ammonia
(refrigerant) with water (absorbent), and water (refrigerant) with
lithium bromide (absorbent).
The main difference with absorption cycle, is that in adsorption cycle, the refrigerant (adsorbate) could be ammonia, water, methanol, etc., while the adsorbent is a solid, such as silica gel, activated carbon, or zeolite, unlike in the absorption cycle where absorbent is liquid.
The reason adsorption refrigeration technology has been
extensively researched in recent 30 years lies in that the operation of
an adsorption refrigeration system is often noiseless, non-corrosive and
environment friendly.
Gas cycle
When the working fluid is a gas that is compressed and expanded but does not change phase, the refrigeration cycle is called a gas cycle. Air
is most often this working fluid. As there is no condensation and
evaporation intended in a gas cycle, components corresponding to the
condenser and evaporator in a vapor compression cycle are the hot and
cold gas-to-gas heat exchangers in gas cycles.
The gas cycle is less efficient than the vapor compression cycle because the gas cycle works on the reverse Brayton cycle instead of the reverse Rankine cycle.
As such, the working fluid does not receive and reject heat at constant
temperature. In the gas cycle, the refrigeration effect is equal to the
product of the specific heat of the gas and the rise in temperature of
the gas in the low temperature side. Therefore, for the same cooling
load, a gas refrigeration cycle needs a large mass flow rate and is
bulky.
Because of their lower efficiency and larger bulk, air cycle coolers are not often used nowadays in terrestrial cooling devices. However, the air cycle machine is very common on gas turbine-powered jet aircraft
as cooling and ventilation units, because compressed air is readily
available from the engines' compressor sections. Such units also serve
the purpose of pressurizing the aircraft.
Thermoelectric refrigeration
Thermoelectric cooling uses the Peltier effect to create a heat flux between the junction of two types of material. This effect is commonly used in camping and portable coolers and for cooling electronic components
and small instruments. Peltier coolers are often used where a
traditional vapor-compression cycle refrigerator would be impractical or
take up too much space, and in cooled image sensors as an easy, compact
and lightweight, if inefficient, way to achieve very low temperatures,
using two or more stage peltier coolers arranged in a cascade refrigeration
configuration, meaning that two or more Peltier elements are stacked on
top of each other, with each stage being larger than the one before it,
in order to extract more heat and waste heat generated by the previous
stages. Peltier cooling has a low COP (efficiency) when compared with
that of the vapor-compression cycle, so it emits more waste heat (heat
generated by the Peltier element or cooling mechanism) and consumes more
power for a given cooling capacity.
A strong magnetic field is applied to the refrigerant, forcing
its various magnetic dipoles to align and putting these degrees of
freedom of the refrigerant into a state of lowered entropy.
A heat sink then absorbs the heat released by the refrigerant due to
its loss of entropy. Thermal contact with the heat sink is then broken
so that the system is insulated, and the magnetic field is switched off.
This increases the heat capacity of the refrigerant, thus decreasing
its temperature below the temperature of the heat sink.
Because few materials exhibit the needed properties at room temperature, applications have so far been limited to cryogenics and research.
Other methods
Other methods of refrigeration include the air cycle machine used in aircraft; the vortex tube used for spot cooling, when compressed air is available; and thermoacoustic refrigeration using sound waves in a pressurized gas to drive heat transfer and heat exchange; steam jet cooling
popular in the early 1930s for air conditioning large buildings;
thermoelastic cooling using a smart metal alloy stretching and relaxing.
Many Stirling cycle heat engines can be run backwards to act as a refrigerator, and therefore these engines have a niche use in cryogenics. In addition, there are other types of cryocoolers such as Gifford-McMahon coolers, Joule-Thomson coolers, pulse-tube refrigerators and, for temperatures between 2 mK and 500 mK, dilution refrigerators.
Elastocaloric refrigeration
Another potential solid-state refrigeration technique and a relatively new area of study comes from a special property of super elastic materials. These materials undergo a temperature change when experiencing an applied mechanical stress (called the elastocaloric effect). Since super elastic materials deform reversibly at high strains, the material experiences a flattened elastic region in its stress-strain curve caused by a resulting phase transformation from an austenitic to a martensitic crystal phase.
When a super elastic material experiences a stress in the austenitic phase, it undergoes an exothermicphase transformation
to the martensitic phase, which causes the material to heat up.
Removing the stress reverses the process, restores the material to its
austenitic phase, and absorbs heat from the surroundings cooling down the material.
The most appealing part of this research is how potentially
energy efficient and environmentally friendly this cooling technology
is. The different materials used, commonly shape-memory alloys,
provide a non-toxic source of emission free refrigeration. The most
commonly studied materials studied are shape-memory alloys, like nitinol and Cu-Zn-Al. Nitinol is of the more promising alloys with output heat at about 66 J/cm3 and a temperature change of about 16–20 K. Due to the difficulty in manufacturing some of the shape memory alloys, alternative materials like natural rubber have been studied. Even though rubber may not give off as much heat per volume (12 J/cm3
) as the shape memory alloys, it still generates a comparable
temperature change of about 12 K and operates at a suitable temperature
range, low stresses, and low cost.
The main challenge however comes from potential energy losses in the form of hysteresis,
often associated with this process. Since most of these losses comes
from incompatibilities between the two phases, proper alloy tuning is
necessary to reduce losses and increase reversibility and efficiency.
Balancing the transformation strain of the material with the energy
losses enables a large elastocaloric effect to occur and potentially a
new alternative for refrigeration.
Fridge Gate
The
Fridge Gate method is a theoretical application of using a single logic
gate to drive a refrigerator in the most energy efficient way possible
without violating the laws of thermodynamics. It operates on the fact
that there are two energy states in which a particle can exist: the
ground state and the excited state. The excited state carries a little
more energy than the ground state, small enough so that the transition
occurs with high probability. There are three components or particle
types associated with the fridge gate. The first is on the interior of
the refrigerator, the second on the outside and the third is connected
to a power supply which heats up every so often that it can reach the E
state and replenish the source. In the cooling step on the inside of the
refrigerator, the g state particle absorbs energy from ambient
particles, cooling them, and itself jumping to the e state. In the
second step, on the outside of the refrigerator where the particles are
also at an e state, the particle falls to the g state, releasing energy
and heating the outside particles. In the third and final step, the
power supply moves a particle at the e state, and when it falls to the g
state it induces an energy-neutral swap where the interior e particle
is replaced by a new g particle, restarting the cycle.
Passive systems
When combining a passive daytime radiative cooling system with thermal insulation and evaporative cooling,
one study found a 300% increase in ambient cooling power when compared
to a stand-alone radiative cooling surface, which could extend the shelf life of food by 40% in humid climates and 200% in desert climates
without refrigeration. The system's evaporative cooling layer would
require water "re-charges" every 10 days to a month in humid areas and
every 4 days in hot and dry areas.
Capacity ratings
The refrigeration capacity of a refrigeration system is the product of the evaporators' enthalpy rise and the evaporators' mass flow rate.
The measured capacity of refrigeration is often dimensioned in the unit
of kW or BTU/h. Domestic and commercial refrigerators may be rated in
kJ/s, or Btu/h of cooling. For commercial and industrial refrigeration
systems, the kilowatt (kW) is the basic unit of refrigeration, except in North America, where both ton of refrigeration and BTU/h are used.
A refrigeration system's coefficient of performance
(CoP) is very important in determining a system's overall efficiency.
It is defined as refrigeration capacity in kW divided by the energy
input in kW. While CoP is a very simple measure of performance, it is
typically not used for industrial refrigeration in North America. Owners
and manufacturers of these systems typically use performance factor
(PF). A system's PF is defined as a system's energy input in horsepower
divided by its refrigeration capacity in TR. Both CoP and PF can be
applied to either the entire system or to system components. For
example, an individual compressor can be rated by comparing the energy
needed to run the compressor versus the expected refrigeration capacity
based on inlet volume flow rate. It is important to note that both CoP
and PF for a refrigeration system are only defined at specific operating
conditions, including temperatures and thermal loads. Moving away from
the specified operating conditions can dramatically change a system's
performance.
Air conditioning systems used in residential application typically use SEER (Seasonal Energy Efficiency Ratio)for the energy performance rating. Air conditioning systems for commercial application often use EER (Energy Efficiency Ratio) and IEER (Integrated Energy Efficiency Ratio) for the energy efficiency performance rating.
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