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)
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.
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 EngineeringStanford 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.
"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 1230Fire 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.
