Use of water as a refrigerant

 

Water is an efficient refrigerant. Cloud cooling can cool water droplets to temperatures as low as  -70 degree Celsius. To effectively use water as a refrigerant we need to understand how it evaporates. 

How does water evaporate?

Water does not need to reach 100 degree Celsius before evaporation. Water is constantly evaporating in seas at temperatures between 0-35 degree Celsius. The reason for this is bubbles that are created at the surface of air water interface due to continual wave motion. 

[Bubbles have been observed when water is stretched to pressures below its vapor pressure. This is about 3 kpa for 25C ]

[Bubbles can also form when air is mixed with water as is the case in the seas where wind velocities are high]

These bubbles implode leading to pressures as large as 2.1 GPA breaking the hydrogen bonds in water & causing it to vaporize. 

https://akshatjiwannotes.blogspot.com/2024/10/what-is-maximum-wall-pressure-developed.html?m=1

Inherent advantages of impure water,Large negative pressure sustained by pure water its significance

Impure water has dissolved particles that can decrease its tensile strength and reduce the pressure needed for it to form vapor bubbles. This is advantageous in the refrigeration process because it will help reduce the energy consumed by the suction unit. 

Pure water in the other hand can sustain tension of upto 200Mpa without breaking. Therefore pure water would need much higher energy to perform as a refrigerant. 

Decrease in pressure required to boil going from Mega Pascal to kilo Pascal as dissolved impurities increase. 

Application of water as a refrigerant

While cloud cooling is a good model for analysing the use of water as a refrigerant a few things need to be kept in mind

Cooling in clouds can simply be achieved by raising of vapour above the ground.  6° drop per kilometer it gains in height. 

Further cooling can be achieved due to large area of expansion available 

Neither of these are true for refrigeration system and therefore external work needs to be done

A suction unit can be used to create negative pressures inside impure water to achieve fast efficient evaporation at low pressures followed by usual refrigeration cycle.

Furthermore the material properties of water can be easily tweaked by addition of particles (gas or solid) that can lower its tensile strength from MPa range to kilo Pascal range helping the cause of cavitation and later condensation (in which particles act as condensation nuclei)

A typical refrigeration cycle will look like this. 

Water moves into the heat exchanger section. It absorbs heat. 

Next it moves through an evaporator.  The evaporator section is kept at a negative pressure. It will cause water to cavitate. 

The bubbles will absorb heat from the fluid as they nucleate. This will decrease the temperature of bulk water. 

The chilled water will circulate around leading to the temperature drop in the unit. 

Brine is already used as a refrigerant. Can we achieve better results?

Temperatures less than 10 degree Celsius should be achievable due to super cooling of droplets

In the condenser section we can play with relative humidity to achieve condensation of vapor to liquid phase at significantly low pressures and temperatures.

Challenges 

Can efficient suction units be designed?

 Will the entire set up need to be put in a vacuum?

At standard temperature & pressure the boiling point of water is 100 degree Celsius but evaporation of H2O is more Complex and can occur by

Air water mixing leading to bubble formation and collapse causing consequent cavitation

Dissolved air decreases tensile strength of water causing bubble nucleation at decreased pressures

Terrestrial evaporation of water is almost always a cause of bubble cavitation as temperatures never reach 100 degree Celsius

Terrestrial cavitation is always aided by dissolved air because pure water can sustain extremely large tensile pressures in the range of tens of megapascals 220 40 Mpa based on different research Publications

Impure water will be a better refrigerant then pure water because it will be able to cavitate easily and be evaporated at temperatures far less than boiling point of water.In fact at any temperature less than the freezing point.

Ammonia versus water

Pure ammonia boils at - 33 degree Celsius what is usually available in the market is aq. solution of ammonia or its compounds so they are not a good representation of ammonia properties. 

Because ammonia boils at low temperatures it is effective as a refrigerant and can participate in usual refrigeration cycle 

[gas expansion leading to cooling -cool gas through heat exchanger leading to heating -hot gas moving through compressors then passing through condenser to expansion valve to repeat the process.]

The reason for low boiling point of liquid ammonia and low melting point of solid ammonia is due to less number of hydrogen bonds than water

 The difference is quite Stark- 33 degree Celsius for ammonia and 100 degree Celsius for water. 

Ammonia though does not fully solve the problem for a refrigerant. It is much better than complex fluorocarbons and can be produced in large quantities for mass usage. It satisfies the most common cooling requirements for households and industrial usage. 

While it can't perform cryogenic cooling none of the other refrigerants work for that temperature range either and nitrogen hydrogen, Helium, are necessary for those applications. Or air works as well in case of a Stirling cryocooler

But even something as simple as ammonia has problems related to its manufacturing some of which are

Extremely high energy consuming production process

Usage of ammonia as a refrigerant will have to complete with ammonia the fertilizer and ammonia the fuel

present ammonia production requires the use of fossil fuels for hydrogen generation

The refrigerant problem 

An alternative is carbon dioxide which is quite easy to create and has few industrial uses like in lasers.

But this brings to the second problem which is handling of ammonia and carbon dioxide for use as a refrigerant

Both ammonia and carbon dioxide need to be converted into a liquid form in their pure state as their use these compounds require pressure and Mega Pascal range to be converted into liquid at ambient temperatures demanding more powerful compressors and more energy . 

References

An Investigation into the Feasibility of the Use of

Water as a Refrigerant

https://scholar.google.co.in/scholar_url?url=https://docs.lib.purdue.edu/cgi/viewcontent.cgi%3Farticle%3D1722%26context%3Diracc&hl=en&sa=X&ei=8vpGZ-GiGqjB6rQPz6e92Ac&scisig=AFWwaea0dC-JrsmqMHvCIumj7A2B&oi=scholarr

The Use of Water as a Refrigerant: 

Impact of Cycle Modifications on Commercial Feasibility

https://minds.wisconsin.edu/handle/1793/7614

DEVELOPMENT OF A WATER VAPOR COMPRESSOR FOR HIGH 

TEMPERATURE HEAT PUMP APPLICATIONS

https://nhsoft.dk/work/FG26/Okt/GL2014Paper003_MRI.pdf

Water as a Refrigerant in Centrifugal Compressor Cooling Systems for Industrial Applications

https://www.intechopen.com/chapters/62663

A comparative study of water as a refrigerant with some

current refrigerants

https://onlinelibrary.wiley.com/doi/abs/10.1002/er.1084

A review of carbon dioxide as a refrigerant in refrigeration technology

https://www.researchgate.net/publication/282513706_A_review_of_carbon_dioxide_as_a_refrigerant_in_refrigeration_technology

brines a large scale cooling solution

https://www.coolingindia.in/brines-a-large-scale-cooling-solution/

The commercial feasibility of water vapor as a refrigerant (Lachner, et al 2007).

https://www.researchgate.net/publication/239342952_The_commercial_feasibility_of_the_use_of_water_vapor_as_a_refrigerant_International_Journal_of_Refrigeration_304_699-708

Using water as a refrigerant in a commercial

chiller

https://www.cibsejournal.com/technical/echiller-using-water-as-a-refrigerant/

Water As A Refrigerant Oxycom Fresh Air BV

https://www.oxy-com.com/blog-news/water-as-a-refrigerant