Phase change air conditioning

Phase c​hange - air conditioning

The following method allows cheap off peak electricity to produce "coolth" at night, for air conditioning during the day.

Phase change - latent heat

When a substance is heated it normally becomes hotter.

However, if it is changing phase: solid to liquid, or liquid to vapour, then it will absorb heat without changing temperature, until the phase change is completed.

The molecular explanation is simple. The molecules are locked in a crystal and have to be vibrated out by vibrating enough to break the bonds.

When a liquid is evaporating the molecules have to move enough to break free of the surface tension holding it in the liquid.

The heat is called latent heat. It is latent, or waiting, and not sensible, because it cannot be sensed by the senses. 

The terms are latent heat of fusion, and latent heat of vaporisation (or latent heat of vaporization). The units are kJ/kg.

 

Water has the highest latent heat of any substance, both for ice to water, and water to steam. 

The same amount of heat energy required to completely melt 1 kilogram of ice at 0oC, is sufficient to increase the temperature of 1 kilogram of water from 0oC to 80oC. And the heat to evaporate or vaporise water would be enough to heat it to 540oC

However, the temperature may not always be the best for storing cold, or heat, so other substance will often be used.

All other substances with an exact MP or BP will have latent heat and can be used for storage.

Credit: Clive Freeman/Biosym technologies science photo library

 

  

Phase Change Materials - PCM

Older examples are: Glauber's salt, Soda Ash, Sodium acetate, Calcium Chloride and paraffin wax are the most commonly used PCM or "Thermal Salts" in solar heating systems. 

However these materials have problems of supercooling etc, so newer materials have been developed.

A company PCP Aust. makes mixtures to store latent heat at different melting points. On the right is a list of their products.

 

Product ID

Latent H

KJ/Kg

Temp range

oC

PC58
226
53 to 63
PC29
188
24 to 34
PC25
150
20 to 30
PC17
145
14 to 20
PC14
145
9 to 19
PC0
333
-4 to 4
PC-4
286
-1 to -9
PC-10
283
-5 to -15
PC-16
289
-11 to -21
PC-21
222
-16 to -26

Phase change air conditioning.

Air conditioners are at their worst in the heat of the day.

The temperature difference is greatest

The electricity as at maximum price.

The way around this is to run the air conditioning at night when the air is coolest, and the electricity is one quarter the price. Cool air can freeze a phase change material. The "coolth" can be stored in frozen solid.

Then during the heat of the day a fan passes hot air over the frozen substance and heat is absorbed in melting it. The air is cooled in the process.

PCP Aust

One idea is to use ice as a clathrate allowing it to freeze at temperatures above zero.