Water or Air: Better Cooling Medium?

Objective

To compare the effectiveness of air vs water as a cooling medium.

Outcome

Students will realize that water is a much better cooling medium than air. Students will realize that air conditioning is an inefficient method to cool buildings.

Requirements

None

Prerequisites

 A conceptual knowledge of the following:

1. Density
2. Specific Heat Capacity
3. Volumetric Heat Capacity
4. Thermal Conductivity
5. Functioning of Airconditioning and Water coolers

Procedure

Step 1: Find out the thermal conductivity (in W/m-K) of

1. air
2. water

Step 2: Estimate the thermal conductivity multiplier for water and air by dividing the higher thermal conductivity value with the lower one.

Step 3: Find out the density (in kg/m³). of 

1. air
2. water 

Step 4: Find out the specific heat capacity (in kW/kg-K) of

1. air
2. water

Step 5: Estimate the volumetric heat capacity (i.e multiplier of density and specific heat capacity) (in kW/m³-K) of

1. air
2. water

Step 6: Estimate the volumetric heat capacity multiplier for water and air by dividing the higher volumetric heat capacity value with the lower one.

Step 7: Ask the students: ‘Which one of water and air can be a better medium for cooling spaces?’ 

Calculations

Table

	Thermal Conductivity (K) (W/m-K)	Specific Heat Capacity (Cp) (kW/kg-K)	Density (⍴) (kg/m3)	Volumetric Heat Capacity (Cpx⍴) (VP) (kW/m3-K)
Air	0.03	1.004	1.225	1.23
Water	0.6	4.186	1000	4186

Step 1: Thermal Conductivity Multiplier = ^{K_Water(W⁄_m-K)}⁄_{KAir(^W⁄m-K)}

= 0.60.03

= 20 times

Hence, Thermal Conductivity of water is 20 times that of air.

Step 2: Volumetric Heat Capacity Multiplier = ^{V_P-Water(kW⁄_m3-K)}⁄_{VP-Air(^kW⁄m³-K)}

= 41861.23

= 3403 times 

Hence, Volumetric Heat Capacity of water is approximately 3400 times that of air.

Step 8: Discuss the following questions to students:

• As the calculations infer that Water is a better conductor of heat, with more density and Volumetric Heat Capacity, why do we use ‘air’ to cool our buildings and why are water coolers used less?
• How did an entire industry get established using this inefficient medium?

References

1. Fairconditioning: Radiant Cooling Animation https://www.youtube.com/watch?v=dYjMBfORftk
2. https://www.theatlantic.com/technology/archive/2017/08/the-moral-history-of-air-conditioning/536364/