Evaporative and Passive CoolingProf. Dr. L.C.M. Itard

Phot

o Su

n Em

its a

Sol

stic

e CM

E by

NAS

A G

odda

rd S

pace

Flig

ht C

ente

r, CC

BY

2.0

http

s://

crea

tivec

omm

ons.

org/

licen

ses/

by/2

.0/

. Cha

nges

mad

e by

Rin

sLin

dem

an, 2

020

Evaporative cooling: Principle

Image by Stux, from Pixabay

00.005

0.010.015

0.020.025

0.030.035

0 5 10 15 20

Part

ial p

ress

ure

wat

er

vapo

ur in

hum

id a

ir at

1 b

ar

[Bar

] Water vapour content in air [g/kg dry air]

Evaporative cooling: Principle

00.005

0.010.015

0.020.025

0.030.035

0 5 10 15 20

Part

ial p

ress

ure

wat

er

vapo

ur in

hum

id a

ir at

1 b

ar

[Bar

]

Water vapout content in air [g/kg dry air]

-10

0

10

20

30

40

50

0 5 10 15 20

Air t

empe

ratu

re (o C

)

Maximum Water vapour content in air at 1 bar [g/kg dry air]

2450

2460

2470

2480

2490

2500

0 0.005 0.01 0.015 0.02

Late

nt h

eat o

f vap

oriza

tion

of w

ater

(kJ/

kgK)

Pressure of water vapour[ bar]

Example: cooling of 1 kg air at 25oC by evaporation of 5 g of water. Final temperature of air?

-Qevap = 0.005 x 2488 = 12.4 kJ-Qair=-Qevap=m.Cp.ΔT =-12.4 kJ

-m= 1-0.005 =0.995 kg ≈ 1 kg (mass of air)-Cp= 1000J/kg = 1kJ/kg (specific heat of air)

-ΔT=-12.4/(1.1)=12.4 K à T =25-12.4 = 12.6oC

Evaporative cooling: examples

Portable cooler Mist machine

Wind Catcher, Yasd, iran by Ivan Mlinaric on commons-wkimedia,org, license CC-BY-2.0

Title: Portable Cooler CoolpressoAuthor: Samsung NewsroomLicense: CC BY-NC-SA 2.0

Title: sundial bridge - mist machineAuthor: Mik ScheperLicense: CC BY-NC-ND 2.0

Warm air

Pictures by TUDelft, Rins Lindeman

Warm air from outside

Cold air to rooms

Simple air handling unit by TUDelft, Rins Lindeman

Passive cooling: outdoor air

Cold and moderate climate countries:• Ventilate with outdoor air

when colder than indoor

All countries:• Ventilate during night with

cold night air

20 oC24 oC

Passive cooling: earth tubes

Warm air inCold air out

Heat transfer to the colder ground

Passive cooling: surface water

Warm air in 35oC 18oC

23oCair out 26oC

Heat exchanger

Primary Energy and CO2-eq emissions of evaporative and passive cooling systems

• Only pump and fan energy• Very little energy use in comparison with active systems• By approximation

• PE=0 kWh; • CO2-eq emissions=0

• For more accurate estimations, electricity needed by pump and fans must be calculated for the chosen configuration

Phot

o Su

n Em

its a

Sol

stic

e CM

E by

NAS

A G

odda

rd S

pace

Flig

ht C

ente

r, CC

BY

2.0

http

s://

crea

tivec

omm

ons.

org/

licen

ses/

by/2

.0/

. Cha

nges

mad

e by

Rin

sLin

dem

an, 2

020

SummaryPassive cooling:

• Evaporative cooling: very efficient• Used in simple systems and in AHU

• Night cooling• Earth tubes• Surface water

• Limitation in maximum temperature