evaporative and passive cooling
TRANSCRIPT
Evaporative and Passive CoolingProf. Dr. L.C.M. Itard
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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