Air can be effectively cooled by exploiting the evaporation of water atomised into very fine droplets: the change in state, from liquid to vapour, absorbs energy from the air, which is consequently cooled. Evaporation of 100 kg/h of water absorbs 69 kW of heat from the air, for power consumption of less than 1 kW!. In an air handling unit, the supply air can be evaporatively cooled and humidified (direct evaporative cooling, DEC). Alternatively, if the outside air humidity is already high, the exhaust air can be cooled by several degrees without limits in terms of humidity, as it is discharged by the AHU; this cooling capacity, using a heat exchanger, can be used to cool the incoming fresh air with an efficiency that depends on the heat recovery unit used, yet easily exceeds 50%! (indirect evaporative cooling, IEC). All this means lower unit energy consumption and smaller dimensions and capacity of the cooling coil and chiller.

2. ENERGY SAVING BY
EVAPORATIVE COOLING IN AHUs
Raul Simonetti – CAREL Industries SpA (Italy)
1. Natural water evaporation & cooling
2. Water atomizers
3. DEC & IEC
4. Energy Saving by DEC/IEC in Australia

3. Natural water evaporation & cooling (1/2)
• Water naturally evaporates when in contact with air
• It requires energy: the latent heat of vaporization
• Latent heat of vaporization  690 W/(L/h)
• This heat is drawn also from the air, which is
humidified and cooled (evaporative cooling)

4. Natural water evaporation & cooling (2/2)
• The evaporation rate (L/h) is “proportional” to the
contact surface air-water
• The wider the surface, the stronger the
evaporation, thus the heat taken from the air and
the evaporative cooling
– 1 L/h  0.69 kW of cooling
– 100 L/h  69 kW of cooling
– and so on

5. Water atomizers: description
• They spray water in very tiny drops (5-50 µm)
• The smaller the drops, the wider their total
surface, the stronger the evaporation and the
evaporative cooling
• Small drops is better!
• Input power: 0.5-10 W/(L/h)

6. Evaporation in AHUs/ducts (1/2)
• Tiny drops fly with the air, do not fall
• Evaporation takes time, thus some drops may not
fully evaporate before the first device
downstream (coil, blower, etc.)
• Evaporation efficiency η = evaporated/sprayed
water = 50%-95% depending on models and
conditions
°C before g/kg before m³/h m/s µm
         
η          
Key:
 = the characteristic increases
 = the characteristic decreases

7. Evaporation in AHUs/ducts (2/2)
• Non-evaporated drops must be collected and
drained not to wet downstream
• Drop separator (30-70 Pa) and drained drop pan
are required

8. WUE: Water-Usage Effectiveness
• WUE = evaporated water / input mains water
• WUE = (evap.’d w. / sprayed w.) x (sprayed w. / input mains w.)
• Typ. WUE with RO system: 10% to 48%
• Typ. WUE with softener: 48% to 95%
Water
treatment
Supply waterMains water
Drain

9. DEC: Direct Evaporative Cooling (1/2)
Evaporation in supply air : cooling + humidification

10. DEC: Direct Evaporative Cooling (2/2)
Overall water consumption may be reduced
To yield 1 kWhcooling, a chiller–based system uses:
EERSI = 3 kWcooling/kWelectric
[e] = 0.3 kWhelectric
(= 1 / 3)
Combined value for the water withdrawal of power plants
in Australia:
46.0 L/kWhelectric
1 kWhcooling generated by a chiller system requires: [a] = 15.3 L of mains w.
(= 1/3 x 46)
If DEC does 1 kWhcooling, the evaporated water in air is:
1 L of evaporated water  0.69 kWhcooling
1.4 L
(= 1 / 0.69)
The amount of mains water supplied to the water-spraying
system is:
WUE of the water atomizer is equal to 80%
[b] = 1.8 L
(= 1.4 / 0.80)
Input energy to DEC @ 10 W/(L/h) [c] = 18 Wh
(= 1.8 x 10)
Input energy saved [e-c] 0.282 kWhelectric/kWhcooling
Mains water SAVED with DEC [a-b] 13.5 L/kWhcooling

11. IEC: Indirect Evaporative Cooling (1/2)
• Evaporation in exhaust before heat exchanger (cross-flow
or run-round coil): 1 + 3 below
• Only sensible cooling is passed on to the supply
• To avoid humidification (e.g. suitable for tropical climate)

12. IEC: Indirect Evaporative Cooling (2/2)
Overall water consumption may be reduced
To yield 1 kWhcooling, a chiller–based system uses:
EERSI = 3 kWcooling/kWelectric
[e] = 0.3 kWhelectric
(= 1 / 3)
Combined value for the water withdrawal of power plants
in Australia:
46.0 L/kWhelectric
1 kWhcooling generated by a chiller system requires: [a] = 15.3 L of mains w.
(= 1/3 x 46)
Heat exchanger’s efficiency: 60% 60%
If IEC does 1 kWhcooling to the outdoor air, the cooling of the
exhaust air is:
and the evaporated water in the exhaust air is:
1 L of evaporated water  0.69 kWhcooling
1.7 kWhcooling
(= 1 / 0.60)
2.5 L
(= 1.7 / 0.69)
The amount of mains water supplied to the water-spraying
system is:
WUE of the water atomizer is equal to 80%
[c] = 3.1 L
(= 2.5 / 0.80)
Input energy to IEC @ 10 W/(L/h) [d] = 31 Wh (= 3.1 x 10)
Mains water SAVED with IEC [a-c] 12.2 L/kWhcooling
Input energy saved [e-d] 0.269 kWhelectric/kWhcooling

13. DEC + IEC
• DEC sprays to the required cooling OR the set %rh
• IEC starts to do more cooling if required
• Water saving: combination

14. Estimations of Energy Saving
50%
To servers
22 °C 45 %rh
From servers
32 °C 25 %rh
• 3 data centres: 50 kW, 100 kW, 500 kW
• Adelaide, Brisbane, Canberra, Darwin, Melbourne, Perth, Sydney
• AHU: 24/7, modulating outdoor air for free cooling, EER = 3
• Water: 2.2 AUD/kL - Electricity: 0.276 AUD/kW
• Installation: 2000 AUD per DEC+IEC

15. Energy Saving by DEC/IEC: ROIs

16. Energy Saving by DEC/IEC: comments (1/3)
Internal heat loads
ROI 50 kW 100 kW 500 kW
Max 1 yr None Adelaide, Perth ALL but Darwin
> 1 to 2 yrs Adelaide, Perth Brisbane, Canberra,
Melbourne, Sydney
ALL but Darwin
> 2 to 3 yrs Canberra,
Melbourne, Sydney
None ALL but Darwin
Darwin, given its outdoor conditions, might have a convenient ROI for internal loads > 500 Kw
• Warm, not damp climate support energy saving by DEC/IEC
• The higher the internal load, the higher the saving, the better
the ROI
• Overall water saving and CO2: next slides

17. Energy Saving by DEC/IEC: comments (2/3)
• DEC/IEC uses water, but reduces electricity input
• This reduces water withdrawals by the power plants
• Overall water consumption is reduced:
CITIES Electricity saved
[kWh/yr]
DEC/IEC
[kL/yr]
Withdrawal
[kL/yr]
Water saved
[kL/yr]
Adelaide 57,520 320 -2,647 -2,327
Brisbane 27,484 327 -1,265 -938
Canberra 33,038 186 -1,520 -1,334
Darwin 5,397 201 -248 -47
Melbourne 34,622 172 -1,593 -1,421
Perth 51,058 527 -2,350 -1,823
Sydney 31,893 362 -1,468 -1,106
Key:
• Data “proportional” to 50kW heat load
• 46 L per kWhelectricity

18. Energy Saving by DEC/IEC: comments (3/3)
The saved electricity also reduces CO2:
CITIES Energy saved [kWh/yr] CO2 reduction [ton eq. /yr]
Adelaide 57,520 -49
Brisbane 27,484 -23
Canberra 33,038 -28
Darwin 5,397 -5
Melbourne 34,622 -29
Perth 51,058 -43
Sydney 31,893 -27
Key:
• Data “proportional” to 50kW heat load
• 852x10-6 ton eq. per kWhelectricity

19. DEC/IEC by water sprayers: conclusions
• Cooling from water evaporation  690 Wcooling/(L/h)
• Reduces electricity consumption because reduces
the load of the cooling coils (chillers)
• Suitable for Australia: YES
• ROIs depend on installation and location, but may
be 1-3 years for internal loads from 50 kW
• Reduces water withdrawal of power plants and
overall water consumption
• Reduces CO2 from power plants