You know that chill you feel on your face on a foggy day? Well, it’s caused by the heat released by your body to the small airborne droplets of moisture that touch your skin and evaporate.
This is evaporative cooling.
Evaporative cooling exploits the natural evaporation of water to provide cooling.
Water naturally evaporates into vapour as long as the air it is in contact with is not saturated, otherwise the air will not accept any more water in the form of vapour, i.e. no more evaporation (cooling) will take place. This means that the air conditions (temperature and humidity) influence the evaporation of water and its cooling effect.
The cooling effect is greater when more water evaporates, therefore, depending on the application and the heat gain to compensate for, it is just a matter of evaporating the right amount of water to provide cooling and reduce (avoid) chiller workload. As regards cooling potential, 1 litre (1 kg) of water that evaporates in 1 hour generates a cooling effect of approx. 0.69 kW: this comes from the water’s latent heat of vaporisation, approx. 2500 kJ/kg, absorbed from the surrounding air in 3600 s (= 1 hour), which corresponds to 2500 (kJ/kg) / 3600 s 0.69 kW/kg. If more cooling were needed, and the air were able to absorb the corresponding amount of vapour, more water would need to evaporate; for instance, 10 kg/h of evaporated water provides 6.9 kW of cooling, 100 kg/h corresponds to 69 kW and so on proportionally.
It is thus evident how evaporative cooling reduces the temperature and increases the humidity content of the air into which water evaporates.
This technique can be used advantageously by evaporating water directly into a supply air stream in dry outdoor conditions (Direct Evaporative Cooling, DEC). On the other hand, if the outdoor conditions are already quite humid, it is better to separate the cooling “part” of the process from the humidification “part”: this can be done by evaporating water into a secondary air stream (e.g. the exhaust air from a ventilation system) before it enters a cross-flow plate heat exchanger or goes through a run-around coil, as the heat exchanger (coil) thus transfers only the cooling effect to the supply air stream without increasing its humidity; if using a cross-flow heat exchanger, recent solutions also wet the secondary ducts in order to exploit the considerable sensible cooling effect on the primary air stream from the water that directly evaporates from the ducts. This second method of evaporative cooling is called indirect evaporative cooling (IEC), as water cools the supply air indirectly via a heat exchanger, regardless of whether this is dry or wet.
The evaporative cooling process is cost effective when the devices that apply it use much less energy than the chillers that would provide the same cooling; its energy-saving potential can be increased by combining DEC/IEC with free cooling, the technique that uses fresh outdoor air to handle part of the cooling load. Obviously, the initial investment must be taken into account in order to understand the potential break-even point (BEP) arising from the savings in electricity.
Also worth highlighting is the fact that the amount of water used by evaporative coolers can be offset by the reduction in ground water for cooling taken in by the power plant, as the chillers will consume less electricity. For the same reason, evaporative cooling can reduce the amount of CO2 generated by the power plant.
DEC - Direct Evaporative Cooling
The attribute “Direct” indicates that water is being sprayed into and evaporates directly in the supply air stream (HU in the picture below): the temperature of the supply air decreases and its humidity increases (approx. -2.5 °C for every additional 1 gvapour/kgdry air). DEC is suitable when adiabatic humidification is feasible for the application and/or the humidity of the supply air can be increased.
DEC can also be deployed by directly atomising water into the controlled environment.
Direct Evaporative Cooling (DEC): HU atomises water directly into the supply air stream, which is cooled and humidified
The cooling effect is approx. 0.69 kWcooling/(kg/h) of evaporated water, which corresponds to the latent heat of vaporisation of water. The total amount of cooling over the year depends both on the application itself, and also on the outdoor conditions: as a general rule, the drier and warmer the air, the more total cooling DEC can provide; vice versa for hot and humid climates. The cooling provided by DEC reduces chiller workload, and thus its power consumption, which translates into energy savings, provided that the DEC energy input and the additional consumption caused by the droplet separator pressure drop (always needed for water atomisers) are less than that of the chiller. A dedicated tool is required to estimate the overall energy savings based on annual climatic conditions of the location in question and the application characteristics (set point, minimum ventilation rates, heat gains, no. of people in the space, etc.)
It is often said that DEC uses too much water. There is no doubt that water is being used, but it’s also true that overall water consumption can be reduced if taking into account that less cooling ground water is used at the electricity power plant.
IEC - Indirect Evaporative Cooling
Water is sprayed into a secondary air stream (e.g. exhaust air) before a cross-flow heat exchanger (rack 1+3 in the picture below): as the heat exchanger only transfers sensible energy, the supply air (e.g. outdoor air) is sensibly cooled without increasing its humidity. The “box” including the heat exchanger and IEC can be considered as a “virtual chiller”:
Rack 1+3 evaporates water into the exhaust air and its cooling only is passed on to the supply air stream, as the plate heat exchanger intercepts humidification
IEC is suitable where adiabatic humidification and/or the increase in humidity of the supply air are not acceptable, i.e. when DEC cannot be used. For instance, when the outdoor climate is already humid enough, such as in sub-tropical and tropical areas, IEC is an opportunity for saving energy. IEC is also used in data centres.
It should also be pointed out that the heat exchanger has a heat-transfer efficiency (commonly 50%-75%, in some cases up to 90+ %) that only partially transfers the cooling from the exhaust air to the incoming outdoor air. This efficiency reduces the evaporative cooling effect; however it may still be effective enough to achieve energy savings by reducing the load on the chiller-based system. Additionally, recent developments include wetting the exhaust ducts on the cross-flow heat exchanger, as the water evaporating from these further increases the cooling on the supply side. Again, in the same way as for DEC, dedicated tools are needed to evaluate the annual energy saving based on the application and local climatic conditions.
DEC + IEC
DEC and IEC can be combined to increase savings (racks 1+3 and 2+4 in the picture above). One possible control strategy, but not the only one available, consists in running DEC until the supply humidity set point is reached, and then starting IEC if more sensible cooling is required.
DEC, IEC or DEC + IEC? How to choose the best solution?
The right choice can only be made using a simulation tool able to calculate the savings based on at least outdoor conditions, set point and internal heat gain; the same tool can also provide an estimate of the sizes of the DEC and IEC systems.
Manual calculations are very difficult and not viable. As a general rule, DEC is suitable for hot and dry climates because in such places humidification tends to be needed in addition to cooling; on the other hand, IEC is suitable for hot and humid climates as it does not increase the humidity of the already humid outdoor air, or for cooling data centres without introducing outdoor air as described above under IEC; finally, the combination DEC + IEC appears to be suitable for a range of climatic conditions, as it can be used both for dry and humid outdoor air.
Whatever the case, evaporative cooling means “green” cooling, because the refrigerant used is water, which does not pollute the environment nor increases the global warming: in fact, after evaporating, the water simply precipitates back down to become ground water again, thus closing the natural water cycle.
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