Adiabatic Cooling (Evaporative Cooling)

Humidifying the extract air upstream of the heat exchanger cools the incoming outside air without active refrigeration — up to 8 K of cooling for free.

How does adiabatic cooling work?

Adiabatic (evaporative) cooling uses the evaporative cooling effect of water. In the ventilation system, the extract air from the building is humidified before the heat exchanger. As the water evaporates, it draws heat from the extract air, which cools by several degrees. This cooled extract air then passes through the rotary or plate heat exchanger of the heat recovery unit, where it cools the warm outdoor air before it enters the rooms as supply air. Because the humidification takes place indirectly in the extract air, the supply air stays dry and hygienically safe. Only the humidifier pump, at around 50 W, needs electricity. The drier the outdoor air, the greater the cooling effect: on Viennese summer days with 35 to 50 % relative humidity, up to 8 K of cooling is achievable. The effective energy efficiency ratio (EER) is around 25, compared with 3.5 for conventional compression cooling.

Which buildings benefit from adiabatic cooling?

The prerequisite is an air handling unit (AHU) with heat recovery into which a humidifier module can be retrofitted. A potable water connection and hygiene monitoring against Legionella are also needed. Typical properties are office buildings, schools and assembly buildings in moderately humid climates, as found across large parts of Austria and Germany. Highly humid climate zones such as coastal regions are unsuitable, because there the air can hardly absorb any more water. Direct humidification of the supply air is to be ruled out for hygiene reasons. Adiabatic cooling covers 40 to 70 % of cooling demand during the shoulder seasons and on dry summer days; a smaller chiller remains sensible for heat peaks.

What does adiabatic cooling actually deliver?

Air conditioning energy falls by 30 to 60 %. The saving is derived from the cooling demand that can be met adiabatically and the difference between the reciprocals of the two EER values. An office building with 80,000 kWh/a cooling demand, of which 50,000 kWh is met adiabatically, saves around 12,300 kWh/a of electricity and at the same time reduces the chiller's peak load. A retrofit humidifier module costs €5,000 to €20,000 depending on plant size. With a service life of 15 years, the measure pays back in 5 to 10 years. The electricity benefit is offset by water consumption, which should be captured in monitoring.

Energy carrier

Strom (Kompressionskälte) → Strom (Befeuchter) + Wasser

Savings potential

30-60 % Klimaenergie in Übergangszeit + trockenen Sommertagen

Worked example

An office building is served by a central ventilation system with a rotary heat exchanger and has an annual cooling demand of 80,000 kWh, so far covered by a compression chiller with an EER of 3.5. A humidifier module is retrofitted on the extract air side. Given the humidity conditions at the site, 50,000 kWh per year of this can be covered adiabatically, mainly during the shoulder seasons and on dry summer days. The adiabatic cooling achieves an effective EER of 25, since only the humidifier pump requires electricity. The calculation is: E_saved = 50,000 × (1/3.5 − 1/25) = 50,000 × (0.286 − 0.040) = 12,286 kWh/a. The building therefore saves around 12,300 kWh of electricity per year. In addition, the chiller's peak load falls, so it can be sized smaller at a later replacement.

Investment & payback

Befeuchtermodul Nachrüst: 5.000-20.000 € je nach RLT-Größe · Service life: 15 a · Payback: 5-10 a

Applicability

Requirements

Exclusion criteria

Typical buildings

Frequently asked questions

What is the difference between direct and indirect adiabatic cooling?

In direct adiabatic cooling, the supply air itself is humidified, which raises indoor humidity and is problematic for hygiene. In the indirect variant, the extract air is humidified and transfers its coolness to the supply air via the heat exchanger, so the supply air stays dry. In office buildings and schools, only indirect extract air humidification is recommended.

How much electricity does adiabatic cooling save?

Adiabatic cooling reduces air conditioning energy by 30 to 60 % during the shoulder seasons and on dry summer days. An office building with 80,000 kWh/a cooling demand, of which 50,000 kWh is met adiabatically, saves around 12,300 kWh of electricity per year. The effective EER of adiabatic cooling is around 25, because only the humidifier pump at roughly 50 W needs electricity.

How much does it cost to retrofit adiabatic cooling in a ventilation system?

A humidifier module for the extract air side of an existing AHU costs €5,000 to €20,000 depending on plant size. A potable water connection and hygiene monitoring come on top. With a service life of 15 years, the payback period is 5 to 10 years. The economics improve the higher the previous cooling electricity consumption and the drier the outdoor air at the site.

Does adiabatic cooling work in high humidity?

Only to a limited extent. The cooling effect depends on how much water the extract air can still absorb. At low outdoor humidity, around 35 to 50 % in a Viennese summer, up to 8 K of cooling is possible. In highly humid climate zones such as coastal regions, performance drops sharply. A conventional chiller therefore remains necessary as a supplement for muggy days.

Which hygiene regulations apply to adiabatic cooling in Austria and Germany?

The relevant standards are VDI 6022 on the hygiene of air handling systems and ÖNORM H 6020 for ventilation technology in Austria. They require regular checks of the humidifier water, avoidance of stagnation and Legionella monitoring. VDI 3803 describes the technical design. Humidification may only take place in the extract air so that no aerosols reach the supply air.

Related measures

Standards & sources

Metering points in the EDM Toolbox

Temperatursensoren: Außen-T+rH, Zuluft, Abluft vor/nach Befeuchter. Wasser- + Stromverbrauch des Befeuchters. Vergleich Klimaanlagen-Strom mit/ohne Adiabate.

All measures