5.3 Free Cooling, Economizers, Humidity & Water

Free Cooling and Economizers

Free cooling (economization) uses cool outdoor conditions to remove heat without running mechanical refrigeration compressors — the single biggest lever for lowering PUE after airflow management. In cool climates a facility can run 'chiller-off' for much of the year. There are two families.

Air-Side Economizer

An air-side economizer brings filtered outside air directly into the white space when ambient temperature and humidity fall within the ASHRAE allowable envelope, and exhausts warm air outdoors. Compressors shut off entirely. It offers the biggest savings but requires high-efficiency filtration (dust, pollutants, salt) and careful humidity control, and it exposes the room to outdoor contaminants and humidity swings. Northern-climate hyperscalers use it to reach sub-1.10 PUE in winter.

Water-Side Economizer

A water-side economizer keeps outside air out of the white space. Instead, cool cooling-tower water is passed through a plate-and-frame heat exchanger (or the chiller's condenser barrel in free-cooling/thermosiphon mode) to produce chilled water without running the chiller compressor. It avoids contamination and humidity problems but is slightly less effective than air-side because of the extra heat-exchange step. It suits climates with cold, dry winters and facilities that already use towers.

Adiabatic / Evaporative Cooling

Adiabatic (evaporative) cooling exploits latent heat: water sprayed or wicked into an airstream evaporates, absorbing heat and lowering the air's dry-bulb temperature toward its wet-bulb temperature. Direct evaporative cooling adds moisture to the supply air; indirect systems evaporate water on the outside of a heat exchanger so the IT air stays dry. Evaporative/adiabatic cooling can slash or eliminate compressor energy in hot, dry climates, but it consumes significant water — the classic trade-off between energy (PUE) and water (WUE).

Why Humidity Control Matters: ESD vs Condensation

Data-centre air must be kept within a humidity band, and the exam wants you to know both failure modes:

• Too dry (low RH): risk of electrostatic discharge (ESD). Dry air lets static charge build on people and equipment; a discharge can damage sensitive electronics. This is why RH is not allowed to fall too low.
• Too humid (high RH): risk of condensation and corrosion. If a surface or cooling coil falls below the dew point, water condenses — potentially onto live electronics — and high humidity accelerates corrosion and hygroscopic-dust problems.

The dew point is the temperature at which air becomes saturated and water begins to condense. Modern ASHRAE guidance expresses humidity limits partly in dew-point terms rather than pure RH, because RH alone is misleading across temperatures. Practically, operators keep RH within roughly 20-80% (recommended) and avoid chilling coils below the room dew point so they do not strip moisture and then have to re-humidify — a wasteful contest between dehumidification and humidification that arises when multiple units hold mismatched set points.

Water Systems and Make-Up Water

Any evaporative process — cooling towers, adiabatic coolers, humidifiers — consumes water. Make-up water replaces what is lost to evaporation, drift, and blowdown in a cooling tower. Key considerations the exam associates with tower/water systems:

• Water quality and treatment chemistry (hardness, conductivity, biocides) to prevent scale, fouling, and microbial growth.
• Supply continuity — loss of make-up water directly halts heat rejection, so backup water tanks or secondary feeds are common in resilient designs.
• Legionella control — mandatory under standards such as ASHRAE 188, because warm tower water can breed Legionella bacteria.

Water dependency is a growing sustainability and resilience concern, especially in drought-prone regions, which is why some operators deliberately choose air-cooled (waterless) plants despite lower energy efficiency.

WUE: Measuring Water Efficiency

Water Usage Effectiveness (WUE) is the water counterpart to PUE. Defined by The Green Grid and standardised in ISO/IEC 30134-9, it is:

WUE = annual site water consumption (litres) / IT equipment energy (kWh), expressed in L/kWh.

A lower WUE is better; a fully air-cooled facility can approach 0 L/kWh on-site. Source-WUE additionally counts the water used upstream to generate the facility's electricity, so a site that saves energy with evaporative cooling but draws grid power still carries an upstream water footprint. The exam pairs WUE with PUE (Total Facility Energy / IT Energy) and CUE (Carbon Usage Effectiveness, kg CO2e / IT kWh) as the three Green Grid / ISO 30134 sustainability KPIs.

The Energy-vs-Water Trade-Off

The recurring theme: economizers and evaporative cooling cut energy (PUE) but often raise water use (WUE), and vice-versa. A hot-dry site might run adiabatic cooling for a superb PUE while consuming heavy make-up water; a water-scarce site might accept a higher PUE to run waterless air-cooled chillers. CDCP scenario questions test whether you can reason about this trade-off given a climate and a stated sustainability priority.

A Worked Efficiency Example

Suppose a facility draws 2000 kW total and its IT load is 1250 kW. Its PUE = 2000 / 1250 = 1.6, meaning 0.6 W of overhead (mostly cooling, plus UPS and lighting losses) per watt of IT. If the operator seals airflow leaks, raises the cold-aisle set point to 26 °C, and enables an economizer, cooling overhead might fall so that total draw drops to 1650 kW at the same IT load, giving PUE = 1650 / 1250 = 1.32 — a large operating saving with no new IT. If that economizer is evaporative, however, the site now consumes make-up water: at, say, 4.5 million litres per year against 10.95 million IT kWh, WUE = 4,500,000 / 10,950,000 ≈ 0.41 L/kWh. The lesson the exam drives home is that a single number never tells the whole story — you weigh PUE, WUE, and CUE together against the local climate, water availability, and carbon intensity of the grid.

Free-Cooling Hours and Set Points

The number of economizer hours a site can claim each year depends directly on how high its supply temperature and how wide its allowable envelope are. Designing to ASHRAE Class A3 or A4 (allowable up to 40-45 °C) and running warm supply within the recommended band unlocks far more free-cooling hours than a conservative A1 design held at 20 °C. This is why the modern efficiency playbook is: manage airflow first (Delta-T), then raise set points within the recommended envelope, then maximise economizer operation — in that order — while keeping humidity between the ESD floor and the condensation/dew-point ceiling.