Cooling and Control Systems

When free cooling stops paying off: payback by Mexican climate zone

5 min read By Noxtel
Ilustración: Cuándo el free cooling deja de ser rentable: payback por zona climática mexicana

Free cooling uses outside air to cool the DC when the ambient temperature and humidity allow it. It drastically reduces the electrical consumption of the cooling system. The question is not whether it works, but in which Mexican zones it is cost-effective and when it stops being so.

Cost-effectiveness depends on the annual hours during which outdoor conditions allow direct, partial, or indirect free cooling. Each Mexican climate zone has a different profile.

How free cooling works in a DC

A traditional cooling system cools the DC air through chillers that consume a lot of electricity. Free cooling incorporates outside air into the cooling process when its conditions (temperature and humidity) are compatible with the DC operating ranges, defined by ASHRAE.

  • Direct free cooling (outside air): fans draw outside air directly into the room. Requires filtration and humidification/dehumidification.
  • Indirect free cooling (air-to-water or air-to-glycol): a heat exchanger transfers the cold from outside air to the internal water or glycol loop without mixing the two air streams.
  • Partial free cooling: the mixed system uses mechanical cooling and free cooling simultaneously to reach the setpoint.

    • Mexican climate zones relevant to a DC

    Mexico has notable climatic diversity. For free cooling purposes in data centers, it makes sense to group by temperature and humidity ranges throughout the year.

  • Temperate zone (Mexico City, Puebla, Toluca, Guadalajara in high areas): average annual temperature between 12 and 22 °C, moderate humidity. Extended free cooling period, between 7 and 10 months per year.
  • Mediterranean/cool semi-arid zone (Tijuana): average temperature between 14 and 22 °C with low humidity, large night-time thermal differential due to Pacific influence. Excellent potential for night-time free cooling, between 9 and 11 months per year with good heat exchanger performance.
  • Semi-desert zone (Monterrey, Chihuahua, Hermosillo, Saltillo): average temperature between 18 and 28 °C, low humidity. Free cooling viable between 4 and 6 months per year, given that desert summer nights exceed the ASHRAE TC 9.9 A1 standard operating limits; the rest of the year the system requires mechanical cooling support, with attention to dust and daytime heat peaks.
  • Humid tropical zone (Veracruz, Villahermosa, Mérida, Cancún): average temperature between 24 and 30 °C, high relative humidity. Free cooling limited by humidity, between 2 and 5 months per year.
  • Dry tropical zone (La Paz, Los Cabos): average temperature between 20 and 28 °C, low humidity. Similar to semi-desert with a better night-time temperature profile.
  • Cold mountain zone (Toluca, Pachuca, high areas of Chihuahua): low temperatures most of the year. Free cooling during 10 to 12 months, with altitude limitations on compression equipment.

    • When free cooling stops being cost-effective

    The cost-effectiveness of free cooling depends on comparing the expected electrical savings to the additional system investment. There are three cases where the math does not close.

  • Persistently high humidity: if average relative humidity exceeds 75% for most of the year, direct free cooling is not viable and indirect free cooling has low performance. The free cooling system is underused and payback extends beyond the equipment’s service life.
  • Outside temperature almost always outside the useful range: in tropical zones with annual average above 26 °C, the free cooling windows are short and the infrastructure investment is not recovered.
  • Severe dust and contamination: if outside air requires high-efficiency filtration with frequent filter replacement, the free cooling OPEX can exceed the electrical savings.
  • DC too small: free cooling infrastructure investment has a minimum CAPEX floor that is not justified for a DC with less than 50 kW of IT load. Payback extends beyond reasonable.

    • Payback estimate by zone

    For a reasonable estimate, three variables are used: annual free cooling hours possible, electrical savings per free cooling hour versus mechanical cooling, and cost of additional infrastructure.

    A mid-sized DC with 200 kW of IT load in a temperate zone (Mexico City) can expect between 5,000 and 6,000 annual hours of free cooling. Estimated electrical savings per hour, assuming average CFE industrial tariff and cooling consumption of approximately 70 kW, is on the order of 8 to 12 USD per hour. That yields annual savings between 40,000 and 70,000 USD.

    With an additional CAPEX for indirect free cooling between 80,000 and 180,000 USD, realistic payback in the temperate zone is between 2 and 3.5 years, which is still an excellent return for infrastructure that lasts 10-15 years. In the humid tropical zone with only 2,000 annual hours of free cooling, the same system has a payback of 5 to 8 years, and in some cases the math does not close.

    Operational recommendation

    Before specifying a free cooling system, it makes sense to obtain the climate data of the specific site (not the generic zone) and calculate the annual hours within the recommended ASHRAE range. The official source in Mexico is the Servicio Meteorológico Nacional of CONAGUA.

    With that information, system sizing and payback estimation are straightforward. Skipping that step leads to installing free cooling on a site where it is not cost-effective, or not installing it on a site where the investment is recovered in less than a year.

    Free cooling is not a universal improvement. It is a tool with a very defined geographic profile and return profile. The informed decision is the one made with the site’s climate data in hand.

    Sources

    [1] ASHRAE TC 9.9 — Thermal Guidelines for Data Processing Environments (recommended operating ranges) — https://tpc.ashrae.org/?cmtKey=fd4a4ee6-96a3-4f61-8b85-43418dfa988d

    [2] IEA — Data Centres and Data Transmission Networks (global sector energy consumption profile) — https://www.iea.org/energy-system/buildings/data-centres-and-data-transmission-networks

    [3] CONAGUA — Servicio Meteorológico Nacional (official Mexican climate data) — https://www.gob.mx/conagua/acciones-y-programas/servicio-meteorologico-nacional

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