Cooling and Control Systems

When free cooling stops being profitable: 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 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 regions of Mexico it is profitable and when it stops being so.

Profitability depends on the annual hours in which outdoor conditions allow direct, partial, or indirect free cooling. Each climate zone in Mexico has a different profile.

How free cooling works in a DC

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

  • Direct free cooling (outside air): fans bring outside air directly into the room. It requires filters and humidification/dehumidification.
  • Indirect free cooling (air-water or air-glycol): a heat exchanger transfers the cold from the outside air to the internal water or glycol circuit 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 for a DC

    Mexico has notable climatic diversity. For data center free cooling purposes, it is convenient to group by temperature and humidity ranges throughout the year.

  • Temperate zone (Mexico City, Puebla, Toluca, high-altitude Guadalajara): annual average temperature between 12 and 22 degrees C, moderate humidity. Extensive free cooling period, between 7 and 10 months per year.
  • Mediterranean / cool semi-arid zone (Tijuana): average temperature between 14 and 22 degrees C with low humidity, large nighttime temperature differential due to Pacific influence. Excellent nighttime free cooling potential, 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 degrees C, low humidity. Free cooling viable between 4 and 6 months per year, since desert summer nights exceed the ASHRAE TC 9.9 A1 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, Merida, Cancun): average temperature between 24 and 30 degrees 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 degrees C, low humidity. Similar to semi-desert with a better nighttime temperature profile.
  • Cold mountain zone (Toluca, Pachuca, high-altitude Chihuahua): low temperatures almost year-round. Free cooling for 10 to 12 months, with the limitation of altitude on compression systems.

    • When free cooling stops being profitable

    Free cooling profitability depends on the comparison between the expected electrical savings and the additional system investment. There are three cases where the math does not work out.

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

    • Payback estimation by zone

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

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

    With an additional CAPEX for indirect free cooling between USD 80,000 and 180,000, the realistic payback in a temperate zone is between 2 and 3.5 years, which is still an excellent return for infrastructure that lasts 10-15 years. In a humid tropical zone with only 2,000 annual free cooling hours, 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 is convenient to obtain climate data for the specific site (not for the generic zone) and calculate the annual hours within the recommended ASHRAE range. The official source in Mexico is the Servicio Meteorologico Nacional of CONAGUA.

    With that information, system sizing and payback estimation are straightforward. Skipping that step leads to installing free cooling in a site where it is not profitable, or not installing it in 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 site-specific 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 energy consumption profile) — https://www.iea.org/energy-system/buildings/data-centres-and-data-transmission-networks

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

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