# A hybrid cooling system cut direct water use at an operating national-laboratory data center.

DOE reports that NREL’s thermosyphon hybrid system avoided 2.10 million gallons over two years and cut data-center water use in half by using dry heat rejection when weather allowed while retaining a cooling tower for peak conditions.

Canonical: https://brightaifuture.com/discoveries/nrel-thermosyphon-water-savings
Format: discovery
Source publication: 2019-05-01
Bright publication: 2026-09-19
Substantive update: None recorded
Evidence and review: Deployed; confidence: high; approved; ai-assisted. AI-assisted editorial comparison with the cited original and supporting public sources, bounded claims and explicit status labels. Bright did not independently audit the underlying records.

## The human problem

Evaporative cooling can reject heat efficiently but requires a continuing water supply, especially during hot conditions when local systems may already be stressed.

## The prior constraint

NREL’s warm-water liquid-cooled HPC system could reuse some heat, but the remaining heat still relied heavily on evaporative towers.

## AI’s actual role

The HPC facility supports advanced computing; the documented result is conventional thermal engineering and measured operating water reduction, not an AI-model outcome.

## The documented result

A May 2019 DOE case study reports that the roof-mounted thermosyphon cooler avoided 1.16 million gallons in its first operating year and 2.10 million gallons over two years, cutting data-center water use in half. Controls route heat to campus reuse first, dry rejection when conditions permit, and the existing open cooling tower for remaining and peak loads.

## Why it may matter

It is an operating example of a hybrid strategy that reduces direct cooling water without pretending the wet system disappeared or one design fits every climate.

## Limitations

The result comes from a 10,000-square-foot national-laboratory HPC facility, not a hyperscale commercial campus.

The system retained full cooling-tower capacity for the hottest hours and did not eliminate direct water use.

DOE reported the case; Bright did not audit meters, loads, weather normalization, or later performance.

## Unresolved questions

How has the water reduction changed with later workloads, weather and equipment?

What are the complete energy, refrigerant, maintenance and source-water tradeoffs?

Which climates and loop temperatures can reproduce the operating hours in dry mode?

## Provenance and history

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  "dates": {
    "eventDate": null,
    "publicationDate": "2019-05-01",
    "captureDate": "2026-09-19",
    "lastReviewedDate": "2026-09-19"
  },
  "provenance": {
    "origin": "editorial",
    "externalId": "https://www.energy.gov/sites/default/files/2019/05/f63/data-center-water-efficiency-0.pdf"
  },
  "revisions": [
    {
      "id": "revision:data-centers-nrel-thermosyphon-water-savings-01",
      "recordedAt": "2026-09-19",
      "summary": "Published the two-year DOE operating case with its facility scale, retained wet-cooling capacity, and direct-water boundary explicit.",
      "sourceIds": [
        "source-doe-nrel-thermosyphon"
      ]
    }
  ],
  "corrections": []
}

## Original sources

- [Using Thermosyphon Hybrid Cooling System to Optimize Data Center Water Efficiency](https://www.energy.gov/sites/default/files/2019/05/f63/data-center-water-efficiency-0.pdf)

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