Using Thermosyphon Hybrid Cooling System to Optimize Data Center Water Efficiency (DOE/GO-102019-5172, May 2019)

Overview

The 10,000 ft2 high-performance computing (HPC) data center in NREL's ESIF building runs on warm-water, component-level liquid cooling and reports an annualized average PUE of 1.036. Evaporative cooling rejects heat effectively and cheaply but needs a large, steady water supply, more than 2 million gallons per year here. So in 2016 NREL, with Johnson Controls and Sandia, installed a thermosyphon cooler (TSC) on the roof. The TSC is an advanced dry cooler that uses refrigerant in a passive cycle to dissipate heat, and together with the existing evaporative towers it forms a hybrid system.

The heat-rejection hierarchy is as follows. First, heat in the energy recovery water (ERW) loop is passed where possible to the building process hot water loop to heat the building. When reuse potential is exhausted, warm ERW is dissipated through the TSC when temperatures permit. The remaining heat is transferred through the cooling tower heat exchanger to the tower water open loop. The system uses wet cooling when it is hot and dry and dry cooling otherwise, and the TSC is designed to cool the primary loop safely and prevent freeze-ups even in subfreezing ambient conditions. Air-cooled heat exchangers use only dry bulb temperature and need no water, but they use fan energy.

The TSC saved 1.16 million gallons of water in its first year and 2.10 million gallons over two years, roughly halving the data center's water use, equivalent to more than three Olympic-size pools. Installation needed only a 1-day data center outage. The system is additive: in most cases enough cooling tower capacity is needed to handle the entire IT load in the hottest hours of the year when the TSC cannot contribute, which adds resilience. NREL now performs tower preventive maintenance on days when energy reuse and the TSC can handle all heat rejection. The benefits apply to sites with year-round heat rejection load and loop temperatures high relative to ambient. Detailed results are in report NREL/TP-2C00-72196.

Key points

1. The NREL ESIF HPC data center reports an annualized average PUE of 1.036.
2. Before the TSC, the 10,000 ft2 HPC data center's evaporative cooling used more than 2 million gallons of water per year.
3. The TSC saved 1.16 million gallons of water in its first year and 2.10 million gallons over two years, roughly halving data center water use, equal to more than three Olympic-size pools.
4. Heat-rejection hierarchy: energy-recovery water first heats the building, then the thermosyphon dry cooler rejects heat when ambient conditions allow, and the cooling towers take the remainder.
5. Installing the TSC required only a 1-day data center outage.
6. Cooling towers must still be sized for the full IT load in the hottest hours, when the TSC cannot contribute; this adds resilience.
7. The TSC is designed to cool the primary loop safely and prevent freeze-ups even in subfreezing ambient conditions.
8. The approach suits sites with year-round heat rejection load and loop temperatures high relative to ambient; NREL does tower preventive maintenance on days when energy reuse and the TSC handle the whole load.
9. The work won the 2018 DOE FEMP Federal Energy and Water Management Award and the Data Center Dynamics 2018 Eco-Sustainability award.

Using it in design

• Arrange the heat-rejection hierarchy as in the scheme: heat reuse first, then dry cooling, evaporative towers last; write the control logic in that order.
• Evaluate a hybrid dry/wet system to lower water use (WUE) in warm-water liquid-cooled facilities with high heat-rejection loop temperatures.
• Still size cooling towers for the full IT load in the hottest hours of the year even with a TSC; the dry cooler's contribution is zero then.
• In climates with freezing risk, add to the specification the design and controls needed to protect the primary loop from freezing.
• Schedule tower preventive maintenance for days when heat reuse and the dry cooler can handle the load alone, and note that installation downtime can be short (1 day here).

Who should read it

Mechanical engineers designing heat rejection and water efficiency for warm-water liquid-cooled data centers.

Document contents

1. Rounding Out NREL's Data Center Sustainability Story
2. How TSC Compares to Traditional Cooling Options
3. How It Works: An Efficient Hybrid Cooling System
4. Beyond NREL
5. References

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Publication details

Authors

U.S. DOE Federal Energy Management Program (FEMP)

Publisher

U.S. DOE FEMP / NREL

Year

2019

Document language

English

Source type

Government / public

Access

Open licence — downloadable

Licence

U.S. public domain — U.S. Government work; free to reuse

Licence evidence

www.energy.gov

Citation