Data Center Master List of Energy Efficiency Measures, Version 2.0 (September 2020)

Overview

The Master List is described as a living document of best-practice measures for owners, operators and qualified assessors. It can be used on its own for in-house improvements or to feed an assessment report for an outside organisation. Measures are arranged hierarchically in seven categories: energy efficiency management, the IT power distribution chain, IT equipment, lighting, air management, cooling the data center space, and the central cooling plant. Each carries a number such as EEM 5-1.2, so a recommendation can be cited unambiguously. The Center of Expertise also offers companion tools that draw on the list: the DC Pro scoping tool that estimates PUE, an Excel air management tool, and a Word energy assessment report template.

The power chain section focuses on loading and voltage. Most power conversion equipment is said to have an efficiency sweet spot near 75% load, though the actual curve should be obtained from the manufacturer. Two parallel double-conversion UPS units sized so that either can carry everything, with a 20% safety factor, each run at about 40% load. Three equal units in an N+1 arrangement run at 53% and four at 60%, and unneeded modules in a 2N system can be deactivated. Higher voltage means lower current and lower losses, so the list favours 480 V over 208 V UPS, 240 V over 208 V IT supply, and mentions IT power supplies that accept 277 V directly, removing the PDU transformer.

Air management and space cooling make up the longest stretch. The list pushes higher supply air temperature, noting that enclosed architectures tolerate supply close to the top of the recommended intake range, while open layouts are governed by the hottest intake. It recommends controlling CRAC/CRAH units on rack inlet temperature instead of return air, using about 0.05 in. of water of raised-floor plenum pressure, and relaxing humidity control: the ASHRAE recommended range runs from a 15.8 °F dew point to a 59 °F dew point and 60% RH, yet many sites still hold 40-55% RH. IT servers that accept A2 or A3 allowable temperatures extend economizer hours.

The central plant chapter covers chillers, cooling towers, thermal storage and hydronics. Variable-speed chillers usually peak at 60-80% of full load while constant-speed chillers peak at full load. Efficiency rises about 1.5% per 1 °F drop in condenser water temperature and about 1% per 2 °F rise in chilled water supply temperature, though chilled water below a 59 °F dew point causes unintended dehumidification. The list also proposes chiller-free plants with cooling towers alone in moderate climates, thermal energy storage to time-shift chiller load, and water-efficient tower operation by not blowing down prematurely.

The document is oriented toward assessments and retrofits rather than being a design standard. Measures are brief, and the disclaimer states that the content is guidance prepared for the U.S. Department of Energy. Liquid cooling appears only as two short measures, direct-to-chip and immersion, justified by water's much higher heat capacity and the resulting drop in pump power compared with fan power. Waste heat recovery is limited to low-grade uses such as heating office space through air-to-air exchangers, run-around coils, heat pipes or heat pumps.

Key points

1. Power conversion equipment is most efficient at around 75% load; two parallel UPS units with a 20% safety factor each run at only about 40%.
2. With N+1 redundancy, three equal parallel UPS systems run at 53% load and four at 60%, better than 2N pairs.
3. A standard double-conversion UPS without an input filter draws current with 12-13% THD; a filter brings it to 5% or less but costs efficiency.
4. Raised-floor plenum static pressure of about 0.05 in. of water gives good perforated-tile airflow with limited leakage; use higher open-area tiles rather than more pressure.
5. ASHRAE recommended humidity runs from a 15.8 °F dew point (about 8% RH) to a 59 °F dew point and 60% RH; the A1 allowable upper limit is a 63 °F dew point and 80% RH.
6. Variable-speed chillers usually peak at 60-80% of full load; constant-speed chillers peak at full load.
7. Chiller efficiency improves about 1.5% for each 1 °F lower condenser water temperature and about 1% for each 2 °F higher chilled water supply temperature.
8. In moderate climates a plant with only cooling towers can cool year-round, eliminating chillers; where chillers remain, air-cooled units used only in hot weather can be cost-effective.
9. Cooling tower water use falls when blowdown is not started prematurely and when tower water is warm relative to dry-bulb temperature, because more heat leaves by convection.

Using it in design

• Compare the planned UPS module count and redundancy scheme against the load curve: check the per-unit load at day-one and at full build, and consider modular units or switching off surplus modules.
• Specify 480 V UPS and 240 V (or 277 V) IT distribution where possible, and review whether each load actually needs UPS backup.
• Put the control sensors of CRAC/CRAH units at representative rack inlets, raise the supply temperature, and confirm alarm limits and IT power cable ratings allow the warmer aisles.
• Set humidity control with a wide dead band based on dew point instead of 40-55% RH, and avoid chilled water colder than a 59 °F dew point on coils that would dehumidify unintentionally.
• Test a chiller-free or mostly-free-cooling plant against the local climate, and size chiller and tower setpoints using the efficiency sensitivities given per degree.
• Use the EEM numbers as a checklist when writing an energy assessment or retrofit action plan.

Who should read it

Owners, operators and energy assessors who prepare data center efficiency action plans, and designers who need a practical measure checklist for retrofits.

Document contents

1. Purpose and How to use this Document
2. Category 1: Data Center Energy Efficiency Management
3. Category 2: IT Power Distribution Chain
4. Category 3: IT Equipment
5. Category 4: Lighting
6. Category 5: Air Management
7. Category 6: Cooling the Data Center Space
8. Category 6.2: Liquid-Based Cooling
9. Category 7: Central Cooling Plant
10. Category 7.2: Chilled Water Plant
11. Category 7.3: Cooling Tower Plant
12. Category 7.4: Pumps and Hydronic Distribution
13. Glossary

Publication details

Authors

LBNL Center of Expertise for Energy Efficiency in Data Centers; Kim Traber

Publisher

Lawrence Berkeley National Laboratory (Center of Expertise for Energy Efficiency in Data Centers) / U.S. DOE FEMP

Year

2020

Document language

English

Source type

Government / public

Access

Citation only

Licence

Licence conditions not established — confirm the conditions with the publisher

Licence evidence

datacenters.lbl.gov

Citation