ASHRAE's energy standard for data centers turns three

Overview

In this 2019 article in Consulting-Specifying Engineer, Bill Kosik looks at ASHRAE Standard 90.4-2016 from a practitioner's view three years after its release. He explains its relationship to 90.1 and its two compliance metrics: the mechanical load component (MLC) and the electrical loss component (ELC).

Design MLC is the sum of cooling, pump, heat-rejection fan and AHU fan design power divided by design ITE power, checked at 100% and 50% ITE load against Table 6.2.1. Annualized MLC requires an hourly energy simulation, also checked at 100% and 50% ITE load. ELC covers incoming service, UPS and ITE distribution: single-feed (N, N+1) UPS is evaluated at 100%/50% load and active dual-feed (2N, 2N+1) at 50%/25%. Separate ELC tables apply for ITE loads under 100 kW and 100 kW and above, and mechanical distribution losses are limited to 2%. Annualized air-system MLC values assume a 20 F supply-air temperature rise and 85 F return air.

The author also describes the trade-off and renewable-offset provisions and the proposed 2019 addenda that would drop design MLC, tighten limits and switch to a four-point annualized MLC. In the proposal, annualized MLC is computed over 25/50/75/100% ITE load, with ITE energy at X% equal to design ITE load x 8,760 h x X%. Kosik cautions that IT power does not scale linearly with computational load. For compliance, consult the standard's edition in force and the edition adopted by the jurisdiction.

Key points

1. Design MLC = (cooling + pump + heat-rejection fan + AHU fan design power) / design ITE power, checked at 100% and 50% ITE load against Table 6.2.1.
2. Annualized MLC requires an hourly energy simulation, also checked at 100% and 50% ITE load.
3. ELC covers incoming service, UPS and ITE distribution; single-feed (N, N+1) UPS is evaluated at 100%/50%, active dual-feed (2N, 2N+1) at 50%/25% load.
4. Separate ELC tables apply for ITE loads under 100 kW and 100 kW and above; mechanical distribution losses are limited to 2%.
5. Annualized air-system MLC values assume a 20 F supply-air temperature rise and 85 F return air.
6. Proposed 2019 addenda: annualized MLC over 25/50/75/100% ITE load, with ITE energy at X% = design ITE load x 8,760 h x X%.
7. The proposed addenda would drop design MLC and tighten limits.
8. The author cautions that IT power does not scale linearly with computational load.

Using it in design

• Implement the 90.4 MLC and ELC calculators with the correct test load points per UPS redundancy class (N/N+1 vs 2N).
• Expose the 20 F delta-T and 85 F return assumptions as editable inputs and flag when a design deviates.
• Check which edition (2016 or later) the user's jurisdiction adopts.
• Do not scale IT power linearly with computational load; use real equipment data at part-load points.

Who should read it

Mechanical and electrical designers calculating data centre energy compliance, and energy modellers.

Related standards

• ASHRAE 90.4-2016
• ASHRAE 90.1-2016

Publication details

Authors

Bill Kosik

Publisher

Consulting-Specifying Engineer

Year

2019

Document language

English

Source type

Expert / community

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation