Using Network Modeling to Test for ASHRAE 90.4 Standard Compliance (2023 ASHRAE Winter Conference paper)

Overview

The 2023 ASHRAE Winter Conference paper by Aaron P. Wemhoff, Russ Tipton and Mark Seymour shows how a network (component-connection) model of both the cooling and the electrical systems can compute the Mechanical Load Component (MLC) and Electrical Loss Component (ELC) required by ASHRAE Standard 90.4. Compliance is assessed by comparing MLC and ELC (or their sum) with maximum values set by climate zone and by UPS configuration.

The worked example is a 1 MW data centre near Ashburn, Virginia with DX CRAC cooling and a non-redundant UPS chain, evaluated with hourly weather and the Standard's load points. For the 1 MW CRAC design in Climate Zone 4A, annualized MLC = 7,008,000 kWh / 21,900,000 kWh = 0.32 versus an allowed 0.18 (ITE >300 kW). These data show the numerical outputs of the method plainly.

ELC at 100% load = 0.1399 (incoming service) + 0.1365 (UPS) + 0.0026 (ITE distribution) = 0.279, versus an allowed 0.265 for single-feed UPS, IT >=100 kW. ELC at 50% load is 0.2745 versus an allowed 0.189, so the design fails at both load points. In the method, ELC is evaluated at the hour of peak outdoor enthalpy; 2N and 2(N+1) systems use 50%/25% load points instead of 100%/50%.

PSU losses inside IT equipment are excluded from ELC, and the UPS segment is modelled as rectifier efficiency times inverter efficiency. The authors argue that standardised tools would encourage states to adopt 90.4. Since the example is a single facility in one climate zone, the results cannot be generalised; the main contribution is showing how the method exposes non-compliance. These assumptions are stated in line with the Standard's definitions.

Key points

1. Compliance is assessed by comparing MLC and ELC (or their sum) with maximum values set by climate zone and UPS configuration.
2. For the 1 MW CRAC design in Climate Zone 4A, annualized MLC = 7,008,000 / 21,900,000 kWh = 0.32 versus an allowed 0.18 (ITE >300 kW).
3. ELC at 100% load = 0.1399 + 0.1365 + 0.0026 = 0.279 versus an allowed 0.265 for single-feed UPS, IT >=100 kW.
4. ELC at 50% load = 0.2745 versus an allowed 0.189, so the design fails at both load points.
5. ELC is evaluated at the hour of peak outdoor enthalpy; 2N and 2(N+1) systems use 50%/25% load points.
6. PSU losses inside IT equipment are excluded from ELC; the UPS segment is modelled as rectifier x inverter efficiency.
7. The example is a 1 MW data centre near Ashburn, Virginia with DX CRAC cooling and a non-redundant UPS chain.
8. The authors argue that standardised tools would encourage states to adopt 90.4.

Using it in design

• The tool can compute 90.4 MLC and ELC directly from its mechanical and electrical one-lines: segment losses, UPS efficiency at 100/50/25% load and hourly weather by climate zone.
• Show pass/fail against the table limits as a design-rule warning.
• Apply 50%/25% load points for 2N and 2(N+1) configurations.
• Leave PSU losses inside IT equipment out of the ELC calculation.

Who should read it

Energy modellers assessing ASHRAE 90.4 compliance, electrical and mechanical designers and compliance consultants.

Related standards

• ASHRAE Standard 90.4-2019
• ASHRAE Standard 90.4-2022

Publication details

Authors

Aaron P. Wemhoff; Russ Tipton; Mark Seymour

Publisher

ASHRAE

Year

2023

Document number

Proceedings of the 2023 ASHRAE Winter Conference; NSF PAR 10435236

Event

ASHRAE 2023 Winter Conference

Document language

English

Source type

Standard

Access

Free to read

Licence

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

Citation