Analysis of Data Center Architectures Supporting Open Compute Project (OCP)

Overview

The paper examines what Open Compute racks, with consolidated rack-level power shelves and battery backup units (BBUs), mean for the electrical system upstream of the rack. Costed reference designs for a 9.6 MW, 10 kW/rack facility compare a traditional 2N (tier 3) design, an OCP-specific 1N design, an OCP-specific 2N design with no central UPS, and a simplified 2N design keeping one UPS-backed path so traditional and OCP loads can coexist. The cost basis is actual project costs normalised in EUR/W from MV switchgear through the server power supplies.

Relative to traditional 2N, OCP-specific 1N saves 45% of capex and OCP-specific 2N saves 25%; the simplified mixed-load 2N carries a 3% premium over OCP-specific 2N. Simplified 2N with 100% OCP loads is 1.95 EUR/W, adding 0.21 EUR/W for 100% traditional loads. In the waterfall from traditional 2N to OCP 1N, removed redundancy saves 0.79 EUR/W (31%), UPS removal 0.20 EUR/W (8%) and server PSU consolidation 0.31 EUR/W (12%), against a 0.14 EUR/W (5%) premium for rack BBUs. Architecture simplification (no cross-ties, extra breakers, load bank or second UPS) accounts for 0.36 EUR/W, 14 of the 25 percentage points saved by OCP 2N, independent of load type.

PSU cost per IT watt is 0.07 EUR x redundancy 2 x oversizing 3 = 0.42 EUR for traditional, versus (0.06 + 0.03 shelf) x 1.2 = 0.11 EUR (1N) or 0.22 EUR (2N) for OCP. Varying traditional PSU oversizing from 2 to 4.5 gives a premium of 5-15% at a 50/50 load mix and 8-26% at 100% traditional load. Battery assumptions are VRLA, 5 min, centralised in the UPS for traditional and lithium-ion, 4 min, rack-based for OCP, both at 25 C. Li-ion stores the same energy in about one third of the footprint and weight of VRLA, enabling rack-level batteries. The paper adds that battery chemistry should be considered separately from battery location and tabulates pros and cons of central versus rack-level storage.

Key points

1. Reference: 9.6 MW at 10 kW/rack; cost from MV switchgear to server PSUs in EUR/W.
2. Capex versus traditional 2N: OCP-specific 1N saves 45%, OCP-specific 2N saves 25%.
3. Simplified mixed-load 2N costs a 3% premium over OCP-specific 2N.
4. Simplified 2N with 100% OCP loads is 1.95 EUR/W; +0.21 EUR/W for traditional loads.
5. Waterfall 2N to OCP 1N: redundancy removal 0.79 EUR/W (31%), UPS removal 0.20 (8%), PSU consolidation 0.31 (12%) savings; rack BBUs 0.14 (5%) premium.
6. Architecture simplification is 0.36 EUR/W, 14 of OCP 2N's 25 points.
7. PSU cost per IT watt: traditional 0.42 EUR; OCP 0.11 (1N) or 0.22 EUR (2N).
8. Li-ion stores the same energy in about one third of VRLA's footprint and weight.

Using it in design

• Offer reference architectures 'traditional 2N', 'OCP 1N', 'OCP 2N (no central UPS, 1N BBU)' and 'simplified 2N mixed-load (single UPS path)' in the topology builder; let a rack type declare power shelf, BBU runtime and PSU redundancy.
• Flag OCP 1N as not concurrently maintainable; when OCP racks have no BBU require an upstream UPS on at least one path.
• Warn when the declared tier or redundancy target conflicts with the chosen topology.
• In the capex calculator break costs into MV/LV/pod/rack levels, make server PSU cost and oversizing user inputs, and show savings from removing cross-ties, load banks and the second UPS as a separate line.

Who should read it

Electrical designers and investment decision makers evaluating OCP or rack-level power architectures.

Related standards

• OCP Open Rack
• Uptime Institute Tier Standard: Topology
• ANSI/TIA-942

Publication details

Authors

Kevin Brown; Wendy Torell

Publisher

Schneider Electric

Year

2016

Document number

WP 228

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

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

Citation