Diesel Rotary UPS (DRUPS) vs. Static UPS: A Quantitative Comparison for Cooling and IT Applications

Overview

The paper compares low-voltage DRUPS with low-voltage static UPS for a 3 MW, 500-rack facility in a 2N architecture covering IT and cooling loads, with chilled-water buffer tanks providing thermal ride-through in the static case. It challenges common DRUPS claims about efficiency, scalability, space and cooling energy, arguing they rest on old SCR-based static UPS curves and long VRLA autonomy. The base case is 3 MW IT at 6 kW per rack, 945 RT of cooling, N+1 water-cooled chilled water at 8-15 C, 2N electrical, a 5-minute static UPS battery and 2 x 28.6 m3 tanks for 8 minutes of ride-through.

At 47% IT utilisation, DRUPS part-load efficiency is 93% (226 kW loss) versus 97% (93 kW) for static; on the cooling side 91% (115 kW) versus 96% (15 kW). Total losses are 341 kW versus 108 kW, 68% lower for static. Plant floor area including maintenance access is about 1,128 m2 for DRUPS versus 1,176 m2 for static with thermal storage, only 4.3% more. DRUPS ride-through is about 8-12 seconds from utility loss to completed diesel start. The 10-year TCO is $82,001,841 for DRUPS and $76,109,746 for static, a 7.2% saving, with capex 27.8% and maintenance/cyclical replacement 25.2% lower for static.

The conclusion is conditional: including colocation rental gains (assumed $1,285 per rack per month) cuts the static saving to 3.7%, and for a 2N IT-only case DRUPS TCO becomes 10.6% better. The low-voltage limit is 6,300 A (about 4.36 MVA at 400 V), so no more than two 2 MVA DRUPS can be paralleled on one common bus. Li-ion batteries are cited as using 50-80% less space than VRLA. The study is vendor-authored with Singapore pricing converted to USD; appendices cover IT-only, cooling-only and tri-redundant variants.

Key points

1. Base case: 3 MW IT (500 racks at 6 kW), 945 RT cooling, 2N, static UPS 5 min battery, 2 x 28.6 m3 tanks for 8 min.
2. DRUPS ride-through about 8-12 seconds.
3. At 47% IT utilisation: DRUPS 93% (226 kW loss) vs static 97% (93 kW); cooling side 91% (115 kW) vs 96% (15 kW).
4. Total losses 341 kW vs 108 kW; static 68% lower.
5. Plant area: DRUPS about 1,128 m2 vs static with thermal storage 1,176 m2 (4.3% more).
6. 10-year TCO: DRUPS $82,001,841 vs static $76,109,746 (7.2% saving).
7. With colocation rental the static saving drops to 3.7%; for 2N IT-only, DRUPS is 10.6% better.
8. LV limit 6,300 A (about 4.36 MVA at 400 V): at most two 2 MVA DRUPS on one bus.

Using it in design

• Store full UPS efficiency curves (10/25/50/75/100% load), ride-through, footprint and service clearance, and a technology field (static VRLA, static Li-ion, DRUPS) rather than one peak value.
• Compute losses at the actual per-module utilisation implied by the redundancy scheme (2N typically below 50%).
• When cooling is not on UPS, require a thermal ride-through strategy (buffer tank volume versus generator start and chiller restart).
• Warn when a LV bus paralleling DRUPS exceeds 6,300 A / 4.36 MVA and suggest MV distribution; include battery replacement every 5 years, engine overhaul and optional space rental value in the TCO tool.

Who should read it

Designers and decision makers choosing UPS technology and redundancy architecture.

Related standards

• IEC 62040-3
• IEC 88528-11
• ISO 8528-1

Publication details

Authors

Han Ming Kuang; Hng Yong Way; Sanjeet Sandhu

Publisher

Schneider Electric

Year

2016

Document number

WP 222

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