PUE Improvement Claim for a 1.2 MW Hall
A 1.2 MW average IT load consumes 10.5 GWh per year. The facility total is 16.8 GWh per year, with cooling 4.2 GWh, UPS losses 0.8 GWh, lighting 0.3 GWh, and other infrastructure 1.0 GWh. A vendor proposes hot-aisle containment and EC CRAH fans, claiming a 25% reduction in cooling energy and a resulting PUE of 1.25. The site has no electrical sub-metering on CRAH fans, chillers, or pumps; only the main utility meter and UPS output are logged. Chilled-water supply is fixed at 7°C and the chillers are staged manually. The IT load is expected to grow 10% next year. The CFO wants a payback calculation before approving the project. Project cost is USD 300,000; power is USD 0.10/kWh.
1. Calculate current PUE and the PUE after a 25% cooling reduction; assess whether the 1.25 claim is internally consistent.
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Current PUE = 16.8/10.5 = 1.60. A 25% cooling reduction saves 0.25×4.2 = 1.05 GWh, giving 15.75 GWh total and PUE = 15.75/10.5 = 1.50. The vendor's 1.25 would require total energy of 1.25×10.5 = 13.125 GWh, a 3.675 GWh cut. That is 87.5% of all cooling energy, far beyond a 25% cooling reduction. The claim is internally inconsistent unless the baseline or the scope is different. Note also that 4.2 GWh cooling is only 25% of facility energy; chillers, pumps, towers, and CRAH fans must all be included.
2. Recommend a metering and measurement plan, and calculate annual savings and simple payback for the 25% cooling reduction.
Read the worked answer
Savings = 1.05 GWh/year × USD 0.10/kWh = USD 105,000/year. Simple payback = 300,000/105,000 ≈ 2.9 years, before utility incentives or load growth. Metering is inadequate: install kWh meters on chillers, chilled-water pumps, condenser-water pumps, cooling-tower fans, and CRAH fans, plus UPS output and lighting. Log kW, kWh, supply/return temperatures, flow, and outdoor wet-bulb. A temporary power logger on the CRAH panel can validate the fan savings. Without submetering, the PUE and savings cannot be verified.
3. What additional measures could close the gap toward PUE 1.45, and how does 10% IT growth affect the analysis?
Read the worked answer
To approach 1.45, combine containment with higher chilled-water supply temperature (subject to dew point and coil capability), optimized chiller staging, variable tower-fan control, free cooling where available, and CRAH fan VFDs. A 10% IT growth raises IT energy to about 11.55 GWh; if cooling does not scale proportionally, PUE may improve, but capacity and redundancy must be checked. Recalculate savings on the grown load and avoid a single-point PUE claim.