Cooling retrofit: trading kilowatt-hours for potable water
Northvale Data Centres operates a 6 MW IT load colocation facility in a region classified as extremely high water stress, where the utility has imposed a 15% annual water reduction target on large consumers. The site uses evaporative cooling: PUE 1.18, but 42 million litres of potable water a year. Engineering proposes a closed-loop air-cooled retrofit that would raise PUE to 1.42 and add about 12 GWh of electricity annually; the local grid emits 480 gCO2e/kWh. Two anchor tenants have public 2030 targets: one reports both energy and water under a global ESG framework, the other applies an internal carbon price of EUR 80 per tonne but tracks no water metric. The CFO wants the decision framed in cost terms; the sustainability lead argues that improving PUE by consuming potable water in a water-stressed basin is a false economy. The cooling units reach end of life in six months, and the board wants a recommendation before then.
1. How should the team frame and analyse the cooling decision before any option is selected?
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Do not accept the binary framing. Engineering has presented two options; the decision is broader, and a third route (reclaimed or greywater feed to evaporative cooling, possibly with a smaller hybrid unit) may satisfy the water target without the energy penalty. Reframing the option set is the first analytical step, not an optional extra.
Start from materiality, then quantify. In an extremely water-stressed basin, potable water withdrawal is material to the business and to the two anchor tenants in different ways - one reports water, one does not. So the appraisal must produce both metrics for every option, over the full asset life:
- Annual energy (GWh) and scope 2 emissions at 480 gCO2e/kWh. The retrofit's 12 GWh equates to roughly 5,800 tCO2e a year, which at the tenant's EUR 80 shadow price is about EUR 460k of internalised cost.
- Absolute water withdrawal by source (potable, reclaimed, mains), plus WUE in litres per kWh, so a future change in load does not silently invalidate the comparison.
- Whole-life cost including the utility's penalty exposure and the risk of a tightening water target.
Test against constraints. The 15% water reduction is an external commitment, not a preference; an option that breaches it needs an explicit, minuted exception. Conversely, a PUE of 1.42 may conflict with what tenants can report.
Insist on paired metrics. PUE alone must never be the decision metric here, because it is precisely the metric that rewards water consumption. Report energy, emissions and water together in every board paper, and state the trade-off in a single table rather than in prose.
2. The carbon-focused tenant asks for a contractual guarantee of PUE below 1.25. How do you respond?
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Recognise the trap before negotiating. PUE is a ratio of total facility power to IT power. It rises when IT load falls, so a contractual PUE ceiling creates an incentive to keep the hall full, or to consume water to hit the number - the opposite of what the tenant wants. Guaranteeing it in a water-stressed site is also a commitment the operator may only be able to keep by reverting to evaporative cooling.
Counter-propose a sustainability schedule rather than a single ratio. Offer commitments that are both measurable and hard to game:
- Emissions per unit of delivered compute (or per workload), reported on a stated method, with the grid factor and reporting period fixed in the contract.
- Absolute water withdrawal and WUE, with a reduction trajectory aligned to the utility target.
- A metering and data-sharing clause so the tenant can assure its own disclosure.
- A roadmap with milestone dates for efficiency measures (raised supply temperature, free cooling, heat reuse), with remedies tied to milestones rather than to a ratio.
Clarify the accounting question early. Ask whether the tenant's target is set on location-based or market-based scope 2. If they intend to claim renewable matching, the electricity volume matters more than the PUE number, and the conversation changes.
Escalate with a commercial view. Losing an anchor tenant is a real cost, so present account management with the option set: an achievable schedule, a pricing adjustment for the retrofit, or a carefully bounded PUE commitment with an explicit water caveat. Document that the operator refused an unbounded guarantee and why - that record protects against a later greenwashing challenge.
3. How should the chosen option be embedded in governance and in the service value chain so the trade-off is not lost after the board decision?
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Put the criteria where decisions are actually made. Sustainability criteria belong in the design and transition stages of the facility change value stream, as mandatory fields in the change record - energy, emissions and water impact for every material change - not as a paragraph added to board papers after the engineering choice is made.
Make ownership explicit. Name an accountable role for the site's sustainability objectives, with the utility's 15% water target recorded as an external commitment the site is measured against, alongside any tenant contractual commitments.
Extend into the supply chain. The retrofit's performance depends on the installer and maintenance regime: add reporting obligations, commissioning evidence and audit rights to the maintenance contract, and require metered water and energy data at the granularity the reporting needs.
Build an audit trail for the trade-off. Record the options considered, the metrics used, the reason the chosen option was preferred, and who approved it. Reviewers and assurance providers will ask why an option with worse PUE was selected; a documented materiality rationale answers that, an undocumented one looks like cherry-picking.
Set review triggers, not just an annual cycle. Reopen the decision if the grid emission factor moves materially, if the water-stress classification or the utility target changes, if a reclaimed-water supply becomes available, or if a tenant's reporting requirement shifts from location-based to market-based accounting. These are the events that can turn a correct decision into a wrong one.