Dawid Kropiwnicki and Nick Remington from Black & White Engineering explain how the utilisation of waste heat can further improve PUE.
The quest to improve power usage effectiveness (PUE) has been one of the long-standing driving forces in data centre design, procurement and operation.
PUE is a standard efficiency metric for energy use in data centres. It is the ratio of total facility energy to IT equipment energy used in a data centre.
The application of PUE for data centre facility is multifaceted, including consideration of maximising IT yield, greater utilisation of available utility, operational expenditure forecasting and indirectly as a sustainability metric. Furthermore, legislative PUE targets are now in place in some locations.
The European Energy Efficiency Directive (EED), effective from May 2024 in Germany, now stipulates PUE targets as well as waste heat percentage contributions to be provided to a local district heat network operator (DHNO).
German Energy Efficiency Act (EnEfG) Requirements:
- Monitoring: 15th May 2024 – Report on key metrics (PUE, temperature set points, ERE, water usage, renewable energy). DC > 0.5 MW.
- Monitoring: 1st July 2025 – All data centre operators must ensure their facilities are provided with an energy management system.
- Monitoring: 1st January 2026 – All operators will have to validate / certify their energy management system.
- PUE: <1st July 2026, PUE < 1.3, > 1st July 2026, PUE < 1.2.
- Heat Export: 1st July 2027 – Data centres must provide 15% Energy Reuse Factor. DC > 1 MW.
- Heat Export: 1st of July 2028 – Data centres must provide 20% Energy Reuse Factor. DC > 1 MW.
An R&D case study has considered the opportunity to utilise the waste heat to aid in meeting these targets. The considered project is a notional 54 MWIT data centre in Germany, deploying high efficiency free-cooling air-cooled chillers. Waste heat is in the form of low-grade heat and does not include for heat pump technology to provide temperature elevation.
EN 50600’s definition of PUE does not consider the use of waste heat in determining a data centre’s PUE. However, in response to the EED’s stipulated PUE compliance and percentage contribution of heat export requirement, this study has continued to assess this opportunity. Any application of heat export to improve PUE and therefore deviation from EN 50600 shall be discussed with the local authority having jurisdiction.
Under the EED, waste heat is required to be provided to a local DHNO, unless no such operator exists or the heat supplied is not required. A local DHNO’s heat demand profile, (typically heating and hot water) has a higher demand during the peak winter months, reducing in the off-peak months to a base load during the summer. Typically, summer months have low heating demand due to external conditions negating heating loads.
Conversely, the PUE profile through the year is seasonal and a function of the free-cooling operation of the air-cooled chiller, the largest contributor to PUE after the IT load.
The above shows the optimum period of the year where chiller compressor power consumption can be omitted through heat rejection to a DHNO, is simultaneously when DHNOs do not require the available waste heat. Until the above profiles have greater convergence, PUE improvements are limited. It should be noted, alternative annual applications such as swimming pools, greenhouses, aquaculture and industrial processes would increase this heat profile baseload.
Using the same notional project case study, B&W have assessed hypothetical percentage increases in the DHNO base load, artificially increasing the opportunity for heat rejection to the DHNO. From a practical, physical and commercial viability perspective, high percentage heat export values are more difficult to achieve in practice and are only considered here to test this PUE improvement hypothesis.
Upon analysing the results, increasing the DHNO baseload from 0% to 100% IT load yields up to 3.3% reduction in annualised PUE, while the impact to peak PUE is more substantial, at 21.2%. When the parasitic PUE component (or PUE overhead) is considered in isolation, increasing the DHNO baseload from 0% to 100% IT load, offers up to 17.9%in the annualised parasitic PUE and an impressive 63.5% in the peak parasitic PUE.
On average, for every 10% baseload waste heat increase, the parasitic annualised PUE improved by ~2%. These results account for the increase in annual pump power consumption to serve the DHNO.
Conclusion
The opportunity to improve PUE through exporting waste heat is possible, subject to local acceptance and deviation from EN 50600.
The PUE improvement is heavily reliant on the DHNO usage profiles. To fully realise this opportunity, DHNOs are required to accept more heat for a larger period of the year, minimising chiller compressor power consumption. Alternative applications for the waste heat could also be considered.
Although PUE improvement is available, the viability of higher percentage values of heat export is required from a physical and commercial perspective.
