The Physical World Behind AI, the Tech Innovations library
Definitions and research
The words the library uses, in plain terms, and the reports behind the articles, so you can read the originals for yourself.
Definitions and units
- kW (kilowatt): a rate of using electricity. Heat made by equipment is measured the same way, because the electricity becomes heat.
- kWh (kilowatt hour): an amount of electricity, one kilowatt used for one hour.
- Rack: one cabinet of computing equipment in a data centre. Rack power, in kW, tells you how much heat the cabinet makes.
- Direct-to-chip cooling: liquid flowing through a metal cold plate fixed to a processor, carrying its heat away.
- Hot aisle and cold aisle: rows of racks arranged so that cool air arrives on one side and hot air leaves on the other, without mixing.
- Free cooling (economiser): using cool outside air or water to remove heat, with little or no mechanical refrigeration.
- Cooling tower: equipment that removes heat by evaporating a small part of the water passing through it.
- PUE (Power Usage Effectiveness): total facility energy divided by the energy used by the IT equipment. Closer to 1 means less energy spent on cooling and other overheads.
- WUE (Water Usage Effectiveness): water used by the facility in relation to the energy used by the IT equipment, in litres per kWh.
Research worth reading
| Report | Publisher | Date | Why it is useful |
|---|---|---|---|
| Best Practices Guide for Energy-Efficient Data Center Design | NREL for the US Department of Energy | Revised July 2024 | A clear engineering guide to air management, free cooling and liquid cooling |
| AI Data Center Energy Performance Framework | PNNL, ASHRAE and NEMA | Undated web page, read 29 October 2026 | Recommended practice for cooling and energy in AI data centres |
| Liquid Cooling in Data Centres | IEA 4E Technology Collaboration Programme | 22 June 2026 | Estimates of the energy savings from liquid cooling and the barriers to using it |
| Cooling Water Efficiency Opportunities for Federal Data Centers | US Department of Energy | Updated 28 April 2026 | How cooling towers use water and how water use is measured |
- Best Practices Guide for Energy-Efficient Data Center Design: https://www.energy.gov/sites/default/files/2024-07/best-practice-guide-data-center-design_0.pdf
- AI Data Center Energy Performance Framework: https://www.ashrae.org/technical-resources/topics-and-initiatives/ai-data-center/energy-and-thermal-efficiency
- Liquid Cooling in Data Centres: https://www.iea-4e.org/edna/publications/liquid-cooling-in-data-centres/
- Cooling Water Efficiency Opportunities for Federal Data Centers: https://www.energy.gov/cmei/femp/cooling-water-efficiency-opportunities-federal-data-centers
How the library checks its sources
Every figure in an article comes from a source that is named, dated and linked beside it. Links are opened before an article is published and again at each review. A source whose link no longer opens is removed.