Nearly every kilowatt-hour a data center consumes ends up as heat. In a conventional facility, that heat is rejected to the atmosphere through cooling towers, dry coolers or condensers. In a country where buildings need heating for much of the year, throwing that energy away is an obvious missed opportunity.
Heat reuse captures data center waste heat and delivers it to something useful: the data center's own offices, a neighbouring building, domestic hot water, greenhouses, or a district energy network. The concept is simple. The engineering, economics and contracts are where projects succeed or fail.
Where the heat is and how warm it is
The value of waste heat depends heavily on its temperature. Higher temperatures are easier and cheaper to use.
| Heat source | Typical temperature range | Usefulness |
|---|---|---|
| Air-cooled hot aisle return | About 30 to 45 °C (86 to 113 °F) | Low grade; needs heat pump for most uses |
| Chilled water return from CRAH units | About 18 to 30 °C (64 to 86 °F) | Low grade; heat pump required |
| Condenser water from chillers | About 30 to 40 °C (86 to 104 °F) | Low grade; heat pump or heat recovery chiller |
| Direct-to-chip liquid cooling return | Often about 40 to 50 °C (104 to 122 °F) or more, depending on class | Medium grade; low-temperature heating directly, heat pump for higher |
| Immersion cooling return | Similar to or above direct-to-chip | Medium grade |
Ranges vary with design. The general rule is that liquid cooling produces warmer, more concentrated heat than air cooling, which is why heat reuse and liquid cooling are often discussed together. See direct-to-chip liquid cooling.
Upgrading heat with heat pumps
Most building heating systems need water at higher temperatures than air-cooled data centers produce. Older hydronic systems may need 70 to 80 °C (158 to 176 °F) supply on cold days, while newer low-temperature systems may run at 40 to 55 °C (104 to 131 °F). District networks vary from ambient-temperature loops to high-temperature hot water systems.
Heat pumps bridge the gap. A water-to-water heat pump or heat recovery chiller takes heat from data center water and raises its temperature. The coefficient of performance (COP) depends on lift, the temperature difference between source and sink. Warmer source water and lower supply temperatures mean smaller lift, higher COP and better economics.
In practice, the simplest heat reuse is a heat recovery chiller serving the data center's own building or campus, replacing part of the boiler load. Larger projects export heat to neighbours or district networks.
Connecting to district energy
District energy systems distribute hot water (and sometimes chilled water) through underground pipes to many buildings. Several Canadian cities have district systems, and some newer networks operate at low temperatures that suit waste heat sources. Connecting a data center typically involves:
- A heat exchanger or heat pump station at the data center
- Metering of exported energy
- Piping to the district network
- Controls agreed between the data center operator and the utility
- A commercial agreement setting price, quantity, availability and responsibilities
See district energy connections for commercial buildings for interconnection basics.
Measuring reuse: ERF
The Energy Reuse Factor (ERF), developed by The Green Grid and also included in the ISO/IEC 30134 series, measures the fraction of data center energy reused outside the data center:
ERF = reused energy / total data center energy
An ERF of 0 means no reuse; higher values mean more reuse. ERF is reported alongside PUE because reusing heat does not change PUE. See power usage effectiveness.
Practical barriers
Mismatched timing
Data centers produce heat steadily all year. Heating demand peaks in winter and drops in summer. Without a summer heat sink, such as domestic hot water, pools, industrial processes or thermal storage, reuse is limited to the heating season. The data center must always retain full heat rejection capacity for times when the heat customer does not need it.
Distance
Heat is expensive to move. Long pipe runs add capital cost and heat loss. Projects work best when a heat user is close by, or when a district network passes nearby.
Reliability and responsibility
The data center's first priority is cooling. Heat reuse systems must never compromise IT cooling, so designs keep conventional heat rejection in place and allow the reuse system to be isolated without affecting the data hall. Conversely, a heat customer relying on data center heat needs backup heating in case the data center reduces load or shuts down.
Commercial structure
Questions that often stall projects:
- Who owns and maintains the heat pumps and interconnection?
- How is heat priced relative to natural gas or electricity?
- What happens if either party's needs change?
- Who pays for piping across property lines?
Economics
The value of reused heat depends on what it displaces. Displacing natural gas heating provides carbon reductions that may be valued under federal and provincial carbon pricing and building performance programs, while the heat pump electricity has a cost. Utility incentive programs in some provinces may help. A life cycle cost analysis is essential.
Design considerations for heat-ready data centers
Even if no heat customer exists today, a new data center can be designed to make reuse easier later:
- Choose higher water temperatures and wider delta T where IT equipment allows
- Plan space and connection points for heat recovery chillers or heat pumps
- Include valved, capped tees on condenser or facility water headers
- Meter heat rejection so future export can be quantified
- Consider liquid cooling for high-density zones, which produces warmer heat
Frequently asked questions
Does heat reuse lower my PUE?
No. PUE measures energy use within the data center. Heat reuse is captured by ERF and by carbon accounting. Heat pumps used for reuse should be metered separately so they are not counted as data center overhead.
Is heat reuse practical for a small server room?
Sometimes. A small room's heat can be recovered for the building it sits in, for example with a heat pump serving the building heating loop. The savings are modest, but so is the cost when the building already has suitable systems.
Is there a regulatory requirement for heat reuse in Canada?
Requirements are evolving and vary by jurisdiction. Some municipalities and provinces encourage waste heat use through district energy policy and building performance programs. Check current local requirements at the planning stage.
Key takeaways
- Data centers convert nearly all their energy into heat that can be reused for building heating.
- Liquid-cooled systems produce warmer heat that is easier to reuse; air-cooled heat usually needs heat pumps.
- District energy networks and on-site heat recovery chillers are the main reuse paths.
- Seasonal mismatch, distance, reliability and commercial terms are the main barriers.
- Design new facilities to be heat-ready even if no heat customer exists yet.
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