Heat Reuse & Waste-Heat Recovery (Engineering)

A 100 MW IT load rejects very close to 100 MW of heat — essentially all the electrical energy that enters the building leaves it as low-grade warmth. That continuous, year-round source sits next to cities that burn gas to stay warm, so the pull to sell it is obvious, and in 2026 it is increasingly a legal obligation rather than an option. The binding constraint is grade, not quantity: heat at 30 °C needs paid electrical lift for a customer asking for 80 °C, while the same joules delivered at the required temperature can be a tradable commodity when that customer actually needs them. And the grade you can deliver was largely decided one chapter upstream, when you set the facility-water supply temperature in Chapter 5.7.

The engineering falls into four questions: how the grades are defined and what each is good for; how a heat pump upgrades a sub-usable return temperature to a district-heating supply, and what that lift costs in COP and electricity; where the reuse heat exchanger ties into the loop, and why warm-water DLC is what makes the exercise viable at all; and the offtake chicken-and-egg, the scheduling and counterparty problem that kills more heat-reuse projects than any thermodynamic limit. Economics, contract structures, carbon-accounting, and regulatory treatment are deferred to Chapter 15.5; here we engineer the heat.

Waste heat is graded by temperature because temperature, not energy content, is what an offtaker's distribution network and emitters require. A district-heating network has a supply-temperature spec set by its generation: legacy 2nd/3rd-generation networks run 70–90 °C (and some older systems higher), modern 4th-generation networks run 30–70 °C, and emerging 5th-generation (ambient-loop) networks run 10–30 °C and put a heat pump at every building. Your captured stream is only useful if it can meet the network it is feeding, directly or after a lift. Everything below follows from that one matching condition.

The grade you can capture is dictated by the cooling modality, which is why the cooling-architecture chapters are upstream of this one. Air cooling and rear-door heat exchangers (Chapter 5.2, Chapter 5.3) hand you a diffuse, low-grade stream — return air or RDHx water around 30–40 °C — spread over large flow rates and large surface areas, which is expensive to concentrate. Direct-to-chip liquid (Chapter 5.4) concentrates almost all the heat into a single small-flow, high-temperature stream: NVIDIA's Rubin liquid-cooling example describes about 45 °C coolant entering and about 55 °C leaving the fully liquid-cooled chip; other D2C products require their own OEM TCS/FWS limits and selected ΔT. That concentration, not any extra heat, is what makes DLC the enabler of credible reuse — it delivers usable grade in a form a heat exchanger can grab cheaply.

The table sorts halls by cooling modality. If your hall is air-cooled, you are in the top row — a large heat pump, a marginal COP, and an offtaker who must be either very close or very forgiving. If your hall runs warm-water DLC, you are in the second row — the row where a warmer source can make the numbers work by shortening the required heat-pump lift, because a modest lift from the selected validated return condition to a 4th-generation network's 60–70 °C supply can make the captured heat worth selling if the selected heat-pump curve supports, for example, COP 4–6 at those actual conditions and the customer takes the heat. This is the quiet reason that the cooling decision in Chapter 5.4 and the loop-temperature decision in Chapter 5.7 are the real heat-reuse decisions. By the time you reach this chapter, the grade is mostly already set.

A heat pump moves heat from a cooler source to a warmer sink and consumes electricity to do it. For waste-heat upgrade, the source is your DLC or RDHx return and the sink is the district-heating supply. The governing number is the coefficient of performance (COP): useful heat delivered per unit of electrical work. COP falls as the temperature lift (sink temperature minus source temperature) rises — this is Carnot's tax, and it is unavoidable. A heat pump lifting a 50 °C source to a 65 °C sink (a 15 °C lift) might run at a COP of 5–6; the same pump lifting a 30 °C air-side source to an 80 °C legacy network (a 50 °C lift) drops toward a COP of 2.5–3.5. Industrial high-temperature heat pumps for district heating typically span a COP of roughly 2.5–6 across this range.

The COP sets both the electricity you burn and the carbon you account for, which is why it dominates the economics. At a COP of 5, you deliver 5 MW of heat for 1 MW of electricity; the captured 4 MW of waste heat plus 1 MW of pump work leave as sellable heat. At COP 3, fixed 5 MW delivery requires 5/3 MW of electricity, about two-thirds more unit input than COP 5, and on a power-bound site that 1 MW is megawatts you could have spent on accelerators. A low-COP heat-reuse scheme is a tax on the IT capacity of the building — the clearest POWER-BOUND consequence in the cooling stack — and the warmer the source water, the smaller that tax. Designing the DLC loop warm (Chapter 5.7) is therefore the biggest lever on heat-reuse economics, because it shrinks the lift before the pump ever runs.

Heat reuse adds a third thermal domain to the building. The technology-cooling loop (chip to CDU) and the facility-water loop (CDU to heat rejection) already exist from Chapter 5.6 and Chapter 5.7. Heat reuse interposes a third loop — the heat-recovery loop — that runs from the reuse HX, optionally through a heat pump, into a hot-water buffer, and across an isolation heat exchanger to the offtaker's network. The interface point — the boundary where your water meets the district utility's water — is a hydraulically and contractually critical plane, and it is almost always a plate heat exchanger that keeps the two networks chemically and pressure-isolated. Neither side wants the other's water, the other's chemistry, or the other's pressure transients; the HX is the demarcation.

The integration's hardest engineering problem is that the data center can keep throwing off heat while the city stops asking for it — source heat and customer demand vary independently. An AI hall rejects near-baseload heat 8,760 hours a year; district-heating demand can swing from a January peak to a July trough as space-heating loads disappear, and the summer trough can fall below the heat the data center produces. The plant must therefore be designed to reject the heat anyway when there is no buyer — the dry coolers and towers of Chapter 5.8 never leave the building, because heat reuse is an opportunistic heat sink layered on top of a mandatory one, never a replacement for it. A common mistake is to size the rejection plant assuming the offtaker absorbs the load; the offtaker disappears every summer and on every demand dip, and the GPUs do not stop. Warm-water DLC is the enabler not only because it produces usable grade, but because the same warm loop lets compressor-free equipment do both jobs: the recovery exchanger takes the grade the offtaker can use, and the dry cooler rejects whatever is left — the whole load, on the days there is no buyer. A passive dry cooler exchanging with cold winter ambient removes heat; it never upgrades it, so any lift the network's supply spec demands comes from a heat pump you size and power.

Most data-center waste heat is still dumped for want of coordination, not for want of thermodynamics. Heat reuse requires two parties with very different planning horizons to arrive at the same place at the same time. The data center is built in 18–36 months against an interconnection slot and a depreciation clock. A district-heating network expansion, a new greenhouse, or an industrial-process tie-in is a multi-year municipal or industrial capital project with its own permitting, its own financing, and its own committee. Each side is reluctant to commit capital before the other has: the utility will not lay pipe to a heat source that may never deliver; the operator will not buy heat pumps and reuse HXs for an offtaker that may never connect. That is the offtake chicken-and-egg, and it is an engineering constraint, not just a commercial one, because the loop you build must survive the period when one side exists and the other does not.

The engineering responses are concrete. Plumb for reuse without committing to it: install the tie-in stubs, reserve the plant-room footprint and electrical headroom for a future heat pump, and route the pipe-rack so a reuse HX can drop in — because the irreversible substrate (loop taps, space, power) is cheap at construction and expensive to retrofit, exactly the reversible-vs-irreversible discipline from Chapter 1.1. Design the plant to run profitably with zero offtake on day one: the mandatory rejection plant must stand alone, so the reuse loop can sit dormant for a year or three until the offtaker materializes. Decouple the capital split contractually so neither party fronts the other's risk — the model that broke the deadlock at scale is Stockholm Exergi's Open District Heating, where the operator invests in the heat pumps and the utility invests in the network connection, and the heat trades as a standardized, temperature-indexed product. That platform connects 30-plus data centers across 16 providers in Eurelectric’s May 2026 report (p. 48), and a single site can earn on the order of SEK 2M per MW per year for heat it would otherwise have thrown away. Stockholm Exergi’s heat-recovery page gives that payment example and makes it dependent on outdoor temperature. Transfer agreed availability, temperatures, metering boundary and connection date to the contract in 15.5; another city’s revenue does not price this interface.

Two 2026 thresholds bound the engineering: EU EED Article 26 makes facilities above 1 MW assess waste-heat recovery, and Germany's EnEfG sets a minimum energy-reuse factor of 10% for data centers starting operation from 1 July 2026, rising to a planned 15% and 20% for the 2027 and 2028 cohorts. Use Chapter 15.5 to establish the applicable heat-reuse duty, cost-benefit assessment and offtake contract. Hand over source and delivered heat, temperature limits, annual coincidence, electrical headroom, fallback capacity and connection schedule. A reuse obligation supplies neither a buyer nor a heat-pump COP; those remain explicit engineering and commercial inputs.

Unit input = 320/4.00 = 80.0 kW; recovered heat = 320 − 80.0 = 240 kW; source flow = 240 × 60/(4.18 × 10.0) = 344 L/min. Source balance: 240 recovered + 83.0 rejected = 323 kW. Delivery balance: 240 source + 80.0 electrical = 320 kW. The separate 80.0 kW IT air duty remains on room cooling.

Annual source recovery is 480 MWh, delivery 640 MWh, unit electricity 160 MWh and auxiliary electricity 20.0 MWh. System COP = 640/(160 + 20.0) = 3.56. Available annual source heat is 323 × 8,760 = 2,829.48 MWh; normal rejection receives 2,349.48 MWh, about 2.35 GWh. At zero offtake the full 323 kW returns to the independent sink; an 83.0 kW sink fails by 240 kW.

Select the branch with the 90.0 kW electrical allowance and full fallback. Crossover unit COP = 320/(90.0 − 10.0) = 4.00. At COP 3.00, total input is about 117 kW, exceeding the allowance by 27 kW: reduce delivered duty or change the selection. ASHRAE/NEMA/PNNL framework provides system-boundary context; this chapter owns the balance, 15.5 values offtake and 13.5 accepts fallback.

Choose heat recovery only with a closed source, delivery and auxiliary-energy balance and a sink whose hourly demand overlaps the heat. Retain rejection for the no-offtake state; an attractive seasonal heat sale does not remove a year-round cooling obligation.