Module 4 · Unit 5
Read the unit content below, then use the case studies and key terms to review what you have learned.
By the end of this session, learners should be able to:
District-heating systems can use heat that would otherwise be released to:
Recovering suitable surplus heat can:
Recovery should not reduce incentives to improve the source process itself.
Recovered heat is not automatically renewable, low-carbon or low-cost.
Surplus heat
Heat leaving a process that is not currently used by that process.
Waste heat
A common term for heat that would otherwise be rejected to the environment.
Recovered heat
Surplus heat that is captured and put to a useful purpose.
Screening principle
Use the heat hierarchy:
Heat export should use genuinely available surplus heat—not create additional energy consumption merely to produce saleable heat.
Source temperature high enough
If the source temperature is high enough (e.g., industrial flue gases or waste-to-energy), it can be transferred to the network through a heat exchanger.
Source temperature too low
Urban sources like data centers or sewage systems often provide low-grade heat. When the source temperature is lower than the network's required flow, large-scale heat pumps are commonly required to raise the temperature to a usable level.
Supply-line integration
Used when the recovered-heat system can deliver the required supply temperature.
Possible benefit: directly displaces central heat production.
Important checks:
Return-line integration
May be used when the source can raise return-water temperature but cannot meet supply temperature.
Possible benefit: preheats water before central generation.
Possible disadvantage:
Return-line integration is project-specific rather than inherently good or bad. Assess the effect on plant efficiency, network ΔT, hydraulic capacity and dispatch before selecting it.
Storage can help when:
Monitor:
Storage shifts heat in time. It does not solve insufficient annual energy, unsuitable temperature or long-term source unreliability.
| Criterion | Questions to ask |
|---|---|
| Temperature | Minimum, normal and maximum source temperature? |
| Capacity | Recoverable kW or MW? Annual MWh? |
| Availability | Hours per day, days per year and seasonal pattern? |
| Demand match | Is network demand present when the source operates? |
| Distance | Pipe length, route and elevation? |
| Reliability | Planned shutdowns, process variability and expected site lifetime? |
| Heat quality | Clean water, contaminated water, air or process gas? |
| Integration need | Heat exchanger, heat pump, storage or backup? |
| Carbon benefit | Which existing heat source will be displaced? |
| Commercial interface | Ownership, pricing, guarantees and maintenance responsibility? |
| Economic potential | Calculate the technically recoverable and usable heat, then assess whether its recovery is economically viable. |
Potential sectors include:
Source forms
flue gas;
cooling water;
hot product or process streams;
condensate;
exhaust air;
refrigeration condenser heat.
Possible strengths
Possible limitations
Potential sectors include:
Typical characteristics:
Heat may be recovered from:
Key screening questions
What temperature is available?
Is the cooling load stable throughout the year?
Is a heat pump required?
Is the heat-network connection close enough?
Can the data centre reject heat independently if the network is unavailable?
Who owns and operates the recovery equipment?
How will source availability be guaranteed?
Main principle
The district-heating connection must never compromise data-centre cooling or server availability.
Wastewater can provide a comparatively stable low-temperature source.
Possible recovery locations include:
A typical system includes:
Key risks
Possible warning patterns
First checks
A pattern is abnormal only when it differs from the approved control sequence.
Source availability
Delivered heat
Conversion equipment
Network response
Environmental reporting
Carbon benefit should be calculated using an agreed methodology—not inferred from heat-meter data alone.
Source
source-side recoverable heat: 2 MW
source temperature: 30°C
availability: 24 hours/day
network supply/return: 70/40°C
connection distance: 800 m
Proposed arrangement
Questions
Can the heat be used directly?
Why is a heat pump required?
What data are needed to estimate COP and annual delivery?
What happens when network demand is low?
Which party must maintain backup heat rejection?
What evidence is needed before approving the project?
A reference collection of real-world projects for student research.
Each entry gives the project name, core parameters, and a link to find more detail.
Several projects appear in more than one section (e.g., Vojens, Braedstrup, Varanto) because real DH decarbonisation projects typically combine storage, renewables, and waste heat rather than using a single technology in isolation - worth highlighting when comparing case studies.
Denmark's largest heat-pump-based waste-heat recovery scheme: low-grade heat (-27 °C) from Meta's hyperscale Tietgenbyen data centre is boosted by large ammonia heat pumps to 70-75 °C DH supply temperature. Phase 1 (2020) delivered-24 MW/ -100,000-125,000 MWh/yr to - 7,000 homes; full build-out targets -45 MW and 215,000 MWh/yr, covering over 12,000 homes.
https://www.ramboll.com/en-us/projects/energy/meta-surplus-heat-to-district-heating
and
Surplus heat from data centre used for district heating
Follow-on project to the Meta case: surplus heat from industrial curing/drying ovens at a metal-can manufacturing plant (Envases) is now captured and exported into Fjernvarme Fyn's DH network, illustrating replication of a data-centre heat-recovery business model to conventional industry.
Closing the Loop: HOW INDUSTRIAL-UTILITY SYMBIOSIS IN ODENSE FORGES A BLUEPRINT FOR DECARBONIZATION
As well as being a TES case study (Section 1), Varanto is designed specifically to store surplus/waste heat from Vantaa's waste-to-energy incineration and low-price-hour electric boilers - a good cross-cutting example linking waste heat recovery with seasonal storage.
Varanto – The Cavern Thermal Energy Storage
A city-wide programme in which Stockholm Exergi pays data-centre tenants for their waste heat, integrating multiple facilities' surplus heat into the metropolitan DH grid; a useful example of a commercial/contractual model for waste-heat sourcing (vs. a single flagship project).
Home
Recovers heat at -1,000 °C lost from a steel annealing oven and feeds it into both the municipal DH network and the plant's own process/space heating; project delivered by Kyotherm with Schneider Electric, saving -4,000 t CO2/yr.
https://www.renewablethermal.org/wp-content/uploads/2022/03/Press-release-Kyotherm-Waste-Heat-project.pdf
Waste heat from the LUMI pre-exascale supercomputer (hosted by CSC - IT Center for Science) is recovered and fed into the local district heating network, an example of waste heat recovery from HPC/data infrastructure outside a major metropolitan area.
https://www.datacenterdynamics.com/en/news/lumi-supercomputer-use-district-heating-cooling/
Thermal energy leaving a process, installation or service activity that is not used for its primary purpose. “Waste heat”, “excess heat” and “surplus heat” are not fully interchangeable in engineering, contracts and legislation, so the intended meaning should be stated. Once useful heat has been captured and delivered to another process or network, “recovered heat” is usually the clearest operational term. It is not automatically renewable, low-cost, low-carbon or continuously available.
A regulatory accounting concept for heat or cold generated as a by-product that would otherwise be dissipated and that cannot reasonably be avoided or used on site after appropriate efficiency and heat-integration measures. Eligibility depends on the applicable jurisdiction and methodology. The broader engineering inventory of rejected heat can therefore be larger than the amount eligible to be reported as “waste heat and cold” under energy-policy rules.
The source is the process or system from which heat can be recovered; the sink is the network, building or process that can use it. Integration requires compatibility in temperature, capacity, timing, location, reliability and commercial conditions. A large source is not a viable opportunity unless a suitable sink exists or can be created.
Theoretical potential is the total heat rejected within a stated boundary. Technically recoverable potential accounts for access, heat-exchanger limits, process constraints and feasible recovery equipment. Usable or integrable potential additionally accounts for sink temperatures, timing, connection losses, upgrading and network constraints. Economic potential is the portion that meets specified cost, risk and investment criteria. These levels must not be reported as if they were equivalent.
Recoverable thermal power is the instantaneous or rated rate of useful heat recovery, expressed in kW or MW at stated conditions. Recoverable energy is the heat available over a period, such as MWh per year, after applying the source profile and availability. Annual energy cannot be inferred reliably from peak power without operating-hour and part-load data.
A decision sequence that first avoids unnecessary heat generation, improves process efficiency and considers feasible on-site heat recovery before exporting residual heat. External district-heating use should be compared with internal process heating, preheating, drying, absorption cooling and other higher-value uses. The hierarchy is especially important when determining whether heat is genuinely unavoidable for regulatory accounting.
A practical classification including industrial processes, data centres, commercial refrigeration, wastewater systems, power generation, waste-to-energy plants, electrolysers and other urban infrastructure. Categories help organise screening, but each site must be characterised by measured temperature, flow, operating schedule, process constraints and ownership rather than by generic assumptions.
A structured technical, economic, environmental and contractual evaluation of a candidate source. It should cover recoverable power and energy, temperature profiles, source and demand coincidence, connection route, return-temperature compatibility, heat-pump and auxiliary energy, storage and backup needs, process risks, metering, permissions, ownership, emissions accounting, investment performance and uncertainty. A screening study should clearly separate measured data from estimates and assumptions.
Recoverable heat from process streams, cooling water, condensate, exhaust gases, furnaces, compressors or refrigeration within manufacturing and processing facilities. Temperature and continuity vary widely, and recovery can affect the host process, product quality, pressure levels, maintenance or future production. Pinch analysis or another systematic process-integration method should be used where appropriate before assuming heat is available for export.
Heat removed from information-technology equipment and supporting systems by the data-centre cooling system. Available temperature depends strongly on air cooling, rear-door heat exchangers, direct-to-chip liquid cooling, immersion cooling and the selected recovery boundary. IT load can provide a comparatively continuous source, but heat quantity, temperature, free-cooling operation, redundancy rules and future computing load must be verified. Heat pumps are often needed for conventional district-heating temperatures.
Condenser heat rejected by refrigeration equipment serving supermarkets, cold stores, ice rinks or industrial cooling processes. Output follows the cooling load and includes both extracted cooling energy and compressor input. Availability can be high in warm periods when district-heating demand is low, so local sinks, storage or low-temperature networks may be important. Refrigerant system design and heat-recovery pressure levels must remain compatible with safe refrigeration operation.
Recovery of sensible heat from raw wastewater, sewer flows, treatment-plant effluent or sludge-processing systems, normally through a heat exchanger and heat pump. Temperature and flow are site- and season-specific. Design must consider minimum sewer temperature, biological-treatment effects, hydraulic restrictions, fouling, corrosion, maintenance access, environmental permits and whether heat extraction is located upstream or downstream of treatment.
Potential recovery from condenser systems, flue gases, cooling circuits or other residual streams associated with electricity or waste treatment. Useful heat deliberately produced by cogeneration is not automatically “waste heat”. Accounting and feasibility must distinguish useful cogenerated heat from genuinely residual heat, and must consider flue-gas condensation, corrosion, emissions controls, plant dispatch and the long-term role of the host facility.
Heat generated by electrolysis stacks, power electronics, gas compression and auxiliary cooling systems. Its temperature and recoverable share depend on electrolyser technology, operating pressure, load factor and cooling design. The opportunity may be attractive near heat networks, but project appraisal must reflect uncertain hydrogen-production schedules, electricity-market operation and possible changes in plant scale.
Heat rejected by metro systems, hospitals, laundries, commercial buildings, telecommunications facilities, transformer stations, cooling plants and similar urban activities. Individual sources may be smaller or spatially distributed, so aggregation through an ambient loop, local low-temperature network or shared heat pump may be more suitable than a long dedicated connection to a high-temperature network.
The risk that recoverable heat declines or disappears because the host process changes, improves efficiency, relocates, closes or changes operating hours. Heat-recovery infrastructure often has a longer lifetime than the source contract. Feasibility studies and agreements should therefore consider alternative sources, asset reuse, termination provisions, stranded-asset exposure and realistic residual value.
Use of a source at a sufficient temperature to meet the sink requirement without a heat pump. “Direct” refers to temperature upgrading and does not necessarily mean direct hydraulic mixing: a heat exchanger may still separate process and district-network fluids. Pumps, controls and temperature losses remain relevant.
Sequential use of heat by progressively lower-temperature sinks, for example process heating followed by district heating and then low-temperature preheating. Cascading can improve overall exergy use, but the sequence must respect source ownership, simultaneous demand, contamination barriers and the effect of each use on the temperature available to the next sink.
Raising recovered heat to a useful delivery temperature, most commonly with a heat pump. Upgrading requirements depend on source temperature, network supply and return conditions, heat-exchanger approach temperatures and operating mode. A heat pump is not always required, and it is not automatically the largest capital-cost item; connection pipework, civil works and source modifications can dominate.
Separation of the source, recovery and district-network fluid circuits by heat exchangers or hydraulic interfaces. It protects pressure regimes and water quality and allows independent flow control, but adds approach-temperature loss and pumping demand. The need for separation depends on network rules, fluid compatibility and safety requirements.
The physical location and hydraulic arrangement where recovered heat enters the district network or plant. Options include connection to supply, return, a local branch, a generation plant header or an ambient loop. The preferred point depends on source temperature, network pressure, flow direction, return-temperature effects, local capacity, control authority and the ability to prevent unwanted circulation.
The ability of a source to deliver useful heat at the required network injection temperature while accepting the available return temperature. Compatibility varies by season and operating state. A source may be usable directly at low network temperatures but require a heat pump, cascading connection or return-line integration at higher temperatures.
Potentially recoverable heat that cannot be accepted because demand, storage capacity, connection flow, temperature conditions or equipment availability are insufficient. Curtailment should be quantified separately from unavoidable technical losses. It is a key indicator for storage sizing, dispatch optimisation and contract design.
Backup capacity covers unplanned or planned unavailability of a main source, while peak-load capacity covers demand above the economical or technical output of base-load sources. The functions can be provided by the same asset but are conceptually different. Backup and peak equipment may use electricity, renewable fuels, stored heat, conventional boilers or other sources; it is not necessarily gas-fired.
Planned verification that the recovery system is installed, configured and able to meet specified hydraulic, thermal, electrical, control, metering, safety and communication requirements. Tests should cover normal and failure modes, source and network interlocks, heat-meter data, temperature and flow performance, alarms, backup operation and documentation. Acceptance criteria and witness responsibilities should be agreed before testing begins.
The practice-question document includes answers and explanations for self-study.
When you are ready, take the Unit 5 quiz.