How to Calculate DAC Waste-Heat Utilisation
Heat supply dominates the energy balance of solvent-based direct air capture (DAC) systems. Many projects co-locate with industrial facilities to harvest low- or medium-grade waste heat, but stakeholders need a clear metric showing what fraction of the DAC heat duty can realistically be served. This guide defines a utilisation factor that blends available waste heat, delivery efficiency, and auxiliary penalties.
The method pairs naturally with regeneration energy benchmarking in the DAC regeneration energy guide and storage sizing exercises using the Thermal Storage Sizing calculator. Together they offer a coherent view of heat sourcing, buffering, and dispatch.
Definition
The waste-heat utilisation factor is the percentage of a DAC plant's thermal demand that can be met using recovered waste heat after accounting for delivery losses and auxiliary energy penalties. It is expressed over a consistent time base—typically an hour or a day—and can be scaled to annual outlooks if both supply and demand scale proportionally.
A utilisation factor below 100% signals the need for supplementary heat sources such as electric heaters, heat pumps, or additional waste-heat streams. A factor above 100% indicates surplus heat that could be diverted to storage, district energy, or other co-located processes.
Variables and units
Track variables in megawatt-hours (MWh) over the same time base to maintain comparability. Where plant data logs in megajoules or BTU, convert to MWh using standard factors.
- Qavail – Available waste heat (MWh).
- Qdemand – DAC process heat demand over the interval (MWh).
- ηdel – Delivery efficiency accounting for piping and exchanger losses (%).
- Paux – Auxiliary electricity penalty expressed as thermal equivalent (MWh).
- U – Utilisation factor (%).
If auxiliary power is measured in MWhelectric, convert to thermal by dividing by heater efficiency or multiplying by heat pump coefficient of performance, depending on the auxiliary technology supplying the deficit.
Formula
We first adjust available waste heat by delivery efficiency to find usable heat. We then subtract auxiliary penalties, clamp negative values to zero, and divide by the DAC heat demand.
Qdelivered = Qavail × ηdel
Qnet = max(0, Qdelivered − Paux)
U = 100 × min(1, Qnet ÷ Qdemand)
The min() operator caps utilisation at 100% to prevent exaggerated coverage. Reporting the residual deficit or surplus provides actionable insight into whether to add storage, reroute heat, or trim auxiliary loads.
Step-by-step workflow
1. Characterise heat supply
Measure or model the waste-heat profile from host facilities—steel reheaters, data center condensers, or geothermal brines. Use hourly data to capture variability. Record temperature levels to ensure compatibility with DAC sorbent regeneration requirements.
2. Estimate delivery efficiency
Calculate expected losses from piping length, insulation quality, and exchanger approach temperature. If detailed models are absent, use empirical ranges (85–95%) and refine after commissioning tests by comparing supply and return temperatures.
3. Quantify auxiliary penalties
Convert blower, pumping, or heat pump electricity into an equivalent thermal penalty. For electric heaters, divide by efficiency; for heat pumps, divide by coefficient of performance. Include parasitic loads from solvent circulation if they materially heat the fluid.
4. Compute utilisation and balance
Apply the formula to obtain U along with any deficit (Qdemand − Qnet) or surplus (Qnet − Qdemand). Use the balance to size supplemental heaters or storage.
5. Iterate with operations
Recompute weekly during ramp-up and whenever host facility loads shift. Integrate the calculation into your data historian so dispatch teams can plan DAC throughput around heat availability, similar to how operators manage carbon intensity using the levelised cost of hydrogen framework for electrolysers.
Validation
Validate utilisation estimates by comparing the calculated Qnet against measured regeneration duty during steady-state operation. If actual steam or hot water consumption exceeds the model, revisit delivery efficiency assumptions and auxiliary power measurements. Cross-check with energy balances used in your carbon accounting, especially if reporting under low-carbon product standards.
Run sensitivity analyses: vary ηdel by ±5% and Paux by expected maintenance ranges to understand headroom. When utilisation hovers near 100%, small degradations can create deficits; storage or redundancy may be warranted.
Limits and caveats
The calculation assumes that available heat and DAC demand overlap temporally. Seasonal or daily mismatches may reduce practical utilisation even when averages suggest full coverage. Incorporate storage modelling when profiles are misaligned.
Quality of heat matters. Low-temperature streams may not reach sorbent regeneration setpoints without heat pumps, raising auxiliary penalties. Likewise, fouling or corrosion in exchangers can degrade efficiency over time; schedule periodic recalculations after maintenance.
Embed: DAC waste-heat utilisation calculator
Use the embedded calculator to turn hourly waste-heat profiles and auxiliary loads into a utilisation factor for investor updates, operational dashboards, and permitting submissions.