Project Center · Annual matching
Annual load overlap hours, storage, and HX vs heat pump
Screening benefits are limited by how often waste heat and heat demand overlap—not by rating × 8760. Direct heat exchange mostly moves waste heat to users; a closed booster heat pump adds compressor electricity into the useful heat, so the bill must subtract that power. Pair with Start a Project overlap fields and heat exchangers.
1 · What overlap hours mean
Overlap hours answer: in one year, how many hours are waste heat and useful-heat demand both present? Only that window feeds the annual energy account in Project Center screening.
Overlap ≠ plant operating hours
A factory may run many hours with refrigeration on but no hot-water draw—or hot water needed while the refrigeration plant is off. Those hours do not count as overlap.
Low / medium / high bins
Project Center splits overlap into three load bins for screening (not a weather forecast term). Defaults scale design waste-heat availability and heat demand (about 0.6× / 1.0× / 1.1× on the source side and 0.5× / 1.0× / 1.1× on demand). Each bin contributes roughly min(available, demand) × hours.
Sum of the three bins
Low + medium + high overlap hours are the time window used for annual matched heat. The total must be ≤ 8760 and should not exceed confirmed annual availability. Do not fill all three toward 8760 unless the site truly aligns almost continuously.
2 · Example: ammonia reject heat → process hot water
Food plant: design recoverable reject heat ≈ 500 kW; design hot-water demand ≈ 400 kW. Illustrative overlap bins (similar to Project Center demos):
| Bin | Hours / year | Typical scene |
|---|---|---|
| Low | 800 h | Nights / weekends: both sides present but light |
| Medium | 2 500 h | Day shift: reject heat and hot water largely together |
| High | 1 200 h | Peak season: both sides near full load when they overlap |
Sum ≈ 4 500 h/year. The remaining ~4 000+ hours are often misaligned or idle—they do not enter the annual useful-heat total by default. Filling medium as 8 760 would roughly double the paper savings.
3 · Direct HX vs closed booster heat pump
The same overlap window feeds annual hours, but what the user receives and what the bill must subtract depend on recovery topology.
A · Direct heat exchanger (temperature already enough)
Useful heat to the sink ≈ heat taken from the waste-heat stream (after approach / losses). Almost no booster electricity. Outlet temperature cannot meaningfully exceed the source condensing / saturation temperature minus approach—see heat exchangers. Prefer HX when temperatures allow; keep it as the fair baseline against booster options.
B · Closed booster heat pump (need a temperature lift)
Steady-state intuition (losses aside): Quseful ≈ Qfrom waste heat + Welectric, or COP ≈ Quseful / Welectric. Electricity both increases useful heat and adds cost. Benefits must subtract booster kWh × tariff (plus any refrigeration power penalty and recovery auxiliaries, minus condenser-fan savings). Project Center uses an incremental boundary vs incumbent heat—not “free heat from COP”.
C · Hybrid (direct first, booster for the gap)
Recover what fits by HX with little or no lift electricity; use a closed booster only for the remaining duty or hotter stream. Often the best energy balance when loads span a temperature range.
| Item | Direct HX | Booster heat pump |
|---|---|---|
| Where waste heat goes | Through the exchanger to users | Into the booster evaporator |
| Where electricity goes | Little / auxiliaries only | Into the compressor—and into useful heat |
| Useful heat from | ≈ waste heat | ≈ waste heat + electricity |
| Role of overlap hours | How long heat can be moved | How long heat can be moved and lifted |
| Benefit sensitivity | Whether ΔT / flow really exchanges | COP, power price, whether the lift is worth buying |
One-sentence memory: a heat exchanger moves waste heat; a heat pump moves waste heat and folds electricity into useful heat—so never credit the extra heat without the power cost.
4 · Where thermal storage helps
Storage addresses time misalignment. It does not turn 4 500 overlap hours into 8 760.
Without storage
In a bin where available waste heat exceeds demand, the surplus is unused heat for that period—annual matched heat and savings fall.
With storage (HX path)
You can park recovered heat that users cannot take yet, then discharge when demand returns—limited by storage capacity (kWh) and charge/discharge practicality.
With storage (heat-pump path)
Storage still mainly shifts thermal timing. Electricity is still paid when the booster runs. Do not assume “heat in the tank means the kWh were free.” Screening may credit a limited recovery of unused heat from storage capacity; fine dispatch and cycle counts are not fully monetized without more data.
When storage is not enough
If overlap is chronically low, enlarge storage alone rarely fixes the economics—first confirm that process demand can really accept the waste-heat grade and schedule.
Need a capacity-level tank size? Open the Thermal Storage Agent (water sensible screening). For stratified-tank physics, use ASWT.
5 · How to fill Project Center fields
Practical order for the wizard’s operating-overlap step.
- Judge temperature first: if direct HX can meet the sink, think overlap as “hours both streams are present for exchange.”
- If a booster is required: still enter hours when waste heat and heat demand coexist. Do not inflate overlap because electricity also becomes heat.
- Split low / medium / high: when unsure, put most hours in the medium bin; keep the sum honest.
- Storage: fill capacity when shifts clearly misalign; otherwise leave blank and read the no-storage baseline first.
- Read results by topology: HX path → matched heat; booster path → matched useful heat minus incremental electricity cost.
Open Start a Project (phase-change / reject-heat path), size storage with the Thermal Storage Agent, or return to the Project Center.
6 · Quick FAQs
“So only the ~4 500 overlap hours count for benefits?”
Yes for the annual matched-heat time window in screening. Storage may reclaim a limited share of heat that was available but not simultaneous with demand—it does not invent full-year alignment.
“On a heat pump, isn’t useful heat larger than waste heat alone?”
Yes: Quseful ≈ Qfrom source + Welectric. That extra heat is paid for as electricity. Incremental COP and net benefit already treat power as a cost, not free energy.
“Should I enter plant operating hours into the overlap fields?”
No. Enter hours when waste heat and demand overlap. Sensible single-phase paths may use annual operating hours differently; phase-change / reject-heat screening wants overlap bins.
“Where next on this site?”
Overlap bins, storage credits, and topology electricity boundaries here match Open Thermal AI Project Center screening intuition. They are not a substitute for measured load profiles, OEM performance maps, or contract energy models.