Sizing a custom 18–150 kW natural gas CHP unit around heat demand

Size a custom 18-150kW natural gas CHP generator against the heat your facility can absorb during its intended operating hours. The strongest starting point is a sustained thermal load at a compatible temperature, supported by simultaneous electricity demand. Peak boiler capacity and annual gas consumption alone cannot establish that operating point: both can conceal long periods when recovered heat has nowhere useful to go.

A suitable unit often supplies the persistent portion of heat demand while boilers cover peaks and maintenance periods. The final electrical rating follows from the selected engine’s recoverable heat output, site conditions, and control limits. Establishing those relationships first prevents an apparently attractive electrical capacity from becoming an oversized thermal installation.

Define the heat load at the connection point

Build the demand profile around the circuit that will actually receive CHP heat. Separate space heating, domestic hot water, process heating, and any other proposed heat sinks. Each has different schedules and temperature requirements. Combining them into one annual energy figure hides whether the loads occur together and whether the same recovery circuit can serve them.

Measured flow and supply-to-return temperature difference provide a useful basis for calculating delivered heat. For water circuits, thermal power follows mass flow multiplied by specific heat capacity and temperature difference. Use the appropriate fluid properties where glycol is present, and confirm meter placement: measurements across a boiler header can include circulation or losses that do not represent useful demand at the proposed CHP connection.

Where only fuel records exist, converting boiler fuel consumption into useful heat requires an efficiency assumption suited to actual operation. Nameplate efficiency does not capture cycling, standby losses, or changing return temperatures. Treat that conversion as an estimate until measurements establish the load more reliably.

Plot a heat-load duration curve to reveal how much demand persists across operating hours. Then retain the chronological profile as well. A duration curve sorts loads and loses their sequence, so it cannot reveal repeated short demand windows, overnight shutdowns, or the timing needed to evaluate thermal storage.

Temperature compatibility determines usable recovery

Total recoverable heat is only useful if it can cross the heat exchanger and meet the receiving circuit’s requirements. Engine jacket-water recovery and exhaust recovery operate under different temperature constraints. Their outputs should be stated separately, with the conditions under which each contribution is available.

A heating loop with a high return temperature leaves less temperature difference for heat transfer. That can reduce delivered heat or push the engine cooling circuit toward its operating limit. A nominal thermal rating measured with cooler return water therefore cannot be carried directly into a hotter process loop. Required supply temperature, maximum return temperature, flow range, and heat-exchanger approach temperature belong in the sizing specification.

For a process requiring temperatures beyond the proposed hot-water circuit’s capability, count only the heat that the selected recovery arrangement can deliver at that condition. Do not add all jacket-water and exhaust energy into a single usable figure without confirming the hydraulic arrangement and temperature levels.

The heat exchanger also separates engine coolant from the facility circuit. Its pressure drop, fouling allowance, materials, and fluid compatibility affect both heat delivery and pumping requirements. An undersized exchanger can make a correctly sized engine appear thermally inadequate; enlarging the engine would leave the underlying restriction unresolved.

Convert sustained heat demand into candidate electrical ratings

Once the receiving conditions are defined, request performance data for candidate units within the 18–150 kW electrical range. Use thermal output delivered at the agreed site interface, rather than heat available upstream of piping and exchanger losses. The heat-to-power relationship varies with engine design, loading, and recovery configuration, so one fixed ratio should not represent the whole range.

A preliminary relationship is:

Candidate electrical output = sustained usable thermal demand ÷ recoverable heat-to-electric-power ratio at the intended operating point.

This calculation identifies a starting capacity. The resulting candidate still needs evaluation against its actual load curve, minimum continuous operating load, and site derating. Gas composition and available pressure, ambient temperature, altitude, and cooling conditions should be included in the quoted performance basis wherever they affect output.

Compare candidates over the chronological demand profile. For each interval, useful CHP heat is limited by both available recovery and the connected heat demand, with storage handled separately. Also record boiler contribution, rejected heat, electrical generation, auxiliary consumption, and starts. This exposes a larger unit that produces more energy on paper while spending substantial time curtailed or rejecting heat.

Product specifications need careful interpretation during this comparison. The listing for 30-90kW biogas generator sets identifies model AMC30GFJ-PZ with rated output of 30 kW (37.5 kVA), 400 V, and 50 Hz. Those figures describe electrical ratings; they do not establish natural-gas suitability or recoverable CHP heat. Fuel compatibility and thermal performance require separate confirmation before such a specification enters a natural-gas CHP sizing calculation.

Resolve low-load operation before increasing capacity

Part-load performance determines whether a unit sized for sustained demand remains useful when that demand falls. Obtain electrical efficiency, recoverable thermal output, allowable temperatures, and minimum continuous load across the proposed operating range. A full-load efficiency figure offers little guidance for a machine expected to spend much of its schedule at reduced output.

If demand repeatedly falls below the unit’s usable thermal output at minimum load, the practical options include scheduled shutdown, storage, a smaller unit, or multiple modules. Frequent starting introduces warm-up periods and changes the relationship between running hours and delivered energy. The operating model should include those periods instead of assuming immediate steady-state recovery.

Thermal storage suits a timing mismatch when surplus heat can serve a later load. Size usable storage energy from the surplus accumulated over the charging period and the permitted temperature range. Tank volume alone is insufficient: stratification, minimum useful discharge temperature, and charging power determine how much stored energy reaches the load. Storage cannot resolve a persistent seasonal surplus without a later heat demand.

Multiple smaller units offer staged output when demand varies widely. Their benefit depends on each module’s minimum load, hydraulic arrangement, and sequencing logic. Compare that arrangement with a single unit using the same demand data, including auxiliary loads and available installation space.

Electrical constraints can change a heat-led selection

Overlay site electricity demand on the proposed thermal operating schedule. Where export is unavailable or restricted, low electrical demand can force the CHP unit to reduce output even while heat demand remains high. An electrical load-duration curve alone misses this problem because it does not show whether electricity and heat are required simultaneously.

Distinguish generator-terminal output from net electricity available to the facility. Pumps, ventilation, controls, and other package auxiliaries consume power. Compare net output with the site load, and specify how the controller responds when electrical demand falls: modulation, shutdown, permitted export, or another agreed operating mode.

Heat-led operation also needs a defined response when the receiving circuit stops accepting heat. Maintaining engine cooling may require a heat-rejection path, but rejected heat must remain outside useful-energy calculations. Boiler staging should accommodate CHP warm-up and outages without causing unstable return temperatures or unnecessary simultaneous firing.

Specify performance at the site interface

A reviewable sizing proposal states the selected electrical rating together with guaranteed thermal delivery at defined supply and return temperatures, fuel conditions, and loading points. It also identifies minimum stable operation, auxiliary consumption, control priorities, and the boundary between package equipment and site piping.

Commissioning measurements should use those same boundaries: net electrical output, delivered heat, fuel input, and receiving-circuit temperatures under representative load. If measured heat delivery falls short, flow, exchanger performance, return temperature, and engine loading provide distinct explanations. A capacity increase should follow evidence that the engine itself is undersized, rather than compensate for a restriction in heat transfer or control.