A surprising fact: a 10 MW electrical demand can be paired with a CHP system as small as 500 kW if the usable thermal load is low, because CHP sizing is fundamentally thermal-limited, not just electrical. Many engineers overlook that the heat-to-power ratio (HPR) directly caps the electric capacity derived from recovered heat.
The Formula
The core of the CHP sizing screening model is:
CHP Size (kW) = min(Electrical Demand, Thermal-Limited Electric Capacity)
Thermal-Limited Electric Capacity = Usable Thermal Demand / Heat-to-Power Ratio
Variables:
- Electrical Demand (kW): The facility's peak or average electrical load. This sets the upper bound for CHP electric capacity; you never want to generate more electricity than you can use on-site (or export if allowed).
- Usable Thermal Demand (kWth): The thermal load that can actually be recovered and used (e.g., process heat, space heating, hot water). This is not the total thermal demand but the portion that aligns with CHP operation. In Imperial units, 1 MMBtu/h = 293.071 kWth.
- Heat-to-Power Ratio (HPR, kWth/kWe): The ratio of thermal output to electric output for the CHP technology. For example, a reciprocating engine might have HPR = 1.5, a microturbine HPR = 2.0. A higher HPR means more heat per unit electricity, so for a given thermal load, the thermal-limited electric capacity is lower.
Why min()? The CHP system cannot exceed either limit: you can't sell more electricity than the load (or grid allows), and you can't recover more heat than the facility can use. The smaller of the two constraints determines the viable size.
Worked Example 1: Thermal-Limited Scenario
Consider a food processing plant with:
- Electrical Demand: 2,000 kW
- Usable Thermal Demand: 1,500 kWth (converted from 5.12 MMBtu/h)
- HPR: 2.5 kWth/kWe (typical for a gas turbine)
Step 1: Compute Thermal-Limited Electric Capacity:
Thermal-Limited Capacity = 1,500 / 2.5 = 600 kW
Step 2: CHP Size = min(2,000, 600) = 600 kW
The CHP system is thermal-limited. Even though the plant could use 2,000 kW of electricity, the usable heat only supports 600 kW of electric generation. Installing a larger system would waste heat and reduce economic viability.
Worked Example 2: Electrical-Limited Scenario
An office building with:
- Electrical Demand: 500 kW
- Usable Thermal Demand: 800 kWth (2.73 MMBtu/h)
- HPR: 1.2 kWth/kWe (efficient reciprocating engine)
Step 1: Thermal-Limited Capacity = 800 / 1.2 = 666.67 kW
Step 2: CHP Size = min(500, 666.67) = 500 kW
Here the electrical demand is the binding constraint. The building can recover more heat than needed to support 500 kW of generation, but the system should not exceed the electrical load to avoid exporting power (unless export is valued).
What Engineers Often Miss
Thermal load usability is not total thermal load. Many projects fail because they input total boiler capacity instead of the actual recoverable and usable thermal demand. If the plant only needs heat for 8 hours a day, the usable thermal demand should reflect that, not peak capacity.
Heat-to-power ratio varies with part-load operation. The HPR is not constant; at partial load, efficiency drops and the HPR may increase. A screening model uses a fixed HPR, but detailed design must account for off-design performance.
Seasonal and diurnal load profiles matter. A CHP sized for winter thermal load may be oversized in summer. The screening model uses a single point; real projects require hourly or monthly load duration curves to avoid oversizing.
Try the Calculator
Use the Cogeneration CHP Sizing Calculator to quickly evaluate your facility's potential CHP size. Input your electrical and thermal loads, adjust the heat-to-power ratio, and get an immediate screening result.
Top comments (0)