CHP Systems and Renewable Energy
CHP fundamentals
Combined heat and power recovers waste heat from on-site generation. Prime movers include reciprocating engines (strong electrical efficiency plus recoverable jacket and exhaust heat), gas turbines and microturbines (high-temperature exhaust usable for steam or absorption cooling), fuel cells (high electrical efficiency, low emissions), and back-pressure steam turbines. Total CHP efficiency = (electrical output + useful thermal output) / fuel input, and well-matched systems reach 75 to 90%, compared with roughly 33 to 45% for grid power plus separate boilers.
Load matching is the design question
The real question is not how much can be generated but what the coincident thermal-to-electric ratio is. Reciprocating engines produce roughly 1 to 1.5 units of recoverable heat per unit of electricity; turbines are more electric-heavy. If thermal load is small, excess heat is dumped and overall efficiency collapses. Sizing to the base thermal load (thermal-led) maximizes efficiency; sizing to the electrical peak (electric-led) usually forces power export and standby and interconnection charges. The spark spread between fuel cost per kWh and purchased power price is the screening metric.
Renewables and traps
PV is rated in kW-DC at standard test conditions; annual output = kW x 8,760 x capacity factor, with typical U.S. capacity factors of 15 to 22%. Performance ratio captures soiling, wiring, inverter, and temperature losses. Traps: confusing module efficiency with capacity factor, and assuming nameplate kW equals annual kWh.
Worked example
A 1 MW CHP unit with 40% electrical and 45% recovered thermal efficiency: fuel input = 1,000 / 0.40 = 2,500 kW; useful thermal = 0.45 x 2,500 = 1,125 kW; total efficiency = 85%.