Both installations must deliver the same electricity demand E and heat demand H. The left plant is a cogeneration (CHP) unit: fuel spins a turbine to make electricity, and the exhaust steam — which a normal power plant would just dump into a cooling tower — is instead routed to a heat-recovery loop that supplies a district-heating network. The right side is separate production: a stand-alone power plant makes only electricity (dumping its waste heat up the cooling tower), and a stand-alone boiler burns extra fuel just to make heat.
Fuel_CHP = E/effE + max(0, H − E/effE·effH)/effBoiler
Fuel_sep = E/effE_sep + H/effBoiler
savings % = 1 − Fuel_CHP / Fuel_sep
- Electricity/heat demand — the fixed loads both plants must meet; raising heat demand past what the turbine's exhaust can supply forces the CHP unit to fire an auxiliary "topping" boiler too (shown as a second, smaller fuel stream).
- Turbine electrical efficiency — a CHP turbine sacrifices some electrical efficiency (steam is extracted before full expansion) compared with a condensing-only power plant, fixed here at 0.50 for the separate side.
- Heat recovery efficiency — how much of the turbine's rejected heat is actually captured as usable district heat rather than vented; a higher value means less needs to come from the topping boiler, and it needs venting once heat available exceeds heat demanded.
Real-world relevance: because the CHP unit reuses heat that a separate power plant would waste through its cooling tower, it needs noticeably less total fuel to deliver the same electricity and heat — the gap you see in "Fuel saved by CHP" is the practical case for district cogeneration over separate generation.