A combined heat and power (CHP) plant generates electricity and useful heat from a single fuel source at the same time. Instead of letting heat escape as waste, the way a conventional power station does, a CHP plant captures that thermal energy and puts it to work. This simple principle pushes total energy efficiency above 80 percent, compared to roughly 50 percent when electricity and heat are produced separately.
The Basic Principle: One Fuel, Two Outputs
Every power plant that burns fuel produces enormous amounts of heat. In a traditional setup, that heat exits through exhaust stacks and cooling towers, wasted. A CHP plant reroutes that heat through recovery equipment so it can be used for space heating, hot water, steam for industrial processes, or even cooling. The electricity goes where electricity always goes: into the grid or directly into the building. But the thermal energy, which would otherwise be lost, gets a second life.
A CHP system has four core parts: a prime mover (the engine or turbine that burns fuel), a generator (which converts mechanical energy into electricity), a heat recovery unit (which captures exhaust heat), and an electrical interconnection (which ties the system into the broader power network). The type of prime mover is what defines the system.
Three Main Ways CHP Plants Generate Power
Gas Turbines With Heat Recovery
A combustion turbine burns fuel, typically natural gas, to spin a shaft connected to a generator. The exhaust gases leaving the turbine are extremely hot, often above 500°C. Rather than venting those gases, the plant channels them through a heat recovery steam generator. Inside this unit, the hot exhaust heats water into steam or simply heats water directly. That steam or hot water then flows to wherever thermal energy is needed: a factory floor, a hospital’s heating system, or a district heating network serving an entire neighborhood.
Reciprocating Engines With Heat Recovery
These work like large versions of a car engine. They burn fuel in cylinders, and the mechanical motion drives a generator. Heat is recovered from two places: the hot exhaust stream and the engine’s cooling system (the jacket water that keeps the engine from overheating). Reciprocating engines are common in smaller CHP installations because they start up quickly, respond well to changing electrical loads, and work efficiently even at partial output.
Steam Turbines
This approach flips the sequence. A boiler burns fuel to produce high-pressure steam first. That steam spins a turbine connected to a generator, producing electricity. The steam that exits the turbine, now at lower pressure but still carrying significant thermal energy, is routed to heating systems or industrial processes. Steam turbine CHP plants are especially common in industries that need large amounts of process steam, like paper manufacturing or chemical production.
How Waste Heat Gets Captured
The heat recovery unit is what separates a CHP plant from a regular power plant. In gas turbine and engine systems, the most common setup is a waste heat boiler. Hot exhaust gases pass over tubes filled with pressurized water. The water absorbs the heat and turns to steam, which can then drive additional equipment or be piped directly to buildings.
When exhaust temperatures are lower, some plants use an organic Rankine cycle instead. This works on the same principle as a steam turbine, but replaces water with a fluid that has a much lower boiling point. That means the system can extract useful energy from heat sources that aren’t hot enough to boil water efficiently. Another variation, the Kalina cycle, uses a water-ammonia mixture as the working fluid for even better performance at moderate temperatures.
In reciprocating engine systems, heat also comes from the engine block cooling water and the oil cooling circuit. These lower-temperature heat streams are ideal for heating buildings or preheating water in industrial settings.
What Fuels CHP Plants Use
Natural gas dominates, fueling about 72 percent of CHP capacity. But roughly 15 percent of systems run on biomass, biogas, landfill gas, or waste fuels, and that share is growing. Biogas from food waste, animal manure, and wastewater treatment plants can feed directly into many existing CHP engines and turbines without major modifications.
Hydrogen is emerging as a future fuel. Most existing turbines and engines can already handle hydrogen blended in at 20 to 40 percent of the fuel mix. All major engine and turbine manufacturers are developing units capable of running on 100 percent hydrogen, with targets around 2030. The U.S. Department of Energy’s equipment catalog already lists four CHP systems rated for pure hydrogen, alongside 32 systems designed for hydrogen blends and 42 built for digester gas.
Why CHP Is So Much More Efficient
When you generate electricity at a distant power plant and heat your building with a separate boiler, you’re running two independent energy systems, each with its own losses. The power plant wastes heat. Electricity loses energy traveling through transmission lines. Your boiler burns fuel with its own inefficiencies. Combined, these separate systems convert only about 50 percent of the fuel’s energy into something useful.
A CHP plant, by producing both outputs on-site from one fuel source, eliminates transmission losses and captures heat that would otherwise be wasted. Total system efficiency tops 80 percent. In carbon terms, a typical 1-megawatt CHP system produces about 4,200 tons of CO₂ per year, roughly half the 8,300 tons that separate grid electricity and a conventional boiler would emit to deliver the same energy.
Where CHP Plants Are Used
CHP plants operate across a wide range of scales. Large industrial facilities like refineries, paper mills, and chemical plants use multi-megawatt systems where the steam does double duty in manufacturing processes. Hospitals, universities, and military bases often run CHP to ensure reliable power and heating from a single source. District energy systems pipe hot water or steam from a central CHP plant through underground networks to heat dozens or even hundreds of buildings across a city center.
At the small end, micro-CHP units sized for individual homes have been in development since the 1990s. These use technologies like Stirling engines, small internal combustion engines, or fuel cells as their prime movers. Fuel cell micro-CHP systems are particularly promising because they convert fuel to electricity through an electrochemical reaction rather than combustion, reaching electrical efficiencies of 35 to 45 percent. That’s roughly double what small combustion-based systems achieve.
Trigeneration: Adding Cooling to the Mix
Some CHP plants go a step further and produce chilled water for air conditioning, a setup called trigeneration or CCHP (combined cooling, heat, and power). The trick is an absorption chiller, which uses heat instead of electricity to drive a cooling cycle. Where a conventional air conditioner compresses refrigerant with an electric motor, an absorption chiller uses a chemical process: an absorbent fluid bonds with refrigerant vapor, effectively compressing it by changing it from gas to liquid. A small pump moves this mixture to a generator chamber, where recovered heat from the CHP system boils off the refrigerant. The refrigerant then condenses and evaporates in a loop that pulls heat from the building, just like a standard chiller would.
This is especially valuable in climates where heating demand drops in summer. Instead of leaving the CHP plant’s thermal output unused for months, the absorption chiller converts that heat into cooling, keeping the system productive year-round.