Combined Heat and Power Unit

Overview

A combined heat and power unit, also called a cogeneration unit, burns fuel once and sells the energy twice. A reciprocating gas engine drives an electrical generator, and the heat that an ordinary generator would throw away is captured and piped to the site as hot water. Power-only generation typically converts about 40 percent of the fuel into electricity and loses the rest as warm air and hot exhaust. A CHP unit recovers most of that loss, so the total useful output reaches roughly 85 to 90 percent of the fuel energy. That single number is the reason the technology exists.

The package is built around the Gas Engine Prime Mover and the Alternator. Around them sit four heat recovery stages, a hydraulic Heating Circuit Module, a Control and Electrical cabinet, and an Acoustic Enclosure that keeps the noise in and the weather out. A unit of this kind serves hospitals, district heating schemes, greenhouses, leisure centres and any site that has a steady year-round demand for both electricity and heat.

Prime mover and generator

The engine is a spark-ignition reciprocating unit running on natural gas at a fixed 1500 rpm, which gives 50 Hz directly from a four-pole alternator. The Engine Block is a cast iron V-block with wet Cylinder Liner bores and a water jacket that doubles as the first heat source. Each cylinder carries its own Cylinder Head with two intake and two exhaust valves, and the Piston Assembly drives the Crankshaft through a forged Connecting Rod. A pair of Turbocharger units boost the air and gas charge, which lets a given engine frame make more power without growing in size.

The Alternator is a brushless synchronous machine that shares the Stator Assembly and Rotor Assembly used across the heavy rotating-machine catalogue. A Automatic Voltage Regulator holds terminal voltage steady as the load swings, and a flexible Flexible Coupling joins the alternator to the engine flywheel so torsional pulses do not crack the shafts. Mechanical power becomes electrical power here, and everything downstream of this point is about catching the heat.

Jacket water heat recovery

The first and largest low-grade heat stream comes from the engine cooling water. Instead of dumping it through a radiator, the Jacket Water Heat Recovery stage passes the hot jacket water through a Jacket Plate Exchanger, transferring the heat into the building circuit while keeping the engine coolant separate. A Jacket Water Pump circulates around 30 cubic metres per hour, and a Thermostat Valve holds the engine outlet near 90 C, which is hot enough to be useful but well below the boiling point inside the pressurized loop. The jacket water typically supplies water at 90 C flow and 70 C return, the standard temperatures for commercial heating circuits.

Exhaust gas heat recovery

The hottest energy leaves through the exhaust at 450 to 500 C, and recovering it is what separates a real CHP unit from a plain genset with a coolant tap. The Exhaust Gas Heat Recovery stage runs the gas through a Exhaust Gas Heat Exchanger, a finned tube boiler that boosts the heating water to its final temperature. Downstream of it a Condensing Economizer cools the flue gas below its dew point, condensing water vapour out of the stream and harvesting the latent heat that a conventional boiler wastes up the stack. A Exhaust Bypass Valve diverts the gas straight to atmosphere when the site cannot accept any more heat, which protects the engine from overheating during low-demand periods.

Oil, charge-air and heating circuit

Two smaller streams round out the recovery. The Lube Oil and Intercooler Recovery stage cools the lube oil through a Oil Cooler Exchanger and takes the first stage of charge-air heat through an Intercooler Exchanger, both of which preheat the return water before it reaches the jacket stage. An Lube Oil Pump built from a Helical Gear Pair keeps oil pressure at around 5 bar.

All the recovered heat collects in the Heating Circuit Module, a hydraulic skid that delivers hot water to the site. A Heating Circuit Pump drives the distribution loop, motorized three-way valves stage the flow between the recovery exchangers and the building, and an Expansion Vessel absorbs the volume change as the water heats and cools. When demand falls below the unit output, a Emergency Dump Radiator rejects the surplus so the engine can keep generating electricity even when the site needs no more heat.

Control, enclosure and exhaust

The Control and Electrical cabinet runs the whole package. A CHP Controller sequences the start, ramps the engine to speed, and a Synchronizer matches voltage, frequency and phase before the Main Circuit Breaker closes the unit onto the grid. A Protection Relay trips the breaker on grid faults, and the Power and Heat Meter tracks both exported power and delivered heat.

The complete machine sits inside an Acoustic Enclosure lined for sound, bringing the noise down to 65 to 75 dBA at one metre, and stands on Anti-Vibration Mount isolators that keep engine vibration out of the building. The spent exhaust finally passes through the Exhaust Line, where a Exhaust Silencer quiets it and a Oxidation Catalyst cleans up carbon monoxide and unburned hydrocarbons before the gas reaches the flue.