chp - Combined Heat and Power
A boiler which generates electricity from the hot gas.
An operating Whispergen CHP boiler unit
Typically a household boiler will use gas (or electricity) to heat water for use around the house. In a conventional heating system the boiler boils water which is then stored in the large hot tank for use as and when needed. More modern systems do away with the hot tank and simply heat water on demand - called combi boilers.
In both examples the boiler can be a condensing boiler. These increase efficiency by pre-heating the incoming cold water with the excess hot exhaust gas. However, a recent invention is the CHP unit. This is a condensing boiler which also generates electricity. This electricity can be used by the house or sold back to the grid.
The electricity is generated by a tiny efficient engine (called a Stirling engine) integrated into the unit. This engine can produce up to a 1kwh of electrical power. This electrical power is then fed back into the grid, with the electrical power being sold per-unit to the local electrical supplier.
The inset figure on the right shows an operational Whispergen CHP boiler.