Technical Details

The components of a mini CHP system
A Combined heat and power system is made up of the following key components (summarised in the diagram below).

1. Prime Mover - an engine to drive the generator
2. Generator - to produce electricity, which is fed into the building's power distribution system
3. Heat Recovery - to recover usable heat from the engine
4. Control System - to maintain safe and efficient operation
5. Enclosure - to achieve physical and environmental protection for the engine and operators, and to reduce noise.

Prime Movers
The prime mover is the engine. Prime movers for a small-scale CHP scheme may be chosen from:

• Gas fired spark ignition engines
• Compression ignition engines (diesel)
• Gas turbines

Gas turbines in the UK at present are generally only economically applicable for higher power applications.
The two main types of fuel are natural gas and diesel oil. The main fuel available for natural gas installations is delivered by the national pipeline system, but other gas fuels, biogas, landfill gas or LPG may be available at some sites or as standby.

Generator
Generators create electricity by moving an electrical conductor through a magnetic field. The electricity produced can be used directly at the point of installation.

Heat Recovery System
CHP engines produce heat as well as movement as the result of the combustion of fuel. The heat that is not converted to electrical output may be recovered. Heat is recovered from exhaust gases, and from the engine cooling system.
Heat recovery, normally by heat exchanger to the appropriate building services, is worthwhile in direct proportion to the amount of heat that can be used. The overall thermal efficiency of the CHP scheme may be as high as 90% where suitable uses can be made of the heat. In many cases, however, it is not possible to use all the available heat, and a lower efficiency will be achieved.

Control System
Control, instrumentation and monitoring of CHP systems is concerned with:

• Ensuring that the CHP engine runs safely and efficiently
• Integrating the heat produced with the building heat systems
• Synchronising, paralleling and disconnecting the generator safely
• Monitoring the CHP plant for performance, maintenance and accounting purposes.

Systems may be based on relay or solid state logic systems but are increasingly designed around user-friendly microprocessor controllers communicating with remote PC based control and monitoring stations, often at the CHP plant supplier's offices.

Enclosures and Mountings
Engines used for CHP always require some form of enclosure and mounting to provide protection from the elements and to reduce noise and vibration.

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