WHAT TO CONSIDER IN A CHP APPLICATION

An embedded CHP set is really a specialised type of engine driven electricity generator, so many of the considerations are electrical. This section is intended to help you to think about some of the aspects of any application in order of importance.

Whether a CHP set can be installed and make economies in energy use will be driven by a number of preliminary considerations:
1. Do the site's tariffs for gas and electricity utilities demonstrate that economies can be made?
2. Is there a reasonably consistent electrical demand for a substantial proportion of the day?
3. Is there a heat demand for a substantial portion of the year?
4. Are those demands large enough to justify the cost of installing a machine?
5. Are there short-term periods in the year when electricity is prohibitively expensive?
6. Is there a source of cheap fuel?

   Some other points which may influence the analysis:

7. Does the site have a three-phase power supply?
8. Does the site need a standby electrical power supply?
9. Is the size of the site's supply transformer restricting expansion and the cost of upgrading is considerable?
10. Is there space for the equipment?

A hierarchy of specialisations can be constructed, starting from a simple diesel generator to work in standby duty, and ranging through embedded generators using engines driven by different fuels. This schedule gives some idea of that hierarchy:

Standby generator ("island" operation).

Embedded generator (synchronised to grid on continuous duty or for "peak lopping" only).

Compression ignition engine (diesel).

Spark ignition engine -

Natural gas

Biogas (various sludge gases and landfill gases)

LPG

Engine cooled by radiator and engine driven fan.

Engine cooled by heat exchanger and fitted with electrically driven pump.

Extra heat recovered from exhaust.

Following on this analysis, a decision can be made about the type of machine to be installed, whether it be a peak lopping diesel set or a fully heat recovered, embedded CHP set. Fuel source must also be decided at an early point in the analysis.

The majority of locations where CHP is applicable will have a core demand of electricity. This core demand is for those pieces of equipment which are switched on continuously such as circulating water pumps, internal lights, personal computers, mainframe computers etc. Overlaying the core demand are the constantly fluctuating loads due to the switching on and off of the myriad items of electrically driven equipment - small heaters, kettles etc. The core demand figure can be obtained from an analysis of the site's maximum demand figures and will generally show a minimum figure as occurring during the summer period. Maximum demand reports are generally kept by the energy or utilities manager of the site.

The machine size is usually determined on this basis, that is the core electrical load during the lightest loaded part of the year. It is rare for the machine size to be determined any other way as electricity is the highest value component of the products of a CHP set. The value of that component can be assessed at what it would have cost had it been purchased from the site's electricity supplier, who, note, may not be the same body as is responsible for the actual connections. The value can be at the price inclusive of any levies which might be applicable.

Having decided on the notional electrical output of the machine, it is then necessary to decide on heat recovery figure. This can range from nothing (the engine would be cooled conventionally with a fan and radiator), through merely recovering the engine cooling jacket heat, up to recovering both the jacket heat plus all that is in the exhaust.

Some engines, being turbocharged, may also have a source of heat from the turbocharger intercooler. This may be useable, but is often of such low grade as to be of no value. A separate radiator would then be necessary to dispose of this heat.

Once the fuel, the electrical output of the machine and an approximate heat recovery figure ascertained, then other details can be considered.

Such details are :	- Mechanical -	Special provisions for biogases Enclosure required / not required / level of attenuation necessary Air routing through enclosure Ducted in and out Ducted in only, exhausted to room Ducted out only, induced from room Temperature of cooling water on / off / temperature rise Water flowrate through machine Percentage of vibration isolation necessary. Dump heat system required / not required Heat dumped via air blast radiator or to alternative heat exchanger Customer preferred engine Grade of exhaust silencing system Output controlled on external influences Power export prevention Heat output restriction Consideration of local skill level, component support available when selecting engine for overseas application.
	- Electrical -	Synchronous or asynchronous generation Relay or plc based panel Always embedded or occasional standby duty Location of "G59" breaker (q.v.) Remotely monitored operation Startup and Shutdown arrangements Panel location On machine Nearby on wall In separate room

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