How do I connect a CHP system to my existing plant?

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Answer by Dale Blundell (Atkins)

Hydraulic / Thermal:

• The primary issue is whether the CHP system is to operate as lead ‘boiler’ in parallel with the peaking boilers, or whether the system is placed in series with the boilers, pre-heating the return water to minimise or hold-off the peaking boilers.
• For a low pressure how water (LPHW) system, there would typically be a heat exchanger separating the existing and the new hydraulic systems.
• Where the interface is a steam system, a waste heat boiler or steam generator may be employed.
• The steam conditions need to match the existing system. Modifications to existing systems may need to be undertaken to accommodate the new installation.

Electrical considerations:

• Design of connections to existing switchgear will need to consider the cable route, method of fixing and heat dissipation, cable sizing and whether there is space within the electrical panels to accommodate the new connection and isolation switches.
• The capacity and protection systems of the existing switchgear needs to be assessed. If export is planned, then the incoming supply cables need to be suitable to carry this load.
• If connection is to be made to a High Voltage system, then consideration needs to be made for transformers, HV switchgear and possibly ring-main units. HV systems may also require maintenance personnel to be qualified to operate or work on HV systems – this in turn may incur additional costs.
• If the plant is to operate both in parallel with the grid and in isolation (island mode), then earthing arrangements need to be switched accordingly to ensure only one earthing point at any one time.

Gas considerations:

• CHP units consume more gas per kW of heat output than a typical boiler. It is therefore important to understand if the gas pipework has sufficient capacity to deliver the extra gas.

Exhaust flue considerations:

• CHP units have limits on the back pressure, which is partly determined by the size of the flue duct. If the route is long, then the duct may need to be increased in size to compensate. In addition, twin walled ducting may need to be employed to restrict the heat loss from the gases to maintain flue gas buoyancy for adequate dispersion, and to avoid cooling the gases below dew point to minimise corrosive condensation forming.  Silencers will need to be incorporated in the flue system to reduce noise levels.