All combustion-based electricity generators create waste heat as a by-product.  In Combined Heat and Power (CHP), the generators are located close to a process that can make use of the waste heat, thereby increasing overall efficiency.  CHP is also called “cogeneration”.

Heat users can be district heating schemes, industrial process plants, or commercial buildings.  Using absorption chilling, the heat can be used to provide cooling.  This is sometimes known as “trigeneration”, or CCHP.

How can Flexitricity work with CHP generators?

Flexitricity increases CHP revenue by exploiting flexibility in the generation.  Flexibility can come from:

• Thermal storage, such as hot water stores, phase change materials, or inherent thermal inertia in a heat distribution system;
• Extra capacity which is not used all year round, including “N+1” generator configurations, steam extraction systems, and units which run limited hours during off-peak seasons.
• CHP generators operating on certain shift patterns.

There are two ways for a CHP generator to use its flexibility:

1. CHP generators which can supply short-duration increases in generation can earn revenue from reserve services.  Flexitricity can arrange this with no involvement from the Electricity Supplier which purchases the generation.
2. CHP generators which cannot provide reserve can nevertheless increase their trading income by dynamically adjusting their generation to the most lucrative times.  Flexitricity will need to involve the Electricity Supplier which purchases the generation at an early stage.

Steam extraction (“pass-out”) CHP

In steam extraction CHP, heat is provided by extracting steam from an intermediate stage in the steam turbine.  By reducing the extraction flow, electricity generation increases.  Heat demand can be satisfied either by drawing down the thermal inertia in the system, or by operating standby boilers.

This ability to quickly increase generation for short periods can be extremely valuable.  The greater the increase, the greater the revenue, but it isn’t necessary to run the turbine at maximum power.

Reciprocating engines, gas turbines, and “back pressure” CHP

In most types of CHP, turning up the generation means turning up the heat output.  Thermal storage or thermal inertia can be flexed so that electricity generation is maximum when the value of electricity is maximum.  This also enables the generator to exploit every opportunity to provide heat using the CHP generator instead of a standby boiler.

In small CHP generators based on reciprocating engines and gas turbines, it is normal for each engine or turbine to be operated either at maximum continuous rating, or not at all.  Flexibility is found during periods when generators are not required to operate continuously.

Working within constraints

All CHP generators have limitations on the flexibility they provide, including:

• The generator must not miss any opportunities to supply low-carbon heat to its heat customers;
• Qualification as “good quality CHP” must be unaffected;
• Heat or coolth supplied must stay within temperature boundaries;
• Excessively frequent generator starts must be avoided.

It is essential to identify these limits on a site-by-site basis, and work within them.  By live measurement and continual re-forecasting of a CHP generator’s ability to operate flexibly, Flexitricity can ensure that generators exploit their flexibility while prioritising the core business activity of low-carbon heat and power supply.

 

Picture supplied by: www.carboncare.co.uk

 

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